Methylation sets the rhythm

The research group of Prof. Steven Brown at the Institute of Pharmacology & Toxicology, has shown for the first time a direct connection between circadian behaviour and plastic DNA methylation. In their recent Nature Neuroscience paper, Prof. Brown’s group studied sleep-wake rhythms in mammals. While sleep-wake rhythms are largely genetically determined, they are also known to be influenced by external factors such as sunlight. The master clock tissue regulating circadian behaviour in mammals, which is located in the suprachiasmatic nucleus (SCN), was the focus of Brown’s research.

In their experiments, the researchers exposed mice to different day lengths (i.e. 22, 24 and 26 h days) for 6 weeks to align their sleep-wake rhythms to an external rhythm; the so-called entraining phase. The mice were subsequently kept in complete darkness for 10 days with no external factors to influence  their circadian behaviour. Brown and colleagues showed that in these entrained mice their sleep-wake rhythms were stable for months (i.e. mice entrained for 22 h days kept the 22 h day rhythm, and those entrained for 24 h days stayed in the 24 h rhythm).

Direct connection between DNA methylation and circadian behaviour

The research group then moved on to look for differences in the SCN between the groups of entrained mice at the molecular level. Interestingly, the genes involved in the circadian rhythm were differently regulated between mice entrained for short days compared to those accustomed to longer days. Over 1,200 regions were found to be differentially methylated when comparing the 22 h day entrained mice with the 24 h day mice. Many of these differentially regulated genes are involved in neurophysiological processes including synaptogenesis, axonal guidance and neurohormone signalling. The methylation effects were reversible after re-entrainment of mice to 24 h days, showing that the changes are plastic.

A subsequent experiment proved a direct connection between DNA methylation and circadian behaviour. The SCN of mice were infused with Zebularine, a blocker of DNA methylation. The results showed that the circadian behaviour of these mice hardly changed with new entrainment, indicating that methylation is necessary to modify the genetically determined length of the circadian oscillator.

Circadian behavior is light-reprogrammed by plastic DNA methylation.
Abdelhalim Azzi, Robert Dallmann, Alison Casserly, Hubert Rehrauer, Andrea Patrignani, Bert Maier, Achim Kramer & Steven A Brown. Nat Neurosci. 2014 Mar;17(3):377-82 PubMed abstract