Retroviruses and mouse embryos: a model system in which to study gene expression in development and differentiation.

Early mouse embryos exposed to Moloney leukaemia virus (M-MuL V) produce substrains of mice, designated Mov-1 to Mov-14, that transmit the virus genetically from one generation to the next. In some substrains the inserted viral genome becomes activated at specific stages of embryogenesis and the available evidence suggests that these viral genomes are developmentally regulated. The effect of cellular differentiation on virus expression was investigated by introducing M-MuL V into preimplantation or postimplantation mouse embryos, or into embryonal carcinoma cells (EC cells) in tissue culture. Whereas preimplantation embryos or EC cells did not permit virus expression, efficient replication occurred in postimplantation embryos or in differentiated cells. The viral genomes introduced into early embryos were highly methylated and non-infectious when analysed in the adult. In contrast, viral genomes introduced into postimplantation embryos remained unmethylated and were infectious in a transfection assay. Similarly, de novo methylation occurred in undifferentiated EC cells but not in differentiated derivatives. These results demonstrate an efficient de novo methylation activity which appears to be involved in the repression of genes introduced into pluripotent embryonic cells and is not observed in cells of the postimplantation embryo or in differentiated cells growing in culture. Integration of M-MuL V into the germ line can lead to recessive lethal mutations. This has been shown for the Mov-13 substrain, as animals homozygous at the Mov-13 locus die between Days 13 and 14 of embryogenesis. This suggests that viral integration occurred in a chromosomal region that is active during, and crucial for, embryonic development.