What are the genetic changes related to homocystinuria ?
Mutations in the CBS, MTHFR, MTR, MTRR, and MMADHC genes cause homocystinuria.  Mutations in the CBS gene cause the most common form of homocystinuria. The CBS gene provides instructions for producing an enzyme called cystathionine beta-synthase. This enzyme acts in a chemical pathway and is responsible for converting the amino acid homocysteine to a molecule called cystathionine. As a result of this pathway, other amino acids, including methionine, are produced. Mutations in the CBS gene disrupt the function of cystathionine beta-synthase, preventing homocysteine from being used properly. As a result, this amino acid and toxic byproducts substances build up in the blood. Some of the excess homocysteine is excreted in urine.  Rarely, homocystinuria can be caused by mutations in several other genes. The enzymes made by the MTHFR, MTR, MTRR, and MMADHC genes play roles in converting homocysteine to methionine. Mutations in any of these genes prevent the enzymes from functioning properly, which leads to a buildup of homocysteine in the body. Researchers have not determined how excess homocysteine and related compounds lead to the signs and symptoms of homocystinuria.