Hypothesis

We propose that restoring the full long‑patch base excision repair (LP‑BER) cassette—FEN1, PCNA, Pol ε, and LIG1—in post‑mitotic neurons will recapitulate germline‑grade DNA repair capacity without forcing cells back into the cell cycle.

Mechanistic Basis

Germ cells retain a proliferative‑state repair toolkit that includes the LP‑BER factors coupled to replication machinery. In neurons, exit from the cell cycle coincides with transcriptional silencing of these factors, leaving only the short‑patch BER initiators (OGG1, APE1, Pol β). This creates a bottleneck after incision, allowing abasic sites to persist and trigger inflammatory pathways. Beyond polymerase activity, PCNA acts as a chromatin hub: it recruits histone chaperones CAF‑1 and ASF1, facilitating nucleosome re‑assembly after repair. Germline cells therefore not only excise damage but also restore chromatin integrity rapidly. Neurons lose this coupling, leading to chronic chromatin loosening, ectopic transcription of repeat elements, and STING‑mediated inflammation.

Experimental Design

1. Vector construction – Create a self‑limiting AAV9 cassette under the synapsin promoter. Each LP‑BER gene is fused to a degron (e.g., dTAG‑7) that allows rapid proteasomal clearance upon addition of a small molecule, limiting expression to a 6‑hour window after oxidative challenge.
2. Animal model – Inject vectors into the hippocampus of 18‑month‑old C57BL/6 mice. Controls receive AAV9 expressing GFP or isolated OGG1.
3. Oxidative challenge – Administer a low dose of kainic acid to provoke ROS production without causing excitotoxic cell death.
4. Readouts –
   • Measure 8‑oxoguanine levels by immuno‑dot blot and qPCR‑based lesion assay at 0, 6, 24 h.
   • Assess LP‑BER flux using a plasmid‑based reporter restored only when long‑patch synthesis occurs.
   • Monitor cell‑cycle re‑entry via EdU incorporation and phospho‑histone H3 (Ser10) immunostaining.
   • Evaluate neuroinflammation (GFAP, Iba1, cytosolic dsDNA, STING phosphorylation) and cognitive performance in Morris water maze.
   • Chromatin recovery examined by MNase sensitivity and H3K9ac ChIP‑seq at promoter regions of interferon‑stimulated genes.

Expected Outcomes and Falsifiability

• Supporting evidence: Neurons expressing the inducible LP‑BER cassette show ≥50 % reduction in 8‑oxoG, restored LP‑BER reporter activity, unchanged EdU/H3‑phospho signals, lowered STING activation, and improved spatial memory relative to controls.
• Falsifying evidence: No significant decline in 8‑oxoG despite LP‑BER expression, or detection of EdU‑positive neurons indicating unwanted S‑phase entry, or exacerbation of inflammatory markers, would refute the hypothesis that germline‑like LP‑BER can be decoupled from proliferation in post‑mitotic neurons.

Potential Caveats

• Over‑expression of PCNA may sequester p21 and inadvertently affect cyclin‑dependent kinase activity; the degron system mitigates this by limiting duration.
• AAV9 tropism may favor astrocytes; using a neuron‑specific promoter and verifying transduction efficiency via immunostaining is essential.
• Chronic oxidative models might overwhelm the short repair window; adjusting degron kinetics or using inducible promoters (Tet‑ON) can be tested in follow‑up studies.