Retroviral Vectors in Gene Therapy

Viral vectors are the most effective means to deliver genes into cells. Evolution of viruses over the years has enabled them to adopt several strategies not only to enter but also infect a wide range of cells. Making use of this property, scientists have manipulated viruses to express therapeutic genes. These viruses serve as vehicles for gene delivery and are referred to as viral vectors. The ones that are currently used in gene therapy include retroviral vectors, lentiviral vectors, adenoviral vectors, adeno-associated viral vectors, and foamy viral vectors. Each of them has their own salient features that are both beneficial and harmful. Retroviral vectors belonging to the family Retroviridae were among the first viral vectors used in gene therapy clinical trials. Their genetic material is in the form of RNA. The ability of retroviruses to integrate into the host genome makes them a permanent resident of the cell. The Retroviridae family is further divided into two subfamilies and seven genera, out of which gammaretrovirus, lentivirus, and foamy virus are the most widely used. Gammaretroviral vectors which have been a part of nearly 21% of gene therapy clinical trials were developed from the prototype Moloney murine leukemia virus and hence referred to as MoMLV-based retroviral vectors. The gammaretroviral vector genome ranges in size from 9 to 11 kb and is composed of two long terminal repeats (LTRs) one each at the 3′ and 5′ ends and three essential genes gag, pol, and env which code for proteins required for viral packaging. Reports of insertional mutagenesis and clonal proliferation due to integration of gammaretrovirus into the LMO2 proto-oncogene raised concerns about safety of its application for human gene therapy. Another drawback of this virus is that it can transduce only rapidly proliferating cells. So, in order to successfully target terminally differentiated or largely quiescent cells such as stem cells, lentiviruses are used as they could infect both dividing and nondividing cells fairly efficiently. Unlike gammaretroviruses, lentiviruses do not require cells to be in active mitosis while entering. Over the past three decades, three different generations of lentiviral vectors have been developed, each generation significantly improved over the preceding one. Safety has always been a concern with the use of all viral vectors due to the adverse events reported such as immune response in the case of adenoviral vectors and insertional mutagenesis in the case of retroviral vectors. Recently, third-generation self-inactivating (SIN) lentiviral vectors have been proven to be very efficient and also safer when compared to gammaretroviral vectors which were used in earlier clinical trials. Despite integrating into active transcription units, lentiviral vectors were reported to have a safer integration profile compared to gammaretroviral vectors. Foamy viruses belong to the genera of spumavirus. Analysis of integration sites of foamy viruses in HSCs has shown a unique and safe integration profile compared to both gamma and lentiviruses. Reports from ongoing clinical trials might answer emerging questions related to their safety and efficacy in human gene therapy applications.