Reply to “…and secreted tumour suppressors”

Loss of nuclear BRCA1 expression in breast cancers is associated with a highly proliferative tumor phenotype

Recent studies have suggested that BRCA1-associated hereditary breast cancer may be a more aggressive form of disease than sporadic breast cancer. BRCA1-associated breast cancer has been reported to be significantly associated with grade 3 disease. Because grade 3 disease indicates a poor prognosis, this implies that women with a germ-line mutation in BRCA1 who develop breast cancer may have a poorer prognosis than those with sporadic disease. However, little is known about the association of BRCA1 expression with biological markers of prognosis. The present study examined the expression of BRCA1 in a total of 40 archival breast tumor specimens from three patient cohorts (sporadic, familial, and early onset breast cancer) to determine localization of the protein. Furthermore, BRCA1 staining was compared with expression of markers of tumor biology. We found that BRCA1 is generally located in the nucleus and the cytoplasm of normal and malignant breast tissue. Nuclear staining for BRCA1 was observed in most sporadic tumors, but nuclear BRCA1 was reduced or absent in the majority of familial and early onset breast tumors. Although no correlation was found between nuclear BRCA1 expression and estrogen and progesterone status, a significant inverse correlation was found between nuclear BRCA1 and expression of the proliferation marker Ki-67 (P = 0.01). Our findings suggest that tumors associated with a germ-line mutation in one of the breast cancer genes may be highly proliferative and support the view that loss of BRCA1 expression in breast tumors may lead to a more aggressive tumor phenotype.

Functions of the BRCA1 and BRCA2 genes

Mutations in the BRCA1 and BRCA2 genes confer a high risk of breast cancer development. Both genes encode very large proteins of unknown function but recent results suggest that they may have roles in transcriptional regulation and DNA repair. These advances offer the prospect of understanding not only the normal cellular function of these genes but also how their loss leads to tumour formation.

BRCA1 is a cell cycle-regulated nuclear phosphoprotein

We have characterized the BRCA1 gene product by using four polyclonal antibodies raised against peptides from four different regions of the protein. The antibodies specifically recognize an approximately 220-kDa BRCA1 protein that is predominantly expressed in the nucleus of both normal and neoplastic breast cancer cells. It is a serine phosphoprotein that undergoes hyperphosphorylation during late G1 and S phases of the cell cycle and is transiently dephosphorylated early after M phase. We propose that BRCA1 is a phosphoprotein that alters in a qualitative and quantitative manner during cell cycle progression.

Cloning, chromosomal mapping and expression pattern of the mouse Brca2 gene

A proportion of human breast cancers result from an inherited predisposition to the disease. Mutations in the BRCA2 gene confer a high risk of breast cancer and are responsible for almost half of these cases. The recent cloning of the human BRCA2 gene has revealed that it encodes a large protein having little significant homology to known proteins. Here we describe the mouse Brca2 gene. The gene maps to mouse chromosome 5, consistent with its location on human chromosome 13q12. We have sequenced cDNA for the entire 3329 amino acid Brca2 protein and this has revealed that, like Brca1, Brca2 is relatively poorly conserved between humans and mice. Brca2 is transcribed in a diverse range of mouse tissues, and the pattern of expression is strikingly similar to that of Brca1. Taken together, our data highlight some intriguing similarities between two genes involved in inherited breast cancer susceptibility.

Evidence for a transcriptional activation function of BRCA1 C-terminal region

Mutations in BRCA1 account for 45% of families with high incidence of breast cancer and for 80-90% of families with both breast and ovarian cancer. BRCA1 protein includes an amino-terminal zinc finger motif as well as an excess of negatively charged amino acids near the C terminus. In addition, BRCA1 contains two nuclear localization signals and localizes to the nucleus of normal cells. While these features suggest a role in transcriptional regulation, no function has been assigned to BRCA1. Here, we show that the C-terminal region, comprising exons 16-24 (aa 1560-1863) of BRCA1 fused to GAL4 DNA binding domain can activate transcription both in yeast and mammalian cells. Furthermore, we define the region comprising exons 21-24 (aa 1760-1863) as the minimal transactivation domain. Any one of four germ-line mutations in the C-terminal region found in patients with breast or ovarian cancer (Ala-1708-->Glu, Gln-1756 C+, Met-1775-->Arg, Tyr-1853 ->Stop), had markedly impaired transcription activity. Together these data underscore the notion that one of the functions of BRCA1 may be the regulation of transcription.

Subcellular localization and analysis of apparent 180-kDa and 220-kDa proteins of the breast cancer susceptibility gene, BRCA1

The breast cancer susceptibility gene BRCA1 encodes an 1863-amino acid protein that acts as a tumor suppressor. The biochemical function of BRCA1 is unknown, and there are conflicting results describing its subcellular location. We have identified a 220-kDa protein, which is reactive with three antibodies raised against the amino- and carboxyl-terminal regions of BRCA1. Immunoflourescence staining with an antibody to the carboxyl terminus of BRCA1 localized the protein to the nucleus of breast, ovarian, and cervical carcinoma-derived cell lines. A similar result was observed by biochemical subcellular fractionation that indicated that the 220-kDa protein was localized primarily to the nucleus of cell lines established from breast carcinomas. In addition to the 220-kDa protein, one antibody, C-20, also recognized a 180-kDa protein in MDA-MB-468 total cell lysates that was not detected by the other two antibodies. Several observations suggest the 180-kDa protein is the epidermal growth factor (EGF) receptor: (i) C-20 reacted avidly with a 180-kDa protein immunoprecipitated by an antibody to the EGF receptor; (ii) an EGF receptor antibody detected a 180-kDa protein immunoprecipitated by C-20; (iii) the affinity purified EGF receptor was both immunoprecipitated and detected on immunoblots by the C-20 antibody but not another BRCA1 antibody; (iv) similar phosphopeptide maps were generated from the EGF receptor and the 180-kDa protein immunoprecipitated by C-20, and this peptide map was distinct from the 220-kDa phosphoprotein; and (v) the C-20 immunizing peptide bears sequence identity to the EGF receptor. These results indicate that BRCA1 is a 220-kDa nuclear protein and that the 180-kDa protein reported previously may be unrelated to BRCA1.