A RAD51 assay feasible in routine tumor samples calls PARP inhibitor response beyond BRCA mutation

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) are effective in cancers with defective homologous recombination DNA repair (HRR), including BRCA1/2-related cancers. A test to identify additional HRR-deficient tumors will help to extend their use in new indications. We evaluated the activity of the PARPi olaparib in patient-derived tumor xenografts (PDXs) from breast cancer (BC) patients and investigated mechanisms of sensitivity through exome sequencing, BRCA1 promoter methylation analysis, and immunostaining of HRR proteins, including RAD51 nuclear foci. In an independent BC PDX panel, the predictive capacity of the RAD51 score and the homologous recombination deficiency (HRD) score were compared. To examine the clinical feasibility of the RAD51 assay, we scored archival breast tumor samples, including PALB2-related hereditary cancers. The RAD51 score was highly discriminative of PARPi sensitivity versus PARPi resistance in BC PDXs and outperformed the genomic test. In clinical samples, all PALB2-related tumors were classified as HRR-deficient by the RAD51 score. The functional biomarker RAD51 enables the identification of PARPi-sensitive BC and broadens the population who may benefit from this therapy beyond BRCA1/2-related cancers.

Multicenter Phase II Study of Lurbinectedin in BRCA-Mutated and Unselected Metastatic Advanced Breast Cancer and Biomarker Assessment Substudy

PURPOSE

This multicenter phase II trial evaluated lurbinectedin (PM01183), a selective inhibitor of active transcription of protein-coding genes, in patients with metastatic breast cancer. A unicenter translational substudy assessed potential mechanisms of lurbinectedin resistance.

PATIENTS AND METHODS

Two arms were evaluated according to germline BRCA1/2 status: BRCA1/2 mutated (arm A; n = 54) and unselected ( BRCA1/2 wild-type or unknown status; arm B; n = 35). Lurbinectedin starting dose was a 7-mg flat dose and later, 3.5 mg/m² in arm A. The primary end point was objective response rate (ORR) per Response Evaluation Criteria in Solid Tumors (RECIST). The translational substudy of resistance mechanisms included exome sequencing (n = 13) and in vivo experiments with patient-derived xenografts (n = 11) from BRCA1/2-mutated tumors.

RESULTS

ORR was 41% (95% CI, 28% to 55%) in arm A and 9% (95% CI, 2% to 24%) in arm B. In arm A, median progression-free survival was 4.6 months (95% CI, 3.0 to 6.0 months), and median overall survival was 20.0 months (95% CI, 11.8 to 26.6 months). Patients with BRCA2 mutations showed an ORR of 61%, median progression-free survival of 5.9 months, and median overall survival of 26.6 months. The safety profile improved with lurbinectedin dose adjustment to body surface area. The most common nonhematologic adverse events seen at 3.5 mg/m² were nausea (74%; grade 3, 5%) and fatigue (74%; grade 3, 21%). Neutropenia was the most common severe hematologic adverse event (grade 3, 47%; grade 4, 10%). Exome sequencing showed mutations in genes related to the nucleotide excision repair pathway in four of seven tumors at primary or acquired resistance and in one patient with short-term stable disease. In vivo, sensitivity to cisplatin and lurbinectedin was evidenced in lurbinectedin-resistant (one of two) and cisplatin-resistant (two of three) patient-derived xenografts.

CONCLUSION

Lurbinectedin showed noteworthy activity in patients with BRCA1/2 mutations. Response and survival was notable in those with BRCA2 mutations. Additional clinical development in this subset of patients with metastatic breast cancer is warranted.

Triterpenoid Hydroxamates as HIF Prolyl Hydrolase Inhibitors

Pentacyclic triterpenoid acids (PCTTAs) are pleiotropic agents that target many macromolecular end-points with low to moderate affinity. To explore the biological space associated with PCTTAs, we have investigated the carboxylate-to-hydroxamate transformation, discovering that it de-emphasizes affinity for the transcription factors targeted by the natural compounds (NF-κB, STAT3, Nrf2, TGR5) and selectively induces inhibitory activity on HIF prolyl hydrolases (PHDs). Activity was reversible, isoform-selective, dependent on the hydroxamate location, and negligible when this group was replaced by other chelating elements or O-alkylated. The hydroxamate of betulinic acid (5b) was selected for further studies, and evaluation of its effect on HIF-1α expression under normal and hypoxic conditions qualified it as a promising lead structure for the discovery of new candidates in the realm of neuroprotection.

Tri-methylation of histone H3 lysine 4 facilitates gene expression in ageing cells

Transcription of protein coding genes is accompanied by recruitment of COMPASS to promoter-proximal chromatin, which methylates histone H3 lysine 4 (H3K4) to form H3K4me1, H3K4me2 and H3K4me3. Here, we determine the importance of COMPASS in maintaining gene expression across lifespan in budding yeast. We find that COMPASS mutations reduce replicative lifespan and cause expression defects in almost 500 genes. Although H3K4 methylation is reported to act primarily in gene repression, particularly in yeast, repressive functions are progressively lost with age while hundreds of genes become dependent on H3K4me3 for full expression. Basal and inducible expression of these genes is also impaired in young cells lacking COMPASS components Swd1 or Spp1. Gene induction during ageing is associated with increasing promoter H3K4me3, but H3K4me3 also accumulates in non-promoter regions and the ribosomal DNA. Our results provide clear evidence that H3K4me3 is required to maintain normal expression of many genes across organismal lifespan.

Multicenter Phase II Study of Lurbinectedin in BRCA-Mutated and Unselected Metastatic Advanced Breast Cancer and Biomarker Assessment Substudy

Purpose

This multicenter phase II trial evaluated lurbinectedin (PM01183), a selective inhibitor of active transcription of protein-coding genes, in patients with metastatic breast cancer. A unicenter translational substudy assessed potential mechanisms of lurbinectedin resistance.

Patients and Methods

Two arms were evaluated according to germline BRCA1/2 status: BRCA1/2 mutated (arm A; n = 54) and unselected (BRCA1/2 wild-type or unknown status; arm B; n = 35). Lurbinectedin starting dose was a 7-mg flat dose and later, 3.5 mg/m² in arm A. The primary end point was objective response rate (ORR) per Response Evaluation Criteria in Solid Tumors (RECIST). The translational substudy of resistance mechanisms included exome sequencing (n = 13) and in vivo experiments with patient-derived xenografts (n = 11) from BRCA1/2-mutated tumors.

Results

ORR was 41% (95% CI, 28% to 55%) in arm A and 9% (95% CI, 2% to 24%) in arm B. In arm A, median progression-free survival was 4.6 months (95% CI, 3.0 to 6.0 months), and median overall survival was 20.0 months (95% CI, 11.8 to 26.6 months). Patients with BRCA2 mutations showed an ORR of 61%, median progression-free survival of 5.9 months, and median overall survival of 26.6 months. The safety profile improved with lurbinectedin dose adjustment to body surface area. The most common nonhematologic adverse events seen at 3.5 mg/m² were nausea (74%; grade 3, 5%) and fatigue (74%; grade 3, 21%). Neutropenia was the most common severe hematologic adverse event (grade 3, 47%; grade 4, 10%). Exome sequencing showed mutations in genes related to the nucleotide excision repair pathway in four of seven tumors at primary or acquired resistance and in one patient with short-term stable disease. In vivo, sensitivity to cisplatin and lurbinectedin was evidenced in lurbinectedin-resistant (one of two) and cisplatin-resistant (two of three) patient-derived xenografts.

Conclusion

Lurbinectedin showed noteworthy activity in patients with BRCA1/2 mutations. Response and survival was notable in those with BRCA2 mutations. Additional clinical development in this subset of patients with metastatic breast cancer is warranted.

Nitrogen isotope signature evidences ammonium deprotonation as a common transport mechanism for the AMT-Mep-Rh protein superfamily

Ammonium is an important nitrogen (N) source for living organisms, a key metabolite for pH control, and a potent cytotoxic compound. Ammonium is transported by the widespread AMT-Mep-Rh membrane proteins, and despite their significance in physiological processes, the nature of substrate translocation (NH₃/NH₄⁺) by the distinct members of this family is still a matter of controversy. Using Saccharomyces cerevisiae cells expressing representative AMT-Mep-Rh ammonium carriers and taking advantage of the natural chemical-physical property of the N isotopic signature linked to NH₄⁺/NH₃ conversion, this study shows that only cells expressing AMT-Mep-Rh proteins were depleted in ¹⁵N relative to ¹⁴N when compared to the external ammonium source. We observed ¹⁵N depletion over a wide range of external pH, indicating its independence of NH₃ formation in solution. On the basis of inhibitor studies, ammonium transport by nonspecific cation channels did not show isotope fractionation but competition with K⁺. We propose that kinetic N isotope fractionation is a common feature of AMT-Mep-Rh-type proteins, which favor ¹⁴N over ¹⁵N, owing to the dissociation of NH₄⁺ into NH₃ + H⁺ in the protein, leading to ¹⁵N depletion in the cell and allowing NH₃ passage or NH₃/H⁺ cotransport. This deprotonation mechanism explains these proteins' essential functions in environments under a low NH₄⁺/K⁺ ratio, allowing organisms to specifically scavenge NH₄⁺. We show that ¹⁵N isotope fractionation may be used in vivo not only to determine the molecular species being transported by ammonium transport proteins, but also to track ammonium toxicity and associated amino acids excretion.

Is it important to assess the ascending aorta after tetralogy of Fallot repair?

INTRODUCTION AND OBJECTIVES

Aortic dilatation can develop late after tetralogy of Fallot repair. Its extension beyond the aortic root is not clearly understood. We aimed to assess the prevalence and predictors of ascending aorta dilatation to set up an imaging protocol.

METHODS

In this prospective study including adult patients after tetralogy of Fallot repair followed at a referral center, we assessed the aorta by cardiovascular magnetic resonance and defined ascending aorta dilatation as an observed-to-expected ratio >1.5.

RESULTS

We included 78 adults (mean age 31±10 years; 56% female), with a mean follow-up of 23±7 years since tetralogy of Fallot repair. The prevalence of ascending aorta dilatation was 11.5%. The ascending aorta was larger than the sinuses of Valsalva in 12.8% of cases. Patients with ascending aorta dilatation were older, predominantly male, with later repair and larger left ventricular mass and volumes. By multivariate analysis left ventricular mass index (LVMI) was the only factor independently associated with ascending aorta dilatation (odds ratio 1.10, 95% confidence interval 1.01-1.20, p=0.03). A cut-off value of ≥57.9 g/m² for LVMI had 89% sensitivity and 71% specificity for ascending aorta dilatation.

CONCLUSIONS

Ascending aorta assessment as part of a routine cardiovascular magnetic resonance study after tetralogy of Fallot repair is recommended to screen for future aortic complications, particularly in males and older patients, and those with later repair and larger left ventricles. LVMI assessment has potential as a screening tool for ascending aorta dilatation with future clinical implications.

Aortic dilatation after tetralogy of Fallot repair: A ghost from the past or a problem in the future?

INTRODUCTION AND AIMS

Intrinsic aortopathy can lead to dilatation late after tetralogy of Fallot (TOF) repair. Its extent and prevalence are not known. We aimed to assess aortic dimensions and elasticity and to find predictors of aortic dilatation.

METHODS

A total of 126 adults were prospectively included after TOF repair and compared to 63 gender- and age-matched controls. Transthoracic echocardiography was used to assess aortic diameters at the level of the sinuses of Valsalva and ascending aorta and aortic dilatation was defined as z-score >+2. M-mode parameters of the ascending aorta were used to calculate strain, distensibility and stiffness index.

RESULTS

TOF patients (mean age 30±9 years; 52% male) had a complete repair at a median age of five (2-49) years; mean follow-up time since repair was 23±7 years. The prevalence of aortic dilatation at the sinuses of Valsalva and ascending aorta was 29% and 24%, respectively. Compared to controls, TOF patients had a higher ascending aorta z-score, lower strain (6.4% [0.0-61.5] vs. 15.2% [0.0-45.0], p<0.01) and higher stiffness index (7.3 [0.8-23.6] vs. 3.1 [0.9-14.1], p<0.01). On multivariate analysis male gender was strongly associated with sinuses of Valsalva dilatation (odds ratio 6.3, 95% confidence interval 1.5-26.3, p=0.01).

CONCLUSIONS

The prevalence of aortic dilatation late after TOF repair is significant, with a larger and stiffer ascending aorta. Male gender appears to influence aortic root dilatation. This aortopathy requires careful follow-up in order to prevent future complications.

Percutaneous Closure of a Fistulous Giant Coronary Aneurysm

Giant coronary artery aneurysms larger than 50 mm are rare and associated with important complications: namely, rupture. Its workup requires comprehensive imaging and standard treatment is surgical exclusion. We present a 60-year-old patient with previous ostium secundum atrial septal defect surgical closure diagnosed with a giant proximal right coronary artery aneurysm (70 x 62 mm) fistulizing into the right atrium. Percutaneous closure of its aortic origin with an atrial septal occluder was successfully performed, and thrombosis of the aneurysm confirmed on angiography and echocardiogram. This case depicts an innovative, minimally invasive approach to this worrisome entity.

RAD51 foci as a functional biomarker of homologous recombination repair and PARP inhibitor resistance in germline BRCA-mutated breast cancer

Background

BRCA1 and BRCA2 (BRCA1/2)-deficient tumors display impaired homologous recombination repair (HRR) and enhanced sensitivity to DNA damaging agents or to poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi). Their efficacy in germline BRCA1/2 (gBRCA1/2)-mutated metastatic breast cancers has been recently confirmed in clinical trials. Numerous mechanisms of PARPi resistance have been described, whose clinical relevance in gBRCA-mutated breast cancer is unknown. This highlights the need to identify functional biomarkers to better predict PARPi sensitivity.

Patients and methods

We investigated the in vivo mechanisms of PARPi resistance in gBRCA1 patient-derived tumor xenografts (PDXs) exhibiting differential response to PARPi. Analysis included exome sequencing and immunostaining of DNA damage response proteins to functionally evaluate HRR. Findings were validated in a retrospective sample set from gBRCA1/2-cancer patients treated with PARPi.

Results

RAD51 nuclear foci, a surrogate marker of HRR functionality, were the only common feature in PDX and patient samples with primary or acquired PARPi resistance. Consistently, low RAD51 was associated with objective response to PARPi. Evaluation of the RAD51 biomarker in untreated tumors was feasible due to endogenous DNA damage. In PARPi-resistant gBRCA1 PDXs, genetic analysis found no in-frame secondary mutations, but BRCA1 hypomorphic proteins in 60% of the models, TP53BP1-loss in 20% and RAD51-amplification in one sample, none mutually exclusive. Conversely, one of three PARPi-resistant gBRCA2 tumors displayed BRCA2 restoration by exome sequencing. In PDXs, PARPi resistance could be reverted upon combination of a PARPi with an ataxia-telangiectasia mutated (ATM) inhibitor.

Conclusion

Detection of RAD51 foci in gBRCA tumors correlates with PARPi resistance regardless of the underlying mechanism restoring HRR function. This is a promising biomarker to be used in the clinic to better select patients for PARPi therapy. Our study also supports the clinical development of PARPi combinations such as those with ATM inhibitors.

Robust singlet fission in pentacene thin films with tuned charge transfer interactions

Singlet fission, the spin-allowed photophysical process converting an excited singlet state into two triplet states, has attracted significant attention for device applications. Research so far has focused mainly on the understanding of singlet fission in pure materials, yet blends offer the promise of a controlled tuning of intermolecular interactions, impacting singlet fission efficiencies. Here we report a study of singlet fission in mixtures of pentacene with weakly interacting spacer molecules. Comparison of experimentally determined stationary optical properties and theoretical calculations indicates a reduction of charge-transfer interactions between pentacene molecules with increasing spacer molecule fraction. Theory predicts that the reduced interactions slow down singlet fission in these blends, but surprisingly we find that singlet fission occurs on a timescale comparable to that in pure crystalline pentacene. We explain the observed robustness of singlet fission in such mixed films by a mechanism of exciton diffusion to hot spots with closer intermolecular spacings.

Quantifying the relative effects of environmental and direct transmission of norovirus

Norovirus is a common cause of outbreaks of acute gastroenteritis in health- and child-care settings, with serial outbreaks also frequently observed aboard cruise ships. The relative contributions of environmental and direct person-to-person transmission of norovirus have hitherto not been quantified. We employ a novel mathematical model of norovirus transmission, and fit the model to daily incidence data from a major norovirus outbreak on a cruise ship, and examine the relative efficacy of potential control strategies aimed at reducing environmental and/or direct transmission. The reproduction number for environmental and direct transmission combined is [Formula: see text] [6.1,9.5], and of environmental transmission alone is [Formula: see text] [0.9,2.6]. Direct transmission is overwhelmingly due to passenger-to-passenger contacts, but crew can act as a reservoir of infection from cruise to cruise. This is the first quantification of the relative roles of environmental and direct transmission of norovirus. While environmental transmission has the potential to maintain a sustained series of outbreaks aboard a cruise ship in the absence of strict sanitation practices, direct transmission dominates. We find that intensive promotion of good hand washing practices may prevent outbreaks. Isolation of ill passengers and cleaning are beneficial, but appear to be less efficacious at outbreak control.

Abstract P2-09-25: Clinical and pathologic characteristics of breast cancers determined to be HER2-positive by fluorescence in-situ hybridization (FISH) using alternative chromosome 17 probes

Background:

Based on updated 2013 ASCO/CAP guideline for HER2 testing, cases with a HER2/CEP17 ratio &lt; 2.0 but with an average HER2 copy number &gt; 4.0 and &lt;6.0 signals/cell are considered equivocal. In such cases, HER2 testing using alternative chromosome 17 probes was proposed as one way to resolve the equivocal FISH results. Using the alternative probe method increases the number of cancers categorized as HER2 positive but brings to question if these cancers truly represent HER2 amplified breast cancers and derive the same benefit from anti-HER2 therapies.

Methods:

Since 2013, all breast cancers at our institution that were HER2 equivocal by traditional FISH but classified as HER2 positive using the alternative probe method were assessed for clinical and pathologic features including histologic type and grade, TNM stage, HER2: alternative probe ratio, treatment, and clinical outcome.

Results:

We identified 24 invasive breast cancers considered HER2 positive by the alternative probe method: 23 (96%) were estrogen receptor-positive (ER+) and 20 (83%) were progesterone receptor- positive. Histologically, only 2 were invasive lobular carcinomas; all others were ductal or had ductal and lobular features. Most cancers (63%) had low or intermediate histologic grade: Grade 1 (n=3); Grade 2 (n=12); Grade 3 (n=9). Clinical information was available for 18 patients: 2 had metastatic disease, 1 had a local recurrence after mastectomy and 15 patients had early stage disease; 9 with node negative disease and 6 with nodal involvement. HER2 IHC was equivocal (2+) in 16 (66.7%) cases, positive (3+) in 4 (16.7%) cases, and negative (0 or 1+) in 4 (16.7%) cases. The average HER2 copy number was 4.77, the average HER22:p53 ratio was 2.61. Repeat HER2 testing on a 2nd tumor sample was performed in 8 cases: HER2-positivity was confirmed in only 2 (25%) cases and by the alternative probe only. Treatment information was available for 17 patients: 1 had T1aN0M0 lesion and did not get chemotherapy, 16 received chemotherapy and 13 received trastuzumab-based chemotherapy. Eleven patients with early stage disease received chemotherapy and trastuzumab. Of these patients, 10/11 were ER+, 7/11 were node negative and 5/11 had grade 2 tumors, yet only one tumor was assessed by oncotype recurrence score ( RS = 29). Three patients received chemotherapy and trastuzumab in the neoadjuvant setting: 1 had a complete pathologic response, 1 a partial response, and 1 has not yet gone to surgery. One additional patient received neoadjuvant chemo alone and achieved a partial response.

Conclusions:

Breast cancers considered HER2+ by the alternative probe method but not by traditional FISH are almost always ER-positive and most have low or intermediate histologic grade. Repeat HER2 testing on a subsequent tumor sample did not confirm HER2-positivity in 75% of cases. Almost all patients with early stage disease received chemotherapy and trastuzumab based on the alternative probe results without molecular assessment to predict chemotherapy response. Intrinsic molecular subtyping using PAM50 analysis on these cancers is underway to determine how many are HER2-enriched by molecular assessment.

Citation Format: Desai NV, Torous V, Cruz C, Schnitt SJ, Tung N. Clinical and pathologic characteristics of breast cancers determined to be HER2-positive by fluorescence in-situ hybridization (FISH) using alternative chromosome 17 probes [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 P2-09-25.

Continuous Flow Peritoneal Dialysis: A New Double Lumen Catheter

Continuous flow peritoneal dialysis (CFPD) is a therapy originally utilized in the sixties. It was then abandoned because of technical reasons, but, today, a new interest in this technique is emerging, because of new technical solutions and new hardware capabilities. CFPD is a peritoneal dialysis technique in which a certain amount of fluid is maintained in the peritoneal cavity, while a continuous inflow and outflow is provided via twin catheters or through a double lumen catheter. In this paper a new double lumen catheter is presented. The catheter is characterized by the presence of a diffuser in the inflow lumen, while a standard coiled shape characterizes the outflow lumen. The diffuser allows the use of high dialysate flows without peritoneal damage and with an excellent distribution of the fluid. The other feature of the catheter is the removable hub which allows for an easy subcutaneous tunneling of the catheter with a subsequent connection to the y segment. The special shape also guarantees a minimum recirculation during treatment. Data obtained in the first implanted catheter showed a progressive increase in small solute clearances in relation to an increase of the flow and the tidal volume in the peritoneal cavity. In particular, urea clearances up to 48 ml/min and creatinine clearances up to 39 ml/min were obtained. No major complications were observed after one year of use of the catheter.

Neurogenic stuttering: a review of the literature

INTRODUCTION

Neurogenic stuttering is a disorder of neurologic origin in the rhythm of speech during which the patient knows exactly what he wants to say but is unable to because of an involuntary prolongation, cessation or repetition of a sound.

AIM

To assemble new insights regarding the epidemiology, pathophysiology, diagnosis, evaluation and treatment of neurogenic stuttering.

DEVELOPMENT

A review of all PubMed and Scopus published articles between January 2000 and September 2016 was performed. Thirty-three publications were analyzed. Neurogenic stuttering is a rare entity whose epidemiological incidence is yet not fully established. It is correlated with several neurological diseases and with several possible localizations within the nervous system. Notwithstanding the recent advances in the understanding of the underlying mechanism, it is not yet possible to establish a single pathophysiological mechanism of neurogenic stuttering. The differential diagnosis is complex and requires the detailed knowledge of other language disorders. The treatment is currently based on specific speech language therapy strategies.

CONCLUSION

Neurogenic stuttering is a complex disorder which is not fully understood. Additional studies might help to better explain the underlying pathophysiological mechanism and to open doors to novel therapeutic methods.

Intra- and inter-specific variations in chitin in lichens along a N-deposition gradient

The mechanisms of nitrogen (N) tolerance in lichens are not yet fully understood. Here, we investigated how the increase of chitin content is related with N excess at inter- and intra-specific levels, by using species with differing ecological N tolerances (the tolerant Xanthoria parietina and Parmotrema hypoleucinum and the sensitive Evernia prunastri and Usnea sp.) and thalli of X. parietina and P. hypoleucinum from sites with different availabilities of N of agricultural origin (livestock), as confirmed by lichen N content and δ¹⁵N. Nitrogen, chitin (N-containing compound), and ergosterol contents were measured in lichen thalli. Nitrogen and chitin contents were higher in tolerant species than those in sensitive ones (inter-specific level) and in thalli collected from the N-polluted site than in thalli from the clean site (intra-specific level). We suggest that chitin contributes to N stress tolerance in lichens, and that excess N can be partially stored as chitin (non-toxic form) in the cell walls of tolerant species.

CDK12 Inhibition Reverses De Novo and Acquired PARP Inhibitor Resistance in BRCA Wild-Type and Mutated Models of Triple-Negative Breast Cancer

Although poly(ADP-ribose) polymerase (PARP) inhibitors are active in homologous recombination (HR)-deficient cancers, their utility is limited by acquired resistance after restoration of HR. Here, we report that dinaciclib, an inhibitor of cyclin-dependent kinases (CDKs) 1, 2, 5, and 9, additionally has potent activity against CDK12, a transcriptional regulator of HR. In BRCA-mutated triple-negative breast cancer (TNBC) cells and patient-derived xenografts (PDXs), dinaciclib ablates restored HR and reverses PARP inhibitor resistance. Additionally, we show that de novo resistance to PARP inhibition in BRCA1-mutated cell lines and a PDX derived from a PARP-inhibitor-naive BRCA1 carrier is mediated by residual HR and is reversed by CDK12 inhibition. Finally, dinaciclib augments the degree of response in a PARP-inhibitor-sensitive model, converting tumor growth inhibition to durable regression. These results highlight the significance of HR disruption as a therapeutic strategy and support the broad use of combined CDK12 and PARP inhibition in TNBC.