A mutant ASXL1-EHMT complex contributes to heterochromatin dysfunction in clonal hematopoiesis and chronic monomyelocytic leukemia

Abstract

ASXL1is one of the three most frequently mutated genes in age-related clonal hematopoiesis (CH), with the others beingDNMT3AandTET21,2. CH can progress to myeloid malignancies including chronic monomyelocytic leukemia (CMML), and is also strongly associated with inflammatory cardiovascular disease and all-cause mortality in humans3,4,5. DNMT3A and TET2 regulate DNA methylation and demethylation pathways respectively6,7, andDNMT3AandTET2loss-of-function mutations in CH reduce DNA methylation in heterochromatin, allowing de-repression of silenced elements in heterochromatin8,9,10. In contrast, the mechanisms that connect mutant ASXL1 and CH are not yet fully understood. CH/CMML-associatedASXL1mutations encode C-terminally truncated proteins that enhance the deubiquitinase activity of the ASXL-BAP1 “PR-DUB” deubiquitinase complex, which removes mono-ubiquitin from H2AK119Ub11,12,13. Here we show that ASXL1 mutant proteins interact with the EHMT1-EHMT2 methyltransferase complex, which generates H3K9me1 and me2, the latter a repressive modification in constitutive heterochromatin. Compared to cells from age-matched wildtype mice, we found that expanded myeloid cells from old (>18-month-old)Asxl1tm/+mice14, a heterozygous knock-in mouse model of CH, display genome-wide decreases of H3K9me2, H3K9me3 and H2AK119Ub as well as an associated increase in expression of transposable elements (TEs) and satellite repeats. Increased TE expression was also observed in monocytes fromASXL1-mutant CMML patients compared to monocytes from healthy control individuals. Our data suggest that mutant ASXL1 proteins compromise the integrity of both constitutive and facultative heterochromatin in an age-dependent manner, by reducing the levels of H3K9me2/3 and H2AK119Ub respectively. The resulting increase in TE expression can alter the expression of nearby genes and promote the expression of inflammation-associated and interferon-inducible genes (ISGs).