The Consequence of Nucleotide Substitutions in the Triosephosphate Isomerase (TPI) Gene Promoter

ABSTRACT: Mutations at −5A→G, −8→GA within the cap proximal element (CPE), and −24T→G within the TATA box of the triosephosphate isomerase (TPI) gene promoter have been identified in populations with a wide geographical distribution. These mutations lie within, or in close proximity to, known cis-active elements in the TPI gene promoter. To determine the functional significance of mutation at these sites, which remains controversial, their effect on the expression of erythrocyte TPI enzyme activity was studied in 110 healthy unrelated subjects. The −5G mutation did not alter erythrocyte TPI level, whereas the −8A mutation was accompanied by a significant reduction in enzyme activity to around 90% and 76% of normal erythrocyte TPI activity in heterozygotes and homozygotes, respectively. The −8A−24G genotype was associated with 75% of normal TPI activity in a heterozygote studied, implying that substitution of G at position −24 within the canonical TATA motif causes an additive decrease in TPI gene transcription in erythroid cells. A DNA–protein complex of 125kDa which was competitively blocked by specific unlabelled oligomers was demonstrated at the CPE and TATA box by electrophoretic mobility shift analysis. These findings provide direct evidence that TPI promoter mutations are linked to a reduction of TPI enzyme activity in vivo.