homocystinuria caused by cystathionine beta-synthase deficiency
What is Homocystinuria Caused by Cystathionine Beta-Synthase Deficiency?
Homocystinuria caused by cystathionine beta-synthase deficiency, or CBS deficiency, is an inherited metabolic disease in which affected people cannot process certain amino acids, the building blocks of proteins, due to the lack of a particular enzyme. The disease causes an array of symptoms including eye problems, skeletal abnormalities, an increased risk for dangerous blood clots, and in some people it causes developmental delay or intellectual disability.
Not all people with the disease exhibit all of the symptoms described below, and the severity of the symptoms varies widely from person to person. In some cases, people will reach adulthood before developing any symptoms.
There are two forms of the disease: a type that responds well to treatment with vitamin B6 and a type that does not. People with B6-responsive CBS deficiency sometimes, but not always, have a less severe course of the disease than those with the B6-non-responsive form.
The majority of people with CBS deficiency are nearsighted as infants. Those who go untreated will experience a dislocation of the lenses in their eyes (ectopia lentis) before the age of 8.
CBS deficiency can cause abnormal blood clots called thromboembolisms. If these blood clots form in or travel to the heart, brain, or other vital organs, they can be fatal. Thromboembolisms are frequently the cause of death in people with the disease.
People with the disease tend to be tall and thin with long arms and legs and an angular appearance. They are more likely than average to develop fragile, porous bones, a condition known as osteoporosis, at an early age. Roughly half of people with CBS deficiency will show signs of osteoporosis by their teenage years. They are also prone to scoliosis, a curvature of the spine.
The IQs of some people with CBS deficiency have been recorded as high as 138, which is well above the general population average of 100 and would be considered “gifted.” The average IQ score for a person with the disease, however, ranges between 57 and 79, depending on the genetic mutations he or she inherited. These IQs are generally considered borderline to mild intellectual disability. IQs for people with the disease have been recorded as low as 10, which indicates profound disability.
The disease can cause neurological or mental health problems such as seizures, anxiety, depression, obsessive-compulsive behavior, and psychosis. Roughly half of people with CBS deficiency have some kind of clinically significant psychiatric illness.
The disease can also cause abnormal muscle twitching, inflammation of the pancreas, the loss of skin color, and liver problems.
How common is Homocystinuria Caused by Cystathionine Beta-Synthase Deficiency?
The exact prevalence of homocystinuria caused by CBS deficiency is unknown. Studies have reported that 1 in every 250,000 people in the general population is affected by the disease, but this figure is widely believed to be too low. Several studies estimate the populations in particular countries and regions as follows: Ireland (1 in 65,000), Germany (1 in 17,800), Norway (1 in 6,400), and Qatar (1 in 3,000).
How is Homocystinuria Caused by Cystathionine Beta-Synthase Deficiency Treated?
A main goal of treatment for a person with homocystinuria caused by CBS deficiency is to control the concentration of homocysteine in the blood, thereby preventing symptoms of the disease. Medical professionals can help design a particular diet which includes vitamins and supplements such as vitamin B6, vitamin B12, folate, and betaine, and avoids certain types of food, notably those with the amino acid methionine. It is important that this treatment begin in infancy so as to limit any damage to the child’s body and intellect.
In order to lower the risk of blood clots, people with the disease should avoid unnecessary surgery and women should not take oral contraceptives. Women with the disease who become pregnant should work with their doctor to avoid pregnancy-related clotting problems.
Typically eye surgery is performed to correct the lens of the eye in people who have ectopia lentis.
What is the prognosis for a person with Homocystinuria Caused by Cystathionine Beta-Synthase Deficiency?
Homocystinuria with CBS deficiency is thought to shorten the lives of people affected by the disease, however there is no known average lifespan. About 1 in 4 people with the disease will die before age 30 from a blood clot. The outlook is much better when the diagnosis is made early in life and the person's diet can be carefully planned.