Selective Gene Amplification in Mammalian Cells after Exposure to 60 Co g Rays, 241 Am a Particles, or uv Light

Abstract

Simian Virus 40 wild type (SV40)-transformed Chinese hamster embryo cells (Co631) contain about five viral copies integrated per cell genome. These SV40 sequences were used as endogenous indicator genes to study the response of mammalian cells to radiation at the gene level. An increase in copy number was detected by dispersed cell blotting and Southern analysis in combination with specific DNA hybridization. All types of radiation tested induce a 15- to 25-fold amplification of SV40 sequences without producing intact virus. The amplification is dose dependent and increases with time after irradiation: a maximum effect is observed at Day 3 after alpha particle or uv exposure and at Day 6 after gamma-ray exposure. A RBE of 6 can be calculated for alpha particles if amplification rates at Day 3 are compared. However, when the maximum effect is considered independent of time, no difference between different types of radiation is observed. Southern blots of genomic DNA show that not all integrated SV40 sequences are amplified upon radiation. Amplified sequences are found either in restriction fragments of relatively high molecular weight or in unit size fragments. SV40 amplification is selective in that the amplification of other genes, e.g., of alpha-actin, dhfr (dihydrofolate reductase), and of two oncogenes of the ras family (Kirsten ras and Harvey ras), was below detection level.

Molecular Mechanisms of Ultraviolet Radiation Carcinogenesis

“…Exposure of simian virus 40 (SV40)-transformed CHO cells to UV radiation resulted in amplification of integrated SV40 sequences (Lavi and Etkin, 1981). Lucke-Huhle et al (1986) showed that UV-irradiation of SV-40-transformed CHO cells resulted in a 15-to 25-fold amplification of SV40 sequences without producing intact virus. This amplification was selective in that other genes, such as DHFR, K-ras, Ha-ras and aactin, were not expressed in abnormally high copy numbers.…”

UV-induced early-domain binding factor as the limiting component of simian virus 40 DNA amplification in rodent cells.

“…Nonpermissive rodent cells (e.g., the SV40-transformed Chinese hamster embryo cell line Co631) can be induced to asynchronous SV40 replication by treatment with any one of a number of DNA-damaging agents (12,21,23,25,26,35). The induced replication resembles that in permissive cells: T antigen is required, and initiation occurs at the SV40 origin.…”

Binding of a sequence-specific single-stranded DNA-binding factor to the simian virus 40 core origin inverted repeat domain is cell cycle regulated.

“…Because of limited information on the human replicon and how origins of replication are controlled in cis, viral systems have proven to be most useful to the study of mechanisms of replication and gene amplification (5,15,17,23,24,27,29,33,38,39,44,53,54,60). Papovaviruses and parvoviruses can be induced to replicate in various cell lines (permissive or nonpermissive) when the cells are treated with a variety of DNA-damaging agents (38,53,60).…”