Mechanisms that couple irregular development of fetal melanoblasts to premature exhaustion of adult melanocyte stem cells

PROJECT SUMMARY Maintenance of tissue function during adulthood, and hence suppression of tissue degeneration and disease, depends on maintenance of stem cell populations. Adult stem cells are epigenetically programmed and a culmination of a series of developmental decisions initiated in the embryo. In utero environmental exposures on the embryo can influence adult and late-life disease, likely in part via effects on stem cell development that are transmitted to maintenance and function of stem cells in the adult. However, the molecular links between embryonic development and long-term maintenance of stem cell function and phenotype in adults are poorly defined. We will employ lineage-specific genetic inactivation of a histone chaperone to understand how embryonic developmental integrity of melanoblasts (Mb) impacts on maintenance adult melanocyte (Mc) stem cells (McSC). The histone chaperone HIRA deposits histone variant H3.3 into active genes, promoters and enhancers. Through in vitro and in vivo studies and single cell RNA-seq of mouse embryo melanoblasts (Mbs) from wild type mice and mice lacking expression of HIRA in embryonic Mbs, we have uncovered a role for HIRA in sustaining the PAX3/SOX10-MITF Mb specification pathway. Inactivation of HIRA in Mbs depletes the number of Mbs in early/mid stage embryos. However, this embryonic defect is rescued by birth and young mice exhibit normal numbers of melanocytic cells, and only a very subtle pigmentation defect. Nevertheless, in new-born mice, Hira knock out (KO) melanocytic cells exhibit a higher frequency of telomere-associated DNA damage foci, indicating that Hira knock out McSC and/or melanocytes harbor molecular damage, even in new-born mice. Indeed, melanoblasts and melanocytes from new-born Hira KO mice respond poorly to pro-proliferative challenge in vitro and in vivo, and these mice show marked accelerated McSC and melanocyte depletion and dramatically accelerated hair greying during adulthood. Building on these extensive preliminary data, we will investigate the role of HIRA in differentiation and development of the melanocytic lineage, and investigate the links between abnormal embryonic development and adult stem cell depletion during adulthood and aging. Dysregulation of the PAX3/SOX10-MITF signaling pathway contributes to developmental disorders and melanoma. These studies to define HIRA's role in the PAX3/SOX10-MITF axis can promote therapeutic interventions to combat these developmental and neoplastic disorders. Moreover, completion of these Specific Aims will address how the integrity of embryonic development of tissue specific stem cells impacts maintenance of those stem cells during adulthood.