Adventures With Parasites, Metabolism, Inborn Errors, and Cancer: The 2023 Lasker-Koshland Special Achievement Award in Medical Science

In this Viewpoint, Lasker Award winner Piet Borst looks back over a 50-year career in scientific research, including work with trypanosomatids, mechanisms of drug resistance in cancer cells, and inborn errors of metabolism.

Markedly increased risk of postoperative bleeding complications during perioperative bridging anticoagulation in general and visceral surgery

BACKGROUND

Increasing numbers of patients receiving oral anticoagulants are undergoing elective surgery. Low molecular weight heparin (LMWH) is frequently applied as bridging therapy during perioperative interruption of anticoagulation. The aim of this study was to explore the postoperative bleeding risk of patients receiving surgery under bridging anticoagulation.

METHODS

We performed a monocentric retrospective two-arm matched cohort study. Patients that received perioperative bridging anticoagulation were compared to a matched control group with identical surgical procedure, age, and sex. Emergency and vascular operations were excluded. The primary endpoint was the incidence of major postoperative bleeding. Secondary endpoints were minor postoperative bleeding, thromboembolic events, length of stay, and in-hospital mortality. Multivariate analysis explored risk factors of major postoperative bleeding.

RESULTS

A total of 263 patients in each study arm were analyzed. The patient cohort included the entire field of general and visceral surgery including a large proportion of major oncological resections. Bridging anticoagulation increased the postoperative incidence of major bleeding events (8% vs. 1%; p < 0.001) as well as minor bleeding events (14% vs. 5%; p < 0.001). Thromboembolic events were equally rare in both groups (1% vs. 2%; p = 0.45). No effect on mortality was observed (1.5% vs. 1.9%). Independent risk factors of major postoperative bleeding were full-therapeutic dose of LMWH, renal insufficiency, and the procedure-specific bleeding risk.

CONCLUSION

Perioperative bridging anticoagulation, especially full-therapeutic dose LMWH, markedly increases the risk of postoperative bleeding complications in general and visceral surgery. Surgeons should carefully consider the practice of routine bridging.

Looking back at multidrug resistance (MDR) research and ten mistakes to be avoided when writing about ABC transporters in MDR

This paper presents a personal, selective, and sometimes critical retrospective of the history of ABC transporters in multidrug resistance (MDR) of cancer cells, overrepresenting discoveries of some early pioneers, long forgotten, and highlights of research in Amsterdam, mainly focussing on discoveries made with disruptions of ABC genes in mice (KO mice) and on the role of ABC transporters in causing drug resistance in a mouse model of mammary cancer. The history is complemented by a list of erroneous concepts often found in papers and grant applications submitted anno 2020.

The malate-aspartate shuttle (Borst cycle): How it started and developed into a major metabolic pathway

This article presents a personal and critical review of the history of the malate–aspartate shuttle (MAS), starting in 1962 and ending in 2020. The MAS was initially proposed as a route for the oxidation of cytosolic NADH by the mitochondria in Ehrlich ascites cell tumor lacking other routes, and to explain the need for a mitochondrial aspartate aminotransferase (glutamate oxaloacetate transaminase 2 [GOT2]). The MAS was soon adopted in the field as a major pathway for NADH oxidation in mammalian tissues, such as liver and heart, even though the energetics of the MAS remained a mystery. Only in the 1970s, LaNoue and coworkers discovered that the efflux of aspartate from mitochondria, an essential step in the MAS, is dependent on the proton‐motive force generated by the respiratory chain: for every aspartate effluxed, mitochondria take up one glutamate and one proton. This makes the MAS in practice uni‐directional toward oxidation of cytosolic NADH, and explains why the free NADH/NAD ratio is much higher in the mitochondria than in the cytosol. The MAS is still a very active field of research. Most recently, the focus has been on the role of the MAS in tumors, on cells with defects in mitochondria and on inborn errors in the MAS. The year 2019 saw the discovery of two new inborn errors in the MAS, deficiencies in malate dehydrogenase 1 and in aspartate transaminase 2 (GOT2). This illustrates the vitality of ongoing MAS research.

PXE, a Mysterious Inborn Error Clarified

Ever since Garrod deduced the existence of inborn errors in 1901, a vast array of metabolic diseases has been identified and characterized in molecular terms. In 2018 it is difficult to imagine that there is any uncharted backyard left in the metabolic disease landscape. Nevertheless, it took until 2013 to identify the cause of a relatively frequent inborn error, pseudoxanthoma elasticum (PXE), a disorder resulting in aberrant calcification. The mechanism found was not only biochemically interesting but also points to possible new treatments for PXE, a disease that has remained untreatable. In this review we sketch the tortuous road that led to the biochemical understanding of PXE and to new ideas for treatment. We also discuss some of the controversies still haunting the field.

Selective Loss of PARG Restores PARylation and Counteracts PARP Inhibitor-Mediated Synthetic Lethality

Inhibitors of poly(ADP-ribose) (PAR) polymerase (PARPi) have recently entered the clinic for the treatment of homologous recombination (HR)-deficient cancers. Despite the success of this approach, drug resistance is a clinical hurdle, and we poorly understand how cancer cells escape the deadly effects of PARPi without restoring the HR pathway. By combining genetic screens with multi-omics analysis of matched PARPi-sensitive and -resistant Brca2-mutated mouse mammary tumors, we identified loss of PAR glycohydrolase (PARG) as a major resistance mechanism. We also found the presence of PARG-negative clones in a subset of human serous ovarian and triple-negative breast cancers. PARG depletion restores PAR formation and partially rescues PARP1 signaling. Importantly, PARG inactivation exposes vulnerabilities that can be exploited therapeutically.

Selected Alkylating Agents Can Overcome Drug Tolerance of G0-like Tumor Cells and Eradicate BRCA1-Deficient Mammary Tumors in Mice

Purpose: We aimed to characterize and target drug-tolerant BRCA1-deficient tumor cells that cause residual disease and subsequent tumor relapse.Experimental Design: We studied responses to various mono- and bifunctional alkylating agents in a genetically engineered mouse model for BRCA1/p53-mutant breast cancer. Because of the large intragenic deletion of the Brca1 gene, no restoration of BRCA1 function is possible, and therefore, no BRCA1-dependent acquired resistance occurs. To characterize the cell-cycle stage from which Brca1^-/-;p53^-/- mammary tumors arise after cisplatin treatment, we introduced the fluorescent ubiquitination-based cell-cycle indicator (FUCCI) construct into the tumor cells.Results: Despite repeated sensitivity to the MTD of platinum drugs, the Brca1-mutated mammary tumors are not eradicated, not even by a frequent dosing schedule. We show that relapse comes from single-nucleated cells delaying entry into the S-phase. Such slowly cycling cells, which are present within the drug-naïve tumors, are enriched in tumor remnants. Using the FUCCI construct, we identified nonfluorescent G₀-like cells as the population most tolerant to platinum drugs. Intriguingly, these cells are more sensitive to the DNA-crosslinking agent nimustine, resulting in an increased number of multinucleated cells that lack clonogenicity. This is consistent with our in vivo finding that the nimustine MTD, among several alkylating agents, is the most effective in eradicating Brca1-mutated mouse mammary tumors.Conclusions: Our data show that targeting G₀-like cells is crucial for the eradication of BRCA1/p53-deficient tumor cells. This can be achieved with selected alkylating agents such as nimustine. Clin Cancer Res; 23(22); 7020-33. ©2017 AACR.

HELB Is a Feedback Inhibitor of DNA End Resection

DNA double-strand break repair by homologous recombination is initiated by the formation of 3' single-stranded DNA (ssDNA) overhangs by a process termed end resection. Although much focus has been given to the decision to initiate resection, little is known of the mechanisms that regulate the ongoing formation of ssDNA tails. Here we report that DNA helicase B (HELB) underpins a feedback inhibition mechanism that curtails resection. HELB is recruited to ssDNA by interacting with RPA and uses its 5'-3' ssDNA translocase activity to inhibit EXO1 and BLM-DNA2, the nucleases catalyzing resection. HELB acts independently of 53BP1 and is exported from the nucleus as cells approach S phase, concomitant with the upregulation of resection. Consistent with its role as a resection antagonist, loss of HELB results in PARP inhibitor resistance in BRCA1-deficient tumor cells. We conclude that mammalian DNA end resection triggers its own inhibition via the recruitment of HELB.

Subunit composition of VRAC channels determines substrate specificity and cellular resistance to Pt-based anti-cancer drugs

Although platinum‐based drugs are widely used chemotherapeutics for cancer treatment, the determinants of tumor cell responsiveness remain poorly understood. We show that the loss of subunits LRRC8A and LRRC8D of the heteromeric LRRC8 volume‐regulated anion channels (VRACs) increased resistance to clinically relevant cisplatin/carboplatin concentrations. Under isotonic conditions, about 50% of cisplatin uptake depended on LRRC8A and LRRC8D, but neither on LRRC8C nor on LRRC8E. Cell swelling strongly enhanced LRRC8‐dependent cisplatin uptake, bolstering the notion that cisplatin enters cells through VRAC. LRRC8A disruption also suppressed drug‐induced apoptosis independently from drug uptake, possibly by impairing VRAC‐dependent apoptotic cell volume decrease. Hence, by mediating cisplatin uptake and facilitating apoptosis, VRAC plays a dual role in the cellular drug response. Incorporation of the LRRC8D subunit into VRAC substantially increased its permeability for cisplatin and the cellular osmolyte taurine, indicating that LRRC8 proteins form the channel pore. Our work suggests that LRRC8D‐containing VRACs are crucial for cell volume regulation by an important organic osmolyte and may influence cisplatin/carboplatin responsiveness of tumors.

ATP-binding Cassette Subfamily C Member 5 (ABCC5) Functions as an Efflux Transporter of Glutamate Conjugates and Analogs

The ubiquitous efflux transporter ABCC5 (ATP-binding cassette subfamily C member 5) is present at high levels in the blood-brain barrier, neurons, and glia, but its in vivo substrates and function are not known. Using untargeted metabolomic screens, we show that Abcc5(-/-) mice accumulate endogenous glutamate conjugates in several tissues, but brain in particular. The abundant neurotransmitter N-acetylaspartylglutamate was 2.4-fold higher in Abcc5(-/-) brain. The metabolites that accumulated in Abcc5(-/-) tissues were depleted in cultured cells that overexpressed human ABCC5. In a vesicular membrane transport assay, ABCC5 also transported exogenous glutamate analogs, like the classic excitotoxic neurotoxins kainic acid, domoic acid, and NMDA; the therapeutic glutamate analog ZJ43; and, as previously shown, the anti-cancer drug methotrexate. Glutamate conjugates and analogs are of physiological relevance because they can affect the function of glutamate, the principal excitatory neurotransmitter in the brain. After CO2 asphyxiation, several immediate early genes were expressed at lower levels in Abcc5(-/-) brains than in wild type brains, suggesting altered glutamate signaling. Our results show that ABCC5 is a general glutamate conjugate and analog transporter that affects the disposition of endogenous metabolites, toxins, and drugs.

Abstract 736: Combining PD1- and CTLA4-inhibiting antibodies with cisplatin or PARP inhibition in an attempt to eradicate BRCA1-deficient mouse mammary tumors

Solid tumors are usually not eradicated by conventional chemotherapy resulting in disease relapse and mortality. An intriguing example is BRCA-associated breast cancer. A genetically engineered mouse model for BRCA1-associated breast cancer (K14Cre;Brca1F/F;p53F/F), which highly resembles disease in human patients, can be used to study therapy escape and residual disease in BRCA-deficient tumors. Due to the BRCA inactivation, the tumors that arise in this model lack homologous recombination-directed DNA repair, an Achilles heel that has provided a therapeutic opportunity to eradicate tumors with DNA damage-inducing agents. Despite repeated sensitivity some residual cancer cells escape the deadly effect of anticancer therapy and lead to disease relapse. A special histopathological feature of BRCA1-mutated tumors in both humans and mice is the presence of infiltrating lymphocytes. This may be explained by the high frequency of genomic alterations of BRCA1-mutated tumors resulting in the generation of many neo-antigens. Our hypothesis is that blocking inhibitory T-cell signaling, using antibodies directed against CTLA4 and PD1, may increase the activity of the T-cell compartment towards a diverse pool of antigens, and successfully eliminate residual tumor cells in combination with DNA damage-inducing drugs. In particular, we will present data of combining CTLA4- and PD1- targeting antibodies with PARP inhibition or cisplatin to eliminate residual tumor cells in this mouse model for BRCA1-mutated breast cancer.

Citation Format: Sohvi Blatter, Charlotte Guyader, Aslı Küçükosmanoğlu, Stephan Freriks, Karin de Visser, Piet Borst, Sven Rottenberg. Combining PD1- and CTLA4-inhibiting antibodies with cisplatin or PARP inhibition in an attempt to eradicate BRCA1-deficient mouse mammary tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 736. doi:10.1158/1538-7445.AM2015-736

Abstract 3607: Loss-of-function screens using haploid KBM7 and HAP1 cells to identify mechanisms of anti-cancer drug resistance

The lack of biomarkers to predict anti-cancer therapy response remains a major obstacle in the treatment of various cancers. This handicap not only affects novel targeted therapies. Also for classical chemotherapeutic drugs such as DNA cross-linking agents, topoisomerase inhibitors or microtubule-targeting compounds, which are all frequently used in the clinic, we do not understand well why some patients have a major benefit of the therapy whereas others do not. A decreased intracellular drug accumulation has been proposed as one potential mechanism, but the clinical relevance of known uptake and efflux mechanisms is still controversial for various chemotherapeutic drugs. To identify novel factors that may be relevant for the intracellular concentration of drugs, we carried out loss-of-function screens using haploid KBM7 and HAP1 cells. In particular, we used the PARP inhibitor olaparib, the topoisomerase I inhibitor topotecan, platinum drugs, and the microtubule-targeting compounds docetaxel and vinorelbine to select resistant clones. We will present results from these screens and their validation, including independent cell lines and TCGA data sets.

Citation Format: Nora M. Gerhards, Charlotte Guyader, Vincent A. Blomen, Aslı Küçükosmanoğlu, Olaf van Tellingen, Daniel J. Vis, Lodewyk F. Wessels, Thijn R. Brummelkamp, Piet Borst, Sven Rottenberg. Loss-of-function screens using haploid KBM7 and HAP1 cells to identify mechanisms of anti-cancer drug resistance. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3607. doi:10.1158/1538-7445.AM2015-3607

REV7 counteracts DNA double-strand break resection and affects PARP inhibition

Error-free repair of DNA double-strand breaks (DSBs) is achieved by homologous recombination (HR), and BRCA1 is an important factor for this repair pathway. In the absence of BRCA1-mediated HR, the administration of PARP inhibitors induces synthetic lethality of tumour cells of patients with breast or ovarian cancers. Despite the benefit of this tailored therapy, drug resistance can occur by HR restoration. Genetic reversion of BRCA1-inactivating mutations can be the underlying mechanism of drug resistance, but this does not explain resistance in all cases. In particular, little is known about BRCA1-independent restoration of HR. Here we show that loss of REV7 (also known as MAD2L2) in mouse and human cell lines re-establishes CTIP-dependent end resection of DSBs in BRCA1-deficient cells, leading to HR restoration and PARP inhibitor resistance, which is reversed by ATM kinase inhibition. REV7 is recruited to DSBs in a manner dependent on the H2AX-MDC1-RNF8-RNF168-53BP1 chromatin pathway, and seems to block HR and promote end joining in addition to its regulatory role in DNA damage tolerance. Finally, we establish that REV7 blocks DSB resection to promote non-homologous end-joining during immunoglobulin class switch recombination. Our results reveal an unexpected crucial function of REV7 downstream of 53BP1 in coordinating pathological DSB repair pathway choices in BRCA1-deficient cells.

Defining the sequence requirements for the positioning of base J in DNA using SMRT sequencing

Base J (β-D-glucosyl-hydroxymethyluracil) replaces 1% of T in the Leishmania genome and is only found in telomeric repeats (99%) and in regions where transcription starts and stops. This highly restricted distribution must be co-determined by the thymidine hydroxylases (JBP1 and JBP2) that catalyze the initial step in J synthesis. To determine the DNA sequences recognized by JBP1/2, we used SMRT sequencing of DNA segments inserted into plasmids grown in Leishmania tarentolae. We show that SMRT sequencing recognizes base J in DNA. Leishmania DNA segments that normally contain J also picked up J when present in the plasmid, whereas control sequences did not. Even a segment of only 10 telomeric (GGGTTA) repeats was modified in the plasmid. We show that J modification usually occurs at pairs of Ts on opposite DNA strands, separated by 12 nucleotides. Modifications occur near G-rich sequences capable of forming G-quadruplexes and JBP2 is needed, as it does not occur in JBP2-null cells. We propose a model whereby de novo J insertion is mediated by JBP2. JBP1 then binds to J and hydroxylates another T 13 bp downstream (but not upstream) on the complementary strand, allowing JBP1 to maintain existing J following DNA replication.

BRCA2-deficient sarcomatoid mammary tumors exhibit multidrug resistance

Pan- or multidrug resistance is a central problem in clinical oncology. Here, we use a genetically engineered mouse model of BRCA2-associated hereditary breast cancer to study drug resistance to several types of chemotherapy and PARP inhibition. We found that multidrug resistance was strongly associated with an EMT-like sarcomatoid phenotype and high expression of the Abcb1b gene, which encodes the drug efflux transporter P-glycoprotein. Inhibition of P-glycoprotein could partly resensitize sarcomatoid tumors to the PARP inhibitor olaparib, docetaxel, and doxorubicin. We propose that multidrug resistance is a multifactorial process and that mouse models are useful to unravel this.

Abstract 3771: Splenic macrophages induce chemotherapy resistance via DNA damage repair

The development of resistance to chemotherapy is one of the most important obstacles to continued effective treatment of cancer in patients. We recently identified an important network that is responsible for reversible systemic resistance to chemotherapy. Mesenchymal stem cells (MSCs), activated by chemotherapy, secrete two specific polyunsaturated fatty acids that confer resistance to a broad spectrum of anti-cancer agents. These two distinct platinum-induced polyunsaturated fatty acids (PIFAs), 12-S-keto-5,8,10-heptadecatrienoic acid (KHT) and 4,7,10,13-hexadecatetraenoic acid (16:4(n-3)), can independently induce chemotherapy resistance at picomolar concentrations. Here we show that these PIFAs do not induce resistance to the tumor cells directly but rather function systemically via F4/80+/CD11blow/Ly6G- splenocytes to enhance DNA damage repair within tumors. We found that the PIFAs only induce resistance against DNA damaging agents but not against non-DNA damaging chemotherapeutics. When comparing tumors from cisplatin and PIFAs treated mice with cisplatin alone treated mice we found overall less DNA damage and a quicker restoration of DNA damage. In BRCA1-/-/p53-/- mice lacking homologous recombination, PIFAs were unable induce chemotherapy resistance.

Several lines of evidence identified F4/80+/CD11blow/Ly6G- splenocytes as key mediators within this mechanism of systemic chemoresistance. First, PIFAs were unable to induce resistance in splenectomized tumor-bearing mice. Second, administration of conditioned medium from splenocytes (sCM) activated by the PIFAs to splenectomized mice was able to re-introduce systemic chemotherapy resistance in various mouse models (mean tumor volume in mm3 8 days after treatment ± SEM control: 290.7 ± 33.4, cisplatin: 118.2 ± 8.9 and cisplatin + sCM: 261.1 ± 29.7). Third, analysis of the different cell types present in the spleen indicated that F4/80+/CD11blow expressing cells were able to induce systemic resistance in splenectomized mice. Finally, administration of liposomal clodronate to tumor-bearing mice treated with chemotherapy and PIFAs was able to inhibit induction of resistance. Given the fact that 12-S-HHT is a natural ligand of the leukotriene B4 receptor 2 (BLT2) we investigated if signaling via BLT2 is responsible for 12-S-HHT-induced chemoresistance. Both genetic loss of BLT2 or inhibition of BLT2 using LY255283 was able to prevent 12-S-HHT-mediated chemoresistance. Taken together, our findings provide a novel role for the spleen in inducing a systemic protection against chemotherapy via enhancing DNA damage repair.

Citation Format: Julia M. Houthuijzen, Laura G.M, Daenen, Jeanine M.L. Roodhart, Klaas M. Govaert, Michelle E. Smith, Juergen Thomale, Sahar J. Sadatmand, Hilde Rosing, Fabian Kruse, Nico van Rooijen, Jos H. Beijnen, Piet Borst, Sven Rottenberg, Bodduluri Haribabu, Emile E. Voest. Splenic macrophages induce chemotherapy resistance via DNA damage repair. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3771. doi:10.1158/1538-7445.AM2014-3771

ABCC6-mediated ATP secretion by the liver is the main source of the mineralization inhibitor inorganic pyrophosphate in the systemic circulation-brief report

OBJECTIVE

Mutations in ABCC6 underlie the ectopic mineralization disorder pseudoxanthoma elasticum (PXE) and some forms of generalized arterial calcification of infancy, both of which affect the cardiovascular system. Using cultured cells, we recently showed that ATP-binding cassette subfamily C member 6 (ABCC6) mediates the cellular release of ATP, which is extracellularly rapidly converted into AMP and the mineralization inhibitor inorganic pyrophosphate (PPi). The current study was performed to determine which tissues release ATP in an ABCC6-dependent manner in vivo, where released ATP is converted into AMP and PPi, and whether human PXE ptients have low plasma PPi concentrations.

APPROACH AND RESULTS

Using cultured primary hepatocytes and in vivo liver perfusion experiments, we found that ABCC6 mediates the direct, sinusoidal, release of ATP from the liver. Outside hepatocytes, but still within the liver vasculature, released ATP is converted into AMP and PPi. The absence of functional ABCC6 in patients with PXE leads to strongly reduced plasma PPi concentrations.

CONCLUSIONS

Hepatic ABCC6-mediated ATP release is the main source of circulating PPi, revealing an unanticipated role of the liver in systemic PPi homeostasis. Patients with PXE have a strongly reduced plasma PPi level, explaining their mineralization disorder. Our results indicate that systemic PPi is relatively stable and that PXE, generalized arterial calcification of infancy, and other ectopic mineralization disorders could be treated with PPi supplementation therapy.

P-glycoprotein ABCB1: a major player in drug handling by mammals

Mammalian P-glycoproteins are active drug efflux transporters located in the plasma membrane. In the early nineties, we generated knockouts of the three P-glycoprotein genes of mice, the Mdr1a, Mdr1b, and Mdr2 P-glycoproteins, now known as Abcb1a, Abcb1b, and Abcb4, respectively. In the JCI papers that are the subject of this Hindsight, we showed that loss of Mdr1a (Abcb1a) had a profound effect on the tissue distribution and especially the brain accumulation of a range of drugs frequently used in humans, including dexamethasone, digoxin, cyclosporin A, ondansetron, domperidone, and loperamide. All drugs were shown to be excellent substrates of the murine ABCB1A P-glycoprotein and its human counterpart, the MDR1 P-glycoprotein, ABCB1. We found that the ability of ABCB1 to prevent accumulation of some drugs in the brain is a prerequisite for their clinical use, as absence of the transporter led to severe toxicity or undesired CNS pharmacodynamic effects. Subsequent work has fully confirmed the profound effect of the drug-transporting ABCB1 P-glycoprotein on the pharmacokinetics of drugs in humans. In fact, every new drug is now screened for transport by ABCB1, as this limits oral availability and penetration into sanctuaries protected by ABCB1, such as the brain.

Abstract 5584: Finding predictive markers for tumor response to classical cytotoxic drugs

As attempts to find predictive markers for cytotoxic drugs have been relatively unsuccessful using patient tumor samples (1), we are using a mouse model for BRCA1-deficient mammary tumors to search for such markers. Although these tumors originate in inbred mice from an inactivating deletion of the Brca1 and p53 genes, their response to doxorubicin (2), docetaxel (2), topotecan (3) and PARP inhibitors (4) varies. We have shown that in this tumor model 5 of the 22 tumors respond poorly to docetaxel, because of upregulated drug-transporting P-glycoproteins (5). These 5 were not found by standard gene expression analytical tools, such as significance analysis of microarrays (SAM), and we have shown that SAM only detects resistance markers if present in more than half the tumor samples (5). Better algorithms for the detection of resistance determinants in a subfraction of tumors are being developed by De Ronde and Wessels (5). We are still searching for the cause of the poor docetaxel response in 17 of the 22 tumors.

1. Borst,P. and Wessels,L. (2010). Do predictive signatures really predict response to cancer chemotherapy? Cell Cycle 9, 4836-4840.

2. Rottenberg,S., et al (2007). Selective induction of chemotherapy resistance of mammary tumors in a conditional mouse model for hereditary breast cancer. Proc. Natl. Acad. Sci. U. S. A. 104, 12117-12122.

3. Zander,S., et al (2010). Sensitivity and acquired resistance of BRCA1; -53-deficient mouse mammary tumors to the topoisomerase I inhibitor topotecan. Cancer Res. 70, 1700-1710.

4. Rottenberg,S., et al (2008). High sensitivity of BRCA1-deficient mammary tumors to the PARP inhibitor AZD2281 alone and in combination with platinum drugs. Proc. Natl Acad. Sci. U. S. A 105, 17079-17084.

5. Rottenberg,S., et al (2012). Impact of intertumoral heterogeneity on predicting chemotherapy response of BRCA1-deficient mammary tumors. Cancer Res 72, 2350-2361.

Citation Format: Piet Borst, Sven Rottenberg. Finding predictive markers for tumor response to classical cytotoxic drugs. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5584. doi:10.1158/1538-7445.AM2013-5584

Loss of 53BP1 causes PARP inhibitor resistance in Brca1-mutated mouse mammary tumors

Inhibition of PARP is a promising therapeutic strategy for homologous recombination-deficient tumors, such as BRCA1-associated cancers. We previously reported that BRCA1-deficient mouse mammary tumors may acquire resistance to the clinical PARP inhibitor (PARPi) olaparib through activation of the P-glycoprotein drug efflux transporter. Here, we show that tumor-specific genetic inactivation of P-glycoprotein increases the long-term response of BRCA1-deficient mouse mammary tumors to olaparib, but these tumors eventually developed PARPi resistance. In a fraction of cases, this resistance is caused by partial restoration of homologous recombination due to somatic loss of 53BP1. Importantly, PARPi resistance was minimized by long-term treatment with the novel PARP inhibitor AZD2461, which is a poor P-glycoprotein substrate. Together, our data suggest that restoration of homologous recombination is an important mechanism for PARPi resistance in BRCA1-deficient mammary tumors and that the risk of relapse of BRCA1-deficient tumors can be effectively minimized by using optimized PARP inhibitors.

SIGNIFICANCE

In this study, we show that loss of 53BP1 causes resistance to PARP inhibition in mouse mammary tumors that are deficient in BRCA1. We hypothesize that low expression or absence of 53BP1 also reduces the response of patients with BRCA1-deficient tumors to PARP inhibitors.

Glucosylated hydroxymethyluracil, DNA base J, prevents transcriptional readthrough in Leishmania

Some Ts in nuclear DNA of trypanosomes and Leishmania are hydroxylated and glucosylated to yield base J (β-D-glucosyl-hydroxymethyluracil). In Leishmania, about 99% of J is located in telomeric repeats. We show here that most of the remaining J is located at chromosome-internal RNA polymerase II termination sites. This internal J and telomeric J can be reduced by a knockout of J-binding protein 2 (JBP2), an enzyme involved in the first step of J biosynthesis. J levels are further reduced by growing Leishmania JBP2 knockout cells in BrdU-containing medium, resulting in cell death. The loss of internal J in JBP2 knockout cells is accompanied by massive readthrough at RNA polymerase II termination sites. The readthrough varies between transcription units but may extend over 100 kb. We conclude that J is required for proper transcription termination and infer that the absence of internal J kills Leishmania by massive readthrough of transcriptional stops.

Binding of the J-binding protein to DNA containing glucosylated hmU (base J) or 5-hmC: evidence for a rapid conformational change upon DNA binding

Base J (β-D-glucosyl-hydroxymethyluracil) was discovered in the nuclear DNA of some pathogenic protozoa, such as trypanosomes and Leishmania, where it replaces a fraction of base T. We have found a J-Binding Protein 1 (JBP1) in these organisms, which contains a unique J-DNA binding domain (DB-JBP1) and a thymidine hydroxylase domain involved in the first step of J biosynthesis. This hydroxylase is related to the mammalian TET enzymes that hydroxylate 5-methylcytosine in DNA. We have now studied the binding of JBP1 and DB-JBP1 to oligonucleotides containing J or glucosylated 5-hydroxymethylcytosine (glu-5-hmC) using an equilibrium fluorescence polarization assay. We find that JBP1 binds glu-5-hmC-DNA with an affinity about 40-fold lower than J-DNA (~400 nM), which is still 200 times higher than the JBP1 affinity for T-DNA. The discrimination between glu-5-hmC-DNA and T-DNA by DB-JBP1 is about 2-fold less, but enough for DB-JBP1 to be useful as a tool to isolate 5-hmC-DNA. Pre-steady state kinetic data obtained in a stopped-flow device show that the initial binding of JBP1 to glucosylated DNA is very fast with a second order rate constant of 70 μM(-1) s(-1) and that JBP1 binds to J-DNA or glu-5-hmC-DNA in a two-step reaction, in contrast to DB-JBP1, which binds in a one-step reaction. As the second (slower) step in binding is concentration independent, we infer that JBP1 undergoes a conformational change upon binding to DNA. Global analysis of pre-steady state and equilibrium binding data supports such a two-step mechanism and allowed us to determine the kinetic parameters that describe it. This notion of a conformational change is supported by small-angle neutron scattering experiments, which show that the shape of JBP1 is more elongated in complex with DNA. The conformational change upon DNA binding may allow the hydroxylase domain of JBP1 to make contact with the DNA and hydroxylate T's in spatial proximity, resulting in regional introduction of base J into the DNA.

Lack of ABCG2 shortens latency of BRCA1-deficient mammary tumors and this is not affected by genistein or resveratrol

In addition to their role in drug resistance, the ATP-binding cassette (ABC) transporters ABCG2 and ABCB1 have been suggested to protect cells from a broad range of substances that may foster tumorigenesis. Phytoestrogens or their metabolites are substrates of these transporters and the influence of these compounds on breast cancer development is controversial. Estrogen-like properties might accelerate tumorigenesis on the one hand, whereas their proposed health-protective properties might antagonize tumorigenesis on the other. To address this issue, we used a newer generation mouse model of BRCA1-mutated breast cancer and examined tumor latency in K14cre;Brca1(F/F); p53(F/F), Abcb1a/b(-/-);K14cre;Brca1(F/F); p53(F/F), or Abcg2(-/-);K14cre;Brca1(F/F); p53(F/F) animals, fed with genistein- or resveratrol-supplemented diets. Ovariectomized K14cre;Brca1(F/F); p53(F/F) animals were included to evaluate whether any estrogen-mimicking effects can restore mammary tumor development in the absence of endogenous estrogens. Compared with the ABC transporter proficient model, ABCG2-deficient animals showed a reduced median tumor latency of 17.5 days (P < 0.001), whereas no significant difference was observed for ABCB1-deficient animals. Neither genistein nor resveratrol altered this latency reduction in Abcg2(-/-);K14cre;Brca1(F/F); p53(F/F) animals. Ovariectomy resulted in nearly complete loss of mammary tumor development, which was not restored by genistein or resveratrol. Our results show that ABCG2 contributes to the protection of genetically instable epithelial cells against carcinogenesis. Diets containing high levels of genistein or resveratrol had no effect on mammary tumorigenesis, whether mice were lacking ABCG2 or not. Because genistein and resveratrol only delayed skin tumor development of ovariectomized animals, we conclude that these phytoestrogens are no effective modulators of mammary tumor development in our mouse model.

Cancer drug pan-resistance: pumps, cancer stem cells, quiescence, epithelial to mesenchymal transition, blocked cell death pathways, persisters or what?

Although chemotherapy of tumours has scored successes, drug resistance remains the major cause of death of cancer patients. Initial treatment often leaves residual disease, from which the tumour regrows. Eventually, most tumours become resistant to all available chemotherapy. I call this pan-resistance to distinguish it from multi-drug resistance, usually describing resistance caused by upregulation of drug transporters, such as P-glycoprotein. In this review, I discuss mechanisms proposed to explain both residual disease and pan-resistance. Although plausible explanations are at hand for residual disease, pan-resistance is still a mystery. My conclusion is that it is time for a major effort to solve this mystery using the new genetically modified mouse tumour models that produce real tumours resembling cancer in human patients.

Abstract LB-392: Loss of Rev7 causes PARP inhibitor resistance in BRCA1;p53-deficient mouse mammary tumor cells

We have previously shown that mammary tumors spontaneously arising in a genetically engineered mouse model for human BRCA1-associated breast cancer (K14Cre;Brca1F/F;p53F/F) are highly sensitive to the poly(ADP-ribose) polymerase (PARP) inhibitor olaparib due to their deficiency in homology-directed DNA repair (Rottenberg et al. PNAS 2008;105:17079-84). Despite the initial sensitivity, tumors are not eradicated and eventually develop drug resistance. In this project we aim to identify mechanisms that explain this resistance. In particular we are interested in alterations that result in restoration of DNA repair by homologous recombination (HR). For this purpose we performed a loss-of-function shRNA screen using Brca1; p53-deficient cell lines derived from a mouse mammary tumor. The CGH profile of these cell lines is highly similar to that of the original tumor. Moreover, these cells have successfully been transplanted orthotopically into syngeneic mice and they recapitulate the morphology and olaparib response of the donor tumor. In our screen we included 2000 hairpins from the Sigma Mission library which target 409 mouse genes involved in the DNA damage response. As a result we found enrichments for hairpins targeting Rev7 in olaparib-surviving colonies. This finding was validated with 3 individual hairpins which caused substantially decreased Rev7 mRNA levels. Moreover, Rev7-deficient Brca1−/−;p53−/− tumor cells are less sensitive to doxorubicin, carboplatin and cisplatin. Loss of Rev7 rescued mES cells from Brca1 deficiency and also correlated with the formation of Rad51 foci in both Brca1-deficient mES cells and Brca1−/−;p53−/− tumor cells after irradiation. We therefore think that homology-directed DNA repair is restored in Rev7-depleted Brca1−/−;p53−/− cells. We are currently confirming this finding using HR reporter assays and we are investigating whether the loss of Rev7 causes olaparib resistance in mouse tumors. Our data suggest that absence of REV7 function, like lack of 53BP1, contributes to PARP inhibitor resistance in vivo.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-392. doi:1538-7445.AM2012-LB-392

Impact of intertumoral heterogeneity on predicting chemotherapy response of BRCA1-deficient mammary tumors

The lack of markers to predict chemotherapy responses in patients poses a major handicap in cancer treatment. We searched for gene expression patterns that correlate with docetaxel or cisplatin response in a mouse model for breast cancer associated with BRCA1 deficiency. Array-based expression profiling did not identify a single marker gene predicting docetaxel response, despite an increase in Abcb1 (P-glycoprotein) expression that was sufficient to explain resistance in several poor responders. Intertumoral heterogeneity explained the inability to identify a predictive gene expression signature for docetaxel. To address this problem, we used a novel algorithm designed to detect differential gene expression in a subgroup of the poor responders that could identify tumors with increased Abcb1 transcript levels. In contrast, standard analytical tools, such as significance analysis of microarrays, detected a marker only if it correlated with response in a substantial fraction of tumors. For example, low expression of the Xist gene correlated with cisplatin hypersensitivity in most tumors, and it also predicted long recurrence-free survival of HER2-negative, stage III breast cancer patients treated with intensive platinum-based chemotherapy. Our findings may prove useful for selecting patients with high-risk breast cancer who could benefit from platinum-based therapy.

Drug resistance in the mouse cancer clinic

Drug resistance is one of the most pressing problems in treating cancer patients today. Local and regional disease can usually be adequately treated, but patients eventually die from distant metastases that have become resistant to all available chemotherapy. Although work on cultured tumor cell lines has yielded a lot of information on potential drug resistance mechanisms, it has proven difficult to translate these results to clinical drug resistance in patients. The controversy regarding the contribution of ABC transporters to drug resistance in patients is one example. The study of genetically engineered mouse models (GEMMs), which closely resemble cancer in human patients, can help to bridge this gap. In models for BRCA1- or BRCA2-associated breast cancer, we observed a substantial synergy between the defect in homology-directed DNA repair and sensitivity to DNA-targeting drugs. Nevertheless, tumors are not easily eradicated and eventually drug resistance develops. In this review we will discuss the use of the new generation mouse models to address major clinical problems, such as mechanisms of drug resistance, predicting chemotherapy response or characterizing the nature of residual tumor cells that escape eradication. Moreover, we will address the contribution of ABC transporters to drug resistance in our model.

ABCC6 does not transport vitamin K3-glutathione conjugate from the liver: relevance to pathomechanisms of pseudoxanthoma elasticum

Vitamin K is a cofactor required for gamma-glutamyl carboxylation of several proteins regulating blood clotting, bone formation and soft tissue mineralization. Vitamin K3 is an important intermediate during conversion of the dietary vitamin K1 to the most abundant vitamin K2 form. It has been suggested that ABCC6 may have a role in transporting vitamin K or its derivatives from the liver to the periphery. This activity is missing in pseudoxanthoma elasticum, a genetic disorder caused by mutations in ABCC6 characterized by abnormal soft tissue mineralization. Here we examined the efflux of the glutathione conjugate of vitamin K3 (VK3GS) from the liver in wild type and Abcc6(-/-) mice, and in transport assays in vitro. We found in liver perfusion experiments that VK3GS is secreted into the inferior vena cava, but we observed no significant difference between wild type and Abcc6(-/-) animals. We overexpressed the human ABCC6 transporter in Sf9 insect and MDCKII cells and assayed its vitamin K3-conjugate transport activity in vitro. We found no measurable transport of VK3GS by ABCC6, whereas ABCC1 transported this compound at high rate in these assays. These results show that VK3GS is not the essential metabolite transported by ABCC6 from the liver and preventing the symptoms of pseudoxanthoma elasticum.

Transportomics: screening for substrates of ABC transporters in body fluids using vesicular transport assays

The ATP-binding cassette (ABC) genes encode the largest family of transmembrane proteins. ABC transporters translocate a wide variety of substrates across membranes, but their physiological function is often incompletely understood. We describe a new method to study the substrate spectrum of ABC transporters: We incubate extracts of mouse urine with membrane vesicles prepared from Spodoptera frugiperda Sf9 insect cells overproducing an ABC transporter and determine the compounds transported into the vesicles by LC/MS-based metabolomics. We illustrate the power of this simple "transportomics" approach using ABCC2, a protein present at sites of uptake and elimination. We identified many new substrates of ABCC2 in urine. These included glucuronides of plant-derived xenobiotics, a class of compounds to which humans are exposed on a daily basis. Moreover, we show that the excretion of these compounds in vivo depends on ABCC2: compared to wild-type mice, the urinary excretion of several glucuronides was increased up to 20-fold in Abcc2(-/-) mice. Transportomics has broad applicability, as it is not restricted to urine and can be applied to other ATP-dependent transport proteins as well.

The structural basis for recognition of base J containing DNA by a novel DNA binding domain in JBP1

The J-binding protein 1 (JBP1) is essential for biosynthesis and maintenance of DNA base-J (β-d-glucosyl-hydroxymethyluracil). Base-J and JBP1 are confined to some pathogenic protozoa and are absent from higher eukaryotes, prokaryotes and viruses. We show that JBP1 recognizes J-containing DNA (J-DNA) through a 160-residue domain, DB-JBP1, with 10 000-fold preference over normal DNA. The crystal structure of DB-JBP1 revealed a helix-turn-helix variant fold, a 'helical bouquet' with a 'ribbon' helix encompassing the amino acids responsible for DNA binding. Mutation of a single residue (Asp525) in the ribbon helix abrogates specificity toward J-DNA. The same mutation renders JBP1 unable to rescue the targeted deletion of endogenous JBP1 genes in Leishmania and changes its distribution in the nucleus. Based on mutational analysis and hydrogen/deuterium-exchange mass-spectrometry data, a model of JBP1 bound to J-DNA was constructed and validated by small-angle X-ray scattering data. Our results open new possibilities for targeted prevention of J-DNA recognition as a therapeutic intervention for parasitic diseases.

Tumor-initiating cells are not enriched in cisplatin-surviving BRCA1;p53-deficient mammary tumor cells in vivo

Although many breast cancers respond to chemotherapy or hormonal therapy, lack of tumor eradication is a central clinical problem preceding the development of drug resistant tumors. Using the K14cre;Brca1(F5-13/F5-13);p53(F2-10/F2-10) mouse model for hereditary breast cancer, we have previously studied responses of mammary tumors to clinically relevant anti-cancer drugs, including cisplatin. The BRCA1- and p53-deficient tumors generated in this model are hypersensitive to cisplatin and never become resistant to this agent due to the large, irreversible deletion in Brca1. We show here that even dose-dense treatment with a maximum tolerated dose of cisplatin does not result in complete tumor eradication. To explain this result we have addressed the hypothesis that the lack of eradication of drug-sensitive tumors is due to increased in vivo chemotherapy resistance of tumor-initiating cells (TICs). Using the CD24 and CD49f cell surface markers which detect normal mouse mammary stem cells, we have identified tumor-initiating cells in BRCA1- and p53-deficient tumors. In addition to the Lin⁻/CD24(+)/CD49f(+) subpopulation, we show that a larger population of Lin⁻/CD24(+)/CD49f-cells also has tumor-initiating capability in at least two serial orthotopic transplantations, suggesting that these are not more differentiated transit-amplifying cells. However, we did not find an enrichment of TICs in cisplatin-treated tumor remnants. We conclude that in this model the tolerance of the cisplatin-surviving cells cannot be attributed to special biochemical defense mechanisms of TICs.

Abstract A14: Lack of tumor eradication of chemotherapy-sensitive BRCA1;p53-deficient mouse mammary tumors

Many breast cancers respond to chemotherapy or hormonal therapy, however, the lack of tumor eradication is a central clinical problem preceding the development of drug resistant tumors. We have previously studied responses of mammary tumors generated in the K14cre;Brca1F5-13/F5-13;p53F2-10/F2-10 mouse model for hereditary breast cancer to clinically relevant anti-cancer drugs such as doxorubicin, topotecan, cisplatin and the poly(ADP-ribose) polymerase (PARP) inhibitor olaparib (1-4). The BRCA1- and p53-deficient tumors generated in this model are hypersensitive to these drugs and never become resistant to cisplatin due to the large, irreversible deletion in Brca1 (5).

We show here that even dose-dense treatment with a maximum tolerable dose of cisplatin does not result in tumor eradication. This is not due to the trivial possibility that the drug-resistant remnants are poorly accessible to drug, since we found a homogeneous distribution of the platinum-DNA adducts throughout the tumor. To explain this result we have addressed the hypothesis that the lack of eradication of drug-sensitive tumors is due to increased in vivo chemotherapy resistance of tumor-initiating cells (TICs). Using the CD24 and CD49f cell surface markers which detect normal mouse mammary stem cells, we have identified TICs in BRCA1- and p53-deficient tumors. In addition to the Lin-/CD24+/CD49f+ subpopulation, we show that a larger population of Lin-/CD24+/CD49f-cells also has tumor-initiating capability in at least two serial orthotopic transplantations, suggesting that these are not more differentiated transit-amplifying cells. However, we did not find an enrichment of TICs in cisplatin-treated tumor remnants.

We conclude that in this model the resistance of cisplatin-surviving cells cannot be attributed to special biochemical defense mechanisms of TICs. Instead we will present data supporting the hypothesis that surviving TICs arrest in their cell cycle and ‘hibernate’ until the drug is gone. Using Brca1−/−;p53−/− cell lines derived from the mouse tumors we found that only Brca1−/−;p53−/− cells that were in the G0/G1 phase of the cell cycle 24h after cisplatin treatment were capable of forming new colonies, whereas cells in G2 or M were not. Hence, a p53-independent arrest in G0/G1 appears to underlie the lack of eradication of residual Brca1−/−;p53−/− cells.

Citation Information: Clin Cancer Res 2010;16(7 Suppl):A14

Abstract PL6-1: Resistance to targeted therapies in a realistic mouse breast cancer model

The contribution of ABC-transporters to resistance in patients has remained controversial. We have tackled this problem in a mammary mouse tumor model. These Brca1−/−, p53−/− tumors arise “spontaneously” and closely resemble human breast cancer in BRCA1+/− carriers (Rottenberg, PNAS 2007;104:12117–22; 2008;105:17079–84). The tumors respond to chemotherapeutic agents used in human breast cancer, but eventually end up being resistant to most drugs.

Resistance to doxorubicin in this tumor model is nearly always due to P-glycoprotein upregulation at the transcriptional level. A 5-fold increase in the low basal level of P-glycoprotein RNA in these tumors is sufficient for complete resistance. These levels of P-glycoprotein are not detectable with standard immunocytochemistry using the C219 Mab (Pajic, Cancer Res 2009;69:6396–404). Crossing in null alleles for Mdr1a/b made the tumors hypersensitive to doxorubicin, docetaxel, and to the PARP inhibitor olaparib. In about half of the mice treated with topotecan, resistance is associated with upregulation of Abcg2 (Bcrp). Tumors in Abcg2−/− mice take longer to become resistant (Zander, Cancer Res 2009 in press). Notwithstanding the complete remissions often obtained with some drugs, we are rarely able to eradicate the tumor. This is not due to the hypothetical unique properties of tumor stem cells. Supported by Dutch Cancer Foundation and EU Framework Program.

Citation Information: Clin Cancer Res 2010;16(7 Suppl):PL6-1

Sensitivity and acquired resistance of BRCA1;p53-deficient mouse mammary tumors to the topoisomerase I inhibitor topotecan

There is no tailored therapy yet for human basal-like mammary carcinomas. However, BRCA1 dysfunction is frequently present in these malignancies, compromising homology-directed DNA repair. This defect may serve as the tumor's Achilles heel and make the tumor hypersensitive to DNA breaks. We have evaluated this putative synthetic lethality in a genetically engineered mouse model for BRCA1-associated breast cancer, using the topoisomerase I (Top1) poison topotecan as monotherapy and in combination with poly(ADP-ribose) polymerase inhibition by olaparib. All 20 tumors tested were topotecan sensitive, but response heterogeneity was substantial. Although topotecan increased mouse survival, all tumors eventually acquired resistance. As mechanisms of in vivo resistance, we identified overexpression of Abcg2/Bcrp and markedly reduced protein levels of the drug target Top1 (without altered mRNA levels). Tumor-specific genetic ablation of Abcg2 significantly increased overall survival of topotecan-treated animals (P < 0.001), confirming the in vivo relevance of ABCG2 for topotecan resistance in a novel approach. Despite the lack of ABCG2, a putative tumor-initiating cell marker, none of the 11 Abcg2(-/-);Brca1(-/-);p53(-/-) tumors were eradicated, not even by the combination topotecan-olaparib. We find that olaparib substantially increases topotecan toxicity in this model, and we suggest that this might also happen in humans.

Targeted metabolomics identifies glucuronides of dietary phytoestrogens as a major class of MRP3 substrates in vivo

BACKGROUND & AIMS

The physiologic function of the efflux transporter Multidrug Resistance Protein 3 (MRP3) remains poorly defined. In vitro, MRP3 transports several glucuronidated compounds, but the compounds transported under physiologic conditions are unknown. Knowledge of the compounds transported by MRP3 in vivo would greatly contribute to the elucidation of the physiologic function of this transport protein.

METHODS

We used targeted metabolomics to identify substrates of MRP3 in vivo. Liquid chromatography coupled to mass spectrometry was used to specifically screen in plasma and urine of mice for compounds containing a glucuronic acid moiety.

RESULTS

We found that several highly abundant compounds containing a glucuronic acid moiety have a much lower abundance in plasma and urine of Mrp3((-/-)) than of wild-type mice. We identified these as phytoestrogen-glucuronides, and we show that MRP3 transports these compounds at high rates and with high affinity in vitro.

CONCLUSIONS

We have identified the efflux transporter MRP3 as a major factor in the disposition of phytoestrogens, a class of compounds to which mammals are exposed via food of plant origin. Our targeted metabolomics approach is not restricted to MRP3 but applicable to many other transport proteins for which knockout mouse models are available. Similar screens could be developed for sulpho- and glutathione-conjugates, further increasing the potential of identifying new physiologic transporter substrates.

Moderate increase in Mdr1a/1b expression causes in vivo resistance to doxorubicin in a mouse model for hereditary breast cancer

We have found previously that acquired doxorubicin resistance in a genetically engineered mouse model for BRCA1-related breast cancer was associated with increased expression of the mouse multidrug resistance (Mdr1) genes, which encode the drug efflux transporter ATP-binding cassette B1/P-glycoprotein (P-gp). Here, we show that even moderate increases of Mdr1 expression (as low as 5-fold) are sufficient to cause doxorubicin resistance. These moderately elevated tumor P-gp levels are below those found in some normal tissues, such as the gut. The resistant phenotype could be completely reversed by the third-generation P-gp inhibitor tariquidar, which provides a useful strategy to circumvent this type of acquired doxorubicin resistance. The presence of MDR1A in drug-resistant tumors with a moderate increase in Mdr1a transcripts could be shown with a newly generated chicken antibody against a mouse P-gp peptide. Our data show the usefulness of realistic preclinical models to characterize levels of Mdr1 gene expression that are sufficient to cause resistance.

Evidence that J-binding protein 2 is a thymidine hydroxylase catalyzing the first step in the biosynthesis of DNA base J

The genomic DNA of kinetoplastid parasites contains a unique modified base, beta-d-glucosyl-hydroxymethyluracil or base J. We recently reported that two proteins, called J-binding protein (JBP) 1 and 2, which regulate the levels of J in the genome, display features of the family of Fe(II)-2-oxoglutarate dependent dioxygenases and are likely to be the enzymes catalyzing the first step in J biosynthesis. In this study, we examine the effects of replacing the four conserved residues critical for the activity of this class of enzymes on the function of Leishmania tarentolae JBP2. The results show that each of these four residues is indispensable for the ability of JBP2 to stimulate J synthesis, while mutating non-conserved residues has no consequences. We conclude that JBP2, like JBP1, is in all probability a thymidine hydroxylase involved in the biosynthesis of base J.

Base J: discovery, biosynthesis, and possible functions

In 1993, a new base, beta-d-glucopyranosyloxymethyluracil (base J), was identified in the nuclear DNA of Trypanosoma brucei. Base J is the first hypermodified base found in eukaryotic DNA. It is present in all kinetoplastid flagellates analyzed and some unicellular flagellates closely related to trypanosomatids, but it has not been found in other protozoa or in metazoa. J is invariably present in the telomeric repeats of all organisms analyzed. Whereas in Leishmania nearly all J is telomeric, there are other repetitive DNA sequences containing J in T. brucei and T. cruzi, and most J is outside telomeres in Euglena. The biosynthesis of J occurs in two steps: First, a specific thymidine in DNA is converted into hydroxymethyldeoxyuridine (HOMedU), and then this HOMedU is glycosylated to form J. This review discusses the identification and localization of base J in the genome of kinetoplastids, the enzymes involved in J biosynthesis, possible biological functions of J, and J as a potential target for chemotherapy of diseases caused by kinetoplastids.

Contribution of the drug transporter ABCG2 (breast cancer resistance protein) to resistance against anticancer nucleosides

We have studied the potential contribution of ABCG2 (breast cancer resistance protein) to resistance to nucleoside analogues. In cells transfected with DNA constructs resulting in overexpression of human or mouse ABCG2, we found resistance against cladribine, clofarabine, fludarabine, 6-mercaptopurine, and 6-mercaptopurine riboside in both MDCKII and HEK293 cells and against gemcitabine only in HEK293 cells. With Transwell studies in MDCK cells and transport experiments with vesicles from Sf9 and HEK293 cells, we show that ABCG2 is able to transport not only the nucleotide CdAMP, like several other ATP-binding cassette transporters of the ABCC (multidrug resistance protein) family, but also the nucleoside cladribine itself. Expression of ABCG2 in cells results in a substantial decrease of intracellular CdATP, explaining the resistance against cladribine. The high transport rate of cladribine and clofarabine by ABCG2 deduced from Transwell experiments raises the possibility that this transporter could affect the disposition of nucleoside analogues in patients or cause resistance in tumors.

Does the absence of ABCC6 (multidrug resistance protein 6) in patients with Pseudoxanthoma elasticum prevent the liver from providing sufficient vitamin K to the periphery?

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disease characterized by a progressive mineralization of connective tissue, resulting in skin, arterial and eye disease. Classical PXE is caused by mutations in the ABCC6 gene, which encodes a member of the ABCC (MRP) family of organic anion transporters. Recent studies on Abcc6(-/-) mice show that the absence of ABCC6 in the liver is crucial for PXE and confirm the "metabolic disease hypothesis" for PXE, which states that tissue calcification is due to the absence of a plasma factor secreted from the basolateral hepatocyte membrane. We propose that this plasma factor is vitamin K (precursor). We propose that vitamin K (precursor) is secreted by ABCC6 from the liver as a glutathione--(or glucuronide)--conjugate and that this supplements the vitamin K need of peripheral tissues that receive insufficient vitamin from the diet, because dietary vitamin K is effectively extracted from blood by the liver. Peripheral tissue vitamin K is needed for the gamma-carboxylation of glutamate residues in proteins known to be required for counteracting calcification of connective tissue throughout the body. Our hypothesis explains the known facts of PXE and also explains why PXE-like symptoms can occur in patients with mutations in the gamma-glutamyl carboxylase gene (encoding the enzyme responsible for protein carboxylase) and in rats treated with vitamin K antagonists. The hypothesis implies that the symptoms of PXE can be prevented or mitigated by providing patients (intravenously) with a form of plasma vitamin K (precursor) that can be used by peripheral tissues.

How do real tumors become resistant to cisplatin?

Lab research on cultured tumor cells selected for resistance to platinum compounds has turned up a diverse array of resistance mechanisms. In contrast, we recently found that mouse mammary tumors containing irrepairable null alleles of the Brca1 gene do not become resistant to cisplatin ever, although they invariably become resistant to a variety of other anti-cancer drugs. Each new treatment with cisplatin shrinks the tumor to a very small remnant, but relapse always occurs. The BRCA1 missing in these mouse tumors is essential for the homology-directed DNA repair (HR) that allows error-free repair of the duplex breaks caused by the excision of platin-DNA adducts. The mouse tumor results therefore raise the question whether the cisplatin resistance mechanisms identified in vitro can actually overcome an irreversible defect in DNA repair in real tumors. This question is underlined by recent analyses of tumor samples of patients with ovarian cancer that have uncovered a new platin resistance mechanism: these tumors were initially sensitive to platin through a defect in the BRCA2 gene, also required for HR, like BRCA1. Resistance in these patients,-after an initial response of the tumor,-was due to secondary mutations in the defective BRCA2 gene, restoring BRCA2 function.(1,2) These clinical observations show the overriding importance of a functional HR system for tumor cells to survive platin-induced DNA lesions. Taken together with the mouse mammary tumor data, these observations raise the possibility that proliferating cells have no readily available mechanism to escape from cisplatin DNA damage once their HR is irreversibly inactivated.