Integration-specific retrovirus expression in embryonal carcinoma cells.

The undifferentiated embryonal carcinoma (EC) cell line PCC4 aza 1 was infected with a selectable amphotropic retrovirus. Although EC cells are generally refractory to retroviral gene expression, we found that approximately 1 of 5,000-10,000 recombinant proviruses was transcribed in these cells. Cells containing an active recombinant provirus were cloned and expanded. Nucleic acid analysis revealed approximately one intact provirus per cell. Viral RNA levels were different for each cell clone examined [between 0.05% and 0.5% of the poly(A)+ RNA], and transcription was initiated and terminated in the long terminal repeats as in permissive differentiated cells. The cells retained an undifferentiated phenotype and remained positive for stage-specific embryonic antigen 1 and negative for H-2 surface antigens. The cells retained the block to the expression of other proviruses integrated at other chromosomal locations. The data suggest that a cis-acting genetic element, at or near the chromosomal site of integration, or mutations in the viral control elements themselves may be responsible for provirus expression in these cells.