Self-complementary adeno-associated virus

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]

Like AAV, scAAV is a member of the family Parvoviridae, commonly known as parvoviruses. These viruses are nonenveloped, single-strand DNA (ssDNA) viruses. Within Parvoviridae, scAAV further belongs to the Dependovirus genus, characteristically defined by an inability to replicate on their own. In nature, these viruses depend on another virus to provide replication machinery; adeno-associated virus can only replicate during an active infection of adenovirus or some types of herpesvirus. In lab use, this obstacle is overcome by addition of the helper plasmids, which exogenously expresses replication genes which AAV itself lacks.[14]

As a dependovirus, scAAV remains in a latent state within the cell until the cell experiences certain permissive conditions. These can include presence of a helper virus infection (such as adenovirus) or other toxic events such as exposure to UV light or carcinogens.[14] Because the endogenous rep ORF has been replaced with transgene, exogenously provided rep genes encode the proteins required for genome replication and other viral life cycle components. The ITRs located 5' and 3' of the viral genome serve as the origin of replication.[15]

Like the rep ORF, scAAV's cap ORF has been replaced by transgene and therefore is provided exogenously in a lab environment.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]