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Bloom syndrome

Types and clinical signs

This is a severe disease that causes delayed growth in infancy, very short stature in later life, a characteristic facial appearance, sensitivity to sunlight that causes a characteristic rash on the face, and a tendency to develop cancerous tumors at a young age. All these result from the defective production of a protein whose function is to repair the damage that occurs spontaneously in DNA.

Inheritance pattern

Autosomal recessive.



Associated features that can be demonstrated in tests performed during pregnancy

There are no specific ultrasound signs and no 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 Bloom syndrome.

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 Bloom syndrome in the partners' families, consanguinity between the parents, if present, etc. and is established within genetic counseling.

Molecular genetic information

The gene for the disease

The gene, BLM, is responsible for producing the protein DNA helicase RevQ protein-like-2.


The long arm of chromosome 15 - 15q26.

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

The disease is common in Ashkenazi Jews. The carrier frequency in the healthy Ashkenazi Jewish population is approximately 1:100. Unlike cystic fibrosis, where there are a number of mutations in the gene that causes the disease, in Bloom syndrome, only one mutation common to all affected individuals and carriers has been found in the Ashkenazi Jewish population. In Jews of other ethnic groups, carriers cannot be identified. It is therefore advisable to perform the test in couples when one of the partners is of full or partial Ashkenazi origin.

Carrier status testing in relatives of patients with Bloom syndrome

It is important that relatives of patients with Bloom syndrome 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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