Gene therapy using adeno-associated virus vectors

Abstract

The unique life cycle of adeno-associated virus (AAV) and its ability to infect both nondividing and dividing cells with persistent expression have made it an attractive vector. An additional attractive feature of the wild-type virus is the lack of apparent pathogenicity. Gene transfer studies using AAV have shown significant progress at the level of animal models; clinical trials have been noteworthy with respect to the safety of AAV vectors. No proven efficacy has been observed, although in some instances, there have been promising observations. In this review, topics in AAV biology are supplemented with a section on AAV clinical trials with emphasis on the need for a deeper understanding of AAV biology and the development of efficient AAV vectors. In addition, several novel approaches and recent findings that promise to expand AAV's utility are discussed, especially in the context of combining gene therapy ex vivo with new advances in stem or progenitor cell biology.

FIG. 1.

Map of the wild-type AAV-2 genome. (A) Rep and Cap genes flanked by ITRs. The different Rep and Cap transcripts are produced from their respective promoters (P5, P19, and P40). The star indicates the alternative ACG codon used to produce VP3. (B) Secondary structure of an AAV-2 ITR showing the RBEs (RBE, GAGCGAGCGAGCGCGC; RBE′, CTTG) and the TRS (GTTGG).

FIG. 2.

AAV life cycle. AAV undergoes productive infection in the presence of adenovirus coinfection. This is characterized by genome replication, viral gene expression, and virion production. In the absence of adenovirus, AAV can establish latency by integrating into chromosome 19 (AAVS1). The latent AAV genome can be rescued and replicated upon superinfection by adenovirus. Both stages of AAV's life cycle are regulated by complex interactions between the AAV genome and AAV, adenoviral, and host proteins.

FIG. 3.

(Left) trans-Splicing approach. The head-to-tail formation of two different AAV vector results in functional product after splicing. (Right) Comparison of scAAV and rAAV vectors.