A Role for the Histone Chaperone Hira in Muscle Hypertrophy, Cellular Stress Responses, and Developmental Gene Expression

Chromatin modifications play a pivotal role in regulating gene expression. The deposition of histone variants by histone chaperones into the nucleosome plays a large role in influencing gene expression. Histone incorporation can be separated into two categories: replication-coupled and replication independent. The histone chaperone HIRA deposits the variant histone H3.3 into promoters and gene bodies of active genes in a replication-independent manner. HIRA is also responsible for H3.3 deposition into the “bivalent” promoters of regulatory genes in embryonic stem cells, required for transcription restart after DNA repair, and required for the formation of senescence associated heterochromatin foci. Hira null mutation in mice resulted in embryonic lethality by E11 which largely resulted from gastrulation defects including abnormalities in the heart. Cardio- and skeletal myocytes are post mitotic cells and thus the majority of chromatin remodeling should be performed in a replication-independent manner. Additionally, skeletal myocytes must alter gene expression in response to physiological signals. Because of this we hypothesized that HIRA is likely to play a large role in epigenetically regulating gene expression in myocytes. The objective of this study was to determine the consequence of HIRA ablation in cardio- and skeletal myocytes in vivo. We accomplished this by using Myf6-cre to delete Hira from myofibers, and αMHC-cre for cardiomyocytes, both of which remove Hira after these cells have terminally differentiated. Both HIRA CKO cardio- and skeletal myocytes exhibited hypertrophy, sarcolemmal damage, upregulation of fetal/developmental genes, and downregulation of genes associated with responses to cellular stresses and DNA damage. This resulted in focal replacement fibrosis and altered cardiac function in the heart, while mice lacking HIRA from myofibers exhibited decreased body weight, increased lean mass, increased grip strength and endurance, increased abundance of type I fibers, and centralized nuclei. Comparative analysis of gene expression sets suggest that loss of HIRA impaired transcriptional response to cellular stresses. The major discrepancies in these phenotypes can largely be attributed to each tissues mode of regeneration in which cardiomyocytes lack regenerative potential while skeletal myocytes can rapidly regenerate damage. Thus HIRA is an important factor in epigenetically maintaining myocyte homeostasis.