What are the genetic changes related to autosomal recessive congenital methemoglobinemia ?
Autosomal recessive congenital methemoglobinemia is caused by mutations in the CYB5R3 gene. This gene provides instruction for making an enzyme called cytochrome b5 reductase 3. This enzyme is involved in transferring negatively charged particles called electrons from one molecule to another. Two versions (isoforms) of this enzyme are produced from the CYB5R3 gene. The soluble isoform is present only in red blood cells, and the membrane-bound isoform is found in all other cell types.  Each hemoglobin molecule contains four iron atoms, which are needed to carry oxygen. In normal red blood cells, the iron in hemoglobin is ferrous (Fe2+), but it can spontaneously become ferric (Fe3+). When hemoglobin contains ferric iron, it is methemoglobin. The soluble isoform of cytochrome b5 reductase 3 changes ferric iron back to ferrous iron so hemoglobin can deliver oxygen to tissues. Normally, red blood cells contain less than 2 percent methemoglobin.  The membrane-bound isoform is widely used in the body. This isoform is necessary for many chemical reactions, including the breakdown and formation of fatty acids, the formation of cholesterol, and the breakdown of various molecules and drugs.  CYB5R3 gene mutations that cause autosomal recessive congenital methemoglobinemia type I typically reduce enzyme activity or stability. As a result, the enzyme cannot efficiently change ferric iron to ferrous iron, leading to a 10 to 50 percent increase in methemoglobin within red blood cells. This increase in methemoglobin and the corresponding decrease in normal hemoglobin reduces the amount of oxygen delivered to tissues. The altered enzyme activity affects only red blood cells because other cells can compensate for a decrease in enzyme activity, but red blood cells cannot.  Mutations that cause autosomal recessive congenital methemoglobinemia type II usually result in a complete loss of enzyme activity. Cells cannot compensate for a complete loss of this enzyme, which results in a 10 to 70 percent increase in methemoglobin within red blood cells. This increase in methemoglobin and the corresponding decrease in normal hemoglobin leads to cyanosis. The lack of enzyme activity in other cells leads to the neurological features associated with type II. Researchers suspect that the neurological problems are caused by impaired fatty acid and cholesterol formation, which reduces the production of a fatty substance called myelin. Myelin insulates nerve cells and promotes the rapid transmission of nerve impulses. This reduced ability to form myelin (hypomyelination) leads to a loss of nerve cells, particularly in the brain. The loss of these cells likely contributes to the encephalopathy and movement disorders characteristic of autosomal recessive congenital methemoglobinemia type II.