K. Higasa et al., Molecular basis of hereditary methaemoglobinaemia, types I and II: two novel mutations in the NADH-cytochrome b(5) reductase gene, BR J HAEM, 103(4), 1998, pp. 922-930
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.