Analysis of spontaneous mutations in a chromosomally located hsv-1 thymidine kinase (TK) gene in a human cell line

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Abstract
We have developed a system for studying spontaneous mutations at a chromosomally located single-copy HSV-1 thymidine kinase (TK) gene in the human 143TK − cell line. The neo gene, which confers resistance to the antibiotic G418, was placed next to theTK gene for the purpose of screening out gross chromosomal alterations.TK − mutations were selected using the anti-TK nucleotide analogs trifluorothymidine, acyclovir, and DHPG 9-(1,3 dihydroxy-2-propoxymethyl)-guanine either separately, or in combination to eliminate leaky mutations. Analysis of theTK − mutations by Southern blotting revealed that the majority had undetectable alterations of less than 50 base pairs. The results using the methylationsensitive enzymes HpaII,AvaI, andSmaI suggest that the inactivation of the TK gene was not due to extensive methylation, although specific methylation of a limited number of MspI sites cannot be ruled out. Reversion studies, however, showed that of 16 mutants analyzed, about half had a very high reversion frequency (approximately 10−2 This suggests that inactivation of theTK gene may have occurred by a variety of mutational events.