Sustained Polyclonal Hematopoietic Repopulation after Successful SCID-X1 Gene Therapy by Means of a Non Random Integrating Pseudotyped Gammaretrovector.

The integration of a retroviral vector as used in hematopoietic gene therapy trials produces a transition sequence from the vector DNA into the genomic DNA and may thus serve as a stable molecular marker unique for each cell clone. High sensitive linear amplification mediated PCR (LAM-PCR) allows the detection of specific retroviral integration sites. Thus it is possible to determine the clonal composition of the hematopoietic system in vivo (1). We could show that the hematopoietic repopulation in human SCID-X1 patients was derived from various, long-lived progenitor cell clones indicating retroviral transduction into pluripotent cells (manuscript submitted). In two cases of lymphoproliferative disorder after successful SCID-X1 gene therapy integration of the retroviral vector into the LMO-2 oncogene was probable the main reason for malignancy (2). The distribution of integration sites over the whole genome, the potential preference for integration at certain loci and which cells receive genetic correction and engraftment are therefore of considerable interest. Recently, another gene therapy trial has successfully corrected 4 infants suffering from SCID-X1. A GALV-pseudotyped MLV-based vector carrying the therapeutic common gamma chain gene was used for transduction of autologous CD34+ cells. The patients did not receive any conditioning therapy before transplantation. We analyzed lymphoid and myeloid DNA from all patients. The transduction efficiency of T lymphocytes and myeloid cells was up to 100 and 1 %, respectively. In vivo clonality analysis of CD3+ cells showed a polyclonal composition 1 to 2 years after transplantation. The myeloid repopulation also consisted of various different clones. These data may indicate transduction of pluripotent and long term active stem or progenitor cells. We here report on more than 300 sequenced integration sites of the patients, whereas 250 sequences could be assigned unequivocally to a unique locus. So far, no vector integration in the LMO-2 oncogene could be detected in this trial, and the patients do not reveal any other evidence for malignancy or clonal deformation of their stem cell compartment. We could show that integration of the mammalian gammaretroviral vector in this gene therapy trial is not random. Integration of the vector happens generally within or close to specific regions of genes. We found common integration sites (CIS) in RefSeq gene regions. The targeted RefSeq genes were classified according to the Gene Ontology database. Our data strongly support the presumption that curative gene therapeutic treatment requires a sustained polyclonal contribution of ex vivo manipulated stem and progenitor cells and provide an important insight into the integration manner of GALV-pseudotyped MLV-based vectors.