Usher Syndrome: Type IIIA

Usher syndrome type III (USH3) is an inherited disorder caused by defects in nerve cell communication in the inner ear and the retina. Children affected by USH3 have normal hearing and vision at birth but will lose both over time. They become deaf by mid-to-late adulthood. Vision loss typically begins with night blindness during teenage years, followed by peripheral vision loss and finally blindness by middle age. Some of them may also have mild problems with balance. Intellectual function and life expectancy are not affected. USH3 is caused by pathogenic variants in the CLRN1 and HARS genes.

USH3 is inherited in an autosomal recessive manner. This type of inheritance requires the presence of two copies of a pathogenic variant in the gene for a person to have the genetic disease. Both parents must be carriers of a pathogenic variant in the gene in order to be at risk to have an affected child. The child must inherit a pathogenic variant from each carrier parent in order to be affected. There is a 1 in 4 chance that a baby will inherit two mutated copies of the gene and be affected when both parents are carriers.

There are generally no signs or symptoms associated with being a carrier for USH3. However, the risk to have a child affected with USH3 is increased. Testing of reproductive partners is recommended for carriers of USH3.

Usher syndrome is estimated to occur in approximately 3 to 6 per 100,000 newborns in Northern Europe and at least 4 in 100,000 newborns in the general US population. Usher syndrome is divided into three major types, I, II, and III. Type III is frequently seen in Ashkenazi Jewish and Finnish population.

Genetics Home Reference: Usher syndrome (http://ghr.nlm.nih.gov/)
Genetic Alliance: Usher syndrome (http://www.diseaseinfosearch.org/)
Foundation fighting for blindness (https://www.blindness.org/)

OMIM [USH32A: 276902] (http://www.ncbi.nlm.nih.gov/omim)

• Full gene sequencing