Determination of esterase D (EsD) genotype in cases with trisomy 13

To provide a rationale for gene dosage study of esterase D (EsD) in a case with trisomy 13 where the determination of gene dosage effects is complicated owing to the dimeric structure of EsD and the diversity in catalytic activity between different EsD phenotypes, we assayed red cell EsD activities and analyzed its isozyme patterns using starch gel electrophoresis and isoelectric focusing in five cases with partial trisomy 13 (Cases 1, 2 and 3) and full trisomy 13 (Cases 4 and 5). From mean EsD activities of three common EsD phenotypes (1, 2-1 and 2) in a total of 197 normal individuals, theoretical values of the total EsD activity and activity ratios of respective EsD isozymes were calculated for each genotype expected in a trisomy case. The observed total EsD activities and activity ratios of respective EsD isozymes, which were measured on starch gel zymograms by a spectrophotometric scanner, in the five cases led us to assume that Case 1 had EsD genotype 1-1-1, Case 2 genotype 2-2-1 and the remaining three cases genotype 2-1-1. The interpretations in three cases (Cases 1, 2 and 4) were further justified by the study of EsD phenotype in their parents. Isoelectric focusing, on the other hand, was insufficient to distinguish between genotypes 2-2-1 and 2-2 or between genotypes 2-1-1 and 2-1. These results suggested that the assay of EsD activity along with analysis of EsD isozyme pattern by starch gel electrophoresis is practically the best procedure to identify EsD genotype in a trisomy individual. Furthermore, this procedure proved to be useful to ascertain the parental origin of nondisjunction in some cases with trisomy 13.