Enzymatic defect in "X-linked" sideroblastic anemia: molecular evidence for erythroid delta-aminolevulinate synthase deficiency.
Recently, the human gene encoding erythroid-specific delta-aminolevulinate synthase was localized to the chromosomal region Xp21-Xq21, identifying this gene as the logical candidate for the enzymatic defect causing "X-linked" sideroblastic anemia. To investigate this hypothesis, the 11 exonic coding regions of the delta-aminolevulinate synthase gene were amplified and sequenced from a 30-year-old Chinese male with a pyridoxine-responsive form of X-linked sideroblastic anemia. A single T----A transition was found in codon 471 in a highly conserved region of exon 9, resulting in an Ile----Asn substitution. This mutation interrupted contiguous hydrophobic residues and was predicted to transform a region of beta-sheet structure to a random-coil structure. Prokaryotic expression of the normal and mutant cDNAs revealed that the mutant construct expressed low levels of enzymatic activity that required higher concentrations of pyridoxal 5'-phosphate to achieve maximal activation than did the normal enzyme. The amino acid substitution occurred in the exon containing the putative pyridoxal 5'-phosphate binding site and may account for the reduced ability of the cofactor to catalyze the formation of delta-aminolevulinic acid.
Cotter
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Division of Medical and Molecular Genetics, Mount Sinai School of Medicine, New York, NY 10029.
Baumann
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Bishop
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D000624:5-Aminolevulinate Synthetase / Q000172:deficiency* / Q000235:genetics; D000756:Anemia, Sideroblastic / Q000201:enzymology*; D001483:Base Sequence; D006801:Humans; D006860:Hydrogen Bonding; D008297:Male; D008969:Molecular Sequence Data; D009154:Mutation; D009838:Oligodeoxyribonucleotides / Q000737:chemistry; D011487:Protein Conformation; D016415:Sequence Alignment; D014960:X Chromosome
D009838:Oligodeoxyribonucleotides; D000624:5-Aminolevulinate Synthetase