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Homocystinuria: Types, Causes, Symptoms, Treatments, Risk Factors, Diagnosis, Complications, Prevention

What Is Homocystinuria?

Homocystinuria (HCY) is a rare condition that interferes with your body’s ability to break down a protein from the food you eat. Protein is made of smaller building blocks called amino acids.

It is an inherited disorder that keeps the body from processing the essential amino acid methionine. Amino acids are the building blocks of protein. Methionine occurs naturally in various proteins. Infants need it for growth, and adults need it to maintain their body’s nitrogen balance.

The illness usually affects infants during the first few years of life, and the more rare forms of the disorder can lead to children being underweight. If it’s left untreated, homocystinuria can have serious and sometimes fatal complications.

Symptoms of Homocystinuria

The symptoms will depend on the type of homocystinuria. Symptoms generally develop during the first years of life. However, some people experience symptoms during adulthood. Symptoms are often vague and difficult to detect. The most common forms of this disorder may involve the following symptoms:

  • dislocation of the lenses in the eyes
  • nearsightedness
  • abnormal blood clots
  • osteoporosis, or weakening of the bones
  • learning disabilities
  • developmental problems
  • chest deformities, such as a protrusion or a caved-in appearance of the breastbone
  • long, spindly arms and legs
  • scoliosis
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Less common variations involve these additional signs and symptoms:

  • megaloblastic anemia, an anemia involving larger-than-normal red blood cells
  • seizures
  • failure to thrive
  • intellectual disabilities
  • movement and gait abnormalities

Diagnosing Homocystinuria

Your child’s doctor may look for certain signs to determine whether you or your child has this condition. An extremely thin or tall child is more likely to have the condition. Additionally, their doctor may search for signs such as chest deformities, a curvature of the spine, and dislocated lenses in the eyes. An eye examination can reveal a dislocated lens if your child experiences double or significantly impaired vision.


Your doctor may also order a series of tests to determine if your child is affected. These tests may include:

  • genetic testing to look for one of the genes involved in the disorder
  • an amino acid screen of the blood and urine to check for excess homocysteine
  • a test to determine the body’s response to consuming methionine
  • a liver biopsy and enzyme assay to check enzymatic activity

Other tests that may be done to determine the impact of the disease include X-rays to look for signs of osteoporosis, a skin biopsy, and a fibroblast culture.

Types of Homocystinuria

Numerous variations of homocystinuria exist. They range from common to rare. No specific names exist for these variations. Instead, they’re distinguished by their symptoms.

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Infants who are affected by the common form of this disorder generally experience mild symptoms. In fact, they may not even have symptoms that require treatment until they’re older. However, the less common forms of this disorder have been known to cause more serious developmental problems, including impaired intellectual capability.

Causes of Homocystinuria

Certain genetic mutations present at birth cause this disease. More than 150 mutations that cause homocystinuria have been found in the gene cystathionine beta-synthase, which is also known as the CBS gene. The CBS gene holds instructions for making an enzyme that uses vitamin B-6 to metabolize the amino acids homocysteine and serine. The mutations prevent the normal functioning of the CBS gene. This results in a buildup of homocysteine and other toxins that damage the nervous system, which includes the brain, and the vascular system.

In rare cases, mutations in other genes like MTHFR, MTR, or MTRR cause the disorder. Homocystinuria is an autosomal recessive trait. This means that for a child to have the signs or symptoms of this condition, they would need to inherit the mutated gene from both parents.

Homocystinuria Treatment

There’s no cure for homocystinuria. High doses of vitamin B-6 are a successful treatment for about half of the people with this disorder. If you respond well to this supplementation, it’s likely that you’ll have to use daily vitamin B-6 supplements for the rest of your life.

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Alternatively, if you don’t experience positive results from this therapy, your doctor may recommend eating a diet low in foods containing the amino acid methionine. People diagnosed at an early stage have had positive responses after switching to this diet.

Your doctor may also recommend taking betaine (Cystadane). Betaine is a nutrient that works to remove homocysteine from the blood. Taking a folic acid supplement and adding the amino acid cysteine to the diet are helpful.

Homocystinuria Prevention

This disorder occurs as a result of genetic mutations. This can make it challenging to prevent. You should consider going to a genetic counselor if a history of homocystinuria runs in your family. Prospective parents can use this method to analyze the risks of having a child who inherits the disorder.

Requesting an intrauterine diagnosis of the disorder is another possible way to prevent this condition. This requires testing a culture of amniotic cells or villi for the presence of the genetic mutation.

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