Functional examination of the H3K4 histone methyltransferases setd1b and Mll2 in myeloid neoplasia

Abstract

The enzymes that methylate histone tails are central to epigenetic regulation and frequently mutated in many cancers. All expressed genes are trimethylated at histone 3 lysine 4 (H3K4me3) on promoter nucleosomes. The responsible enzymatic machinery belongs to the trithorax group of proteins. We have made conditional mouse lines for all six H3K4 methyltransferases and are systematically examining their roles during development, in the germline and in the adult. These six include the major leukemogenic factor Mll1, its sister gene Mll2, and Setd1b. Here, we present work based on our recent findings that the loss of Setd1b in adult mice provokes myeloid neoplasia with features of human chronic myelomonocytic leukemia. Upon conditional mutagenesis induced by tamoxifen mice die within 30 weeks due to severe bone marrow failure. This phenotype is reflected in dysplastic morphology and altered counts of circulating blood cells including anemia, thrombo- and lymphocytopenia as well as increased numbers of neutrophil granulocytes, monocytes and immature blasts. The analysis of hematopoietic stem cells (HSCs) in the bone marrow of knockout mice unveils an enriched population of short-term HSCs (Lin- Sca1+ c-Kit+ CD34+ Flt3-) and skewed differentiation towards myelopoiesis that is evident from a gain in granulocyte monocyte progenitors (Lin- Sca1- c-Kit+ CD34+ CD16/32+). Consistent with extramedullary hematopoiesis knockout mice develop extensive splenomegaly with an enriched HSC and myeloid (CD11b+ Gr-1+) compartment. In ongoing bone marrow transplantation studies the apparent neoplastic phenotype is similarly reproduced and clearly indicates an intrinsic requirement of Setd1b in hematopoietic stem and progenitor cells. In contrast to Setd1b, conditional deletion of Mll2 leads to milder but still significant hematopoietic deficiencies and therefore implies specific and non-redundant roles of the different H3K4 methyltransferases in the mouse. Our findings, together with the known role of Mll1 in the maintenance of HSCs, emphasize the relevance of elucidating the roles of epigenetic players in hematopoiesis and the mechanisms underlying oncogenic transformations.