Chromosomal gene transfer of human cytosol thymidine kinase into mouse cells: integration or association of the transferred gene with a non-homologous mouse chromosome.

If a chromosomal fragment transferred into recipient cells were integrated or strongly associated with a specific recipient chromosome it should segregate with this chromosome in hybrid cells. In order to corroborate this prediction we studied two independent mouse cell clones (“transferent clones”) which had taken up by chromosomal gene transfer a human chromosomal fragment carrying the gene for cytosol thymidine kinase (TKs, E.C.No.2.7.1.75). The following results were obtained. 1. Ten somatic cell hybrids isolated after fusion of transferent mouse clones and chinese hamster cells expressed functional human TKs and mouse galactokinase (GALK, E.C.No.2.7.1.6) activity. Counterselected derivatives of all clones had lost human TKs but still expressed mouse GALK. Recently both mouse genes for TKs and GALK have been assigned to mouse chromosome 11 (Kozak and Ruddle, 1977a, McBreen et al., 1977). Therefore our results argue against integration or association of the human gene for TKs at the site of the homologous defective mouse TKs-GALK-region in the genome of transferent clones. 2. In three somatic cell hybrids isolated after fusion of microcells from two different transferent mouse clones with established chinese hamster cells only human TKs but not mouse GALK was expressed. Karyotypic analysis of one hybrid suggested the presence of at least one copy of mouse chromosome 9 per hybrid cell. Chromosome 9 and the mouse isozyme activity of mannose phosphate isomerase which had been previously assigned to a gene locus on mouse chromosome 9 were missing in a subclone counter-selected against the presence of TKs activity. These findings suggested that the transferred human gene for TKs may be integrated or strongly associated with mouse chromosome 9 in this transferent mouse clone. 3. Two other somatic cell hybrids were analyzed which were derived from a different phenotypically stable transferent mouse clone by similar microcell fusion with the chinese hamster cells. Although these hybrids expressed human TKs activity, no mouse chromosome could be detected in these clones. Possibly after fusion of microcells derived from certain transferent mouse clones the transferred human chromosomal fragment could become translocated from a mouse chromosome to the chinese hamster genome.