Infectivity and methylation of retroviral genomes is correlated with expression in the animal.

We studied mechanisms controlling gene expression during animal development using retroviruses as model genes. For this, substrains of mice have been previously derived carrying the Moloney leukemia virus (M-MuLV) in their germ line. Virus activation occurs in some of these substrains at different stages of development, resulting in two classes of viral genomes. The genetically transmitted (endogenous) copy is present in every cell, whereas somatically acquired ("exogenous") copies are carried only in cells that were superinfected. We compared these two classes of M-MuLV genomes using two parameters. DNA sequences of the endogenous M-MuLV genome in all mouse substrains were highly methylated in GCGC, the recognition sequence of the restriction enzyme Hha I, and were not infectious (specific infectivity less than 10(-7) pfu per proviral genome) in a DNA transfection assay. In contrast, the "exogenous" copies were hypomethylated and infectious. These parameters are strongly correlated to genome activity in the animal: only tissues carrying exogenous copies express virus-specific RNA. With the assumption that gene expression of transfected DNA is controlled by mechanisms that are relevant for gene expression in the animal, our results suggest that DNA methylation plays a causative role in gene regulation during development and differentiation.