FUNCTIONAL ANALYSIS OF THE BRCA1 GENE PRODUCT

David M. Livingston*, Roger Greenberg, Shridar Ganesan, Jean Feunteun**, Sharon Cantor, Stoil Dimitrov, Ronny Drapkin, Vladimir Joukov, Chryssa Kanellopoulou, Wael Elshamy, Yi-Li Yang, and Bing Xia.
*david_livingston@dfci.harvard.edu

Dana-Farber Cancer Institute, 44 Binney St., Boston, MA 02115
and Harvard Medical School, Boston, MA and
**Institute Gustave Roussy, Villejuif, France

The BRCA1 protein is an 1863 aa product of a human breast and ovarian cancer suppression gene, germ line mutations in which are responsible for relatively high penetrance breast and ovarian carcinoma development in families. For the most part, only the female members of such families develop disease, despite the fact that the gene is expressed in most proliferating cells of both females and males. Among the most prominent questions are: how does BRCA1 function biochemically, how do its biochemical operations contribute to a state of tumor suppression, why is disease confined to women bearing germ line inactivating BRCA1 mutations, and why, when tumors do arise, are the breast and ovary the two target organs. Clarity has not been reached on any of these questions.

There is abundant evidence that BRCA1 plays a number of roles in the maintenance of genome integrity. It is a necessary participant in the response to DNA damage, operating in both the repair of double strand breaks (DSB) and in the enaction of certain cell cycle checkpoint responses. Where studied, considerable circumstantial evidence has been obtained implying that at least part of its repair and/or checkpoint response function is linked to its tumor suppressing activity. In this regard, a few years ago, we suggested from the results of a genetic study of BRCA1 DSBR function that BRCA1 protein might, at least in part, operate as a signal integrating (scaffold) element capable of coordinating a number of functions delivered by BRCA1- associating proteins on behalf of a

coordinated DSBR response (1). At the hands of assays that analyze events which occur in a timely manner at the site of DNA damage (2), we have very recently obtained evidence that supports this hypothesis. Complementary evidence supporting a BRCA1/smart scaffold hypothesis has also been obtained by others (3).

In the recent past, we also reported that BRCA1 function is necessary for the maintenance of the heterochromatin superstructure of the inactive X chromosome (Xi) in female somatic cells (4). As part of this work, a biochemical association of BRCA1- containing multiprotein complexes and XIST RNA was detected. Moreover, in cells acutely or chronically deprived of a normal complement of BRCA1 function, a reporter gene embedded in Xi became partially activated, suggesting that BRCA1 function contributes to the efficient silencing of certain Xi genes. Since a series of BRCA1 -/- human and murine breast carcinomas were found to lack a classical Xi structure, these findings leave open the possibility-albeit clearly do not prove- that BRCA1-> Xi function contributes to BRCA1 mediated breast  (and ovarian) cancer suppression. Recent work on this aspect of BRCA1 function will be discussed.

REFERENCES:

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2. Rogakou,  E.P., Boon, C.,  Redon,  C., and Bonner, W.M.  J(1999). Cell Biol. 146, 905-916.           
3. Foray, N., Marot,  D., Gabriel, A., Randrianarison, V., Carr, A.M., Perricaudet, M., Ashworth,  A.,  Jeggo,  P. (2003) EMBO J. 22, 2860-2871.
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