RNASEH2B

RNASEH2B is the non-catalytic accessory subunit of the heterotrimeric human RNase H2 endoribonuclease, functioning in ribonucleotide excision repair (RER) at sites of replication (PMID:19015152, PMID:35179959). It nucleates assembly of the active enzyme: RNASEH2B and RNASEH2C form a soluble B/C sub-complex that serves as the docking site for catalytic RNASEH2A, and its PIP-box motif confers PCNA binding to the holoenzyme (PMID:19015152). RNASEH2B is rate-limiting for assembly of the active trimer, and the stability of the entire complex—including RNASEH2A and RNASEH2C protein levels—depends on intact RNASEH2B [PMID:33981319, PMID:bio_10.1101_2024.12.16.628316]. Functionally, loss of RNASEH2B impairs RER and sensitizes cells to PARP inhibition through PARP trapping, a vulnerability that can be reversed by co-deletion of the neighboring 13q14 gene RB1 via E2F1-driven BRCA2 upregulation and restored homologous recombination (PMID:35179959). Increased RNase H2 levels restrain replication fork stalling caused by camptothecin, hydroxyurea, or oncogenic HRAS, linking the complex to replication stress tolerance [PMID:bio_10.1101_2024.12.16.628316]. RNASEH2B is a cause of Aicardi-Goutières syndrome; its disease-associated mutations act largely without abolishing catalytic activity, instead reducing protein expression and destabilizing the complex, and patient cells exhibit mitochondrial dysfunction and cytoplasmic mtDNA accumulation (PMID:19015152, PMID:36430958, PMID:33981319).

1. 2008 High

   Established RNASEH2B as a structural, non-catalytic subunit that nucleates RNase H2 assembly and recruits the complex to PCNA, and showed that AGS mutations do not act by abolishing enzymatic activity—reframing how these mutations cause disease.

   Evidence Biochemical reconstitution of the human RNase H2 trimer, in vitro activity assays, PCNA binding assays, and analysis of AGS-linked mutant proteins

   PMID:19015152

   Open questions at the time

   • Did not define the non-enzymatic mechanism by which RNASEH2B mutations cause disease
   • PIP-box contribution to in vivo fork recruitment not quantified
   • No structure of the assembled trimer reported in this study
2. 2021 Low

   Connected patient RNASEH2B mutations to destabilization of the whole complex, showing subunit interdependence whereby loss of RNASEH2B lowers RNASEH2A and depletes RNASEH2C protein.

   Evidence NGS, Western blot and RT-qPCR quantification in patient cells with computational structural modeling of destabilizing mutations

   PMID:33981319

   Open questions at the time

   • Structural predictions not validated by in vitro reconstitution or mutagenesis
   • Single patient case
   • Functional RER consequence not directly measured
3. 2022 High

   Placed RNASEH2B loss in a cancer therapeutic context, showing impaired RER sensitizes cells to PARP inhibition via PARP trapping and mapping a resistance mechanism through co-deleted RB1 and E2F1-driven BRCA2.

   Evidence Prostate cancer loss-of-function models, PARP inhibitor sensitivity assays, RB1/BRCA2 epistasis, and ATR inhibitor rescue experiments

   PMID:35179959

   Open questions at the time

   • Generality across other tumor types not established
   • Direct biochemical link between RER intermediates and PARP trapping not resolved
   • In vivo therapeutic validation limited
4. 2022 Medium

   Linked RNASEH2B mutation to mitochondrial dysfunction in AGS, with elevated ROS, 8-oxoGuanine, VDAC1, and cytoplasmic mtDNA implicating mtDNA release in disease pathogenesis.

   Evidence TEM, flow cytometry, Seahorse, immunofluorescence and mtDNA quantification in patient lymphoblastoid lines versus controls

   PMID:36430958

   Open questions at the time

   • No genetic rescue or reconstitution to confirm RNASEH2B as direct cause
   • Causal chain from RER defect to mtDNA release not demonstrated
   • Single-lab, patient-cell observations
5. 2023 Medium

   Defined a splicing mechanism of pathogenicity, showing an intronic variant causes intron retention, premature termination, and reduced RNASEH2B protein.

   Evidence Splice analysis, exome/genome sequencing, and mRNA expression quantification in patient blood

   PMID:36775013

   Open questions at the time

   • Single case
   • Downstream effect on complex assembly not measured
   • No functional RER readout
6. 2024 Medium

   Showed RNASEH2B is rate-limiting for active heterotrimer assembly and that increased RNase H2 levels limit replication fork stalling under genotoxic and oncogenic stress.

   Evidence Inducible overexpression, S9.6 RNA:DNA hybrid immunofluorescence, DNA fiber assays, and oncogene-induced replication stress models (preprint)

   PMID:bio_10.1101_2024.12.16.628316

   Open questions at the time

   • Not yet peer-reviewed
   • Paradoxical basal increase in RNA:DNA hybrids upon overexpression unexplained
   • Mechanism of fork protection beyond hybrid resolution unclear
7. 2025 Medium

   Extended the spectrum of splicing-based loss-of-function mechanisms, identifying a variant that creates a new splice site causing frameshift and protein truncation.

   Evidence RNA analysis of splice site usage and next-generation sequencing in a single case

   PMID:40750230

   Open questions at the time

   • Single case
   • Effect on complex stability and RER not directly assessed
   • No cellular phenotype reported

• How RER failure mechanistically drives innate immune activation and mtDNA release in AGS, and whether the PARP-trapping vulnerability generalizes as a therapeutic strategy, remain open.
• Causal pathway from nucleic-acid intermediates to immune signaling not reconstituted
• No structure of the AGS-mutant trimer linking destabilization to phenotype
• Therapeutic generalizability beyond prostate cancer untested