Alteration of gammaretroviral vector integration patterns by insertion of histone and leucine zipper into integrase

Retroviral vectors show long-term gene expression in gene therapy through the integration of transgenes into the human cell genome. Murine leukemia virus (MLV), a well-studied gammaretrovirus, has been often used as a representative retroviral vector. However, frequent integrations of MLV-based vectors into transcriptional start sites (TSSs) could lead to the activation of oncogenes by enhancer effects of the genetic components within the vectors. Therefore, the MLV integration preference for TSSs limits its wider use in clinical applications. To reduce the integration preference of MLV-based vectors, we attempted to perturb the structure of the viral integrase that plays a key role in determining integration sites. For this goal, we inserted histones and leucine zippers, having DNA-binding property, into internal sites of MLV integrase. This integrase engineering yielded multiple mutant vectors that showed significantly different integration patterns compared with that of wild-type vector. Some mutant vectors did not prefer the key regulatory genomic domains of human cells, TSSs. Moreover, a couple of engineered vectors did not integrate into the genomic sites near the TSSs of oncogenes. Overall, this study suggests that structural perturbation of integrase is a simple way to develop safer MLV-based retroviral vectors for use in clinical applications.