World Homocystinurias Awareness Day Date in the current year: May 18, 2026

Homocystinuria (HCU) is a disorder in which the essential amino acid methionine is not metabolized properly. Under normal conditions, methionine is converted into homocysteine. Homocysteine can then be converted into cystathionine, the precursor to the amino acid cysteine, or it can be converted back into methionine. In patients with HCU, however, one of these processes is disrupted, resulting in the accumulation of homocysteine in the body.

The term “homocystinuria” indicates an increased level of homocysteine in the urine. It is used in the plural form in the phrase “World Homocystinurias Awareness Day” because elevated homocysteine levels can have several causes. Classical HCU is caused by a deficiency of the enzyme cystathionine beta-synthase, which catalyzes the conversion of homocysteine into cystathionine. Another form of HCU is caused by a deficiency of methionine synthase, which catalyzes the conversion of homocysteine back into methionine. Deficiencies of several other enzymes can also cause HCU.

HCU is a genetic disorder that is inherited in an autosomal recessive pattern. This means that a child must inherit two copies of the defective gene, one from each parent, for symptoms to manifest. In many cases of classical homocystinuria, symptoms are not evident at birth and typically become apparent in early childhood, although elevated homocysteine levels are present from infancy. Depending on the underlying metabolic defect, some forms of homocystinuria can present much earlier with severe symptoms.

HCU primarily affects the brain, eyes, blood, and skeleton. The most common eye abnormalities in HCU patients include ectopia lentis (misplacement of the lens) and severe, progressive nearsightedness that begins in childhood. Musculoskeletal symptoms include tall stature, long limbs, a sunken or protruding chest, high-arched feet, knock knees, scoliosis, and premature osteoporosis. Brain-related symptoms include developmental delay, intellectual disability, seizures, behavioral problems, and psychiatric disorders such as depression, anxiety, and OCD. Finally, patients with HCU are at high risk for thrombosis, or blood clots.

HCU is incurable, but treatable. Those with mild classical HCU are typically treated with high doses of vitamin B6 to stabilize cystathionine beta-synthase and ensure proper enzyme function. These patients need to take vitamin supplements for life. Patients who do not respond to vitamin B6 treatment are usually prescribed a low-methionine diet that restricts or excludes foods high in methionine, such as meat, poultry, fish, dairy, and eggs. This diet is often supplemented with protein drinks that contain other essential amino acids and folic acid.

World Homocystinurias Awareness Day was created by a group of patient organizations working to raise global awareness of homocystinurias and their impact on the lives of patients and their families and caregivers. You can get involved by learning more about homocystinurias and sharing what you’ve learned with others, donating to an organization that supports patients or funds research, and spreading the word on social media with the hashtags #WorldHomocystinuriasAwarenessDay, #WHAD, and #GoBlue4HCU.