Genetic alterations of integrated avian sarcoma virus DNA sequences in transformed rat cells

Molecular biology of cell nonpermissiveness to retroviruses. Has the time come?

The problem of host cell nonpermissiveness to retrovirus infection is characterized and illustrated on several retroviral models, including the role of viral receptors, cell fusion, and endogenous retroviral genomes as modifiers of the outcome of retroviral infection. Special attention is paid to different barriers against the infection of mammalian cells with avian leukosis/sarcoma viruses (ALV/ASV). Even when avian retroviruses become integrated in mammalian cells, several blocks at the level of provirus expression, processing of viral RNAs, and posttranslational modification prevent virus production in such virogenic cells. The significance of these blocks and new strategies making it possible to overcome some of them are discussed in relation to the development of ALV/ASV-based vectors suitable for gene therapy in mammals.

The methylation state of the proviruses in avian sarcoma virus transformed chick and rat cells

The endogenous viruses in the avian cells are not completely methylated, nor are the Schmidt-Ruppin RSV-D (SRD) proviruses in the infected cells completely unmethylated. Avian sarcoma proviruses integrated in rat transformed cloned cells are heavily methylated. In these cells, a region in the 3' end of the env gene is unmethylated in all the src-containing proviruses but not in the transformed defective (td) proviruses. A possible role for the hypomethylation of the 3' end of the env region is proposed.