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Albinism

Clinical types

There are many types:
  • Eye involvement only (ocular).

  • Eye and skin involvement (oculocutaneous).

  • Many syndromes featuring albinism along with other characteristics such as deafness, intestinal problems (Hermansky-Pudiak type), immune system disorders (Chediak Higashi syndrome) and other syndromes such as Prader Willi syndrome, etc.

Clinical signs

The common denominator in most types is the absence of pigment mainly in the fundus of the eye, which can be seen on examination by an ophthalmologist. In most cases, reduced pigmentation of the skin and sometimes of the hair is also present. The majority of cases are in this category, but there is probably genetic heterogeneity among these too. In other words, a number of different genes can cause albinism affecting the skin and eyes. For example, one type of albinism features a deficiency in the enzyme tyrosinase, the level of which can be measured in the patient-s hair, whereas in another type of albinism the level of this enzyme is normal, etc.

There are rare types of albinism in which there are other features such as red hair, deafness, etc. In these cases it is important to establish the exact diagnosis in genetic institutes, after examination by a geneticist.

Inheritance pattern

In cases in which there are no metabolic effects other than albinism, the inheritance is Autosomal recessive. There is a relatively rare type of albinism that is restricted to the eyes and is associated with deafness - this is transmitted by X-linked inheritance.

Penetrance

Varies according to type.

Associated features that can be demonstrated by imagery

Usually there are no defects that can be identified by ultrasound examination during pregnancy.

What is the risk of recurrence in a subsequent pregnancy?

According to the type of inheritance. The inheritance is autosomal recessive in most cases, and a couple with one affected child has a 25% risk in each subsequent pregnancy of having another affected child. With regard to more distant relatives, the risk of having a child with the same condition is relatively low. Healthy siblings of patients have a risk of less than 1 in 500 of having an affected child, and the risk is even lower for more distant relatives.

Molecular genetic information

The genes for the disease

A number of genes associated with albinism have been found - in this table the different types of disease have been arranged by gene type:

Genes associated with different types of albinism
Type of albinism (clinical term) Name of gene
OCA1 (OCA1A and OCA1B) Tyrosinase gene - chromosome 11
OCA2 P gene - chromosome 13
OCA3 TRP1 gene - chromosome 9
Hermansky-Pudlak Syndrome HPS gene - chromosome 10
Chediak Higashi Syndrome CHS gene - chromosome 11
X-linked ocular albinism OA1 gene - X chromosome


Genetic testing

Diagnostic testing


In certain ethnic groups it is sometimes possible to attempt to identify the mutation by direct testing in order to identify mutations in patients.

In Jews of North African origin with oculocutaneous albinism, a common mutation that can be tested for with relative ease has been identified.

In Ashkenazi Jews there are a number of common mutations that can also be identified with relative ease: See: Testing the disease-causing gene for mutations that are common in a specific ethnic group - autosomal recessive diseases.

If the mutation is not detected in such a test, or the patient is a member of an ethnic group in which no common characteristic mutation is known, the genetic sequence of a number of possible genes can be examined. This is an expensive and time-consuming process that is not routinely performed apart from in special settings at centers engaged in albinism research. See: Finding the defect (mutation) by establishing the gene sequence - autosomal recessive diseases. In all cases it is first necessary to establish a diagnosis with counseling and testing at a genetic institute.

Carrier testing

To identify genes in the patient-s family

If the mutation has been found, other carriers in the extended family can be identified with relative ease. If the mutation has not been found, the indirect tests specified below in the section on fetal testing can be used provided that there are enough affected family members. See: Indirect testing for genetic markers in a family that has a number of patients - when there are a number of genes that can each cause the disease - all genes having been located / identified / mapped - autosomal recessive diseases. However, because of low risk and limited possibility of testing the partners, these tests are not usually performed.

Carrier testing for partners of carriers in the general population

When a mutation has been found in a gene responsible for albinism in a patient in a specific family, an attempt can be made to identify the mutations that are common in that gene in the ethnic groups of the partners of the carriers. If a mutation is not found in a partner, this does not completely rule out the possibility that he or she is a carrier. In order to ascertain carrier status in members of the general population, the sequence of the gene that is responsible for albinism in the affected family member must be examined. This is a time-consuming and expensive test and it is not performed routinely at the present time. See: Finding the defect (mutation) by establishing the gene sequence - autosomal recessive diseases.

Fetal testing

When a couple has had one affected child, a relatively simple genetic test can be used for prenatal testing of the fetuses in subsequent pregnancies. This is on condition that the mutation in the albinism gene is found in the affected child and his or her DNA is retained. If the mutation is not found and there are many affected individuals in the family, indirect tests can also be employed. See: Indirect testing for genetic markers in a family that has one or more patients - when there are a number of genes that can cause the disease - autosomal recessive diseases.

Alternatively, a fetal skin biopsy test can be performed to test for cutaneous albinism in utero. This test involves a 5% risk for miscarriage due to the procedure itself.

If the patient has another problem apart from albinism or the mutation is not found, it is necessary to establish the diagnosis within counseling and testing at a genetic institute.
 
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