Учебники / Genetic Hearing Loss Willems 2004

A deletion of one nucleotide (207delC) was found in homogygosity in a Spanish family SDP26, resulting in a frameshift after amino acid I69 just after the transmembrane domain, addition of 18 novel amino acids, and premature termination of TMPRSS3 (30).

The D103G missense mutation was found in heterozygosity in a Greek pedigree K208. The other mutant allele in this family was 207delC. The D103G mutation a ects an Asp residue of the LDLRA domains that is well conserved. 3D modeling of the mutation suggested that this substitution impairs the Ca2+-binding site of the LDLRA domain (30).

From the large number of patients now screened for TMPRSS3 mutations, 28 polymorphisms or rare sequence variants were also identified and are shown in Table 2. These include the five amino acid substitutions: V53I, G111S, D173N, I253V, A426T (30). Two of the missense variants, D173N and A426T, were observed only once in heterozygosity in di erent patients (1/896 Caucasian chromosomes). By direct sequencing, no other changes were found in the patients heterozygous for D173N and A426T. These sequence variants cannot formally be ruled out as mutations without a functional assay. However, D173 is not well conserved between SRCR domains, within SRCR domains of TTSPs (Fig. 3f), and the mouse Tmprss3 has an N at this position. While A426 is relatively well conserved in TTSPs, it is not well conserved in serine protease domains in general and 3D modeling of the conservative substitution of A426 by threonine in TMPRSS3 does not have any obvious deleterious e ect (30).

Thus the analysis of TMPRSS3 to date indicates that the frequency of TMPRSS3 mutations in a European childhood deaf population is approximately 0.4% (4 in 1024 deaf alleles) after exclusion of the common 35delG GJB2 mutation. However, the estimate in the Pakistani and Indian Muslim population is approximately 3% (5/160), and 5% (2/39) in Tunisian families. In both these populations, GJB2 mutations were not excluded and thus TMPRSS3 mutations are still a significant cause of deafness. This is supported by the fact that the IVS4-6G>A (DF1/DFNB8) and C407R (PKSN37 and PKSN18b) mutations were found in 1 of 160 and 1 of 200 Muslim Indian control chromosomes, respectively (5,31).

VII. FUTURE INVESTIGATIONS

The following studies will enhance our understanding of the molecular pathophysiology of TMPRSS3-related deafness, and may allow the introduction of new therapeutic possibilities.

1.Cloning of the mouse homolog of TMPRSS3 and determining its temporal and spacial expression pattern, particularly in the ear.

The generation of a targeted disruption of Tmprss3 in mice will provide an outstanding animal model to study the molecular pathology of this particular recessive deafness.

2.Functional analysis of TMPRSS3 protein and its di erent domains. Identifying the substrates for the TMPRSS3, its interacting proteins, and the pathways of its involvement. Furthermore, determining the intracellular localization of the protein and its biosynthetic pathways. This knowledge may provide targets for therapeutic interventions.

3.Examining the involvement of di erent alleles of TMPRSS3 in hearing loss in older adults.

4.Determining the physiological role of TMPRSS3 protein outside the ear.

5.Evaluating the involvement of other TTSPs in hereditary deafness.

The study described above provides another example of the utility of results of exploration of the human genome in terms of DNA markers and nucleotide sequences that, when coupled with excellent clinical material and careful phenotyping, result in the determination of disease-related genes. Functional analysis of the predicted proteins adds to our understanding of complex processes such hearing.

ACKNOWLEDGMENTS

We are grateful to the patients and their family members for their participation in the described studies. We thank all clinicians who collected patients’ samples and performed clinical and audiological laboratory investigations. We also thank our collaborators N. Shimizu, J. Kudoh, B. Bonne-Tamir, A. Gal, and members of our laboratories particularly M.L. Guipponi and M. Wattenhofer. The laboratory of SEA is supported by grants 31.57149.99 from the Swiss FNRS, 98-3039 from the OFES/EU, the Foundation Child Care, and funds from the University and Cantonal Hospital of Geneva. The laboratory of HSS is supported by the National Health and Medical Research Council of Australia (project grant 215305 and fellowship 171601), by the Nossal Leadership award from the Walter and Eliza Hall Institute of Medical Research, and a grant from the Rebecca L. Cooper Foundation.

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ment in this group (1). The disease is characterized by isolated endochondral bone sclerosis of the labyrinthine capsule leading to hearing loss. Auditory impairment is heralded by the appearance of otosclerotic foci that invade the stapediovestibular joint (oval window) and interfere with free motion of the stapes (2). Mean age-of-onset is in the third decade, and 90% of a ected persons are under 50 years of age at the time of diagnosis (3,4). In approximately 10% of persons with clinically significant otosclerosis, a profound sensorineural hearing loss develops across all frequencies (3,5), reflecting either mechanical or toxic damage to the inner ear as otosclerotic foci invade the cochlear endosteum and encroach on the membranous labyrinth. While the sensorineural component of the hearing loss cannot be corrected, stapes microsurgery has proven to be a highly successful means to restore the normal conduction mechanism and can improve hearing thresholds by as much as 50 dB (5).

III.A POSSIBLE VIRAL ETIOLOGY?

Retroviral infections are believed to play a role in several bone diseases (6,7) and consistent with this hypothesis are several studies that suggest a viral etiology in the pathogenesis of otosclerosis. By immunofluorescence with polyclonal and monoclonal antibodies against mumps, rubella, and measles viruses, the presence of viral antigen, most commonly measles, has been found in otosclerotic foci (8). PCR amplification of the measles nucleocapsid gene also has been used to confirm the presence of retroviral RNA in temporal bone specimens from persons with otosclerosis but not in histologically negative controls. These results suggest that a viral insult, most likely measles, triggers the development of otosclerotic foci in susceptible individuals. Several investigators believe that after the middle ear mucosa becomes infected, viral particles invade the bone of the labyrinth via lymphatic or pericapillary spaces with otosclerosis developing as a consequence of the induced inflammation (9,10).

A causal role for measles in otosclerosis, however, has not been proven. Grayeli et al. (11) could not detect measles virus in any bone samples or primary bone cultures in 35 persons with otosclerosis, and the disparate ethnic-based epidemiological data for otosclerosis and measles suggest that the detection of measles in otosclerotic foci may represent a secondary, unrelated event. Measles is a highly infectious disease and a major cause of morbidity and mortality in all ethnic groups worldwide (12,13). In contrast, otosclerosis shows a racial bias. Histological evidence of otosclerosis is found in approximately 2.5% of whites (1) but in only 1% of blacks (14). Among

the Japanese, its prevalence is approximately one-half that in whites (15), and among South American Indians, its prevalence drops to only 0.04% (16).

IV. THE GENETICS OF OTOSCLEROSIS

The etiology of otosclerosis is unknown and its genetics is poorly understood. Reports of an inherited disease that probably represents otosclerosis date to the mid-nineteenth century when Toynbee described a familial pattern of conductive hearing loss (17). In his catalogue of 1837, he noted that thickening of the anterior two-thirds of the stapedial footplate resembled ivory and originated from the vestibular surface of the labyrinth. In 1876, Magnus documented a family in which the father and seven of 13 children had conductive hearing impairment, verified in one child to be due to ankylosis of the stapes (18). Eighteen years later, Politzer coined the term ‘‘otosclerosis,’’ in reference to a ‘‘disease that has its seat in the labyrinthine capsule [and] leads, through new formation and growth of osseous tissue, to ankylosis of the stapes in the fenestra ovalis’’ (19).

Although the genetics of otosclerosis is controversial, the majority of studies indicate autosomal dominant inheritance, a conclusion first reached by Albrecht (20). This observation was supported by Larsson’s analyses of 262 probands in which he also recognized that penetrance is incomplete (21,22). In a detailed study of 150 probands, Morrison calculated the fractions of a ected first-, second-, and third-degree relatives, and noted that the observed ratios were consistent with autosomal dominant inheritance with 40% penetrance (23). Other studies have confirmed these findings (24–26). Detailed mathematical calculations by Larsson (22) and Gapany-Gapana- vicius (27) suggest that other modes of transmission are unlikely, but Baurer and Stein have postulated digenic recessive inheritance based on a study of 94 families (24). Hernandez-Orozco and Courtney also favor digenic inheritance, but of a dominant X-linked gene and an autosomal recessive gene (25).

Lack of a positive family history in 40–50% of cases has added to the heritability controversy (28–30). These cases have been hypothesized by some to represent an autosomal recessive form of otosclerosis in which the heterozygote is identifiable only by histological examination of the temporal bones (31).

In the 1960s and 1970s it was often tried to explain complex genetic characteristics with monogenic concepts. As described higher, this has also been done for otosclerosis. However, little further understanding was gained from these studies. Nowadays, complex diseases are looked upon di erently, and analyzed without prior assumption on the mode of inheritance.

V.MONOGENIC AND COMPLEX DISEASES

The spectrum of human diseases forms a continuum between purely genetic and purely environmental conditions (Fig. 1). At one extreme are purely genetic conditions typified by the monogenic diseases showing autosomal dominant, recessive, or X-linked inheritance. Although complicating factors such as reduced penetrance can be present, a disease is not considered truly complex unless di erent genes or environmental factors also impact phenotype. At the other extreme are purely environmental diseases, including many types of infectious illnesses.

With respect to hearing, while most cases of congenital deafness have a single cause that is either monogenic (e.g., a GJB2 mutation) or environmental (prematurity, infections), the etiology in late-onset deafness, such as agerelated hearing impairment, otosclerosis, or Menie`re’s disease, is complex. In many of these cases, there is thought to be an interaction between several genes and di erent environmental triggers.

Large autosomal dominant families segregating otosclerosis are very rare—in the majority of cases there are only a few, if any, other a ected family members. While this presentation may reflect reduced penetrance, it is more consistent with the interaction of other complicating factors. Otosclerosis can therefore be considered a complex disease with rare monogenic autosomal dominant families. Much can be learned from the analysis of monogenic otosclerosis that may be applicable to otosclerosis as a complex

Figure 1 The spectrum of human diseases forms a continuum between purely genetic and purely environmental conditions.

Figure 2 Strategies used to study monogenic versus complex diseases.

disease, but in general, the analysis of these two types of otosclerosis will require di erent research strategies (Fig. 2).

A.Monogenic Forms of Otosclerosis

Genes responsible for monogenic forms of otosclerosis can be identified by a positional cloning strategy in large families segregating autosomal dominant otosclerosis (12 or more persons). Initially, a genome-wide scan is completed