Deafness, Autosomal Recessive 2

A number sign (#) is used with this entry because of evidence that autosomal recessive nonsyndromic deafness-2 (DFNB2) is caused by homozygous or compound heterozygous mutation in the myosin VIIA gene (MYO7A; 276903) on chromosome 11q13.

Allelic disorders include autosomal dominant deafness-11 (DFNA11; 601317) and Usher syndrome type IB (USH1B; see 276900).

Clinical Features

Guilford et al. (1994) reported a consanguineous family from southern Tunisia in which 22 individuals had autosomal recessive nonsyndromic sensorineural deafness. All had profound deafness, and 4 also had vertigo. The age of onset of deafness was variable and reported to range from birth to age 16 years.

Riazuddin et al. (2008) reported a consanguineous Pakistani family in which several members had nonsyndromic sensorineural deafness. None had vestibular or retinal abnormalities.

Hildebrand et al. (2010) reported 3 sibs, born of consanguineous Iranian parents, with autosomal recessive nonsyndromic deafness-2. Onset of hearing loss occurred between ages 7 months and 7 years. Audiologic testing revealed hearing loss at all frequencies, although low frequency hearing was less impaired. All had normal vestibular function, and funduscopic examination and visual acuity tests excluded retinitis pigmentosa in all patients at ages 39, 31, and 42 years, respectively. One patient had a milder phenotype, with later onset and less severe impairment, suggesting the presence of a genetic modifier. The findings confirmed that nonsyndromic hearing loss can be caused by mutation in the MYO7A gene.

Clinical Variability

Zina et al. (2001) reevaluated the family reported by Guilford et al. (1994) and Weil et al. (1997). Since the original reports, 5 patients had developed mild retinal degeneration in addition to the progressive deafness. Fundus examination of 1 patient showed spicule pigmentary changes consistent with retinal dystrophy. Another previously unaffected family member, homozygous for the mutation, had retinitis pigmentosa. Seven patients had abnormal vestibular function as assessed by caloric tests. Zina et al. (2001) concluded that some patients in this Tunisian family had features consistent with Usher syndrome type IB. The findings suggested that other factors must modulate the expression of the phenotype.

Mapping

By linkage analysis of a consanguineous Tunisian family with autosomal recessive neurosensory deafness, Guilford et al. (1994) found linkage to chromosome 11q13 (maximum lod score of 10.63 at marker D11S527). Homozygosity mapping refined the location of the locus to a 6-cM interval that also contained the olfactory marker protein gene (OMP; 164340). The murine homolog of OMP is tightly linked to the autosomal recessive deafness shaker-1 gene in mice, suggesting that the deafness in this Tunisian family is the human homolog of the mouse shaker-1 mutation.

Molecular Genetics

In affected members of the Tunisian family reported by Guilford et al. (1994), Weil et al. (1997) identified a homozygous mutation in the MYO7A gene (276903.0010).

In affected members of 2 Chinese families with DFNB2, Liu et al. (1997) identified homozygous or compound heterozygous mutations in the MYO7A gene (276903.0007-276903.0009).

In affected members of a consanguineous Pakistani family with DFNB2, Riazuddin et al. (2008) identified a homozygous mutation in the MYO7A gene (276903.0018).

In 3 sibs, born of consanguineous Iranian parents, with DFNB2, Hildebrand et al. (2010) identified a homozygous mutation in the MYO7A gene (R395H; 276903.0021).

Animal Model

Shaker-1 (sh1) homozygous mice show hyperactivity, head-tossing, and circling due to vestibular dysfunction, as well as neuroepithelial-type cochlear defects involving dysfunction and progressive degeneration of the organ of Corti. Gibson et al. (1995) described 3 different mutations in the Myo7a gene that segregated with the disorder in mice. All the mutations were located in the region encoding the myosin head. The sh1 phenotype differs from that of Usher syndrome in humans by the absence of retinal degeneration. Weil et al. (1995) noted that one form of human neurosensory recessive deafness without retinal dystrophy, DFNB2, maps to 11q in the same general region as USH1B and may represent the human equivalent of sh1.