Analysis of second-step mutations of class II and class III CHO aprt heterozygotes: chromosomal differences in deletion frequencies.
We have determined the nucleotide sequence surrounding a BclI restriction fragment length variation near the aprt gene of CHO cells. By BclI digestion of the PCR-amplified DNA from a variety of APRT-deficient mutants derived from CHO cells, we were able to infer the following. First, all three heterozygotes of class II, which are known to undergo the second mutational step via a large deletion event occurring at high frequency, are mutant at the chromosome Z4-linked allele, and wild type at the Z7 allele. Second, both class-III heterozygotes, which mutate to the APRT- phenotype at low frequency, are mutant at the Z7 allele, wild type at the Z4 allele. A total of 12 class-I lines, defined as having already undergone a deletion event and yielding fully APRT- mutants at low frequency were all found to have lost the Z7-linked allele. We conclude that the Z7-linked allele is substantially more susceptible to mutation by the large deletion event than is the Z4-linked allele. This supports a hypothesis we advanced earlier to explain the existence of the class-III heterozygotes but does not support previous work suggesting that a chromosomal inversion break-point junction near the Z4-linked aprt allele is responsible for the high frequency deletion event.
Institut du cancer de Montréal, Québec, Canada.
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Belouchi
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Bradley
D000228:Adenine Phosphoribosyltransferase / Q000235:genetics*; D000483:Alleles; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D002872:Chromosome Deletion; D006224:Cricetinae; D003412:Cricetulus; D005260:Female; D005787:Gene Frequency; D008040:Genetic Linkage; D006579:Heterozygote; D008969:Molecular Sequence Data; D009154:Mutation; D010053:Ovary; D016133:Polymerase Chain Reaction; D012150:Polymorphism, Restriction Fragment Length
D000228:Adenine Phosphoribosyltransferase