Gene Therapy Replaces Daily Cysteamine in Rare Lysosomal Disease

Cystinosis is a rare autosomal recessive lysosomal storage disorder caused by loss-of-function mutations in CTNS, the gene encoding cystinosin, a lysosomal membrane transporter that exports cystine. Without functional cystinosin, cystine crystals accumulate in virtually every organ, causing renal Fanconi syndrome in infancy, progression to end-stage kidney disease, corneal damage, myopathy, neurological deficits, endocrinopathies, and premature death. The only approved treatment, cysteamine, reduces intracellular cystine but must be taken every 6 hours orally plus hourly eye drops during waking hours; it delays but cannot halt disease progression.

Preclinical work in Ctns−/− mice established that transplanting HSPCs expressing functional Ctns could integrate bone marrow–derived macrophages into diseased tissues, where they corrected neighboring cells via tunneling nanotubes—subcellular bridges enabling lysosomal transfer. Building on this, the UCSD team developed CTNS-RD-04: autologous CD34+ HSPCs transduced ex vivo with a self-inactivating lentiviral vector carrying CTNS cDNA. Six adults (ages 20–46) with infantile cystinosis were enrolled between July 2019 and May 2022. After G-CSF/plerixafor mobilization and leukapheresis, cells were transduced and reinfused following myeloablative busulfan conditioning. Two lentiviral vector constructs were used across the cohort (pCCL-CTNS for patients 1–3; pCDY-CTNS for patients 4–6), and LentiBOOST was added for patients 5–6 to improve transduction efficiency. Cell doses ranged from 3.63 to 9.59 × 10⁶ CD34+ cells/kg with vector copy numbers (VCN) of 0.59–2.91 at infusion.

All six patients achieved neutrophil engraftment at a median of 13 days and platelet engraftment at 19.5 days. VCNs remained stable through 24 months (0.51–2.67 copies/diploid genome), and CTNS expression in peripheral blood mononuclear cells rose 11–49-fold above baseline at 24 months. Integration site analysis across all patients revealed highly polyclonal profiles (396–85,868 unique sites per patient) with no monoclonal expansions or leukoproliferative events. WBC cystine levels declined from baseline in five of six patients; patient 4, who had the lowest VCN (0.59), was the exception. Tissue cystine crystal reductions were observed in rectal and skin biopsies and by corneal confocal microscopy in evaluable patients. Over 216 adverse events were recorded, predominantly mild or moderate and attributable to busulfan conditioning or underlying cystinosis. Four severe AEs occurred (appendicitis, two CKD-worsening episodes, and pre-existing coronary artery disease requiring hospitalization); none were attributed to CTNS-RD-04. Two patients contracted SARS-CoV-2 post-infusion, delaying immune reconstitution.

The mechanistic rationale distinguishes this approach from simple enzyme replacement: engrafted macrophages physically integrate into tissues and transfer functional lysosomes to host cells via tunneling nanotubes, potentially providing durable, body-wide cystine clearance. Discontinuation of oral cysteamine before infusion and eye drops one month post-conditioning—if sustained—would dramatically reduce treatment burden. Five of six patients enrolled in a 13-year long-term follow-up study, enabling assessment of durability.

This first-in-human HSC gene therapy for cystinosis demonstrates an acceptable safety profile and promising biomarker efficacy over up to five years. Larger, controlled trials with younger patients (before extensive organ damage) are needed to determine whether functional benefits—preserved GFR, corneal clarity, muscle strength—are clinically meaningful and durable.