Chromosome-mediated gene transfer results in two classes of unstable transformants

The human thymidine kinase gene has been transferred from HeLa S3 cells to mouse LM(TK-) cells via isolated metaphase chromosomes. Efficient transfer of the thymidine kinase gene (1.8 X 10(-5) colonies per recipient cell) was obtained when the donor chromosomes were precipitated with calcium phosphate and the recipient cells were treated with 10% (vol/vol) dimethyl sulfoxide. Thirty-five independent cell lines were analyzed in detail. Cytologically detectable donor chromosome fragments were observed in 14% of the cell lines. Many of the transformed cell lines were also found to express the human genes for galactokinase (23% of the transformed cell lines) and procollagen type I (69% of the transformed cell lines), which are syntenic to thymidine kinase on human chromosome 17. On the basis of stability analyses, three classes of transformed cell lines were defined and characterized. One class of transformants was stable, showing no loss of the transferred phenotype in the absence of selection. A second group of transformants was unstable, losing the thymidine kinase phenotype at a rate of 1.5-2.5% per day. This group of transformants was found to possess large donor chromosome fragments (macrotransgenomes) and relatively low levels of donor gene activity. The third group of transformants lost the thymidine kinase phenotype rapidly, at a rate of 6-10% per day. These cell lines contained small, cytologically undetectable transgenomes (microtransgenomes) and overexpressed the transferred thymidine kinase gene.