The CBS gene encodes the enzyme called cystathionine beta-synthase. This enzyme acts in a chemical pathway and is responsible for using vitamin B6 to convert the amino acids homocysteine and serine to a molecule called cystathionine. Another enzyme then converts cystathionine to the amino acid cysteine, which is used to build proteins or is broken down and excreted in urine. Additionally, other amino acids, including methionine, are produced in this pathway.Genetics Home Ref
More than 150 mutations that cause homocystinuria have been identified in the CBS gene. The most common mutation substitutes the amino acid threonine for the amino acid isoleucine at position 278 in the enzyme (written as Ile278Thr or I278T; rs5742905). Another common mutation, which is the most frequent cause of homocystinuria in the Irish population, replaces the amino acid glycine with the amino acid serine at position 307 (written as Gly307Ser or G307S; rs121964972). These mutations disrupt the normal function of cystathionine beta-synthase. As a result, homocysteine and other potentially toxic compounds build up in the blood, and homocysteine is excreted in urine. Researchers have not determined how excess homocysteine leads to the signs and symptoms of homocystinuria.Genetics Home Ref
Homocystinuria is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. Most often, the parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but do not show signs and symptoms of the condition.
Although people who carry one mutated copy and one normal copy of the CBS gene do not have homocystinuria, they are more likely than people without a CBS mutation to have shortages (deficiencies) of vitamin B12 and folic acid.Genetics Home Ref