BioWorld International Correspondent

LONDON - It is possible to monitor the circadian clock of humans using skin cells in culture, a study in Switzerland has shown. The method makes it possible to distinguish the behavior of cells from people with very different daily behaviors - whether early-rising "larks" or late-to-bed "owls."

The discovery will allow ready analysis of the genes involved in controlling the human circadian clock, as well as assessment of new therapies to treat sleep disorders and possibly some types of mental illness.

Steven Brown, section leader of the Chronobiology and Sleep Research Group at the University of Zurich, told BioWorld International: "This finding means we can find the molecular bases for why human beings show different daily behaviors, simply using skin cells. Skin cells do not determine behavior, but they contain the same molecular mechanisms as the neurons that do, and that makes them quite useful."

The technique used is important, he added, because understanding what is wrong in different people is the first step toward designing good therapies.

An account of the study appeared in the Jan. 28, 2008, issue of the Proceedings of the National Academy of Sciences, in a paper titled "Molecular insights into human daily behaviour."

In humans, the "central clock," located in the suprachiasmatic nucleus of the brain, governs the pattern of daily behavior. Recent discoveries also have shown that virtually every cell in the body retains the molecular clock mechanism that is present in the suprachiasmatic nucleus.

Brown likens the suprachiasmatic nucleus to the atomic clock, by which everyone else (in this case, the cells of the body) sets the time of their watches.

Brown and his colleagues already had shown that they could see differences in the responses to light of cells from mice that had genetically engineered mutations affecting their circadian clocks.

The team hypothesized that they would be able to do the same with skin cells from humans with divergent daily behavior.

To obtain their study subjects, the researchers advertised on television at between 3 a.m. and 4 a.m., to catch both those who stay up late and those who rise exceptionally early. Subjects completed a questionnaire designed to assess their "chronotype," and then had a skin biopsy.

The researchers grew the fibroblasts from the skin biopsies, and then infected the cells with a genetically modified nonreplicating lentivirus, which inserted a reporter gene that was under the control of a circadian clock promoter gene into the genomes of the cells. As a result, whenever the cellular clock genes begin to express their products, the promoter sequence would trigger expression of the reporter gene.

For the study, the reporter gene was the firefly luciferase gene. Measurements showed that the cells would glow and become extinguished, with a period that reflected the type of behavior of the person from which they had come.

"We found that these genetically engineered peripheral cells acted as a very good proxy of the central clock that determines human behaviors," Brown noted.

Thus the period of cells from people who tend to stay up very late was in general greater than 24 hours, while that of the cells from people who go to bed early and get up early was in general less than 24 hours.

Further experiments showed that there were many other causes for the tendency of the cells' clocks to run fast or slow. For example, if the amplitude of the oscillation was very high, those cells were much more resistant to signals to change (such as different light and dark phases).

Brown said: "Although we did not investigate this aspect in this study, different amplitudes - which translates into propensity for change - may explain why some people seem to be more susceptible to jet lag than others. Jet lag probably relates to the ability of a person's internal clock to reset time after a change in time zones."

The team also was able to show that cells from different people had different sensitivity to input stimuli (such as light and dark phases).

For the first time, Brown concluded, it now will be possible to ask what are the molecular differences between the clock mechanisms of different individuals.

"This technique also gives us the ability to diagnose different kinds of chronobiological syndromes on the molecular level, and provides us with an easy system in which any potential treatments can be tested," Brown said.

He plans further investigations of the molecular differences between cells from individuals of different behaviors, including those from people who suffer from different types of depression. The team also wants to carry out genetic linkage and genomewide association studies to detect variations in DNA that might be responsible for different chronotypes.