MLL3 Deletion Enhances HSC Self-Renewal

Mixed lineage leukemia 3 ( MLL3 ) is one of the most commonly mutated genes in human cancer, and it is believed to function as a tumor suppressor. It is one of many genes affected as a result of mono-allelic 7q deletions that occur commonly in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). MLL3 encodes a histone methyltransferase that catalyzes the monomethylation of Histone 3 lysine 4 (H3K4), a known feature of enhancer elements. MLL3 also nucleates a COMPASS-like complex that removes repressive H3K27me3 marks, recruits the histone acetyltransferase p300/CBP to enhancers and promotes transcript elongation at promoters. For these reasons, MLL3 binding at cis-regulatory elements is thought to potentiate enhancer activity and transcription. Previous studies have shown that Mll3 deficiency causes a mild myeloproliferative disorder in mice (PLoS ONE 11:e0162515), and shRNA knockdown of Mll3 has been shown to cooperate with loss of Nf1 and p53 to promote AML (Cancer Cell 25:652). In the latter study, Mll3 silencing was associated with a hematopoietic stem cell (HSC)-like gene expression signature within the AML cells. These studies implicate MLL3 as a hematopoietic tumor suppressor, though its precise function in HSC self-renewal and lineage commitment has not been described.

To better understand how Mll3 regulates HSC self-renewal, we developed a novel loss-of-function mouse allele in which a 5-base pair deletion and frameshift mutation was created near the N-terminus of the protein. Homozygous Mll3^-/- mice are viable through most of gestation, but they die just prior to birth. Heterozygous Mll3^+/- mice are born at normal Mendelian ratios, their development is normal after birth and they are fertile. To test whether Mll3 regulates hematopoiesis, we determined the frequencies and absolute numbers of HSCs, lineage restricted hematopoietic progenitors (HPCs), and granulocyte-monocyte progenitors (GMPs) in control, Mll3^+/- and Mll3^-/- fetal livers (E18.5) and control and Mll3^+/- adult bone marrow. Heterozygous and homozygous Mll3 deletion caused a modest increase in HSC numbers and a modest decrease in HPC frequency in fetal livers. Mll3 heterozygosity resulted in a significant expansion of the HSC population in adult bone marrow. To test whether these increases in HSC numbers reflected enhanced self-renewal, we performed serial transplantation assays with fetal and adult HSCs. While primary transplants showed only a modest increase in repopulating activity in Mll3^+/- and Mll3^-/- HSCs, this effect was greatly enhanced in secondary transplants. Thus, Mll3 deficiency enhances HSC self-renewal capacity. Despite this phenotype, RNAseq showed no differences in the transcriptomes of wild type and Mll3^+/- HSCs or HPCs. This suggests that Mll3 loss may reduce the probability of lineage commitment during successive HSC divisions without altering the commitment pathways themselves. Ongoing experiments will test this hypothesis. Enhanced self-renewal, particularly under conditions of hematopoietic stress, may convey a selective advantage to Mll3 -heterozygous HSCs and enhance progression to MDS.