Dose-dependent functions of SWI/SNF BAF in permitting and inhibiting cell proliferation in vivo

SWI/SNF complexes regulate transcription through chromatin remodeling and opposing gene silencing by Polycomb-group (PcG) proteins. Genes that encode SWI/SNF subunits are frequently mutated in human cancer. The selective advantage, subunit bias, and common heterozygosity of such mutations remains poorly understood. Here, we characterized how functional loss of various SWI/SNF subunits and PcG EZH2 affect proliferation-differentiation decisions in vivo, making use of the reproducible development of the nematode C. elegans. We applied a lineage-specific genetics strategy to create partial or complete SWI/SNF subunit loss, as well as double gene knockout with PcG EZH2. Our data show that a high SWI/SNF BAF dosage is needed to oppose Polycomb-mediated transcriptional repression and to arrest cell division during differentiation. In contrast, even in the absence of the PcG EZH2-related methyltransferase, a low level of the SWI/SNF BAF complex is necessary and sufficient to sustain cell proliferation and hyperplasia. Our data provide experimental support for the theory that during carcinogenesis partial SWI/SNF BAF loss-of-function mutations are selected because they eliminate a tumor suppressor activity while maintaining an essential transcription regulatory function.