Abstract
The retinoblastoma (Rb) tumor suppressor acts with a number of chromatin cofactors in a wide range of species to suppress cell proliferation. The Caenorhabditis elegans retinoblastoma gene and many of these cofactors, called synMuv B genes, were identified in genetic screens for cell lineage defects caused by growth factor misexpression. Mutations in many synMuv B genes, including lin-35/Rb, also cause somatic misexpression of the germline RNA processing P granules and enhanced RNAi. We show here that multiple small RNA components, including a set of germline-specific Argonaute genes, are misexpressed in the soma of many synMuv B mutant animals, revealing one node for enhanced RNAi. Distinct classes of synMuv B mutants differ in the subcellular architecture of their misexpressed P granules, their profile of misexpressed small RNA and P granule genes, as well as their enhancement of RNAi and the related silencing of transgenes. These differences define three classes of synMuv B genes, representing three chromatin complexes: a LIN-35/Rb-containing DRM core complex, a SUMO-recruited Mec complex, and a synMuv B heterochromatin complex, suggesting that intersecting chromatin pathways regulate the repression of small RNA and P granule genes in the soma and the potency of RNAi. Consistent with this, the DRM complex and the synMuv B heterochromatin complex were genetically additive and displayed distinct antagonistic interactions with the MES-4 histone methyltransferase and the MRG-1 chromodomain protein, two germline chromatin regulators required for the synMuv phenotype and the somatic misexpression of P granule components. Thus intersecting synMuv B chromatin pathways conspire with synMuv B suppressor chromatin factors to regulate the expression of small RNA pathway genes, which enables heightened RNAi response. Regulation of small RNA pathway genes by human retinoblastoma may also underlie its role as a tumor suppressor gene.
Highlights
The tumor suppressor protein Rb is a chromatin factor that functions in transcriptional repression of cell cycle regulatory genes as well as other genes
In C. elegans, explicit chromatin marks deposited by the MES-4 histone methyltransferase and the MRG1 chromodomain protein allow germline expression of particular suites of target genes
We demonstrate that one of the complexes closely counteracts the activity of MES-4 and MRG-1, whereas another complex interacts with additional regulators that are yet to be identified
Summary
The tumor suppressor protein Rb (retinoblastoma) is a chromatin factor that functions in transcriptional repression of cell cycle regulatory genes as well as other genes. Rb interacts with other chromatin factors (e.g., condensin II) and participates in other chromatin functions such as chromosome condensation and maintaining genome stability [6,7]. Even though it has been studied as a cell cycle regulator for two decades, the functions of Rb are clearly much broader. Genes encoding Rb (lin-35), the rest of the core DRM/dREAM complex, and Rb-recruited repressive chromatin factors all belong to the class of synMuv B (synthetic multivulva B) genes, mutations in which cause a Muv (Multivulva) phenotype when combined with a mutation in a synMuv A gene.
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