Targeted knockout of BRG1 potentiates lung cancer development

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.