NM_006019.4(TCIRG1):c.2236+1G>A

For TCIRG1 c.2236+1G>A, a canonical splice donor variant expected to produce non-functional a3 subunit of the vacuolar H+-ATPase in osteoclasts, ex vivo hematopoietic stem/progenitor cell (HSPC) gene therapy with lentiviral TCIRG1 delivery represents the most promising curative approach beyond allogeneic HSCT. TCIRG1 mutations account for >50% of autosomal recessive osteopetrosis (ARO) cases. Because osteoclasts derive from the hematopoietic lineage, correcting HSPCs with a functional TCIRG1 transgene can restore osteoclast acidification capacity. This approach avoids the graft-versus-host disease risk, donor availability constraints, and high transplant-related mortality (20-40%) associated with allogeneic HSCT in ARO.

Preclinical proof-of-concept has been demonstrated in the oc/oc mouse model (TCIRG1-null): transplantation of lentiviral-corrected HSPCs restored osteoclast function and normalized bone density. Rocket Pharmaceuticals is developing RP-L401 (formerly TGRN-001), a lentiviral HSPC gene therapy for TCIRG1-deficient ARO, which has received FDA Rare Pediatric Disease and Orphan Drug designations. The program uses a codon-optimized TCIRG1 cDNA driven by a ubiquitous promoter (PGK or EFS) in a self-inactivating lentiviral vector. Clinical trials are in early-phase development. Allogeneic HSCT remains the current standard of care but has significant limitations: matched donors are unavailable for ~30% of patients, and transplant mortality in ARO (particularly with mismatched donors) remains 20-40% due to graft failure and organ damage from the dense bone marrow environment.

Lentiviral HSPC gene therapy requires myeloablative conditioning (typically busulfan-based) to create marrow space, which carries toxicity risks especially in the dense, fibrotic marrow environment of ARO patients. Engraftment may be impaired by the osteosclerotic niche. The bone marrow space in severe ARO is progressively obliterated, and delayed treatment reduces the likelihood of successful engraftment — early intervention (ideally before 3-6 months of age) is critical. It remains uncertain whether gene-corrected osteoclasts can remodel already pathologically dense bone, or whether intervention must precede the establishment of severe skeletal disease. Long-term transgene expression from integrated lentivirus is generally durable, but insertional mutagenesis risk is non-zero. For the specific c.2236+1G>A splice variant, confirming complete loss of exon inclusion (rather than residual cryptic splicing) is important for genotype-phenotype correlation.