A novel nine base deletion mutation in NADH-cytochrome b5 reductase gene in an Indian family with recessive congenital methemoglobinemia-type-II

Recessive hereditary methemoglobinemia (RCM) associated with severe neurological abnormalities is a very rare disorder caused by NADH- cytochrome b5 reductase (cb5r) deficiency (Type II). We report a case of 11 month old male child who had severe mental retardation, microcephaly and gross global developmental delay with methemoglobin level of 61.1%. The diagnosis of NADH-CYB5R3 deficiency was made by the demonstration of significantly reduced NADH-CYB5R3 activity in the patient and intermediate enzyme activity in both the parents. Mutation analysis of the CYB5R gene revealed a novel nine nucleotide deletion in exon 6 leading to the elimination of 3 amino acid residues (Lys173, Ser174 and Val 175). To confirm that this mutation was not an artifact, we performed PCR-RFLP analysis using the restriction enzyme Drd I. As the normal sequence has a restriction recognition site for Drd I which was eliminated by the deletion, a single band of 603-bp was seen in the presence of the homozygous mutation. Molecular modeling analysis showed a significant effect of these 3 amino acids deletion on the protein structure and stability leading to a severe clinical presentation. A novel homozygous 9 nucleotide deletion (p.K173–p.V175del3) is shown to be segregated with the disease in this family. Knowing the profile of mutations would allow us to offer prenatal diagnosis in families with severe neurological disorders associated with RCM — Type II.