METTL23

METTL23 is an SAM-dependent protein arginine methyltransferase that catalyzes asymmetric dimethylation of histone H3 at arginine 17 (H3R17me2a), functioning in chromatin regulation during zygotic reprogramming and in retinal neuronal homeostasis (PMID:28930672, PMID:36099048). In oocytes, METTL23-mediated H3R17me2a is required for histone H3.3 incorporation into the male pronucleus and recruits the GSE–Tet3 complex to paternal chromatin, enabling active DNA demethylation via 5-hydroxymethylcytosine accumulation (PMID:28930672). In retinal ganglion cells, loss of METTL23 reduces H3R17 dimethylation, impairs pS2 transcription, and derepresses NF-κB-driven neuroinflammatory signaling (TNF-α, IL-1β), leading to ganglion cell death and a glaucoma phenotype (PMID:36099048). METTL23 also physically interacts with the transcription factor GABPA and co-regulates its transcriptional targets, linking its methyltransferase activity to broader transcriptional control (PMID:24501276).

1. 2014 Medium

   Establishing that METTL23 functions as a nuclear transcriptional co-regulator resolved its initial cellular role: interaction with GABPA and modulation of GABPA-dependent promoters showed METTL23 participates directly in gene regulation rather than acting solely as a structural or metabolic enzyme.

   Evidence Co-immunoprecipitation with GABPA, overexpression/siRNA knockdown with THPO reporter and ATP5B expression assays in human cells

   PMID:24501276

   Open questions at the time

   • Whether the GABPA interaction depends on METTL23 catalytic activity was not tested
   • No genome-wide identification of METTL23-GABPA co-regulated targets
   • Reciprocal validation of the GABPA interaction with endogenous proteins not shown
2. 2014 Low

   Demonstrating that disease-associated truncating mutations disrupt METTL23 localization and its predicted SAM-binding domain supported, but did not prove, an enzymatic methyltransferase function.

   Evidence Frameshift/nonsense mutation analysis with cellular localization imaging and 3D structural modelling

   PMID:24626631

   Open questions at the time

   • No direct enzymatic assay was performed; catalytic activity remained computationally predicted
   • Putative heat-shock-protein substrates were identified only by expression correlation, not by biochemical demonstration
   • Single study without independent replication
3. 2017 Medium

   Ruling out eEF1A as a substrate narrowed the search for METTL23's bona fide targets and distinguished it from related methyltransferases that act on translation factors.

   Evidence CRISPR/Cas9 knockout of METTL23 in K562 cells followed by targeted mass spectrometry of eEF1A methylation

   PMID:28663172

   Open questions at the time

   • Only one candidate substrate was excluded; systematic substrate identification was not performed
   • Single cell line tested
4. 2017 High

   Identifying METTL23 as a histone H3R17 asymmetric dimethyltransferase and showing this mark is essential for paternal pronuclear H3.3 incorporation and GSE–Tet3-mediated DNA demethylation established the enzyme's catalytic specificity and its biological role in zygotic epigenetic reprogramming.

   Evidence In vitro methyltransferase assay, PRMT inhibitor (TBBD) treatment, oocyte depletion with H3.3/5hmC immunofluorescence, co-IP of METTL23–GSE–Tet3 complex in mouse oocytes

   PMID:28930672

   Open questions at the time

   • Whether METTL23 has additional non-histone substrates in oocytes was not addressed
   • Structural basis of METTL23 specificity for H3R17 over other arginine residues is unknown
   • Mechanism by which H3R17me2a promotes H3.3 incorporation is not defined
5. 2022 High

   Demonstrating that METTL23 loss in retinal ganglion cells reduces H3R17me2a, silences pS2, and derepresses NF-κB-driven neuroinflammation leading to glaucoma extended the enzyme's physiological significance beyond early development to adult neuronal survival.

   Evidence Mettl23 knock-in and knockout mouse models with histone methylation assays, gene expression profiling, and NF-κB pathway activity measurement in retina

   PMID:36099048

   Open questions at the time

   • Direct chromatin occupancy of METTL23 at the pS2 or NF-κB target loci has not been shown
   • Whether the glaucoma phenotype is cell-autonomous to retinal ganglion cells versus involving glia was not fully resolved
   • Therapeutic rescue of the phenotype by restoring METTL23 activity was not demonstrated

• A comprehensive substrate map for METTL23 beyond histone H3R17 is lacking, and the structural basis for its arginine specificity, its relationship to other PRMTs, and whether its interaction with GABPA requires catalytic activity remain unresolved.
• No crystal structure or cryo-EM structure of METTL23 is available
• Genome-wide chromatin profiling of H3R17me2a deposited by METTL23 has not been performed
• Whether METTL23 catalytic activity and GABPA co-regulatory function are mechanistically linked is untested