Gene amplification in methotrexate-resistant mouse cells
The site of association of the human transgenome and host murine chromosomes was determined in several subclones of a stable human/mouse transformed cell line. Chromosomes were transferred from each of three transformed subclones into Chinese hamster recipient cells, and selection was applied for the expression of human transgenome-encoded HPRT. A series of trispecific microcell hybrids was isolated and characterized for each subclone. Evidence is presented that, within a given transformed subclone, only a single host (murine) chromosome was associated with the human transgenome. This contrasts with previous results which utilized a newly stabilized transformed cell line as the microcell donor and in which a variety of chromosomal sites of association existed. The results presented here support the view that the heterogeneity of transgenome association (integration) sites in newly stabilized transformants was due to the fact that these populations were multiclonal mixtures resulting from independent stabilization events. The initial heterogeneity in the population was subsequently reduced upon prolonged cultivation, as a subset of the original population became predominant.