Key takeaways

  • Homocystinuria is a genetic disorder in which the body cannot break down a particular amino acid, called methionine.
  • Symptoms may depend on the type of homocystinuria a person has. More common symptoms may include nearsightedness, dislocation of the eye’s lens, and brittle bones.
  • Treatment for homocystinuria may depend on the type a person has, but can include vitamin B6 supplements or dietary changes.

The disorder passes from parents to children in an autosomal recessive manner, which means that two copies of an irregular gene must be present for it to develop.

Healthcare professionals may be able to identify some types of homocystinuria through screening newborns, which can help them initiate early treatment and improve a person’s outlook.

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In people with homocystinuria, the body is unable to break down certain proteins. Specifically, the condition affects the metabolism of an amino acid called methionine.

Amino acids are essential human building blocks. Methionine is a naturally occurring protein involved in certain cellular processes, such as the regulation of nitrogen in the body. All amino acids contain nitrogen, making nitrogen an important part of human DNA.

There are several different forms of homocystinuria, which differ in their symptoms.

In the United States, doctors screen for homocystinuria at birth. However, screening may not detect all forms of homocystinuria. A person with the condition may, therefore, not receive a diagnosis for several years following birth.

According to a 2022 review, newborn screenings identify classical homocystinuria in about 1 out of every 200,000 to 335,000 babies worldwide. The article suggests it may be more common in certain parts of the world, including:

  • Ireland, where screenings find 1 in 65,000
  • Germany, where screenings find 1 in 17,800
  • Qatar, where screenings find 1 in 1,800

Homocystinuria occurs due to variations in the following genes:

  • cystathionine beta-synthase (CBS)
  • methylenetetrahydrofolate reductase (MTHFR)
  • 5-methyltetrahydrofolate-homocysteine methyltransferase reductase (MTRR)
  • 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR)
  • metabolism of cobalamin associated D (MMADHC)

As homocystinuria is genetically autosomal recessive, a child must inherit a copy of the mutated gene for homocystinuria from each parent to develop the condition.

Parents who carry just one mutated gene may not have any symptoms of homocystinuria.

According to researchers, a child of parents who are both carriers of the gene has:

  • a 25% chance of having homocystinuria
  • a 25% chance of being unaffected and not carrying the mutated gene
  • a 50% of being a carrier of the gene with no symptoms

Although some people may show symptoms of homocystinuria in infancy or early in life, others may not show signs until adulthood. The symptoms may depend on the type of homocystinuria.

In most cases of homocystinuria, the enzyme cystathionine beta-synthase (CBS) is not effective. As a result, the body cannot break down the amino acids methionine and homocysteine. Symptoms of this may include:

Symptoms that may occur due to less common types of homocystinuria include:

  • learning disabilities
  • infants not growing or gaining weight at the expected rate
  • megaloblastic anemia, in which the number of red blood cells is lower than usual, and the blood cells are unusually large
  • movement and gait problems
  • seizures

Complications

The most serious complication of homocystinuria is blood clots, which can be life threatening and increase the risk of stroke.

Other complications may include:

Key terms explained

  • Strabismus: A condition in which the eyes do not point in the same direction.
  • Optic atrophy: The degeneration or damage of optic nerve fibers.

Signs that may lead a doctor to test for homocystinuria include skeletal irregularities, such as a child being extremely thin and too tall for their age or not growing as expected.

A doctor may also look for signs of structural irregularities of the chest, spinal curvature, and dislocated eye lenses.

Eye exams may reveal vision problems, while X-rays may show signs of weak bones. Genetic testing, amino acid screening, liver biopsies, and liver enzyme tests can also help a doctor confirm a diagnosis of homocystinuria.

Genetic testing

Healthcare professionals may recommend genetic testing for anyone with a family history of homocystinuria who wants to have children.

However, because this condition is a result of a genetic mutation, it is not possible to prevent it. The only reason for genetic testing is to give people an idea of their risk of passing on a gene mutation that can lead to homocystinuria.

If doctors identify homocystinuria through screening newborns, early treatment can significantly improve a person’s outlook.

There is no cure for homocystinuria. Treatments may vary depending on the type of homocystinuria a person has. Treatment options may include:

  • vitamin B6 supplementation
  • a low protein diet with amino acid supplementation
  • betaine, a nutrient that helps remove homocysteine from the bloodstream

Treatment may also focus on managing symptoms and complications. People can speak with a healthcare professional to understand the most suitable treatment for them.

Homocystinuria refers to certain genetic condition that can affect a person’s vision, growth, and development.

People with homocystinuria can lead active lives if they receive a diagnosis early enough, and the management of the disorder is successful.

Although there is no cure for homocystinuria, treatments are available to help people manage the condition.