The BOD1L subunit of the mammalian SETD1A complex sustains the expression of DNA damage repair genes despite restraining H3K4 trimethylation

SETD1A is the histone 3 lysine 4 (H3K4) methyltransferase central to the mammalian version of the highly conserved eight subunit Set1 complex (Set1C) that apparently conveys H3K4 trimethylation (H3K4me3) onto all active Pol II promoters. Accordingly, the Setd1a mouse knock-out dies in early embryogenesis at the epiblast stage and mouse embryonic stem cells (ESCs) die when SETD1A is removed. We report that ESC death is accompanied by loss of expression of DNA repair genes and accumulating DNA damage. BOD1L and BOD1 are homologs of the yeast Set1C subunit, Shg1, and subunits of the mammalian SETD1A and B complexes. We show that the Shg1 homology region binds to a highly conserved central alpha-helix in SETD1A and B. Like mutagenesis of Shg1 in yeast, conditional mutagenesis of Bod1l in ESCs promoted increased H3K4 di- and tri-methylation but also, like loss of SETD1A, loss of expression of DNA repair genes, increased DNA damage and cell death. In contrast to similar losses of DNA repair gene expression, the converse changes in H3K4 methylation after loss of SETD1A or BOD1L implies that H3K4 methylation is not essential for expression of the target DNA repair genes. Because BOD1L becomes highly phosphorylated after DNA damage and acts to protect damaged replication forks, the SETD1A complex, and BOD1L in particular, are key nodes for the DNA damage repair network.