Monogenic nonsyndromic otosclerosis: audiological and linkage analysis in a large Greek pedigree

Genetic Heterogeneity and Core Clinical Features of NOG-Related-Symphalangism Spectrum Disorder

OBJECTIVES

To better distinguish NOG-related-symphalangism spectrum disorder (NOG-SSD) from chromosomal 17q22 microdeletion syndromes and to inform surgical considerations in stapes surgery for patients with NOG-SSD.

BACKGROUND

Mutations in NOG cause a variety of skeletal syndromes that often include conductive hearing loss. Several microdeletions of chromosome 17q22 lead to severe syndromes with clinical characteristics that overlap NOG-SSD. Isolated deletion of NOG has not been described, and therefore the contribution of NOG deletion in these syndromes is unknown.

METHODS

Two families with autosomal dominant NOG-SSD exhibited stapes ankylosis, facial dysmorphisms, and skeletal and joint anomalies. In each family, NOG was evaluated by genomic sequencing and candidate mutations confirmed as damaging by in vitro assays. Temporal bone histology of a patient with NOG-SSD was compared with temporal bones of 40 patients diagnosed with otosclerosis.

RESULTS

Family 1 harbors a 555 kb chromosomal deletion encompassing only NOG and ANKFN1. Family 2 harbors a missense mutation in NOG leading to absence of noggin protein. The incus-footplate distance of the temporal bone was significantly longer in a patient with NOG-SSD than in patients with otosclerosis.

CONCLUSION

The chromosomal microdeletion of family 1 led to a phenotype comparable to that due to a NOG point mutation and much milder than the phenotypes due to other chromosome 17q22 microdeletions. Severe clinical findings in other microdeletion cases are likely due to deletion of genes other than NOG. Based on temporal bone findings, we recommend that surgeons obtain longer stapes prostheses before stapes surgery in individuals with NOG-SSD stapes ankylosis.

Genetics of otosclerosis

OBJECTIVES

Otosclerosis is a major cause of acquired hearing loss in adult life affecting exclusively the human temporal bone. Until recently, the etiopathogenesis of otosclerosis was still a matter of debate. Genetic research, however, has evolved enormously the last years and unveiled important clues regarding the cause of otosclerosis. The objective of this article is to review the genetics of otosclerosis with special attention for the links to the bone homeostasis of the otic capsule.

DATA SOURCES

A detailed literature study was performed focusing on the recent genetic findings in otosclerosis and the special bone turnover of the otic capsule. A PubMed search and own research data were used to bring the relevant information for this review together.

CONCLUSION

Unlike all other bones in the human skeleton, the otic capsule undergoes very little remodeling after development, possibly due to local inner ear factors. Otosclerosis is a process of pathologic increased bone turnover in the otic capsule, which in most cases leads to stapes fixation, resulting in a conductive hearing loss. Although environmental factors such as estrogens, fluoride, and viral infection have been implicated, it is clear that genetic factors play a significant role in the manifestation of otosclerosis. From a genetic viewpoint, otosclerosis is considered to be a complex disease with rare autosomal dominant forms caused by a single gene. Already, 7 monogenic loci have been published, but none of the genes involved have been identified. For the complex form of otosclerosis, caused by an interaction between genetic and environmental factors, the first susceptibility genes were identified by case-control association studies. All 3 replicated genes, TGFB1, BMP2, and BMP4, are a part of the transforming growth factor-beta1 pathway. Data from both genetic association studies and gene expression analysis of otosclerotic bone showed that the TGF-beta1 pathway is most likely an important factor in the pathogenesis of otosclerosis.

An overview of the etiology of otosclerosis

Otosclerosis is the primary disease affecting the homeostasis of otic capsule and is among the most common causes of acquired hearing loss. Otosclerosis is considered as a multifactor disease, caused by both genetic and environmental factors. The aim of the present review is to summarize and analyze the bibliographic data, associated with the etiology of the disease. In some cases, the otosclerosis has an autosomal dominant mode of inheritance with incomplete penetrance. Genetic studies reveal the occurrence of at least nine chromosomal loci as candidate genes of the disease. The localized measles virus infection of the otic capsule has been postulated as a possible etiological theory. The role of hormonal factors, immune and bone-remodeling system in the etiopathogenesis of otosclerosis and the association of the disease with the disorders of the connective tissue are the issues of the present study. Despite the extensive research, many etiological factors and theories have been suggested and the process of development of the otosclerosis remains unclear.

Phenotype description of a Dutch otosclerosis family with suggestive linkage toOTSC7

This study reports on a clinical investigation of a Dutch family that shows suggestive linkage to OTSC7. Cross-sectional as well as longitudinal analyses of audiometric data were performed. Also, high-resolution computed tomography (CT) images of the temporal bones from genetically affected family members were obtained to study the incidence and extent of otospongiotic foci. Audiometric data showed a considerable degree of phenotypic variability. Cross-sectional regression analysis did not show age-dependent progression of bone conduction (BC), air conduction (AC), and air-bone gap (ABG) levels. Longitudinal analysis of audiometric follow-up data of one family member showed age-dependent progression of AC, BC, and ABG levels. High-resolution CT images revealed an otospongiotic focus in six of six (100%) clinically affected individuals that carried the disease haplotype. In none of the clinically unaffected family members that showed linkage to OTSC7, an otospongiotic focus was detected by CT. In conclusion, hearing impairment in the present otosclerosis family seems to be variable in terms of onset age and level of progression. Long-term audiometric data of one patient proved to be valuable in understanding progression of hearing impairment in this individual. The detection rate of otospongiotic foci in our study group is similar compared to previous reports on CT data in consecutive otosclerosis patients who had stapes replacing surgery.

The role of mtDNA mutations in the pathogenesis of age-related hearing loss in mice carrying a mutator DNA polymerase gamma

Mitochondrial DNA (mtDNA) mutations may contribute to aging and age-related diseases. Previously, we reported that accumulation of mtDNA mutations is associated with age-related hearing loss in mice carrying a mutator allele of the mitochondrial Polg DNA polymerase. To elucidate the role of mtDNA mutations in the pathogenesis of age-related hearing loss or presbycusis, we performed large scale gene expression analysis to identify mtDNA mutation-responsive genes and biological process categories associated with mtDNA mutations by comparing the gene expression patterns of cochlear tissues from 9-month-old mitochondrial mutator and control mice. mtDNA mutations were associated with transcriptional alterations consistent with impairment of energy metabolism, induction of apoptosis, cytoskeletal dysfunction, and hearing dysfunction in the cochlea of aged mitochondrial mutator mice. TUNEL staining and caspase-3 immunostaining analysis demonstrated that the levels of apoptotic markers were significantly increased in the cochleae of mitochondrial mutator mice compared to age-matched controls. These observations support a new model of how mtDNA mutations impact cochlear function whereby accumulation of mtDNA mutations lead to mitochondrial dysfunction, an associated impairment of energy metabolism, and the induction of an apoptotic program. The data presented here provide the first global assessment at the molecular level of the pathogenesis of age-related disease in mitochondrial mutator mice and reveal previously unrecognized biological pathways associated with mtDNA mutations.

A seventh locus for otosclerosis, OTSC7, maps to chromosome 6q13–16.1

Otosclerosis is a common form of hearing impairment among white adults with a prevalence of 0.3-0.4%. It is caused by abnormal bone homeostasis of the otic capsule that compromises free motion of the stapes in the oval window. Otosclerosis is in most patients a multifactorial disease, caused by both genetic and environmental factors. In some cases, the disease is inherited as a monogenic autosomal dominant trait, sometimes with reduced penetrance. However, families large enough for genetic linkage studies are extremely rare. To date, five loci (OTSC1-5) have been reported, but none of the responsible genes have been cloned yet. An additional locus, OTSC6, has been reported to the HUGO nomenclature committee but the relevant linkage study has not been published. In this study, a genome-wide linkage study was performed in a large Greek pedigree segregating autosomal dominant otosclerosis. A seventh locus, OTSC7, was localized on chromosome 6q13-16.1 with a multipoint LOD score of 7.5 in the 13.47 cM region defined by markers D6S1036 (centromeric) and D6S300 (telomeric). Linkage analysis of this new locus in 13 smaller Belgian and Dutch families has identified one family from The Netherlands in which allele segregation suggests linkage to this region. The overlap between the critical regions of these two families is a 1.06 Mb interval between the genetic markers D6S1036 (centromeric) and D6S406 (telomeric) on chromosome 6q13.

Current research in otosclerosis

PURPOSE OF REVIEW

The aim of this article is to summarize and put into historical perspective current advances in research in otosclerosis, a disorder of the human temporal bone with a hereditary predisposition that is among the most common causes of acquired hearing loss.

RECENT FINDINGS

Genetic studies have revealed that otosclerosis is heterogeneous, with evidence for defects in at least seven genes associated with six distinct chromosomal loci. Measurements of high levels of osteoprotegerin expression in the normal otic capsule and soft tissues of the cochlea provide the first molecular insight as to why the normal otic capsule remodels minimally, if at all. Osteoprotegerin knockout mice provide the best available animal model to date to study abnormal otic capsule remodeling that closely resembles otosclerosis. There is mounting evidence that the measles virus plays an important role in pathogenesis of otosclerosis although the mechanisms by which the virus results in otosclerosis remain unknown. Quantitative measures of angiogenesis can reliably distinguish between clinical and histological otosclerosis. Advances in the emerging field of osteoimmunology will likely impact and benefit from the research in otosclerosis.

SUMMARY

Insights into molecular mechanisms that inhibit extensive remodeling in the normal otic capsule, and understanding of how these mechanisms are dysregulated in otosclerosis will allow future design of rational treatment strategies for otosclerosis.

The Phenotype of the First Otosclerosis Family Linked to OTSC5

OBJECTIVE

To report the audiometric and radiologic findings in the first otosclerosis family linked to OTSC5.

STUDY DESIGN

A clinical investigation of a family linked to OTSC5, including analyses of audiometric data and of high-resolution computed tomography (CT) images of the temporal bones from genetically affected family members.

SETTING

Tertiary referral center.

PATIENTS

Family members from a four-generation pedigree with otosclerosis segregating as an autosomal dominant trait.

MAIN OUTCOME MEASURE(S)

Pre-surgery pure tone audiometric data. Classification of otosclerotic foci on high-resolution spiral CT images of the temporal bones of genetically affected individuals.

RESULTS

Audiometric data showed a considerable degree of phenotypic variability. Cross-sectional regression analysis did not disclose any clear age dependence of threshold-related data. Systematic differences between mean parameter values relating to the thresholds in the best or the worst ear were found. High-resolution CT images revealed a fenestral otosclerotic focus in seven of nine (78%) clinically affected individuals and cochlear foci in one of these seven patients.

CONCLUSION

The phenotype of OTSC5 seems to be variable. Additional long-term audiometric data are needed to construct age-related typical audiograms, which may also facilitate the comparison between phenotypes of the different otosclerosis loci. The detection rate of otospongiotic foci in our study group is similar or lower compared with previous reports on CT data in consecutive otosclerosis patients who had stapes replacing surgery.