HDAC9 (histone deacetylase 9; HGNC:14065, NCBI Gene ID 9734) is a human locus mapped to chromosome band 7p21.1 that encodes full-length Class IIa HDAC9 (1,011 aa) & catalytic-domain-lacking splice isoform MITR (MEF2-Interacting Transcription Repressor, 590 aa). HDAC9 acts in MEF2 transcriptional repression in cardiomyocytes/denervated muscle, FOXP3 deacetylation in Tregs & vascular smooth muscle calcification/atherosclerosis, together with HDAC5 (Synergistic Class IIa Cardiac Hypertrophy Brake Gene), HDAC4 (Class IIa Paralog Gene), and FOXP3 (Regulatory T-Cell Factor Repressed by HDAC9). Encodes Class IIa histone deacetylase 9 (and its splice isoform MITR), a key repressor of cardiac hypertrophy and major genetic risk locus for large-artery ischemic stroke.
MEF2 transcriptional repression in cardiomyocytes/denervated muscle, FOXP3 deacetylation in Tregs & vascular smooth muscle calcification/atherosclerosis
Clinical / Therapeutic Role
7p21.1 GWAS variants are the strongest genetic risk factor for large-vessel atherosclerotic ischemic stroke & Moyamoya disease
Gene Structure & Expression
Genomic Organization & Cytogenetic Locus: 25-exon mega-gene spanning >900 kb on chromosome 7p21.1 that undergoes extensive alternative splicing to produce full-length 1,011-residue HDAC9 as well as the abundant exon-12-truncated isoform MITR (MEF2-Interacting Transcription Repressor), which completely lacks the C-terminal pseudo-deacetylase domain yet potently represses MEF2 by recruiting CtBP and HP1.
Cellular & Tissue Expression Profile: Expressed at highest levels in cardiac myocytes, skeletal muscle fibers (rapidly down-regulated upon motor nerve denervation to allow acetylcholine receptor gene induction), vascular smooth muscle cells, macrophages, and FOXP3+ regulatory T lymphocytes.
Molecular Function & Pathways
Biochemical & Physiological Mechanism: In the heart, HDAC9/MITR acts alongside HDAC5 as a signal-responsive brake on MEF2: stress-activated CaMKII/PKD phosphorylates conserved N-terminal serines (Ser218, Ser448, Ser611) to export HDAC9 from the nucleus; in regulatory T cells, HDAC9 destabilizes FOXP3 acetylation; and in arterial wall smooth muscle cells and macrophages, elevated HDAC9 promotes osteogenic transdifferentiation (RUNX2 calcification), IKK-alpha/beta activation, and suppresses ABCA1/ABCG1 cholesterol efflux.
Clinical & Therapeutic Significance
Human Genetic Variants & Disease Phenotypes: Hdac9-knockout mice are hyper-responsive to calcineurin and pressure overload, developing massive cardiomegaly; in human population genetics, non-coding variants at the 7p21.1 HDAC9 locus (such as rs2107595, which disrupts E2F3/RB1-mediated repression of HDAC9 in macrophages and vascular smooth muscle) represent the single strongest genome-wide association for large-artery atherosclerotic ischemic stroke, carotid plaque instability, and myocardial infarction.
Therapeutic & Clinical Pharmacology: Selective inhibition of Class IIa HDAC9 (with TMP195) stabilizes atherosclerotic fibrous caps, reduces vascular inflammation, and expands immunosuppressive FOXP3+ regulatory T cells in preclinical stroke and colitis models.