Abstract
The conversion of methionine to inorganic sulfate involves the formation of homocysteine encompassing transmethylation followed by transsulfuration. Several inherited enzyme deficiencies within this pathway have been described.
Those causing hypermethioninemia may be confused with the many known secondary causes of increased methionine demanding diagnostic expediency. Most of the disorders have been described in small numbers of patients so that the full clinical spectrum of these is not known.
Exceptions are methionine adenosyltransferase (MAT) I/III deficiency and cystathionine ß-synthase deficiency which causes classical homocystinuria, characterized primarily by an increased risk of thrombosis and embolism, lens dislocation, and other connective tissue involvement and cognitive impairment. While methionine adenosyltransferase I/III deficiency is only symptomatic in some patients causing different neurological problems and glycine N-methyltransferase deficiency affects liver function, other diseases causing hypermethioninemias may be associated with a multisystem disease of varying severity and progression.
MAT II deficiency can be associated with thoracic aortic aneurysms in some heterozygotes for MAT2 mutations. Methanethiol oxidase deficiency causes cabbage-like breath odor (extraoral halitosis).
The association of mercaptopyruvate sulfur transferase deficiency with cognitive impairment, as the only disease characteristic, is questionable. Isolated sulfite oxidase deficiency is characterized by refractory convulsions in early infancy, brain atrophy, severe psychomotor retardation, and lens dislocation.
Ethylmalonic encephalopathy is a severe disorder manifesting with seizures, developmental delay and cognitive impairment, orthostatic acrocyanosis and petechia due to vasodilation, failure to thrive, and chronic hemorrhagic diarrhea. Measurement of plasma and urine amino acids and total homocysteine can detect many of the disorders described in this chapter, while other tests are necessary for others.
Confirmatory tests are enzyme assays and/or mutation analysis. Treatment combines one or more of dietary restriction of precursors, substitution of essential products, pharmacologic doses of cofactors, and binding and removing of harmful metabolites.
Early diagnosis and early treatment favor better outcome.