Usher Syndrome, Type Iic

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Retrieved

2019-09-22

Source

Omim

Trials

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Genes

MYO7A, PCDH15, HARS1, PLD4, USH1C, ADGRV1, USH2A, CDH23, CEP250, ARSG, PROM1, CLRN1, ZDHHC24, CDH23-AS1, TRNS2, BBS1, GUCA1A, WHRN, USH1G, PDZD7, CIB2, GJB2, ESPN, HTC2, GC, CEP78, ABHD12, PCARE, GPR166P, LGR6, INTS2, VN1R17P, MRGPRX1, FADS6, MRGPRX3, MRGPRX4, ASIC5, GPR151, OXER1, LCA5, GPRC6A, VEZT, ACTB, MYO15A, PRPH2, ENG, FGF3, GBX2, MEF2C, MYO5B, NOTCH2, NUCB2, OMP, PEX6, RCVRN, RPGR, PHPT1, SOX3, STATH, USH1E, WFS1, FZD4, OTOF, CIB1, ASAH1, NINL, LPAR3, IMMT

Drugs

Adeno-associated virus serotype 8 containing the human RdCVF sequence and the human RdCVFL sequence, Combination of three adeno-associated viral vectors of serotype 8 containing the 5'-, the body- and the 3'- coding sequences of human CEP290 fused to inteins, Lentiviral vector containing the human MYO7A gene, Mixture of two adeno-associated viral vectors serotype 8 containing the 5'-half sequence of human MYO7A gene and the 3'-half sequence of human MYO7A gene

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A number sign (#) is used with this entry because Usher syndrome type IIC (USH2C) is caused by homozygous or compound heterozygous mutation in the ADGRV1 gene (602851) on chromosome 5q14. It is also caused by biallelic digenic mutation in the ADGRV1 and PDZD7 (612971) genes.

Description

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.

For a discussion of genetic heterogeneity of Usher syndrome type II, see USH2A (276901).

Clinical Features

Ebermann et al. (2009) reported a German brother and sister with USH2C. The patients were 43 and 50 years old, respectively, at the time of the examination. Both patients had congenital, bilateral, symmetric, moderate to severe hearing loss with a mildly downsloping pure tone audiogram. The woman had slightly better visual acuity and later onset of visual defects than her brother, but both had retinitis pigmentosa.

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 contrast to previous studies, they found marked phenotypic variation in retinal function including the macular area in 3 sibs with Usher syndrome 2C who had the same GPR98 mutation (602851.0004). Two sibs, aged 48 and 52 years, had severe rod cone degeneration, whereas the third, aged 51 years, demonstrated only a slight visual handicap with moderately reduced rod and cone function according to full-field electroretinography, remaining function in the macular region according to multifocal electroretinography, visual acuity of 20/20, and almost normal macular structure according to optical coherence tomography.

Mapping

Pieke-Dahl et al. (1993) observed a family in which 4 of 7 sibs had characteristics of Usher syndrome type II, i.e., deafness and retinitis pigmentosa with no vestibular dysfunction; nonlinkage to markers spanning the 1q32-q41 region, where the USH2A (276901) gene had been thought to be located, indicated further heterogeneity in Usher syndrome. Scrutiny of the phenotype suggested that the retinitis pigmentosa was of milder type than in the usual cases of Usher syndrome type II and that mild vestibular abnormalities were present.

Pieke-Dahl et al. (2000) reported linkage analysis in the family they reported in 1993 and in another large family with the same phenotype. A 2-point lod score of 3.1 was obtained with marker D5S484. A total of 9 unrelated 5q-linked families were subsequently identified, producing a maximum combined multipoint lod score of 5.86. Haplotype analysis of 5q markers indicated that this Usher syndrome locus, designated USH2C, is flanked by markers D5S428 and D5S433.

Pieke-Dahl et al. (2000) reported that 3 families with an Usher syndrome type II phenotype did not link to 5q. In 2 of these families, there was severe early-onset retinitis pigmentosa and dental enamel anomalies.

Molecular Genetics

Weston et al. (2004) analyzed the GPR98, or VLGR1, gene for mutations in chromosome 5q14-linked Usher syndrome (USH2C) and found mutations limited to 1 isoform, VLGR1b (602851.0002-602851.0005).

In affected members of a large consanguineous Tunisian family with Usher syndrome type II, originally designated as USH2B, Hmani-Aifa et al. (2009) identified a homozygous mutation in the GPR98 gene (Y6044C; 602851.0006). Heterozygous mutation carriers were unaffected. The family also segregated nonsyndromic retinitis pigmentosa-40 (RP40; 613801) that was caused by a homozygous mutation in the PDE6B gene (180072.0007). One family member who was doubly homozygous for both mutations had a more severe ocular phenotype. Two family members who were doubly heterozygous for both mutations were unaffected at ages 82 and 65 years, respectively. Hmani-Aifa et al. (2009) commented that consanguinity can increase familial clustering of multiple hereditary diseases within the same family. The family had originally been reported by Hmani et al. (1999). Hmani-Aifa et al. (2009) concluded that the USH2B locus at chromosome 3p23-p24.2 does not exist and therefore withdrew the locus designation.

Hilgert et al. (2009) identified a homozygous 136-kb deletion in the GPR98 gene (602851.0007) in 6 affected members of an extended consanguineous Iranian family with USH2C, including 3 men and 3 women.

Ebermann et al. (2009) identified pathogenic mutations in the GPR98 gene in 2 unrelated men with USH2C. One German man and his affected sister were compound heterozygous for GPR98 mutations (602851.0008 and 602851.0009). The findings indicated that men can also be affected with USH2C, and that males and females with GPR98 mutations show a typical USH2C phenotype.

In 10 of 31 French USH2 patients who were not linked to the USH2A locus (608400), Besnard et al. (2012) identified mutations in the GPR98 gene. In 2 of the 10 patients, only 1 deleterious mutation was identified in GPR98; screening for large genomic rearrangements revealed a large duplication involving several exons of GPR98 in 1 of the patients. In the other patient, the PDZD7 gene (612971) was analyzed, but no mutations were found. Besnard et al. (2012) concluded that GPR98 mutations account for a small but significant proportion of mutations causing USH2 (6.4%).

Usher Syndrome Type IIC, GPR98/PDZD7 Digenic

In a 51-year-old German man with type II Usher syndrome, who was negative for mutation in the Usher syndrome genes USH2A (608400), WHRN (607928), and CLRN1 (606397), Ebermann et al. (2010) identified a heterozygous frameshift mutation in the GPR98 gene (602851.0010) and a heterozygous frameshift mutation in the PDZD7 gene (612971.0002). No second mutant allele was detected in GPR98 or PDZD7. His unaffected sister was heterozygous for the PDZD7 mutation but did not carry the GPR98 mutation. The patient had onset of night blindness in early childhood, with retinitis pigmentosa diagnosed at 25 years of age and a posterior polar cataract at age 51 years. Funduscopic examination showed pale optic discs, narrowing of retinal vessels, and moderate bone spicule formation. Electroretinography revealed highly reduced photopic responses, and Goldmann visual field was 8 degrees.

Genotype/Phenotype Correlations

In 3 sibs with USH2C 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. A coincidental finding in an USH2C patient was that oral intake of antihistamines was associated with temporary resolution of the macular cystic change.

Abadie et al. (2012) analyzed the audiologic findings in 100 USH2 patients, including 88 with USH2A (608400) mutations, 10 with GPR98 mutations, and 2 with WHRN (607928) 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%) 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.