Lentiviral ex vivo TCIRG1 gene therapy for TCIRG1 c.1674-1G>A: autologous HSC approach as alternative to allogeneic HSCT

For TCIRG1 c.1674-1G>A, a pathogenic splice acceptor variant predicted to disrupt exon 15 splicing and abolish a3 subunit function in osteoclasts, lentiviral ex vivo gene therapy using autologous CD34+ HSPCs transduced with a functional TCIRG1 transgene offers a potentially safer alternative to allogeneic hematopoietic stem cell transplantation (HSCT). Autosomal recessive osteopetrosis (ARO) due to TCIRG1 mutations accounts for ~50% of all ARO cases and is lethal without treatment. While allogeneic HSCT is curative by replacing the osteoclast lineage, it carries significant morbidity from graft-versus-host disease, graft failure, and conditioning-related toxicity. Ex vivo lentiviral gene correction of autologous HSPCs eliminates donor-related risks while providing functional osteoclasts derived from the corrected stem cell compartment.

Preclinical studies have demonstrated that lentiviral TCIRG1 gene transfer into Tcirg1-deficient murine HSPCs rescues osteoclast function and corrects the osteopetrotic phenotype after transplantation (Moscatelli et al., Hum Gene Ther 2018; PMID:29706106). Patient-derived CD34+ cells transduced with lentiviral TCIRG1 vectors show restored osteoclast resorption activity in vitro, confirming that gene addition restores the functional deficit (Lanzi et al., Bone 2020; PMID:32169673). The splice variant c.1674-1G>A disrupts the invariant AG at the exon 15 splice acceptor, expected to cause exon skipping and loss of the a3 subunit of the vacuolar H+-ATPase, which is essential for osteoclast acid secretion and bone resorption. ClinVar classifies this as Pathogenic. The Rocket Pharmaceuticals RP-L401 program represents clinical-stage lentiviral TCIRG1 gene therapy development, with IND-enabling studies completed.

No clinical efficacy data for lentiviral TCIRG1 gene therapy are available yet. ARO patients typically present in infancy with life-threatening complications (pancytopenia, cranial nerve compression, hepatosplenomegaly), requiring urgent intervention — the manufacturing timeline for autologous gene therapy (cell collection, transduction, quality release) may be incompatible with the clinical urgency in severely affected neonates. Conditioning regimens, though potentially reduced compared to allogeneic HSCT, are still required for engraftment and carry toxicity risks in fragile infants. Insertional mutagenesis risk from lentiviral integration, while lower than with gamma-retroviral vectors, cannot be entirely excluded. Long-term durability of transgene expression from the lentiviral-transduced HSC compartment needs verification. For patients with an available matched sibling donor, allogeneic HSCT remains the established standard with good outcomes, so gene therapy may be most valuable for patients lacking suitable donors.