Usher syndrome is a clinically and genetically heterogeneous autosomal recessive disorder characterized by sensorineural hearing deficiencies at birth and later development of progressive retinitis pigmentosa (RP). It is the most frequent cause of combined deafness and blindness in ... Usher syndrome is a clinically and genetically heterogeneous autosomal recessive disorder characterized by sensorineural hearing deficiencies at birth and later development of progressive retinitis pigmentosa (RP). It is the most frequent cause of combined deafness and blindness in adults and affects 3 to 6% of children born with hearing impairment. In brief, patients with Usher syndrome type II have mild hearing impairment with normal vestibular responses. Type II is the most common of the 3 Usher syndromes (Eudy et al., 1998). See 276900 for clinical characterization of Usher syndrome types I, II, and III. - Genetic Heterogeneity of Usher Syndrome Type II Usher syndrome type II is genetically heterogeneous. USH2C (605472) can be caused by mutation in the GPR98 gene (602851) or by biallelic digenic mutation in the GPR98 and PDZD7 (612791) genes. USH2D (611383) is caused by mutation in the WHRN gene (607928). The locus designation USH2B has been withdrawn; see HISTORY.
Eudy et al. (1998) characterized type IIa Usher syndrome as showing moderate to severe sensorineural hearing loss as well as progressive retinitis pigmentosa.
Blanchet et al. (1992) described a patient with presumably type II Usher syndrome ... Eudy et al. (1998) characterized type IIa Usher syndrome as showing moderate to severe sensorineural hearing loss as well as progressive retinitis pigmentosa. Blanchet et al. (1992) described a patient with presumably type II Usher syndrome who had apparent acceleration of retinitis pigmentosa following cytotoxic chemotherapy for non-Hodgkin lymphoma. Robbins et al. (1984) claimed that lymphoid and fibroblast cell lines derived from patients with Usher syndrome are hypersensitive to DNA-damaging agents such as x-ray. Lymphoblastoid cells from dominantly inherited RP showed no hypersensitivity. Thus, the DNA-damaging effects of chemotherapy may have had particularly deleterious consequences for this patient. Based on the observation that both sperm and retina are rich in docosahexaenoic acid (DHA) and that blood levels of DHA in RP patients are less than normal, Connor et al. (1997) studied the lipid composition of erythrocytes and sperm in 26 patients with RP and in 8 healthy men. The sperm of patients with RP had a much lower DHA concentration, a lower desmosterol-to-cholesterol ratio, reduced motility, abnormal structure, and lower sperm counts compared with that in normal subjects. Patients with Usher syndrome type II exhibited the most pronounced reductions of DHA in sperm. Sperm DHA concentration was positively correlated to sperm motility, to sperm count, and to the desmosterol-to-cholesterol ratio. Lower erythrocyte DHA was also observed in RP patients. Connor et al. (1997) concluded that sperm of patients with RP, particularly those with Usher syndrome type II, may have an abnormal lipid composition that is associated with reduced motility. Hmani-Aifa et al. (2002) reported a family from Tunisia in which 7 members were affected with Usher syndrome type IIa. Audiometry tests showed a moderate to profound bilateral sensorineural hearing loss, and ophthalmologic examination detected RP with the appearance of night blindness in the first or second decade. There was no vestibular dysfunction. Mutation analysis detected a mutation in the usherin gene that segregated with the disorder. Hmani-Aifa et al. (2002) compared the findings in this family with those in a family with Usher syndrome type IIb. The hearing loss in type IIa was greater than that in type IIb at low frequencies (0.25, 0.5, and 1 kHz). In addition, the onset of RP was slightly later in type IIb, occurring in the late second or early third decade. Iannaccone et al. (2004) found that the kinetics of Goldmann visual field (GVF) decline in 19 patients with Usher syndrome type II were, on average, very similar to those in other forms of RP. Their findings suggested the existence of stereotypical disease mechanism(s) that may characterize most patients with RP and related conditions once their degenerative process has become symptomatic. Malm et al. (2011) evaluated visual function, comprising both the severity of the rod cone degeneration and the function in the macular region, in 12 patients genotyped as Usher syndrome 1B, 1D, 1F, 2A, 2C, or 3A, including 3 families with affected sibs, and confirmed phenotypic heterogeneity between sibs with the same genotype and between patients with different genotypes. In all patients examined with ERG, the 30 Hz flicker response revealed remaining cone function. Optical coherence tomography (OCT) demonstrated loss of foveal depression with distortion of the foveal architecture in the macula of all patients. The foveal thickness ranged from 159 to 384 micrometers and was not correlated with retinal function.
To investigate genotype/phenotype correlations, Aller et al. (2004) screened 191 unrelated Spanish patients with syndromic or nonsyndromic retinal diseases, or with nonsyndromic hearing impairment, for the 2299delG (608400.0001) and C759F (608400.0006) mutations in the USH2A gene. They found ... To investigate genotype/phenotype correlations, Aller et al. (2004) screened 191 unrelated Spanish patients with syndromic or nonsyndromic retinal diseases, or with nonsyndromic hearing impairment, for the 2299delG (608400.0001) and C759F (608400.0006) mutations in the USH2A gene. They found that the 2299delG mutation was present in patients with clinical signs of Usher syndrome type II or of atypical Usher syndrome, whereas the C759F mutation, whether or not it was associated with the 2299delG mutation, was identified in cases with nonsyndromic retinitis pigmentosa. Aller et al. (2004) concluded that sensorineural hearing loss in patients with RP may depend on the nature and association of the USH2A allelic variants present. They recommended that patients with nonsyndromic RP and a USH2A mutation should be examined for auditory function. In 3 sibs with USH2C (605472) and 14 patients with USH2A, Schwartz et al. (2005) investigated the retinal disease expression. The sibs with USH2C showed abnormal photoreceptor-mediated function in all retinal regions, and there was greater rod than cone dysfunction. USH2A had a wider spectrum of disease expression and included patients with normal function in some retinal regions. When abnormalities were detected, there was more rod than cone dysfunction. Retinal microstructure in both USH2C and USH2A shared the abnormality of loss of outer nuclear layer thickness. Cystic macular lesions complicated the central retinal structure in both genotypes. Bernal et al. (2005) studied 28 Spanish patients with Usher syndrome type II, identifying 10 different pathogenic mutations and 17 polymorphisms in the USH2A gene. They observed discordant phenotypes in sib pairs from 2 unrelated families and noted that Liu et al. (1999) had reported clinical differences in monozygotic twins with Usher syndrome type II and had suggested that variation in the expression of the USH2A gene is not determined simply by genetic factors. Ebermann et al. (2010) studied 2 French Canadian sisters with USH2A who were homozygous for the 4338delCT mutation in the USH2A gene (608400.0003), and in 1 of the sisters, who had earlier onset and more severe retinal disease, they identified an additional de novo heterozygous frameshift mutation in the PDZD7 gene (612971.0001). The PDZD7 mutation was not present in the other sister, who had a much milder retinal phenotype. Ebermann et al. (2010) concluded that PDZD7 is a retinal disease modifier in patients with USH2A. Abadie et al. (2012) analyzed the audiologic findings in 100 USH2 patients, including 88 with USH2A mutations, 10 with GPR98 mutations, and 2 with WHRN mutations. The median age of diagnosis of hearing loss was 5 years (range, 8 months to 31 years), but some patients may have had earlier onset. Usher syndrome was diagnosed at a median age of 34.5 years. Most patients (76%) patients had moderate hearing loss and most (66%) had a gently down-sloping audiogram. The median pure tone average (PTA) was 59.7 dB. There were no significant differences between patients with USH2A and GPR98 mutations, but the GPR98 cases had a higher proportion of severe hearing loss (40%) compared to USH2A cases (16%). Among all groups, cut-off frequencies were noted at 500-1000 Hz. There was some intrafamilial variability. Overall, Abadie et al. (2012) concluded that it is not possible to predict the mutated gene from audiograms in patients with USH2.
Among 96 patients with Usher syndrome type IIa, Eudy et al. (1998) identified 3 mutations in the USH2A gene (608400.0001-608400.0003), all of which resulted in frameshifts and premature terminations. A 2299delG mutation (608400.0001) was the most frequent mutant ... Among 96 patients with Usher syndrome type IIa, Eudy et al. (1998) identified 3 mutations in the USH2A gene (608400.0001-608400.0003), all of which resulted in frameshifts and premature terminations. A 2299delG mutation (608400.0001) was the most frequent mutant allele, occurring in 21 cases. In a mutation search of 57 independent USH2A probands, Weston et al. (2000) identified 15 mutations in the USH2A gene. The 2299delG mutation was the most frequent mutant allele, observed in 31 of 192 alleles (16%). In 12 unrelated patients with Usher syndrome, each with 1 mutation in exons 1 to 21 of the USH2A gene, van Wijk et al. (2004) identified a second pathogenic USH2A mutation in the 51 novel exons that they identified. The novel mutations included 3 different truncating mutations and 2 missense mutations (see, e.g., 608400.0007-608400.0009). The presence of pathogenic mutations in the novel exons indicated that at least 1 of the putative long isoforms of the USH2A protein plays a role in both hearing and vision. Aller et al. (2006) identified mutations in the USH2A gene in 14 of 32 unrelated Spanish patients with Usher syndrome, nonsyndromic retinal degeneration, or nonsyndromic deafness in whom 2 disease-causing mutations could not be found after screening the first 21 exons of the USH2A gene. Analysis of the 51 new exons identified by van Wijk et al. (2004) and the 2 new exons identified by Adato et al. (2005) yielded 14 novel mutations, including 7 missense, 5 frameshift, 1 duplication, and 1 putative splice-site mutation. Most of the patients had previously been reported by Aller et al. (2004). All of the individuals with 2 mutations were clinically diagnosed with Usher syndrome type IIa. To explore the spectrum of USH2A disease-causing mutations among Scandinavian USH2 cases, Dreyer et al. (2008) performed extensive DNA sequence analysis of the full-size USH2A gene in patients from 118 unrelated families, of which 27 had previously been found to carry mutations in exons 1 to 21. In all, 122 USH2A DNA sequence alterations were identified, of which 57 were predicted to be pathogenic, 7 were considered to be of uncertain pathogenicity, and 58 were predicted to be benign variants. Of 36 novel pathogenic USH2A mutations, 31 were located in exons 22 to 73, specific to the long isoform (see, e.g., 608400.0013). USH2A mutations were identified in 89 (75.4%) of 118 families. In 79 (88.8%) of these 89 families, 2 pathogenic mutations were identified, whereas in 10 families (11.2%) the second mutation remained unidentified. In 5 (4.2%) of the 118 families the USH phenotype could be explained by mutations in the CLRN1 gene (606397). Yan et al. (2009) identified mutant USH2A alleles in 12 (60%) of 20 American patients of European ancestry with Usher syndrome type IIa. Seven (35%) patients had only 1 pathogenic mutation, and 8 patients did not have USH2A mutations. There were 5 novel mutations and 5 previously reported mutations, consisting of 3 missense, 3 frameshift, and 4 nonsense. The 2299delG mutation was the most common, accounting for 38.9% of mutant alleles. - Genetic Heterogeneity of USH2 In 31 French patients with USH2 who were not linked to the USH2A locus (608400), Besnard et al. (2012) analyzed the GPR98 and WHRN genes: 10 patients were found to have mutations in GPR98, and 2 had mutations in WHRN. Analysis of the PDZD7 gene was performed in the remaining 19 patients, but no deleterious mutations were detected. Subsequent reassessment of the phenotype in the 19 mutation-negative patients revealed that 12 had atypical audiologic and/or ophthalmologic impairment, and 2 had clinical features such as renal impairment or dysmorphism that indicated the possibility of another syndrome. Besnard et al. (2012) concluded that GPR98 mutations account for a small but significant proportion of mutations causing USH2 (6.4%), and that mutations in WHRN account for a very small proportion (1.3%).
Dreyer et al. (2001) presented data indicating that the widespread geographic distribution of the USH2A 2299delG mutation (608400.0001) is the result of an ancestral mutation that spread throughout Europe and into the New World as a result of ... Dreyer et al. (2001) presented data indicating that the widespread geographic distribution of the USH2A 2299delG mutation (608400.0001) is the result of an ancestral mutation that spread throughout Europe and into the New World as a result of migration. Various studies had reported a range of frequencies (from 0.16 to 0.44) among patients with Usher syndrome, depending on the geographic origin of the patients. Dreyer et al. (2001) performed haplotype analysis on DNA samples from 116 unrelated patients with Usher syndrome type IIa; the patients were from 14 countries and represented 148 2299delG alleles. On the basis of 6 single-nucleotide polymorphisms (SNPs) within the USH2A gene, 12 core haplotypes were observed in a panel of normal chromosomes. However, in their patient analysis, only 1 core haplotype was associated with the 2299delG mutation. Ouyang et al. (2004) confirmed that 2299delG is the most common mutation in USH2A, accounting for 77.5% of pathologic alleles. In 5 of the 24 patients, the 2299delG mutation was present in homozygous state; in 3 it was present in compound heterozygous state with other mutations; and in 16 it was present in heterozygous state. Ebermann et al. (2009) identified a homozygous USH2A mutation (4338delCT; 608400.0003) in 4 of 9 French Canadian families with Usher syndrome type IIa from Quebec and New Brunswick, the former Acadia. Affected individuals from 2 additional families carried the mutation in heterozygosity. Altogether, the 4338delCT mutation accounted for 10 (55.6%) of 18 disease alleles. Haplotype analysis indicated a founder effect. The findings indicated that the Acadian and Quebec populations share common ancestors.