Characterization of mammary tumors from Brg1 heterozygous mice

Abstract

Mammalian SWI/SNF-related complexes have been implicated in cancer based on some of the subunits physically interacting with retinoblastoma (RB) and other proteins involved in carcinogenesis. Additionally, several subunits are mutated or not expressed in tumor-derived cell lines. Strong evidence for a role in tumorigenesis in vivo, however, has been limited to SNF5 mutations that result primarily in malignant rhabdoid tumors (MRTs) in humans and MRTs as well as other sarcomas in mice. We previously generated a null mutation of the Brg1 catalytic subunit in the mouse and reported that homozygotes die during embryogenesis. Here, we demonstrate that Brg1 heterozygotes are susceptible to mammary tumors that are fundamentally different than Snf5 tumors. First, mammary tumors are carcinomas not sarcomas. Second, Brg1 þ /À tumors arise because of haploinsufficiency rather than loss of heterozygosity. Third, Brg1 þ /À tumors exhibit genomic instability but not polyploidy based on array comparative genomic hybridization results. We monitored Brg1 þ /À , Brm À/À double-mutant mice but did not observe any tumors resembling those from Snf5 mutants, indicating that the Brg1 þ /À and Snf5 þ /À tumor phenotypes do not differ simply because Brg1 has a closely related paralog whereas Snf5 does not. These findings demonstrate that BRG1 and SNF5 are not functionally equivalent but protect against cancer in different ways. We also demonstrate that Brg1 þ /À mammary tumors have relatively heterogeneous gene expression profiles with similarities and differences compared to other mouse models of breast cancer. The Brg1 þ /À expression profiles are not particularly similar to mammary tumors from Wap-T121 transgenic line where RB is perturbed. We were also unable to detect a genetic interaction between the Brg1 þ /À and Rb þ /À tumor phenotypes. These latter findings do not support a BRG1-RB interaction in vivo. IntroductionMuch work has focused on the role of mammalian SWI/ SNF-related complexes in cancer. SWI/SNF-related subunits physically interact with a number of proteins encoded by tumor-suppressor genes and proto-oncogenes (Muchardt and Yaniv, 2001;Roberts and Orkin, 2004). For example, BRM and BRG1 bind retinoblastoma (RB) and are required to repress the activity of E2F1, inhibit the transcription of cyclins A and E and mediate G 1 cell-cycle arrest in vitro. BRG1 and SNF5 can also act upstream of RB by activating the expression of several cyclin-dependent kinase inhibitors (p15 INK4b , p16 INK4A or p21 CIP1/WAF1 ), which leads to the inhibition of CDK2 and CDK4 and accumulation of the hypophosphorylated form of RB that mediates G 1 arrest.In addition to being associated with cancer-related proteins, the BRM, BRG1, SNF5, BAF155 and BAF250 subunits are mutated or not expressed in various tumor-derived cell lines (Muchardt and Yaniv, 2001). When tumor-derived cell lines are cultured, however, deletions and epigenetic alterations are selected for and accumulate. Because of this caveat, it is crucial to identify and characterize mu...

SWI/SNF Deficiency Results in Aberrant Chromatin Organization, Mitotic Failure, and Diminished Proliferative Capacity

“…This suggests a mechanism for the observed mitotic abnormalities, because disruption of such domains compromises mitotic fidelity. It has been recently reported that tumors arising in Brg1 ϩ/Ϫ mice, although not mimicking specific pathways, are best characterized by genomic instability (Bultman et al, 2008). Importantly, this study also concludes that tumor formation in Brg1 ϩ/Ϫ mice occurs due to haploinsufficiency rather than loss of heterozygosity (Bultman et al, 2008), suggesting a lack of selection or proliferative advantage with the complete ablation of Brg1.…”

“…It has been recently reported that tumors arising in Brg1 ϩ/Ϫ mice, although not mimicking specific pathways, are best characterized by genomic instability (Bultman et al, 2008). Importantly, this study also concludes that tumor formation in Brg1 ϩ/Ϫ mice occurs due to haploinsufficiency rather than loss of heterozygosity (Bultman et al, 2008), suggesting a lack of selection or proliferative advantage with the complete ablation of Brg1. Thus, in the context of Brg1 deficiency, resultant dispersion of pericentric heterochromatin domains and mitotic dysfunction could potentially represent the underlying key etiological feature relevant to tumorigenesis.…”

The SWI/SNF complex and cancer

“…As cited above, the BAF47 knockout mouse is the most tumorigenic model reported to date . Knockout experiments with BRG1 and BRM have revealed that loss of either ATPase can potentiate cancer development in mice Bultman et al, 2000Bultman et al, , 2008Glaros et al, 2008). However, interpretation of the results of these experiments is complicated by the possibility that BRG1 and BRM can potentially compensate for each other in certain circumstances.…”

“…Knockout of a single allele of BRG1 results in spontaneous tumor development in about 10% of BRG1 þ /À mice within a year (Bultman et al, 2000(Bultman et al, , 2008. These tumors originated from the milk line and stained for histopathology markers indicative of mammary tumors.…”

Srg3, a mouse homolog of BAF155, is a novel p53 target and acts as a tumor suppressor by modulating p21WAF1/CIP1 expression

“…Consistently, Brg1 heterozygous mice are predisposed to subcutaneous glandular tumors with histological features similar to breast adenocarcinomas (Bultman et al, 2000). High incidence of mammary tumors or lung cancers was also reported in the Brg1 mutant mice (Bultman et al, 2008;Glaros et al, 2008). Snf5 heterozygous mice are prone to tumors resembling malignant rhabdoid tumors (Klochendler-Yeivin et al, 2000;Roberts et al, 2000;Guidi et al, 2001).…”