Mutational analysis of the avian adenovirus CELO, which provides a basis for gene delivery vectors

The avian adenovirus CELO is being developed as a gene transfer tool. Using homologous recombination in Escherichia coli, the CELO genome was screened for regions that could be deleted and would tolerate the insertion of a marker gene (luciferase or enhanced green fluorescent protein). For each mutant genome, the production of viable virus able to deliver the transgene to target cells was monitored. A series of mutants in the genome identified a set of open reading frames that could be deleted but which must be supplied in trans for virus replication. A region of the genome which is dispensable for viral replication and allows the insertion of an expression cassette was identified and a vector based on this mutation was evaluated as a gene delivery reagent. Transduction of avian cells occurs at 10- to 100-fold greater efficiency (per virus particle) than with an adenovirus type 5 (Ad5)-based vector carrying the same expression cassette. Most important for gene transfer applications, the CELO vector transduced mammalian cells as efficiently as an Ad5 vector. The CELO vector is exceptionally stable, can be grown inexpensively in chicken embryos, and provides a useful alternative to Ad5-based vectors.