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|Year : 2003 | Volume
| Issue : 20 | Page : 19--28
Genetic influences in individual susceptibility to noise : A review
RR Davis1, P Kozel2, LC Erway3,
1 Hearing Loss Prevention Section, National Institute for Occupational Safety and Health, Cincinnati; Department of Biological Sciences, University of Cincinnati, Cincinnati, OH, USA
2 National Institute of Deafness and Other Communication Disorders, Rockville, MD, USA
3 Department of Biological Sciences, University of Cincinnati, Cincinnati, OH, USA
R R Davis
Hearing Loss Prevention Section, Engineering and Physical Hazards Branch, Division of Applied Research and Technology, National Institute for Occupational Safety and Health. 4676 Columbia Parkway, Cincinnati, OH 45226
Individual animals and humans show differing susceptibility to noise damage even under very carefully controlled exposure conditions. This difference in susceptibility may be related to unknown genetic components. Common experimental animals (rats, guinea pigs, chinchillas, cats) are outbred-their genomes contain an admixture of many genes. Many mouse strains have been inbred over many generations raeducing individual variability, making them ideal candidates for studying the genetic modulation of individual susceptibility. Erway et al. (1993) demonstrated a recessive gene associated with early presbycusis in the C57BL/6J inbred mouse. A series of studies have shown that mice homozygous for Ahlallele are more sensitive to the damaging effects of noise.
Recent work has shown that mice homozygous for Ahl are not only more sensitive to noise, but also are probably damaged in a different manner by noise than mice containing the wild-type gene (Davis et al., 2001).
Recent work in Noben-Trauth's lab (Di Palma et al., 2001) has shown that the wild-type Ahl gene codes for a hair cell specific cadherin. Cadherins are calcium dependent proteins that hold cells together at adherins junctions to form tissues and organs. The cadherin of interest named otocadherin or CDH23, is localized to the stereocillia of the outer hair cells. Our working hypothesis, suggests that otocadherin may form the lateral links between stereocilia described by Pickles et al (1989). Reduction of, or missing otocadherin weakens the cell and may allow stereocilia to be more easily physically damaged by loud sounds and by aging.
|How to cite this article:|
Davis R R, Kozel P, Erway L C. Genetic influences in individual susceptibility to noise : A review.Noise Health 2003;5:19-28
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Davis R R, Kozel P, Erway L C. Genetic influences in individual susceptibility to noise : A review. Noise Health [serial online] 2003 [cited 2020 Jul 9 ];5:19-28
Available from: http://www.noiseandhealth.org/text.asp?2003/5/20/19/31692
It is a well-known phenomenon that workers exposed to the same level of noise exhibit different levels of noise-induced hearing loss. These differences are often dismissed as having genetic origins. In theory, any gene which weakens the ear functionally or structurally would make the ear more susceptible to noise damage. Practically, gene mutations having a phenotype which is expressed throughout the body would probably shorten the organism's life. Therefore a more useful strategy would be to look for genes encoding proteins specific for the ear. Using powerful molecular techniques it is now possible to observe these genotypic differences in tissue samples obtained from the living organism.
The objectives of this paper are to review the history of a certain noise susceptibility gene Ahl (and PMCA2); to review our classical and molecular results; and present a hypothetical, physiological model of how we believe this gene functions to modulate susceptibility to noiseinduced hearing loss.
Sexual reproduction in nature normally results in high genetic diversity. In a natural environment genetic diversity favours increased survival of some offspring. Sexual reproduction results in chromosome pairs - one half set from father, one half set from mother (humans have 23 pairs, mice 20 pairs). Of the 30,000 or so human genes, each parent likely contributes different alleles. Statistically, lethal recessive genes are masked by pairing with dominant genes. By chance, recessive genes are sometimes paired and phenotypes such as albino are expressed. In a natural environment, an albino mouse would not have a high likelihood of survival to reproductive age.
Dominant alleles, e.g. Wardenburg Syndrome, are expressed in one half of the offspring of an affected parent. Recessive alleles result in deafness only when both parents possess at least one copy of the recessive allele.
The picture becomes even more complex when modifier genes are added. Modifier genes alter the phenotypic expression of other genes. This may be through regulation of the gene, its messenger RNA or the resulting protein.
Wild type genomes are laborious for conducting research. Because of the inherent genetic diversity it becomes very difficult to trace the function of genes to the next generation. For this reason mammalian geneticists have developed a number of inbred strains of mice over the past 100 years. Inbred strains are developed through 20 or more generations of brother-sister matings. Any two individuals in an inbred strain are as statistically alike as identical twins. By mating siblings, the diversity of the genome is decreased until copies of the same chromosome are present in each chromosome pair. Inbreeding leads to pairings of dominant with dominant genes and recessive with recessive genes. This makes genetic research easier, but may lead to situations where viable offspring are not produced due to lethal combinations of recessive genes.
Discovery of the gene for age-related hearing loss (Ahl)
In the early 1990s Erway became involved in an ongoing project studying the genetics of aging mice at the Jackson Laboratory, Bar Harbor, Maine. The study followed five inbred strains and their ten F1 hybrids for the entire life of the mice-normally 2 to 3 years. Erway tested the inbred mice and hybrids for their ability to generate the Auditory Brainstem Response (ABR) at approximately one, two and three years of age. The ABR is an electrical evoked potential event recorded from electrodes located on the head in response to a click or very short tone burst. Hundreds of responses are collected and averaged to reduce variability and make the signal clearer. The amplitude and latency of the ABR are correlated with the hearing threshold of the organism. Erway discovered that certain inbred mouse strains and hybrids were predisposed to early presbycusis-the loss of hearing due to aging.
Among the strains displaying early hearing loss was C57BL/6J (abbreviated B6). Johnson et al. (1993) demonstrated that presbycusis in this strain was associated a single gene located on Chromosome 10. They named this gene agerelated hearing loss or Ahl.
Discovery of a gene for Ahl and for NIHL
The National Institute for Occupational Safety and Health (NIOSH) became interested in Ahl in mice as an explanation for differences in worker susceptibility to noise-induced hearing loss. A partnership was established between NIOSH researchers and researchers at the University of Cincinnati to conduct a number of collaborative noise experiments.
The first experiment was designed to demonstrate that Ahl modulated the susceptibility of mouse ears to noise-induced hearing loss (Erway et al.,1995). Four groups of mice were exposed to broadband noise. Group one consisted of mice of the inbred strain C57BL/6J (abbreviated B6) homozygous for the Ahl gene (Ahl/Ahl). Group two consisted of mice of the inbred strain CBA/CaJ (abbreviated CBA) homozygous for the wild-type gene (+/+). Group three consisted of mice of a hybrid cross between B6 and CBA which was hypothesized to be heterozygous for Ahl (+/Ahl). The fourth group consisted of mice of a hybrid cross between B6 and the inbred strain DBA/2J, which was also hypothesized, based on Erway et al. (1993), to be homozygous for Ahl (Ahl/Ahl). Mice were tested for the ability to produce the ABR pre-exposure and at 8 hours, 1 day, 3 days, 7 days, 14 days, 30 days and 60 days post exposure.
The data indicated that ABR thresholds of mice homozygous and heterozygous for the wild type gene recover from the noise exposure in 3-7 days [Figure 1]. By contrast ABR thresholds of mice homozygous for Ahl (both the B6 inbred and the B6 × DBA/2J hybrid) do not recover from the noise exposure. In addition, these two strains subsequently exhibit further age related hearing loss, while the wild-type inbred and hybrid mice do not.
These findings strongly support the conclusion that the Ahl gene on chromosome 10 increases the vulnerability in mice to both age-related hearing loss and noise-induced hearing loss from an early age.
Demonstration that susceptibility is a genetic phenomenon
Although individuals within a strain are statistically as genetically alike as identical twins, there are many differences between strains. To determine that increased susceptibility to noise-induced hearing loss was an inheritable factor a technique from classic genetics for tracking a recessive gene-a back test-cross was utilized (Davis et al., 1999).
The use of a back test cross introduces a new variable-hybrid vigour. It is known that increased genetic diversity results in greater phenotypic variability. A hybrid back or test cross thus results in hybrid vigour which increases the variability of the individual to the damaging effects of noise.
The test cross began with the mating of a B6 parent with a CBA parent, similar to the last experiment. All offspring would then have a genotype of +/Ahl. These offspring were then crossed with B6 inbred mice (Ahl/Ahl) resulting in progeny, half of who were heterozygous for Ahl (+/Ahl) and half who were homozygous for AN N ((AN/AN). (AN/AN). (AN/AN). (AN/AN). (AN/AN). (AN/AN). (AN/AN). ). If AN N is truly a genetic factor, one half of the progeny should be more susceptible to NIHL and the other half should not.
The back cross mice were exposed for 8 hours to 110 dB SPL noise and tested for the ability to generate the ABR at 8 hr, 1 day, 3 days, 7 days, 14 days and 30 days. The variability of the results required the use of cluster analysis to group the data. Initially, four clusters emerged [Figure 2]. Three of the four clusters were collapsed by this analysis, providing two equal clusters. These clusters divided the group about in half (30:31) very close to the expected genotypes. This is the expected ratio based on AN N being a recessive gene. This is further evidence that AN N is a gene and follows Mendel's laws.
B6 and CBA differ in 2 ways
The third experiment in the series returned to the original inbred strains: C57BL/6J (B6) and CBA/CaJ (CBA) (Davis et al., 2001). Groups of B6 and CBA mice were exposed to differing levels of broadband noise for the same length of time. This allowed a dose-response curve to be defined for each strain.
The resulting dose-response curves showed that B6 was about 6 dB more sensitive to the effects of noise at 16 kHz [Figure 3]. In addition the slopes of the curves for B6 and CBA were not parallel indicating that the mechanism of damage was not the same. The slope of the doseresponse curve for B6 is shallower than the CBA curve. This indicates that damage occurs over a larger 9 dB dynamic range for B6. CBA on the other hand is damaged over a narrower approximately 6 dB range. This can be interpreted as a metabolic failure in B6 and a catastrophic failure in CBA.
In 1997 while studying the molecular genetics of recessive deaf-wadler (dfw) Konrad NobenTrauth et al., (1997) observed a modifier gene which affected the phenotype of deaf-waddler. In mice homozygous for modifier deaf waddler (mdfw/mdfw) and deaf waddler heterozygous (dfw/+) the mice exhibit the deaf waddler phenotype. They demonstrated that the allele modifier deaf waddler (mdfw) physically mapped to chromosome 10 in a region associated with Ahl. Recently, DiPalma et al (2001) showed that the wild-type gene for mdfw or Ahl codes for one of the cadherins. Cadherins anchor into the actin cytoskeleton of a cell, cross the plasma membrane and attach to an identical cadherin of a neighbouring cell. Cadherins are well-known for "gluing" the same cell types together to form tissues. Cadherins form adherins junctions and synapses. They are also highly dependent upon calcium to maintain the "ball-and-socket" conformation necessary for their function. Recently the three-dimensional extracellular structure of c-cadherin has been described (Boggon et al, 2002).
Street et al. (1998) showed that dfw is a mutation of the Plasma Membrane ATPase Isoform 2 (PMCA2) pump protein. The gene encoding PMCA2 is known to physically map to chromosome 7. The question becomes: how can a gene on chromosome 10 modify the phenotypic expression of a gene physically located on chromosome 7? The important link may be through calcium. PMCA2 is important in transporting calcium from inside of the cell to outside of the cell. Yamoah et al (1998) showed that depriving haircells of Ca++ leads to disarray of stereocilia. He also showed that the microenvironment around the stereocillia is high in Ca++ concentration.
Around 1999 a collaboration began between NIOSH and the University of Cincinnati with Kozel who had produced a PMCA2 knockout mouse (Kozel et al., 1998). This gene codes the same protein as the spontaneous mutant deaf waddler (dfw). Kozel noted that the homozygote offspring were ataxic. Erway showed through ABR measurements that they were also deaf. Molecular genetics showed that the heterozygotes (pmca2/+) produce one-half as much messenger RNA as wild type mice but had normal hearing and balance. The question was: are heterozygotes more susceptible to noise? There were two confounding factors that needed to be controlled for. First, the strain on which the knockout was generated was known to have Ahl in its genome. Second, the strain was something of a hybrid so the noise would have to be increased to produce damage.
The first problem was solved by breeding the PMCA2 knockout with the inbred CAST/Ei strain (abbreviated CAST). CAST had been used previously and known not to contain Ahl. The second problem was solved by a series of pilot exposures which resulted in a range of noise levels which produced reproducible, statistically significant threshold shifts.
The PMCA2 knockout and Ahl genotypes were detected through molecular markers. Ultimately, the study showed that heterozygotes of PMCA2 knockouts are more susceptible to noise than their homozygote wild-type litter mates (Kozel et al., 2002). The original mouse strain required only a four hour noise exposure but the hybrid mice of this study required 8 hours of exposure to 110, 113 or 116 dB SPL noise.
Our model is based on some speculation. Currently, neither the cellular location, nor the function of otocadherin have been proven. Based on current knowledge of other cadherins, the model hypothesizes that otocadherin is a structural protein, probably important for the structure of the stereocilia. In other tissues, cadherin inserts through the cell membrane, often attaching to the actin cytoskeleton. A cadherin dimer binds in a ball-and-socket arrangement to attach two neighbouring cell membranes together in an adherins junction. The cadherins rely heavily on calcium ions to maintain their functional integrity (typically 7 calcium ions per cadherin molecule) and the calcium concentration in the endolymph - the environment of the stereocilia - is low. The Plasma Membrane Calcium ATPase, Isoform 2 (PMCA2) pump is localized to the stereocilia of the hair cells. Using ATP, PMCA2 pumps calcium out of the stereocilia into the endolymph. It is thought that PMCA2 is able to generate a microenvironment of high calcium concentration around the stereocilia (Yamoha et al., 1998). PMCA2 is probably also important for removing intracellular calcium which might be toxic to the cell. There is evidence that noise overexposure results in high levels of calcium inside of the stereocilia and hair cell (Fridberger et al, 1998). The model hypothesizes that otocadherin is located in the so-called lateral links of the stereocilia (Pickles et al., 1989) [Figure 4]. Rather than holding cells together, the otocadherins hold the stereocilia together. This also places the otocadherins outside of the tip link transducer.
[Figure 5] is a schematic diagram of three normal stereocilia. The circle with the double arrows represents PMCA2. The shading around the stereocilia indicates that a normal concentration of calcium is maintained by PMCA2. The lines with the ball-and-socket joints are the otocadherin. The two dimers fit together and hold the stereocilia together as a unit. This schematic represents the presumed normal environment in the cochlea of CBA/CaJ inbred mice.
[Figure 6] is a schematic of the homozygous Ahl situation. In this model, the homozygous gene mutation Ahl (aka mdfw), results in an otocadherin protein which is less effective; is no longer able to structurally maintain the stereocilia lateral link or may not be present at all. The PMCA2 pump functions normally and is able to maintain a normal calcium concentration. An overload of acoustic energy applied to the system results in damage to the stereocilia and outer hair cell due to lack of otocadherin buttressing. The model hypothesizes that this environment is present in the cochlea of C57BL/6J inbred mice.
[Figure 7] is a schematic diagram of the PMCA2 knockout mouse. In this case the model hypothesizes that a lowering of calcium concentration due to a heterozygous PMCA2 gene results in decreased effectiveness of otocadherin in the lateral links and other calcium dependent proteins. Presumably some percentage of the otocadherin molecules would still be functional, since about 50% of the PMCA2 pumps are present, but the stereocilia would be compromised. Thus the amount of acoustic energy required to damage the hair cell is much higher than the homozygous Ahl but less than the normal case.
This also explains how Ahl (or modifier deaf waddler) can interact with the deaf waddler phenotype [Figure 8]. Since the deaf waddler gene produces an ineffective calcium pump, presumably even small fluctuations in calcium availability in the endolymph will affect the ability of the otocadherins to form lateral links between stereocilia. If the otocadherins are compromised through Ahl- mdfw the ear will be extremely vulnerable to noise. The "modification" of deaf-waddler is actually a change in calcium level which is modifying the function of otocadherin and weakening the stereocilia.
Recent research in other laboratories support the hypothesis that the lateral links are composed of otocadherin. DeBrouwer et al (2003) show that the length of the cadherin diamer is about that of the lateral links in the stereocilia.
The cadherins are an interesting group of proteins. Although fundamental for their role in maintaining tissue integrity some members of the group may also have a role in signalling.
Much needs to be learned about genes modifying worker susceptibility to NIHL. A significant proportion of this work can be carried out in model systems such as inbred mice.
There may be many genes like Ahl or PMCA2 which play a role in worker susceptibility to NIHL. Finding these genes could be a challenge since susceptibility implies that the worker must be exposed to noise. A non-noise exposed relative would not exhibit the hearing loss phenotype and would be mis-categorized as a normal genotype. Currently molecular genetic and statistical tools are being developed which may make these challenges approachable. Important work awaits.
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