CYB5R3 – Recessive Congenital Methemoglobinemia

Recessive congenital methemoglobinemia is caused by biallelic loss‐of‐function variants in CYB5R3, encoding NADH‐cytochrome b5 reductase, which catalyzes reduction of methemoglobin to hemoglobin. Clinically, type I presents with lifelong cyanosis due to erythrocyte‐restricted enzyme deficiency, whereas type II manifests with generalized enzyme deficiency, severe neurodevelopmental impairment, microcephaly, hypotonia, and corpus callosum hypoplasia in addition to cyanosis and hypoxemia.

Genetic evidence for CYB5R3 involvement in methemoglobinemia is robust: over 78 distinct pathogenic variants have been reported in more than 100 unrelated individuals worldwide, including nonsense, missense, splice‐site, frameshift, and small‐deletion alleles. Biallelic CYB5R3 variants have been identified in single cases (e.g., c.478C>T (p.Arg160Ter) (PMID:10874300)) and cohorts (seven families with type I harboring seven novel variants (PMID:11159544)).

Inheritance is autosomal recessive, with segregation confirmed in multiple sibships and 19 additional affected relatives across families (PMID:2107882). Case reports describe homozygous missense mutations such as c.653T>C (p.Leu218Pro) in a six-year-old with isolated cyanosis (PMID:19579085) and compound heterozygous stop codons c.229C>T (p.Gln77Ter) and c.478C>T (p.Arg160Ter) in a patient with type II neurodevelopmental delay (PMID:10874300).

Functional assays vigorously support a loss‐of‐function mechanism: in vitro enzyme activity is reduced to ≤10% of normal for missense and splice‐site mutants, nonsense alleles yield truncated proteins, and heterologous expression studies demonstrate impaired NADH binding, thermostability, and electron transfer (PMID:8874222; PMID:30309019). Detailed biochemical and structural analyses of p.Ser128Pro and p.Arg192Cys variants delineate disrupted FAD/NADH‐binding domains and conformational instability.

Animal and cell models further recapitulate the human phenotype: CYB5R3‐deficient zebrafish and murine cells exhibit elevated methemoglobin, cyanosis, neurodevelopmental defects, and rescue with ascorbic acid or riboflavin. Concordance between genotype, enzyme deficiency, and clinical severity underscores the critical role of CYB5R3 dosage in erythrocytes (type I) versus systemic tissues (type II).

Key Take-home

CYB5R3 testing provides a definitive molecular diagnosis for methemoglobinemia, enables early therapeutic interventions (methylene blue, antioxidants), informs prognosis for neurological involvement, and guides reproductive counseling.

References

• Human mutation • 2000 • A case of methemoglobinemia type II due to NADH-cytochrome b5 reductase deficiency: determination of the molecular basis. PMID:10874300
• Pediatric hematology and oncology • 2009 • A novel L218P mutation in NADH-cytochrome b5 reductase associated with type I recessive congenital methemoglobinemia. PMID:19579085
• Blood • 2001 • Seven new mutations in the nicotinamide adenine dinucleotide reduced-cytochrome b(5) reductase gene leading to methemoglobinemia type I. PMID:11159544
• Blood • 1996 • Two novel mutations in the reduced nicotinamide adenine dinucleotide (NADH)-cytochrome b5 reductase gene of a patient with generalized type, hereditary methemoglobinemia. PMID:8874222
• Journal of clinical medicine • 2018 • Cellular and Molecular Mechanisms of Recessive Hereditary Methaemoglobinaemia Type II. PMID:30309019

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