Genetic Analysis of Azaguanine Resistance I N an Established

RESISTANCE to a guanine analogue, 8-azaguanine (AG) in mammalian cell lines has been the subject of a good deal of investigation in recent years. Resistant lines have been isolated, found to be stable in the absence of analogue, and a reverse selective system has been developed ( SZYBALSKI and SZYBALSKA 1962). In established mouse cell lines, several discrete levels of resistance have been described. LITTLEFIELD ( 1963) has studied two levels of resistance, while DAVIDSON et al. (1962) detected a third higher level of resistance in a murine line of different origin. The former author observed that lines resistant to a low (0.1 pg/ml) level of AG had levels of hypoxanthine-guanine phosphoribosyltransferase (HGPRT;EC 2.4.2.8) approximately ‘/3 that of the wild type and a second level (1 pg/ml) had no detectable enzyme, whereas the latter found that all three levels studied possessed no detectable enzyme. LITTLEFIELD proposed on the basis of his results that the low level might represent the property of a heterozygote or hemizygote. Extensive chromosome studies on drug resistant cell lines have been performed in an established pig kidney cell line, but not in the mouse. It was found that no significant difference existed in overall number or karyotype pattern between wild type and mutant (HARRIS and RUDDLE 1960). Because of the fact that areas of uncertainty exist, it was thought propitious to perform systematic studies of the AG analogue resistance system in an established mouse line. This choice was based on the fact that although the material is unfavorable from the karyotypic standpoint, several of the investigations cited above have been carried out using cells from this species. Therefore, karyotypic analysis, biochemical studies and mutation rate estimates have been performed in an effort to develop a genetical model of this system. The results will be reported in this paper.