7.1.2.1 Viral Genome

It is a double stranded DNA, about 36 kb in size. Each end is characterized by having 100 base sequence that is inverted repeats of the sequence present on the other end. Inside the genome there are early and late genes. The early regions include regions called E1A1 and E1B to E3.  They can be expressed to form m-RNA molecules. The m-RNA is produced in an over lapping way, giving rise to proteins that are subunits of one another. The E1A region is transcribed first immediately after the entry of the virus to the nucleus. It is essential for viral DNA replication. The E3 region plays a role in pathogenesis.

To use the adenovirus as a vector in gene therapy, the early regions E1A and B must be deleted from the genome therefore preventing viral replication. Such vectors are capable of infecting a cell only once and no viral propagation occurs. Then followed by inserting the desired gene in place of the deleted region. This deletion can make room for inserting a foreign desired DNA gene, up to a size of about 6kb.

The adenoviral vector used in gene therapy has to be constructed to contain two parts, first the viral DNA vector and the second is a packaging cell line. The adenoviral DNA vector is a plasmid DNA that contains a portion of the viral genome after deleting E1A region and replaced by the gene of interest. This adenovirus vector can be produced using either in vitro ligation or homologous recombination.  (Fig. 12).

Figure 12 : adenovirus vector