Homocystinuria, also called cystathionine beta-synthase deficiency, is a genetically inherited disorder which causes dysfunctional metabolism of an amino acid called methionine. Children born with this disorder are at risk of intellectual disability, seizures, and thrombosis if not diagnosed and treated quickly. There is no cure for homocystinuria, but early treatment can prevent the worst of the symptoms developing.
Homocystinuria is inherited in an autosomal fashion. This means that the genes which are involved in the development of the disorder are not located on sex chromosomes. In addition the disorder is recessive, which means for a child to be born with the disorder both parents must carry and pass on a defective copy of the gene. The disorder is rare, with a global frequency of approximately 1 in 344,000.
Cystathionine beta-synthase deficiency is a multi-systemic disorder, meaning that it affects many different bodily systems. People with this disorder cannot properly metabolize methionine. This leads to a buildup of a protein called homocysteine, which damages elastic and collagen fibers of many types of muscles, and is toxic in brain tissue. People with the disorder have a reduced life expectancy, particularly if diagnosis is not made early in life.
Symptoms of homocystinuria are apparent in the central nervous system and cardiovascular system, as well as in muscles and connective tissue. Possible symptoms of the disorder include intellectual disability, increased risk of thrombosis, increased risk of psychiatric disorders, seizures, glaucoma, nearsightedness, atrophy of eye muscles, and arterial swelling. Children born with the disorder also tend to share certain physical characteristics, including long limbs, knock-knees, and high-arched feet.
Homocystinuria is not curable, but can be managed with an appropriate diet and certain types of medication. Around 50% of people with the disorder can be treated effectively with regular high doses of vitamin B6, and require no further treatment. People who do not respond positively to this treatment may need to follow a special diet. The recommended diet is low in protein, and particularly low in methionine and certain other amino acids. This diet helps reduce the rate at which homocysteine accumulates in the body.
The low-methionine diet must be used in conjunction with a medication called betaine, which converts homocysteine to methionine. This further reduces the homocysteine accumulation rate, allowing methionine to be incorporated into the body’s protein reserves. The medication constantly converts small amounts of homocysteine into methionine, but cannot keep up with a rapid pace of homocysteine production; therefore the medication must be used in conjunction with the low-methionine diet. The medication and diet together can prevent toxic levels of homocysteine accumulation, and prevent symptoms of disease from developing.