BRCA1 regulates human mammary stem cell self-renewal and differentiation Free

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The epithelial components of the mammary gland are thought to arise from stem cells with the capacity for self-renewal and multilineage differentiation. These cells and/or their immediate progeny may be the targets for transformation, generating tumors which are driven by cells retaining stem cell properties. BRCA1 is an important susceptibility gene for human breast cancer. Germ line mutations in this gene confer a substantial lifetime risk for developing breast cancers which tend to express a “basal” phenotype which is ER PR and HER2 negative. Since this phenotype is also characteristic of breast stem cells, we hypothesized that BRCA1 is involved in stem cell function. In order to characterize this role, we have utilized both in vitro suspension based culture systems as well as NOD-SCID mouse models to examine the role of BRCA1 in regulating human mammary stem cell self-renewal and differentiation. By qRT-PCR BRCA1 m-RNA levels substantially increase as primitive mammary cells in mammospheres are induced to differentiate on collagen substrata. Reduction of BRCA1 expression utilizing an siRNA lenti virus vector increases mammosphere initiating cells by 75% and increases the stem/progenitor cell marker ALDH1 in these cells 3 fold. Furthermore, knockdown of BRCA1 expression in mammosphere initiating cells increases the generation of ALDH1+ ER- stem/progenitor cells in the acini of mammary gland outgrowths generated from the cells in humanized mammary fat pads of NOD-SCID mice. In order to determine whether stem cell expansion was also characteristic of breast tissue from BRCA1 carriers, we performed immunostaining for ALDH1 and ER-ALPHA in 13 samples obtained from mastectomy specimens of confirmed BRCA1 carriers. In 5 of the 13 samples we detected clusters of ALDH1+ ER- cells replacing acini. This pattern was not found in any of 10 breast samples obtained from normal women. These studies suggest that loss of BRCA1 function may produce a block in cell differentiation and increase in self-renewal of mammary stem cells, resulting in expansion of the stem cell pool. Since BRCA1 is also involved in DNA repair, this may generate genetically unstable stem cells which are targets for further carcinogenic events. This results in the generation of breast cancers which display an aggressive “basal” stem cell-like phenotype.