title: Phenotypic variation associated with molecular alterations at a cluster of thymidine kinase genes.
abstract: Genetic variation was studied in several mouse L cell lines containing tandemly repeated herpes simplex virus thymidine kinase (TK) genes introduced by DNA-mediated gene transfer. Variants were obtained after alternate positive and negative selection for TK expression. Three classes of molecular alteration are described. One class consisted of a concerted wave of hypermethylation affecting many sites in all or nearly all of the TK genes. This resulted in genetically stable TK- variants. Of five TK+ transformants from independent transfer experiments, only one, named HM, showed this class of methylation. Hypermethylation was a reproducible phenomenon in HM, yielding TK- variants after selection with either bromodeoxyuridine or acycloguanosine [Acyclovir or 9-(2-hydroxyethy-oxymethyl)guanine]. A second class of alteration consisted of methylation affecting some, but not all, genes in the cluster. This happened in all TK+ (HAT [hypoxanthine-aminopterin-thymidine]-resistant) cell lines investigated, and this second class of methylation was incapable of generating TK- variants. Neither type of methylation was accompanied by genomic rearrangements. The third class of molecular alteration was found among TK+ (HAT-resistant) back revertants of hypermethylated HM TK- derivatives. It consisted of a 10-fold amplification of the hypermethylated TK genes. Demethylation of hypermethylated HM variants was not observed. Thus, hypermethylation in this system can be compensated for by amplification but cannot be reversed.
mesh_terms: D000818:Animals; D004274:DNA, Recombinant; D005784:Gene Amplification; D005786:Gene Expression Regulation; D005796:Genes; D007739:L Cells / Q000201:enzymology; D008745:Methylation; D051379:Mice; D010641:Phenotype; D018139:Simplexvirus / Q000235:genetics; D013937:Thymidine Kinase / Q000235:genetics*
chemical_list: D004274:DNA, Recombinant; D013937:Thymidine Kinase