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Blind mice provide insight into Usher’s syndrome
Researchers have identified yet another gene responsible
for Usher’s syndrome.
The findings now bring to seven the number of genes so far implicated
in an inherited disease that steals a patient’s sight and
hearing at the same time. As the world of sight and sound slowly
disappear, the patient becomes not only physically dependent but
also psychologically traumatised.
Although there is now no treatment for Usher’s syndrome, the
new research results – from the University of California at
Los Angeles – may help lay the groundwork for an eventual
treatment.
In the July 2003 issue of Nature Genetics (Vol. 34, No. 3, pp313-319)
researchers in the Department of Neurobiology at UCLA’s Jules
Stein Eye Institute reported that they had found a gene –
"SLC4A7" – has a role in the development of Usher’s
syndrome.
The gene appears to encode a protein regulator responsible for maintaining
pH (H+) balance within cells. Mice lacking this regulator develop
blindness and auditory impairment due to degeneration in sensory
receptors in the eye and inner ear – just like that observed
in patients with Ushers.
The protein regulator encoded by the SLC4A7 gene is a sodium bicarbonate
co-transporter known as "NBC3." NBC3’s central function
is in pH regulation, a critical physiological activity that maintains
acidity within strict limits in cells to permit normal functioning.
Consequently, disruption of pH regulation may cause serious dysfunction
within the individual cell and within tissue.
Biology has devised many types of pH regulators; often, cells may
carry a repertoire of alternative regulators depending on specific
physiological requirements. The American work, led by Dean Bok,
PhD, has shown that the visual and auditory systems have a specific
requirement for NBC3; this preference essentially turns out to be
an Achilles heel.
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Usher’s
syndrome named for British ophthalmologist
Usher’s
syndrome, named for the British ophthalmologist Charles Usher,
has been described and characterised clinically since the
late 1800s and early 1900s.
In general there are three types of Ushers syndrome:
Type
1 is characterised by a congenital, severe to profound and
pre-verbal deafness so thatpatients rarely learn to talk;
Type 2 a milder (post-verbal) hearing loss with a later onset
of retinal degeneration;
Type 3 distinguished from type 2 by a rapid and more progressive
hearing loss.
Usher’s
syndrome is diagnosed through a series of tests including
visual field and electroretinogram, retinal examination, auditory
tests and balance tests for patients older than ten years.
Usher’s is an inherited genetic disease for which devising
a treatment begins with identifying the genes at fault. Several
regions of the human genome have been associated with the
disease, and to date, mutations in seven distinct genes are
known to cause Usher’s syndrome.
Ushers is classified as an autosomal recessive disorder; as
such, an individual must receive a faulty copy of the gene
from both parents for the disease to occur.
At present, no genetic testing for Ushers is available outside
laboratory-based programmes conducted by academic researchers
in the United States and Europe.
In the United States, the prevalence of Usher’s syndrome
is estimated to be 4.4 per 100,000 of population.
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Whereas
other tissue systems may use their repertoire of alternatives as
a back-up should one fail, the visual and auditory systems both
malfunction when NBC3 malfunctions. Without access to a back up,
the visual and auditory tissues become affected while the other
organ systems appear wholly unconcerned.
The gene SLC4A7, which produces NBC3, has been shown to function
in several tissues including testis, spleen, ovary, small intestine,
heart and muscle. However, when SLC4A7 is damaged, these tissues
operate normally – only the visual and auditory systems are
affected.
Dr. Bok’s team was able to demonstrate this phenomenon by
disrupting the SLC4A7 gene in mice. Mice without a correct copy
of the SLC4A7 gene grew and developed normally; however, by six
months of age, these mice had suffered severe degeneration in the
photoreceptors of their retina and showed impaired auditory responses.
The SLC4A7 deficient mice also provide the first documented example
of inner ear hair cell degeneration due to the loss of function
of a pH regulator.
The researchers achieved their results using what is known as a
targeted genetics approach. An artificial SLC4A7 gene was built
in the lab but constructed in such a way as to be non-functioning
when implanted into the mouse. This construct was then delivered
into mouse embryonic stem cells using an electrical pulse.
By a process of homologous recombination, the artificial gene built
in the lab was then able to insert itself into the genome of the
embryonic stem cells, trading places with the normal gene. After
careful analysis of the embryonic stem cells to determine which
ones carried the artificial gene, positive stem cells were then
injected into a blastocyst which, once implanted into recipient
female mice, would give birth to pups carrying the artificial gene.
These animals could not now produce any of the NBC3 protein regulators
and therefore mimicked the symptoms of Usher’s syndrome as
observed in humans.
The researchers then proceeded to compare mice with NBC3 versus
mice without NBC3. At four months of age, a fundus photograph of
animals with and without NBC3 showed that animals missing the protein
regulator had "marked blood vessel attenuation and diffuse
granularity as typically observed in retinitis pigmentosa,"
the researchers wrote.
Examinations of the animals’ retinas further supported the
evidence that without NBC3, serious degeneration was occurring in
ocular tissues. In mice lacking the protein, "the outer segments
[of the retina] were shorter and the outer nuclear layer had become
thinner by approximately one layer of nuclei," the researchers
found.
By six months of age animals lacking the NBC3 protein "had
lost around 60% of outer nuclear layer thickness in the inferior
retina and around 75% of its thickness in the superior hemisphere,"
the researchers said. The researchers also presented electroretinogram
analysis confirming what had been observed histologically.
In terms of hearing dysfunction, Dr. Bok’s team performed
a series of auditory brainstem response tests to record the effect
of a missing NBC3 protein on auditory function in the mice from
which the SLC4A7 gene had been removed.
The brainstem response test revealed a mild auditory impairment,
the reason for which became apparent upon histological analysis
of the inner ear. By the time the mice had become three months old,
the researchers could see severe degeneration and loss of both inner
and outer hair cells in those animals lacking the SLC4A7 gene.
The associated impairment of both auditory and retinal function
suggests that this valuable colony of engineered mice lacking the
SLC4A7 gene represent a new model for Usher syndrome. The development
of animal models in any disease is a significant milestone on the
path to discovering and testing therapies to treat a disorder.
In demonstrating how the lack of NBC3 pH regulator in mice leads
to a pathology similar to Usher’s in humans, the American
researchers have provided a further tool with which to screen Usher
patients for possible mutations in the SLC4A7 gene.
The SLC4A7 gene is located on human chromosome number 3. Such a
discovery makes SLC4A7 a candidate gene for type 2 Usher’s
syndrome, one of three types of the condition. Previous studies
have mapped Type 2 Usher’s to chromosome 3 in a consanguineous
Tunisian family whose members suffered from significant auditory
and retinal impairment.
Undoubtedly, Usher’s syndrome, with seven causative genes
already identified, is a complex disease. And, of course, there
is much more laboratory research and development required before
researchers develop a full understanding of the underlying molecular
biology of Usher’s. Nevertheless, the work of Dr. Bok and
his team represents a significant contribution to the development
of much-needed therapies for a truly tragic condition.
Dean
Bok, PhD
UCLA, Los Angeles,
California, US
bok@jsei.ucla.edu
http://www.neurobio.ucla.edu/webfacul/pbok/pdbframe.html
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