A comparison of foamy and lentiviral vector genotoxicity in SCID-repopulating cells shows foamy vectors are less prone to clonal dominance.

Hematopoietic stem cell (HSC) gene therapy using retroviral vectors has immense potential, but vector-mediated genotoxicity limits use in the clinic. Lentiviral vectors are less genotoxic than gammaretroviral vectors and have become the vector of choice in clinical trials. Foamy retroviral vectors have a promising integration profile and are less prone to read-through transcription than gammaretroviral or lentiviral vectors. Here, we directly compared the safety and efficacy of foamy vectors to lentiviral vectors in human CD34(+) repopulating cells in immunodeficient mice. To increase their genotoxic potential, foamy and lentiviral vectors with identical transgene cassettes with a known genotoxic spleen focus forming virus promoter were used. Both vectors resulted in efficient marking in vivo and a total of 825 foamy and 460 lentiviral vector unique integration sites were recovered in repopulating cells 19 weeks after transplantation. Foamy vector proviruses were observed less often near RefSeq gene and proto-oncogene transcription start sites than lentiviral vectors. The foamy vector group were also more polyclonal with fewer dominant clones (two out of six mice) than the lentiviral vector group (eight out of eight mice), and only lentiviral vectors had integrants near known proto-oncogenes in dominant clones. Our data further support the relative safety of foamy vectors for HSC gene therapy.