Analysis of the Developmental and Physiological Roles of Histone Deacetylases 1 and 2

Histone deacetylases (HDAC) 1 and 2 are highly similar enzymes that help regulate chromatin structure as the core catalytic components of co-repressor complexes. Although tissue specific deletion of HDAC1 and HDAC2 has demonstrated functional redundancy, germline deletion of HDAC1 in the mouse causes early embryonic lethality, whereas HDAC2 does not. To address the unique requirement for HDAC1 in early embryogenesis I have generated conditional knock-out mouse embryonic stem (mES) cells in which HDAC1 or HDAC2 genes can be inactivated. Deletion of HDAC1, but not HDAC2, causes a significant reduction in the HDAC activity of Sin3A, NuRD and CoREST co-repressor complexes. This reduced co-repressor activity results in a specific 1.6-fold increase in histone H3 K56 acetylation, providing genetic evidence that H3K56Ac is a substrate of HDAC1. In culture, loss of HDAC1 but not HDAC2 leads to precocious mES cell differentiation. This genetic study of HDAC1 and HDAC2 in mES cells, which mimic the embryonic epiblast, has identified a unique requirement for HDAC1 in the optimal activity of HDAC1/2 co-repressor complexes and cell fate determination during differentiation. Given previous demonstrations of the roles of HDAC1/2 known co-repressor complexes in T cell development and the functional redundancy between HDAC1 and -2 in a number of tissues, a conditional knock-out approach was undertaken in murine T cells. I have demonstrated that HDAC1 and -2 exert pleiotropic effects on T-cell development. I have also shown, for the first time in a physiological system, that HDAC1/2 activity is critical for maintaining genome stability. The occurrence of tumours (and the developmental block) is dose dependent, occurring in cells with the least amount of deacetylase activity, indicating that regulation of the acetyl-proteome, a balance between HAT and HDAC enzymes is crucial for normal cell development and viability.