Gene therapy for severe combined immunodeficiency X1
Ex vivo retroviraly-mediated gc gene transfer into CD34 (+) bone marrow precursor cells led to the correction of the immunodeficiency in 9 out of 10 patients with X-linked severe combined immunodeficiency. Follow-up now reaches more than 7 years for the first 2 treated patients in whom the thymus is still functional as attested by the presence of TRECS in peripheral blood and by a broadly diversified TCR Vb repertoire. Among these patients, three (P4, P5 and P10) developed at 30 to 34 months after gene therapy a monoclonal T cell proliferation leading to clinical manifestations requiring a chemotherapy. P4 died unfortunately but for P5 and P10 these two patients are doing well and P5 is off treatment with a good immunological recovery. Genetic analysis of the malignant cells showed that in the two first cases the retroviral vector had integrated within or upstream of the LMO2 locus causing an insertional activation of LMO2 transcription from the affected allele. The last case revealed the involvement of several targeted sites, but the exact contribution of these different sites to the leukemia occurrence is still under investigation. A large scale analysis of retroviral integration patterns has been performed on patients’ PBMCS by LAM-PCR to assess the risk factors associated with the LTR’s strong enhancer effect of the MLV-based retroviral vector. 704 unique integration sites (IS) have been obtained and among them, 572 could be mapped unequivocally to the human genome. Most of these insertions (63%) were located in the vicinity of 10kb or within the coding sequence of a known gene and a significant peak of insertion frequency was related closely to the transcription start site (28%). Among the 572 IS, 160 were clustered in form of common integration sites (CIS). The non-random and clustered character of IS distribution and the likely in vivo selection advantage of clones with certain integrants in engrafted patient T cells was further underlined by the observation that 16 CIS (formed by 62 IS) were found to be hit 3 or more times. In the affected genomic regions, a high number of the nearest genes to the integrated vector are involved in human oncogenic process.