Opening a Door to PARP Inhibitor-Induced Lethality in HR-Proficient Human Tumor Cells
Authors: Elodie Hatchi, David M. Livingston
Journal: Cancer Cell (2020)

PARP inhibition (PARPi) kills tumor cells defective in homologous recombination-based repair (HR−) but not their HR+ competent counterparts. In this issue of Cancer Cell, it is shown that, when EZH2 is functionally silenced, HR+, CARM1-high, high-grade serous ovarian cancer cells become PARPi sensitive, undergo mitotic catastrophe, and die. PARP inhibition (PARPi) kills tumor cells defective in homologous recombination-based repair (HR−) but not their HR+ competent counterparts. In this issue of Cancer Cell, it is shown that, when EZH2 is functionally silenced, HR+, CARM1-high, high-grade serous ovarian cancer cells become PARPi sensitive, undergo mitotic catastrophe, and die. Mammalian cells repair double-strand DNA breaks (DSBs) by multiple modalities, the most prominent being homologous recombination (HR) and non-homologous end-joining (NHEJ). The former is an error-free process; the latter is not. In the practice of targeted cancer therapeutics, a synthetic lethal relationship exists in certain HR-incompetent cells that renders them PARP inhibitor-sensitive (Bryant et al., 2005Bryant H.E. Schultz N. Thomas H.D. Parker K.M. Flower D. Lopez E. Kyle S. Meuth M. Curtin N.J. Helleday T. Specific killing of BRCA2-deficient tumours with inhibitors of poly(ADP-ribose) polymerase.Nature. 2005; 434: 913-917Crossref PubMed Scopus (3232) Google Scholar, Farmer et al., 2005Farmer H. McCabe N. Lord C.J. Tutt A.N.J. Johnson D.A. Richardson T.B. Santarosa M. Dillon K.J. Hickson I. Knights C. et al.Targeting the DNA repair defect in BRCA mutant cells as a therapeutic strategy.Nature. 2005; 434: 917-921Crossref PubMed Scopus (4153) Google Scholar). By contrast, in HR-competent (HR+) cells, PARPi (PARP inhibition) is therapeutically ineffective, thereby ruling out the application of PARPi-driven synthetic lethality in that setting. Because of the potential clinical effectiveness of PARPi leading to synthetic lethality in tumor cells, a desirable goal in HR+ and PARPi-resistant tumor cells is to be able to render them PARPi-sensitive. In this issue of Cancer Cell, Karakashev and colleagues report a novel molecular route through which PARPi-driven lethality can be induced in HR+ tumor cells. Its pursuit has, for the first time, resulted in PARPi-induced lethality of formerly HR+ human tumor cells (Karakashev et al., 2020Karakashev S. Zhao B. Lin J. Wu S. Fatkhutdinov N. Park P.-H. Fukumoto T. Kossenkov A. Zhang R. EZH2 inhibition sensitizes CARM1-high, homologous recombination proficient ovarian cancers to PARP inhibition.Cancer Cell. 2020; 37 (this issue): 157-167Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar). This suggests that, like their HR-defective counterparts, certain HR+ human cancers such as BRCA1 +/+ high-grade serous ovarian cancer (HGSOC) can be rendered candidates for such a therapeutic approach. A reproducible ability to generate synthetic lethality in HR+ cells following PARPi exposure might create a major new treatment opportunity. Underlying this recent development are new mechanistic insights that depend upon detailed aspects of how DSB repair decisions are made. More specifically, new observations that make it possible for one to "force" HR-competent cells to adopt a preference for NHEJ that is accompanied by significantly increased sensitivity to PARPi constitute the generic basis of the Karakashev et al. work (Karakashev et al., 2020Karakashev S. Zhao B. Lin J. Wu S. Fatkhutdinov N. Park P.-H. Fukumoto T. Kossenkov A. Zhang R. EZH2 inhibition sensitizes CARM1-high, homologous recombination proficient ovarian cancers to PARP inhibition.Cancer Cell. 2020; 37 (this issue): 157-167Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar). Mammalian tumor cells are commonly confronted by spontaneously arising DSBs, and current clinical practice offers an opportunity to generate a specific therapeutic response in certain cancers that are innately HR-defective. Administration of a PARPi agent may result in abundant tumor cell killing, the potential for a life-prolonging clinical response, and limited drug toxicity. However, until now, it has not been feasible to invoke such a rational strategy when the relevant tumor cells are HR-competent. Therefore, a key question is whether one can convert these particular cancer cells to a PARP-sensitive phenotype and, if so, does this represent a step toward the induction of clinically relevant synthetic lethality? Others have shown that the HR-NHEJ balance in mammalian cells can be biochemically perturbed by altering their ability to perform 5′-3′ end resection at a DSB that is required to initiate RAD51-dependent, strand-invasion-dependent HR (Ingram et al., 2019Ingram S.P. Warmenhoven J.W. Henthorn N.T. Smith E.A.K. Chadwick A.L. Burnet N.G. Mackay R.I. Kirkby N.F. Kirkby K.J. Merchant M.J. Mechanistic modelling supports entwined rather than exclusively competitive DNA double-strand break repair pathway.Sci. Rep. 2019; 9: 6359Crossref PubMed Scopus (16) Google Scholar, Li and Xu, 2016Li J. Xu X. DNA double-strand break repair: a tale of pathway choices.Acta Biochim. Biophys. Sin. (Shanghai). 2016; 48: 641-646Crossref PubMed Scopus (27) Google Scholar). A naturally occurring multi-subunit complex—shieldin (Noordermeer et al., 2018Noordermeer S.M. Adam S. Setiaputra D. Barazas M. Pettitt S.J. Ling A.K. Olivieri M. Álvarez-Quilón A. Moatti N. Zimmermann M. et al.The shieldin complex mediates 53BP1-dependent DNA repair.Nature. 2018; 560: 117-121Crossref PubMed Scopus (211) Google Scholar) (Setiaputra and Durocher, 2019Setiaputra D. Durocher D. Shieldin - the protector of DNA ends.EMBO Rep. 2019; 20: e47560Crossref PubMed Scopus (93) Google Scholar)(Mirman et al., 2018Mirman Z. Lottersberger F. Takai H. Kibe T. Gong Y. Takai K. Bianchi A. Zimmermann M. Durocher D. de Lange T. 53BP1-RIF1-shieldin counteracts DSB resection through CST- and Polα-dependent fill-in.Nature. 2018; 560: 112-116Crossref PubMed Scopus (158) Google Scholar)—protects DNA broken ends from 5′-3′ end resection at DSBs, thereby interfering with HR and enabling NHEJ, a 53BP1-dependent DSB repair mechanism. With HR blocked at such a DSB, NHEJ, which is largely resection-independent, can proceed. Therefore, when sufficiently abundant, shieldin (Setiaputra and Durocher, 2019Setiaputra D. Durocher D. Shieldin - the protector of DNA ends.EMBO Rep. 2019; 20: e47560Crossref PubMed Scopus (93) Google Scholar) or one of its subunits, e.g., REV7 (aka MAD2L2) (Boersma et al., 2015Boersma V. Moatti N. Segura-Bayona S. Peuscher M.H. van der Torre J. Wevers B.A. Orthwein A. Durocher D. Jacobs J.J.L. MAD2L2 controls DNA repair at telomeres and DNA breaks by inhibiting 5′ end resection.Nature. 2015; 521: 537-540Crossref PubMed Scopus (170) Google Scholar, Xu et al., 2015Xu G. Chapman J.R. Brandsma I. Yuan J. Mistrik M. Bouwman P. Bartkova J. Gogola E. Warmerdam D. Barazas M. et al.REV7 counteracts DNA double-strand break resection and affects PARP inhibition.Nature. 2015; 521: 541-544Crossref PubMed Scopus (332) Google Scholar), inhibits resection and promotes NHEJ-type DSB repair and certain error-prone outcomes with which it is associated. In those cancer cells where the HR process is defective, e.g., as a consequence of an inherited or acquired mutation in a gene required for HR, such as BRCA1, BRCA2, PALB2, etc., exposure to PARPi is predicted to be synthetically lethal. Better still, this principle is now the basis for an established and effective form of cancer therapy. While this form of treatment represents a major advance in the field, a sizeable fraction of cancers—even of breast and ovarian origin—are HR+ and, therefore, PARPi resistant. Clearly, overcoming this hurdle would add a new opportunity for employing PARPi therapy in certain common and aggressive HR+ cancers that currently remain difficult to treat by a targeted approach such as a synthetically lethal one. In a search for a solution to this problem in HGSOC, Karakashev et al. have made progress. First, they focused on the existing knowledge that NHEJ can be activated by shieldin and that, when resection is blocked, HR amplitude may fall (Karakashev et al., 2020Karakashev S. Zhao B. Lin J. Wu S. Fatkhutdinov N. Park P.-H. Fukumoto T. Kossenkov A. Zhang R. EZH2 inhibition sensitizes CARM1-high, homologous recombination proficient ovarian cancers to PARP inhibition.Cancer Cell. 2020; 37 (this issue): 157-167Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar). To promote suitably high shieldin expression, they chose a convenient, drug-based trigger and found that in HGSOC cells with relatively high expression of the histone methyltransferase, CARM1, small-molecule-based EZH2 inhibition triggered a marked increase in MAD2L2/REV7 transcription. In return, they then observed that patient-derived HGSOC cell xenografts that had been PARPi-resistant now manifested PARPi sensitivity accompanied by mitotic catastrophe and tumor cell death. These results strongly imply that the above-noted pathway of events driven by combined EZH2i and PARPi in CARM1-high, HR+, PDX HGSOC cells results in synthetic tumor cell lethality. The underlying mechanism involves an EZH2i-promoted HR to NHEJ switch. In support of this conclusion, Karakashev et al. showed that KU70 depletion leading to NHEJ inhibition reduced PARPi sensitivity and the associated cell death. Furthermore, in keeping with the claim that shieldin action contributes to EZH2-inhibition-driven PARPi hypersensitivity, MAD2L2 depletion significantly decreased NHEJ activity induced by an EZH2-inhibiting compound and, importantly, blunted PARPi sensitivity. The synthetic lethal dependence in HGSOC cells upon high CARM1 expression is also interesting. In the A1847 HGSOC cell line, colony-forming sensitivity to PARPi alone was not linked to its CARM1 status. Indeed, the Olaparib IC50 in A1847 was comparable to that of an isogenic CARM1 knockout line. However, when a PARPi agent was combined with an EZH2i drug, this combination elicited greater tumor cell growth inhibition in CARM1-high cells than did PARPi alone. It also inhibited CARM1-high cells better than CARM1-low cells. Moreover, when combined, the EZH2i and PARPi agents triggered greater chromosomal disorder and mitotic catastrophe in CARM1-high tumor cells than they did individually. Since mitotic catastrophe spells tumor cell death, a CARM1-high phenotype is a necessary precondition to synthetic lethality in this setting. If these findings also apply to CARM1-high human HGSOC cells freshly isolated from patients, their clinical applicability to BRCA1-competent HGSOC therapy will gain justification, and the pathway to clinical trials of this novel therapeutic approach will, ideally, be shortened. One also wonders whether, with the assistance of EZH2 inhibition, any tumor types other than HR+ HGSOC can be forced to cede their ability to perform HR repair in favor of NHEJ. And, if so, will the outcome be PARPi sensitivity? Similarly, will these putative PARPi-sensitive candidate cancers also share a CARM1-high phenotype, as was the case in HGSOC? Finally, using pooled RNA CRISPR screening, can new combinations of gene perturbation events be discovered that culminate in novel forms of PARPi-driven cancer cell synthetic lethality? If so, increasing opportunities for eliciting PARPi-based and yet other forms of synthetic-lethality-based cancer therapy may emerge. Elodie Hatchi declares no competing interests. David Livingston is a consultant to Constellation Pharma and the Novartis Institute of Biomedical Research. He is also a member of the External Advisory Boards of the MIT Cancer Center and the Sidney Kimmel Cancer Center at the Johns Hopkins Medical Center and advises Cancer Research UK and the Pezcoller Foundation (Trento, IT). EZH2 Inhibition Sensitizes CARM1-High, Homologous Recombination Proficient Ovarian Cancers to PARP InhibitionKarakashev et al.Cancer CellJanuary 30, 2020In BriefKarakashev et al. show that CARM1 promotes EZH2-mediated epigenetic silencing of the shieldin complex protein MAD2L2. Inhibition of EZH2 induces MAD2L2 expression and non-homologous end-joining in CARM1-high, homologous recombination proficient ovarian carcinoma cells, sensitizing them to PARP inhibitors. Full-Text PDF Open Archive