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The Yin and Yang of Treating BRCA-Deficient Tumors

2008; Cell Press; Volume: 132; Issue: 6 Linguagem: Inglês

10.1016/j.cell.2008.03.006

1097-4172

Richard W. Martin, Philip P. Connell, Douglas K. Bishop,

BRCA gene mutations in cancer

The myriad changes that occur during the malignant progression of cancer cells present challenges to both clinicians and basic scientists. Two new studies in Nature underscore the central role of genome instability in tumor biology (Edwards et al., 2008Edwards S.L. Brough R. Lord C.J. Natrajan R. Vatcheva R. Levine D.A. Boyd J. Reis-Filho J.S. Ashworth A. Nature. 2008; 451: 1111-1115Crossref PubMed Scopus (703) Google Scholar, Sakai et al., 2008Sakai W. Swisher E.M. Karlan B.Y. Agarwal M.K. Higgins J. Friedman C. Villegas E. Jacquemont C. Farrugia D.J. Couch F.J. et al.Nature. 2008; 451: 1116-1120Crossref PubMed Scopus (713) Google Scholar). These reports describe secondary changes in the BRCA2 locus that restore the wild-type reading frame and contribute to the development of resistance to chemotherapeutic agents. The myriad changes that occur during the malignant progression of cancer cells present challenges to both clinicians and basic scientists. Two new studies in Nature underscore the central role of genome instability in tumor biology (Edwards et al., 2008Edwards S.L. Brough R. Lord C.J. Natrajan R. Vatcheva R. Levine D.A. Boyd J. Reis-Filho J.S. Ashworth A. Nature. 2008; 451: 1111-1115Crossref PubMed Scopus (703) Google Scholar, Sakai et al., 2008Sakai W. Swisher E.M. Karlan B.Y. Agarwal M.K. Higgins J. Friedman C. Villegas E. Jacquemont C. Farrugia D.J. Couch F.J. et al.Nature. 2008; 451: 1116-1120Crossref PubMed Scopus (713) Google Scholar). These reports describe secondary changes in the BRCA2 locus that restore the wild-type reading frame and contribute to the development of resistance to chemotherapeutic agents. Tumorigenesis involves numerous genetic alterations, some of which are selected for if they promote tumor growth, survival, metastasis, or resistance to treatment (Merlo et al., 2006Merlo L.M. Pepper J.W. Reid B.J. Maley C.C. Nat. Rev. Cancer. 2006; 6: 924-935Crossref PubMed Scopus (1079) Google Scholar, Vogelstein and Kinzler, 2004Vogelstein B. Kinzler K.W. Nat. Med. 2004; 10: 789-799Crossref PubMed Scopus (3129) Google Scholar). The multi-hit model for tumor progression has particular relevance to tumor suppressors, such as the BRCA genes. Women with BRCA mutations are highly susceptible to developing breast or ovarian cancers. BRCA protein is thought to suppress malignancies by promoting homologous recombination-dependent repair of DNA. Loss of BRCA function is thought to lead to malignancy as a consequence of subsequent mutations caused by alternative repair pathways compensating for the primary repair defect. Two groups recently reported that secondary mutations at the BRCA2 locus, which restore the wild-type reading frame, in BRCA-deficient tumors may contribute to the development of resistance to chemotherapy (Edwards et al., 2008Edwards S.L. Brough R. Lord C.J. Natrajan R. Vatcheva R. Levine D.A. Boyd J. Reis-Filho J.S. Ashworth A. Nature. 2008; 451: 1111-1115Crossref PubMed Scopus (703) Google Scholar, Sakai et al., 2008Sakai W. Swisher E.M. Karlan B.Y. Agarwal M.K. Higgins J. Friedman C. Villegas E. Jacquemont C. Farrugia D.J. Couch F.J. et al.Nature. 2008; 451: 1116-1120Crossref PubMed Scopus (713) Google Scholar). Homologous recombination is required for repair of the frequent spontaneous lesions that arise during DNA replication. As a consequence, homologous recombination is also essential for cell viability and proliferation. Given that a reduction in homologous recombination is disadvantageous to malignant progression, BRCA-deficient backgrounds select for mutations that promote the growth and survival of tumors. BRCA-mediated homologous recombination is also important for the response of cells to DNA-damaging agents that cause double-strand breaks or replication forks to stall or collapse. Therefore, BRCA-defective tumors can be hypersensitive to cancer therapies that function by inducing recombinogenic lesions (Foulkes, 2006Foulkes W.D. Fam. Cancer. 2006; 5: 135-142Crossref PubMed Scopus (110) Google Scholar). Correspondingly, cells impaired in homologous recombination are hypersensitive to DNA-crosslinking drugs such as mitomycin C and cisplatin (Bhattacharyya et al., 2000Bhattacharyya A. Ear U.S. Koller B.H. Weichselbaum R.R. Bishop D.K. J. Biol. Chem. 2000; 275: 23899-23903Crossref PubMed Scopus (482) Google Scholar); these cells are also hypersensitive to agents that inhibit poly(ADP-ribose) polymerase (PARP), because inhibition of PARP leads to collapsed DNA replication forks (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. Nature. 2005; 434: 913-917Crossref PubMed Scopus (3154) Google Scholar, Farmer et al., 2005Farmer H. McCabe N. Lord C.J. Tutt A.N. Johnson D.A. Richardson T.B. Santarosa M. Dillon K.J. Hickson I. Knights C. et al.Nature. 2005; 434: 917-921Crossref PubMed Scopus (4057) Google Scholar). Secondary genetic or epigenetic changes that affect homologous recombination or cellular responses to DNA damage (such as apoptosis or cell-cycle checkpoints) can modify BRCA phenotypes (Martin et al., 2007Martin R.W. Orelli B.J. Yamazoe M. Minn A.J. Takeda S. Bishop D.K. Cancer Res. 2007; 67: 9658-9665Crossref PubMed Scopus (119) Google Scholar and references therein). In the recent studies from the labs of Ashworth and Taniguchi, the genetic basis of acquired resistance to chemotherapy was investigated using the BRCA2 mutant human pancreatic cancer cell line CAPAN1 (Edwards et al., 2008Edwards S.L. Brough R. Lord C.J. Natrajan R. Vatcheva R. Levine D.A. Boyd J. Reis-Filho J.S. Ashworth A. Nature. 2008; 451: 1111-1115Crossref PubMed Scopus (703) Google Scholar, Sakai et al., 2008Sakai W. Swisher E.M. Karlan B.Y. Agarwal M.K. Higgins J. Friedman C. Villegas E. Jacquemont C. Farrugia D.J. Couch F.J. et al.Nature. 2008; 451: 1116-1120Crossref PubMed Scopus (713) Google Scholar). CAPAN1 cells harbor two copies of a frameshift allele of BRCA2 that expresses an N-terminal protein fragment. In both studies, drug-resistant cells were selected using a BRCA-specific agent (cisplatin or PARP inhibitor). This selection yields cells that acquire drug resistance from pseudo-reversion of BRCA2 via secondary frameshift mutations. All the revertant BRCA2 alleles express between two and six BRC repeats, a nuclear localization sequence, and the TR2 domain, which is thought to regulate RAD51 loading onto single-stranded DNA (ssDNA). Interestingly, all revertants isolated using cisplatin selection (Sakai et al., 2008Sakai W. Swisher E.M. Karlan B.Y. Agarwal M.K. Higgins J. Friedman C. Villegas E. Jacquemont C. Farrugia D.J. Couch F.J. et al.Nature. 2008; 451: 1116-1120Crossref PubMed Scopus (713) Google Scholar) contain the ssDNA-binding domain of BRCA2, whereas most revertants isolated with PARP inhibitors (Edwards et al., 2008Edwards S.L. Brough R. Lord C.J. Natrajan R. Vatcheva R. Levine D.A. Boyd J. Reis-Filho J.S. Ashworth A. Nature. 2008; 451: 1111-1115Crossref PubMed Scopus (703) Google Scholar) lack much or all this region. Previous observations suggested that the OB folds of BRCA2, which bind ssDNA, are required for its repair function (Saeki et al., 2006Saeki H. Siaud N. Christ N. Wiegant W.W. van Buul P.P. Han M. Zdzienicka M.Z. Stark J.M. Jasin M. Proc. Natl. Acad. Sci. USA. 2006; 103: 8768-8773Crossref PubMed Scopus (78) Google Scholar). However, the isolation of functional revertants lacking this domain indicates that either the OB folds are dispensable for DNA binding or that direct BRCA2-DNA interaction is not required for function. Another intriguing observation concerns the fraction of selected lines carrying BRCA2 revertants. In the case of the PARP inhibitor, all selected lines contain BRCA2 reversions. In contrast, only half (6/12) of the cisplatin-selected lines are BRCA2 revertants; the mechanisms of cisplatin resistance for the remaining lines are unknown. Importantly, the amount of homologous recombination activity of the cisplatin-resistant lines depends on the status of BRCA2. Taken together, these observations suggest that PARP inhibitors exert a more specific selective pressure for the restoration of homologous recombination activity than does cisplatin. Underscoring the physiological relevance of genomic instability in BRCA2-deficient cells, Edwards et al. detect short regions of sequence homology that flank the deletion sites in revertant clones. This finding indicates that micro-homology-dependent end-joining compensates for the deficiency in homologous recombination in CAPAN1 cells. These secondary mutations are examples of the genetic instability caused by compensatory error-prone repair mechanisms. Both papers also demonstrate that revertants of BRCA2 can occur clinically following treatment of ovarian cancer with platinum-based therapy. Sakai et al. describe a recurrent ovarian tumor containing an intragenic deletion that restores expression of functional BRCA2 via activation of cryptic splice sites. Furthermore, both groups describe BRCA2 revertants in cancers refractory to treatment from patients that carried frameshift BRCA2 mutations similar to those present in CAPAN1 cells. These studies demonstrate a compelling model for the processes by which resistance can be acquired. It will be interesting to determine whether this mechanism of acquired resistance can be generalized to other BRCA genotypes, as well as other cancer susceptibility genes. As the authors point out, the frameshift mutation of BRCA2 may be particularly prone to reversion. The results also suggest a potential for choosing clinical treatments that are tailored to particular types of BRCA mutations. The studies by Sakai and Edwards emphasize the opposing consequences of treating repair-deficient cancers with agents that induce DNA damage. Although treatment kills a large fraction of tumor cells, it also selects for cells with restored repair functions and resistance to therapy. The development of revertant clones likely occurs via mechanisms that compensate for essential house-keeping functions of the deficient repair pathways. Some of the mutagenesis may also be caused by the therapies themselves. This phenomenon, however, does not necessarily require mutagenesis or selection by a DNA-damaging therapy. For example, overexpression of RAD51 is frequently observed in BRCA mutant cells (Martin et al., 2007Martin R.W. Orelli B.J. Yamazoe M. Minn A.J. Takeda S. Bishop D.K. Cancer Res. 2007; 67: 9658-9665Crossref PubMed Scopus (119) Google Scholar). Although RAD51 overexpression can partially compensate for the deficiency in homologous recombination, it might also promote a deregulated mutagenic version of homologous recombination that drives further malignant progression. It is interesting to consider that a cell with a profound repair defect, such as a BRCA−/−cell that is newly derived from a BRCA+/− heterozygote, might represent an intermediate cell type en route to a malignant cancer cell with partially or completely restored capacity for repair and relatively less genome instability (Figure 1).