Histopathology of the stapes in osteogenesis imperfecta

Increased cochlear otic capsule thickness and intracortical canal porosity in the oim mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI or brittle bone disease) is a group of genetic disorders of the connective tissues caused mainly by mutations in the genes encoding collagen type I. Clinical manifestations of OI include skeletal fragility, bone deformities, and severe functional disabilities, such as hearing loss. Progressive hearing loss, usually beginning in childhood, affects approximately 70% of people with OI with more than half of the cases involving the inner ear. There is no cure for OI nor a treatment to ameliorate its corresponding hearing loss, and very little is known about the properties of OI ears. In this study, we investigate the morphology of the otic capsule and the cochlea in the inner ear of the oim mouse model of OI. High-resolution 3D images of 8-week old oim and WT inner ears were acquired using synchrotron microtomography. Volumetric morphometric measurements were conducted for the otic capsule, its intracortical canal network and osteocyte lacunae, and for the cochlear spiral ducts. Our results show that the morphology of the cochlea is preserved in the oim ears at 8 weeks of age but the otic capsule has a greater cortical thickness and altered intracortical bone porosity, with a larger number and volume density of highly branched canals in the oim otic capsule. These results portray a state of compromised bone quality in the otic capsule of the oim mice that may contribute to their hearing loss.

The impact of stapes surgery on osteogenesis imperfecta: a retrospective comparison of operative outcomes with those for patients with otosclerosis

BACKGROUND

While stapes surgery has long been employed as a means of improving hearing in patients with otosclerosis, it remains controversial as to whether such surgical intervention is of value in patients with osteogenesis imperfecta (OI).

AIMS

This study was therefore designed to determine whether stapes surgery could still be reliably performed in OI patients as it is in otosclerosis patients.

MATERIALS AND METHODS

Seventeen OI patients and 18 matched otosclerosis patients were included. In both of these groups, 22 ears underwent primary stapes surgery. Audiometric follow-up data for patients in these two treatment groups were available for between 2 weeks and 25 years post-surgery.

RESULTS

We detected significant improvements in both the short- and long-term mean air conduction threshold and reduced air-bone gap (ABG) in both treatment groups as measured via postoperative follow-up audiometry. There was no difference between the OI and otosclerosis patient groups with respect to short- and long-term postoperative audiometric. Conclusions and significance: The majority of patients with both OI and otosclerosis that underwent stapes surgery achieved beneficial gains in hearing function. We did not observe any differences in short- or long-term post-operative hearing gains or ABG reductions between the two groups.

Prolyl 3-hydroxylase-1 null mice exhibit hearing impairment and abnormal morphology of the middle ear bone joints

Prolyl 3-hydroxylase1 (P3H1) is a collagen modifying enzyme which hydroxylates certain prolines in the Xaa position of conventional GlyXaaYaa triple helical sequence. Recent investigations have revealed that mutations in the LEPRE1 (gene encoding for P3H1) cause severe osteogenesis imperfecta (OI) in humans. Similarly LEPRE1 knockout mice display an OI-like phenotype. Significant hearing loss is a common problem for people with osteogenesis imperfecta. Here we report that hearing of the P3H1 null mice is substantially affected. Auditory brainstem responses (ABRs) of the P3H1 null mice show an average increase of 20-30 dB in auditory thresholds. Three dimensional reconstructions of the mutant middle ear bones by Micro-scale X-ray computed tomography (Micro-CT) demonstrate abnormal morphology of the incudostapedial and incudomalleal joints. We establish the LEPRE1 knockout mouse as a valuable model system to investigate the mechanism of hearing loss in recessive OI.

Hearing loss in osteogenesis imperfecta: characteristics and treatment considerations

Osteogenesis imperfecta (OI) is the most common heritable disorder of connective tissue. It is associated with fractures following relatively minor injury, blue sclerae, dentinogenesis imperfecta, increased joint mobility, short stature, and hearing loss. Structures in the otic capsule and inner ear share in the histologic features common to other skeletal tissues. OI is due to mutations involving several genes, the most commonly involved are the COL1A1 or COL1A2 genes which are responsible for the synthesis of the proalpha-1 and proalpha-2 polypeptide chains that form the type I collagen triple helix. A genotype/phenotype relationship to hearing loss has not been established in OI. Hearing loss is commonly found in OI with prevalence rates ranging from 50 to 92% in some studies. Hearing loss in OI may be conductive, mixed, or sensorineural and is more common by the second or third decade. Treatment options such as hearing aids, stapes surgery, and cochlear implants are discussed.

The skeletal dysplasias

The skeletal dysplasias (osteochondrodysplasias) are a heterogeneous group of more than 350 disorders frequently associated with orthopedic complications and varying degrees of dwarfism or short stature. These disorders are diagnosed based on radiographic, clinical, and molecular criteria. The molecular mechanisms have been elucidated in many of these disorders providing for improved clinical diagnosis and reproductive choices for affected individuals and their families. An increasing variety of medical and surgical treatment options can be offered to affected individuals to try to improve their quality of life and lifespan.

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.

A review on the genetics of otosclerosis

BACKGROUND

The aetiology of otosclerosis is not fully understood despite intensive research. It is, however, certain that a genetic component plays a significant role in the manifestation of otosclerosis, although the precise mode of inheritance is still uncertain.

OBJECTIVE OF REVIEW

To provide an up-to-date review for the genetics of otosclerosis. The mode of inheritance, chromosomal and linkage studies are presented. In addition, the possible genetic relationship between otosclerosis and osteogenesis imperfecta, and the association between otosclerosis and specific human leucocyte antigen types are described.

TYPE OF REVIEW

Systematic analysis of the literature was focused on any information related to the genetics of otosclerosis.

SEARCH STRATEGY

A MEDLINE search (1960-2007) was undertaken to perform a comprehensive review. Articles were also identified through searches of the files of authors.

RESULTS

The majority of epidemiological studies on families with otosclerosis suggest an autosomal dominant mode of inheritance with reduced penetrance of approximately 40%. Genetic linkage studies have demonstrated the presence of six loci (OTSC1, OTSC2, OTSC3, OTSC4, OTSC5 and OTSC7) located on chromosomes 15q, 7q, 6p, 16q, 3q and 6q respectively. Although these loci have been mapped, no causative genes have been identified, and we have little idea of the molecular process involved in this disease. While clinical similarities and some unreplicated genetic association studies suggest an aetiological relationship between otosclerosis and osteogenesis imperfecta-type I, there is no definite evidence of a common pathological process between the two diseases.

CONCLUSIONS

Otosclerosis may be considered as a complex disease with relatively common monogenic forms. Knowledge of these genes could lead to substantial improvements in our ability to diagnose and possibly even prevent or treat this type of hearing deterioration.

Is otosclerosis a generalized connective tissue disorder?

It has been suggested that otosclerosis might be caused by a generalized disorder in the connective tissue.

The biophysical and biochemical properties of skin biopsies from twelve patients with otosclerosis and twelve age- and sex-matched controls were investigated. No differences were found in skin strength and extensibility, skin thickness, collagen content, the relationship between collagen type I and type III, reducible collagen cross-links and molecular stability of collagen type I of samples from patients with otosclerosis as compared with those from the controls. The present study does not support the suggestion that otosclerosis might be a generalized connective tissue disorder.

Studies of Otic Capsule Morphology and Gene Expression in the Mov13 Mouse – An Animal Model of Type I Osteogenesis Imperfecta

Type I osteogenesis imperfecta (OI) is a disorder of skeletal bones characterized by bone fragility and blue sclera, which can result from mutations in genes encoding for type I collagen--the COL1A1 and COL1A2 genes. Fifty percent of patients with type I OI develop hearing loss and associated histopathological changes in the otic capsule that are indistinguishable from otosclerosis, a major cause of acquired hearing loss. In an attempt to elucidate molecular and cellular mechanisms of hearing loss in type I OI, we have studied the Mov13 mouse, which has served as an animal model of type I OI by virtue of exhibiting variable transcriptional block of the COL1A1 gene. We studied the morphometry of the Mov13 otic capsule and compared expression levels of 60 genes in the otic capsule with those in the tibia and parietal bone of the Mov13 and wild-type mice. The degree of transcriptional block of the COL1A1 gene and its downstream effects differed significantly between the bones examined. We found that expression levels of bone morphogenetic protein 3 and nuclear factor kappa-B1 best distinguished Mov13 otic capsule from wild-type otic capsule, and that osteoprotegerin, caspase recruitment domain containing protein 1, and partitioning defective protein 3 best distinguished Mov13 otic capsule from Mov13 tibia and parietal bone. Although the Mov13 mouse did not demonstrate evidence of active abnormal otic capsule remodeling as seen in type I OI and otosclerosis, studying gene expression in the Mov13 mouse has provided evidence that osteocytes of the otic capsule differ from osteocytes in other bones.

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.

Is it necessary to screen for hearing loss in the paediatric population with osteogenesis imperfecta?

The aim of the study was to assess the necessity of a screening service to detect early hearing loss in the paediatric population with osteogenesis imperfecta. Twenty-two children were assessed over a 5-year period. Five children (22.7%) had normal hearing. Fourteen (63.6%) had conductive hearing loss, with 12 children in this group having otitis media with effusion (OME); all had resolution of hearing loss with appropriate therapy. Two children had persistent conductive losses unrelated to OME. Three children (13.6%) had sensorineural hearing loss, with one being detected at the age of 1 year. Existing evidence suggests that hearing loss associated with osteogenesis imperfecta has its onset in the second to third decade of life. Contrary to this, hearing loss was detected in 77.3% (17) of this population with a median and mean age of 9 years. This study would suggest that routine screening is worthwhile in children with osteogenesis imperfecta.

Hearing loss in Finnish adults with osteogenesis imperfecta: a nationwide survey

Hearing loss, bone fragility, and blue sclerae are the principal clinical features in osteogenesis imperfecta (OI), a genetic disorder of connective tissue. In a nationwide search, an audiometric evaluation of 133 adult patients was performed. According to the criteria introduced by Sillence, type I was the most common form of OI. Of the patients with normal hearing on audiometry, 17.1% reported subjective hearing loss, and 19.1% of the patients with impaired hearing did not recognize it. On audiometry, 57.9% of the patients had hearing loss, which was progressive, often of mixed type, and mostly bilateral, and began in the second to fourth decades of life. The frequency or severity of the hearing loss was not correlated with any other clinical features of OI. Hearing loss is common, affecting patients with all types of OI. Subjective misjudgment of hearing ability supports the need for repeated audiometry in all OI patients. A baseline study at the age of OI years followed by audiograms every third year thereafter is recommended.

Results of stapes surgery on patients with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous group of connective tissue disorders. The classic triad of blue sclerae, spontaneous fractures and hearing loss is known as the Van der Hoeve and De Kleyn syndrome. Between 1989 and 2000, six patients with OI presented with conductive hearing loss. Five of them proceeded to stapedotomy. All the patients who had surgery had significant hearing gain. None of the patients had any complications. This study presents a higher incidence of spontaneous fractured crura as the cause of the conductive hearing loss than previously reported, and that the presence of a fractured crura with mobile footplate can be anticipated by the presence of a large conductive hearing loss. The pre- and postoperative results are presented and support the view that stapes surgery in OI can have encouraging results, provided the operator anticipates the possibility of a fractured crura and a mobile footplate.

A clinical and audiological investigation of osteogenesis imperfecta

Fifty-six patients with osteogenesis imperfecta underwent clinical and audiological assessment. They completed questionnaires regarding their physical and otological disabilities, and attended various centres for audiological testing. It was found that 31 patients had a hearing loss. Hearing loss began in the second and third decades as a conductive loss, and progressed to a mixed loss. Eight patients, found in all age groups, had a pure sensorineural loss in one or both ears. Patients with osteogenesis imperfecta congenita suffered more fractures, became more physically disabled and had more severe hearing loss than those with osteogenesis imperfecta tarda. Tympanometry was found to give unpredictable results and was unhelpful in assessing middle ear function in these patients.

Pathology of otosclerosis: a review

Otosclerosis is a bone disorder of unknown etiology confined to the otic capsule. Failure of remodeling of newly formed vascular, woven bone (otospongiosis) results in sclerotic bone (otosclerosis) with abnormal osteons. Involvement of the oval window causes conductive hearing loss. Electron microscopic, histochemical, and biochemical studies identify normal cellular and matrix components of otosclerotic bone without providing clues to the abnormal bone formation and resorption. Plastic-embedded, nondecalcified histologic sections with in vivo tetracycline labels permit the study of mineralization rates to separate this disorder from other bone dyscrasias that have similar histopathologic appearances. Characterization of the cells, matrix, and their mediators can yield an understanding of abnormalities that disorder bone.