Methemoglobinemia In Rhesus Monkeys Due To M-Hemoglobinopathy

M-hemoglobinopathies in humans are usually due to amino acid changes, which tend to stabilize the heme iron in ferric state and inaccessible to methemoglobin reductase. Consequently, blood methemoglobin concentrations are elevated despite normal methemoglobin reductase activity. No naturally-occurring animal models for M-hemoglobinopathies have been described.

In contrast to humans where thousands of hereditary disorders and genetic predispositions to disease have been described, inborn errors of metabolism in non-human primates have been very rarely documented (e.g. globoid cell leukodystrophy). As part of an NIH-supported National Referral Center for Animal Models of Human Genetic Disease, which focuses on homologous hereditary diseases in dogs and cats, we present here the discovery and characterization of an M-hemogobinopathy in rhesus monkeys (Macaca mulatta).

Adult rhesus monkeys of Chinese origin with persistent cyanotic mucous membranes and healthy controls were studied from two institutions. Despite the apparent cyanosis, these primates did clinically not show any other overt signs. Routine complete blood cell counts, chemistry screens and urinalyses as well as cardiopulmonary examinations did not reveal any abnormalities. Fresh EDTA blood samples were analyzed to determine the degree and cause of methemoglobinemia and results were compared to healthy rhesus monkeys from the same institutions.

Blood methemoglobin concentrations were persistently increased to 15-30% in affected monkeys, while control animals had <1% methemoglobin levels. Methemoglobin (cytochrome b5) reductase activities in erythrocytes were repeatedly measured in vitro and found to be normal compared to controls. Based upon the hemoglobin-oxygen dissociation curves and p50 values (14.8 mmHg; compared to 14.5 and 14.6 mmHg for controls) the oxygen transport function was normal. Both cation-exchange as well as reversed-phase and high performance liquid chromatography analyses revealed normal elution patterns of hemoglobin isotype and globin chains. DNA studies identified a disease-associated polymorphism in the alpha globin gene (CAC → CAA) in the affected animals. This change in the alpha globin protein was confirmed by mass spectrometry (α78 His → Gln).

Our findings indicate an electrophoretically silent α-globin polymorphism is associated with methemoglobinemia in these rhesus monkeys. This is the first description of methemoglobinemia in a non-human primate and represents the first animal model of an M-hemoglobinopathy. While clinical signs were relatively mild in these rhesus monkeys, our findings represent one of the first inborn errors of metabolism described in non-human primates, thus emphasizing how unusual disorders caused by “experiments of nature” provide rare opportunities to gain a better understanding of both medicine and biology. Rhesus monkeys represent unique, valuable animal models for developing and testing new treatment options for hemoglobinopathies.