Abstract B001: A mutant ASXL1-EHMT complex contributes to heterochromatin dysfunction in clonal hematopoiesis and chronic monomyelocytic leukemia

Abstract Background: ASXL1 is one of the three most frequently mutated genes in age-related clonal hematopoiesis (CH), with the others being DNMT3A and TET2. CH frequently progresses to myeloid malignancies including chronic monomyelocytic leukemia (CMML), and is also strongly associated with inflammatory cardiovascular disease and all-cause mortality in humans. DNMT3A and TET2 regulate DNA methylation and demethylation pathways respectively, and loss-of-function mutations of both genes reduce DNA methylation in heterochromatin. In contrast, the mechanisms that connect mutant ASXL1 and CH are not yet fully understood. ASXL1 is a regulatory component of the ASXL1-BAP1 deubiquitinase complex, which removes ubiquitin from mono-ubiquitinated H2A lysine 119 (H2AK119Ub). Recurrent frameshift and nonsense ASXL1 mutations in CH and CMML generate C-terminally truncated proteins that improve BAP1 enzymatic activity. Methods: To relate ASXL1 mutations to CH, we performed the protein immunoprecipitation followed by a mass spectrum to identify proteins interacting with mutant ASXL1. Utilizing a heterozygous Asxl1 mutant knock-in mouse model to recapitulate ASXL1-driven clonal hematopoiesis, we characterized alterations in histone modifications within expanded myeloid populations using CUT&RUN, with Drosophila S2 cells as spike-ins for normalization. Differential gene expression and transposable elements (TEs) activity were quantified in ASXL1-mutant chronic myelomonocytic leukemia (CMML) patients using RNA sequencing (RNA-seq) to assess the broader impact of ASXL1 mutations on transcriptional regulation. Results: 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 Asxl1 mutant mice 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 from ASXL1-mutant CMML patients compared to monocytes from healthy controls. 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. This increase in TE expression correlated with increased expression of nearby genes, including many interferon-inducible (inflammation-associated) genes (ISGs). Conclusion: Our findings provide new insights into how ASXL1 gain-of-function mutations might contribute mechanistically to the progression of CH and myeloid malignancies in aging. We show that even heterozygous expression of truncated ASXL1 proteins can lead to reduced levels of heterochromatin-associated histone modifications, weakening heterochromatin integrity and promoting increased expression of transposable elements that are normally silenced in heterochromatin. Citation Format: Zhen Dong, Hugo Sepulveda, Leo Josue Arteaga Vazquez, Mrinal Patnaik, Samuel A Myers, Anjana Rao. A mutant ASXL1-EHMT complex contributes to heterochromatin dysfunction in clonal hematopoiesis and chronic monomyelocytic leukemia [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Methylation, Clonal Hematopoiesis, and Cancer; 2025 Feb 1-4; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2025;85(3 Suppl):Abstract nr B001.