Учебники / Genetic Hearing Loss Willems 2004

174	Lalwani and Mhatre

antibody, is characterized by the presence of type III and type IV fibrocytes. The type III fibrocytes are rich in actin and other contractile/contractionassociated proteins (21,22). The postulated role for these cells includes providing anchorage for surrounding cells and/or developing or reacting to tension generated in the basilar membrane–spiral ligament complex. Maintenance of this tension is likely more crucial in the high-frequency- sensitive basal turn of the cochlea where the basilar membrane is wider relative to the apex. This may be the simple explanation for the observed high-frequency hearing impairment in DFNA17.

VI. HEREDITARY MACROTHROMBOCYTOPENIAS

Hereditary macrothrombocytopenias (MTCPs) are a family of related disorders inherited in an autosomal dominant manner, characterized by large platelets with or without leukocyte inclusion bodies, and a reduction in total number of platelets. These syndromes include May-Hegglin anomaly (MHA), Sebastian syndrome (SBS), Fechtner syndrome (FTNS), Epstein syndrome (EPS), and Alport syndrome with platelet defects (APSM) (Table 1). While sharing the presence of MTCP, each of these syndromes is distinguished from the others by the presence or absence of nephritis, cataracts, and deafness. Molecular genetic analysis has recently identified mutations in MYH9 as the pathogenic cause of each of the five inherited MTCPs (Table 2) (4,5,23). Single base pair missense or nonsense mutations in both the motor and rod domain have been linked to MCTP and DFNA17. Mutations in the head domain are proposed to a ect the motor function while the rod domain mutations are thought to a ect the assembly of the

Table 1 Clinical Features Associated with the Autosomal Dominant MTCP

Disorder		MTCP	LI	HI	Nephritis	Cataracts
	a	+	+
MHA
SBS		+	+
Brodiea		+		+
FTNS		+	+	+	+	+
EPS		+		+	+	+
APSM		+		+	+	+

MHA, May-Hegglin anomaly; SBS, Sebastian syndrome; FTNS, Fechtner syndrome; APSM, Alport syndrome with platelet defect; EPS, Epstein syndrome; MTCP, macrothrombocytopenia; LI, leukocyte inclusions; HI, hearing impairment.

a In SBS and FTNS, the inclusion are smaller and less organized in comparison to MHA.

MYH9			175
Table 2 Reported Amino Acid Changes in MYH9 and the Associated
Phenotype
AA change	Nt change	Exon	Diagnosis
N93K	C279G	1	MHA
A95T	G283A	1
S96L	C287T	1	Fe/Ep, Ep
K371N		10	MHA/SBS
R702C	C2104T	16	FTNS; EPS; APSM
R705H	G2111A	16	DFNA17
S1114P		25	APSM
T1155I		25	MHA
R1165C		26	MHA
R1165L	G3494T	26
R1400W		30
D1424H	G4270C	30	FTNS, MHA
D1424N	G4270A	30	FTNS, SBA, MHA
D1424Y	G4270T	30	MHA
E1841K		38	FTNS, MHA, MHA/SBS
R1933X	C5797T	40	MHA, MHA/SBS, FTNS
	5779delC	40

AA, amino acid; Nt, nucleotide; MHA, May-Hegglin anomaly; SBS, Sebastian syndrome; FTNS, Fechtner syndrome; APSM, Alport syndrome with platelet defect; EPS, Epstein syndrome.

myosin complex. The clinical symptoms associated with MYH9 mutations serve to identify the critical biological role of MYH9 in the a ected target organs. However, the precise mechanism of pathogenesis in the a ected target organs remains to be determined. The finding of mutations at the same location causing di erent phenotypes suggests that the genetic background may be influential in determining the clinical severity associated with a particular MYH9 mutation.

VII. SUMMARY

Mutations in MYH9 are associated with syndromic and nonsyndromic deafness. The mechanism by which MYH9 dysfunction leads to hearing loss and the observed inner ear histopathology, cochleosaccular dysplasia (CSD), in the DFNA17 proband in some and hereditary MCTPs in others remains to be determined. Future development of animal models with loss of function of Myh9 will facilitate a direct and e ective means of under-

176	Lalwani and Mhatre

standing the role of MYH9 and its mutant allele in hearing and its dysfunction.

ACKNOWLEDGMENTS

This study was supported in part by grants from the National Institute on Deafness and Other Communication Disorders, National Institute of Health (K08 DC 00112 to AKL); American Hearing Research Foundation; National Organization for Hearing Research; Deafness Research Foundation; and Hearing Research, Inc.

REFERENCES

1.Lalwani AK, Linthicum FH, Wilcox ER, et al. A five-generation family with late-onset progressive hereditary hearing impairment due to cochleosaccular degeneration. Audiol Neuro-Otol 1997; 2:139–154.

2.Lalwani AK, Luxford WM, Mhatre AN, et al. A new locus for nonsyndromic hereditary hearing impairment, DFNA17, maps to chromosome 22 and represents a gene for cochleosaccular degeneration [letter]. Am J Hum Genet 1999; 64:318–323.

3.Lalwani AK, Goldstein JA, Kelly MJ, et al. Human Nonsyndromic Deafness DFNA17 is due to a mutation in nonmuscle myosin MYH9. Am J Hum Genet 2000; 67:1121–1128.

4.Kelley MJ, Jawien W, Ortel TL, Korczak JF. Mutation of MYH9, encoding non-muscle myosin heavy chain A, in May-Hegglin anomaly. Nat Genet 2000; 26:106–108.

5.Seri M, Cusano R, Gangarossa S, et al. Mutations in MYH9 result in the MayHegglin anomaly, and Fechtner and Sebastian syndromes. The May-Hegglin/ Fechtner Syndrome Consortium. Nat Genet 2000; 26:103–105.

6.Dunham I, Shimizu N, Roe BA, et al. The DNA sequence of human chromosome 22 [see comments] [published erratum appears in Nature 2000 Apr 20;404(6780):904]. Nature 1999; 402:489–495.

7.Friedman TB, Sellers JR, Avraham KB. Unconventional myosins and the genetics of hearing loss. Am J Med Genet 1999; 89:147–157.

8.Sellers JR. Myosins: a diverse superfamily. Biochim Biophys Acta 2000; 1496: 3–22.

9.Saez CG, Myers JC, Shows TB, Leinwand LA. Human nonmuscle myosin heavy chain mRNA: generation of diversity through alternative polyadenylylation. Proc Natl Acad Sci USA 1990; 87:1164–1168.

10.Simons M, Wang M, McBride OW, et al. Human nonmuscle myosin heavy chains are encoded by two genes located on di erent chromosomes. Circ Res 1991; 69:530–539.

MYH9

177

11.Maupin P, Phillips CL, Adelstein RS, Pollard TD. Di erential localization of myosin-II isozymes in human cultured cells and blood cells. J Cell Sci 1994; 107:3077–3090.

12.Phillips CL, Yamakawa K, Adelstein RS. Cloning of the cDNA encoding human nonmuscle myosin heavy chain-B and analysis of human tissues with isoform-specific antibodies. J Muscle Res Cell Motil 1995; 16:379–389.

13.Kolega J. Cytoplasmic dynamics of myosin IIA and IIB: spatial ‘‘sorting’’ of isoforms in locomoting cells. J Cell Sci 1998; 111:2085–2095.

14.Kolega J. Fluorescent analogues of myosin II for tracking the behavior of di erent myosin isoforms in living cells. J Cell Biochem 1998; 68:389–401.

15.Kelley CA, Sellers JR, Gard DL, et al. Xenopus nonmuscle myosin heavy chain isoforms have di erent subcellular localizations and enzymatic activities [published erratum appears in J Cell Biol 1997 Jul 14;138(1):215]. J Cell Biol 1996; 134:675–687.

16.Gulick AM, Rayment I. Structural studies on myosin II: communication between distant protein domains. Bioessays 1997; 19:561–569.

17.Rayment I. The structural basis of the myosin ATPase activity [see comments]. J Biol Chem 1996; 271:15850–15853.

18.Houdusse A, Kalabokis VN, Himmel D, Szent-Gyorgyi AG, Cohen C. Atomic structure of scallop myosin subfragment S1 complexed with MgADP: a novel conformation of the myosin head. Cell 1999; 97:459–470.

19.Patterson B, Ruppel KM, Wu Y, Spudich JA. Cold-sensitive mutants G680V and G691C of Dictyostelium myosin II confer dramatically di erent biochemical defects. J Biol Chem 1997; 272:27612–27617.

20.Suzuki J, Aikawa M, Isobe M, et al. Altered expression of smooth muscle and nonmuscle myosin heavy chain isoforms in rejected hearts: a sensitive marker for acute rejection and graft coronary arteriosclerosis. Transplant Proc 1995; 27:578.

21.Henson MM, Henson OW Jr, Jenkins DB. The attachment of the spiral ligament to the cochlear wall: anchoring cells and the creation of tension. Hearing Res 1984; 16:231–242.

22.Henson MM, Burridge K, Fitzpatrick D, et al. Immunocytochemical localization of contractile and contraction associated proteins in the spiral ligament of the cochlea. Hearing Res 1985; 20:207–214.

23.Heath KE, Campos-Barros A, Toren A, et al. Nonmuscle myosin heavy chain IIA mutations define a spectrum of autosomal dominant macrothrombocytopenias: May-Hegglin anomaly and Fechtner, Sebastian, Epstein, and Alportlike syndromes. Am J Hum Genet 2001; 69:1033–1045.

12

Mitochondrial Hearing Loss

Nathan Fischel-Ghodsian

Cedars-Sinai Medical Center and David Geffen School of Medicine at UCLA, Los Angeles, California, U.S.A.

I.INTRODUCTION

The last decade has led to the identification of several mitochondrial DNA mutations associated with hearing loss. Since the only known function of the human mitochondrial chromosome is to participate in the production of chemical energy through oxidative phosphorylation, it was not unexpected that mitochondrial mutations interfering with energy production could cause systemic neuromuscular disorders, which have as one of their features hearing impairment. Surprisingly, however, inherited mitochondrial mutations have also been found to be a cause of nonsyndromic hearing loss, and predispose to aminoglycoside-induced hearing loss, while acquired mitochondrial mutations have been proposed as one of the causes of presbycusis. This chapter will first give a short background review of mitochondrial genetics, outline the di erent mitochondrial mutations associated with hearing loss, and discuss the clinical relevance of diagnosing these mutations. The latter part of the chapter will concentrate on the fact that the clinical expression of these mitochondrial mutations is dependent on environmental exposures and nuclear-encoded modifier genes. Preventive and therapeutic strategies will depend on identification and avoidance of the environmental exposures, and the identification of the nuclear-encoded modifier genes. Experimental approaches to identify these modifier genes will be presented.

179

180	Fischel-Ghodsian

II.NORMAL MITOCHONDRIAL GENETICS

There are hundreds of mitochondria in each cell, which help in the protection against reactive oxygen species, are intimately involved in the regulation of cell death, and serve a variety of metabolic functions, the most important being the synthesis of ATP by oxidative phosphorylation. Each mitochondrion contains in its matrix 2–10 mitochondrial chromosomes. Each of these mitochondrial DNA molecules in humans is 16,569 bp long, double-stranded, forms a closed circle, and replicates and is transcribed within the mitochondrion in ways reminiscent of its bacterial origin. The mitochondrial DNA molecule encodes 13 mRNA genes, as well as two rRNAs and 22 tRNAs, which are required for assembling a functional mitochondrial protein-synthesizing system. The 13 mRNAs are translated on mitochondrion-specific ribosomes, using a mitochondrion-specific genetic code, into 13 proteins. These proteins interact with approximately 60 nuclear encoded proteins to form the five enzyme complexes required for oxidative phosphorylation. These complexes are bound to the mitochondrial inner membrane, and are involved in electron transport and ATP synthesis (reviewed in Ref. 1).

Mitochondrial DNA is transmitted exclusively through mothers. This leads to the expectation that a defect in a mitochondrial gene should lead to disease equally in both sexes, but can only be transmitted through the maternal line. Normally, most healthy individuals appear to have only a single mitochondrial DNA genotype, i.e., are homoplasmic, but in many mitochondrial disease states the mitochondrial DNA population is mixed, i.e., heteroplasmic (2). The amount of heteroplasmy varies from tissue to tissue, and for cells within a tissue, and the severity of the symptoms does not always correlate well with the proportion of mutant mitochondrial chromosomes. While for most of the multisystemic mitochondrial syndromes the homoplasmic state would presumably be lethal, homoplasmy of mutant mitochondrial DNA is observed for two tissue-specific diseases, the ocular disorder Leber’s hereditary optic neuroretinopathy (3) and maternally inherited hearing loss.

III.HEARING IMPAIRMENT DUE TO MITOCHONDRIAL MUTATIONS

Hearing loss can be due to both inherited and acquired, heteroplasmic and homoplasmic, mitochondrial mutations. These data have recently been reviewed (4), and are summarized with the inclusion of the most recent data in Table 1.

Mitochondrial Hearing Loss					181
Table 1 Mitochondrial Mutations and Hearing Impairment
Hearing impairment	Mutations identified	Inherited	Acquired	Homoplasmy	Heteroplasmy
Syndromic
Syst. neuromuscular	Deletions, A3243G, . . .	Rare	Usually	No	Yes
Diabetes + deafness	A3243G-tRNAleu(UUR)	Yes	Possible	No	Yes
	Deletion/rearrangement	Yes	Not observed	No	Yes
	A8296G-tRNAlys	Yes	Not known	No	Yes
	T14709C in the tRNAglu	Yes	Not observed	No	Yes
PPK + deafness	A7445G—noncoding	Yes	Not observed	Yes	Minimal
Nonsyndromica	A1555G-12S rRNA	Yes	Not observed	Yes	Minimal
	A7445G—noncoding	Yes	Not observed	Yes	Minimal
	Cins7472-tRNAser(UCN)	Yes	Not observed	Nearly	Yes
	T7511C-tRNAser(UCN)	Yes	Not observed	Nearly	Yes
Ototoxic	A1555G-12S rRNA	Yes	Not observed	Yes	No
	DT961Cn	Yes	Possible	Yes	‘‘Multiple’’
Presbycusis	‘‘Random’’	Not known	Yes	No	Yes

aA pathogenic role has been proposed, but not been established, for the T7510C and G7444A sequence changes (see text).

A.Mitochondrial Mutations and Syndromic Hearing Loss

Systemic neuromuscular syndromes such as Kearns-Sayre syndrome, mitochondrialencephalomyopathy, lactic acidosis, and strokelike episodes (MELAS), and mitochondrial encephalomyopathy with ragged red fibers (MERRF), have hearing loss frequently as one of their clinical signs (5–7). In these cases the heteroplasmic mutation can be found generally at highest levels in nerves and muscle. Because of the higher energy requirements of muscle and nervous tissue, and the fact that small numbers of dysfunctional muscle and nerve cells can interrupt the function of many neighboring normal cells, mitochondrial DNA mutations in those tissues are thought to be particularly harmful. It is not unexpected that generalized neuronal dysfunction is also expressed in the auditory system.

In 1992 several families with diabetes mellitus and sensorineural hearing loss were described, and surprisingly were found to have inherited the heteroplasmic A3243G mutation in the gene for tRNAleu(UUR) the very same mutation associated with the systemic MELAS syndrome (8,9). In none of these cases were other neurological symptoms present. One family had instead of the 3243 mutation a heteroplasmic large deletion/insertion event (10), and more recently the heteroplasmic point mutations T14709C in the tRNAglu gene and A8296G in the tRNAlys gene were also found to be associated with maternally inherited diabetes and deafness (11,12). This association between diabetes mellitus, hearing loss, and mitochondrial mutations has been confirmed in population studies of diabetic patients (13–20). Kadowaki et al., for example, found the 3243 mutation in 2–6% of diabetic

182	Fischel-Ghodsian

patients in Japan, and in three of five patients with diabetes and deafness. Twenty-seven of their 44 patients with diabetes and the 3243 mutation had hearing loss (15). The hearing loss is sensorineural and usually develops only after the onset of diabetes. In the non-Japanese populations examined, the 3243 mutation accounts for less than 1% of the diabetic patients (17–19). More recently, the phenotype of diabetes in those patients was shown to be of the nonobese type, with relatively young age of onset in most patients, a low frequency of diabetic proliferative retinopathy, and a high frequency of macular pattern dystrophy, diabetic nephropathy, and associated neuromuscular conditions (20–23). In most of these cases the heteroplasmic A3243G mutation was found (20,21), while in the other cases a heteroplasmic large deletion was identified (22,23). In addition, diabetes mellitus, diabetes insipidus, optic atrophy, and deafness have been well described as the Wolfram syndrome, a usually autosomal recessive condition (24), but which may also occur as a consequence of mitochondrial deletions (25,26).

Some of the mutations described below in the nonsyndromic section have been found associated occasionally with other symptoms. Most prominently, the A7445G in the tRNAser gene mutation was initially described as a nonsyndromic deafness mutation, but was subsequently found to be also associated with the skin condition palmoplantar keratoderma (PPK) in at least some of the cases (27–29). Another mutation in the tRNAser gene has also been found in one family to be associated with ataxia and myoclonus (30). The most common nonsyndromic mutation, the A1555G mutation in the 12S rRNA gene, has been described in one family with Parkinson’s disease, in another with a constellation of spinal and pigmentary disturbances, and in one case of a woman with a restrictive cardiomyopathy (31– 33). It remains, however, not unlikely that these associations occurred by chance and are not causally related. Similarly, the homoplasmic mitochondrial sequence change A5568G in the tRNAtrp gene was proposed as pathogenic for a family with hearing loss and hypopigmentation, but the evidence for pathogenicity is speculative at this time (34).

B.Mitochondrial Mutations and Nonsyndromic Hearing Loss

The first mutation associated with nonsyndromic deafness was identified in an Arab-Israeli pedigree, when the striking pattern of transmission only through mothers was noted (35,36). Most of the deaf family members had onset of severe to profound sensorineural hearing loss during infancy, but a minority of family members had onset during childhood or even adulthood (37). The homoplasmic A1555G mutation in the mitochondrial 12S ribosomal RNA gene was identified as the pathogenic mutation (36). Initially, in

Mitochondrial Hearing Loss

183

all additional pedigrees and individual patients with the same A1555G mutation the hearing loss occurred only after aminoglycoside exposure, as described below (37a,38–42). However, subsequently a significant number of pedigrees were described in Spain, with family members who went deaf with and without aminoglycosides (43,44). It is interesting that the age of onset of hearing loss in the Spanish families was rarely congenital, which is di erent from the Arab-Israeli pedigree. In particular the study by Estivill et al. is remarkable for two reasons, both of which indicate a higher than previously expected frequency of this mutation. First, it describes 19 families with the A1555G mutation out of a total of 70 families with sensorineural hearing loss collected. Even if the selection of families led to a bias toward families with multiple a ected individuals, and even when only the individuals without aminoglycoside exposure are considered, this represents an unexpectedly high frequency of familial sensorineural hearing loss due to the A1555G mutation. Second, the fact that the mutation was identified on di erent haplotypes, a finding supported by the study of Torroni et al., indicates that it is likely that this mutation exists in other populations as well, and may not be rare (44,45). Similarly, in Mongolia the mutation appears to be common, although it is not clear to what extent this is a selection bias due to aminoglycoside exposure (46). However, unpublished results from screening of hearing-impaired populations in other parts of the world seem so far to indicate a very low frequency of the A1555G mutation. Despite that, most recently individual families with the A1555G and nonsyndromic, nonototoxic hearing loss have been described in di erent places in the world (47–50).

Another close to homoplasmic inherited mutation leading to hearing loss is the A7445G mutation. It was first described in a family from Scotland, and confirmed and established in two unrelated pedigrees from New Zealand and Japan (27–29). In the New Zealand and Japanese pedigrees, a mild form of the skin condition palmoplantar keratoderma also segregates in the maternal line (29). Interestingly, the penetrance of this mutation for hearing loss in the Scottish pedigree is quite low, while in the New Zealand and Japanese pedigrees it is very high. Thus, in similarity to the Arab-Israeli pedigree, the mitochondrial mutation by itself does not appear to be su cient to cause hearing loss, but requires additional genetic or environmental factors, which seem to be rare in the Scottish pedigree and common in the New Zealand and Japanese pedigrees. The di erence in penetrance in this situation appears to be due to a di erence in mitochondrial haplotype. In the New Zealand pedigree complete sequencing of the mitochondrial DNA revealed three additional sequence changes in complex I protein genes, two of which have also been labeled as secondary Leber’s hereditary optic neuroretinopathy mutations (28). Since these or similar sequence changes