Regulation of SOX Proteins by Methylation-dependent Proteolysis in Stem Cells and Development

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Project Summary The CRL4 ubiquitin ligase complexes regulate many important biological processes such as DNA replication, DNA repair, transcriptional regulation, and epigenetic inheritance of DNA methylation. We recently found that CRL4 also regulates the self-renewal and pluripotency of embryonic stem cells (ESCs) through important stem cell proteins such as SOX2 (SRY-box2). SOX2 is a dose-dependent master transcriptional factor that plays a key role in regulating the self-renewal and pluripotency or multipotency of ESCs, iPSCs, and many fetal and adult stem cells. In ESCs, an increase of SOX2 promotes development into ectoderm and mesoderm lineages, while loss or reduction of SOX2 induces differentiation into endoderm and trophectoderm lineages. Even at the 4-cell embryonic stage, the heterogeneous binding of SOX2 to target genes determines the first lineage decision. In human, loss-of-function mutations on a single Sox2 allele is sufficient to cause the familial anophthalmia/microphthalmia syndrome associated with seizures, brain abnormalities, slow growth, delayed motor skills and learning disability. Conversely, gene amplification and over-expression of SOX2 are frequently associated with many poorly differentiated cancers including lung, esophagus, brain, and breast cancers. However, it remains unclear how the SOX2 protein level is regulated in various stem/progenitor cells during development and tissue homeostasis. We recently found that the protein stability of SOX2 is regulated by a novel CRL4-based proteolytic mechanism using lysine methylation as a proteolytic trigger in ESCs and other related cells. Genetic mutation of this particular CRL4 function impairs embryonic development. We propose to unravel the function and regulation of this novel and important CRL4-based proteolytic mechanism in regulating self-renewal and pluripotency of ESCs and during embryonic development. Our specific aim 1 is to define the roles of CRL4-based ubiquitin E3 ligases that target the methylated SOX2 protein for degradation. Our specific aim 2 is to determine how the levels of methylated SOX2 proteins are dynamically regulated during the self-renewal of ESCs. In our specific aim 3, we propose to examine how the function of SOX2 is regulated in animal development using specific genetic mutants defective in the methylation- dependent proteolysis pathway. Since SOX2 is central to many stem cells and pathological loss or elevation of SOX2 levels underlies many diseases, our studies should reveal a novel paradigm by which the self-renewal and pluripotency or multipotency of various stem cells are regulated.