Coordinated expression of Rb gene family in the mammary gland

Rb deletion in mouse mammary progenitors induces luminal-B or basal-like/EMT tumor subtypes depending on p53 status

Breast cancer is a highly heterogeneous disease, with several different subtypes being characterized by distinct histology, gene expression patterns, and genetic alterations. The tumor suppressor gene retinoblastoma 1 (RB1) is frequently lost in both luminal-B and triple-negative tumor (TNT; i.e., estrogen receptor-, progesterone receptor-, and human epidermal growth factor receptor 2-negative) breast cancer subtypes. However, a causal role for RB1 loss in different subtypes remains undefined. Here we report that deletion of Rb alone or together with its relative p107 in mouse mammary stem/bipotent progenitor cells induced focal acinar hyperplasia with squamous metaplasia. These lesions progressed into histologically diverse, transplantable mammary tumors with features of either luminal-B or TNT subtypes. The TNTs included basal-like tumors as well as tumors that exhibited epithelial-to-mesenchymal transition (EMT). The EMT-type tumors and a subset of the basal-like tumors, but not luminal-B-like tumors, expressed mutant forms of the tumor suppressor p53. Accordingly, targeted deletion of both Rb and p53 in stem/bipotent progenitors led to histologically uniform, aggressive, EMT-type tumors. Reintroduction of Rb into these tumor cells suppressed growth in vitro and tumor formation in vivo. These results establish a causal role for Rb loss in breast cancer in mice and demonstrate that cooperating oncogenic events, such as mutations in p53, dictate tumor subtype after Rb inactivation.

GFP reporter mice for the retinoblastoma-related cell cycle regulator p107

The RB tumor suppressor gene is mutated in a broad range of human cancers, including pediatric retinoblastoma. Strikingly, however, Rb mutant mice develop tumors of the pituitary and thyroid glands, but not retinoblastoma. Mouse genetics experiments have demonstrated that p107, a protein related to pRB, is capable of preventing retinoblastoma, but not pituitary tumors, in Rb-deficient mice. Evidence suggests that the basis for this compensatory function of p107 is increased transcription of the p107 gene in response to Rb inactivation. To begin to address the context-dependency of this compensatory role of p107 and to follow p107 expression in vivo, we have generated transgenic mice carrying an enhanced GFP (eGFP) reporter inserted into a bacterial artificial chromosome (BAC) containing the mouse p107 gene. Expression of the eGFP transgene parallels that of p107 in these transgenic mice and identifies cells with a broad range of expression level for p107, even within particular organs or tissues. We also show that loss of Rb results in the upregulation of p107 transcription in specific cell populations in vivo, including subpopulations of hematopoietic cells. Thus, p107 BAC-eGFP transgenic mice serve as a useful tool to identify distinct cell types in which p107 is expressed and may have key functions in vivo, and to characterize changes in cellular networks accompanying Rb deficiency.

High expression during neurogenesis but not mammogenesis of a murine homologue of the Deleted in Breast Cancer2/Rhobtb2 tumor suppressor

Deleted in Breast Cancer-2, Dbc2, identified as a candidate tumor suppressor gene for breast cancer and other human malignancies, is a member of a novel gene family encoding putative small GTPases designated as Rhobtb. In this report, we describe the spatio-temporal expression of mDbc2/Rhobtb2 mRNA during mammary gland development and embryogenesis in the mouse. We have first determined that mDbc2/Rhobtb2 transcripts are expressed at low, but seemingly constant levels during mammopoiesis. Secondly, we have found that mDbc2/Rhobtb2 is expressed highly and specifically in the central and peripheral nervous systems during mouse embryogenesis. While our results do not reveal any connection between elevated expression of mDbc2/Rhobtb2 and a specific stage of mammary gland development, they strongly support a role for this gene during development of the nervous system.

Conserved and specific functions of mammalian ssu72

We describe the cloning and characterization of a human homolog of the yeast transcription/RNA-processing factor Ssu72, following a yeast two-hybrid screen for pRb-binding factors in the prostate gland. Interaction between hSsu72 and pRb was observed in transfected mammalian cells and involved multiple domains in pRb; however, so far, mutual effects of these two factors could not be demonstrated. Like the yeast counterpart, mammalian Ssu72 associates with TFIIB and the yeast cleavage/polyadenylation factor Pta1, and exhibits intrinsic phosphatase activity. Mammals contain a single ssu72 gene and a few pseudogenes. During mouse embryogenesis, ssu72 was highly expressed in the nervous system and intestine; high expression in the nervous system persisted in adult mice and was also readily observed in multiple human tumor cell lines. Both endogenous and ectopically expressed mammalian Ssu72 proteins resided primarily in the cytoplasm and only partly in the nucleus. Interestingly, fusion to a strong nuclear localization signal conferred nuclear localization only in a fraction of transfected cells, suggesting active tethering in the cytoplasm. Suppression of ssu72 expression in mammalian cells by siRNA did not reduce proliferation/survival, and its over-expression did not affect transcription of candidate genes in transient reporter assays. Despite high conservation, hssu72 was unable to rescue an ssu72 lethal mutation in yeast. Together, our results highlight conserved and mammalian specific characteristics of mammalian ssu72.

Dynamic regulation of the Stra13/Sharp/Dec bHLH repressors in mammary epithelium

Mammary gland development is a dynamic process involving cyclical proliferation, cellular differentiation, and cell death. In this study, we have determined that expression of the Stra13/Sharp/Dec basic helix-loop-helix (bHLH) family is dynamically regulated in mammary epithelium. In cultured HC11 cells, epidermal growth factor (EGF) treatment rapidly induces Stra13 protein accumulation, which is blocked by the synthetic glucocorticoid, dexamethasone. Neither the induction of Stra13 by EGF nor its repression by dexamethasone correlates with changes in Stra13 mRNA levels. During mouse mammary gland development in vivo, Stra13 is highly expressed in epithelial ducts during puberty, and strongly induced in both ducts and alveoli during early involution, while the related Sharp-1 gene is highly expressed only during late stages of involution. Together, these data indicate that Stra13/Dec/Sharp-family bHLH repressors are dynamically regulated during mammary gland development and may function to regulate apoptosis in this tissue.