Arrest of the cell cycle by the tumour-suppressor BRCA1 requires the CDK-inhibitor p21WAF1/CiPl

Abstract

Much of the predisposition to hereditary breast and ovarian cancer has been attributed to inherited defects in the BRCA1 tumour-suppressor gene. The nuclear protein BRCA1 has the properties of a transcription factor, and can interact with the recombination and repair protein RAD51. Young women with germline alterations in BRCA1 develop breast cancer at rates 100-fold higher than the general population, and BRCA1-null mice die before day 8 of development. However, the mechanisms of BRCA1-mediated growth regulation and tumour suppression remain unknown. Here we show that BRCA1 transactivates expression of the cyclin-dependent kinase inhibitor p21WAF1/CIP1 in a p53-independent manner, and that BRCA1 inhibits cell-cycle progression into the S-phase following its transfection into human cancer cells. BRCA1 does not inhibit S-phase progression in p21-/- cells, unlike p21+/+ cells, and tumour-associated, transactivation-deficient mutants of BRCA1 are defective in both transactivation of p21 and cell-cycle inhibition. These data suggest that one mechanism by which BRCA1 contributes to cell-cycle arrest and growth suppression is through the induction of p21.

BRCA1 physically associates with p53 and stimulates its transcriptional activity

“…Both proteins can activate expression of the cell cycle inhibitor p21 (El-Deiry et al, 1993;Somasundaram et al, 1997). Because both BRCA1 and p53 are implicated in regulation of gene transcription, control of cell growth and response to DNA damage, we sought to investigate the possibility of a functional cross-talk between these two proteins.…”

“…Thus it is clear that BRCA1 can activate p21 by a p53-independent pathway that maps to a region distinct from the p53 DNA binding site Somasundaram et al, 1997 and can coactivate p53-dependent gene expression in general only in the presence of p53 binding sites (Figures 1 and 4).…”

“…The role of p53 as a transcription factor is well-established (Vogelstein and Kinzler, 1992) and appears to be important for its ability to arrest cell cycle progression as well as to induce apoptosis (Levine, 1997). BRCA1 has been postulated to function as a coactivator of transcription, based on its localization as a component of RNA polymerase II holoenzyme and transactivation function (Chapman and Verma, 1996;Monteiro et al, 1996;Somasundaram et al, 1997;Scully et al, 1997a). The observation that BRCA1 is a transcriptional coactivator of p53, including strong upregulation of expression of the bax gene, provides a novel mechanism for apoptosis induction and tumor suppression by BRCA1.…”

“…The Luciferase T7 control DNA was obtained from Promega. Wild-type and mutant BRCA1 (Dexon11, D500 ± 1863, D1312 ± 1863, P1749R, Y1853insA and Q1756insC) were described previously (Thakur et al, 1997;Somasundaram et al, 1997). The p53 cDNA was ampli®ed using the primers 5'-AAGCTTGCCACCATGGAGGAGCCGCAGTCA-3' and 5'-ATGCGGCCGCTCAGTCTGAGTCAG-3' and the human p53 cDNA as template.…”

“…Both p53 and BRCA1 are physically-altered by the cellular response to DNA damage, p53 by stabilization and BRCA1 by hyperphosphorylation (Kastan et al, 1991;Scully et al, 1997b). Both p53 and BRCA1 can activate p21 as a common target gene (El-Deiry et al, 1993;Somasundaram et al, 1997). In this study we explored the possibility that BRCA1 and p53 may participate in a common pathway of growth regulation.…”