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03.21.2022
Researchers at the University of California, Berkeley, have found
that a drug once widely used to wean alcoholics off of drinking helps to
improve sight in mice with retinal degeneration.
The drug may revive sight in humans with the inherited disease retinitis
pigmentosa, and perhaps in other vision disorders, including
age-related macular degeneration.
A group of scientists led by Richard Kramer
UC Berkeley professor of molecular and cell biology, had previously
shown that a chemical — retinoic acid — is produced when light-sensing
cells in the retina, called rods and cones, gradually die off. This
chemical causes hyperactivity in retinal ganglion cells, which
ordinarily send visual information to the brain. The hyperactivity
interferes with their encoding and transfer of information, obscuring
vision.
He realized, however, that the drug disulfiram — also called Antabuse —
inhibits not only enzymes involved in the body’s ability to degrade
alcohol, but also enzymes that make retinoic acid. In new experiments,
Kramer and collaborator Michael Goard, who directs a lab at UC Santa
Barbara (UCSB), discovered that treatment with disulfiram decreased the
production of retinoic acid and made nearly-blind mice much better at
detecting images displayed on a computer screen.
Kramer suspects that retinoic acid plays an identical role in people
with vision loss. But experiments measuring retinoic acid in the eye
have not been done on humans because they would be too invasive.
Disulfiram — which is already approved for use by the Food and Drug Administration (FDA) —could establish that link. The researchers are planning to partner with ophthalmologists to conduct a clinical trial of disulfiram on patients with retinitis pigmentosa. The trial would be carried out on a small set of people with advanced, but not yet complete, retinal degeneration.
"There may be a long window of opportunity in which suppressing retinoic
acid with drugs like disulfiram could substantially improve low vision
and make a real difference in people’s quality of life," said Kramer,
the CH and Annie Li Chair in Molecular Biology of Diseases at UC
Berkeley and a member of the campus’s Helen Wills Neuroscience
Institute. "Because the drug is already FDA-approved, the regulatory
hurdles are low. It wouldn’t be a permanent cure, but right now there
are no available treatments that even temporarily improve vision."
Kramer, Goard and their colleagues — Michael Telias, a former UC
Berkeley postdoctoral fellow now at the University of Rochester Medical
Center, and Kevin Sit of UCSB — will publish their findings March 18 in
the journal Science Advances.
Kramer acknowledged that disulfiram may not be for everyone. When
combined with alcohol consumption, the drug can have severe side
effects, including headache, nausea, muscle cramps and flushing.
"If you’re on the drug, and you backslide and take a drink, you will
immediately get the worst hangover of your life," he said, “and that is
what makes it a strong deterrent for drinking alcohol."
But if disulfiram can improve vision, more targeted therapies could be
sought that don't interfere with alcohol breakdown or other metabolic
functions. The researchers have already tested an experimental drug
named BMS 493 that inhibits the receptor for retinoic acid, and they
have also used an RNA interference technique — a type of gene therapy —
to knock down the receptor. Both of these procedures also dramatically
improved vision in mice with retinitis pigmentosa.
Photoreceptor breakdown
Three years ago, Kramer and his colleagues reported that
retinoic acid generated sensory noise that interfered with remaining
vision in mice with retinitis pigmentosa in the same way that ringing in the ears, known
as tinnitus, can interfere with hearing in people who are losing
vibration-sensitive cells in the inner ear. They showed that inhibiting
the retinoic acid receptor reduced the noise and increased simple light
avoidance behaviors in those mice.
But do mice treated with the drugs actually see better?
The new study provides evidence that they do. First, when the mice were
young and had healthy retinas, they were trained to recognize and
respond to a simple image of black and white stripes displayed on a
computer screen. A month later, after most of the rods and cones had
degenerated, the image was shown once again. The investigators found
that mice treated with disulfiram or BMS 493 responded quite well, even
if the image was blurry. By contrast, mice receiving a placebo failed to
respond, even if the image was crisp and clear.
In a second type of study, the scientists used a special microscope and a
fluorescent protein indicator to light up and examine the responses of
thousands of cells in the brain to much more complex visual scenes — a
Hollywood movie clip, replayed many times. Individual cells in the
brains of vision-impaired mice with retinitis pigmentosa responded preferentially to
particular frames in the movie, and their responses were much stronger
and more reliable than those of mice that had been treated with
disulfiram or BMS 493.
The responses were so reliable, Kramer said, that the investigators
could deduce which specific scene had triggered the cell’s response, but
only in the mice that had been treated with one of the drugs.
Both the behavioral results and the brain imaging results suggest that the drugs improve vision and not just light detection.
“Treated mice really see better than mice without the drugs. These
particular mice could barely detect images at all at this late stage of
degeneration. I think that that's quite dramatic," Kramer said.
In 2019, Kramer and his team laid out the mechanism behind hyperactivity
caused by degeneration. They found that retinoic acid, which is
well-known as a signal for growth and development in embryos, floods the
retina when photoreceptors — the rods, sensitive to dim light, and the
cones, needed for color vision — die. That's because photoreceptors are
packed with light-sensitive proteins called rhodopsin, which contain
retinaldehyde. When the retinaldehyde can no longer be absorbed by rods
and cones, it is converted to retinoic acid by an enzyme called
retinaldehyde dehydrogenase.
The retinoic acid, in turn, stimulates the retinal ganglion cells by
adhering to retinoic acid receptors. It’s these receptors that make
ganglion cells hyperactive, creating a constant buzz of activity that
submerges the visual scene and prevents the brain from picking out the
signal from noise. Drug developers could seek to prevent this by
developing chemicals to stop production of retinoic acid by
retinaldehyde dehydrogenase, or chemicals that interfere with the
retinoic acid receptor.
"If a vision-impaired human were given disulfiram, and their vision got
better, even a little bit, that would be a great outcome in itself. But
it would also strongly implicate the retinoic acid pathway in vision
loss," Kramer said. "And that would be an important proof of concept
that could drive new drug development and a whole new strategy for
helping to improve vision."
The work was supported by grants awarded to Kramer from the National
Institutes of Health (R01EY024334, P30EY003176) and the Foundation for
Fighting Blindness and to Goard from the National Institutes of Health
(R01NS121919) and National Science Foundation (NeuroNex #1707287).
Co-authors of the study are Telias, Daniel Frozenfar, Benjamin Smith and
Arjit Misra of UC Berkeley and Sit of UC Santa Barbara. Telias and Sit
are co-first authors; Goard and Kramer are co-senior authors.
Source: News Release
University of California - Berkeley
March 18, 2022
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