Methemoglobinemia due to NADH-cytochrome b5 reductase deficiency is an autosomal recessive disorder characterized clinically by decreased oxygen carrying capacity of the blood, with resultant cyanosis and hypoxia (review by Percy and Lappin, 2008). There are 2 types of methemoglobin reductase deficiency. In type I, the defect affects the soluble form of the enzyme, is restricted to red blood cells, and causes well-tolerated methemoglobinemia. In type II, the defect affects both the soluble and microsomal forms of the enzyme and is thus generalized, affecting red cells, leukocytes, and all body tissues. Type II methemoglobinemia is associated with mental deficiency and other neurologic symptoms. The neurologic symptoms may be related to the major role played by the cytochrome b5 system in the desaturation of fatty acids (Vives-Corrons et al., 1978; Kaplan et al., 1979).

Methemoglobinemia is a clinical condition in which more than 1% of hemoglobin is oxidized to methemoglobin, a type of hemoglobin that contains the ferric (Fe3+) form of iron. Patients with hemoglobin M are cyanotic but otherwise asymptomatic. If the mutation occurs in the hemoglobin alpha subunit (141800), cyanosis is apparent at birth, whereas if the beta chain is affected, cyanosis appears later or intensifies when beta subunit production increases. If a newborn carries a fetal M hemoglobin (gamma subunit; 142250), cyanosis disappears when the complete gamma-beta-switch occurs (summary by Mansouri and Lurie, 1993).

Neonatal cyanosis is characterized by symptoms in the fetus and neonate that gradually abate by 5 to 6 months of age. The disorder is caused by a defect in the fetal hemoglobin chain, which causes reduced affinity for oxygen due to steric inhibition of oxygen binding and/or due to increased oxidation of the fetal hemoglobin molecule to methemoglobin (Hb FM), which has decreased oxygen-binding capacity. Some patients develop anemia resulting from increased destruction of red cells containing abnormal or unstable hemoglobin. The cyanosis resolves spontaneously by 5 to 6 months of age or earlier, as the adult beta-globin chain (HBB; 141900) is produced and replaces the fetal gamma-globin chain (summary by Crowley et al., 2011).

Methemoglobinemia and ambiguous genitalia (METAG) is due to isolated 17,20-lyase deficiency, defined by apparently normal 17-alpha-hydroxylase activity but severely reduced 17,20-lyase activity of the CYP17A1 enzyme (609300), which results in sex steroid deficiency but normal glucocorticoid and mineralocorticoid reserve. The clinical phenotype is characterized by male undermasculinization, with absent or disturbed pubertal development in both 46,XY and 46,XX individuals. Mild to severe methemoglobinemia has been reported in these patients (Idkowiak et al., 2012). Other autosomal recessive methemoglobinemias include types I and II (see 250800), caused by mutation in the CYB5R3 gene (613213). Isolated 17,20-lyase deficiency can also be caused by mutation in the CYP17A1 gene (609300), and mutation in the POR gene can manifest clinically as isolated 17,20-lyase deficiency (see 124015.0016).

Methemoglobinemia is a clinical condition in which more than 1% of hemoglobin is oxidized to methemoglobin, a type of hemoglobin that contains the ferric (Fe3+) form of iron. Patients with hemoglobin M are cyanotic but otherwise asymptomatic. If the mutation occurs in the hemoglobin alpha subunit, cyanosis is apparent at birth, whereas if the beta chain (141900) is affected, cyanosis appears later or intensifies when beta subunit production increases. If a newborn carries a fetal M hemoglobin (gamma subunit; 142250), cyanosis disappears when the complete gamma-beta-switch occurs (summary by Mansouri and Lurie, 1993).