Self-complementary adeno-associated virus
Self-complementary adeno-associated virus (scAAV) is a
In gene therapy application utilizing rAAV, the virus transduces the cell with a single stranded DNA (ssDNA) flanked by two inverted terminal repeats (ITRs). These ITRs form hairpins at the end of the sequence to serve as primers to initiate synthesis of the second strand before subsequent steps of infection can begin. The second strand synthesis is considered to be one of several blocks to efficient infection.[7] Additional advantages of scAAV include increased and prolonged transgene expression in vitro and in vivo, as well as "higher in vivo DNA stability and more effective circularization."[8]
In gene therapy
scAAV is an attractive vector for use in gene therapy for many reasons. Its parent vector, AAV, is already being used in clinical trials.[9] Due to a variety of scAAV serotypes available, scientists can choose a serotype which has properties desirable for their therapy. Selecting only a subset of cells improves specificity and lowers the risk of being inhibited by the immune system. Different scAAV and AAV serotypes can efficiently transfect a variety of cellular targets.[10][11] Like all vector-based approaches to gene therapy, one obstacle in translating therapies from pre-clinical trials into a human clinical application will be the production of large quantities of highly concentrated virus.[12] One disadvantage that scAAV faces is that due to robust gene expression, transgene products delivered via scAAV elicit a stronger immune response than those same transgenes delivered via a single-stranded AAV vector.[13]
Virus classification
Like AAV, scAAV is a member of the family
Viral replication
As a dependovirus, scAAV remains in a
Viral packaging
Like the rep ORF, scAAV's cap ORF has been replaced by transgene and therefore is provided exogenously in a lab environment. The genes encoded in this ORF build capsid proteins and are responsible (along with intracellular processing) for conveying target specificity. Rep proteins participate in the integration of the genome into preformed capsids.[15] Despite the fact that scAAV is designed to form dsDNA upon infection, the two complementary strands are not packaged in a double stranded manner. Parvoviruses package their viral genome such that the ssDNA bases come in contact with the amino acids on the inside of the viral capsid. Thus the sequence of scAAV is likely unwound by a virally encoded DNA helicase before being packaged into viral protein capsid.[7]