Identification and characterization of the novel FAD-binding lobe G75S mutation in cytochrome b(5) reductase: an aid to determine recessive congenital methemoglobinemia status in an infant

NADH-cytochrome b(5) reductase deficiency results clinically in either type I or type II recessive congenital methemoglobinemia. The more severe type II form is associated with a global deficiency of cytochrome b(5) reductase and is characterized by cyanosis with neurological dysfunction. In contrast, the only symptom for type I is cyanosis. We have identified a novel G to A mutation at position 15,635 in the DIAI gene of a 4-month-old baby that results in a glycine to serine substitution at codon 75 in the cytochrome b(5) reductase protein. The G75S mutation, located in the FAD-binding lobe of cytochrome b(5) reductase, was found in association with the previously described V252M variant. The V252M mutation is present in the NADH-binding domain and associated with both types I and II recessive congenital methemoglobinemia. Since the G75S and V252M mutations represent radical changes in differing regions of cytochrome b(5) reductase, generating and characterizing these variants singly and in combination using a rat heterologous expression system would provide insight into the differences between types I and II disease at the molecular level. Although all three variants were found to retain stoichiometric levels of FAD with spectroscopic and thermodynamic properties comparable to those of native cytochrome b(5) reductase, all exhibited decreased catalytic efficiency and reduced protein stability reflecting the position of the mutations in the primary structure. The G75S variant retained only 11% of the catalytic efficiency of the wild-type enzyme. Thus, cytochrome b(5) reductase deficient patients who are heterozygous for either FAD- or NADH-binding lobe mutations can exhibit the clinically less severe type I phenotype.

Recessive congenital methaemoglobinaemia: functional characterization of the novel D239G mutation in the NADH-binding lobe of cytochrome b5 reductase

Type I recessive congenital methaemoglobinaemia (RCM), caused by the reduced form of nicotinamide adenine dinucleotide (NADH)-cytochrome b(5) reductase (cytb(5)r) deficiency, manifests clinically as cyanosis without neurological dysfunction. Two mutations, E255- and G291D, have been identified in the NADH-binding lobe of cytb(5)r in previously reported patients, and we have detected a further novel mutation, D239G, in this lobe in two unrelated Irish families. Although one family belongs to the genetically isolated Traveller Community, which separated from the general Irish population during the 1845-48 famine, the D239G mutation was present on the same haplotype in both families. Three known cytb(5)r mutations were also identified, including the R159- mutation, which causes loss of the entire NADH-binding lobe and had previously been reported in an individual with type II RCM. Characterization of the three NADH-binding lobe mutants using a heterologous expression system revealed that all three variants retained stoichiometric levels of flavin adenine dinucleotide with spectroscopic and thermodynamic properties comparable with those of native cytb(5)r. In contrast to the E255- and G291D variants, the novel D239G mutation had no adverse impact on protein thermostability. The D239G mutation perturbed substrate binding, causing both decreased specificity for NADH and increased specificity for NADPH. Thus cytb(5)r deficient patients who are heterozygous for an NADH-binding lobe mutation can exhibit the clinically less severe type I phenotype, even in association with heterozygous deletion of the NADH-binding lobe.

Expression of a novel P275L variant of NADH:cytochrome b5 reductase gives functional insight into the conserved motif important for pyridine nucleotide binding

The clinical disorder of recessive congenital methemoglobinemia (RCM, OMIN 250800) is associated with mutations in NADH:cytochrome b5 reductase (cb5r) and manifests as cyanosis from birth. Screening a cyanotic infant indicated elevated methemoglobin levels and decreased cb5r activity suggesting RCM. Sequencing the DIA1 gene encoding cb5r revealed a novel mutation, C27161T (NCBI accession number: NT_011520), resulting in replacement of proline at amino acid 275 with leucine (P275L). To understand how this mutation would affect cb5r's function, the P275L variant was expressed in a heterologous expression system and spectroscopic, thermodynamic, and thermostability studies were performed. The leucine substitution at residue 275 was found to significantly decrease the affinity towards the physiological reducing substrate, NADH, without affecting the activity of the P275L variant. From the rat model, residue 275 is predicted to be part of a conserved "CGPPPM" motif important for the binding and correct positioning of the NADH reducing substrate. Thus P275 influences the interaction with NADH which was confirmed by the change in affinity towards the physiological reducing substrate.