Abstract 134: Reciprocal Cardiac Chromatin Regulation by Ctcf and Hmgb2

Abstract

Listen

Translate article

Chromatin remodeling plays an essential role in cardiac gene reprogramming under pathological conditions. However, we currently lack a detailed understanding of how this process is structurally coordinated across the genome. Previous RNA and proteomic analyses of an in vivo model of heart failure performed by our group identified the chromatin structural proteins CTCF and high mobility group protein B2 (HMGB2) as regulators of heart disease. CTCF is an essential factor in the maintenance of global chromatin organization through the facilitation of long-range interactions and chromatin looping. HMGB2 is a non-nucleosomal structural protein that packages nucleosomes into higher order structures. An unbiased genomic analysis of ~100 strains of mice treated with isoproterenol (ISO) followed by transcriptome and phenotype analyses revealed a general down-regulation of CTCF at the mRNA level across the different strains after ISO treatment, whereas the HMGB2 response was strongly influenced by common genetic variation (i.e. between strains). ChIP-seq data, ChIP-reChIP-PCR experiments (detection of concomitant presence of two proteins at a given locus) and immunofluorescence showed CTCF and HMGB2 bind the same regions of the genome, yet they do not physically co-localize in myocyte nuclei. Loss of HMGB2, which induces hypertrophy in myocytes, causes de-condensation of chromatin, as directly measured by micrococcal nuclease digestion; CTCF knockdown has no effect on global chromatin accessibility by this assay. Examination of other chromatin structural proteins like histone H1 suggests this interaction between CTCF and HMGB2 is specific. Lastly, we showed that CTCF and HMGB2 regulate each other’s expression as well as the transcription of cardiac hypertrophy genes and ribosomal RNA. Our data show that basal HMGB2 (but not CTCF) expression correlates with cardiac mass. The response of HMGB2 levels to heart failure stimuli is highly influenced by genetics, whereas the decrease in CTCF following (ISO) is unaffected by genetic variation. These findings reveal molecular mechanisms of chromatin compaction in cardiac cells, linking them with genetic variability, and revealing a reciprocal relationship between HMGB2 and CTCF in cardiac hypertrophy.