4 NEW MUTATIONS IN THE NADH-CYTOCHROME B5 REDUCTASE GENE FROM PATIENTS WITH RECESSIVE CONGENITAL METHEMOGLOBINEMIA TYPE-II

Recessive congenital methemoglobinemia (RCM) due to NADH-cytochrome b5 reductase (cytb5r) deficiency leads to two different types of disease s. In the type I form, cyanosis is the only symptom, and the soluble e nzyme is defective in red blood cells. In the type II form, cyanosis i s associated with severe mental retardation and neurologic impairment; the enzymatic defect is systemic, involving both soluble and membrane -bound isoforms. We characterized mutations responsible for cytb5r def iciency in three unrelated patients with severe RCM type II. The first patient presented a homozygous exon 5 skipping. The only mutation det ected was a homozygous G to C transversion at position +8, downstream from the 5' splice site of exon 5. We suggest that this unusual mutati on might be responsible for the abnormal splicing of the primary trans cripts, resulting in frameshift with premature STOP codon. The second mutation found corresponds to a homozygous C to T transition changing the Arg-218 codon to a premature STOP codon in exon 8. The third case was a compound heterozygote, carrying two different mutant alleles in the cyb5r gene. One allele presented a missense mutation with replacem ent of Cys-203 (TGC) by Arg (CGC) in exon 7. The second allele carried a 3-bp deletion (TGA) of nucleotides 815 to 817, modifying two contig uous codons in exon 9 of the cDNA with loss of Met-272. These results confirm the genetic polymorphism of cytb5r gene mutations identified i n RCM type II, as observed for the mutations described in the RCM type I, and shed light on the molecular bases of the two different disease s associated with cytb5r deficiency. (C) 1995 by The American Society of Hematology.