Usher syndrome type 3 (USH3) is an autosomal recessive disorder characterized by progressive hearing loss, severe retinal degeneration, and variably present vestibular dysfunction, assigned to 3q21-q25. Here, we report on the positional cloning of the USH3 gene. By haplotype and linkage-disequilibrium analyses in Finnish carriers of a putative founder mutation, the critical region was narrowed to 250 kb, of which we sequenced, assembled, and annotated 207 kb. Two novel genes-NOPAR and UCRP-and one previously identified gene-H963-were excluded as USH3, on the basis of mutational analysis. USH3, the candidate gene that we identified, encodes a 120-amino-acid protein. Fifty-two Finnish patients were homozygous for a termination mutation, Y100X; patients in two Finnish families were compound heterozygous for Y100X and for a missense mutation, M44K, whereas patients in an Italian family were homozygous for a 3-bp deletion leading to an amino acid deletion and substitution. USH3 has two predicted transmembrane domains, and it shows no homology to known genes. As revealed by northern blotting and reverse-transcriptase PCR, it is expressed in many tissues, including the retina.

Usher syndrome (USH) refers to genetically and clinically heterogeneous autosomal recessive disorders with combined visual and hearing loss. Type I (USH1) is characterized by a congenital, severe to profound hearing loss and absent vestibular function; in type II (USH2) the hearing loss is congenital and moderate to severe, and the vestibular function is normal. Progressive pigmentary retinopathy (PPR) is present in both types. A third type (USH3) differing from USH2 by the progressive nature of its hearing loss has been suggested. USH3 has previously been estimated to comprise 2% of all USH. However, based on clinical criteria, in Finland 42% of USH patients have progressive hearing loss suggesting enrichment of an USH3 gene. We excluded the four previously mapped USH regions as the site of the USH3 disease locus. Systematic search for USH3 by genetic linkage analyses in 10 multiple affected families using polymorphic microsatellite markers revealed significant linkage with markers mapping to chromosome 3q. Pairwise lod scores at zero recombination distance were 7.87 for D3S1308, and 11.29 for D3S1299, incorporating the observed linkage disequilibrium. Conventional multipoint linkage analysis gave a maximum lod score of 9.88 at D3S1299 assigning USH3 to the 5 cM interval between markers D3S1555 and D3S1279 in 3q21-25.(ABSTRACT TRUNCATED AT 250 WORDS)

Usher's syndrome, type 3 (USH3) is characterized by progressive hearing loss. Usher's syndrome, type 3 has been supposed to be rare, occurring in 2% to 4% of all patients with Usher's syndrome. In a nationwide study we collected data on 229 patients with Usher's syndrome in Finland. Definite cases of USH3 were found in 30 (13%) of the 229 patients. An additional 61 patients had clinical evidence of earlier progression of their hearing impairment. We suggest that 91 (40%) of the 229 patients with Usher's syndrome represent cases of USH3.

Usher syndrome is a recessive hereditary disease group with clinical and genetical heterogeneity leading to handicapped hearing and visual loss until middle age. It is the most common cause for deaf-blindness. Three distinct phenotypes and five distinct genotypes are already known. In Finland the distribution of known Usher types is different than elsewhere. Usher syndrome type III (USH3) is common in Finland and it is thought to include 40% of patients. Progressive hearing loss is characteristic of USH3. Elsewhere USH3 has been regarded as a rarity covering only several percent of the whole Usher population. The aim of this paper is to describe, for the first time, the course of visual handicap and typical refractive errors in USH3 and compare it with other USH types. From a total patient sample consisting of 229 Finnish USH patients, 200 patients' visual findings were analyzed in a multicenter retrospective follow-up study. The average progress rate during a 10-year follow-up period in different USH types was similar. The essential progress occurred below the age of 40 and was continuous up to that age. Visual acuity dropped below 0.05 (severely impaired) at the age of 37 and the visual fields were of tubular shape without any peripheric islands at the average age of 30. Clinically significant hypermetropia with astigmatism seems to be a pathognomonic clinical sign of USH3.

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