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Bradley Fasulo
Dr. Ely
BIOL 303
1 November 2014
Hemizygosity of Genes Associated with Hearing Loss
Zygosity is the term used to describe how genes for a trait are paired on both
chromosomes; hemizygosity is when only one gene for a particular trait from a
chromosome is present (exemplified by the presence of only one X chromosome in males).
When both genes are present, and at least one of the genes present is healthy, there is less
potential to have a problem because the healthy gene can be expressed. But in the case of
hemizygosity, one of the genes is missing. If the remaining gene is not healthy and does not
have the proper code for normal function, the expression of the gene is often detrimental.
Thus hemizygosity can cause a variety of disorders including hearing loss.
Hearing is a process that requires the transmission of sound through the outer ear,
into the external auditory canal, where it then causes the tympanic membrane (eardrum)
to vibrate. This vibration continues to the middle ear then to the inner ear- through the
Figure 1. The inner structure of an ear (Virtual Medical Centre 2007)
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three ossicles (the bones in the ear), a process that also serves to open part of the cochlea.
This window allows for the transmission of the vibration into the spiral of Corti, where the
stereocilia are located. The stereocilia are tiny hair like structures that vibrate in the fluid
in the spiral of Corti, a movement that is translated into electrical stimuli which are then
sent to the brain (National Institute of Health 2012). Sensorineural Hearing Loss (SNHL) is
a specific form of hearing loss caused by damage to the inner ear, including damage to the
hair cells receiving stimuli, the nerves that carry information from these cells to the brain,
or the brain itself. It can be brought on by physical damage or can be present at birth due to
genetic defects (National Institute of Health 2014). SNHL can be syndromic or
nonsyndromic, meaning that it can be caused by a disorder and can be accompanied by
other symptoms, or it can be present alone. Of the many genes that help code for some
aspect of hearing, hemizygosity at any of them has the potential to make the auditory
process much less effective.
Figure 2. SNHL individuals with STRC deletions and/or sequence changes in respective exons (Vona 2014)
No.
Allele 1
Allele 2
Exon
Interpretation
DFNB16 patients
1, 2, 95
STRC gene deletion
STRC gene deletion
Homozygous deletions
3
STRC gene deletion
c.2726A>T, p.H909L
9
Heterozygous deletion and hemizygous pathogenic
mutation
4
STRC gene deletion
c.4918C>T, p.L1640F
26
Heterozygous deletion and hemizygous pathogenic
mutation
6
STRC gene deletion
c.4402C>T,
p.R1468X
23
Heterozygous deletion and hemizygous pathogenic
mutation
24
c.2303_2313+1del12, p.G768Vfs*77
c.5125A>G,
p.T1709A
6, 28
Compound heterozygous pathogenic mutations
19
Heterozygous deletion and hemizygous variant
Heterozygous deletion, mutation, and variant carriers
5
STRC gene deletion
c.3893A>G,
p.H1298R
7, 94
STRC gene deletion
Normal
25
c.2640G>T, p.E880D; c.3893A>G,
p.H1298R
c.3893A>G,
p.H1298R
16
c.5180A>G, p.E1727G
Normal
28
Heterozygous pathogenic mutation
22, 23,
26
c.3893A>G, p.H1298R
Normal
19
Heterozygous variant
Heterozygous deletion
8, 19
Heterozygous pathogenic mutation and homozygous
variant
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Located at 15q15.3 is a key region in coding for hearing, as it contains the gene for
stereocilin (STRC) (Vona 2014). Stereocilin is a gene that codes for the stereocilia, the tiny
hairs in the inner ear. B. Vona et al. (2014) published a paper on research they conducted
on 95 individuals with SNHL. Only one patient, patient 95, had syndromic SNHL. They set
out with the purpose of attempting to determine the importance of the role of the STRC
gene in hearing by testing for mutations in the subjects suffering from SNHL. They
extracted the DNA from the individuals from the blood, and checked for copy number
variations. They sequenced the STRC gene using Sanger sequencing and nested PCR (to
exclude a pseudogene just upstream). Figure 2 shows the results of the sequencing, in
individuals where the STRC gene was affected. These individuals all had mutations which
caused them to suffer from mild to severe SNHL. The homozygous deletion in patients 1, 2,
and 95 is a large problem not only for them as individuals, but for their offspring, who will
become hemizygous individuals prone to the same hearing loss from which many of the
participants of this study suffer. It is the pairing of the heterozygous deletion and the
mutation that really creates problems, though in some of the heterozygous individuals with
a normal gene the mutated gene is also expressed and causes SNHL. Homozygous deletions
and hemizygous mutations both impair not only the individual but also his/her offspring,
who are guaranteed to inherit at least one mutated gene or inherit a chromosome where
the gene has been deleted.
Figure 3 Bilateral pure tone audiograms from individuals with biallelic STRCmutations. Above the
audiograms are the patient number, sex, and age. Circles in the audiograms represent the right, and
crosses, the left ear, respectively. If multiple audiograms were present, an age range is listed and the
average of all thresholds is depicted. Created with AUDIOGRAMMAKER software (Vona 2014)
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Vona et al. (2014) followed up with the patients who had mutations in both alleles, and
they underwent audiograms and clinical studies. Clinical studies found that many of them
had parents found to be heterozygous deletion carriers, and others had a family history of
hearing loss. The audiograms had downward slopes (figure 3) (as the frequency of sound
increased, hearing sensitivity decreased). Normal hearing should give an almost horizontal
line across the graph around 20 decibels (dB). Patient 1, even at lower frequencies, had
trouble hearing the noises and could only detect sound at at least 30 dB. Patient 1 suffered
a homozygous deletion of the STRC gene. This supports the notion that while STRC may
play an important role in hearing, hearing and the structure of stereocilia are not entirely
dependent on the gene (as patient 1 could still hear).
While Vona et al. (2014) were able to support the hypothesis that the STRC gene
plays a large role in hearing, Perry et al. (2014) conducted a study on people with SNHL
who had deletions at the 18q locus. Deletions at this locus can cause a variety of disorders
and defects, including cognitive impairment, short stature, renal problems, and hearing loss
(Cody 1999). Distal 18q- is the name of the disorder that deals with such deletions and the
ensuing symptoms. Thirty-eight subjects were used, and they were verified to have distal
18q- using karyotypes to physically check for distal 18q deletions and medical records. The
precise breakpoints and genes involved were then determined using microarray analysis.
Similar to the previous study, each of the individuals underwent audiometry tests. Bone
conduction tests were also performed in order to determine whether or not the three
ossicles were the cause of, or at least part of, the SNHL.
Using the audiograms and bone conduction tests, twenty four individuals were
found to have high frequency sensorineural hearing loss (HFSNHL). The rest of the
individuals were considered to have normal hearing. Several of the “normal” subjects also
had deletions in the critical region for high frequency hearing, so it was possible that they
could develop HFSNHL later after the study is conducted. To then decrease the probability
that the HFSNHL is due to age (which is often the cause), the conductors of the study
compared the ages of the individuals found to have HFSNHL. The normal group had an
average age of 12, and the group suffering HFSNHL had an average age of 14.6. The
researchers determined that this was not a significant difference (Perry 2014).
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The reason for the hearing loss was less clear than it was when the STRC gene was
affected because many genes are located on the distal end of 18q, three of which have
functions not yet determined in humans. A recent study was mentioned that points toward
myelin degradation on the auditory nerve that causes age related hearing loss (Xing 2012).
Perry et al. (2014) hypothesized that a congenital deficiency of myelin caused by
hemizygosity of the myelin gene could also result in HFSNHL. MRI data was obtained for 22
of the 24 patients found to have HFSNHL, and there was delayed myelination in the brain
for each of those individuals. These results support their hypothesis that the hemizygosity
of the critical region absent in those 24 individuals plays a part in myelination, both in the
brain and possibly the auditory nerve.
One other disorder in which hearing loss is prevalent among other symptoms is
Fabry disease. Fabry disease is present predominantly in males, as it is an X-linked
syndrome. A lysosomal enzyme that ordinarily metabolizes sphingolipids is defective in
this condition, which results in glycosphingolipid deposits in plasma and lysosomes
(Sweeley 1963). Initial problems of Fabry disease are angiokeratoma (benign skin lesions)
and autonomic deficiency. As glycolipids begin to deposit in microvessels, arteries and
arterioles begin to narrow, leading to complications of brain and heart. It also causes renal
deficiency (Germain 2002). Germaine et al. (2002) conducted research on the impact of
Fabry disease on hearing, a topic of less research in this specific disease. They hypothesized
that deposited glycosphingolipids could alter auditory functions.
Twenty-two males participated in the study, all with Fabry disease. Each individual
underwent extensive audiologic testing, including tympanometry (determining the
flexibility of the eardrum (Hain 2014)), bone conduction tests, and speech audiometry. To
determine amount of hearing loss, the individuals’ results were compared to age
dependent percentiles. Of the 22 patients, 7 complained of progressive deafness, 4 of
sudden deafness, and 3 of a combination of both sudden and progressive deafness (one in
each ear). Six of the 22 patients also complained of tinnitus (ringing in the ears). Figure 4
shows the percentage of subjects who had pure tone thresholds (the softest sound audible
over 50% of the time to an individual) greater than the 95th percentile (the 95th percentile
found using data from the International Organization for Standardization from 1990 at
each of the 5 frequencies) (Germain et al. 2002). Keep in mind that a greater pure tone
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threshold is worse, because in order to hear the particular frequency, the volume must be
greater. For each frequency, more than half of the
Figure 4. Percentage of study subjects with pure-tone thresholds above the 95th percentile for the 0.5, 1, 2, 4
frequencies and PTAs, and above the 90th percentile for the 8 kHz frequency (Germain et al. 2002)
individuals were above the 95th percentile for pure tone threshold, indicating a difficulty in
hearing and hearing loss. PTA represents the pure tone average, the average of the pure
tone thresholds at all of the frequencies tested. All patients also underwent tympanograms
to test middle ear mobility and pressure. Otoacoustic emissions were also tested (sounds
emitted from the inner ear when the cochlea is stimulated by sound, absent when the inner
ear is damaged) (Kemp 2002). The expected results were obtained, as the individuals with
pure tone thresholds above 30 dB from 1 to 4 kilohertz did not have otoacoustic emissions.
Using their previous findings, Germaine et al. (2002) then looked for correlations
between the hearing loss and kidney insufficiency, between hearing loss and
cerebrovascular problems with Fabry disease, and between hearing loss and ventricular
hypertrophy (all other symptoms of Fabry disease). It was found that only 33% of the
individuals had healthy kidneys. In all but one of the patients with normal kidney function,
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there were no hearing abnormalities and pure threshold averages were at normal levels. It
was also found that half of the patients had cerebrovascular disease. The occurrence of
cerebrovascular disease with cochlear problems was significantly more than in those
without cochlear problems. In regards to ventricular hypotrophy, no statistically significant
difference was found between the two groups (Germain et al. 2002).
Each of these three studies focused on genes on a different chromosome to
approach the problem of sensorineural hearing loss. Hearing is a complex process, and it
takes the contribution of many genes to code for just a few small parts like the cochlea and
stereocilia. Thus, mutations in many genes can cause hearing problems. Homozygous
deletions of these genes will be damaging, of course, but heterozygous deletions can be just
as bad due to the presence of recessive mutations or to gene dosage effects. Also,
hemizygous disorders can either directly cause SNHL if they affect a gene that codes for an
auditory organ, such as the stereocilia, or they can indirectly cause SNHL similar to the case
in Fabry disease where it was the deposits of glycolipids that led to hearing loss. Hearing
loss is a serious issue, and studies such as the ones described in this paper are making it
easier to diagnose the causes of hearing loss and to identify susceptible individuals using
mutational screening. Also, by presenting the precise problem caused by these mutations,
these studies all take us one step closer to understanding the human genome.
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Works Cited
Vona B, Hofrichter MAH, Neuner C, Schröder J, Gehrig A, Hennerman JB, Kraus F, ShehataDieler W, Klopocki E, Nanda I, Haaf T (2014) DFNB16 is a frequent cause of
congenital hearing impairment: Implementation of STRC mutation analysis in
routine diagnostics. Clinical Genetics doi: 10.1111/cge. 12332
Perry B, Sebold C, Hasi M, Heard P, Carter E, Hill A, Gelfond J, Hale D, Cody J (2014)
Sensorineural hearing loss in people with deletions of 18q: Hearing in 18q-. Otology
& Neurology 35: 782-786 doi: 10.1097/MAO .0000000000000363
Germain D, Avan P, Chassaing A, Bonfils P (2002) Paatients affected with Fabry Disease
have an increased incidence of progressive hearing loss and sudden deafness: an
investigation of twenty-two hemizygous male patients. BMC Medical Genetics 3 doi:
10.1186/1471-2350-3-10
Hain T (2014, July 26) Conductive Hearing Loss and Middle Ear Disease. Retrieved from
http://www.dizziness-and-balance.com/disorders/hearing/conductive.htm
National Institute of Health (2014) Sensorineural deafness. Medline Plus Retrieved from
http://www.nlm.nih.gov/medlineplus/ency/article/003291.htm
National Institute of Health (2012) Hearing and the cochlea. Medline Plus Retrieved from
http://www.nlm.nih.gov/medlineplus/ency/anatomyvideos/000063.htm
Virtual Medical Centre (2007) Ear. Retrieved from http://www.myvmc.com/anatomy/ear/
Kemp DT (2002) Otoacoustic emissions, their origin in cochlear function, and use
[Abstract]. British Medical Bulletin 63: 223-41
Cody JD, Ghidoni PD, DuPont BR, et al. (1999) Congenital anomalies and anthropometry of
42 individuals with deletions of chromosome 18q. Am J Med Genet;85:455Y62.
Xing Y, Samuvel DJ, Stevens SM, et al. (2012) Age-related changes of myelin basic protein in
mouse and human auditory nerve. PLoS One;7:e34500.
Sweeley CC, Klionsky B (1963): Fabry's disease: classification as a sphingolipidosis and
partial characterization of a novel glycolipid. Journal of Biological
Chemistry, 238:3148-3150.
Germain D (2002): Fabry disease (α-galactosidase A deficiency) : Pathophysiology, clinical
signs and genetics aspects. Journal de la Societe de Biologie, 196:161-173.