Increased rates of spontaneous sister chromatid exchange in lymphocytes of BRCA2^+/− carriers of familial breast cancer clusters

Abstract 

Heterozygous carriers of germ-line mutations of the BRCA2 breast cancer susceptibility gene are predisposed to breast, ovarian, pancreatic and other cancers. The BRCA2 protein is implicated in the maintenance of chromosome stability through its essential function in double-strand DNA repair and recombination. Our previous studies had revealed multiple intrachromosomal rearrangements, duplications, inversions and deletions on 9p23-24 in lymphocytes and fibroblasts of BRCA2^+/− members from independently ascertained familial breast cancer clusters. In pursuit of evaluating if there is a subtle genomic instability in BRCA2^+/− individuals, we have determined frequencies of spontaneous sister chromatid exchanges (SCEs) in BRCA2 wild-types and BRCA2 mutation carriers of two familial breast cancer clusters. Here, we demonstrate an average increase of 65% of spontaneous SCEs in BRCA2^+/− versus BRCA2^+/+ family members. In one cluster, the number of metaphases with multiple SCEs was 5-times higher in BRCA2^+/− compared to wild-type members, while in the second cluster BRCA2^+/− members had 8.9% of metaphases with multiple SCEs compared to a level below detection in BRCA2 wild types. To investigate the correlation between SCE and genomic instability in 9p, we performed fluorescence detection of SCEs and FISH analysis with 9p probes. The frequency of SCE in 9p of BRCA2 mutation carriers was 3–4 fold (P=0.005) higher compared to BRCA2 wild-types. Collectively, the increased rates of SCE in BRCA2 heterozygous mutation carriers indicate a BRCA2 haploinsufficiency, which might be an important factor for the accumulation of structural chromosomal alterations with the consequence of damage in as yet unidentified genes.

Keywords:  BRCA2, Fluorescence in situ hybridisation, Sister chromatid exchange, Homologous recombination, Haploinsufficiency, Familial breast cancer clusters