Genomic alterations in histopathologically normal breast tissue from BRCA1 mutation carriers may be caused by BRCA1 haploinsufficiency

Abstract

Multiple biopsies of normal breast tissue from 10 BRCA1 mutation carriers have been analyzed using array-based comparative genomic hybridization. Normal breast tissue from five age-matched control subjects without a family history of breast cancer was included for reference purposes. We repeatedly found multiple low copy number aberrations at a significantly higher frequency in histopathologically normal tissue from BRCA1 mutation carriers than in normal control tissue. Some of these aberrations were similar across samples from different patients and linked to biological functions such as transcriptional regulation and DNA binding. We also observed a high degree of genomic heterogeneity between samples from the same patient, suggestive of tissue heterogeneity and etiological clonality in the breast epithelium. We show that neither loss of heterozygosity nor promoter methylation of the wild-type BRCA1 allele is the predominant mechanistic origin of the observed genomic instability. Instead, we propose that haploinsufficiency of BRCA1 might be the underlying cause responsible for initiation of breast cancer in these predisposed women, making cells vulnerable to mitotic recombination. We also propose that loss of ERalpha expression is preceded by genetic instability in the initiation of BRCA1-dependent tumorigenesis, indicating that the breast epithelium of BRCA1 mutation carriers may initially be estrogen-responsive. Our results imply that genomic instability instigated by BRCA1 haploinsufficiency may be required for breast cancer initiation in BRCA1 mutation carriers. Finding molecular markers of tumor initiation and progression, for the potential use in early disease detection, may be of great clinical importance for the improved management of at-risk women.

Laser capture microdissection (LCM) and whole genome amplification (WGA) of DNA from normal breast tissue --- optimization for genome wide array analyses

“…Samples of histologically normal breast tissue were collected from women who had undergone a prophylactic mastectomy due to increased risk of developing breast cancer (see Rennstam et al [13] for details). Informed consent forms were signed by all women included in the study and the study was approved by the ethics committee at Lund University, Sweden.…”

BRCA promoter methylation in sporadic versus BRCA germline mutation-related breast cancers

“…However, our findings do not support the general assumption and previous findings reported in the literature that BRCA promoter methylation and BRCA germline mutations are mutually exclusive. In most studies, none of the BRCA -related breast carcinomas showed BRCA promoter methylation [16, 21, 23–26]. Kontorovich et al [20] observed BRCA1 promoter methylation in 3 (6.3%) of 48 BRCA1 -related breast carcinomas, and Tapia et al [17] observed BRCA1 promoter methylation in 2 (66.7%) of 3 observed BRCA1 -related breast carcinomas.…”

Proliferation and ovarian hormone signaling are impaired in normal breast tissues from women with BRCA1 mutations: benefit of a progesterone receptor modulator treatment as a breast cancer preventive strategy in women with inherited BRCA1 mutations

“…Haploinsufficiency was also involved in impaired differentiation of epithelial luminal cells, leading to an expanded luminal progenitor population in BRCA1 mut/+ breast tissues [40]. All these data are consistent with the assumption that BRCA1 mut/+ breast cells are likely to cumulate genomic aberrations during mitotic recombination [38]. Higher proliferative rates caused by the hormone signaling in these cells would therefore explain the sex and organ-specific penetrance of BRCA1 -related cancers.…”

“…Since BRCA1 is involved in DNA repair and cell cycle control, our results suggest that E2 and P4 exposure may enhance proliferation in BRCA1 mut/+ breast tissues, and potentially increase the accumulation of unrepaired mutations and DNA lesions. Indeed, previous studies have shown that BRCA1 mut/+ epithelial breast cells were haploinsufficient for BRCA1 as they displayed genomic alterations [38, 39], including defects in stalled replication fork repair and a higher frequency in fork collapses [39]. Haploinsufficiency was also involved in impaired differentiation of epithelial luminal cells, leading to an expanded luminal progenitor population in BRCA1 mut/+ breast tissues [40].…”