Identification of a novel homozygous mutation as the second cause of Usher syndrome type 4.

Purpose: Usher syndrome is a genetic disease characterized by combined sensorineural hearing loss, retinitis pigmentosa, and vestibular areflexia, with 15 known causative genes. Depending on the severity and onset of the symptoms, 3 different subtypes of the pathology have been classically established, although an increasing number of rare cases are being accumulated as atypical forms. The present work aims to discover the genetic cause in a patient with atypical Usher syndrome, by performing whole exome sequencing in several family members.

Observations: The obtained results identified a novel homozygous missense mutation (p.Asp44Asn) in the gene as the cause of the disease, which was characterized by late-onset progressive symptoms in the patient. A resembling phenotype, recently defined as the novel Usher syndrome type 4, was described in three families sharing another mutation. Both mutations affect two contiguous amino acid residues, which appear to be critical for the correct function of the protein.

Conclusions And Importance: These findings support the identification of the second disease mutation in this gene and a new evidence of the implication of in the genetic basis of Usher syndrome type 4.