April 21, 2000
Gene Mutation Upsets Mammalian Biological Clock
Researchers have pinpointed the cause of a genetic mutation that
switches a hamster's biological clock to a 20-hour day from the normal
24-hour day.
In the April 21, 2000, issue of the journal Science, Joseph
S. Takahashi, a Howard Hughes Medical Institute investigator at
Northwestern University, and his colleagues report that they have
identified the enzyme encoded by the tau gene, the first
single-gene circadian mutation to be discovered in mammals. In 1988,
researchers first described the tau gene in Syrian hamsters that
exhibited a shorter-than-normal biological clock.
“The tau mutant, arguably, has been one of the most significant genetic animal models for the study of circadian rhythms in mammals.”
Joseph S. Takahashi
"The tau mutant, arguably, has been one of the most
significant genetic animal models for the study of circadian rhythms in
mammals," said Takahashi. Identifying the cause of the tau
mutation offers researchers a new tool for understanding biological
clocks in humans, as well as a potential target for drugs that control
the biological clock.
"With the cloning of tau the supply of mammalian clock
mutants has been exhausted for the moment, but there is little doubt
that many of the critical elements of animal clocks have been
identified," writes Michael W. Young of The Rockefeller University in
an editorial that appears in the same issue of Science.
Most biological clocks operate on a 24-hour, or circadian (Latin for
"about a day"), cycle that governs such functions as sleeping and
waking, rest and activity, fluid balance, body temperature, cardiac
output, oxygen consumption and endocrine gland secretion. In mammals,
the main components of the circadian clock are found in cells in the
brain. Inside these cells, the molecular components of the clock are
"rewound" daily by the effects of light and other stimuli.
Takahashi and his Northwestern colleagues, Phillip L. Lowrey,
Kazuhiro Shimomura, Marina P. Antoch, and Peter Zemenides, with Shin
Yamazaki and Michael Menaker at the University of Virginia, and Martin
R. Ralph at the University of Toronto, used genetic and biochemical
techniques to find the enzyme altered by the tau mutation.
"The discovery of the tau mutation by Ralph and Menaker more
than a decade ago was extremely important because it was the first
mutation shown to alter circadian rhythm in a mammal," said Takahashi.
"The major problem in identifying the gene underlying this mutation was
that it was found in Syrian hamsters, which were not among the model
organisms addressed in the Human Genome Project." Thus, said Takahashi,
genetic data and analytical techniques available for studies of mice
and humans were not available for hamster studies.
To overcome these obstacles, Takahashi and his colleagues sought
first to identify a wild-type Syrian hamster strain that was
genetically distinct from all other Syrian hamsters in captivity.
Syrian hamsters in captivity are the offspring of hamsters originally
captured in 1929.
Once they found a wild-type hamster strain from a second capture
made in 1971, they used a genetic subtraction method called genetically
directed representational difference analysis to make detailed
comparisons of the genes of the two strains of hamsters and offspring
that resulted from crosses between the two strains.
Such comparisons enabled the researchers to identify specific
segments of DNA associated with the tau locus. Using these DNA
fragments, the scientists then isolated from collections of hamster
genes larger DNA sequences that they compared to mouse and human genes.
These comparisons showed that the hamster tau gene codes for
CKIe (casein kinase I epsilon)— a type of enzyme that had never
before been associated with the machinery of the mammalian circadian
clock.
Interestingly enough, however, Michael W. Young and colleagues at
The Rockefeller University found that the Drosophila circadian
mutation double-time is also encoded by a casein kinase I that
is most similar to the epsilon form of mammalian CKI.
"Our results, from both genetic linkage analysis and molecular
analysis of the specific gene mutation, provide definitive evidence
that CKIe is a component of the mammalian circadian clock," said
Takahashi.
The researchers then set out to find how the single amino acid
substitutions in CKIe could shorten circadian rhythm. They found that
subtle structural changes introduced by the substituted amino acid
altered the enzyme's ability to function as a biochemical switch. The
mutation rendered the enzyme slower at switching on proteins produced
by a key circadian rhythm gene, called PERIOD, said Takahashi.
The regular rise and fall in levels of these circadian proteins governs
the length of each cycle of the biological clock. The alteration in
CKIe effectively changes the animals' circadian rhythm from 24 to 20
hours.
According to Takahashi, the discovery of the CKIe gene's role
in circadian rhythms offers an unprecedented opportunity for developing
drugs to control the biological clock in humans.
"We now know that there are nine genes governing circadian rhythms,
eight of which code for DNA transcription factors or transcriptional
regulators. CKIe is the only gene that codes for an enzyme,
which is a lot easier to use as a drug target. Such drugs could
conceivably shift a person's biological clock, enabling that person to
more readily adapt to changing schedules due to travel or shift work,
for example."
More speculative, said Takahashi, is the idea that drugs that
control circadian rhythms could be used to treat either seasonal
affective disorder— a depression caused by less natural light in
winter— or psychiatric disorders such as manic depression that
seem to be associated with sleep disorders.
"The discovery of CKIe will also allow us to study the
kinetic features of the circadian rhythm system," added Takahashi.
"Right now, we can draw these nice diagrams of how the system works,
but we still don't have any sense of the rates of the process. We now
have CKIe as a target regulatory enzyme that we can study
further to study what makes the clock go faster or slower in
mammals."
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