High rate of multilocus deletion in a human tumor cell line

Abstract

The nature of recessive mutations at the autosomal locus encoding the purine salvage enzyme adenine phosphoribosyl transferase (APRT) was analyzed in a highly malignant human tumor cell line (the colorectal carcinoma line SW620). Mutant strains resistant to the purine analog 8-azaadenine were obtained in two steps. The first step selection for partial drug resistance produced strains hemizygous for APRT as a result of high frequency loss of one allele. In the second step selection, low frequency base substitutions, small deletions, or insertions produced complete azaadenine resistance. Luria-Delbruck fluctuation analysis of each step of this process indicated that the rate of mutation resulting from allele loss was over 100-fold greater than the rate of mutation resulting from base substitution. There was no reproducible difference in the rate of loss of either of the two APRT alleles even though one maps to a rearranged chromosome. Similarly base substitution rates for the two alleles were not significantly different. Polymorphic loci surrounding APRT on chromosome 16 band q24 were lost together with the selected gene in all isolates while polymorphic loci on the short arm of the chromosome were retained. Thus the high frequency loss of APRT in SW620 appears to be the result of multilocus deletions. SW620 derivatives behaving as heterozygotes were also obtained in the first step selections, but these constituted only 5% of isolates.

The mutation rate and cancer

“…There seems little doubt that some sporadic cancers do have raised intrinsic mutation rates (18)(19)(20). Our models suggest four main ways in which this can occur.…”

Loss of Heterozygosity and Base Substitution at the APRT Locus in Mismatch-Repair-Proficient and -Deficient Colorectal Carcinoma Cell Lines

“…This colorectal carcinoma cell line has no detectable microsatellite instability and low rates of point mutation at selectable loci (Table 1) (4,17). On the other hand, SW620 has a high rate of multilocus deletions that result in loss of one copy of the APRT gene (16,17). Base substitutions at APRT in DLD-1 were compared with those occurring in HUS12, a derivative of SW620 having a single copy of the APRT gene.…”

“…Extent of the LOH events. In the mismatch-repair-proficient line SW620 the loss of one APRT allele occurred at a high rate (6 ϫ 10 Ϫ6 [16]). Using a TaqI polymorphism present in intron 2 of the APRT gene, we demonstrated that there was no difference in the rate of loss of either of the two APRT alleles even though one of the APRT genes was present on a rearranged chromosome.…”

Mutator phenotypes in human colorectal carcinoma cell lines.

“…For example, SW620 cell line shows very low rates of base substitution but exhibits a substantially altered karyotype and high rates of multilocus deletion (25,37). This cell line also displays an unusual mechanism of mutation in the form of multiple base substitutions or fiame-shifts that apparently occur as a result of a single initiating event (25).…”