The Genetic Basis of Human Erythrocyte Pyridoxal Kinase Activity Variation.

Abstract

Previous studies have shown that there is a striking difference in the erythrocyte activity of the human pyridoxal kinase (PdxK) enzyme, with African-Americans having approximately half the mean activity of their Caucasian counterparts. This difference was observed only in erythrocytes, with leukocyte and skin fibroblast PdxK enzyme activities being similar in both ethnic groups (Science187: 1084–1086, 1975). At the time of the original report it was impossible to elucidate the mechanism by which evolution had selectively lowered the enzyme activity in one cell type but not in another. To now determine the genetic basis for this variation, the entire PdxK gene was sequenced to identify candidate mutations in a sample of Caucasian and African-Americans. A number of variations were discovered, with one interestingly being a deletion located in the promoter region. This deletion was found to occur at a gene frequency of 42.7% in Caucasians (n=56), 25.3% in Asians (n=162) and 56.6% in African-Americans (n=334). Sequence analysis of the PdxK promoter using the software program MatInspector (Nucleic Acids Research23: 4878–4884, 1995) revealed that the deletion removed a putative Core-Promoter Binding Promoter (CPBP) transcription factor binding site. The CPBP transcription factor is known to enhance transcriptional activity by at least four fold and is thought to be a general regulator of TATA box-less genes (Journal of Biological Chemistry272: 9573–9580, 1997). The potential CPBP site was located -300bp to the PdxK start codon and was adjacent to a binding site for the erythroid specific transcription factor, Erythroid Krueppel Like Factor (EKLF). This suggested that the presence of the deletion would cause decreased transcription of PdxK in erythroid precursors only. The presence of the deletion element was found to be significantly associated with a reduced erythrocyte PdxK activity (p=0.0017). Although other modifiers of PdxK activity, such as diet, were found to play a role, the identified promoter deletion allows altered erythrocyte specific transcription to exclusively influence erythrocyte PdxK enzyme activity.