Targeted Knockout of BRG1 Potentiates Lung Cancer Development

Abstract

Brahma-related gene 1 (BRG1) is a catalytic subunit of the switch in mating type/sucrose nonfermentation complex and plays an important role in cancer development. Mouse homozygous knockout experiments testing the role of BRG1 in tumorigenesis have been hampered because BRG1 inactivation is embryonic lethal. To bypass this constraint, we developed a lung-specific conditional knockout of BRG1 and examined the effect of BRG1 inactivation in an ethyl carbamate lung carcinogenesis mouse model. We found that the heterozygous loss of BRG1 resulted in increases in both the number and size of tumors when compared with controls. In contrast, when both BRG1 alleles were inactivated, neither the number nor the size of tumors increased compared with controls. In mouse lung tissue where BRG1 was homozygously inactivated, immunostaining for apoptotic markers showed significant increase in Apo-BrdUrd and cleaved caspase-3. These data indicate that a loss of cell viability underlies why biallelic inactivation of BRG1 does not increase tumorigenesis. We also examined mice when exposed to the carcinogen ethyl carbamate and then subjected to BRG1 inactivation. In these cells, loss of BRG1 after carcinogen exposure potentiated tumor development. A subset of tumors retained BRG1 expression, whereas others showed either partial or complete loss of BRG1 expression. Tumors completely devoid of BRG1 expression were significantly larger and expressed higher levels of two markers of proliferation, proliferating cell nuclear antigen and Ki67. Although biallelic inactivation of BRG1 could not initiate tumor development in untransformed cells, our results indicate that transformation and tumor progression are greatly affected by loss of BRG1. [Cancer Res 2008;68(10):3689-96]

The SWI/SNF complex and cancer

“…Biallelic knockout of BRG1 is embryonically lethal (Sumi-Ichinose et al, 1997) (Bultman et al, 2000), and until recently it has been impossible to determine the effects of BRG1 silencing in tumorigenesis. We have now developed a novel mouse model that permits conditional biallelic knockout of BRG1 in lung epithelial cells (Glaros et al, 2008). Our data indicate that the loss of both BRG1 alleles induces apoptosis in non-transformed lung epithelial cells.…”

“…Our data on lung carcinogenesis in transgenic mice suggest that BRM and BRG1 may be involved in different stages of carcinogenesis (initiation versus progression) (Glaros et al, 2008). The unresolved question is the effect of silencing or mutating the genes that encode both of the catalytic SWI/SNF subunits.…”

“…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.…”

“…BRG1 is required for the establishment and maintenance of epithelial polarity (Indra et al, 2005), an observation that is consistent with both the pro-apoptotic effects of BRG1 knockout in the lung as well as with the early embryonic lethality of BRG1 knockout in mice. On the other hand, knockout of both BRG1 alleles in urethane-induced tumors promotes tumorigenesis (Glaros et al, 2008). BRG1-null, urethane-induced lung tumors are more abundant, larger and have a higher index of proliferation than BRG1-positive tumors, demonstrating that the loss of BRG1 promotes tumorigenesis.…”

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).…”