Abstract GS5-05: Resistance to neoadjuvant chemotherapy in triple negative breast cancer mediated by a reversible drug-tolerant state

Approximately 50% of patients with localized triple negative breast cancer (TNBC) have substantial residual cancer burden following treatment with neoadjuvant chemotherapy (NACT), resulting in distant metastasis and death for most of these patients. While genomic and phenotypic intra-tumor heterogeneity are pervasive features of TNBCs at the time of diagnosis, the functional contributions of heterogeneous tumor cell populations to chemoresistance have not been elucidated.

To investigate tumor evolution accompanying NACT, we employed orthotopic patient-derived xenograft (PDX) models of treatment-naïve TNBC, which retain intra-tumor heterogeneity characteristic of human TNBC. We discovered that some PDX models initially exhibited partial sensitivity to standard front-line NACT (Adriamycin plus Cytoxan, AC). Following AC, residual tumors were resistant to chemotherapy but repopulated tumors with chemo-sensitive cells if left untreated, indicating that tumor cells possessed inherent plasticity. To identify the tumor cell subpopulation(s) conferring chemoresistance, we conducted barcode-mediated clonal tracking in three independent PDX models by introducing a high-complexity pooled lentiviral barcode library into PDX tumor cells which were then orthotopically engrafted into recipient mice. Strikingly, residual tumors maintained the same heterogeneous clonal architecture as naïve tumors. Concordantly, whole-exome sequencing revealed conservation of genomic subclonal architecture throughout treatment. These results were corroborated by genomic sequencing of serial biopsies pre- and post-AC obtained directly from TNBC patients enrolled on an ongoing clinical trial at MD Anderson (ARTEMIS; NCT02276443). Together, these studies revealed that genomically distinct pre-treatment subclones were equally capable of surviving AC to reconstitute tumors after treatment.

To identify functional addictions of residual tumor cells, we conducted histologic and transcriptomic profiling. Residual tumors following AC-treatment exhibited extensive fibrotic desmoplasia and tumor cell pleomorphism in both PDX models and in serial biopsies obtained from TNBC patients enrolled on the ARTEMIS trial. Strikingly, these AC-induced features were reverted upon regrowth of residual tumors in PDXs and in patients' tumors. Similarly, residual tumors exhibited unique transcriptomic features, many of which are also de-regulated in cohorts of human TNBCs undergoing chemotherapy treatment. These features were nearly completely reverted after tumors regrew, suggesting that the residual tumor state may be a unique and transient therapeutic window. Gene set enrichment analyses revealed that residual tumors had increased activation of oxidative phosphorylation and decreased glycolytic signaling. Pharmacologic targeting of oxidative phosphorylation with a small-molecule inhibitor of mitochondrial electron transport chain complex I (IACS-010759) significantly delayed the regrowth of AC-treated residual tumors in three independent PDX models. Collectively, these studies reveal that a reversible phenotypic state can confer chemoresistance in the absence of genomic selection and that the residual tumor state is a novel therapeutic window for chemo-refractory TNBC.

Citation Format: Echeverria GV, Ge Z, Seth S, Jeter-Jones SL, Zhang X, Zhou X, Cai S, Tu Y, McCoy A, Peoples M, Lau R, Shao J, Sun Y, Bristow C, Carugo A, Ma X, Harris A, Wu Y, Moulder S, Symmans WF, Marszalek JR, Heffernan TP, Chang JT, Piwnica-Worms H. Resistance to neoadjuvant chemotherapy in triple negative breast cancer mediated by a reversible drug-tolerant state [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr GS5-05.

Abstract P3-07-01: Towards a therapeutically relevant subtyping scheme for triple-negative breast cancer (TNBC), profiling results from A Randomized, TNBC Enrolling trial to confirm Molecular profiling Improves Survival (ARTEMIS)

Triple-negative breast cancer is a highly diverse group of cancers, with poor prognosis, and currently, there are no targeted drugs available in the clinic. In TNBC around 50% percent of the patients respond to chemotherapy, while, the other 50% percent relapse with poor prognosis. There is a need to understand better the targetable mechanisms driving TNBC via integrative analysis of gene-expression, copy-number, and mutational data.

Samples from 220 triple-negative breast cancer (TNBC) pts treated with NACT were prioritized for transcriptomic and genomic profiling. Non-negative matrix factorization was used on array-based profiling to identify six robust (ARTEMIS) subtypes. Comparing ARTEMIS subtypes with Vanderbilt subtypes, revealed significant overlap with 4/6 clusters while identifying two new clusters. Logistic regression on ssGSEA scores vs. subtypes revealed several pathways, selectively enriched specific subtypes. CL1/IM (Immune subtype), was enriched in INFg and INFa, while CL2 (MYC/mTOR), showed enrichment of several proliferation-related pathways. In addition, LAR and M (Mesenchymal) pts formed overlapping clusters, using either method.

Two new subtypes did not associate significantly with any of the previous subtypes. The majority of the tumors from the Vanderbilt BL2 and MSL were reclassified into a CL5 (ANGIO) cluster, which was enriched in angiogenesis geneset, including targetable genes like VEGF and FGFR. Also, an MYO (CL3) subtype was identified, with myogenesis-related genes. Of note, TIL (tumor infiltrating lymphocytes) and LAR quantification using IHC were associated with respective ARTEMIS subtypes. Finally, the IM subtype was significantly associated with higher rates of RCB 0-I and the M (CL4) subtype was associated with higher rates of RCB II-III, irrespective of the neoadjuvant treatment regimen.

ARTEMIS subtypes are a novel classification system for TNBC that is focused on therapeutic translation. Further, we show a possibility to classify previously un-classified (UNS) tumors, which will be validated using additional cohorts (TCGA/METABRIC).

Citation Format: Seth S, Huo L, Rauch G, Lau R, Gilcrease M, Adrada B, Piwnica-Worms H, Symmans WF, Draetta G, Futreal AP, Moulder S, Chang JT. Towards a therapeutically relevant subtyping scheme for triple-negative breast cancer (TNBC), profiling results from A Randomized, TNBC Enrolling trial to confirm Molecular profiling Improves Survival (ARTEMIS) [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-07-01.

Abstract P3-07-03: Withdrawn

This abstract was withdrawn by the authors.

Citation Format: Rinkenbaugh AL, Sinha VC, Zhang X, Shao J, Piwnica-Worms H. Withdrawn [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-07-03.

Abstract P5-05-01: A molecularly annotated collection of breast cancer patient-derived xenograft models aligned with ongoing clinical trials built from fine needle aspiration samples throughout neoadjuvant treatment

BACKGROUND: Patient-derived xenograft (PDX) models of breast cancer replicate the diverse histologic and molecular features of patient tumors and provide a renewable source of human tumor tissue. However, collection of tissue by core needle biopsy is problematic due to patient discomfort, bleeding risk and the limited number of passes a patient can tolerate. Several studies have catalogued the maintenance of molecular features of patient tumors in PDX models of breast cancer.

METHODS: To support the neoadjuvant molecular diagnostic and drug development program in triple negative breast cancer (TNBC), a pilot study was conducted to determine if fine needle aspiration (FNA) could be used for building PDX models. Subsequently, PDX models are being established in alignment with ongoing clinical trials at MDACC. The molecular evolution of patient's tumors, matched with PDXs engrafted from their tumors, is under study throughout the neoadjuvant treatment of TNBC using RNA sequencing, whole-exome sequencing, deep sequencing of cancer genes, and histologic analyses.

RESULTS: To date, 20 established PDX models have been developed and stable PDX models continue to be generated at a rate of 2-3 per month. Several of these models are derived from serial FNAs derived from patients throughout neoadjuvant treatment. These models retain histologic and molecular features of the original patient tumors. Serial patient biopsies, matched with PDX models, have enabled measurement of the mutational and transcriptomic evolution in vivo of TNBC undergoing neoadjuvant treatment.

We have standardized the use of FNAs to generate PDX models both pre- and post-neoadjuvant therapy in the following ongoing neoadjuvant clinical trials:

1. MDACC 2014-0185 (PI Stacy Moulder, 360 patients), 'ARTEMIS: A Randomized TNBC-Enrolling trial to confirm Molecular profiling Improves Survival'

2. MDACC 2014-0045 (PI Jennifer Litton, 20+ patients), 'A pilot study of BMN673 as a neoadjuvant study in patients with a diagnosis of invasive breast cancer and a deleterious BRCA mutation'

CONCLUSION: We demonstrated that PDX models from tissue collected by FNA recapitulate the biology and clinical course of the patient's tumor. Sequencing analyses revealed that neoadjuvant chemotherapy and PDX engraftment enrich for cancer gene mutations. We observe association of the rate of successful PDX engraftment with clinical parameters such as the patient's residual cancer burden (RCB) status at the time of surgery (upon completion of neoadjuvant treatment). In addition, we observe that PDX models derived from serial patient biopsies throughout treatment are more resistant to chemotherapy treatment. These models recapitulate the variety of chemotherapy responses observed in patients with TNBC and serve as powerful tools for preclinical biomarker and discovery studies.

Citation Format: Echeverria GV, Cai S, Tu Y, McCoy A, Lau R, Redwood A, Rauch G, Adrada B, Candelaria R, Santiago L, Thompson A, Litton J, Moulder S, Symmans F, Chang JT, Piwnica-Worms H. A molecularly annotated collection of breast cancer patient-derived xenograft models aligned with ongoing clinical trials built from fine needle aspiration samples throughout neoadjuvant treatment [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-05-01.

Abstract P6-06-01: Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress

Agents that induce replication stress, such as inhibitors of Chk1 or ATR, are advancing in clinical development and are being tested for treatment of various solid tumors, including triple-negative breast cancer (TNBC). While the preclinical data are encouraging, additional studies are needed to predict with precision (i) which patients will most likely benefit from these inhibitors, (ii) the genetic and proteomic contexts in which these inhibitors will provide maximum therapeutic benefit as a single agent, or require additional sensitization via combination with a targeted- or chemotherapeutic agent, and (iii) exactly which targeted/chemotherapeutic agent will provide maximum therapeutic benefit for combination with replication stress inducers. To address these challenges in TNBC, we have attempted to gain a comprehensive understanding of how the DNA damage response pathways regulate TNBC cell survival in response to Chk1 inhibitors, by performing high throughput loss-of-function screens.

We have identified genes whose loss induces death of TNBC cells in the presence of (1) CHK1i alone, (2) chemotherapy alone or (3) CHK1i plus chemotherapy. In addition, given the role of TP53 as the most frequently mutated gene in TNBC, we also determined whether distinct vulnerabilities could be identified in TNBC cells that are p53-proficient versus p53-deficient. Thus, we have also identified the top synthetic lethal interactions that are either common to both p53-proficient and p53-deficient TNBC, or unique to p53-deficient TNBC; we are currently performing in vitro studies to validate the identified mechanisms. We anticipate these studies to be applicable to other agents that induce replication stress and cell cycle checkpoint bypass. Ongoing in vivo preclinical studies, which utilize patient-derived xenografts (PDXs) of TNBC to validate these findings are expected to impact patient selection for clinical trials, and also allow us to predict which chemotherapeutic agents will be most effective for combination with different cell-cycle checkpoint inhibitors.

Citation Format: Redwood AB, Seth S, Cai S, Piwnica-Worms H. Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-06-01.

Abstract P6-03-05: Risk of needle-track seeding with serial ultrasound guided biopsies in triple negative breast cancer

Background: Image-guided percutaneous needle biopsy of the breast is a common procedure. In breast cancer patients (pts) undergoing core biopsies and surgical resection on the same day, the rate of tumor cell displacement along the needle track has been reported to be up to 50%. However, the clinical significance of this finding in triple negative breast cancer (TNBC) patients (pts) undergoing serial biopsies while receiving neoadjuvant chemotherapy (NACT) is unknown. Here we report the incidence of needle-track seeding (NTS) in a cohort of TNBC pts enrolled on a molecular triaging protocol involving serial biopsies of the index breast lesion.

Methods: We reviewed the clinical records of 144 consecutive TNBC pts enrolled on a molecular triaging protocol at MD Anderson Cancer Center. Per protocol, all pts underwent a pre-treatment research biopsy and were initiated on anthracycline based NACT (AC). Pts with inadequate response to front-line NACT were encouraged to undergo additional biopsies of the index breast lesion prior to switching therapies. Serial breast ultrasound (US) was performed to monitor therapeutic response and incidental evidence of needle-track seeding noted on US was documented.

Results: Clinicopathological characteristics of the pts are summarized in Table 1. 89% (128/144) of pts had a diagnostic breast biopsy done at another center prior to presenting at MDACC. To date, we have performed 209 US guided biopsies of index breast lesions in 144 pts. 92% (193/209) of these biopsies were done mainly for research purposes. 1.4% (2/144) of pts were found to have evidence of NTS on follow up US. The first pt had a T1N0 (1.9cm), grade 3, invasive ductal carcinoma (IDC) at diagnosis. She underwent a diagnostic biopsy followed by a research biopsy before initiating AC. She was found to have NTS as well as progression of disease (PD) on follow up US after 2 cycles of AC. The second pt had a T2N0 (3cm), grade 3 IDC at diagnosis. She underwent a diagnostic biopsy at another center, followed by a research biopsy before initiating AC. Like the first pt, she was found to have NTS and PD on follow up US after 2 cycles of AC. Both pts are currently on neoadjuvant clinical trials of novel agents.

Conclusion: The rate of NTS detected on US in TNBC pts undergoing serial biopsies of index breast lesions while receiving NACT is low and further studies are needed to determine the impact of serial biopsies on long term outcomes in TNBC.

Table 1: Patient CharacteristicsCharacteristicN=144Age - Median (years, interquartile range)55 (46-62)Tumor Size Mean (cm, standard deviation)3.4 (2.2)T1 – n(%)35 (24)T2 – n(%)89 (62)T3 – n(%)19 (13)T4 – n(%)1 (1)Clinical Nodal Status Negative – n(%)74 (51)Positive – n(%)70 (49)Grade 1 – n(%)1 (1)2 – n(%)17 (12)3 – n(%)124 (86)Unknown – n(%)2 (1)Histologic Subtype Invasive ductal carcinoma – n(%)121 (84)Invasive lobular carcinoma – n(%)2 (1)Mixed ductal and lobular carcinoma – n(%)3 (2)Metaplastic carcinoma – n(%)13 (9)Not specified – n(%)5 (3)Laterality Right – n(%)72 (50)Left – n(%)72 (50)

Citation Format: Yam C, Santiago L, Candelaria RP, Adrada BE, Rauch GM, Hess KR, Litton JK, Piwnica-Worms H, Mittendorf EA, Ueno NT, Lim B, Murthy RK, Damodaran S, Helgason T, Huo L, Thompson AM, Gilcrease MZ, Symmans WF, Moulder SL, Yang W. Risk of needle-track seeding with serial ultrasound guided biopsies in triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-03-05.

Abstract P4-03-02: Characterizing and targeting chemoresistant subclones in patient-derived xenograft models of triple negative breast cancer

Fifty percent of all triple negative breast cancer (TNBC) patients harbor significant residual tumor burden following treatment with standard neoadjuvant chemotherapy (NACT), resulting in poor prognosis. Recent studies in TNBC have revealed extensive intra-tumoral heterogeneity at the time of diagnosis and throughout disease progression, but the relative contributions of these heterogeneous populations of tumor cells to chemoresistance are not well understood.

The primary tumor, dermal metastasis, and germline reference were obtained from a patient with untreated metastatic TNBC. Tumor cells were engrafted into the humanized mammary fat pads of NOD/SCID mice to establish PDX models of the primary (PIM001-P) and metastatic (PIM001-M) tumors. RNA sequencing and whole-exome sequencing (WES), performed on the patient's primary and metastatic tumors and the first- and third- passage PDX models revealed transcriptomic profiles and subclonal heterogeneity of the patient's tumors were recapitulated in the PDX models.

Treatment of mice engrafted with PIM001-P tumors with NACT (Adriamycin plus cyclophosphamide, AC) resulted in partial response, the magnitude of which was diminished in mice bearing PIM001-M tumors. Tumor subclones were tracked during chemotherapy treatment in mice engrafted with PIM001-P tumors using lentiviral non-targeting DNA barcodes. Residual tumors maintained the clonal architecture of untreated tumors, and deep WES revealed stable maintenance of somatic mutant allele frequencies throughout treatment. Therefore, selection of pre-existing resistant clones did not lead to AC resistance in this model. Interestingly, only 25% of residual tumor clones contributed to primary relapse once treatment was halted, suggesting only a subpopulation of tumor cells was able to reconstitute the tumor.

RNA sequencing and reverse phase protein array revealed that while vehicle-treated and regrown tumors were highly similar, residual tumors harbored a unique profile characterized by numerous significant alterations in RNA and protein levels. Together, these results suggest that residual tumors enter into a transient drug-resistant state that is reversible. Residual tumors were enriched for alterations in pathways such as metabolism, extracellular matrix remodeling, and cell-cell communication. Pharmacologic targeting of the residual tumor state with an inhibitor of mitochondrial oxidative phosphorylation led to significant inhibition of tumor regrowth following AC treatment. Additional vulnerabilities identified in residual tumors are being targeted therapeutically with the goal of eradicating residual tumor cells.

Citation Format: Echeverria GV, Seth S, Ge Z, Sun Y, DiFrancesco E, Lau R, Marszalek J, Moulder S, Symmans F, Heffernan TP, Chang JT, Piwnica-Worms H. Characterizing and targeting chemoresistant subclones in patient-derived xenograft models of triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-03-02.

Abstract GS6-06: Identifying metastatic drivers in patient-derived xenograft models of triple negative breast cancer

Metastases are responsible for the vast majority of deaths due to breast cancer. Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by high rates of metastasis and poor prognosis. We are employing patient derived xenograft (PDX) models of TNBC to identify drivers of metastasis. Tumor samples are obtained from the breast tumors of patients with TNBC and engrafted immediately into the humanized mammary fat pads of immune compromised mice. Lentiviral transduction was employed to express bioluminescent and fluorescent markers in two independent PDX models of TNBC. Using these models, we demonstrated that human breast tumors are capable of completing all stages of the metastatic cascade in mice, and metastatic lesions are observed in organs normally found in patients with metastatic breast cancer including lung, liver, bone, brain, and lymph nodes. Dynamic and reversible epithelial to mesenchymal transition (EMT) was observed as tumors metastasized to lung and were re-passaged to recipient mouse mammary glands. Lung metastases were isolated using bioluminescence imaging and lung metastasis gene expression signatures were generated. Metastasis signatures from two independent PDX models were compared to identify genes that were commonly de-regulated in lung metastases relative to corresponding mammary tumors. Comprehensive gain-of-function screens were then conducted in vivo to identify functional drivers of TNBC metastasis. Carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM5) was identified as a metastatic driver in this screen. CEACAM5 mRNA and protein levels were elevated in lung metastases relative to corresponding mammary gland tumors in mice. In addition, we demonstrated that CEACAM5 expression was upregulated in the lung metastases of breast cancer patients, and its expression inversely correlated with patient survival. Our data indicate that the metastatic function of CEACAM5 is to promote growth of breast tumors in the lung by inducing MET (mesenchymal to epithelial transition).

Citation Format: Powell E, Shao J, Picon HM, Ge Z, Echeverria GV, Peoples M, Bristow C, Cai S, Tu Y, McCoy AM, Piwnica-Worms D, Draetta G, Edwards JR, Moulder SL, Symmans WF, Heffernan TP, Liang H, Piwnica-Worms H. Identifying metastatic drivers in patient-derived xenograft models of triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr GS6-06.

Abstract P1-07-22: Androgen receptor positivity is associated with nodal disease in triple negative breast cancer

Background: Gene expression profiling (GEP) has identified several molecularly distinct subtypes of triple negative breast cancer (TNBC). Currently, GEP-based molecular diagnostics are not routinely used in clinical decision making due to the lack of proven benefit, costs involved and long turnaround time. However, two molecularly distinct subtypes of TNBC, the luminal androgen receptor (AR) and mesenchymal subtypes, have surrogate CLIA-certified immunohistochemical (IHC) markers, AR and vimentin (VM), respectively, which have the potential for application in the clinic. Here we report the rates of AR and VM positivity and their association with clinicopathological characteristics in a cohort of TNBC pts receiving NACT.

Methods: As part of an ongoing molecular triaging protocol, 144 pts with stage I-III TNBC underwent a pretreatment biopsy for molecular characterization (MC) prior to initiating neoadjuvant chemotherapy (NACT). IHC for AR and VM were performed using commercially available antibodies. AR+ and VM+ were defined as ≥10% and ≥50% staining, respectively. Pts were randomized 2:1 to know (intervention arm, n=93) and not know (control arm, n=51) the MC results. The charts of pts randomized to the intervention arm were reviewed. Categorical variables were analyzed using Fisher's exact test. Ordinal and continuous variables were analyzed using the Wilcoxon rank-sum test and Student's t test as appropriate.

Results: 31% (29/93) and 16% (15/93) of pts were AR+ and VM+, respectively. Only 4% (4/93) of pts were both AR+ and VM+. Clinicopathological characteristics are summarized in Table 1. AR+ pts were more likely to have clinically node positive disease as compared to AR- pts (66% vs 34%, p=0.007). There were no significant differences in clinical tumor size or grade between AR+ and AR- pts. VM+ and VM- pts had similar clinicopathological characteristics.

Conclusion: Pts with AR+ TNBC were more likely to have node positive disease. The impact of AR+ on long term outcomes should be investigated in prospective studies.

Table 1: Association between patient characteristics and AR/VM status AR  VM   AR+ (n=29)AR- (n=64)p-valueVM+ (n=15)VM- (n=78)p-valueAge - Median (years, interquartile range)58 (48-65)52 (46-61)0.05855 (48-64)56 (47-62)0.88Clinical Tumor Size      Mean (cm, standard deviation)3.5 (1.8)3.0 (1.8)0.2872.7 (1.7)3.3 (1.9)0.31T1 – n(%)5 (17)21 (33)0.2307 (47)19 (24)0.098T2 – n(%)21 (72)36 (56) 7 (47)50 (64) T3 – n(%)3 (10)7 (11) 1 (7)9 (12) Clinical Nodal Status      Negative – n(%)10 (34)42 (66)0.0078 (53)44 (56)1.00Positive – n(%)19 (66)22 (34) 7 (47)34 (44) Grade      2 – n(%)6 (21)5 (8)0.0763 (20)8 (10)0.293 – n(%)23 (79)59 (92) 12 (80)70 (90) 

Citation Format: Yam C, Huo L, Hess KR, Litton JK, Yang W, Piwnica-Worms H, Mittendorf EA, Ueno NT, Lim B, Murthy RK, Damodaran S, Helgason T, Thompson AM, Santiago L, Candelaria RP, Rauch GM, Adrada BE, Symmans WF, Gilcrease MZ, Moulder SL. Androgen receptor positivity is associated with nodal disease in triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-07-22.

A feasibility study of neoadjuvant talazoparib for operable breast cancer patients with a germline BRCA mutation demonstrates marked activity

This study was undertaken to determine the feasibility of enrolling breast cancer patients on a single-agent-targeted therapy trial before neoadjuvant chemotherapy. Specifically, we evaluated talazoparib in patients harboring a deleterious BRCA mutation (BRCA+). Patients with a germline BRCA mutation and ≥1 cm, HER2-negative primary tumors were eligible. Study participants underwent a pretreatment biopsy, 2 months of talazoparib, off-study core biopsy, anthracycline, and taxane-based chemotherapy ± carboplatin, followed by surgery. Volumetric changes in tumor size were determined by ultrasound at 1 and 2 months of therapy. Success was defined as 20 patients accrued within 2 years and <33% experienced a grade 4 toxicity. The study was stopped early after 13 patients (BRCA1 + n = 10; BRCA2 + n = 3) were accrued within 8 months with no grade 4 toxicities and only one patient requiring dose reduction due to grade 3 neutropenia. The median age was 40 years (range 25–55) and clinical stage included I (n = 2), II (n = 9), and III (n = 2). Most tumors (n = 9) were hormone receptor-negative, and one of these was metaplastic. Decreases in tumor volume occurred in all patients following 2 months of talazoparib; the median was 88% (range 30–98%). Common toxicities were neutropenia, anemia, thrombocytopenia, nausea, dizziness, and fatigue. Single-agent-targeted therapy trials are feasible in BRCA+ patients. Given the rapid rate of accrual, profound response and favorable toxicity profile, the feasibility study was modified into a phase II study to determine pathologic complete response rates after 4–6 months of single-agent talazoparib.

An investigational PARP inhibitor seems safe and possibly effective when given ahead of surgery to women with BRCA-mutated breast cancer. Jennifer Litton and colleagues from the University of Texas MD Anderson Cancer Center in Houston, USA, conducted a small feasibility study to see if patients with stage I-III breast cancer and inherited mutations in BRCA1 or BRCA2 would put off their standard course of chemotherapy ahead of surgery to first receive two months of talazoparib, an experimental inhibitor of poly ADP ribose polymerase (PARP), an enzyme involved in DNA damage repair. The trial was a resounding success. In fact, owing to rapid patient enrollment, decreases in tumor volume among all 13 participants and few signs of serious side effects, the researchers amended the study protocol to give talazoparib for longer and test for therapeutic efficacy.

Abstract P4-06-03: An annotated collection of pre- and post-therapy breast cancer patient-derived xenograft models built from fine needle aspiration samples aligned with ongoing clinical trials documenting response to treatment

BACKGROUND: Patient-derived xenograft (PDX) models of breast cancer replicate the diverse histologic and molecular features of patient tumors and provide a renewable source of human tumor tissue; however collection of tissue by core needle biopsy is problematic due to patient discomfort, bleeding risk and the limited number of passes a patient can tolerate. In addition, FDA guidelines caution that multiple core needle biopsies could lead to an overestimation of the true pCR rate in neoadjuvant trials.

METHODS: To support the neoadjuvant molecular diagnostic and drug development program in TNBC, a pilot study was conducted to determine if fine needle aspiration (FNA) could be used for building PDX models. Prior to engraftment, FNA samples were analysed for cell number and viability.

RESULTS: Six PDX models were successfully generated from eight individual tumor samples. These models retain histologic and molecular features of the original patient tumors as determined by immunohistochemistry, RNA expression profiling, and deep whole-exome and targeted gene sequencing. In addition, the tested PDX models recapitulate the responses to therapies across multiple chemotherapeutic agents.

Based on this success, we have standardized the use of FNAs to generate PDX models both pre- and post-therapy in two ongoing neoadjuvant clinical trials:

1. MDACC 2014-0185 (PI Stacy Moulder, 360 patients), 'Improving outcomes in TNBC using molecular triaging and diagnostic imaging to guide neoadjuvant therapy'

2. MDACC 2014-0045 (PI Jennifer Litton, 20+ patients), 'A pilot study of BMN673 as a neoadjuvant study in patients with a diagnosis of invasive breast cancer and a deleterious BRCA mutation'

 FNA cells (x10^4)Cell viability (%)Total viable cells (x10^4)Study entry biopsy (n=67)144.5050.6544.14Post treatment biopsy (n=16)47.0732.5428.38

To date, treatment-naïve primary tumor samples from 67 patients enrolled onto these neoadjuvant trials, and 16 matched non-responsive post treatment tumor samples have been analysed for cell count and viability (table below) prior to being engrafted into the humanized mammary fat pads of NOD/SCID mice.

CONCLUSION: We have demonstrated success in using FNAs to build PDX models that recapitulate the biology and clinical course of the original tumor. In our pilot study, we successfully generated six PDX models using FNA for TNBC, including some harboring deleterious BRCA1/2 mutations. Because of the high concordance in histologic, genomic, and clinical attributes, we are now using this approach to develop a rich resource of pre- and post-treatment PDX models for the investigation of therapeutic resistance.

Citation Format: Echeverria GV, Chang JT, Cai S, Tu Y, McCoy A, Lau R, Redwood A, Kaffiabasabadi S, Rauch GM, Adrada BE, Jennifer L, Moulder SL, Symmans WF, Piwnica-Worms H. An annotated collection of pre- and post-therapy breast cancer patient-derived xenograft models built from fine needle aspiration samples aligned with ongoing clinical trials documenting response to treatment [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-06-03.

Abstract P5-06-06: Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress

This abstract was not presented at the symposium.

Abstract OT2-01-22: NCT02456857: A phase II trial of liposomal doxorubicin, bevacizumab and everolimus (DAE) in patients (pts) with localized triple-negative breast cancer (TNBC) with tumors predicted insensitive to standard neoadjuvant chemotherapy (NACT)

BACKGROUND: Approximately 50% of TNBC pts treated with standard taxane/anthracycline-based NACT will have chemo-insensitive disease (CID) manifested as extensive residual disease (RCB-II or III) at the time of surgery. 40-80% of these pts will develop recurrence within 3 years of initial diagnosis. Recent advances in molecular profiling have identified subsets of TNBC with distinct, targetable molecular features. We developed a clinical trial to identify and characterize CID (ARTEMIS: A Randomized, TNBC Enrolling trial to confirm Molecular profiling Improves Survival). In the ARTEMIS trial, treatment naïve pts with localized TNBC undergo a pretreatment biopsy and then immediately start their initial phase of anthracycline-based chemotherapy so that the results of the molecular characterization are used in combination with response assessment (clinical exam/diagnostic imaging) to identify CID and inform the second phase of NACT, thus using a 'second hit' strategy in the middle of NACT to overcome drug resistance. The mesenchymal subtypes of TNBC have a high incidence of PI3K pathway activation. Preclinical models demonstrated response to PI3K inhibitors in this subtype. Metaplastic breast cancers make up ∼30% of tumors characterized as 'claudin-low/mesenchymal' by gene signature and are also associated with a high rate of PI3K activating molecular aberrations. A combination regimen of liposomal doxorubicin, bevacizumab and the mTOR inhibitors temsirolimus or everolimus (DAT or DAE) demonstrated response (including durable complete responses) in metastatic metaplastic breast cancer.

PRIMARY OBJECTIVE: Determine the rate of pathologic complete response (pCR/RCB-0) or minimal residual disease (RCB-I) after 4 cycles of DAE for treatment of mesenchymal TNBC deemed to be CID through the ARTEMIS trial

TRIAL DESIGN AND STATISTICAL METHODS: Only pts deemed to have mesenchymal CID on the ARTEMIS trial can enter this non-randomized phase II study. Realizing that pts without response to their initial cycles of chemotherapy have very low chance (5%) of achieving pCR with additional cycles of chemotherapy, it would be clinically meaningful to see pCR in this pt population improved to 20%. Counting pCR (RCB-0) or RCB-I as response, a two-stage Gehan-type design will be employed with 14 pts in the first stage. If at least one pt responds, 23 more pts will be added for a total of 37 pts. This design has a 49% chance of terminating after the first stage if the true response rate is 0.05, 23% chance if the true rate is 0.10, 10% if the true rate is 0.15 and 4% if the true rate is 0.20. If accrual continues to the second stage and a total of 37 pts are enrolled, the 95% confidence interval for a 0.20 response rate will extend from 0.10 to 0.35.

BRIEF ELIGIBILITY CRITERIA: Inclusion: localized TNBC enrolled onto ARTEMIS trial, adequate organ, bone marrow and cardiac parameters Exclusion: metastatic disease, pregnant or lactating pts, medical illness that increases chance of moderate to severe toxicity

CORRELATIVE SCIENCE: Correlate vimentin expression by IHC, mesenchymal signatures and PI3K pathway aberrations with response.

Citation Format: Moulder S, Hess K, Rauch M, Astrada B, Litton J, Mittendorf E, Ueno N, Tripathy D, Lim B, Piwnica-Worms H, Thompson A, Symmans WF. NCT02456857: A phase II trial of liposomal doxorubicin, bevacizumab and everolimus (DAE) in patients (pts) with localized triple-negative breast cancer (TNBC) with tumors predicted insensitive to standard neoadjuvant chemotherapy (NACT) [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr OT2-01-22.

Abstract P6-01-08: Identifying metastatic drivers in patient derived xenograft models of triple negative breast cancer

Metastases are responsible for the vast majority of deaths due to breast cancer. Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by high rates of metastasis and poor response to chemotherapy. We are using patient derived xenograft (PDX) models of TNBC to identify drivers of TNBC metastasis. Using these models, we demonstrated that human breast tumors are capable of completing all stages of the metastatic cascade in mice, and metastatic lesions are observed in organs normally found in patients with metastatic breast cancer including lung, liver, bone, brain and lymph nodes. Lentiviral transduction was employed to express both bioluminescent and fluorescent proteins in three distinct PDX models of TNBC. In this way, metastatic lesions can be isolated using bioluminescent imaging and circulating tumor cells (CTCs) are isolated by flow cytometry. A lung metastasis gene expression signature was generated and comprehensive gain-of-function screens are being conducted in vivo to validate this signature and identify functional drivers of TNBC metastasis.

Citation Format: Powell E, Shao J, Tieu T, Peoples M, Bristow C, Manyam G, Cai S, Tu Y, Edwards JR, Heffernan TP, Piwnica-Worms D, Liang H, Piwnica-Worms H. Identifying metastatic drivers in patient derived xenograft models of triple negative breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-01-08.

Abstract OT2-01-20: Phase IIB study of neoadjuvant panitumumab combined with carboplatin and paclitaxel (PaCT) for anthracycline-resistant triple-negative breast cancer (TNBC)

BACKGROUND: Approximately 50% of patients with TNBC treatedwithstandardtaxane/anthracycline-based neoadjuvant chemotherapy (NACT)have chemo-insensitive disease (CID), i.e., residual disease burden (RCB)-II/III at the time of surgery, and 40-80% of patients develop recurrence within 3 years. Recent developments in molecular profiling have identified subsets of TNBC with distinct, targetable molecular features. We developed a clinical trial to identify and characterize CID (ARTEMIS: A Randomized, TNBC-Enrolling trial to confirm Molecular profiling Improves Survival). In ARTEMIS, patients with localized TNBC will undergo a pretreatment biopsy, then begin anthracycline-based NACT. During NACT, we use molecular profiling and response assessment to identify CID and allocate patients to alternative therapies to overcome CID. Epidermal growth factor receptor (EGFR) is overexpressed in 25-30% of TNBC. In preclinical studies, suppression of EGFR signaling has shown efficacy in controlling cancers through suppression of the stem cell population, enhanced apoptosis via MAPK/PI3K signaling, and modulation of epithelial-mesenchymal transition (EMT). Moreover, in a phase II trial of triple negative inflammatory breast cancer, neoadjuvant PaCT yielded significantly higher pathologic complete response (pCR) rates than historic control. Taken together, we hypothesize that using PaCT to suppress EGFR in TNBC will enhance the pCR rate.

OBJECTIVES: Primary objective: determine pCR and RCB-0/I rates in TNBC patients with CID given PaCT. Secondary objective: determine the benefit of using baseline genomic signatures to develop an alternative second phase of NACT.

TRIAL DESIGN AND STATISTICAL METHODS: Patients with &gt;10% volume reduction for non-CID or &lt;80% for CID will enroll in a biomarker-guided, experimental, nonrandomized phase II study and be given PaCT (panitumumab 2.5 mg/kg, carboplatin AUC 5, paclitaxel 80 mg/m2). Because pCR rates in pts with CID with additional cycles of taxane-based therapy are low (∼5%), a 20% response rate (RCB-0 or RCB-I) will be considered clinically meaningful. A two-stage Gehan-type design will be employed. If at least 1 of 14 patients responds, 23 more patients will be added, for a total of 37 patients. This design has a 49% chance of terminating after the first stage if the true response rate is 0.05, 23% if the rate is 0.10, 10% if the rate is 0.15, and 4% if the rate is 0.20. If accrual continues to the second stage and 37 patients are enrolled, the 95% confidence interval for a 0.20 response rate will be 0.10 to 0.35.

BRIEF ELIGIBILITY CRITERIA: Inclusion: localized TNBC; enrolled in ARTEMIS trial; adequate organ, bone marrow, and cardiac parameters; Exclusion: pregnant or lactating, known or suspected metastasis.

CORRELATIVE SCIENCE: Circulating tumor cells (CTCs) and cell free (cf) DNA in baseline and subsequent blood samples, EGFR expression (immunohistochemistry), stem cell/EMT/apoptosis marker changes in tissue and CTCs, PD-L1 glycosylation for EGFR sensitivity.

Citation Format: Lim B, Helgason T, Hess KR, Piwnica-Worms H, Yang W, Adrada BE, Rauch GM, Gilcrease M, Symmans FW, Huo L, Mittendorf EA, Thompson A, Stacy M-TL, Debu T, Ueno NT. Phase IIB study of neoadjuvant panitumumab combined with carboplatin and paclitaxel (PaCT) for anthracycline-resistant triple-negative breast cancer (TNBC) [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr OT2-01-20.

Phosphorylation of serine 367 of FOXC2 by p38 regulates ZEB1 and breast cancer metastasis, without impacting primary tumor growth

Metastatic competence is contingent upon the aberrant activation of a latent embryonic program, known as the epithelial-mesenchymal transition (EMT), which bestows stem cell properties as well as migratory and invasive capabilities upon differentiated tumor cells. We recently identified the transcription factor FOXC2 as a downstream effector of multiple EMT programs, independent of the EMT-inducing stimulus, and as a key player linking EMT, stem cell traits and metastatic competence in breast cancer. As such, FOXC2 could serve as a potential therapeutic target to attenuate metastasis. However, as FOXC2 is a transcription factor, it is difficult to target by conventional means such as small-molecule inhibitors. Herein, we identify the serine/threonine-specific kinase p38 as a druggable upstream regulator of FOXC2 stability and function that elicits phosphorylation of FOXC2 at serine 367 (S367). Using an orthotopic syngeneic mouse tumor model, we make the striking observation that inhibition of p38-FOXC2 signaling selectively attenuates metastasis without impacting primary tumor growth. In this model, circulating tumor cell numbers are significantly reduced in mice treated with the p38 inhibitor SB203580, relative to vehicle-treated counterparts. Accordingly, genetic or pharmacological inhibition of p38 decreases FOXC2 protein levels, reverts the EMT phenotype and compromises stem cell attributes in vitro. We also identify the EMT-regulator ZEB1-known to directly repress E-cadherin/CDH1-as a downstream target of FOXC2, critically dependent on its activation by p38. Consistent with the notion that activation of the p38-FOXC2 signaling axis represents a critical juncture in the acquisition of metastatic competence, the phosphomimetic FOXC2(S367E) mutant is refractory to p38 inhibition both in vitro and in vivo, whereas the non-phosphorylatable FOXC2(S367A) mutant fails to elicit EMT and upregulate ZEB1. Collectively, our data demonstrate that FOXC2 regulates EMT, stem cell traits, ZEB1 expression and metastasis in a p38-dependent manner, and attest to the potential utility of p38 inhibitors as antimetastatic agents.

Abstract OT2-03-03: Women's triple-negative, first-line treatment: Improving outcomes in triple-negative breast cancer using molecular triaging and diagnostic imaging to guide neoadjuvant therapy

BACKGROUND:

In triple negative breast cancer (TNBC), pathologic compete response/residual cancer burden-0 (pCR/RCB-0) or minimal residual disease (RCB-I) following neoadjuvant chemotherapy (NACT) is associated with a good prognosis. This is in contrast to extensive residual disease (RCB-II-III) which carries approximately a 50% chance of recurrence. These patients have a particularly poor prognosis as there are currently no targeted agents to salvage chemoresistant disease. It is important to predict pCR in order to direct responsive disease toward standard NACT and non-responsive disease (NRD) to therapy on clinical trials.

TRIAL DESIGN:

The use of genomic signatures (JAMA, 2011; 305:1873-81) and imaging to predict response to NACT will be validated, and the clinical impact of selecting patients with predicted NRD for targeted therapy on clinical trial will be determined. Patients will undergo primary tumor biopsy for molecular profiling and will be randomized 2:1 to know the results versus not (control). Following that, all patients will receive 4 cycles of anthracycline-based NACT, with imaging used for response assessment. Patients with molecular/imaging criteria for NRD will be offered enrollment on a clinical trial based upon molecular profiling or based upon physician/patient choice (control).

INCLUSION CRITERIA:

Tumor size ≥1.5 cm diameter; TNBC by standard assays; ≥18 years of age; LVEF ≥50%; adequate organ and bone marrow function

EXCLUSION CRITERIA:

Stage IV disease; invasive cancer within 5 years; excisional biopsy of the primary tumor; features that limit response assessment by imaging; unfit for taxane and/or antracycline regimens; prior anthracycline therapy; ≥grade II neuropathy; Zubrod performance status of ≥2; history of serious cardiac events

PRIMARY AIM:

- Prospectively determine the impact of a molecular diagnostic/imaging platform in patients with localized invasive TNBC

SECONDARY AIMS:

- Compare rates of clinical trial enrollment

- Evaluate disease free survival in the experimental arms compared to control standard NACT

- Perform integrated biomarker analyses and identify therapeutic targets for resistant disease

STATISTICAL METHODS:

A maximum of 360 patients will be randomized (2:1)using a group sequential design with one-sided O'Brien-Fleming boundaries, with two equally spaced binding interim tests for futility and superiority and one final test, having an overall Type I error .05 and power .80 to detect an improvement in pCR/RCB-I from 50% to 64%.

Citation Format: Mitri ZI, Ueno NT, Yang W, Valero V, Litton JK, Murthy RK, Ibrahim NK, Arun BK, Mittendorf EA, Hunt KK, Meric-Bernstam F, Thompson A, Piwnica-Worms H, Tripathy D, Symmans F, Moulder-Thompson S. Women's triple-negative, first-line treatment: Improving outcomes in triple-negative breast cancer using molecular triaging and diagnostic imaging to guide neoadjuvant therapy. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr OT2-03-03.

Abstract P4-07-02: Targeted therapies for TNBC: Exploiting vulnerabilities that arise from DNA damage repair pathway dependencies

We examined the synergistic effects of DNA damage, Chk1 inhibition and poly(ADP-ribose) polymerase (PARP) inhibition in TNBC. This combinatorial targeting allows us to exploit vulnerabilities in two pathways that are often deregulated in TNBCs: DNA damage checkpoint defects due to TP53 deficiency and DNA repair defects due to alterations in homologous recombination repair (HRR). TP53 maintains genome integrity by inhibiting cells that are experiencing genotoxic stress from progressing through the cell cycle, or by inducing apoptosis or senescence. In response to DNA damage, p53 activates gene expression to arrest cells in the G1 phase of the cell cycle and to reinforce the S- and G2-checkpoints. Thus, p53-deficient cells lack a G1 checkpoint and are impaired in their ability to sustain S- and G2-checkpoints. This makes p53-deficient tumors particularly sensitive to agents that abrogate these checkpoints. Because Chk1 inhibitors abrogate both S- and G2-checkpoints, combining Chk1 inhibitors with agents that induce genotoxic stress provides a rational therapeutic strategy for killing p53-deficient TNBC.

Loss of HRR increases dependence of cells on a class of enzymes called PARPs, and Chk1 has also been shown to be important for efficient HRR. Thus, by interfering with HRR, Chk1 inhibitors are predicted to sensitize TNBC cells to PARP inhibitors. We tested the hypotheses that by impairing HRR, Chk1 inhibitors will sensitize TNBCs to PARP inhibition, and that therapies that combine Chk1 inhibitors with PARP inhibitors will be effective at killing TNBCs because they will simultaneously induce checkpoint bypass and block DNA repair. We generated a set of isogenic TNBC cell lines that are p53-proficient (p53WT) or p53-deficient (p53KD), and evaluated their sensitivity to Chk1 inhibitors (LY2606368) and DNA damaging agents (cisplatin). Loss of p53 conferred a dramatic increase in sensitivity to treatment with cisplatin + LY2606368. Surprisingly, inhibition of PARP1 (BMN673) did not increase sensitivity to Chk1 inhibitor ± cisplatin. To determine why Chk1 inhibition did not sensitize cells to PARP inhibition, we evaluated the effect of Chk1 inhibition on the ability of cells to recruit HRR proteins to sites of DNA damage. In line with CHK1 regulating HRR, Chk1 inhibition was associated with an inability of Rad51 to localize to sites of DNA double strand breaks. Interestingly, we also found that upstream of Rad51, there was a significant alteration in the formation of phopho-RPA2 foci in cells treated with the Chk1 inhibitor. On-going studies are evaluating whether there are changes in the kinetics of formation and/or resolution of Rad51 and phospho-RPA2 foci in response to Chk1 inhibition.

Citation Format: Redwood AB, Cai S, Piwnica-Worms H. Targeted therapies for TNBC: Exploiting vulnerabilities that arise from DNA damage repair pathway dependencies. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-07-02.

Abstract P3-06-04: Investigating clonal dynamics in triple negative breast cancer chemoresistance

Approximately 50% of triple-negative breast cancer (TNBC) patients have extensive residual disease following neoadjuvant chemotherapy (NAC). These patients have a four-fold increase in mortality risk and an increased risk of distant metastases within three years (1). Understanding the molecular basis of resistance to NAC is expected to provide opportunities to better treat patients in the primary setting. Extensive intratumoral subclonal heterogeneity has been well documented in primary, treatment-naïve TNBC (2). Subclonal populations harboring distinct molecular profiles may confound targeted therapy strategies, yet the functional impact of subclonal heterogeneity in TNBC resistance to therapy is unknown. We are implementing DNA barcoding to quantitatively track changes in subclonal architecture pre- and post-treatment in patient-derived xenograft (PDX) models of TNBC in order to design novel combination therapies. Such barcoding strategies have been used to monitor clonal dynamics in breast cancer PDXs with great sensitivity (3).

We have established an orthotopic PDX from a treatment-naïve TNBC patient (PIM1, procured from a patient later found to have chemoresistant disease). In order to model chemoresistance, we treated PIM1 with Adriamycin and cyclophosphamide (AC), standard of care NAC for TNBC patients, which resulted in partial response but left residual disease. To characterize subclonal dynamics in response to NAC, we transduced freshly isolated PIM1 cells with a lentiviral library expressing 25 million unique DNA barcodes (Cellecta) using conditions to ensure each transduced cell contained a single unique barcode. Transduced cells were selected with puromycin, then orthotopically implanted into immuno-compromised mice. High-throughput barcode sequencing revealed reproducible maintenance of greater than 60,000 unique barcodes in PDX tumors. Comparison of barcode distribution in tumors treated with vehicle or NAC will reveal whether NAC selects for a subpopulation of cells during the development of resistance. Future directions will include whole-exome and RNA sequencing to characterize genomic changes associated with alterations in barcode distribution in response to NAC treatment. Our ultimate goal is to identify novel combination therapies to eliminate subclones that contribute to chemoresistance in primary TNBC.

References

1. Cortazar P, et al. (Pathological complete response and long-term clinical benefit in breast cancer: the CTNeoBC pooled analysis. The Lancet 384(9938):164-172.

2. Shah SP, et al. (2012) The clonal and mutational evolution spectrum of primary triple-negative breast cancers. Nature 486(7403):395-399.

3. Nguyen LV, et al. (2014) DNA barcoding reveals diverse growth kinetics of human breast tumour subclones in serially passaged xenografts. Nat Commun 5.

Citation Format: Echeverria GV, Seth S, Moulder S, Symmans W, Chang J, Cai S, Heffernan T, Piwnica-Worms H. Investigating clonal dynamics in triple negative breast cancer chemoresistance. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-06-04.

Casein kinase 1 functions as both penultimate and ultimate kinase in regulating Cdc25A destruction

The Cdc25A protein phosphatase drives cell cycle transitions by activating cyclin-dependent protein kinases. Failure to regulate Cdc25A leads to deregulated cell cycle progression, bypass of cell cycle checkpoints and genome instability. Ubiquitin-mediated proteolysis plays an important role in balancing Cdc25A levels. Cdc25A contains a DS₈₂G motif whose phosphorylation is targeted by β-TrCP E3 ligase during interphase. Targeting of β-TrCP to Cdc25A requires phosphorylation of serines 79 (S79) and 82 (S82). Here, we report that casein kinase 1 alpha (CK1α) phosphorylates Cdc25A on both S79 and S82 in a hierarchical manner requiring prior phosphorylation of serine 76 by Chk1 or GSK-3β. This facilitates β-TrCP binding and ubiquitin-mediated proteolysis of Cdc25A throughout interphase and following exposure to genotoxic stress. The priming of Cdc25A by at least three kinases (Chk1, GSK-3β, CK1α), some of which also require priming, ensures diverse extra- and intra-cellular signals interface with Cdc25A to precisely control cell division.

Overview of the baculovirus expression system

Baculoviruses have emerged as a popular system for overproducing recombinant proteins in eukaryotic cells. This unit gives an overview of the baculovirus expression system, including discussion of the baculovirus life cycle, and post-translational modifications that occur in insect cells. In addition, the steps for overproducing proteins in the baculovirus systems are described along with recommendations for choosing an appropriate baculovirus vector and DNA, and reagents and equipment necessary for implementing the whole overexpression system.

Microtubule affinity-regulating kinase 2 functions downstream of the PAR-3/PAR-6/atypical PKC complex in regulating hippocampal neuronal polarity

The PAR-3/PAR-6/atypical PKC (aPKC) complex is required for axon-dendrite specification of hippocampal neurons. However, the downstream effectors of this complex are not well defined. In this article, we report a role for microtubule affinity-regulating kinase (MARK)/PAR-1 in axon-dendrite specification. Knocking down MARK2 expression with small interfering RNAs induced formation of multiple axon-like neurites and promoted axon outgrowth. Ectopic expression of MARK2 caused phosphorylation of tau (S262) and led to loss of axons, and this phenotype was rescued by expression of PAR-3, PAR-6, and aPKC. In contrast, the polarity defects caused by an MARK2 mutant (T595A), which is not responsive to aPKC, were not rescued by the PAR-3/PAR-6/aPKC complex. Moreover, polarity was abrogated in neurons overexpressing a mutant of MARK2 with a deleted kinase domain but an intact aPKC-binding domain. Finally, suppression of MARK2 rescued the polarity defects induced by a dominant-negative aPKC mutant. These results suggest that MARK2 is involved in neuronal polarization and functions downstream of the PAR-3/PAR-6/aPKC complex. We propose that aPKC in complex with PAR-3/PAR-6 negatively regulates MARK(s), which in turn causes dephosphorylation of microtubule-associated proteins, such as tau, leading to the assembly of microtubules and elongation of axons.

14-3-3 binding regulates catalytic activity of human Wee1 kinase

The mitotic inducer Cdc2 is negatively regulated, in part, by phosphorylation on tyrosine 15. Human Wee1 is a tyrosine-specific protein kinase that phosphorylates Cdc2 on tyrosine 15. Human Wee1 is subject to multiple levels of regulation including reversible phosphorylation, proteolysis, and protein-protein interactions. Here we have investigated the contributions made by 14-3-3 binding to human Wee1 regulation and function. We report that the interactions of 14-3-3 proteins with human Wee1 are reduced during mitosis and are stable in the presence of the protein kinase inhibitor UCN-01. A mutant of Wee1 that is incapable of binding to 14-3-3 proteins has lower enzymatic activity, and this likely accounts for its reduced potency relative to wild-type Wee1 in inducing a G(2) cell cycle delay when overproduced in vivo. These findings indicate that 14-3-3 proteins function as positive regulators of the human Wee1 protein kinase.

C-TAK1 regulates Ras signaling by phosphorylating the MAPK scaffold, KSR1

Kinase suppressor of Ras (KSR) is a conserved component of the Ras pathway that interacts directly with MEK and MAPK. Here we show that KSR1 translocates from the cytoplasm to the cell surface in response to growth factor treatment and that this process is regulated by Cdc25C-associated kinase 1 (C-TAK1). C-TAK1 constitutively associates with mammalian KSR1 and phosphorylates serine 392 to confer 14-3-3 binding and cytoplasmic sequestration of KSR1 in unstimulated cells. In response to signal activation, the phosphorylation state of S392 is reduced, allowing the KSR1 complex to colocalize with activated Ras and Raf-1 at the plasma membrane, thereby facilitating the phosphorylation reactions required for the activation of MEK and MAPK.

The human decatenation checkpoint

Chromatid catenation is actively monitored in human cells, with progression from G(2) to mitosis being inhibited when chromatids are insufficiently decatenated. Mitotic delay was quantified in normal and checkpoint-deficient human cells during treatment with ICRF-193, a topoisomerase II catalytic inhibitor that prevents chromatid decatenation without producing topoisomerase-associated DNA strand breaks. Ataxia telangiectasia (A-T) cells, defective in DNA damage checkpoints, showed normal mitotic delay when treated with ICRF-193. The mitotic delay in response to ICRF-193 was ablated in human fibroblasts expressing an ataxia telangiectasia mutated- and rad3-related (ATR) kinase-inactive ATR allele (ATR(ki)). BRCA1-mutant HCC1937 cells also displayed a defect in ICRF-193-induced mitotic delay, which was corrected by expression of wild-type BRCA1. Phosphorylations of hCds1 or Chk1 and inhibition of Cdk1 kinase activity, which are elements of checkpoints associated with DNA damage or replication, did not occur during ICRF-193-induced mitotic delay. Over-expression of cyclin B1 containing a dominant nuclear localization signal, and inhibition of Crm1-mediated nuclear export, reversed ICRF-193-induced mitotic delay. In combination, these results imply that ATR and BRCA1 enforce the decatenation G(2) checkpoint, which may act to exclude cyclin B1/Cdk1 complexes from the nucleus. Moreover, induction of ATR(ki) produced a 10-fold increase in chromosomal aberrations, further emphasizing the vital role for ATR in genetic stability.

The G(2) DNA damage checkpoint delays expression of genes encoding mitotic regulators

Transcriptional control of gene expression contributes to the regulation of diverse cellular processes including cell cycle progression and the cellular response to DNA damage. Global gene expression profiling was performed using p53-deficient human cells to identify genes with G(2)/M-specific and DNA damage-responsive expression. Numerous cell cycle-regulated genes were identified, but surprisingly the analysis failed to identify genes activated by ionizing radiation. Instead, significant delays in expression of G(2)/M-specific genes, including known mitotic regulators, were observed following DNA damage. Thus, in the absence of p53, gene induction does not contribute to the G(2) arrest following DNA damage. Rather, the DNA damage checkpoint elicits a G(2) cell cycle arrest, in part, by delaying accumulation of proteins required in mitosis.

ATR-mediated checkpoint pathways regulate phosphorylation and activation of human Chk1

Chk1 is an evolutionarily conserved protein kinase that regulates cell cycle progression in response to checkpoint activation. In this study, we demonstrated that agents that block DNA replication or cause certain forms of DNA damage induce the phosphorylation of human Chk1. The phosphorylated form of Chk1 possessed higher intrinsic protein kinase activity and eluted more quickly on gel filtration columns. Serines 317 and 345 were identified as sites of phosphorylation in vivo, and ATR (the ATM- and Rad3-related protein kinase) phosphorylated both of these sites in vitro. Furthermore, phosphorylation of Chk1 on serines 317 and 345 in vivo was ATR dependent. Mutants of Chk1 containing alanine in place of serines 317 and 345 were poorly activated in response to replication blocks or genotoxic stress in vivo, were poorly phosphorylated by ATR in vitro, and were not found in faster-eluting fractions by gel filtration. These findings demonstrate that the activation of Chk1 in response to replication blocks and certain forms of genotoxic stress involves phosphorylation of serines 317 and 345. In addition, this study implicates ATR as a direct upstream activator of Chk1 in human cells.

Absence of apparent phenotype in mice lacking Cdc25C protein phosphatase

The Cdc25 family of protein phosphatases positively regulate the cell division cycle by activating cyclin-dependent protein kinases. In humans and rodents, three Cdc25 family members denoted Cdc25A, -B, and -C have been identified. The murine forms of Cdc25 exhibit distinct patterns of expression both during development and in adult mouse tissues. In order to determine unique contributions made by the Cdc25C protein phosphatase to embryonic and adult cell cycles, mice lacking Cdc25C were generated. We report that Cdc25C(-/-) mice are viable and do not display any obvious abnormalities. Among adult tissues in which Cdc25C is detected, its transcripts are most abundant in testis, followed by thymus, ovary, spleen, and intestine. Mice lacking Cdc25C were fertile, indicating that Cdc25C does not contribute an essential function during spermatogenesis or oogenesis in the mouse. T- and B-cell development was also found to be normal in Cdc25C(-/-) mice, and Cdc25C(-/-) mouse splenic T and B cells exhibited normal proliferative responses in vitro. Finally, the phosphorylation status of Cdc2, the timing of entry into mitosis, and the cellular response to DNA damage were unperturbed in mouse embryo fibroblasts lacking Cdc25C. These findings indicate that Cdc25A and/or Cdc25B may compensate for loss of Cdc25C in the mouse.

Immune system dysfunction and autoimmune disease in mice lacking Emk (Par-1) protein kinase

Emk is a serine/threonine protein kinase implicated in regulating polarity, cell cycle progression, and microtubule dynamics. To delineate the role of Emk in development and adult tissues, mice lacking Emk were generated by targeted gene disruption. Emk(-/-) mice displayed growth retardation and immune cell dysfunction. Although B- and T-cell development were normal, CD4(+)T cells lacking Emk exhibited a marked upregulation of the memory marker CD44/pgp-1 and produced more gamma interferon and interleukin-4 on stimulation through the T-cell receptor in vitro. In addition, B-cell responses to T-cell-dependent and -independent antigen challenge were altered in vivo. As Emk(-/-) animals aged, they developed splenomegaly, lymphadenopathy, membranoproliferative glomerulonephritis, and lymphocytic infiltrates in the lungs, parotid glands and kidneys. Taken together, these results demonstrate that the Emk protein kinase is essential for maintaining immune system homeostasis and that loss of Emk may contribute to autoimmune disease in mammals.

Localization of human Cdc25C is regulated both by nuclear export and 14-3-3 protein binding

Entry into mitosis requires activation of the Cdc2 protein kinase by the Cdc25C protein phosphatase. The interactions between Cdc2 and Cdc25C are negatively regulated throughout interphase and in response to G2 checkpoint activation. This is accomplished in part by maintaining the Cdc25 phosphatase in a phosphorylated form that binds 14-3-3 proteins. Here we report that 14-3-3 binding regulates the intracellular trafficking of Cdc25C. Although primarily cytoplasmic, Cdc25C accumulated in the nuclei of leptomycin B (LMB)-treated cells, indicating that Cdc25C is actively exported out of the nucleus. A mutant of Cdc25C that is unable to bind 14-3-3 was partially nuclear in the absence of LMB and its nuclear accumulation was greatly enhanced by LMB-treatment. A nuclear export signal (NES) was identified within the amino terminus of Cdc25C. Although mutation of the NES did not effect 14-3-3 binding, it did cause nuclear accumulation of Cdc25C. These results demonstrate that 14-3-3 binding is dispensable for the nuclear export of Cdc25C. However, complete nuclear accumulation of Cdc25C required loss of both NES function and 14-3-3 binding and this was accomplished both pharmacologically and by mutation. These findings suggest that the nuclear export of Cdc25C is mediated by an intrinsic NES and that 14-3-3 binding negatively regulates nuclear import.

Threonine 68 phosphorylation by ataxia telangiectasia mutated is required for efficient activation of Chk2 in response to ionizing radiation

Eukaryotic cells activate an evolutionarily conserved set of proteins that rapidly induce cell cycle arrest to prevent replication or segregation of damaged DNA before repair is completed. In response to ionizing radiation (IR), the cell cycle checkpoint kinase, Chk2 (hCds1), is phosphorylated and activated in an ataxia telangiectasia mutated (ATM)-dependent manner. Here we show that the ATM protein kinase directly phosphorylates T68 within the SQ/TQ-rich domain of Chk2 in vitro and that T68 is phosphorylated in vivo in response to IR in an ATM-dependent manner. Furthermore, phosphorylation of T68 was required for full activation of Chk2 after IR. Together, these data are consistent with the model that ATM directly phosphorylates Chk2 in vivo and that this event contributes to the activation of Chk2 in irradiated cells.

The Chk1 protein kinase and the Cdc25C regulatory pathways are targets of the anticancer agent UCN-01

A checkpoint operating in the G(2) phase of the cell cycle prevents entry into mitosis in the presence of DNA damage. UCN-01, a protein kinase inhibitor currently undergoing clinical trials for cancer treatment, abrogates G(2) checkpoint function and sensitizes p53-defective cancer cells to DNA-damaging agents. In most species, the G(2) checkpoint prevents the Cdc25 phosphatase from removing inhibitory phosphate groups from the mitosis-promoting kinase Cdc2. This is accomplished by maintaining Cdc25 in a phosphorylated form that binds 14-3-3 proteins. The checkpoint kinases, Chk1 and Cds1, are proposed to regulate the interactions between human Cdc25C and 14-3-3 proteins by phosphorylating Cdc25C on serine 216. 14-3-3 proteins, in turn, function to keep Cdc25C out of the nucleus. Here we report that UCN-01 caused loss of both serine 216 phosphorylation and 14-3-3 binding to Cdc25C in DNA-damaged cells. In addition, UCN-01 potently inhibited the ability of Chk1 to phosphorylate Cdc25C in vitro. In contrast, Cds1 was refractory to inhibition by UCN-01 in vitro, and Cds1 was still phosphorylated in irradiated cells treated with UCN-01. Thus, neither Cds1 nor kinases upstream of Cds1, such as ataxia telangiectasia-mutated, are targets of UCN-01 action in vivo. Taken together our results identify the Chk1 kinase and the Cdc25C pathway as potential targets of G(2) checkpoint abrogation by UCN-01.

DNA damage and replication checkpoints in fission yeast require nuclear exclusion of the Cdc25 phosphatase via 14-3-3 binding

In fission yeast as well as in higher eukaryotic organisms, entry into mitosis is delayed in cells containing damaged or unreplicated DNA. This is accomplished in part by maintaining the Cdc25 phosphatase in a phosphorylated form that binds 14-3-3 proteins. In this study, we generated a mutant of fission yeast Cdc25 that is severely impaired in its ability to bind 14-3-3 proteins. Loss of both the DNA damage and replication checkpoints was observed in fission yeast cells expressing the 14-3-3 binding mutant. These findings indicate that 14-3-3 binding to Cdc25 is required for fission yeast cells to arrest their cell cycle in response to DNA damage and replication blocks. Furthermore, the 14-3-3 binding mutant localized almost exclusively to the nucleus, unlike wild-type Cdc25, which localized to both the cytoplasm and the nucleus. Nuclear accumulation of wild-type Cdc25 was observed when fission yeast cells were treated with leptomycin B, indicating that Cdc25 is actively exported from the nucleus. Nuclear exclusion of wild-type Cdc25 was observed upon overproduction of Rad 24, one of the two fission yeast 14-3-3 proteins, indicating that one function of Rad 24 is to keep Cdc25 out of the nucleus. In support of this conclusion, Rad 24 overproduction did not alter the nuclear location of the 14-3-3 binding mutant. These results indicate that 14-3-3 binding contributes to the nuclear exclusion of Cdc25 and that the nuclear exclusion of Cdc25 is required for a normal checkpoint response to both damaged and unreplicated DNA.

Overproduction of human Myt1 kinase induces a G2 cell cycle delay by interfering with the intracellular trafficking of Cdc2-cyclin B1 complexes

The Myt1 protein kinase functions to negatively regulate Cdc2-cyclin B complexes by phosphorylating Cdc2 on threonine 14 and tyrosine 15. Throughout interphase, human Myt1 localizes to the endoplasmic reticulum and Golgi complex, whereas Cdc2-cyclin B1 complexes shuttle between the nucleus and the cytoplasm. Here we report that overproduction of either kinase-active or kinase-inactive forms of Myt1 blocked the nuclear-cytoplasmic shuttling of cyclin B1 and caused cells to delay in the G2 phase of the cell cycle. The COOH-terminal 63 amino acids of Myt1 were identified as a Cdc2-cyclin B1 interaction domain. Myt1 mutants lacking this domain no longer bound cyclin B1 and did not efficiently phosphorylate Cdc2-cyclin B1 complexes in vitro. In addition, cells overproducing mutant forms of Myt1 lacking the interaction domain exhibited normal trafficking of cyclin B1 and unperturbed cell cycle progression. These results suggest that the docking of Cdc2-cyclin B1 complexes to the COOH terminus of Myt1 facilitates the phosphorylation of Cdc2 by Myt1 and that overproduction of Myt1 perturbs cell cycle progression by sequestering Cdc2-cyclin B1 complexes in the cytoplasm.

A human Cds1-related kinase that functions downstream of ATM protein in the cellular response to DNA damage

Checkpoints maintain the order and fidelity of the eukaryotic cell cycle, and defects in checkpoints contribute to genetic instability and cancer. Much of our current understanding of checkpoints comes from genetic studies conducted in yeast. In the fission yeast Schizosaccharomyces pombe (Sp), SpRad3 is an essential component of both the DNA damage and DNA replication checkpoints. The SpChk1 and SpCds1 protein kinases function downstream of SpRad3. SpChk1 is an effector of the DNA damage checkpoint and, in the absence of SpCds1, serves an essential function in the DNA replication checkpoint. SpCds1 functions in the DNA replication checkpoint and in the S phase DNA damage checkpoint. Human homologs of both SpRad3 and SpChk1 but not SpCds1 have been identified. Here we report the identification of a human cDNA encoding a protein (designated HuCds1) that shares sequence, structural, and functional similarity to SpCds1. HuCds1 was modified by phosphorylation and activated in response to ionizing radiation. It was also modified in response to hydroxyurea treatment. Functional ATM protein was required for HuCds1 modification after ionizing radiation but not after hydroxyurea treatment. Like its fission yeast counterpart, human Cds1 phosphorylated Cdc25C to promote the binding of 14-3-3 proteins. These findings suggest that the checkpoint function of HuCds1 is conserved in yeast and mammals.

UCN-01 abrogates G2 arrest through a Cdc2-dependent pathway that is associated with inactivation of the Wee1Hu kinase and activation of the Cdc25C phosphatase

We have previously demonstrated that UCN-01, a potent protein kinase inhibitor currently in phase I clinical trials for cancer treatment, abrogates G2 arrest following DNA damage. Here we used murine FT210 cells, which contain temperature-sensitive Cdc2 mutations, to determine if UCN-01 abrogates G2 arrest through a Cdc2-dependent pathway. We report that UCN-01 cannot induce mitosis in DNA-damaged FT210 cells at the non-permissive temperature for Cdc2 function. Failure to abrogate G2 arrest was not due to UCN-01-inactivation at the elevated temperature because parental FM3A cells, which have wild-type Cdc2, were sensitive to UCN-01-induced G2 checkpoint abrogation. Having established that UCN-01 acted through Cdc2, we next assessed UCN-01's effect on the Cdc2-inhibitory kinase, Wee1Hu, and the Cdc2-activating phosphatase, Cdc25C. We found that Wee1Hu was indeed inactivated in UCN-01-treated cells, possibly just prior to Cdc2 activation and entry of DNA-damaged cells into mitosis. This inhibition appeared, however, to be a consequence of a further upstream action since in vitro studies revealed purified Wee1Hu was relatively resistant to UCN-01-inhibition. Consistent with such an upstream action, UCN-01 also promoted the hyperphosphorylation (activation) of Cdc25C in DNA-damaged cells. Our results suggest that UCN-01 abrogates G2 checkpoint function through inhibition of a kinase residing upstream of Cdc2, Wee1Hu, and Cdc25C, and that changes observed in these mitotic regulators are downstream consequences of UCN-01's actions.

Replication checkpoint requires phosphorylation of the phosphatase Cdc25 by Cds1 or Chk1

Checkpoints maintain the order and fidelity of events of the cell cycle by blocking mitosis in response to unreplicated or damaged DNA. In most species this is accomplished by preventing activation of the cell-division kinase Cdc2, which regulates entry into mitosis. The Chk1 kinase, an effector of the DNA-damage checkpoint, phosphorylates Cdc25, an activator of Cdc2. Phosphorylation of Cdc25 promotes its binding to 14-3-3 proteins, preventing it from activating Cdc2. Here we propose that a similar pathway is required for mitotic arrest in the presence of unreplicated DNA (that is, in the replication checkpoint) in fission yeast. We show by mutagenesis that Chk1 functions redundantly with the kinase Cds1 at the replication checkpoint and that both kinases phosphorylate Cdc25 on the same sites, which include serine residues at positions 99, 192 and 359. Mutation of these residues reduces binding of 14-3-3 proteins to Cdc25 in vitro and disrupts the replication checkpoint in vivo. We conclude that both Cds1 and Chk1 regulate the binding of Cdc25 to 14-3-3 proteins as part of the checkpoint response to unreplicated DNA.

14-3-3 proteins are required for maintenance of Raf-1 phosphorylation and kinase activity

By binding to serine-phosphorylated proteins, 14-3-3 proteins function as effectors of serine phosphorylation. The exact mechanism of their action is, however, still largely unknown. Here we demonstrate a requirement for 14-3-3 for Raf-1 kinase activity and phosphorylation. Expression of dominant negative forms of 14-3-3 resulted in the loss of a critical Raf-1 phosphorylation, while overexpression of 14-3-3 resulted in enhanced phosphorylation of this site. 14-3-3 levels, therefore, regulate the stoichiometry of Raf-1 phosphorylation and its potential activity in the cell. Phosphorylation of Raf-1, however, was insufficient by itself for kinase activity. Removal of 14-3-3 from phosphorylated Raf abrogated kinase activity, whereas addition of 14-3-3 restored it. This supports a paradigm in which the effects of phosphorylation on serine as well as tyrosine residues are mediated by inducible protein-protein interactions.

Crystal structure of the catalytic domain of the human cell cycle control phosphatase, Cdc25A

Cdc25 phosphatases activate the cell division kinases throughout the cell cycle. The 2.3 A structure of the human Cdc25A catalytic domain reveals a small alpha/beta domain with a fold unlike previously described phosphatase structures but identical to rhodanese, a sulfur-transfer protein. Only the active-site loop, containing the Cys-(X)5-Arg motif, shows similarity to the tyrosine phosphatases. In some crystals, the catalytic Cys-430 forms a disulfide bond with the invariant Cys-384, suggesting that Cdc25 may be self-inhibited during oxidative stress. Asp-383, previously proposed to be the general acid, instead serves a structural role, forming a conserved buried salt-bridge. We propose that Glu-431 may act as a general acid. Structure-based alignments suggest that the noncatalytic domain of the MAP kinase phosphatases will share this topology, as will ACR2, a eukaryotic arsenical resistance protein.

C-TAK1 protein kinase phosphorylates human Cdc25C on serine 216 and promotes 14-3-3 protein binding

Cdc25C is a dual-specificity protein kinase that controls entry into mitosis by dephosphorylating Cdc2 on both threonine 14 and tyrosine 15. Cdc25C is phosphorylated on serine 216 throughout interphase but not during mitosis. Serine 216 phosphorylation mediates the binding of 14-3-3 protein to Cdc25C, and Cdc25C/14-3-3 complexes are present throughout interphase but not during mitosis. Here we report the cloning of a human kinase denoted C-TAK1 (for Cdc twenty-five C associated protein kinase) that phosphorylates Cdc25C on serine 216 in vitro. C-TAK1 is ubiquitously expressed in human tissues and cell lines and is distinct from the DNA damage checkpoint kinase Chk1, shown previously to phosphorylate Cdc25C on serine 216. Cotransfection of Cdc25C with C-TAK1 resulted in enhanced phosphorylation of Cdc25C on serine 216. In addition, a physical interaction between C-TAK1 and Cdc25C was observed upon transient overexpression in COS-7 cells. Finally, coproduction of Cdc25C and C-TAK1 in bacteria resulted in the stoichiometric phosphorylation of Cdc25C on serine 216 and facilitated 14-3-3 protein binding in vitro. Taken together, these results suggest that one function of C-TAK1 may be to regulate the interactions between Cdc25C and 14-3-3 in vivo by phosphorylating Cdc25C on serine 216.

Serine phosphorylation-dependent association of the band 4.1-related protein-tyrosine phosphatase PTPH1 with 14-3-3beta protein

PTPH1 is a human protein-tyrosine phosphatase with homology to the band 4.1 superfamily of cytoskeletal-associated proteins. PTPH1 was found to associate with 14-3-3beta using a yeast two-hybrid screen, and its interaction could be reconstituted in vitro using recombinant proteins. Examination of the interaction between 14-3-3beta and various deletion mutants of PTPH1 by two-hybrid tests suggested that the integrity of the PTP is important for this binding. Although both PTPH1 and Raf-1 form complexes with 14-3-3beta, they appear to do so independently. Binding of 14-3-3beta to PTPH1 in vitro was abolished by pretreating PTPH1 with potato acid phosphatase and was greatly enhanced by pretreating with Cdc25C-associated protein kinase. Thus the association between PTPH1 and 14-3-3beta is phosphorylation-dependent. Two novel motifs RSLS359VE and RVDS853EP in PTPH1 were identified as major 14-3-3beta-binding sites, both of which are distinct from the consensus binding motif RSXSXP recently found in Raf-1. Mutation of Ser359 and Ser853 to alanine significantly reduced the association between 14-3-3beta and PTPH1. Furthermore, association of PTPH1 and 14-3-3beta was detected in several cell lines and was regulated in response to extracellular signals. These results raise the possibility that 14-3-3beta may function as an adaptor molecule in the regulation of PTPH1 and may provide a link between serine/threonine and tyrosine phosphorylation-dependent signaling pathways.

Conservation of the Chk1 checkpoint pathway in mammals: linkage of DNA damage to Cdk regulation through Cdc25

In response to DNA damage, mammalian cells prevent cell cycle progression through the control of critical cell cycle regulators. A human gene was identified that encodes the protein Chk1, a homolog of the Schizosaccharomyces pombe Chk1 protein kinase, which is required for the DNA damage checkpoint. Human Chk1 protein was modified in response to DNA damage. In vitro Chk1 bound to and phosphorylated the dual-specificity protein phosphatases Cdc25A, Cdc25B, and Cdc25C, which control cell cycle transitions by dephosphorylating cyclin-dependent kinases. Chk1 phosphorylates Cdc25C on serine-216. As shown in an accompanying paper by Peng et al. in this issue, serine-216 phosphorylation creates a binding site for 14-3-3 protein and inhibits function of the phosphatase. These results suggest a model whereby in response to DNA damage, Chk1 phosphorylates and inhibits Cdc25C, thus preventing activation of the Cdc2-cyclin B complex and mitotic entry.

Mitotic and G2 checkpoint control: regulation of 14-3-3 protein binding by phosphorylation of Cdc25C on serine-216

Human Cdc25C is a dual-specificity protein phosphatase that controls entry into mitosis by dephosphorylating the protein kinase Cdc2. Throughout interphase, but not in mitosis, Cdc25C was phosphorylated on serine-216 and bound to members of the highly conserved and ubiquitously expressed family of 14-3-3 proteins. A mutation preventing phosphorylation of serine-216 abrogated 14-3-3 binding. Conditional overexpression of this mutant perturbed mitotic timing and allowed cells to escape the G2 checkpoint arrest induced by either unreplicated DNA or radiation-induced damage. Chk1, a fission yeast kinase involved in the DNA damage checkpoint response, phosphorylated Cdc25C in vitro on serine-216. These results indicate that serine-216 phosphorylation and 14-3-3 binding negatively regulate Cdc25C and identify Cdc25C as a potential target of checkpoint control in human cells.

The human Myt1 kinase preferentially phosphorylates Cdc2 on threonine 14 and localizes to the endoplasmic reticulum and Golgi complex

Entry into mitosis requires the activity of the Cdc2 kinase. Cdc2 associates with the B-type cyclins, and the Cdc2-cyclin B heterodimer is in turn regulated by phosphorylation. Phosphorylation of threonine 161 is required for the Cdc2-cyclin B complex to be catalytically active, whereas phosphorylation of threonine 14 and tyrosine 15 is inhibitory. Human kinases that catalyze the phosphorylation of threonine 161 and tyrosine 15 have been identified. Here we report the isolation of a novel human cDNA encoding a dual-specificity protein kinase (designated Myt1Hu) that preferentially phosphorylates Cdc2 on threonine 14 in a cyclin-dependent manner. Myt1Hu is 46% identical to Myt1Xe, a kinase recently characterized from Xenopus laevis. Myt1Hu localizes to the endoplasmic reticulum and Golgi complex in HeLa cells. A stretch of hydrophobic and uncharged amino acids located outside the catalytic domain of Myt1Hu is the likely membrane-targeting domain, as its deletion results in the localization of Myt1Hu primarily to the nucleus.

Identification of a 95-kDa WEE1-like tyrosine kinase in HeLa cells

Human WEE1 (WEE1Hu) was cloned on the basis of its ability to rescue wee1+ mutants in fission yeast [Igarashi, M., Nagata, A., Jinno, S., Suto, K. & Okayama, H. (1991) Nature (London) 353, 80-83]. Biochemical studies carried out in vitro with recombinant protein demonstrated that WEE1Hu encodes a tyrosine kinase of approximately 49 kDa that phosphorylates p34cdc2 on Tyr-15 [Parker, L. L. & Piwnica-Worms, H. (1992) Science 257, 1955-1957]. To study the regulation of WEE1Hu in human cells, two polyclonal antibodies to bacterially produced p49WEE1Hu were generated. In addition, a peptide antibody generated against amino acids 361-388 of p49WEE1Hu was also used. Unexpectantly, these antibodies recognized a protein with an apparent molecular mass of 95 kDa in HeLa cells, rather than one of 49 kDa. Immunoprecipitates of p95 phosphorylated p34cdc2 on Tyr-15, indicating that p95 is functionally related to p49WEEIHu, and mapping studies demonstrated that p95 is structurally related to p49WEE1Hu. In addition, the substrate specificity of p95 was more similar to that of fission yeast p107wee1 than to that of human p49WEE1. Finally, the kinase activity of p95 toward p34cdc2/cyclin B was severely impaired during mitosis. Taken together, these results indicate that the original WEE1Hu clone isolated in genetic screens encodes only the catalytic domain of human WEE1 and that the authentic human WEE1 protein has an apparent molecular mass of approximately 95 kDa.