Contact: Mark Derewicz
University of North Carolina Health Care
UNC researchers find final pieces to the circadian clock puzzle
Sixteen years after scientists found the genes that control the
circadian clock in all cells, the lab of UNC’s Aziz Sancar, M.D.,
Ph.D., discovered the mechanisms responsible for keeping the clock
IMAGE: This is Aziz Sancar, M.D., Ph.D., University of North
Carolina Health Care.
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CHAPEL HILL, NC – Researchers at the UNC School of Medicine have
discovered how two genes – Period and Cryptochrome – keep the
circadian clocks in all human cells in time and in proper rhythm
with the 24-hour day, as well as the seasons. The finding,
published today in the journal Genes and Development, has
implications for the development of drugs for various diseases such
as cancers and diabetes, as well as conditions such as metabolic
syndrome, insomnia, seasonal affective disorder, obesity, and even
“Discovering how these circadian clock genes interact has been a
long-time coming,” said Aziz Sancar, MD, PhD, Sarah Graham Kenan
Professor of Biochemistry and Biophysics and senior author of the
Genes and Development paper. “We’ve known for a while that four
proteins were involved in generating daily rhythmicity but not
exactly what they did. Now we know how the clock is reset in all
cells. So we have a better idea of what to expect if we target
these proteins with therapeutics.”
In all human cells, there are four genes – Cryptochrome, Period,
CLOCK, and BMAL1 – that work in unison to control the cyclical
changes in human physiology, such as blood pressure, body
temperature, and rest-sleep cycles. The way in which these genes
control physiology helps prepare us for the day. This is called the
circadian clock. It keeps us in proper physiological rhythm. When
we try to fast-forward or rewind the natural 24-hour day, such as
when we fly seven time zones away, our circadian clocks don’t let
us off easy; the genes and proteins need time to adjust. Jetlag is
the feeling of our cells “realigning” to their new environment and
the new starting point of a solar day.
Previously, scientists found that CLOCK and BMAL1 work in tandem to
kick start the circadian clock. These genes bind to many other
genes and turn them on to express proteins. This allows cells, such
as brain cells, to behave the way we need them to at the start of a
Specifically, CLOCK and BMAL1 bind to a pair of genes called Period
and Cryptochrome and turn them on to express proteins, which –
after several modifications – wind up suppressing CLOCK and BMAL1
activity. Then, the Period and Cryptochrome proteins are degraded,
allowing for the circadian clock to begin again.
“It’s a feedback loop,” said Sancar, who discovered Cryptochrome in
1998. “The inhibition takes 24 hours. This is why we can see gene
activity go up and then down throughout the day.”
But scientists didn’t know exactly how that gene suppression and
protein degradation happened at the back end. In fact, during
experiments using one compound to stifle Cryptochrome and another
drug to hinder Period, other researchers found inconsistent effects
on the circadian clock, suggesting that Cryptochrome and Period did
not have the same role. Sancar, a member of the UNC Lineberger
Comprehensive Cancer Center who studies DNA repair in addition to
the circadian clock, thought the two genes might have complementary
roles. His team conducted experiments to find out.
Chris Selby, PhD, a research instructor in Sancar’s lab, used two
different kinds of genetics techniques to create the first-ever
cell line that lacked both Cryptochrome and Period. (Each cell has
two versions of each gene. Selby knocked out all four copies.)
Then Rui Ye, PhD, a postdoctoral fellow in Sancar’s lab and first
author of the Genes and Development paper, put Period back into the
new mutant cells. But Period by itself did not inhibit CLOCK-BMAL1;
it actually had no active function inside the cells.
Next, Ye put Cryptochrome alone back into the cell line. He found
that Cryptochrome not only suppressed CLOCK and BMAL1, but it
squashed them indefinitely.
“The Cryptochrome just sat there,” Sancar said. “It wasn’t
degraded. The circadian clock couldn’t restart.”
For the final experiment, Sancar’s team added Period to the cells
with Cryptochrome. As Period’s protein accumulated inside cells,
the scientists could see that it began to remove the Cryptochrome,
as well as CLOCK and BMAL1. This led to the eventual degradation of
Cryptochrome, and then the CLOCK-BMAL1 genes were free to restart
the circadian clock anew to complete the 24-hour cycle.
“What we’ve done is show how the entire clock really works,” Sancar
said. “Now, when we screen for drugs that target these proteins, we
know to expect different outcomes and why we get those outcomes.
Whether it’s for treatment of jetlag or seasonal affective disorder
or for controlling and optimizing cancer treatments, we had to know
exactly how this clock worked.”
Previous to this research, in 2010, Sancar’s lab found that the
level of an enzyme called XPA increased and decreased in synchrony
with the circadian clock’s natural oscillations throughout the day.
Sancar’s team proposed that chemotherapy would be most effective
when XPA is at its lowest level. For humans, that’s late in the
“This means that DNA repair is controlled by the circadian clock,”
Sancar said. “It also means that the circadian clocks in cancer
cells could become targets for cancer drugs in order to make other
therapeutics more effective.”
This research was funded by the National Institutes of Health and
the Science Research Council and Academia Sinica in Taiwan.
Other authors of the Genes and Development paper are UNC
postdoctoral fellows Yi-Ying Chiou, PhD, and Shobban Gaddameedhi,
PhD, and UNC graduate student Irem Ozkan-Dagliyan.
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