THUMPD2

THUMPD2 is an S-adenosylmethionine (SAM)-dependent methyltransferase that, in complex with the co-factor TRMT112, installs the N2-methylguanosine (m2G) modification at position G72 of U6 snRNA — a residue located within the catalytic center of the spliceosome (PMID:37283053, PMID:38165050). TRMT112 stabilizes THUMPD2 protein levels and is required for its activity, and THUMPD2 recognizes U6-specific sequences and structural elements to achieve substrate specificity (PMID:34948388, PMID:38165050). Genetic ablation of THUMPD2 eliminates U6 m2G72, causing thousands of alternative splicing changes and activation of nonsense-mediated mRNA decay, and leads to retinal degeneration in vivo, linking loss of this modification to age-related macular degeneration (PMID:38165050).

1. 2001 Low

   Before any function was known, initial cloning revealed that THUMPD2 (then C2orf8) encodes a protein with three conserved SAM-dependent methyltransferase motifs, establishing it as a candidate RNA or protein methyltransferase.

   Evidence FISH localization, RACE, RT-PCR, Northern blot, and bioinformatic ORF analysis in human cells

   PMID:12063391

   Open questions at the time

   • No enzymatic activity demonstrated; motifs alone do not confirm function
   • Substrate identity unknown
   • No cellular phenotype assessed
2. 2021 Medium

   Identification of THUMPD2 as a TRMT112 interaction partner, and demonstration that TRMT112 stabilizes THUMPD2 protein, placed THUMPD2 within the family of TRMT112-dependent methyltransferases and suggested it requires a co-factor for activity.

   Evidence SILAC-based affinity pulldown of TRMT112 interactors; co-expression stability assays and TRMT112 surface mutagenesis in human cells

   PMID:34948388

   Open questions at the time

   • Catalytic activity of THUMPD2 not directly demonstrated
   • RNA substrate not identified
   • Biological consequence of disruption unknown
3. 2023 High

   Direct demonstration that THUMPD2 is an active m2G methyltransferase that physically associates with U6 snRNA and requires TRMT112 answered the central question of substrate identity and enzymatic activity.

   Evidence TRMT112 interaction network mapping in intact cells; biochemical m2G methyltransferase activity assay; direct THUMPD2–U6 snRNA association

   PMID:37283053

   Open questions at the time

   • Precise target nucleotide on U6 not yet pinpointed in this study
   • Splicing consequences not yet characterized genome-wide
   • Structural basis of U6 recognition unknown
4. 2024 High

   Knockout studies pinpointed position G72 of U6 snRNA as the THUMPD2-dependent m2G site and revealed that loss of this single modification causes widespread alternative splicing defects, NMD activation, and retinal degeneration, establishing the physiological importance of U6 m2G72.

   Evidence THUMPD2 genetic knockout; mass spectrometry RNA modification mapping; transcriptome-wide splicing analysis; NMD pathway activation assay; in vivo retinal phenotyping

   PMID:38165050

   Open questions at the time

   • Structural basis for THUMPD2 recognition of U6-specific sequences undetermined
   • Whether retinal degeneration results from specific mis-spliced transcripts or global splicing dysfunction is unresolved
   • Potential additional RNA substrates beyond U6 not excluded

• Key open questions include the structural mechanism by which THUMPD2–TRMT112 recognizes U6 snRNA, whether THUMPD2 modifies additional RNA targets, and which specific mis-spliced transcripts drive retinal degeneration.
• No crystal or cryo-EM structure of THUMPD2–TRMT112–U6 complex
• Comprehensive substrate profiling beyond U6 snRNA not performed
• Causal link between individual mis-spliced mRNAs and retinal disease phenotype not established