Preliminary studies on the design of retroviral vectors for substitutive gene therapy

: We have studied the effect that exerts on the Moloney murine leukemia virus (Mo-MLV) viral cycle the deletion of the 13 bases that constitute the inverted repeat (IR) present at the external U3 region of the pre-proviral DNA. Whereas supernatants of wild type- and modified virus-producing cells contained similar amounts of viral particles, the deleted viruses showed a 100 to 1000-fold decreased infectivity. To determine how the deletion interfered with the infective capacity of the virus, different steps of the viral cycle were studied using deleted viruses. The deletion affected neither reverse transcription nor the entry of the DNA into the cell nucleus; however, the integration of the pre-proviral DNA into the host genome was reduced to undetectable levels. These results open a pathway to the construction of Mo-MLV-derived retroviral vectors for gene targeting purposes.