Учебники / Genetic Hearing Loss Willems 2004

are not present in the Scottish pedigree (51), mitochondrial haplotype appears to account for the di erences in penetrance in this case.

A third mitochondrial mutation, a cytosine insertion at position 7472 in the tRNAser(UCN) gene, was identified in one large Dutch family (52). The same mutation had been previously described in a Sicilian family with hearing loss, some of the family members also having other neurological symptoms, such as ataxia and myoclonus (30). In the Dutch family, the hearing loss is sensorineural progressive with onset in early adulthood. Most of the individuals over 30 years of age were deaf, indicating that the penetrance in this family is high. The mutation is heteroplasmic, although most individuals have over 90% of abnormal mitochondrial chromosomes in the tissues examined.

More recently, a large African American pedigree with maternal inheritance and nonsyndromic hearing loss has been identified (53), and shown to have a close to homoplasmic T7511C mutation in the tRNAser(UCN) gene (54). The same mutation was subsequently found in a Japanese and two French families (49,55). Also, a British family with a T7510C mutation and nonsyndromic deafness was described, although the mitochondrial chromosome was not fully evaluated (56). Finally, the G7444A substitution has been described in deaf individuals with and without the A1555G mutation, but its pathogenicity has not been established (46).

C.Mitochondrial Mutations and Ototoxic Hearing Loss

Aminoglycoside ototoxicity is one of the most common causes of acquired deafness. Although vestibulocochlear damage is nearly universal when high drug levels are present for prolonged periods, at lower drug levels there appears to be a significant genetic component influencing susceptibility to aminoglycoside ototoxicity. The existence of families with multiple individuals with ototoxic deafness induced by aminoglycoside exposure was noticed early on. Konigsmark and Gorlin in 1976 summarized most existing descriptions of familial aminoglycoside ototoxicity and concluded that inheritance of the predisposition is probably autosomal dominant with incomplete penetrance. However, they also noted that no male-to-male transmission has been seen, and concluded that the inheritance could be multifactorial (57). In 1989 Higashi reviewed the literature, and concluded that the most likely explanation for the maternal inheritance observed is a mitochondrial DNA defect (58). In 1991, Hu et al. in China described another 36 families with maternally transmitted predisposition to aminoglycoside ototoxicity, and concluded also that a mitochondrial defect may be responsible for this predisposition (59). Additional evidence for a genetic basis for aminoglycoside susceptibility comes from animal studies. Maca-

que monkeys are resistant to dihydrostreptomycin, while patas monkeys are highly sensitive to that drug (60).

We analyzed three Chinese families in which several individuals developed deafness after the use of aminoglycosides, and identified the A1555G mutation in the 12S ribosomal RNA gene in all three of them, but not in hundreds of controls (36). In addition, a small proportion of ‘‘sporadic’’ Chinese patients, without a positive family history for aminoglycoside ototoxicity, exhibit this particular mutation (37a). These findings were confirmed in two Japanese families and additional Chinese sporadic cases (38). Subsequently, the same mutation was found in families and sporadic patients with aminoglycoside ototoxicity from Zaire the United States, Mongolia, Spain, South Africa, and Israel (39–44,61). Interestingly, in one streptomycininduced deaf individual with a strong familial history of aminoglyco- side-induced hearing loss and the mitochondrial 1555 mutation, detailed vestibular examination revealed severe hearing loss with completely normal vestibular function (37). This unexpected relative sparing of the vestibular system has recently been confirmed in Japanese patients (62).

Since the A1555G mutation in the mitochondrial 12S rRNA gene accounts only for 17–33% of patients with aminoglycoside ototoxicity, it is possible that other susceptibility mutations can be found in the same gene. DNA from 35 Chinese sporadic patients with aminoglycoside ototoxicity and without the A1555G mutation was analyzed for sequence variations in the 12S rRNA gene. Three sequence changes were found; only one of them, an absence of a thymidine at position 961 with varying numbers of cytosines inserted (DT961Cn), appeared likely to be a pathogenic mutation (63). Analysis of 34 similar U.S. patients of varying ethnic backgrounds did not reveal this mutation, but most recently we identified an Italian family with five maternally related members who all became deaf after aminoglycoside treatment, and were found to have the DT961Cn mutation (64). This sequence change was not found in 799 control individuals (63).

D.Mitochondrial Mutations and Presbycusis

Another condition associated with acquired heteroplasmic mutations and hearing loss is presbycusis. Presbycusis is the hearing loss that occurs with age in a significant proportion of individuals. Since mitochondrial DNA mutations, and the resulting loss of oxidative phosphorylation activity, seem to play an important role in the aging process (reviewed in Ref. 65), it is not unlikely that mitochondrial mutations in the auditory system can also lead to presbycusis. We recently examined the spiral ganglion and membranous labyrinth from archival temporal bones of five patients with presbycusis for mutations within the mitochondrially encoded cytochrome oxidase II gene

hearing loss should probably be screened at least for the presence of the mitochondrial 1555 and 961 mutations, since presence of a mutation will allow counseling to all maternally related relatives to avoid aminoglycosides. Similarly, the description by Estivill et al. (44) indicates that it might not be unreasonable to screen every individual with nonsyndromic hearing loss for the mutation, unless maternal inheritance can clearly be excluded. Since the test is easily done, and since prevention of hearing loss in maternal relatives can easily be accomplished, this may be cost-e ective medical practice. The avoidable hearing loss in at least 40 patients in the report by Estivill et al. is a case in point.

With the exception of aminoglycosides and mitochondrial mutations in the 12S rRNA gene, there are no proven preventive or therapeutic interventions for mitochondrially related hearing impairments. The diagnosis of such defects is, however, useful for genetic counseling and is indicated in all families with an inheritance pattern of hearing loss consistent with maternal transmission, and possibly in all patients who have both diabetes mellitus and adult-onset hearing loss.

V.PATHOPHYSIOLOGY OF MITOCHONDRIAL DEAFNESS MUTATIONS

For aminoglycoside ototoxicity due to the A1555G mutation, it is interesting that this mutation lies exactly in the region of the gene for which the resistance mutations in yeast and Tetrahymena have been described, and in which aminoglycoside binding has been documented in bacteria (68–70). In addition, the mutation makes the mitochondrial RNA gene in this region more similar to the bacterial ribosomal RNA gene (36). Since aminoglycosides are concentrated within cochlear cells, and remain there for prolonged periods (71), we proposed that susceptible individuals with the 1555 mutation have increased binding to aminoglycosides leading to altered protein synthesis in the mitochondria (36), the tissue specificity being due to the concentration of the drug in those cells. Subsequent binding experiments have proven that increased binding to the mitochondrial 12S ribosomal RNA occurs (72). However, when examining lymphoblastoid cell lines of individuals with the 1555 mutation, exposure of the cell lines to high concentrations of neomycin or paromomycin led to a decreased rate of growth in glucose medium, but no mutant proteins were detected (73). Similar results of decreased protein synthesis but no mutant proteins were obtained in Japan using mitochondrial transfer from human skin fibroblast line with the 1555 mutation U0 HeLa cells exposed to very high levels of streptomycin (74). This may indicate that the

e ect of aminoglycosides in these cell lines could be nonspecific and be di erent than in the cochlea, perhaps because of di erent transport of the antibiotic into the mitochondria.

For nonsyndromic hearing loss, at first glance it is possible to speculate that mitochondrial mutations interfere with energy production, that the cochlea is highly dependent on su cient energy production, and that insu cient energy production leads to degeneration of cochlear cells. However, the cochlea is not the most energy-dependent tissue in the body, and in the systemic neuromuscular disorders listed in Sec. III.A, the extraocular muscles appear to be the most energy-sensitive cells, and hearing loss is certainly not the most prominent clinical sign. Thus, to understand the pathophysiological pathways leading from the mitochondrial mutations to hearing loss, two major biological questions need to be answered: Why does the same mutation cause severe hearing loss in some family members but not in others, and why is the ear the only organ a ected?

Study of the mitochondrial mutations leading to hearing loss has led to three possible precipitating factors modulating phenotypic expression, and it is likely that a combination of them play also a significant role in the phenotypic expression of acquired mitochondrial disorders. The first such factor involves environmental agents, and aminoglycosides are the prime example as a triggering event in the case of the 1555 mutation. It is not unlikely that other, as-yet-unrecognized, environmental factors could play similar, but perhaps less dramatic, roles. Diet and drugs a ecting oxygen radical formation and breakdown come to mind. The second factor involves the mitochondrial haplotype, and, as noted above, the 7445 mutation provides a dramatic example of that e ect. The third factor involves nuclear genes. The Arab-Israeli pedigree and some of the Spanish and Italian pedigrees are good examples of the role of nuclear genes. For example, the entire Arab-Israeli family lives in the similar environmental surroundings of a small Arab village in Israel, and all maternal relatives share the same mitochondrial haplotype.

Biochemical di erences between lymphoblastoid cell lines of hearing and deaf family members with the identical mitochondrial chromosomes provide direct support for the role of nuclear factors (73). An extensive ge- nome-wide search has led to the conclusion that this nuclear e ect is unlikely to be due to the e ect of a single nuclear locus but involves a number of modifier genes (75,76). The chromosomal location of one of these modifer genes has been identified, and linkage equilibrium has been obtained in families from varied ethnic backgrounds (76,77). Thus, the model that emerges for explaining penetrance is a threshold model, where a combination of environmental, mitochondrial, and nuclear factors can push a cell over a threshold, with dramatic clinical di erences on either side of this threshold.

enough families have been identified to attempt such e orts for a complex genetic disease, the recent finding of linkage and linkage disequilibrium to a chromosomal region on chromosome 8 is very encouraging (76,77). Analysis of candidate genes in that region, and additional linkage analyses using the chromosome 8 locus as a stratification parameter, will eventually lead to the identification of the chromosome 8 modifier gene and the identification of additional chromosomal loci.

(2)The candidate gene approach: The most likely candidates for being modifiers of the mitochondrial mutated gene are identified and screened for linkage disequilibrium in families and then for mutations. Two initial candidates, the ribosomal protein S12, which physically interacts with the 12S ribosomal RNA, and connexin 26, because of a suggestive linkage result, were screened in this manner and excluded as potential modifier genes (82). More recently we have started a concerted e ort to identify all genes involved in mitochondrial RNA processing and translation. The reasons have been summarized (83), but in short are as follows: (a) all the mitochondrial mutations associated with nonsyndromic hearing loss involve ribosomal or transfer RNA (see Table 1); (b) biochemical analysis of the e ect of these mutations demonstrates a RNA-processing defect or a decrease in translational e ciency (73,84–86); and (c) di erent processing of mitochondrial RNA and protein, leading to tissue-specific defects or functions, has been described (78–81). We have thus proposed that cochlea-specific subunits of mitochondrial ribosomes or RNA-processing proteins interact abnormally with the mitochondrial defect, leading to insu cient oxidative phosphorylation or loss of a secondary function in the cochlea. Similarly, allelic variations in some of these proteins may also be responsible for the penetrance di erences observed between patients.

(3)The mouse model approach: Recently the first mouse model of a naturally occurring pathogenic mitochondrial DNA mutation has been identified (87). In that model a mitochondrial DNA mutation in the tRNAArg gene worsens hearing impairment when combined with the nuclearencoded Ahl gene locus on mouse chromosome 10, which has been described as a major gene for age-related hearing loss in mice (87,88). This mouse model provides a ready experimental model to dissect the complex genetic factors and interactions. It is likely that the pathways in mice and human are similar, and thus the human homologs of all nuclear genes and/or pathways identified in mice will become candidates for testing in humans.

VII. SUMMARY

In conclusion, mitochondrial-related hearing loss can be caused by a variety of mutations, and can present in a variety of clinical forms with di erent

degrees of severity. These mutations are not uncommon and, because of the susceptibility of individuals with the 1555 and 961 mutations and their maternal relatives to aminoglycosides, are important to diagnose. Despite the fact that these mostly homoplasmic mitochondrial mutations represent the simplest model of a mitochondrial DNA disease, it remains unclear how mitochondrial mutations lead to the clinically crucial features of penetrance and tissue specificity: Within the same family some individuals with the mutation can have profound hearing loss while others have completely normal hearing, and only the hearing is a ected although all tissues have the mutation and are dependent on mitochondrial ATP production. Experimental approaches using genetic linkage studies in human families, candidate gene analysis, and spontaneous mouse models of mitochondrial hearing impairment may help in elucidating the pathophysiology between mitochondrial mutations and hearing loss, and eventually provide approaches to prevention and therapies.

ACKNOWLEDGMENTS

The author is supported by NIH/NIDCD grants RO1DC01402 and RO1DC04092.

REFERENCES

1.Attardi G, Schatz G. Biogenesis of mitochondria. Annu Rev Cell Biol 1988; 4:289–333.

2.Wallace DC. Diseases of the mitochondrial DNA. Annu Rev Biochem 1992; 61:1175–1212.

3.Howell N. Mitochondrial gene mutations and human diseases: a prolegomenon. Am J Hum Genet 1994; 55:219–224.

4.Fischel-Ghodsian N. Mitochondrial deafness mutations reviewed. Hum Mutat 1999; 13:261–270.

5.Schon EA, Bonilla E, DiMauro S. Mitochondrial DNA mutations and pathogenesis. J Bioenerg Biomembr 1997; 29:131–149.

6.Chomyn A. The myoclonic epilepsy and ragged-red fiber mutation provides new insights into human mitochondrial function and genetics. Am J Hum Genet 1998; 62:745–751.

7.Sue CM, Lipsett LJ, Crimmins DS, Tsang CS, Boyages SC, Presgrave CM, Gibson WP, Byrne E, Morris JG. Cochlear origin of hearing loss in MELAS syndrome. Ann Neurol 1998; 43:350–359.

8.Reardon W, Ross RJM, Sweeney MG, Luxon LM, Pembrey ME, Harding AE, Trembath RC. Diabetes mellitus associated with a pathogenic point mutation in mitochondrial DNA. Lancet 1992; 340:1376–1379.

9.van den Ouwel JMW, Lemkes HHPJ, Ruitenbeek W, Sandkuijil LA, De Vijlder MF, Struyvenberg PAA, van de Kamp JJP, Massen JA. Mutation in mitochondrial tRNALeu(UUR) gene in a large pedigree with maternally transmitted type II diabetes mellitus and deafness. Nat Genet 1992; 1:368–371.

10.Ballinger SW, Sho ner JM, Hedaya EV, Trounce I, Polak MA, Koontz DA, Wallace DC. Maternally transmitted diabetes and deafness associated with a 10.4 kb mitochondrial deletion. Nat Genet 1992; 1:11–15.

11.Vialettes BH, Paquis-Flucklinger V, Pelissier JF, Bendahan D, Narbonne H, Silvestre-Aillaud P, Montfort MF, Righini-Chossegros M, Pouget J, Cozzone PJ, Desnuelle C. Phenotypic expression of diabetes secondary to a T14709C mutation of mitochondrial DNA: comparison with MIDD syndrome (A3243G mutation): a case report. Diabetes Care 1997; 20:1731–1737.

12.Kameoka K, Isotani H, Tanaka K, Azukari K, Fujimura Y, Shiota Y, Sasaki E, Majima M, Furukawa K, Haginomori S, Kitaoka H, Ohsawa N. Novel mitochondrial DNA mutation in tRNA(Lys) (8296A– > G) associated with diabetes. Biochem Biophys Res Commun 1998; 245:523–527.

13.Oka Y, Katagiri H, Yazaki Y, Murase T, Kobayashi T. Mitochondrial gene mutation in islet-cell-antibody-positive patients who were initially non-insulin- dependent diabetics. Lancet 1993; 342:527–528.

14.Alcolado JC, Majid A, Brockington M, Sweeney MG, Morgan R, Rees, Harding AE, Barnett AH. Mitochondrial gene defects in patients with NIDDM. Diabetologia 1994; 37:372–376.

15.Kadowaki T, Kadowaki H, Mori Y, Tobe K, Sakuta R, Suzuki Y, Tanabe Y, Sakura H, Awata T, Goto Y-I, Hayakawa T, Matsuota K, Kawamori R, Kamada T, Horai S, Nonaka I, Hagura R, Akanuma Y, Yazaki Y. A subtype of diabetes mellitus associated with a mutation of mitochondrial DNA. N Engl J Med 1994; 330:962–968.

16.Katagiri H, Asano T, Ishihara H, Inukai K, Anai M, Yamanouchi T, Tsukuda K, Kikuchi M, Kitaoka H, Ohsawa N, Yazaki Y, Oka Y. Mito-

chondrial diabetes mellitus: prevalence and clinical characterization of diabetes due to mitochondrial tRNALeu(UUR) gene mutation in Japanese patients. Diabetologia 1994; 37:504–510.

17.Sepehrnia B, Prezant TR, Rotter JI, Pettitt DJ, Knowler WC, FischelGhodsian N. Screening for mtDNA diabetes mutations in Pima Indians with NIDDM. Am J Med Genet 1995; 56:198–202.

18.Newkirk JE, Taylor RW, Howell N, Bindo LA, Chinnery PF, Alberti KG, Turnbull DM, Walker M. Maternally inherited diabetes and deafness: prevalence in a hospital diabetic population. Diabetes Med 1997; 14:457–460.

19.Rigoli L, Di Benedetto A, Romano G, Corica F, Cucinotta D. Mitochondrial DNA [tRNA(Leu)(UUR)] in a southern Italian diabetic population [letter]. Diabetes Care 1997; 20:674–675.

20.Guillausseau PJ, Massin P, Dubois-LaForgue D, Timsit J, Virally M, Gin H, et al. Maternally inherited diabetes and deafness: a multicenter study. Ann Intern Med 2001; 134:721–728.

21.Harrison TJ, Boles RG, Johnson DR, LeBlond C, Wong LJ. Macular pattern