Treatment of canine leukocyte adhesion deficiency by foamy virus vectors expressing CD18 from a PGK promoter

Proto-oncogene activation caused by retroviral vector integration can cause malignancies in gene therapy trials. This has led investigators to search for less genotoxic vectors with minimal enhancer activity and a decreased risk of influencing neighboring chromosomal gene expression after integration. We previously showed that foamy virus vectors expressing the canine CD18 gene from an internal murine stem cell virus promoter could cure canine leukocyte adhesion deficiency. Here we have repeated these studies using a foamy virus vector expressing canine CD18 from a phosphoglycerate kinase gene promoter. In vitro analysis showed that this vector did not contain an enhancer that activated neighboring genes, and it expressed CD18 efficiently in canine neutrophils and CD34+ cells. However, dogs that received hematopoietic stem cells transduced with the PGK-CD18 vector continued to suffer from leukocyte adhesion deficiency, and sometimes died prematurely of the disease. These studies show that the phosphoglycerate kinase promoter cannot effectively replace the murine stem cell virus promoter in CD18-expressing foamy virus vectors, and they suggest that vectors containing a strong promoter/enhancer may be necessary for the treatment of human leukocyte adhesion deficiency.