Amplification of a specific set of intracisternal A-particle genes in a mouse plasmacytoma

Intracisternal A-particles (IAPs) derive from endogenous retrovirus-like genes that are present in ca. 1,000 copies per haploid mouse genome. A particular IAP sequence variant that represents only 2% of the endogenous genes was found to be the predominant transcript in five out of seven plasmacytoma lines examined, and these genes have been amplified two- to fourfold in MOPC 315. Restriction enzyme digests of normal and tumor DNAs revealed a set of conserved sites within the population of amplified genes which could be used to generate a consensus restriction enzyme map of the amplified IAP genome. Molecular clones of IAP genes related to those amplified in MOPC 315 were isolated from gene libraries of MOPC 315 and MOPC 104E DNAs. Each clone was characterized for the presence of the restriction enzyme sites conserved in the amplified genes. Of 26 clones isolated from MOPC 315 DNA, 14 contained all of the conserved sites, whereas none of the clones isolated from MOPC 104E DNA contained all of the sites. These results suggested that approximately one-half of the genes isolated from MOPC 315 DNA were members of the amplified IAP gene population, representing newly acquired proviruses in the tumor DNA. Unique cellular flanking sequences were isolated from four of the MOPC 315 clones and four of the MOPC 104E clones. Probes from three of the MOPC 315 clones detected restriction fragments that were unique to MOPC 315 DNA and equivalent in size to the cloned gene, demonstrating that these three IAP genes had integrated into new sites in the plasmacytoma DNA. The other five clones were found to be endogenous to the BALB/c genome. Thus, IAPs represent a significant source of insertional mutations for any cell expressing the retrovirus-like particles. Furthermore, these results suggest that only one or a small number of endogenous IAP sequences are activated to give the homogeneous population of amplified genes.