Of the more than 80 identified serotypes of adenovirus (Ad), 31 are known to infect humans. These viruses cause respiratory infections and may persist in lymphoid tissue without causing recurrent illness. Types 1, 2 and 5 are commonly found in adenoids in latent form, and a high proportion of the population has serum antibodies to them. Indeed, antibodies to types 1 and 2 have already developed in three-quarters of all children by the age of five years (Oswald and Frey, 1962). Thirteen of the 51 human Ad have been classified into groups according to their oncogenic potential (Green and Mackey, 1977). Group A (Ad 12, 18 and 31) are highly oncogenic and rapidly produce tumours in most inoculated baby hamsters. Group B viruses (Ad 3, 7, 11, 14, 16 and 21) induce tumours in only a small fraction of inoculated test animals. Group C viruses (Ad 1, 2, 5 and 6) are not oncogenic in newborn rodents but have been shown to transform cells in culture (Green, 1970). Because of the widespread existence of the adenoviruses in the human population and their oncogenic potential, the possibility exists that these viruses may be responsible for some human tumours. An extensive search for viral sequences in human cancer tissue has been conducted employing the rationale that, like Ad-induced hamster tumours, Ad-induced human tumours should contain viral DNA sequences. The emphasis of this work has been on group A Ad (Mackey, Rigden and Green, 1976). However, searches for group BAd sequences (Mackey et al, 1979; Wold et al, 1979) and group CAd sequences (Green and Mackey, 1977) in human turnouts have also been carried out. To date, no adenovirus-related sequences have been detected in DNA isolated from human tumours. Although none of the adenovirus genome has been found in human tumours, there is some evidence for the presence of some or all of the adenovirus genome in apparently normal mammalian tissue. Analysis of human tonsil DNA by Green et al (1979) revealed multiple copies of the entire Ad 5 genome free in the cells, as well as some Ad 5 sequences possibly integrated into the cellular genome. Furthermore, in a recent report, Frolova and Georgiev (1979) demonstrated the presence of Ad 5-related sequences in the genome of normal rat liver cells. By digesting the cellular DNA with restriction endonuclease Bam HI, separating the fragments on agarose gels and then hybridizing 32P-labelled Ad 5 restriction fragments to the cellular DNA immobilized on nitrocellulose filters, these authors were able to detect at least three regions of homology between the Ad 5 genome and cellular DNA.
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