X-LINKED PYRIDOXINE-RESPONSIVE SIDEROBLASTIC ANEMIA DUE TO A THR(388)-TO-SER SUBSTITUTION IN ERYTHROID 5-AMINOLEVULINATE SYNTHASE

Background. X-linked sideroblastic anemia is usually associated with r educed 5-aminolevulinate synthase activity in erythroid cells, and som e cases are responsive to treatment with pyridoxine, the precursor to the cofactor of the enzyme. The recently identified gene for an erythr oid-specific 5-aminolevulinate synthase isoenzyme and its localization to the X chromosome make it likely that one or more defects in this g ene underlie the anemia. Methods. Using a polymorphic dinucleotide-rep eat sequence in the erythroid 5-aminolevulinate synthase gene, we conf irmed the linkage of this gene to the disorder in a family with X-link ed pyridoxine-responsive sideroblastic anemia. We therefore sought evi dence of a nucleotide-sequence abnormality in the erythroid 5-aminolev ulinate synthase gene by analyzing enzymatically amplified DNA. Result s. DNA-sequencing studies in two affected males and one carrier female in the kindred demonstrated a cytosine-to-guanine change at nucleotid e 1215 (in exon 8). This change results in the substitution of serine for threonine at amino acid residue 388, near the lysine that binds th e pyridoxal phosphate cofactor. In expression studies, the activity of the mutant enzyme was reduced relative to that of the wild type, and this reduction was comparable to that in erythroid cells of the proban d during relapse of the anemia; the enzyme activity expressed in the p resence of pyridoxine was comparable to that in the proband's marrow c ells during remission. Although the affinity of the mutant enzyme for pyridoxal phosphate was not altered, the mutation appears to introduce a conformational change at the active site of the enzyme. Conclusions . We identified a point mutation resulting in an amino acid change nea r the pyridoxal phosphate-binding site of the erythroid 5-aminolevulin ate synthase isoenzyme as the underlying defect in a kindred with X-li nked pyridoxine-responsive sideroblastic anemia.