CAUSE AND HEREDITY
Usher Syndrome is a hereditary disorder which can only develop when both parents each have passed on a ‘fault’ in their DNA to their child. This ‘fault’ is called a mutation: a change in the genetic code. The way of inheriting with Usher Syndrome is autosomal recessive.
FROM DNA TO PROTEIN
The hereditary material of each person is stored in the DNA. The DNA consists of many genes. Each gene contains the code for the production of a protein. Proteins are the building blocks of our bodies.
At this moment, 10 different Usher Syndrome genes are known, all of which are involved in the production of an Usher protein. The various genes are divided into three clinical types (USH 1, USH 2 and USH 3) and the gene is indicated by a letter (A, B, C, etc.).
AUTOSOMAL RECESSIVE INHERITANCE
Autosomal means that the disorder is not gender-specific. Boys and girls have the same chance of having Usher Syndrome.
Recessive means that the mutations are inherited from both parents. The father has one mutation and one healthy copy in one Usher gene. The mother also has one mutation and one healthy copy in the same Usher gene. Both are carriers and do not show any symptoms of Usher Syndrome. The ‘healthy copy’ is dominant. A child of this parents has:
- 25% chance of getting two ‘healthy copies’ from both parents.
- 50% chance of getting one healthy copy and one mutation from its parents. This child will also be carrier, but does not show any symptoms of Usher Syndrome.
- 25% chance of receiving both mutations from both parents. Then the disorder will show.
Usher Syndrome often comes unexpectedly, because most parents do not know that they are carrier. Usher Syndrome is very rare; only one out of 75 people carries a mutation in one of the 10 Usher genes. Examples:
- The parents are both carriers of one mutation in the Usher 1B gene and pass this on to their child. In this case the child has 25% chance to Usher Syndrome type 1B.
- The father is carrier of a mutation in the USH 1B gene en the mother is carrier of a mutation in the USH 2A gene. These mutations can be passed on to their child, but the child will not have Usher Syndrome. The child is carrier of mutations in two different Usher genes though.
DO YOU WAN TOT HAVE A CHILD?
If both parents are carriers of a mutation in the same Usher gene, they have an increased chance of having a child with Usher Syndrome. This chance is 25% for each child (one out of four). This holds for both sons and daughters.