Molecular basis of hereditary methaemoglobinaemia, types I and II: two novel mutations in the NADH-cytochrome b(5) reductase gene

Hereditary methaemoglobinaemia, caused by deficiency of NADH-cytochrome b(5 ) reductase (b5R), has been classified into two types, an erythrocyte (type I) and a generalized (type II). We analysed the b5R gene of two Thai patie nts and found two novel mutations. The patient with type II was homozygous for a C-to-T substitution in codon 83 that changes Arg (CGA) to a stop codo n (TGA), resulting in a truncated b5R without the catalytic portion. The pa tient with type I was homozygous for a C-to-T substitution in codon 178 cau sing replacement of Ala (GCG) with Val (GTG). To characterize effects of th is missense mutation, we investigated enzymatic properties of mutant b5R (A la 178 Val). Although the mutant enzyme showed normal catalytic activity, l ess stability and different spectra were observed. These results suggest th at this substitution influenced enzyme stability due to the slight change o f structure. In conclusion, the nonsense mutation led to type II because of malfunction of the truncated protein. On the other hand, the missense muta tion caused type I, due to degradation of the unstable mutant enzyme with n ormal activities in patient's erythrocytes, because of the lack of compensa tion by new protein synthesis during the long life-span of erythrocytes.