Maintenance of adult cardiac function requires the chromatin factor Asxl2

Abstract

During development and differentiation, cell type-specific chromatin configurations are set up to facilitate cell type-specific gene expression. Defects in the establishment or the maintenance of the correct chromatin configuration have been associated with diseases ranging from leukemia to muscular dystrophy. The heart expresses many chromatin factors, and we are only beginning to understand their roles in heart development and function. We have previously shown that the chromatin regulator Asxl2 is highly expressed in the murine heart both during development and adulthood. In the absence of Asxl2, there is a significant reduction in trimethylation of histone H3 lysine 27 (H3K27), a histone mark associated with lineage-specific silencing of developmental genes. Here we present evidence that Asxl2 is required for the long-term maintenance of ventricular function and for the maintenance of normal cardiac gene expression. Asxl2−/− hearts displayed progressive deterioration of ventricular function. By 10 months of age, there was ~37% reduction in fractional shortening in Asxl2−/− hearts compared to wild-type. Analysis of the expression of myofibril proteins suggests that Asxl2 is required for the repression of βMHC. Asxl2−/− hearts did not exhibit hypertrophy, suggesting that the de-repression of β-MHC was not the result of hypertrophic response. Instead, Asxl2 and the histone methyltansferase Ezh2 co-localize to β-MHC promoter, suggesting that Asxl2 directly represses β-MHC. Interrogation of the CardioGenomics database revealed that ASXL2 is down-regulated in the hearts of patients with ischemic or idiopathic dilated cardiomyopathy. We propose that chromatin factors like Asxl2 function in the adult heart to regulate cell type- and stage-specific patterns of gene expression, and the disruption of such regulation may be involved in the etiology and/or development of certain forms of human heart disease.

Loss of Asxl2 leads to myeloid malignancies in mice

“…Recently, Lai et al 8. showed that deletion of Asxl2 led to reduced global H3K27me3 levels in heart cells.…”

“…Asxl1 loss leads to the development of myeloid malignancies in mice, which is associated with dysregulation of H3K27me3 (refs 6, 7). Lai et al 8. previously reported that ASXL2 regulates gene expression in the heart at cell-type and stage-specific manners.…”

Additional Sex Combs-Like 2 Is Required for Polycomb Repressive Complex 2 Binding at Select Targets

“…To confirm that murine ASXL2 is also associated with chromatin, we expressed FLAG-tagged ASXL2 in HEK293 cells and used biochemical fractionation [20] to separate chromatin-associated proteins from soluble nuclear proteins. Probing the fractions with either the anti-ASXL2 antibody KC17 [21] or with anti-FLAG antibody M2 (Sigma) detected ASXL2 predominantly in the chromatin fraction (Figure 1A). Similar results were obtained with endogenous ASXL2 in murine heart tissue (Figure 1B).…”

Interplay of chromatin modifications and non-coding RNAs in the heart

“…9 In addition, recent studies have expanded the complexity of regulatory determinants that participate in cardiac gene function, for example histone modifying proteins such as EZH2 and ASXL2 specify MHC gene expression in postnatal cardiac homeostasis. 10,11 Human homologs of Drosophila genes (Enhancer of zeste homolog 2 and Additional sex combs-like protein 2) EZH2 and ASXL2 are members of the Polycomb group (PcG) protein family implicated in maintaining gene repressive states by chromatin modification during later stages of heart development.…”

“…1). 11,63 DNase hypersensitive sites are also associated with MHC gene expression at various developmental stages of the heart. 64 Whatever the role of EZH2, showing its involvement in chromatin dependent association with ncRNA is the first step in revealing how MHC isoform expression is regulated in heart disease.…”