SNP Shot: Genomic Insights

Usher syndrome is a genetic disorder causing both hearing and vision loss. The hearing loss is sensorineural, meaning it originates from problems in the inner ear. The vision loss stems from retinitis pigmentosa (RP), an eye disease affecting the retina, the light-sensitive tissue at the back of the eye. RP causes the light-sensing cells in the retina to gradually deteriorate, leading to a progressive loss of sight. Initially, night vision deteriorates, followed by the development of blind spots in peripheral vision. These spots enlarge and merge, resulting in tunnel vision. Cataracts may further impair vision in some cases. However, many individuals with RP retain some central vision throughout their lives.

There are three primary types of Usher syndrome: types I, II, and III. These types are differentiated by the severity of hearing loss, the presence or absence of balance issues, and the age when symptoms appear. Each type is further classified into subtypes based on the specific gene mutation causing the condition.

Individuals with Usher syndrome type I are typically born with severe to profound hearing loss. Vision loss due to retinitis pigmentosa becomes noticeable in childhood. Additionally, type I affects the vestibular system, the inner ear component responsible for balance and spatial orientation. This results in balance problems, causing delays in independent sitting and walking and difficulties with activities like riding a bicycle or playing certain sports.

Usher syndrome type II is characterized by congenital hearing loss and progressive vision loss starting in adolescence or adulthood. The hearing loss ranges from mild to severe, primarily affecting the ability to hear high-frequency sounds. This makes it difficult to distinguish soft, high-pitched speech sounds. The severity of hearing loss can vary significantly, even within the same family, and may worsen over time. Unlike other types, type II does not cause vestibular abnormalities, so balance is not affected.

In Usher syndrome type III, both hearing and vision loss develop later in life. Unlike types I and II, individuals with type III usually have normal hearing at birth. Hearing loss typically starts in late childhood or adolescence, after speech development, and progressively worsens. By middle age, most experience profound hearing loss. Vision loss from retinitis pigmentosa also emerges in late childhood or adolescence. Some individuals with Usher syndrome type III may also experience vestibular problems affecting their balance.

Inheritance:

All types of Usher syndrome are inherited in an autosomal recessive manner. This means that an individual must inherit two copies of a mutated gene, one from each parent, to develop the condition. The parents, who each carry one copy of the mutated gene, are typically asymptomatic.