Учебники / Genetics and Auditory Disorders Keats 2002

Among the 309 HxH matings, 11.7% were consanguineous. If only “genetic” cases were included (that is, those with a nongenetic etiology, such as rubella, were excluded from the analysis), the adjusted rate for recessive pedigrees would have been 13.2%. Chung and Morton (1959) subsequently analyzed Stevenson and Chessman’s data using maximum-likelihood methods and estimated that the proportions of sporadic, recessive and dominant cases were 0.22, 0.56 and 0.22, respectively. Using the theory of detrimental equivalents, they also estimated that genes at 36 ± 12 independent recessive loci contribute to the phenotype. Rose (1975; 1977) applied Morton’s methods to the 1,722 proband matings and 2,355 proband sibships in the Fay data set, together with 12,661 nuclear families collected as part of an Annual Survey of Hearing Impaired Children in 1968, and family histories on 482 students at Gallaudet University. The overall estimates of the proportions of genetic cases were 0.55 in the Fay data, 0.51 in the National Survey and 0.76 among the Gallaudet students. Recessive transmission accounted for 88%, 85% and 78% of the genetic cases respectively. Among the HxH sibships in the Fay data, the overall and adjusted rates of consanguinity were 7.1% and 15.4%. In the two large nationwide data sets, which were collected 100 years apart, the estimated proportion of genetic cases was remarkably consistent, even though the latter is known to have included a large cohort of patients with rubella deafness. The higher proportion of genetic cases, and cases showing apparent dominant transmission among the high-achieving Gallaudet student population, is of interest. Most of these students have at least one deaf parent, and at home as well as at Gallaudet ASL was probably the language of choice. Thus, as might be expected, a familiar language and culture facilitate academic excellence.

Fay’s data set included 1,299 in which both parents were deaf. Among these, the estimated proportion of non-complementary matings (that is, those that can produce only deaf offspring) was 0.042 ± 0.007, while the estimated proportion of complementary matings (that is, all offspring are hearing) was 0.875 ± 0.17. Non-complementary matings refer to those between individuals with the same type of genetic deafness, while complementary matings refer to those between individuals with different types of genetic deafness. Genetic analyses of other large data sets have been reported by Macklin et al. (1946), Sank (1969), Chung and Brown (1970), Furusho (1957), Mori (1959), Marazita et al. (1993) and Liu et al. (1994). The study by Liu et al. (1994) is particularly noteworthy because it involved a clinical survey of 126,876 individuals drawn by a stratified random sampling procedure from the 104 million citizens of Sichuan province in China in 1986 to 87. In the sample, 236 individuals were found to have a hearing loss of 90 dB or more. The overall prevalence of profound deafness was 0.82 per 1,000 and ranged from 0.7 per 1,000 in the predominant Han ethnic group 0.0 among 2,933 Tibetans, to 6.6 per 1,000 among 1,968 members of the Lisu minority group. The prevalence ranged from about 0.5 per 1,000 for subjects less than 30 years of age, to a high of 1.8 per 1,000 for those

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between 30 and 45 years of age. A segregation analysis that included 104 of the cases showed that 71% were genetic, while the remainder were sporadic, being determined largely by nongenetic causes associated with a negligible chance of recurrence within families. Among the genetic cases, 4.2% were associated with distinctive clinical features that permitted the diagnosis of a syndrome, while 89% were nonsyndromic-recessive and 6.8% nonsyndromic-dominant. The reproductive fitness of these cases was estimated to be 0.4 relative to hearing siblings.

The existence of etiologic heterogeneity with both genetic and environmental causes has been a consistent feature of studies of deafness. Among genetic cases, recessive transmission predominates, but the observed and adjusted frequencies of consanguinity are much higher than would be expected from the incidence of the phenotype, yielding estimates of the number of recessive loci that have ranged from 36 to 103, or higher (Chung and Brown 1970; Morton 1991). Data on marriages among the deaf show strong evidence for assortative mating in many but not all countries (e.g., India). When DxD matings have been studied, segregation analysis shows that relatively few couples are capable of producing only affected offspring, a finding that is inconsistent with the assumption that most cases of deafness are caused by recessive mutations at a single locus. Rose (1975; 1977) showed that the observed proportion of non-complementary matings was consistent with the assumption that recessive deafness is caused by equally frequent mutations at about 10 loci. The fact that higher estimates are obtained from consanguinity analysis indicates that the recessive phenotypes are not equally frequent, a conclusion that has been amply verified by recent discovery of the high proportion of recessive deafness that can be attributed to mutations in the connexin 26 gene.

3. Genetic Epidemiology of Deafness

The incidence of profound deafness in the United States is about 0.8 per 1,000 births (Bodurtha and Nance 1988). If lesser degrees (>30 dB) of loss, or unilateral or conductive losses are included, the incidence at birth or early infancy may be as high as 1.5–6 per 1,000. These rates can be influenced greatly by temporal or geographic variation in the frequency of recognized environmental causes including preor postnatal infections, such as rubella, cytomegalic inclusion body virus (CMV), otitis media, meningitis, prematurity, trauma, kernicterus, and exposure to ototoxic drugs. Despite the introduction of rubella immunization programs, the congenital rubella syndrome remains an important cause of deafness. Studies of infants with congenital deafness have also suggested that as many as 12% may be attributable to prenatal CMV infections (Peckham et al. 1987). Aminoglycoside ototoxicity provides a good example of geographic variation in the causes of deafness. Because of the widespread use of these antibiotics in infancy and

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Waardenburg (1951), Pendred (1896), Usher (1914), Treacher Collins (1900), Jervelle and Lange-Nielsen (1957) and Nance (1971), Mendelian transmission is well established and has allowed the mapping, demonstration of heterogeneity and/or cloning of more than 20 relevant genes. Distinctive audiologic or otolaryngologic characteristics, such as lowfrequency (Vanderbilt University Hereditary Deafness Study Group 1968), high-frequency (Nance and McConnell 1974), or progressive hearing loss (Cremers 1979), or the presence of distinctive vestibular, cochlear or ossicular abnormalities, can also be used to characterize different forms of genetic deafness.

3.2 Non-Syndromic Deafness

Dramatic advances have also been made during the past decade in mapping and cloning human genes for nonsyndromic deafness (Mueller, VanCamp, and Lench, Chapter 4). Many factors have contributed to this progress. These include the availability of increasingly dense maps of highly polymorphic markers, and knowledge of mouse homologs for comparative gene mapping. In addition, sophisticated methodologies and linkage programs have been developed which exploit data on sib pairs (Penrose 1935; Kruglyak and Lander 1995), consanguineous probands (Lander and Botstein 1987), large multiplex families (Lathrop et al. 1984) identity by descent (Haseman and Elston 1972), inbred isolates (Guilford et al. 1994) and linkage disequilibrium (Friedman et al. 1994; Blanton et al. 1999).

The recognition of the value of deaf subjects from consanguineous marriages for homozygosity mapping has played a major role in the successful localization of many genes for recessive nonsyndromic deafness. Individuals of this type can be assumed to have inherited two copies of same recessive mutation carried by one of their common ancestors. The disease gene, along with closely linked markers are autozygous, or identical by descent. The mapping strategy involves typing the consanguineous deaf offspring and searching for chromosomal regions in which closely linked polymorphic markers are homozygous. Many of the markers now available are so polymorphic that it is unusual to observe homozygosity. In these circumstances, the observation of even a few deaf offspring of consanguineous marriages who are homozygous for alleles at the same locus may be sufficient to map the gene. When Lander and Botstein (1987) first called attention to the power of this mapping strategy, they advocated the use of isolated probands from consanguineous marriages and showed that as few as six to eight could be sufficient to map a locus. In practice, though, successful examples of homozygosity mapping have usually involved the analysis of multiple affected individuals in large consanguineous kindreds. However, the technique can be used successfully with isolated consanguineous probands, as was shown for biotinidase deficiency. Biotinidase deficiency is a recessively transmitted defect in the recycling of the vitamin

biotin. In the absence of normal biotinidase activity, affected individuals are entirely dependent on their dietary intake of biotin and typically begin to show symptoms of biotin deficiency in early infancy (Wolf et al. 1985). Hearing loss, which is eventually seen in about 60% of untreated patients, is a completely preventable complication of this disease. Subsequent to the introduction of newborn screening for this treatable form of genetic deafness, large numbers of affected individuals have been identified throughout the world. Table 4.1 shows the geotyping results for polymorphic markers in the p21–22 region of chromosome 3 in twelve isolated consanguineous probands with biotinidase deficiency, many of whom had been identified in newborn screening programs. The shaded area indicates the chromosomal regions in which the typed probands exhibited marker homozygosity. Initial analyses of eleven markers localized the gene to a small region containing D3S1286. Subsequent typing of probands P5 and P274 for six additional markers flanking D3S1286 further refined the critical region. Notice that, in contrast to inbred kindreds, the homozygosity involved different alleles in each proband. Proband P5 was the offspring of third cousins once removed and the conserved chromosomal region surrounding the biotinidase locus had been narrowed by recombination during a total of eleven meiotic divisions to a very small interval. By typing only twelve individuals, it was ultimately possible to assign the probable location of the biotinidase gene to the small segment of chromosome 3p between D3S3613 and D3S1286 (Blanton et al. 2000).

4. Functional Genomics of the Ear

As new genes for deafness have been mapped and cloned, the delineation of their base-pair sequences has frequently allowed their function to be surmised by matching the amino acid sequences of their protein products with data bases of genes whose function has already been established. This knowledge is providing dazzling insights into the normal and pathophysiology of hearing.

4.1 Organogenesis

Among genes that can cause deafness, some of the most exciting and potentially significant are those that encode DNA binding transcription factors. These genes produce proteins that bind to specific regulatory sequences on their target genes and act in concert with other transcription factors to promote or inhibit the activities of those genes (Avraham and Hasson, Chapter 2). The miraculous process of organogenesis that leads to the formation of a normal inner, middle and external ear results from a precise cascade of differential gene expression that is controlled by a hierarchy of

DNA binding regulatory genes. The molecular defects in two forms of

genetic deafness have been shown to involve two members of the POU domain family of transcription factors (Griffith and Friedman, Chapter 6). This gene family was originally discovered in studies aimed at understanding how functionally distinct cell types arise within the pituitary gland. Many members of this family are expressed in fetal brain, including the neural tube and otic vesicle. De Kok et al. (1995) showed that the syndrome of X-linked congenital fixation of the stapes footplate with perilymphatic gusher (DFN 3) was caused by mutations in the POU3F4 gene at Xq13.

Mutations at this locus also lead to characteristic morphologic abnormalities in the internal auditory meatus, which can be detected by radio imaging and may contribute to the pathogenesis of the neurosenory component of the hearing loss. Mutations at a second locus POU4F3 on 5q31 have been shown to be the cause of an autosomal-dominant form of hearing loss, DFNA15 (Vahava et al. 1998). The expression of this gene is restricted almost exclusively to the fetal cochlea and knockouts of the murine homolog result in complete absence of the hair cells, with subsequent loss of the cochlear and vestibular ganglia cells. It seems likely that the gene must play some role in sustaining the hair cells, as well as initiating their differentiation in view of the progressive nature of the hearing loss in affected family members. DFNA7, a dominantly inherited form of progressive high-frequency hearing loss, was mapped to 1q21–q23 by Fagerheim et al. (1996). As noted by the authors, this region includes another member of the POU gene family, POU2F1 that is also expressed in the embryonic cochlea of the rat.

4.2 Homeostasis

The high potassium concentration of the cochlear endolymph is a unique feature of the physiology of the ear, upon which health and normal function of the hair cells is dependent. An influx of potassium ions through gated potassium channels is required for transducing physical deflections of the hair cells into nerve impulses that can be processed and transmitted for subsequent neural processing. Several forms of genetic deafness now seem to have as a common denominator a defect in the maintenance of this critical potassium gradient. In the autosomal recessive Jervell and LangeNielsen syndrome, the defect involves one of at least two genes that code for proteins required to form normal potassium channels in the heart as well as the cochlea (Splawski et al. 1997; Duggal et al. 1998). In the heart, expression of the mutant genes leads to a characteristic prolongation of the

QT interval, and a predisposition to syncopal attacks and sudden death.

Heterozygotes for mutations at the KVLQT1 locus on 11p15.2 and the KCNE1 locus on 21q22.1 may exhibit the cardiovascular component of the syndrome without hearing loss. This dominantly transmitted phenotype, Ward-Romano syndrome, can also be caused by mutations involving other ion-channel genes (Wattanasirichaigoon and Beggs 1998). Whether

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homozygosity at these loci will also lead to hearing loss remains to be determined.

The connexins are a somewhat similar class of genes, which code for the proteins that line the intercellular pores of gap junctions, where they facilitate the movement of small ions or molecules between cells. Defects in two members of this family, Cx26 and Cx31, have been identified in patients with deafness (Kelsell et al. 1997; Xia et al. 1998). It is generally believed that, in the cochlea, the connexins facilitate the recycling of potassium ions from the hair cells back to the stria vascularis, where they can be actively transported back into the endolymph. If so, the hearing loss could result from an interference with this normal homeostatic mechanism.

Lastly, it has long been recognized that patients with DFN3 have a mixed hearing loss with a significant sensorineural component in addition to the conductive loss resulting from their congenitally fixed stapes. It had always been assumed that the perilymphatic gusher, which is such a characteristic complication of surgical attempts to mobilize the stapes in this syndrome, must reflect an abnormal communication between the perilymphatic space and the epidural space of the CNS. Recent studies of the temporal bone by CT scans have shown a variety of developmental defects, including enlargement of the internal auditory canal and persistence of the vestibular aqueduct. It seems possible that an abnormal mixing of the high-potassium perilymph with the low-potassium cerebrospinal fluid may exceed the capacity of the stria vascularis to maintain the normal potassium concentration in the perilymph. If so, this mechanism could provide an explanation for the sensorineural component of the hearing loss.

4.3 Energy

Although the neurosensory structures of the organ of Corti are largely avascular, the stria vascularis is a highly vascularized structure, as its name suggests. This component of the cochlea is responsible for maintaining the endolymphatic potential. The high potassium concentration of the endolymph has been likened to a battery (Davis 1965), which stores energy by facilitating the flow of potassium ions across the stereocilia during sound transduction without requiring the active transport of the ions into the hair cell. Although the purpose of the battery is not known with certainty, by limiting the energy requirements of the hair cells it may increase the sensitivity of sound perception by allowing the neurosensory cells in the basilar membrane to function in a microenvironment that is devoid of turbulent blood flow.

In view of the energy requirements to sustain this system, perhaps it should have come as no surprise that genetic defects in the mitochondria are increasingly being recognized as potential causes for deafness (FischelGhodsian, Chapter 7). Hearing loss can be a component of several syndromic forms of mitochondrial disease including MERRF and MELAS, but

there are three other mitochondrial mutations in which deafness plays a much more prominent role. One form of maturity onset diabetes of youth (MODY) has been identified which is caused by an A8334G substitution in the mitochondrial tRNA Leu gene (van den Ouweland et al. 1992). Most of these patients also develop a late-onset hearing loss, which can be quite rapid in its progression. Matrilineal transmission is a characteristic feature of these families. The A1555G substitution in the mitochondrial 12S rRNA gene is now known to be the underlying cause for many cases of aminoglycoside ototoxicity (Fischel-Ghodsian et al. 1995). Aminoglycosides normally bind to bacterial rRNA molecules and exert their therapeutic effect by interfering with normal protein synthesis. The A1555G mutation makes the human mitochondrial ribosome more “bacteria-like” by creating a binding site for streptomycin, where it also interfaces with the fidelity of protein synthesis. The A1555G mutation has been shown to have a high prevalence in deaf populations in Mongolia, China, Japan, and Spain, and also appears to be the cause of hearing loss in some patients with no history of exposure to streptomycin. Whether there are natural compounds, or other toxins that can mimic aminoglycosides is not clear, nor is it clear why the effects of the gene are strictly limited to the ear, but at the same time expression can be so variable. Nuclear or mitochondrial modifier genes have been proposed as one possible explanation for the observed variability. Finally, an A7445G substitution immediately adjacent to the tRNATRY has been identified in several families with matrilineal hearing loss, in some of which ichthyosis was also found (Reid et al. 1994). The A7445G mutation interferes with the normal processing of the polycistronic message coded by the mitochondrial light chain. In a sample of 380 deaf students from Mongolia, loss of the Xba 1 restriction site was found in nine students, but sequencing revealed substitutions involving the 7444 and 7443 residues in addition to nt7445. Because these adjacent mutations all result in deafness, they may well define the binding site for the elusive endonuclease that initiates the processing of the light-strand message. Twelve of the students carried the A1555G substitution including, six who also had the 7444 change. Available clinical and audiologic data suggested that the individuals with the double mutations were more severely affected, raising the possibility of an epistatic interaction in subjects with the double mutant (Pandya et al. 1999).

4.4 Structure

Several genes for deafness code for structural proteins that appear to be required for normal hearing. The X-linked gene Col4A codes for a form of collagen that is deficient in Alport’s syndrome. In the kidney, this protein is an essential constituent of the basal membrane of the glomerulus. In its absence, the membrane becomes porous, allowing proteins and red cells to enter the glomerular filtrate. Although the physiologic role of the protein

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in the cochlea is less well understood, it is known to be present in the basement membrane of the stria vascularis. Alpha tectorin is an important component of the tectorial membrane that supports the hair cells. Precisely how mutations in this gene cause deafness is not yet clear. In a Swedish family, Balciuniene (1998; 1999) found evidence for linkage of hearing loss to the alpha tectorin locus on 11q22 and also to a second locus, DFNA2, on chromosome 1p35.1. Subjects with both mutations had more severe hearing loss than those with single mutations at either locus, suggesting an epistatic interaction between the two loci. The C1057S substitution in the tectorin protein may have predisposed to abnormal crosslinking of the polypeptide.

5. Mating Structure of the Deaf Population

Assortative mating is a distinctive feature of the genetic architecture of deaf populations in many countries. One potent effect of this pattern of mate selection is to bring together rare genes at different loci that would otherwise have a low probability of coexisting in the same individual. Although 90% of deaf individuals in the United States marry deaf partners (Schein and Delk 1974), linguistic homogamy (shared manual communication), rather than phenotypic assortment for deafness, may in fact be the basis for mate selection. The observation suggesting this may be the case is the fact that the hearing partners in deaf-by-hearing matings are often “native signers” who are themselves the offspring of deaf couples. Despite the fact that they are not deaf, these individuals may carry genes for recessive deafness at multiple loci, and matings of this type can sometimes give rise to pseudodominant transmission of deafness.

5.1 Frequency of Common Forms of Deafness

The discovery that mutations in a single gene (connexin 26), are the commonest cause of genetic deafness was unexpected. Estimates of the relative frequency of Cx26 deafness have varied greatly with up to 50% of all childhood deafness being attributed to this cause in some populations (Steel 1998). In others, such as India, Japan, Mongolia and China, the incidence appears to be much lower. Most reported studies have been based on molecular testing of clinic populations, and have not in general involved the random or stratified random sampling of subgroups likely to exhibit different frequencies. Adding to the confusion, probands with no affected siblings have in some reports been designated sporadic cases. The term sporadic refers to cases of deafness in which there is a very low chance of recurrence within the family, comparable to the incidence of deafness in the general population. Most sporadic cases of deafness are caused by environmental etiologies, but some can also represent new dominant mutations.

Although probands with no affected siblings may be sporadic cases, they may also be isolated genetic cases in which by chance only one deaf child