Homocystinuria
Inherited disorder causing homocysteine buildup and multi-system damage.
Homocystinuria (HCU) is an inherited disorder in how the body processes the amino acid methionine, usually caused by a lack of the enzyme cystathionine beta synthase or methionine synthase. It follows an autosomal recessive pattern, so a child must inherit one faulty gene from each parent to develop the condition. This metabolic flaw results in a disorder affecting multiple body systems—connective tissue, muscles, the central nervous system, and the cardiovascular system—marked by a buildup of homocysteine in the blood and extra homocysteine in the urine.
Infants with homocystinuria often appear normal at birth, and any early symptoms tend to be nonspecific. The condition can also stem from deficiencies in vitamins B6, B12, or folate. Diagnosis involves a combined laboratory approach because elevated homocysteine in urine or plasma can have several causes, including cystathionine beta synthase (CBS) deficiency, re-methylation defects (such as cobalamin defects, methionine synthase deficiency, or MTHFR issues), or vitamin deficiencies (riboflavin, B6, folate, B12). Routine metabolic biochemistry can identify CBS deficiency, and genetic testing can screen for known mutations. Initial plasma or urine amino acid analysis often shows high methionine and the presence of homocysteine, which helps distinguish CBS deficiency from re-methylation defects. Organic acid analysis or methylmalonic acid measurement can rule out cobalamin defects or B12 deficiency. Measuring total homocysteine (free plus bound) after reducing disulfide bonds is useful for confirming diagnosis and monitoring treatment.
No cure exists, but many people are treated with high doses of vitamin B6; slightly less than half respond and need lifelong B6 supplementation. Those who do not respond typically benefit from folic acid and betaine (trimethylglycine). Betaine works by converting homocysteine back to methionine, but this is only effective if the amount of methionine to be removed is small, so treatment combines betaine with a low-methionine diet. Adding cysteine to the diet can help reduce oxidative stress, and riboflavin may also be used. For patients with CBS deficiency who do not respond to B6, a low-protein diet low in methionine is recommended, along with supplementation.
- Type
- Inherited metabolic disorder
- Cause
- Deficiency of cystathionine beta synthase or methionine synthase; also vitamin B6, B12, or folate deficiency
- Inheritance
- Autosomal recessive
- Symptoms
- Multi-systemic: connective tissue, muscles, CNS, cardiovascular
- Treatment
- Vitamin B6, folic acid, betaine, low-methionine diet
- Prognosis
- Reduced life expectancy if untreated; approximately 50% experience thrombotic complications before age 30
Lore & Background
Homocystinuria represents a group of hereditary metabolic disorders marked by elevated homocysteine in blood and urine. Infants appear normal initially, with early symptoms vague if present. The disorder is usually caused by deficiency of the enzyme cystathionine beta synthase, but mutations of other enzymes such as methionine synthase, or deficiencies of folic acid, vitamin B12, or pyridoxine (vitamin B6) can also produce symptoms. Diagnosis involves plasma or urine amino acid analysis showing elevated methionine and homocysteine, with genetic testing for known mutations. Total homocysteine analysis after reduction of disulfide bonds is used for confirmation and monitoring.
Reader's Guide
Homocystinuria is significant as a model of how single-enzyme defects can disrupt multiple body systems. Its diagnosis requires a combined laboratory approach to differentiate among CBS deficiency, re-methylation defects, and vitamin deficiencies. Treatment is not curative but includes high-dose vitamin B6 for responders (slightly less than 50%), with folic acid and betaine for non-responders. Betaine promotes conversion of homocysteine back to methionine, but requires a low-methionine diet to avoid toxic methionine levels. The disorder's prognosis is reduced only if untreated; before age 30, nearly one quarter of patients die from thrombotic complications such as heart attack. One theory suggests the Egyptian pharaoh Akhenaten may have had homocystinuria.
Did You Know?
- Homocystinuria is caused by deficiency of cystathionine beta synthase or methionine synthase, or by deficiencies of vitamins B6, B12, or folate.
- Before age 30, almost one quarter of untreated patients die from thrombotic complications like heart attack.
- Slightly less than 50% of patients respond to high-dose vitamin B6 treatment.
- One theory suggests the pharaoh Akhenaten may have had homocystinuria.
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