What is Canavan Disease?
Canavan disease is an inherited, progressive neurodegenerative disease which strikes in early infancy causing progressive mental and physical disabilities. Children with Canavan disease have limited ability to move their hands, hold their head up, or perform even the simplest of tasks. Over a period of time, these children may even lose their sense of sight and the ability to swallow food. Unfortunately, children afflicted with Canavan disease are unable to sit, crawl, walk, or even speak. Although Canavan disease is rare, it is but one form of a group of disorders called leukodystrophies, in which a greater number of children are also affected by structural abnormalities and deterioration of motor, sensory, and intellectual functions.
Children with Canavan disease are unable to produce an enzyme called aspartoacylase. As a result, abnormal fluid collections develop in parts of the brain, particularly in the white matter. The result is a distinctive microscopic change called spongy degeneration which produces the progressive neurological symptoms of Canavan disease.
What Characterizes Leukodystrophies?
In general, leukodystrophies are a group of progressive disorders that affect the brain, spinal cord, and nerves. They result from inherited defects in various enzymes in lipid (fat) metabolism, which affect the production of myelin. Nerve cells need myelin to function properly, just as wires need insulation to transmit electrical signals. In the leukodystrophies, abnormal myelinization occurs, resulting in intellectual deterioration and abnormal muscle tone.
Each of the leukodystrophies affects the chemical composition of the brain and myelin sheath in some way. Occasionally, the leukodystrophies are misdiagnosed, as Multiple Sclerosis and Cerebral Palsy. This is because diagnosis of neurological conditions is often very difficult.
How is Canavan Disease Diagnosed and Treated?
Children with Canavan disease have an elevated level of the chemical, n-acetylaspartic acid (NAA). This compound can be easily measured in the urine or blood. Patients with leukodystrophies other than Canavan disease do not produce excess amounts of NAA although some other neurological conditions such as Alzheimer’s disease may.
Prenatal screening does exist, although not readily available within Canada, the DNA of the fetal cells may be analyzed either through amniocentesis or chronic villus sampling (CVS) to determine if a mutation is present in the Canavan gene.
At the present time, there is no cure or treatment for Canavan disease beyond supporting the child with adequate nutrition, physiotherapy, and treatment for seizures.
Inheritance of Leukodystrophies
Canavan disease is inherited as an autosomal recessive disorder. This means that each parent has contributed one defective gene to their child. With two abnormal genes, the baby cannot produce the necessary enzyme, aspartoacylase, and develops Canavan disease. The parents are clinically and physically normal, because they possess one copy of the normal gene, resulting in production of the enzyme.
For a couple in which both partners are carriers, on average, ½ of their children will also be carriers, ¼ will be affected by the illness, and ¼ will neither be carriers nor affected.
In 1993, researchers were able to identify the deadly gene causing Canavan disease. This quickly enabled researchers to devise carrier screening and prenatal tests. With a simple blood test, it is now possible to screen at-risk populations for Canavan disease and other related Leukodystrophies. Genetic testing is available in Canada through your family physician.