Serine - Proline Replacement at Residue 127 of NADH - Cytochrome b 5 Reductase Causes Hereditary

Hereditary methemoglobinemia is an autosomal recessive disorder characterized by NADH-cytochrome b5 reductase (b5R) deficiency. In an attempt to clarify the molecular mechanisms involved in the enzyme deficiency, we isolated the b5R gene from a patient homozygous for hereditary methemoglobinemia, generalized type, and compared its nucleotide sequence with that of the normal NADHcytochrome b5R gene. Only one difference was observed: a thymidine at the first position of codon 127 (TCT) was altered to a cytidine in the b5R gene of the patient, resulting in replacement of serine with proline. Dot blot hybridization of the amplified DNA samples with allelespecific oligonucleotide probes showed that the proband and her brothers were homozygous for this mutation and