| JMJD2A (also known as KDM4A) is a JmjC‑domain lysine demethylase of the KDM4/JMJD2 family that targets transcriptionally important methyl marks on histones H3 and H1 to remodel chromatin in a context‑dependent manner, linking its catalytic and scaffolding activities to transcriptional repression, replication control, and lineage‑specific gene activation. The full-length protein contains an N‑terminal catalytic module composed of the JmjN and Fe(II)/2‑oxoglutarate–dependent JmjC domains, followed by a C‑terminal region with double PHD fingers and tandem Tudor domains that recognize methyl‑lysine marks and help position JMJD2A at specific chromatin sites, allowing coordinated substrate recognition and demethylation. JMJD2A demethylates trimethylated lysines on histone H3 at Lys9 and Lys36 and on histone H1.4 at Lys26, with high catalytic efficiency toward H3K9me3 and H1.4K26me3 and little or no activity on mono‑ or dimethylated forms, generating formaldehyde and succinate as reaction products and directly reversing repressive heterochromatic marks that recruit HP1 proteins. Demethylation of H3K9me3 by JMJD2A displaces HP1 from chromatin and has been associated with opening of compacted chromatin and earlier replication timing within normally late‑replicating, H3K9me3‑rich domains, indicating that JMJD2A not only shapes transcriptional potential but also modulates replication origin usage in silent regions by antagonizing HP1‑dependent chromatin organization. At selected promoters, JMJD2A also acts as a transcriptional corepressor: it associates in vivo with pRb, HDACs, and NCOR1 and is recruited to E2F‑responsive and ASCL2 promoters, where it promotes histone deacetylation and maintains H3K9 methylation, reinforcing a repressed chromatin state that constrains cell‑cycle gene expression and lineage‑specific transcription. Alternative splicing generates an N‑terminally truncated isoform (ΔN‑JMJD2A) that lacks the catalytic JmjC demethylase domain but retains C‑terminal recognition modules; this isoform accumulates during skeletal muscle differentiation, binds the Myog promoter at the onset of myogenesis, and is required for myotube formation and muscle gene expression, where its presence correlates with loss of H3K9me2/3 and transcriptional activation, consistent with a role in directing other demethylases and coactivators to remove repressive marks at myogenic loci. ΔN‑JMJD2A occupancy is enriched at genes involved in transcriptional control, and exon-specific siRNA knockdown targeting this isoform selectively impairs MyoD‑driven conversion of fibroblasts to muscle cells, reinforcing its function as a non‑catalytic, chromatin‑bound coactivator module embedded in the JMJD2A locus. Dysregulation of JMJD2A expression or activity alters global levels of H3K9me3/H3K36me3 and H1.4K26me3 and contributes to aberrant transcriptional programs, DNA replication stress, and genomic instability, consistent with its dual ability to erase heterochromatic marks, redistribute HP1, and partner with HDACs and pRb/NCOR1 complexes on growth‑regulatory promoters. |
