Ex vivo lentiviral RAG1 gene addition for T-B-NK+ SCID: therapeutic rationale for RAG1 c.1421G>A (p.Arg474His)

Ex vivo lentiviral vector-mediated RAG1 gene addition in autologous hematopoietic stem cells (HSCs) is the leading gene therapy approach for RAG1-deficient SCID, offering a potentially curative alternative to allogeneic HSCT without the risks of graft-versus-host disease and the need for an HLA-matched donor. The p.Arg474His variant, located in the RAG1 core catalytic domain, likely severely impairs V(D)J recombinase activity, resulting in the T-B-NK+ SCID immunophenotype. As an autosomal recessive condition, gene addition of wild-type RAG1 to the patient's own HSCs could restore V(D)J recombination and lymphocyte development.

RAG1 encodes one of two lymphoid-specific recombinases (RAG1/RAG2) essential for V(D)J recombination — the somatic DNA rearrangement process that generates antigen receptor diversity in developing T and B cells. Biallelic loss-of-function RAG1 variants cause T-B-NK+ SCID (absent T and B cells, present NK cells), while hypomorphic variants cause Omenn syndrome or leaky SCID. The p.Arg474His variant affects a residue in the nonamer-binding region (NBR) of the RAG1 core, critical for DNA substrate recognition during V(D)J recombination. Preclinical gene therapy programs using self-inactivating lentiviral vectors with a codon-optimized RAG1 transgene under the endogenous RAG1 promoter or a ubiquitous promoter have demonstrated immune reconstitution in Rag1-knockout mice. The Leiden University Medical Center and Great Ormond Street Hospital groups have been leaders in developing RAG1 gene therapy toward clinical translation. Allogeneic HSCT is the current standard of care but has significant limitations: matched unrelated donor outcomes are suboptimal (60-70% survival for mismatched transplants), and conditioning-related toxicity is substantial in these critically ill infants.

RAG1 expression must be tightly regulated: constitutive high-level RAG1 expression outside of developing lymphocytes could cause genomic instability through aberrant V(D)J-like recombination events, potentially leading to lymphoid malignancy. The choice of promoter (endogenous RAG1 regulatory elements vs. ubiquitous promoters like PGK or EFS) is therefore critical for safety. Insertional mutagenesis risk from lentiviral integration, while lower than with gamma-retroviral vectors (as learned from the X-SCID gene therapy experience), remains a concern requiring long-term monitoring. Myeloablative or reduced-intensity conditioning is still required before autologous HSC gene therapy to create marrow space, exposing SCID infants to chemotherapy toxicity. No clinical trial results for RAG1 gene therapy have been published, though first-in-human trials have been planned. The p.Arg474His variant specifically has not been characterized in functional V(D)J recombination assays in the context of gene therapy complementation.