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Background Mutations in the GJB2 gene cause one form of nonsyndromic recessive deafness. Among Mediterranean Europeans, more than 80 percent of cases of nonsyndromic recessive deafness result from inheritance of the 30delG mutant allele of GJB2. We assessed the contribution of mutations in GJB2 to the prevalence of the condition among Ashkenazi Jews.
Methods We tested for mutations in GJB2 in DNA samples from three Ashkenazi Jewish families with nonsyndromic recessive deafness, from Ashkenazi Jewish persons seeking carrier testing for other conditions, and from members of other ethnic groups. The hearing of persons who were heterozygous for mutations in GJB2 was assessed by means of pure-tone audiometry, measurement of middle-ear immittance, and recording of otoacoustic emissions.
Results Two frame-shift mutations in GJB2, 167delT and 30delG, were observed in the families with nonsyndromic recessive deafness. In the Ashkenazi Jewish population the prevalence of heterozygosity for 167delT, which is rare in the general population, was 4.03 percent (95 percent confidence interval, 2.5 to 6.0 percent), and for 30delG the prevalence was 0.73 percent (95 percent confidence interval, 0.2 to 1.8 percent). Genetic-linkage analysis showed conservation of the haplotype for 167delT but the existence of several haplotypes for 30delG. Audiologic examination of carriers of the mutant alleles who had normal hearing revealed subtle differences in their otoacoustic emissions, suggesting that the expression of mutations in GJB2 may be semidominant.
Conclusions The high frequency of carriers of mutations in GJB2 (4.76 percent) predicts a prevalence of 1 deaf person among 1765 people, which may account for the majority of cases of nonsyndromic recessive deafness in the Ashkenazi Jewish population. Conservation of the haplotype flanking the 167delT mutation suggests that this allele has a single origin, whereas the multiple haplotypes with the 30delG mutation suggest that this site is a hot spot for recurrent mutations.
We have identified two mutant alleles of GJB2 in Ashkenazi Jewish families with nonsyndromic recessive deafness. Our findings on the hearing ability of family members who are heterozygous for one of these mutant alleles suggest that the expression of mutations in GJB2 may be semidominant. We determined the frequencies of these mutant GJB2 alleles in a representative population of Ashkenazi Jews.
Methods
Families with Nonsyndromic Recessive Deafness
Three families participated in genetic studies of nonsyndromic recessive deafness (protocols 97-DC-0180 and 97-DC-N024 of the National Institute on Deafness and Other Communication Disorders). Affected persons had moderate-to-profound congenital sensory-hearing loss across all frequencies, as assessed by pure-tone audiometry and tests of middle-ear immittance. DNA samples were extracted either from whole blood by standard techniques or from cells obtained by application of a swab to the buccal mucosa.13
Detection of Mutations and Genotyping
The GJB2 gene was amplified from DNA samples from members of the three participating families by means of the polymerase chain reaction (PCR) with primer pair 167F (forward) and 452R (reverse) and primer pair 370F (forward) and 889R (reverse), as described by Kelsell et al.9 The numbers refer to the nucleotides at the 5' ends of the oligomers and correspond to nucleotide position numbers in the GJB2 sequence logged in GenBank (accession number, M86849). These PCR primers produce overlapping fragments that cover the entire GJB2 coding region and are small enough (285 and 519 base pairs, respectively) to be suitable for single-strand conformation polymorphism analyses. The PCR fragments were labeled by incorporation of [
33P]deoxycytidine triphosphate, denatured, and separated by electrophoresis at 30 W through 0.5x Mutation Detection Enhancement gels (MDE, FMC BioProducts, Rockland, Me.), 0.6x TBE (1x TBE is 89 mM TRIS base, 89 mM boric acid, and 2 mM EDTA) and 10 percent glycerol at 4°C for four hours. Samples that showed conformation polymorphisms were sequenced directly (ThermoSequenase, Amersham Life Sciences, Arlington Heights, Ill.) with the use of the PCR primers.
Population DNA Samples
As population controls for the samples from the Ashkenazi Jewish families, we used blood samples without personal identifiers from a group of 555 Ashkenazim who were undergoing genetic testing for one or more autosomal recessive diseases (including Tay–Sachs disease, Canavan's disease, Gaucher's disease, and cystic fibrosis).14 No information about a family history of hearing loss was available from these control subjects. Samples from these control subjects have been used previously for population-genetics studies of mutations in the aspartoacylase, breast-cancer susceptibility (BRCA2), and adenomatous polyposis coli (APC) genes.15,16,17 Anonymous DNA samples representing white, Asian, and black racial and ethnic groups were collected at Michigan State University for comparisons of population frequency.
DNA samples were amplified by PCR for GJB2 as described above and then sequenced to determine the presence of the 167delT and 30delG alleles. All the samples were also typed for the genetic markers D13S141, D13S175, and D13S250, which flank GJB2 on chromosome 13, by incorporation of [33P]deoxycytidine triphosphate and resolved on conventional sequencing gels. Tests of association between the mutations in GJB2 and nearby genetic markers were performed with the program EH (Estimate Haplotype frequencies).18 Tests of the significance of differences between carrier rates were calculated with the program 2BY2, which performs Fisher's exact test.18 The age of the 167delT mutation was estimated with the following formula: y = x + (1– x) (1–
)g, where y is the frequency of the ancestral allele among mutation-bearing chromosomes, x is its frequency among normal chromosomes, is the recombination fraction between the marker allele and the mutation, and g is the number of generations.19
Audiologic Analysis
Pure-tone audiometry, testing of middle-ear immittance, and recording of otoacoustic emissions were completed in 10 subjects from Families A and B who had normal hearing and known GJB2 genotypes. These included four parents and five of their offspring who were heterozygous for mutations in GJB2 and one noncarrier offspring who had normal hearing (Subject A-605) (Figure 1). Spontaneous, click-evolved, and distortion-product emissions were recorded, as was efferent suppression of click-evoked otoacoustic emissions.20,21 Distortion-product otoacoustic emissions were obtained with pairs of pure tones. In this study, distortion-product otoacoustic emissions were obtained for eight frequency pairs per octave, with the lower frequency at a sound-pressure level of 65 dB, the higher frequency at 55 dB, and a higher-to-lower-frequency ratio of 1.2:1. The mean amplitudes in carriers of a mutation in GJB2 and in normal control subjects were compared by repeated-measures analysis of variance.
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Genetic Mutations and Frequencies of Alleles
In Families A and B, both from the Ashkenazi Jewish community in North America, and Family C, a European Ashkenazi Jewish family, nonsyndromic recessive deafness was found to be associated with the mutant alleles 30delG and 167delT (Figure 1). We also sought to determine the frequency of these two alleles among Ashkenazi Jews in the New York City area who have participated in genetic testing for autosomal recessive diseases other than deafness.14,15,16,17 In addition, we tested anonymous DNA samples, supplied by the Clinical Genetics Section of Michigan State University, which were obtained from people seeking counseling for disorders other than deafness. These samples were categorized as from whites, Asians (Indians, Japanese, and Koreans), and blacks.
Table 1 summarizes the frequencies of carriers of the 30delG and 167delT mutations in our test populations as well as data on frequencies of carrier status from the literature. The 30delG mutation may account for 85 percent of the alleles causing nonsyndromic recessive deafness in the Mediterranean European population.11 Four carriers of the 30delG mutation were detected among 551 Ashkenazi Jews, and 1 carrier among 173 white persons with DNA samples at Michigan State University. These frequencies are not significantly different from each other or from published frequencies.
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We found 22 carriers of the 167delT mutation among 546 Ashkenazi Jewish samples, suggesting a carrier frequency of 4.03 percent (95 percent confidence interval, 2.5 to 6.0 percent). The carrier rate among Ashkenazi Jews was significantly different from that of any other population reported thus far (P=4.7x10–7 by Fisher's exact test). The 167delT mutation is rare and has not been detected in surveys of randomly chosen subjects from any of the populations listed in Table 1.
High carrier rates are generally attributed to a founder effect in a population and are usually evidenced by conservation of haplotypes with closely linked markers. We determined the genotypes with respect to markers D13S141, D13S175, and D13S250, which span a 2-centimorgan region with GJB2 in the center, in the Ashkenazi Jewish samples. Table 2 and Table 3 summarize the results of a test of association between alleles at the marker loci D13S141 and D13S175 and the mutations in GJB2 (marker D13S250 was less informative since only two alleles were associated with it, and therefore those data were excluded from statistical analyses). There was strong evidence of an association between 167delT and the 3/4 haplotype (D13S141/D13S175) (
2=45.92, 6 df; P=3.1x10–8) (Table 2). There was, however, no evidence of an association between the 30delG allele and any haplotype in the region (2=8.49, 6 df; P=0.204). This result did not merely reflect lack of power to detect an association in a small set of 30delG chromosomes; there are in fact several different 30delG haplotypes (Table 3).
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All the tested offspring had normal pure-tone thresholds (defined as a 15-dB hearing level or better) from 250 to 8000 Hz. Parents' thresholds were normal, with the exception of a loss of low-frequency hearing (<500 Hz) in one ear of one parent and a mild, age-appropriate loss of high-frequency hearing (>4 kHz) in five ears of three parents. Middle-ear immittance and reflexes of the middle-ear muscles were normal in all subjects; no history of middle-ear problems was reported by any of the subjects.
The integrity of the outer hair cells of the organ of Corti can be inferred indirectly from otoacoustic emissions. Otoacoustic emissions are low-amplitude acoustic signals that are generated by the cochlea and that can be measured in the external auditory canal by placing a low-noise probe microphone in the canal.22 These emissions are thought to be associated with normal function of the outer hair cells.23
The results of measurement of distortion-product otoacoustic emissions were informative. In our group of nine control subjects with normal hearing ability, the average sound-pressure level of emissions was 5.88 dB. In contrast, the mean amplitude in the range of 1 to 4 kHz for emissions recorded from the right and left ears was a 0.499-dB sound-pressure level in the nine carrier subjects (four parents and five offspring) (Figure 2). One noncarrier offspring was evaluated, and the sound-pressure level had an emission amplitude of 10.54 dB. Comparison of the amplitudes of distortion-product otoacoustic emissions in carriers and control subjects by repeated-measures analysis of variance showed that the carriers had a significantly lower amplitude (F1,16=11.439, P=0.0038). The amplitude of emissions showed considerable variability across the frequency range, with sharp amplitude notches (shifts of more than 10 dB from one frequency to the next higher frequency) noted in four ears of three of the four parents and in six ears of four of the five carrier offspring. The noncarrier offspring showed no such amplitude notches.
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Discussion
Analysis of DNA from deaf members of three Ashkenazi Jewish families revealed two frame-shift mutations in the GJB2 gene, deletion of a guanine residue in a stretch of six guanines beginning at position 30 (30delG) and deletion of a thymine residue at position 167 (167delT).10 Mutations in GJB2 are rare in the general population, but we observed a carrier rate of 4.76 percent (95 percent confidence interval, 3.1 to 6.9 percent) among Ashkenazi Jews. Moreover, in the general population, the 30delG allele has been determined to be the most common mutation causing nonsyndromic recessive deafness, whereas the 167delT mutation is rare, but in the Ashkenazi Jewish population, we observed the opposite, with the 167delT mutation having a carrier rate of 4.03 percent and the 30delG mutation a rate of 0.73 percent.10,11,12
High carrier rates for recessive genes are caused by population "bottlenecks" (large reductions in the size of a population) and high rates of endogamy (the tendency for marriages to be limited to partners within the same community). When this happens, the haplotype of the region surrounding the mutation is highly conserved because of the small number of mutant chromosomes present in the population at the time of the bottleneck. A population bottleneck in Ashkenazi Jewish history has been posited on the basis of the existence of conserved haplotypes in the regions of other disease-associated genes.28 For example, Risch et al.29 found a high degree of conservation among markers surrounding the gene governing the inheritance of idiopathic torsion dystonia, and on the basis of the degree of conservation, estimated that there have been 12 or 13 generations since the bottleneck. We found a similar degree of haplotype conservation in the region of the 167delT mutant allele. Using the degree of conservation between GJB2 alleles and marker D13S141, and assuming a distance of 1 centimorgan between the two loci, we estimate that there have been seven generations since a population bottleneck.
The conservation of haplotypes surrounding the 167delT mutation suggests that this allele has a single origin. In contrast, the 30delG mutation is associated with several different haplotypes, suggesting that it has multiple origins. The likely explanation is that the 30delG allele has arisen spontaneously several times in Ashkenazi Jewish history. The 30delG mutation has been observed on different haplotype backgrounds in other populations as well, suggesting that it is recurrent and arises at a mutation hot spot associated with the string of six guanine residues at this site.12,19
Identification of these two common mutations in the GJB2 gene should improve genetic counseling in the Ashkenazi Jewish population. The combined carrier rate of 4.76 percent suggests an incidence rate of 0.6 per 1000 for recessive deafness due to mutant alleles of GJB2 in the Ashkenazi Jewish population (with no consideration of possible assortative or consanguineous marriages). The most recent estimates of the incidence of all congenital deafness among Ashkenazi Jews is 1.2 per 1000, of which 38.1 percent is due to nonsyndromic recessive deafness.30,31 Thus, it appears that mutations in GJB2 account for nearly all cases of nonsyndromic recessive deafness in this population. This conclusion is consistent with analyses of the distribution of deaf children among consanguineous Ashkenazi marriages, which show that as few as two to six genetic loci associated with the inheritance of nonsyndromic recessive deafness are present in the Ashkenazi population.30,32
The 4.76 percent carrier rate is similar to that of other genes governing recessive diseases in the Ashkenazi Jewish population, such as Tay–Sachs disease (4 percent), Gaucher's disease (4 to 6 percent), and familial dysautonomia (3 percent).28 For this reason, Ashkenazi Jews may elect to have their carrier status for mutations in the GJB2 gene determined as part of genetic screening. An appropriate test would be one designed to detect both the 167delT and the 30delG mutant alleles.
Supported by intramural funds of the National Institute on Deafness and Other Communication Disorders (NIDCD) (Z01 DC 00039-01, to Dr. Friedman) and by a grant (P0100379, to Dr. Berlin) from the NIDCD. Additional support was provided by the Michigan State University Foundation (to Dr. Friderici), the National Foundation for Jewish Genetic Diseases (to Dr. Ostrer), Kam's Fund for Hearing Research, and the Marriott Foundation.
We are indebted to the members of the participating families.
Source Information
From the Laboratory of Molecular Genetics, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Rockville, Md. (R.J.M., H.J.K., T.B.F.); Kresge Hearing Research Laboratory, Department of Otorhinolaryngology and Biocommunication, Louisiana State University Medical Center, New Orleans (L.J.H., L.G., C.I.B., B.K.); the Department of Pediatrics and Human Development, College of Human Medicine, Michigan State University, East Lansing (K.F., R.F.); the Department of Medical Genetics, University of Antwerp, Antwerp, Belgium (G.V.C.); and the Human Genetics Program, Department of Pediatrics, New York University Medical Center, New York (C.O., H.O.). Other authors were Theresa San Agustin, M.D., Laboratory of Molecular Genetics, National Institute on Deafness and Other Communication Disorders, Rockville, Md., and Jan Dumon, M.D., Department of Medical Genetics, University of Antwerp, Antwerp, Belgium.
Address reprint requests to Dr. Morell at the NIDCD, 5 Research Ct., Rm. 2A19, Rockville, MD 20850.
References
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N Engl J Med 1999;
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Correspondence
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G denotes the 30delG mutation, and 