Hereditary breast and ovarian cancer poses a major health risk due to its prevalence and the severity of its clinical manifestations. Most cases can be attributed to germline mutations in the BRCA1 tumor susceptibility gene. Since the tumors that arise in these patients typically show loss or inactivation of the other BRCA1 allele, the protein product of BRCA1 is believed to function as a tumor suppressor in normal breast and ovarian tissues. This protein has already been implicated in a remarkably broad spectrum of cellular processes, including RNA transcription, chromatin remodeling, mRNA processing, apoptosis, cell cycle checkpoint control, centrosome amplification, and several distinct pathways of DNA repair. The wealth of potential functions ascribed to BRCA1 has made it difficult to discern which of these are responsible for BRCA1-mediated tumor suppression and to determine the mechanism by which loss of BRCA1 activity leads to carcinogenesis.

To investigate the biochemical functions of BRCA1, we have sought to characterize the protein complexes that it forms in vivo and to define the cellular factors that associate with BRCA1. This work has led to the identification of two novel interacting polypeptides: the BRCA1-associated RING domain (BARD1) protein and the C-terminal interacting protein (CtIP). Initial studies of BARD1 have been especially promising. This protein is structurally related to BRCA1 in that it harbors an N-terminal RING domain and two tandem C-terminal BRCT motifs. Most, if not all, of the cellular pools of BRCA1 and BARD1 polypeptides exist in the form of a heterodimer. The interaction between these proteins is mediated by their respective RING domains and it is ablated by tumorigenic mutations in the RING sequences of BRCA1. Moreover, we have observed both germline and tumor-specific mutations of BARD1 in rare cases of breast, ovarian and endometrial carcinoma. Together, these results suggest that the BRCA1/BARD1 heterodimer is the physiological mediator of BRCA1 functions, including those responsible for tumor suppression.

Recent advances in the field of protein ubiquitination have provided important clues about BRCA1 function. In particular, the RING domain is now recognized as an essential component of many ubiquitin E3 ligases, a large group of enzymatic factors that catalyze the final steps of protein ubiquitination. Indeed, the RING sequences of both BRCA1 and BARD1 have been shown to ubiquitinate model substrates in vitro, and this activity is ablated by tumor-associated missense mutations in the BRCA1 motif. Significantly, the E3 ligase activity of the BRCA1/BARD1 heterodimer is dramatically higher than those of the isolated RING domains from either BRCA1 or BARD1. These important studies have uncovered the first catalytic function for the BRCA1 tumor suppressor, and they provide further evidence that the BRCA1/BARD1 complex is the natural mediator of BRCA1 action. Therefore, direct insights into the mechanism of BRCA1-mediated tumor suppression should emerge from further studies of the BRCA1/BARD1 heterodimer and by identification of its enzymatic substrates.

Selected Publications

Baer, R. Protein partners of the BRCA1 tumor suppressor. Breast Disease 10: 23-32 (1998).

Wu, L.C., Wang, Z.W., Tsan, J.T., Spillman, M.A., Phung, A., Xu, X.L., Yang, M.-C.W. Hwang, L.-Y., Bowcock, A.M., Baer, R. Identification of a RING protein that can interact in vivo with the BRCA1 gene product. Nature Genetics 14: 430-440 (1996).

Jin, Y., Xu, X.L., Yang, M.-C.W., Wei, F., Ayi, T.C., Bowcock, A.M., and Baer, R. Cell cycle-dependent colocalization of BARD1 and BRCA1 in discrete nuclear domains. Proc. Natl. Acad. Sci. (USA), 94: 12075-12080 (1997).

Thai, T.H., Du, F., Tsan, J.T., Jin, Y., Phung, A., Spillman, M.A., Massa, H.F., Muller, C.Y., Ashfaq, R., Mathis, J.M., Miller, D.S., Trask, B.J., Baer, R., and Bowcock, A.M. Mutations in the BRCA1-associated RING domain (BARD1) gene in primary breast, ovarian, and uterine cancers. Hum. Mol. Genet. 7: 195-202 (1998).

Yu, X., Wu, L.C., Bowcock, A.M., Aronheim, A., and Baer, R. The carboxy-terminal (BRCT) domains of BRCA1 interact in vivo with CtIP, a protein implicated in the CtBP pathway of transcriptional repression. J. Biol. Chem. 273: 25388-25392 (1998).

Yu, X. and Baer, R. Nuclear localization and cell cycle-specific expression of CtIP, a protein that associates with the BRCA1 tumor suppressor. J. Biol. Chem. 275: 18541-18549 (2000).