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Homocystinuria (classical)
This rare condition results from a deficiency in the cystathionine beta-synthase (CBS) enzyme. It can affect many body systems, including the eyes, bones, brain, and blood vessels. Early diagnosis and management are crucial to reduce severe complications.
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Overview
Homocystinuria (classical) is a rare, inherited metabolic disorder. It occurs when the body lacks a specific enzyme, cystathionine beta-synthase (CBS), which is essential for processing an amino acid called methionine [PMID:33246375]. Amino acids are the building blocks of proteins, and methionine is found in many foods. Without enough functional CBS enzyme, a substance called homocysteine accumulates in the blood and urine, along with methionine and other related compounds. This build-up can be toxic to various tissues and organs throughout the body.
Classical homocystinuria is typically diagnosed through newborn screening programmes in many countries, including the UK, although it can also be identified later in life if symptoms develop. The condition can affect multiple body systems, leading to a wide range of health problems if left untreated. Its exact prevalence varies by population but is estimated to be between 1 in 200,000 and 1 in 335,000 births [PMID:30130217].
Symptoms & clinical features
The symptoms of homocystinuria (classical) can vary significantly among individuals, even within the same family. Symptoms often develop gradually and may not be immediately apparent at birth. One of the most common early signs is dislocation of the lens in one or both eyes (ectopia lentis), which typically occurs after the age of three [PMID:33246375]. This can lead to severe nearsightedness and other vision problems.
Skeletal abnormalities are also common, including long, slender limbs (marfanoid habitus), spinal curvature (scoliosis), and bone fragility, increasing the risk of fractures. Neurological issues can include developmental delays, learning difficulties, and, less commonly, seizures. Individuals with classical homocystinuria also have an increased risk of blood clots (thromboembolism), which can affect arteries and veins, potentially leading to serious complications such as strokes or heart attacks. Skin and hair may also be affected, sometimes appearing fair with a fine texture.
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Affected organs
Homocystinuria (classical) can impact multiple body systems due to the widespread toxic effects of homocysteine accumulation. The eyes are frequently affected, with lens dislocation being a hallmark feature, potentially leading to visual impairment. The skeletal system is also commonly involved, with abnormalities affecting bone structure and strength, such as osteoporosis and increased fracture risk.
The brain and nervous system can be affected, leading to potential developmental delays, cognitive differences, and psychiatric symptoms. The vascular system is particularly vulnerable; the build-up of homocysteine can damage blood vessel linings, significantly increasing the risk of blood clot formation (thrombosis). This heightened risk for blood clots can affect various organs, including the brain, heart, and lungs, making cardiovascular health a key concern.
Risks & severity
The severity of homocystinuria (classical) can vary, influenced by factors such as the specific genetic change and how early treatment is initiated. Untreated individuals often experience progressive complications affecting vision, bone health, and neurological function. The risk of developing blood clots is a major concern and can be life-threatening if not managed appropriately. These vascular events can occur at any age but typically become more frequent in adolescence and adulthood.
Early diagnosis, often through newborn screening, and prompt treatment are crucial for improving outcomes and reducing the severity of complications. With effective management, many individuals can lead relatively healthy lives. However, ongoing monitoring and adherence to treatment are essential to minimise long-term risks. The full spectrum of disease severity ranges from mild, later-onset forms to severe manifestations that appear in early childhood.
Genetic causes
Classical homocystinuria is caused by changes (pathogenic variants) in the *CBS* gene. The *CBS* gene provides instructions for making an enzyme called cystathionine beta-synthase. This enzyme plays a critical role in the metabolic pathway that breaks down the amino acid methionine and converts homocysteine into another compound called cystathionine.
Pathogenic variants in *CBS* lead to either a reduced amount of functional CBS enzyme or a completely non-functional enzyme. This deficiency prevents the proper processing of homocysteine, causing it to accumulate in the body to harmful levels. The specific genetic variant can influence how much residual enzyme activity is present, which in turn can affect an individual's response to vitamin B6 (pyridoxine) treatment, as some variants allow the enzyme to be partially rescued by this vitamin.
- CBS cystathionine beta-synthase
Inheritance pattern
Homocystinuria (classical) is inherited in an autosomal recessive pattern. This means that an individual must inherit two altered copies of the *CBS* gene-one from each parent-to develop the condition. If a person inherits only one altered copy of the *CBS* gene and one normal copy, they are considered a carrier. Carriers typically do not show symptoms of the condition because their single functional copy of the gene is usually sufficient to produce enough CBS enzyme.
When two carriers have a child together, there is a 25% (1 in 4) chance with each pregnancy that the child will inherit two altered copies of the gene and develop homocystinuria. There is a 50% (2 in 4) chance that the child will be a carrier, and a 25% (1 in 4) chance that the child will inherit two normal copies of the gene and neither have the condition nor be a carrier. Family members of someone diagnosed with classical homocystinuria may be offered genetic counselling and testing to understand their carrier status and reproductive risks.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
Diagnosis of homocystinuria (classical) often begins with newborn screening programmes, which test for elevated levels of methionine in blood spots. If the screening test is positive, further diagnostic tests are performed to confirm the diagnosis. These typically include quantitative measurement of amino acids in blood and urine, looking for elevated homocysteine and methionine levels, and reduced cystathionine.
Genetic testing is used to confirm the diagnosis by identifying pathogenic variants in the *CBS* gene. This can be performed via gene sequencing, usually through a blood sample. In the UK, genetic testing for homocystinuria may be requested by a metabolic specialist or clinical geneticist and falls under specific NHS Genomic Medicine Service (GMS) pathways, often relevant to R-code R14 (Metabolic disorders). A confirmed diagnosis allows for prompt initiation of treatment and genetic counselling for the family.
Management & lifestyle
Management of homocystinuria (classical) focuses on reducing homocysteine levels and preventing complications. Treatment plans are individualised but generally involve a combination of dietary modifications and medication. Many individuals respond to high doses of vitamin B6 (pyridoxine), which can help some forms of the CBS enzyme function more effectively. For those who do not respond to vitamin B6, a low-methionine diet is typically recommended, often supplemented with specific amino acid formulas to ensure adequate nutrition.
Other medications, such as betaine, may be used to help lower homocysteine levels by promoting an alternative metabolic pathway. Regular monitoring of blood amino acid levels is essential to adjust treatment as needed. Lifelong follow-up with a metabolic specialist is crucial to manage the condition, monitor for potential complications like eye problems or blood clots, and ensure optimal health outcomes. Lifestyle advice, including regular exercise and avoiding smoking, may also be part of comprehensive care.
UK care pathway
In the UK, individuals suspected of having or diagnosed with homocystinuria (classical) are typically managed through specialist NHS Genomic Medicine Service (GMS) pathways. Following a positive newborn screen or symptomatic presentation, individuals are referred to a metabolic specialist centre. These centres provide expert diagnosis, treatment, and long-term care for rare metabolic conditions.
Genetic testing, often guided by NHS R-codes like R14 for metabolic disorders, is integral to confirming the diagnosis. Clinical geneticists and genetic counsellors play a key role in explaining the genetic basis of the condition, inheritance patterns, and the implications for family members. They also provide support and guidance throughout the diagnostic and management journey, helping families understand the condition and access appropriate services.
Frequently asked questions
What is the main problem in homocystinuria (classical)?
The main problem is that your body cannot properly break down an amino acid called methionine due to a missing or faulty enzyme (cystathionine beta-synthase). This leads to a build-up of a harmful substance called homocysteine in the blood and urine.
How is homocystinuria (classical) usually found?
In the UK and many other countries, it is often detected through routine newborn screening tests, which check a small blood sample from your baby's heel. If the screening suggests a problem, further tests are done to confirm the diagnosis.
Can homocystinuria (classical) be treated?
Yes, it can be treated. Management typically involves a special diet low in methionine, sometimes with amino acid supplements, and medications like vitamin B6 or betaine. Early and consistent treatment is very important to prevent serious health problems.
What happens if homocystinuria (classical) is not treated?
Without treatment, the build-up of harmful substances can lead to various complications. These can include vision problems (like dislocated eye lenses), weak bones, learning difficulties, and a significantly increased risk of blood clots, which can be very serious.
Is homocystinuria (classical) passed down in families?
Yes, it is an inherited condition. It follows an autosomal recessive pattern, meaning a child must inherit a faulty gene from both parents to develop the condition. Parents who each carry one faulty gene usually do not have symptoms themselves.