BRCA2 and genetic instability

VP Yu, MA, MB/BChir, PhD

Hutchison/MRC Cancer Research Centre, Cambridge, Cambridgeshire United Kingdom

AIM: To investigate the role of the breast cancer susceptibility gene, BRCA2, in suppressing genetic instability. METHODS Using a knockout mouse model, which mimics the human truncating mutation, cells derived were subjected to cytogenetic analyses (including spectral karyotyping) and various in vitro DNA repair assays. RESULTS We demonstrated that BRCA2 deficient cells exhibited a profound proliferative defect and accumulated genomic DNA strand breakage during division. Furthermore, abnormal structures or gross chromosomal rearrangements (GCRs) including random translocations, acentric fragments and large deletions, were frequently observed in these cells, suggesting a defect in repairing replicative damage. We proceeded to show that in BRCA2 deficient cells, one of the high-fidelity pathways for repairing double-strand DNA breaks, homologous recombination; was hampered. As a result, when these cells were treated with the DNA cross-linking agent, mitomycin C, aberrant genetic exchange between non-homologous chromosomes took place as an attempt to bypass the defect in homologous recombination, resulting in genetic instability. CONCLUSIONS Cancer-causing mutations frequently arise from gross chromosomal rearrangements (GCRs) such as translocations, which involve genetic exchange between non-homologous chromosomes. In this study, we show that murine BRCA2 has an essential function in suppressing GCR formation after chromosome breakage. Our results indicate that inactivation of BRCA2 greatly enhances the frequency of GCRs that may cause loss of heterozygosity at functionally important genetic loci. We propose a model to explain how such genetic instability may trigger the additional genetic changes necessary for neoplastic transformation in individuals who carry BRCA2 germ-line mutations.

KEY WORDS: Breast cancer susceptibility gene, tumor suppressor, translocations, DNA repair, chromosome structure.

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Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Risk Assessment, Part 1.

This presentation received an honorable mention in our poster contest and was recognized with the Symposium Presidents' Award for Scientific Excellence.