Type I Methemoglobinemia Caused by the Cytochrome b5 Reductase 806C>T Mutation Is Present in the Indigenous Evenk People of Yakutia.

Congenital methemoglobinemia is an autosomal recessive metabolic disorder due to NADH-cytochrome b5 reductase (cytb5r, EC 1.6.2.2) deficiency. This enzyme exists in soluble and membrane-bound forms. The soluble erythrocytic cytb5r isoenzyme is involved in cytochrome b5 reduction and in erythrocyte methemoglobin reduction; the membrane-bound microsomal enzyme participates in a fatty acid desaturation complex and in drug metabolism. The cytb5r isoforms are a product of a single gene locus, DIA1 (or CYB5R3), on chromosome 22. Type I methemoglobinemia, a benign form in which cyanosis is the only phenotype, is characterized by cytb5r deficiency restricted to red blood cells. In less common type II methemoglobinemia, chronic cyanosis is associated with severe neurological and developmental deficits, including mental retardation, microcephaly, generalized dystonia and movement disorders. More then 40 mutations have been reported to date in the DIA1 gene, which either cause type I or type II methemoglobinemia; the majority are missense mutations and are associated with type I disease. Both methemoglobinemia types are sporadic worldwide but are claimed to be endemic among the Yakut people in Siberia, the Aleutians in Alaska and the Navajo tribe in the continental US. In 2006, a new mutation in exon 9 of the CYB5R3 gene (806C>T, Pro269Leu) was identified in 38 patients from the indigenous population of Yakutia in northeastern Siberia, a part of the Russian Federation (1). The frequency of homozygotes was reported to be 1 in 5677. The Sakha region of the Yakutia Republic has an area of 1,200,000 sq miles and a population <1 million composed of 45.5% Yakuts, 41.2% Russians, 3.7% Ukrainians and indigenous people including 1.9% Evenks, 1.2% Evens, 0.1% Dolgans, and 0.1% Yukagirs.

We screened DNA of 162 subjects' samples taken from children of indigenous people from 4 different places in Sakha for the mutation 806C>T in the CYB5R3 gene using the the AluI and AciI restriction enzymes that recognize this mutation, and the results were confirmed by sequencing of the PCR product.

The study sample included 70 Evenks, 35 Evens, 26 Sakha (Yakuts), 23 Yukagirs, 4 Chukchas, 2 Dolgans, 1 Nenets and 1 Tatar. We found 2 806C>T heterozygous samples detected by the AciI restriction enzyme and confirmed by sequencing; both subjects were Evenks. The enzyme AluI produced a partial cut in another 22 samples that could not by confirmed as a true mutation by sequencing. Thus, the analysis with the AluI restriction enzyme frequently falsely identified heterozygotes among these 162 participants.

In screening for the CYB5R3 806C>T mutation, the AciI restriction enzyme should be used instead of the AluI enzyme. Based on these results and according to Hardy-Weinberg equilibrium, the predicted frequency of homozygotes for the CYB5R3 806C>T gene mutation is 1 in 4,444 among the Evenks. These data suggest that the CYB5R3 806C>T mutation may be endemic among the Evenks indigenous people. Further, it remains to be determined whether the CYB5R3 806C>T mutation is also causative of type I methemoglobinemia in the Aleutian and Navajo peoples whose ancestors migrated to North America from Siberia. These data are preliminary and larger population-based studies using the AciI restriction enzyme are planned.

This work was supported by the ASH Visitor Training Program Award and by NHLBI Grant 2 UH1-HL03679.

1) Гaлeeвa H.M., Haзapeнкo Л.П., Haзapeнкo C.A., Tвepcкaя C.M., Пoлякoв A.B. Moлeкyляpнo-гeнeтичecкaя пpичинa нacлeдcтвeн|Ryoй мeтгeмoглoбинeми|Rb лepвoгo типa в Якyти|Rb. Meдицинcкaя Гeнeтикa (2006).

No relevant conflicts of interest to declare.