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What is Homocystinuria?

The disease

HCU-diagram
Homocystinuria is an autosomal recessively inherited disorder. This means that an individual inherits the same defective gene for the same trait from each parent.

 

Homocystinuria is a metabolic disease affecting the trans-sulfuration pathway or methylation pathway.  Classical Homocystinuria, the most common form of Homocystinuria, is caused by a deficiency in the enzyme known as Cystathionine Beta-Synthase (CbS).  The lack of the enzyme prevents the body from processing methionine, an essential amino acid, from food containing protein.  As a result toxic levels of harmful substances accumulate in the body and cause serious health problems.

There are multiple forms of Homocystinuria, each distinguished by their clinical signs and symptoms and underlying genetic basis.

Classical Homocystinuria affects at least 1 in 200,000 to 335,000 people worldwide.  The incidence of the rarer forms of Homocystinuria are unknown.

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The symptoms

Homocystinuria is progressive and affected individuals suffer multi-systemic disorder of the central nervous system (CNS), ocular, skeletal, and cardiovascular system.  Symptoms are highly variable but include intellectual disability, developmental delay and learning difficulties, psychiatric disturbances, behavioural problems, dislocation of the lens of the eye resulting in blindness if untreated, seizures, osteoporosis and skeletal abnormalities, premature heart attack and stroke.  As a whole, the complications of the disease lead to a reduced quality of life and significantly shorter life expectancy.

The treatments

No cure has been discovered for Homocystinuria.  However, nearly one half of individuals affected with Homocystinuria benefit from Vitamin B6 supplementation.  For the remainder, the current mainstay of treatment is a strict low protein diet coupled with supplementation with B6, B12, folate, and Betaine together with a ‘medical formula’.  This treatment regime aides in the restoration of metabolic balance in the affected individual.

Focus of treatment in the newborn period is to ensure the development of normal intelligence and the prevention of the development of other complications of the disease.  When treatment is commenced later, the aim is to prevent further escalation of the complications already present.