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The promoter region of the human GSTP1 gene contains a polymorphic short tandem repeat (STR) locus consisting of pentanucleotide repeat units (ATAAA). This repeat region was originally reported to have allele sizes differing in 5 base pair increments, with 10 alleles observed (Harada et al., 1994). Later, Smith et al., reported it as having an imperfect TAAAA repeat interrupted by a single TA and TAA insertion, with a total of 11 alleles observed. In this work, we report the existence of 20 alleles in a Caucasian population differing in size by 1 to 5 base pair increments and, in one case, by 10 base pairs. This variability was first observed by analysis of the PCR amplified DNA fragments run on native polyacrylamide gels. Genotyping was difficult due to very short migration distances between the alleles, corresponding to differences smaller than the repeat unit. Furthermore, the observation, in some cases, of different heteroduplex patterns in heterozygotes with identical homoduplexes, suggested structural differences between alleles of the same size. Automatic fragment sizing and sequencing analysis eventually revealed that this polymorphism is of a highly complex nature. Allelic designations now proposed are based on the sequence data.

We also report our studies performed on a Northern Portuguese population. 19 alleles were observed and their frequencies were estimated from 196 unrelated individuals. The population showed no deviation from Hardy-Weinberg equilibrium (P=0.162). The heterozygosity observed was 0.74 and the repeat has a polymorphism information content of 0.70. Analysis of nuclear families demonstrated that alleles were inherited in a codominant fashion and in 81 mother/child pairs no germinal instability was detected.