CONICET | Buscador de Institutos y Recursos Humanos

Embryonic Stem (ES) cells are able to self renew and differentiate into the three primary germ layers: ectoderm, mesoderm and endoderm. We found that the mammalian histone deacetylase SIRT6 plays a critical role in embryogenesis. Sirt6 knockout mice from C57BL/6 strain are born at significantly lower Mendelian ratios. Upon differentiation, ES as well as induced Pluripotent Stem (iPS) cells derived from SIRT6 deficient mice generate smaller embryoid bodies (EBs) with underdeveloped endoderm and mesoderm, but overdeveloped neuroectoderm. This morphological phenotype is recapitulated in human EBs with downregulated SIRT6. In support of a SIRT6-dependent regulation of stem cell differentiation, we found teratomas obtained from mouse ES cells lacking SIRT6 to be of significantly smaller size and enriched in neuroectodermal tissue. We found the expression of the core pluripotent factors OCT4, SOX2 and NANOG to be upregulated in SIRT6 depleted EBs. This upregulation correlates with an increased acetylation of H3K56 at the promoters of these pluripotent genes, indicating that SIRT6 functions as an histone deacetylase to repress pluripotent genes during ES cell differentiation. Remarkably, we find bulk genomic levels of 5-hydroxymethylcytosine (5hmC), an epigenetic hallmark for pluripotency, to be increased in SIRT6 deficient ES cells along with elevated levels of the hydroximethylases TET1 and TET2. Importantly, downregulation of TET1 or TET2 fully rescues the differentiation defect in SIRT6 deficient EBs, indicating that this particular epigenetic mark plays a critical, rate limiting role during early embryo differentiation. Therefore, we proposed a model where the deacetylase activity of SIRT6 causes the transcriptional repression of Oct4 and Sox2, which in turn affects the expression of Tet1 and Tet2 as an epistatic pathway required for proper embryogenesis.