Down-regulation of BRCA1 in human sporadic breast cancer; analysis of DNA methylation patterns of the putative promoter region

Abstract

Germ-line alterations of BRCA1 are responsible for about 50% of familial breast cancers. Although its biological function (s) has not yet been fully determined, it has been suggested that it may act as a tumor suppressor gene in breast and ovarian cancers. In sporadic breast cancers alterations of BRCA1 have not been detected and in vitro experiments have indicated that BRCA1 negatively regulates cellular proliferation. The present study was designed to identify and quantify, the BRCA1 mRNA levels, in normal and neoplasic human breast tissue. BRCA1 mRNA molecules were quanti®ed using competitive reverse transcriptase PCR assays. DNA methylation patterns of this gene have been analysed by Southern blot experiments using methylation sensitive restriction enzymes. We found that BRCA1 mRNA levels were signi®cantly lower in sporadic breast cancers (37 cases analysed, 24 cases of invasive ductal carcinomas not otherwise speci®ed (NOS), two lobular carcinomas in situ two medullary carcinomas, four invasive lobular carcinomas, two invasive mucinous carcinomas and three invasive ductal carcinomas with predominantly in situ component) compared with normal breast tissues (P=0.0003). This down-regulation of BRCA1 is observed in all histologic types analysed. In invasive ductal carcinomas NOS, this down-regulation does not correlate with any of the prognostic factors studied (tumor size, node status, histologic grade, hormone receptor status). In the samples analysed, alterations of DNA methylation patterns were not dectected in the vicinity of the major transcription start site. These data suggest the involvement of BRCA1 in the carcinogenesis of these histologic types.

Suppression of ING1 expression in sporadic breast cancer

“…Although the mutation rate is very low, the frequency of down regulation of ING1 in tumours is very substantial, similar to those described for BRCA1 (Thompson et al, 1995;Dobrovic and Simpfendorfer, 1997;Magdinier et al, 1998;Rice et al, 1998;Sourvinos and Spandidos, 1998) and p16 INK4A Brenner et al, 1996;Herman et al, 1995).…”

MTA1-mediated transcriptional repression of BRCA1 tumor suppressor gene

“…On the other hand, the remaining 90% of breast cancers occur sporadically in the absence of any apparent genetic predisposition. In these nonfamilial cases, tumorigenesis is likely not due to mutation but rather to a drastic decrease of BRCA1 mRNA or protein levels (Magdinier et al, 1998;Rio et al, 1999). Multiple studies have shown a correlation between loss of BRCA1 expression and increased metastatic disease.…”

GA-binding protein α/β is a critical regulator of the BRCA1 promoter

“…A series of papers has also suggested that methylation of the BRCA1 promoter may be a signi®cant factor in decreasing BRCA1 expression in sporadic breast tumors, possibly through inactivation of a CREB site (Mancini et al, 1998). However, methylation can account for decreased BRCA1 expression only in a subset of sporadic breast cancer samples (Magdinier et al, 1998;Dobrovic and Simpfendorfer, 1997;Rice et al, 1998;Mancini et al, 1998;Catteau et al, 1999). Overall, these results suggest that sporadic breast cancer may result from defects in the genes regulating the BRCA1 promoter, or that transformation leads to a concomitant down regulation of BRCA1 expression.…”