Advances in the Gene Therapy of Patients with Fanconi Anemia

Fanconi anemia (FA) is a DNA repair syndrome characterized by bone marrow failure, congenital abnormalities and cancer predisposition. Based on previous experimental results showing the in vivo proliferative advantage of gene corrected FA patients' hematopoietic stem cells (HSCs; Rio, Navarro et al. Blood 2017) a gene therapy trial in non-conditioned FA-A patients was initiated in 2016. Six patients have been treated to-date using fresh and cryopreserved CD34⁺ cells mobilized to peripheral blood with G-CSF and plerixafor, and transduced with the PGK-FANCA.Wpre* lentiviral vector. Cell doses infused in four patients with a follow-up of at least 12 months varied from 0.6 to 1.4 million CD34⁺ cells/kg. Transduction efficacies of these samples, determined as vector copies per cell, ranged from 0.17 to 0.53 copies/cell. Despite the absence of patients' conditioning, a marked in vivo expansion of gene-corrected cells was observed in all hematopoietic cell lineages analyzed in BM and PB. Significantly, up to 44% of corrected cells were determined in total PB cells at the most recent follow-up visit (24 month) in the first treated patient. Insertion site analyses in PB cells showed an oligoclonal pattern of hematopoietic reconstitution, and revealed engraftment of multipotent corrected HSCs and no evidence of insertion-site mediated clonal expansion. Functional studies showed significant increases in the resistance of BM progenitors to mitomycin C in all treated patients. Additionally, patients with higher levels of corrected cells also showed significant increases in the chromosomal stability of T cells exposed to diepoxybutane. Finally, analyses discriminating the presence of corrected and uncorrected PB cells in these patients showed marked increases in the total number of corrected leukocytes, contrasting to progressive decreases of uncorrected cells. Our studies demonstrate for the first time that lentiviral-mediated gene therapy results in progressive engraftment and phenotypic correction of HSCs in non-conditioned FA patients, suggesting that this gene therapy approach may constitute a low-toxicity option for the treatment and prevention of BMF in patients with FA.