Histone methyltransferase DOT1L is essential for self-renewal of germline stem cells

Spermatogonial stem cells undergo both self-renewal to maintain the stem cell population and differentiation to produce mature sperm. These processes are controlled by both stem cell-intrinsic and external niche factors. DOT1L, the sole H3K79 methyltransferase, is dispensable for mouse embryonic stem cell self-renewal but instead functions as a barrier to somatic cell reprogramming. Here we show that DOT1L is required for spermatogonial stem cell self-renewal. Mice lacking DOT1L in the germ cells show a failure in the maintenance of spermatogonial stem cells without a block in spermatogenic cell differentiation and thus a progressive loss of germ cells, leading to a Sertoli-cell-only syndrome. Chemical inhibition of DOT1L in cultured stem cells reduces the spermatogonial stem cell activity after transplantation. RNA-seq analysis reveals downregulation of Hoxc cluster genes in DOT1L-inhibited spermatogonia stem cells. Single cell RNA-seq analysis demonstrates that inhibition of DOT1L sequesters spermatogonial stem cells in a primitive state and prevents them from transitioning to a progenitor state. These results identify a new function for DOT1L in adult stem cells and provides a paradigm for regulation of spermatogonial stem cell self-renewal. Self-renewal of spermatogonial stem cells is vital to life-long production of male gametes and thus fertility. However, the underlying mechanisms remain enigmatic. Here, we show that DOT1L, the sole H3K79 methyltransferase, is required for spermatogonial stem cell self-renewal. Mice lacking DOT1L fail to maintain spermatogonial stem cells, characterized by a sequential loss of germ cells from spermatogonia to spermatids and ultimately a Sertoli-cell-only syndrome. Inhibition of DOT1L reduces the stem cell activity after transplantation, prevents spermatogonial stem cells from transitioning to a progenitor state, and sequesters them in a primitive state. Furthermore, DOT1L promotes expression of the fate-determining HoxC transcription factors in spermatogonial stem cells. Our findings identify an essential function for DOT1L in adult stem cells and provide an epigenetic paradigm for regulation of spermatogonial stem cells. Overall design: Mouse testicular cells were prepared from 7-day-old ROSA mice, B6.129S7-Gt(ROSA)26Sor/J (Jackson Laboratory) that expresses a LacZ (ß-galactosidase) reporter gene in all tissues, by enzymatic digestion using trypsin-EDTA (Thermo Fisher Scientific) and cultured on SNL 76/7 cell feeders in a serum-free medium with 10 ng/mL GDNF (R&D Systems) and 0.5 ng/mL FGF2 (Corning). SSCs formed clumps and expanded continuously. After two months in culture, 1 x 10^5 clump-forming cells were seeded on SNL-feeder cells in 24-well plates with 2 µM EPZ5676 (Cayman Chemical) or DMSO (Sigma-Aldrich) as a vehicle control. EPZ5676 vs Control