Resistance to therapy caused by intragenic deletion in BRCA2

Abstract

Cells with loss of BRCA2 function are defective in homologous recombination (HR) and are highly sensitive to inhibitors of poly(ADP-ribose) polymerase (PARP), which provides the basis for a new therapeutic approach. Here we show that resistance to PARP inhibition can be acquired by deletion of a mutation in BRCA2. We derived PARP-inhibitor-resistant (PIR) clones from the human CAPAN1 pancreatic cancer cell line, which carries the protein-truncating c.6174delT frameshift mutation. PIR clones could form DNA-damage-induced RAD51 nuclear foci and were able to limit genotoxin-induced genomic instability, both hallmarks of a competent HR pathway. New BRCA2 isoforms were expressed in the resistant lines as a result of intragenic deletion of the c.6174delT mutation and restoration of the open reading frame (ORF). Reconstitution of BRCA2-deficient cells with these revertant BRCA2 alleles rescued PARP inhibitor sensitivity and HR deficiency. Most of the deletions in BRCA2 were associated with small tracts of homology, and possibly arose from error-prone repair caused by BRCA2 deficiency. Similar ORF-restoring mutations were present in carboplatin-resistant ovarian tumours from c.6174delT mutation carriers. These observations have implications for understanding drug resistance in BRCA mutation carriers as well as in defining functionally important domains within BRCA2.

Mild hyperthermia inhibits homologous recombination, induces BRCA2 degradation, and sensitizes cancer cells to poly (ADP-ribose) polymerase-1 inhibition

“…Our results might have direct clinical relevance, because recent trials demonstrated that both HSP90 and PARP-1 inhibitors are generally well tolerated (35,36). Moreover, because our approach is not based on genetic deficiency, as is the case in hereditary BRCA2-deficient tumors, it is not likely that a previously described mechanism of resistance to the inhibitor will be relevant (37,38). Our results could provide a rational basis for development of therapies for a wide range of tumors, exploiting local, on-demand induction of HR deficiency in combination with PARP-1 inhibitors, ionizing radiation, or other DSB-inducing chemotherapeutic agents.…”

BRCA1 and BRCA2 mutations correlate with TP53 abnormalities and presence of immune cell infiltrates in ovarian high-grade serous carcinoma

“…These may be secondary to contamination by normal cells as described above, or reveal a true difference between germline and tumor DNA suggesting the tumor has undergone a secondary mutation with restoration of the open reading frame and restoration of a functional BRCA1 gene. [62][63][64] We have demonstrated that BRCA abnormalities are only associated with the high-grade serous subtype of ovarian carcinoma. TP53 abnormalities (as detected by immunohistochemistry) and immune cell infiltrates were associated with BRCA mutations; there is at present no data to indicate whether BRCA mutation, TP53 abnormalities, and host immune cell infiltrates, all of which are favorable prognostic factors in univariate analysis in large case series, are of independent prognostic significance.…”

An Emerging Entity: Pancreatic Adenocarcinoma Associated with a Known BRCA Mutation: Clinical Descriptors, Treatment Implications, and Future Directions

“…Notably, all patients treated with a PARP inhibitor experienced progression of disease after several months of therapy, despite an initial clinical and radiographic response. The mechanism of acquired resistance in vivo to PARP inhibition is unknown, but it may potentially occur as a result of a secondary mutation resulting in restored competency of homologous repair in BRCA-mutant cells [13].…”