In 2000 scientists at the University of Utah discovered a family of early risers who typically slept from around five at night to two in the morning. The condition, dubbed familial advanced sleep phase syndrome (FASPS), has allowed researchers studying circadian rhythms to understand how the human body clock works, which could pave the way for future therapies aimed at seasonal affective disorder, jet lag and insomnia.
Now, a new study by a team out of the University of California, San Francisco, which includes members of the group that initially identified FASPS, has determined the operational mechanism by which the gene Per2 is implicated in adjusting the body clock's response to light. Their findings, published in this week's issue of Cell, state that the replacement of one amino acid from among hundreds found in a protein can result in irregular sleep patterns.
"A single amino acid change from serine to glycine, that's enough for all these people who have this mutation to have FASPS," says neurologist and study co-author Ying-Hui Fu.
Based on lab tests of cells from the Utah family, the researchers believe that a single point mutation in Per2 results in the replacement of serine with glycine during transcription. This substitution then prevents a still unknown enzyme from adding a phosphate molecule onto the absent serine, which kicks off a domino effect resulting in lower overall transcription from DNA to mRNA (messenger RNA). This decreased mRNA, in turn, leads to lower amounts of protein when RNA is translated. "The message of this gene doesn't get transmitted appropriately," explains Fu," and therefore the protein level is low."
Previous studies had hinted that Per2's effect on circadian rhythm was an issue of protein stability, a defect that would have been caused during or after translation. "The biggest surprise in this [study] was the change in messenger RNA as opposed to protein stability," notes University of Utah cancer and circadian rhythm specialist David Virshup, who adds that the new paper conflicts with the results of a study published late last year in Genes & Development. Choogon Lee, a biomedical scientist at Florida State University, believes that post-translational effects are definitely not absent based on the raw data. He notes that the decrease in messenger RNA is only 30 percent in the Cell study, but that this results in a protein decrease of 80 to 90 percent. "So, it's not just the transcription decrease," he says. "There must be some protein-stability effect, too."
The San Francisco team, with the aid of University of Utah neurologist Christopher Jones, also studied the effects of Per2 mutation in mice. The scientists added a mutated copy of the Per2 gene to mice that would change the 662nd amino acid in the PER2 protein from a serine to glycine. This triggered a shift in the animals' circadian period, causing the mice to go to sleep nearly two hours earlier than before. When the researchers deleted the natural Per2 genes before inserting the mutated gene, the mice slept and woke nearly four hours earlier than before. "The mutation has a dominant effect over the endogenous Per2, so without endogenous Per2, interference, the phenotype is worse," Fu says. When the researchers inserted a copy of the gene that mimicked the presence of serine, the circadian period lengthened, which, Fu says, "really tells you that this amino acid plays a really critical role. It's almost like a dial in your cell that can turn your [period] short or long."
David Weaver, a neurobiologist at the University of Massachusetts Medical School says he found the new study's findings to be quite surprising. "While the authors can't completely exclude a contribution of post-translational effects on PER2 protein levels," he explains, "their data indicate it has quite an important effect on Per2 transcript levels that I would not have anticipated." He also points out that "while the population affected by FASPS is relatively small, the lessons learned and the mouse models generated will likely be useful in developing methods for resetting the circadian clock."
Fu says that her group is working to develop therapies for modulating the human body clock, irregularities of which are associated with everything from insomnia to seasonal affective disorder and cancer. "We can use our mouse model to screen compounds to see which one can regulate period," Fu says, "to make it normal again."
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