Expression of the adenovirus E4 34k oncoprotein inhibits repair of double strand breaks in the cellular genome of a 293-based inducible cell line

The human adenovirus E4 ORF 6 34 kDa oncoprotein (E4 34k), in concert with the 55 kDa product of E1b, prevents concatenation of viral genomes in infected cells, inhibits the repair of double strand breaks (DSBs) in the viral genome, and inhibits V(D)J recombination in a plasmid transfection assay. These activities are consistent with a general inhibition by the E4 34k and E1b 55k proteins of DSB repair by non-homologous end joining (NHEJ) on extrachromosomal substrates. To determine whether inhibition of NHEJ extends to repair of DSBs in the cell chromosome, we have examined the effects of E4 34k on repair of chromosomal DSBs induced by ionizing radiation in a cell line in which E4 34k expression and biological activity is inducible and E1b 55k is produced constitutively. We demonstrate that in this cell line, induction of E4 34k inhibits chromosomal DSB repair. Recently, it has been shown that in infected cells, E4 34k and the adenovirus E1b 55k proteins cooperate to destabilize Mre11 and Rad50, components of mammalian NHEJ systems. Consistent with this, induction of expression of E4 34k in the inducible cell line also reduces the steady state level of Mre11 protein.