One of the most serious problems in the treatment of advanced stage ovarian cancer is acquired resistance to a key anti-cancer DNA-damaging agent, cisplatin. We have demonstrated that the disruption of the Fanconi anemia (FA)-BRCA pathway causes cisplatin sensitivity in cancer cells. The FA-BRCA pathway is a DNA damage-activated signaling pathway that regulates DNA repair. One of the components of the pathway, BRCA2 (identical to an FA gene, FANCD1), is a gene that is responsible for some familial breast and ovarian cancers, but BRCA2 is also implicated in sporadic (non-familial) ovarian cancer. An immunohistochemical study (Swisher EM, et al. AACR abstract, 2005) showed that a large subset (45%) of sporadic ovarian cancer cases lack BRCA2 protein expression, however, the mechanism of loss is unknown. Ovarian cancer cell lines lacking BRCA2 protein expression can be useful tools to study the mechanism of lack of BRCA2 protein expression in sporadic ovarian cancer. Therefore, we have screened 24 ovarian cancer cell lines by BRCA2 immunoblotting and found one cisplatin-sensitive ovarian cancer cell line in which the BRCA2 protein is undetectable. This cell line, PE01, is derived from ascitic fluid of a patient with ovarian adenocarcinoma which was clinically responsive to cisplatin. PE01 had no mutation in the coding region of BRCA2 gene. BRCA2 mRNA was detectable by RT-PCR and the promoter of BRCA2 was not methylated in PE01. Consistent with the BRCA2 deficiency, DNA damage-inducible foci formation of RAD51 is impaired in PE01. Interestingly, after selection in the presence of cisplatin in vitro for several weeks, the cell line restored BRCA2 protein expression with minimal change of BRCA2 mRNA level, suggesting an involvement of posttranscriptional regulation in BRCA2 expression and a role of BRCA2 in cisplatin resistance. Furtheremore, BRCA2 protein expression and RAD51 foci formation were intact in a cisplatin-resistant ovarian cancer line, PE04, derived from ascites of the same patient at the time of relapse with acquired cisplatin resistance, again suggesting a role of BRCA2 reactivation in cisplatin resistance. Since treatment with proteasome inhibitors did not cause restoration of BRCA2 protein expression in PE01, involvement of increased proteasome-mediated degradation was unlikely. As polysome/monosome distributions of BRCA2 mRNA were similar among PE01, PE04, and cisplatin-surviving PE01 cells, translational regulation was unlikely to be involved in the reactivation of BRCA2. In summary, we found a BRCA2-deficeint ovarian cancer cell line and after selection in the presence of cisplatin, BRCA2 was restored in this line. These cell lines will be useful tools to investigate the mechanism of lack of BRCA2 protein expression in sporadic ovarian cancer cases.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]