REXO4

REXO4 (hPMC2/XPMC2H) is a 3'-5' non-processive exonuclease that maintains genome stability by degrading single- and double-stranded nucleic acid substrates and resolving R-loops (PMID:21602889, PMID:41706852). Its catalytic activity depends on two conserved C-terminal carboxylate residues, and it degrades the RNA strand of R-loops from ends or internal nicks while stimulating RNaseH1 endonuclease activity, establishing an endo/exo-cleavage coupling mode of R-loop processing; REXO4-regulated R-loop regions genome-wide overlap with those controlled by RNaseH1, and REXO4 overexpression counteracts R-loop accumulation caused by RNaseH1 deficiency (PMID:21602889, PMID:41706852). Through this nuclease function REXO4 also participates in DNA damage repair, being required to repair estrogen-induced abasic sites and alkylating-agent-induced lesions, with its loss increasing AP sites and γ-H2AX and sensitizing cells to temozolomide and BCNU (PMID:21602889, PMID:25849309). In an estrogen-signaling context, REXO4 constitutively interacts with ERβ and binds the electrophile/antioxidant response element (EpRE) of antioxidative enzyme genes such as quinone reductase, where its exonuclease activity drives strand-break formation and tamoxifen-dependent co-recruitment of the coactivators PARP-1 and Topoisomerase IIβ to induce gene expression (PMID:10908561, PMID:18663360, PMID:21602889). The REXO4 promoter is itself bound by p53 (PMID:16980608). During interphase REXO4 localizes to the nucleolus and associates with ribosome and rRNA-processing factors, consistent with a role in ribosome biogenesis, and its depletion causes G1/S cell cycle arrest (PMID:39829749). Patient-derived tumor mutations in the enzymatic region impair R-loop cleavage, and a small-molecule inhibitor (REXO4-IN-17) blocks its nuclease activity, with REXO4 interference activating cGAS-mediated antitumor immunity (PMID:41706852).

1. 2000 Medium

   Established REXO4/hPMC2 as a sequence-specific factor at the antioxidant response element and a partner of estrogen receptor beta, linking it to antioxidative gene regulation.

   Evidence Yeast two-hybrid, in vitro binding, and reporter assays for EpRE and ERβ interaction

   PMID:10908561

   Open questions at the time

   • No biochemical activity for the protein itself was defined
   • Mechanism of EpRE binding specificity unresolved
2. 2006 Medium

   Showed the REXO4 gene is a transcriptional target of p53, connecting its expression to stress and DNA-damage signaling.

   Evidence ChIP-chip with gene-specific PCR validation of p53 binding at the XPMC2H promoter CpG island

   PMID:16980608

   Open questions at the time

   • Functional consequence of p53 binding on REXO4 expression not quantified
   • Sequence-independent binding mechanism unexplained
3. 2008 High

   Defined REXO4 as a constitutive ERβ partner required to assemble coactivators at antioxidative gene promoters, giving it a concrete role in inducible cytoprotection.

   Evidence Reciprocal Co-IP, ChIP, and siRNA knockdown in breast epithelial lines with defined PARP-1/TopoIIβ recruitment phenotype

   PMID:18663360

   Open questions at the time

   • Did not yet identify the enzymatic basis of REXO4's contribution
   • How strand breaks couple to coactivator recruitment unresolved
4. 2011 High

   Identified REXO4 as an intrinsic 3'-5' non-processive exonuclease and tied this catalytic activity to gene induction, strand-break formation, and abasic-site repair.

   Evidence In vitro kinetic exonuclease assays with catalytic-residue mutagenesis plus ChIP, western blot, and abasic-site repair assays

   PMID:21602889

   Open questions at the time

   • Physiological nucleic-acid substrates beyond reporter contexts not defined
   • Role of Nrf2 in directing specificity mechanistically incomplete
5. 2015 Medium

   Extended REXO4's repair role to alkylating-agent damage, providing a rationale for targeting it to sensitize tumor cells.

   Evidence siRNA knockdown with viability, AP-site, γ-H2AX, and comet assays in triple-negative breast cancer cells

   PMID:25849309

   Open questions at the time

   • Direct repair substrate at alkylated lesions not biochemically defined
   • Pathway placement relative to known BER/DSB repair unclear
6. 2025 Medium

   Placed REXO4 in the nucleolus and ribosome-biogenesis machinery during interphase and at the perichromosomal layer in mitosis, broadening its cellular role beyond DNA repair.

   Evidence Live-cell imaging, NLS deletion mutagenesis, depletion with cell-cycle flow cytometry, and Co-IP/MS interactome (preprint)

   PMID:39829749

   Open questions at the time

   • Direct rRNA-processing substrate not demonstrated
   • Mechanism linking nucleolar role to G1/S arrest unresolved
   • Preprint, not peer-reviewed
7. 2025 Low

   Identified REXO4 as a stress-granule component with nucleocytoplasmic shuttling, hinting at a cytoplasmic stress-response role.

   Evidence Antibody-guided proximity-labeling proteomics under heat shock in HeLa and RAW264.7 cells

   PMID:40198209

   Open questions at the time

   • No functional validation of REXO4's role in stress granules
   • Single proximity-labeling dataset without orthogonal confirmation
8. 2025 Medium

   Positioned REXO4 downstream of DDX18 in a tumor-promoting axis affecting EMT and MAPK signaling in hepatocellular carcinoma.

   Evidence Reciprocal Co-IP, colocalization, overexpression rescue, and nude mouse xenograft

   PMID:40950356

   Open questions at the time

   • Molecular mechanism connecting REXO4 to MAPK signaling unresolved
   • Whether the effect requires REXO4 nuclease activity not tested
9. 2026 High

   Established REXO4 as an R-loop-resolving nuclease that couples with RNaseH1 genome-wide, and demonstrated its druggability and link to antitumor immunity.

   Evidence In vitro R-loop exonuclease and RNaseH1-stimulation assays, DRIP-seq, patient-variant mutagenesis, small-molecule inhibitor, and cGAS-STING readouts

   PMID:41706852

   Open questions at the time

   • Structural basis of endo/exo coupling with RNaseH1 not resolved
   • How REXO4 is recruited to specific R-loop sites unknown

• It remains unresolved how REXO4's distinct activities — nucleolar rRNA processing, R-loop resolution, estrogen-driven gene induction, and stress-granule association — are coordinated and regulated within a single cell.
• No unifying model of how REXO4 is partitioned among nucleolar, R-loop, and EpRE functions
• Recruitment and regulatory mechanisms across compartments uncharacterized