H4C1

H4C1 encodes a core histone H4 protein involved in chromatin packaging, with the available corpus characterizing it only through disease-associated profiling rather than direct biochemical or structural study. Mass spectrometry of diabetic cardiomyopathy heart tissue identified H4C1 as a target of lysine lactylation, with elevated modification at the K32 site linking histone H4 lactylation to glucose-metabolism-associated chromatin changes (PMID:42029413). In autoimmune inflammatory myopathy, H4C1 is a canonical autoantibody target, and anti-H4C1 immune complexes carrying associated RNAs activate TLR7/8 signaling in PBMCs to drive interferon-α and IL-6 production (PMID:39279150). A CRISPR loss-of-function screen placed H4C1 as a negative regulator of cancer stemness in triple-negative breast cancer, where its deletion enhanced paclitaxel resistance (PMID:37932309), and proteomic profiling found H4C1 enriched in circulating plasma extracellular vesicles during advanced hepatic steatosis (PMID:41301514). No direct enzymatic mechanism, structural model, or biochemical reconstitution of H4C1 itself has been characterized in the available corpus.

1. 2019 Low

   Whether histone H4 carries metabolically driven post-translational marks in cardiac disease was unknown; lactylome profiling established H4C1 as a lactylation substrate at K32 in diabetic cardiomyopathy.

   Evidence LC-MS/MS lactylome-proteome analysis of db/db versus db/m mouse heart tissue with bioinformatic pathway mapping

   PMID:42029413

   Open questions at the time

   • No writer, eraser, or reader of H4-K32 lactylation identified
   • No functional validation that K32 lactylation alters chromatin or transcription
   • Single proteomics dataset without orthogonal confirmation
2. 2023 Low

   The functional role of H4C1 in tumor cell plasticity was unknown; a CRISPR screen showed its loss promotes cancer stemness and chemoresistance, placing it as a negative regulator of stemness in TNBC.

   Evidence Genome-wide CRISPR synthetic lethality screen in TNBC cell lines and orthotopic models with paclitaxel resistance readout

   PMID:37932309

   Open questions at the time

   • No molecular mechanism linking H4C1 loss to stemness
   • No pathway placement for H4C1 in chemoresistance
   • Single lab, no mechanistic follow-up
3. 2024 Low

   Whether histone H4 contributes to inflammatory signaling beyond chromatin was unclear; autoantibody profiling showed anti-H4C1 immune complexes activate TLR7/8 to produce inflammatory cytokines.

   Evidence Autoantibody array profiling of IIM patient IgG plus TLR7/8 activation assays in healthy donor PBMCs

   PMID:39279150

   Open questions at the time

   • Mechanism is indirect via autoantibody-RNA complexes, not H4C1 itself
   • No biochemical reconstitution of the immune complex activity
   • Causal contribution of H4C1 versus bound RNA not separated
4. 2025 Low

   Whether H4C1 is exported in circulating vesicles during liver disease was unknown; EV proteomics showed elevated H4C1 in plasma EVs with advancing hepatic steatosis.

   Evidence Mass spectrometry proteomics of plasma-derived EVs from 70 MASLD patients with imaging correlation

   PMID:41301514

   Open questions at the time

   • No mechanism for selective EV packaging of H4C1
   • Functional role in steatosis progression untested
   • Single preliminary cohort study

• No direct biochemical, structural, or enzymatic study of H4C1 establishes how its modifications or chromatin function mechanistically drive the disease associations observed.
• No structural model or reconstitution of H4C1
• No identified enzymes acting on H4C1 modification sites
• No mechanistic link between chromatin function and the reported phenotypes