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Gaucher disease


Types

This is a disease in which a substance, a glucocerebroside, accumulates mainly in the spleen and bones. The glucocerebroside accumulates because of the deficiency of an enzyme called beta glucosidase, which is responsible for its metabolism.

There are a number of types of Gaucher disease, based on the severity of the clinical signs - all are caused by mutations in the same gene and deficiency of the same enzyme. The degree of deficiency of the enzyme varies with the severity of the mutations, leading to variation in the severity of the disease:

  1. A rare infantile type (infantile/neuropathic type) in which, apart from the accumulation of the glucocerebroside in the spleen, liver and bone, there is also central nervous system involvement, progressive degeneration of the brain and a decrease in its volume. These patients die between 6 and 18 months of age.
  2. Regular type - two types are distinguished here:
    • Regular type with onset in childhood - there is a significant accumulation of the glucocerebroside in the spleen, liver and bone. Signs appear in early childhood, and untreated there are many complications.
    • The adult type - in this type only 30% of patients have signs of the disease and are diagnosed. This is the type that is common in Ashkenazi Jews.

Clinical Signs

There is accumulation of the glucocerebroside in the liver, spleen and bones. The disease manifests as an enlarged spleen and liver (hepatosplenomegaly), impaired function of these organs, and a predisposition for bone fractures.

In patients suffering from the disease it is characteristic to find reduced levels of platelets, which are responsible for blood clotting, and white blood cells, which are active in the immune system.

The white blood cells are scavenged in the enlarged spleen, and excision of the spleen is sometimes required to correct this problem.

Glucocerebroside also accumulates in the bone, which assumes a characteristic shape called the Erlenmeyer-flask deformity, where the bone is widened and has a more translucent center than normal. This occurs mainly in the femur.
The diagnosis can be made in a number of ways. In the past a bone marrow test was performed in order to identify the characteristic cells, known as Gaucher cells (cells that have accumulated the glucocerebroside). The disease can be diagnosed with certainty by measuring the level of the enzyme beta-glucosidase in the blood, or in the fetus in amniotic fluid or chorionic villus cells. Molecular tests assist in identifying carriers, but for identifying patients, the reliability of diagnosis is less than that of the enzyme level test, because not all the mutations that cause the disease are tested for.

The degree of severity of the disease varies according to the mutation. Sometimes it is relatively mild and causes only slight enlargement of the liver and spleen at a late age, and sometimes the onset is in childhood, causing severe damage to these organs. Apart from the infantile type, which is rare, even in the severe childhood type the disease does not cause mental retardation or lead to death, but it does involve intensive medical treatment throughout life, including replacement of the deficient enzyme - this is given intravenously. The treatment is expensive, and approval from the Ministry of Health or HMO is required to obtain it free of charge

Most of the patients affected by this disease are Jews of Ashkenazi origin, of whom most have the mild, adult type. The frequency of carriers is 1:14 in the general healthy Ashkenazi population. As mentioned above, the most common mutation in this population usually causes the mild disease. In other ethnic groups the disease is rarely seen and the carrier frequency is low.


Inheritance pattern

Autosomal recessive.


Example of an Inheritance Pattern for Gaucher disease
Gaucher disease
If two Gaucher carriers have children, there is a 1 in 4 chance that each child born to them will have the disease.

Penetrance

In the infantile and severe childhood types, penetrance and expression are full.

In the adult type, only 30% express the disease, and the rest remain undiagnosed and will not undergo molecular blood tests. The diagnosis in such individuals is only incidental.

Associated features that can be demonstrated in tests performed during pregnancy

The level of the enzyme missing in Gaucher disease can be measured in the amniotic fluid or chorionic villi, allowing a definitive diagnosis to be made in pregnancy. Ultrasound tests are of no use here.

In all these cases it is important to refer the couple for genetic counseling, to which they should bring all data, including the results of other tests performed during the pregnancy such as alpha-fetoprotein, nuchal translucency, etc.

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 be carriers of classical Gaucher disease mutations. If each of the two partners carries a mild mutation (the Ashkenazi mutation, 1226), there is a 25% chance that the fetus will receive two copies of this - one from his/her mother and one from his/her father. In these cases, the disease, if it appears at all, will be relatively mild - only 30% manifest clinical signs. Of these, only 10% need treatment, involving relatively low doses and without complications. 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 Gaucher disease in the partner's families, consanguinity between the parents, if present, etc. The risk is established within genetic counseling.

If one partner carries the mild mutation (1226), and the other carries a different mutation for classical Gaucher disease, they have a 25% chance of having a child with classic Gaucher disease (childhood type).

Molecular genetic information

The gene for the disease

GBA

Location

1q21

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

Mutations in the gene causing Gaucher disease exist in all ethnic groups, but are undoubtedly several times more common in Ashkenazi Jews. The carrier frequency in the healthy Ashkenazi Jewish population is 1:14, as opposed to 1:100 in other ethnic groups. Not only is the frequency different in different ethnic groups, but the mutation itself is also different. Hospital laboratories are capable of identifying more than 96% of carriers in Ashkenazi Jews by testing for 4 - 6 mutations, but only about 50% of carriers in Jews of other ethnic groups.

In view of this, it is very important to ascertain the ethnic origins of the couple, and it is advisable to examine those couples where one or both partners are Ashkenazi Jews.

The carrier test battery in Ashkenazi Jews includes 4 - 6 mutations. Even though most of the affected individuals in this population have the mild type of Gaucher disease, there is not always full correlation between the mutations and the severity of the clinical signs. Because of this it is only possible in some cases to predict the severity of the condition in the fetus. Non-Ashkenazi individuals or those who have only one Ashkenazi parent should undergo a biochemical test in addition to the gene mutation test in order to evaluate the level of activity of the enzyme involved in the disease. When the tests are performed on individuals of non-Ashkenazi origin, 75% - 94% of carriers can be identified.

Carrier status testing in relatives of patients with Gaucher disease

It is important that relatives of patients with Gaucher 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.

Expansion of the battery of mutations tested

It is possible to expand the battery of mutations tested for - there are laboratories that test for additional mutations at extra cost - however, the contribution of these to the detection of carriers is small. See also: Introduction to cystic fibrosis, Gaucher disease, Canavan disease, familial dysautonomia, Bloom syndrome and Fanconi anemia.

Fetal testing

The same as diagnostic testing. As stated above, a definitive fetal diagnosis can be made using an enzyme test carried out on the chorionic villi or amniotic fluid. This is also possible in couples whose mutations have not been identified or where a mutation has been identified in only one where the other is of non-Ashkenazi origin.

Prenatal diagnosis (amniocentesis) can 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, the enzyme tests alone can be used.

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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Comments (2)


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