Loss of proviral sequences in RSV-transformed mouse tumour cells accompanied by the formation of double-minute chromatin bodies.

ABSTRACT

The RVA2 cell line, derived originally from a tumour induced by the Schmidt-Ruppin strain of RSV and passaged repeatedly in vivo and in vitro, was studied. This cell line was originally virogenic and the virus was rescuable with high efficiency from it. When rechecked after prolonged passage history, the efficiency of virus rescue was found to drop to very low levels and from one monocellular clone derived from such cell population no virus was rescued at all. After further 14 in vivo passages of RVA2 cells, no transforming virus rescue was obtained, even in the presence of a helper virus. Nor were the U5 sequences of LTR found in these cells. Karyologic analysis revealed the presence of double-minute chromatin bodies in the RVA2 cell population studied. The frequency of mitoses containing DMs doubled after 14 in vivo passages. The possibility is discussed that DMs as the site of gene amplification also contain amplified cellular oncogenes which might take over direction of the malignant state of the cell in which the original transforming viral gene (v-src) is no longer active or is lost.