Mutated H3 Histones Drive Human Pre-Leukemic Hematopoietic Stem Cell Expansion And Promote Leukemic Aggressiveness

Leukemogenesis is a stepwise progression from mutated, pre-neoplastic hematopoietic stem cells (HSCs) to full-blown leukemia. Our ability to prevent or treat de novo and secondary acute myeloid leukemia (AML) is limited by our incomplete understanding of the epigenetic disruption that is central to this process, including improper histone methylation. We performed a comprehensive analysis of 16 histone H3 genes in 434 primary acute myeloid leukemia (AML) samples and identified mutations in adult and pediatric cases (1.6%), with a higher incidence in secondary AML (s-AML) (9%). These included four novel amino acid substitutions (Q69H, A26P, R2Q and R8H) as well as K27M and K27I in H3.1 and H3.3 genes. These mutations are important early events in leukemogenesis as they were observed in pre-leukemic HSCs in two cases and were in the major clones in every sample. Consistent with a role in pre-leukemic HSC clonal expansion, the mutant histones increased functional human HSC frequency and altered differentiation along the erythroid and myeloid lineages, with activity dependent on the specific mutation (K27M, K27I and Q69H). In established human leukemia, the K27M/I mutant histones amplified leukemic aggressiveness, with increased proliferation, expansion of leukemic progenitor and blast cells, and superior competitiveness in vivo. This was associated with increased expression in genes involved in erythrocyte and myeloid differentiation, correlated with a corresponding decrease in histone H3 K27 tri-methylation and increase in K27 acetylation. While histone mutations can co-occur with alterations in RUNX1, we observed that the functional impact of histone mutations is independent of RUNX1 mutations. Taken together, these data establish the involvement of H3 mutations as early drivers of pre-leukemic HSC expansion and leukemogenesis.