Base Editing (ABE8e) for RAG1 c.1421G>A (p.Arg474His) in Recombinase activating gene 1 deficiency (RAG1-related severe combined immunodeficiency)

Base Editing (ABE8e) via RNP electroporation (ex vivo) delivery is a rationale-driven therapeutic strategy for Recombinase activating gene 1 deficiency (RAG1-related severe combined immunodeficiency) targeting the RAG1 c.1421G>A (p.Arg474His) variant (Pathogenic, missense variant). The editing system (ABE8e-nSpCas9 (adenine base editor)) converts the pathogenic A back to G on the target strand, restoring the wild-type codon. Target tissue: Blood/HSC. Therapeutic goal: Correct biallelic loss-of-function RAG1 mutations in autologous hematopoietic stem cells to restore V(D)J recombination and durable T- and B-cell immunity. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Medium, immunogenicity Low.

1. Molecular basis: RAG1 NM_000448.3(RAG1):c.1421G>A (p.Arg474His) is classified as Pathogenic (ClinVar variation ID 68684). Molecular consequence: missense variant. Protein change: R474H. 2. Epidemiology: RAG1/2 defects account for roughly 10-20% of SCID cases; cohort data from Slavic countries suggest a minimal RAG deficiency incidence of ~1:180,000-1:300,000 live births (PAPER-01, WEB-08, WEB-09). 3. Standard of care: Allogeneic hematopoietic stem cell transplantation (HSCT) is the only established curative treatment; supportive care includes immunoglobulin replacement and antimicrobial prophylaxis (WEB-02, WEB-03, WEB-07, PAPER-03, PAPER-04). 4. Pipeline: An ex vivo autologous CD34+ lentiviral RAG1 gene therapy (LV-RAG1/MB-110) is in Phase I/II clinical testing (NCT04797260) with early reports of successful immune reconstitution in at least one treated infant; additional preclinical work in RAG1-deficient mouse models supports partial to full immune 5. ABE clinical validation: ABE8e (Richter et al. 2020, Nat Biotechnol) achieves ~1.7x higher editing efficiency than ABE7.10. VERVE-101 demonstrated first-in-human LNP-ABE liver editing with 55-66% PCSK9 reduction (Raal et al. 2025, NEJM). Beam Therapeutics is advancing multiple ABE programs.

1. No published data specifically correcting RAG1 c.1421G>A (p.Arg474His) with Base Editing (ABE8e); strategy is based on general principles and must be validated preclinically.
2. PAM availability and bystander base analysis for the specific genomic context have not been performed. If no canonical NGG PAM positions the target within the editing window, PAM-flexible variants (SpRY) may be needed.
3. Long-term durability, off-target genome-wide effects, and immunogenicity in the target patient population require thorough preclinical and clinical evaluation.