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Canavan Disease

Types and clinical signs

This is a severe degenerative disease of the brain that is caused by the accumulation of an abnormal substance called acetylaspartic acid. The disease begins to manifest 2 - 4 months after birth, and the first signs are muscle flaccidity and convulsions. Severe motor and mental retardation develop, and most patients die by the age of three years.

Nowadays there is a trial treatment, still at the research stage, which is being carried out abroad. This involves injecting the missing enzyme, aspartoacylase, into the bloodstream, from where it is transported to the cells. The results are not yet complete, but it appears that the treatment does provide some benefit in all parameters, and arrests the development of the disease to a degree that has yet to be determined. It may be more effective if given before the onset of the signs and complications of the disease.

Inheritance pattern

Autosomal recessive.

Penetrance

Full

Associated features that can be demonstrated in tests performed during pregnancy

There are no specific ultrasound signs or associated defects.

What is the risk of recurrence in a subsequent pregnancy?

For a couple who has already had an affected child, the risk is 25% in every subsequent pregnancy.

This also applies to a couple where both partners have been found to carry mutations for Canavan disease.

When only one parent carries the abnormal gene, there is no risk that any of their offspring will have the disease;
however, in such families there is a 50% chance that the infant will be a carrier, but he himself will be healthy, like the carrier parent.
The risk for more distant relatives depends on the degree of relationship between the relatives and the affected individuals, the ethnic groups of their partners, the presence of family members with Canavan disease in the partner's families, consanguinity between the parents, if present, etc. The risk is established within genetic counseling.

Molecular genetic information

The gene for the disease

The gene for aspartoacylase

Location

The short arm of chromosome 17.

Genetic testing

Diagnostic testing

A direct test can be performed to identify the mutation in the gene by testing for the common mutations. See: Testing the disease-causing gene for mutations that are common in a specific ethnic group - autosomal recessive diseases.

Carrier testing

Among Jews, most of the affected children have been found to be Ashkenazim. Mutations in the gene have also been found in non-Jews and non-Ashkenazi Jews, but they are rare in these groups. The carrier frequency in the healthy Ashkenazi Jewish population is between 1:40 and 1:60. There are two main mutations known today that cause this disease in Ashkenazi Jews. Between them these are responsible for about 98% of the cases of Canavan disease in Israel. In Jews of other ethnic groups, carriers cannot be identified. It is therefore advisable to perform tests in couples when one of the partners is of full or partial Ashkenazi origin.

Carrier status testing in relatives of patients with Canavan disease

It is important that relatives of patients with Canavan disease clarify which mutations are present in the patient or parents in their family in order to ascertain that they are included in the battery of mutations routinely tested for - otherwise it will be necessary to expand the list of mutations tested for and/or examine the partner. This will be discussed in genetic counseling. See also: Introduction to cystic fibrosis, Gaucher disease, Canavan disease, familial dysautonomia, Bloom syndrome and Fanconi anemia.

Fetal testing

Same as the diagnostic test.

Prenatal diagnosis (amniocentesis) can also be offered to each couple who has had an affected child. The best way is after identifying the mutations in the parents, but if these are not found, indirect linkage analysis can be performed. See: Indirect testing for genetic markers in a family that has one or more patients - when there is only one gene that can cause the disease - autosomal recessive diseases.

Preimplantation diagnosis (before the embryonic cells implant in the uterine wall) can also be offered for this disease - this is performed in special centers, mainly overseas, and in special cases this can be considered within genetic counseling.
 
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