title: Heritable alterations at the adenine phosphoribosyltransferase (APRT) locus in human lymphoblastoid cell lines.
abstract: Human lymphoblastoid cell lines derived from WI-L2 exhibit unexpected frequencies of diaminopurine (DAP) resistant mutants. The background mutant fractions of 10(-7) to 10(-8) in untreated cultures are much lower than the frequencies expected for loss of a heterozygous autosomal locus (10(-5) to 10(-6), yet much higher than expected for a homozygous locus (10(-10) to 10(-12). We used aminopterin, adenine and thymidine (AAT) to select DAP-sensitive (DAPS) revertants from one resistant line. The background frequency of DAPR in these revertant cell lines ranged from 3.5 to 6.5 x 10(-4), approximately the square root of 10(-7). Thus these data suggest that both alleles of aprt are inactivated at similarly high frequencies. They also indicate that the DAPS revertants were heterozygotes (aprt +/-) or hemizygotes (aprt +/0) and that WI-L2 was homozygous (aprt+/+). Mutational dose-response studies with X-rays, ethyl methanesulfonate (EMS), and ICR-191 were conducted in 4 of these revertant cell lines. EMS and ICR-191, which induce mainly point mutations, did not induce an increase in mutant fraction. A dose of 200 cGy X-rays, however, induced a frequency of 10(-3). Treatment of DAPR cells with 5-azacytidine induced a significant increase in reversion to DAPS. Southern blot analysis of the aprt gene after digestion with MspI or HpaII also suggests that differential methylation changes may play a major role in the generation of DAP sensitivity and resistance.
mesh_terms: D015075:2-Aminopurine / Q000031:analogs & derivatives / Q000494:pharmacology; D000228:Adenine Phosphoribosyltransferase / Q000235:genetics*; D000585:Aminacrine / Q000031:analogs & derivatives / Q000494:pharmacology; D001374:Azacitidine / Q000494:pharmacology; D015139:Blotting, Southern; D002460:Cell Line; D004247:DNA / Q000187:drug effects / Q000528:radiation effects; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D004351:Drug Resistance / Q000235:genetics; D005020:Ethyl Methanesulfonate / Q000494:pharmacology; D005787:Gene Frequency; D006801:Humans; D009153:Mutagens / Q000494:pharmacology; D009154:Mutation; D009588:Nitrogen Mustard Compounds / Q000494:pharmacology; D014965:X-Rays / Q000009:adverse effects
chemical_list: D009153:Mutagens; D009588:Nitrogen Mustard Compounds; D015075:2-Aminopurine; C007300:2,6-diaminopurine; D000585:Aminacrine; D004247:DNA; C004294:acridine half-mustard; D005020:Ethyl Methanesulfonate; D000228:Adenine Phosphoribosyltransferase; D001374:Azacitidine