Abstract
Genotype–phenotype correlations of humans and dogs with hereditary methemoglobinemia are not yet well characterized. We determined total hemoglobin and methemoglobin (MetHb) concentrations, cytochrome b5 reductase (CYB5R) enzyme activities, genotypes, and clinical signs in 30 dogs with persistent cyanosis without cardiopulmonary disease. Erythrocytic CYB5R enzyme activities were low in all dogs assayed. Owner-reported quality of life ranged from subclinical to occasional exertional syncope. Two previously reported and two novel CYB5R3 missense variants were identified among the methemoglobinemic cohort and were predicted to impair enzyme function. Two variants were recurrent: a homozygous Ile194Leu substitution was found in Pomeranians and other small dogs, and a homozygous Arg219Pro change occurred predominately in pit bull terriers. The other two variants were Thr202Ala and Gly76Ser substitutions in single dogs. Of the two common CYB5R3 genotypes, Arg219Pro was associated with a more severe metabolic phenotype. We conclude that CYB5R3 deficiency is the predominate cause of canine hereditary methemoglobinemia. Although this finding is unlikely to alter the clinical approach to hereditary methemoglobinemia in dogs, it demonstrates the possibility of how genotype–phenotype cohort analysis might facilitate precision medicine in the future in veterinary medicine.
Highlights
Genotype–phenotype correlations of humans and dogs with hereditary methemoglobinemia are not yet well characterized
Eight dogs had a historical diagnosis of persistent, unexplained cyanosis and methemoglobinemia for a variable period of time before inclusion
We characterize a cohort of 30 dogs with persistent methemoglobinemia with respect to clinical spectrum, biochemical derangements, and molecular defects as well as provide phenotype-genotype correlations for two common breed-related CYB5R3 gene variants
Summary
Genotype–phenotype correlations of humans and dogs with hereditary methemoglobinemia are not yet well characterized. We determined total hemoglobin and methemoglobin (MetHb) concentrations, cytochrome b5 reductase (CYB5R) enzyme activities, genotypes, and clinical signs in 30 dogs with persistent cyanosis without cardiopulmonary disease. We conclude that CYB5R3 deficiency is the predominate cause of canine hereditary methemoglobinemia. Deficiency of the soluble type I CYB5R (EC 1.6.2.2), a flavoprotein of the transhydrogenase family of oxidoreductase enzymes, in erythrocytes is the most common cause for hereditary methemoglobinemia in humans (OMIM 250800)[5]. Other causes of hereditary methemoglobinemia in humans are exceedingly rare and include cytochrome b 5 deficiency and hemoglobin M disorders, of which only the former has been suspected in a single dog[15,16,17,18]. Associations between phenotype (clinical signs and biochemical derangements) and genotype have been established in humans but not yet in d ogs[19]
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