Identification of Novel Circadian Biomarkers

Circadian clocks are not only found in discrete areas of the brain, but are found in virtually every organ in our bodies, including the heart, lungs and immune system. Disruptions in circadian clocks, or chronopathology, may underlie various forms of cardiovascular, pulmonary, and metabolic disorders. Over the past two decades, molecular geneticists have "cracked" the clock to reveal its core biochemical mechanisms evident in organisms from fruit flies to humans. These mechanistic insights have led to the discovery of links between clock function and an ever-expanding array of prevalent diseases, including heart, lung, metabolic and sleep disorders. Yet the prevalence of circadian disruption in these patient populations is unclear because current tests are not easily applied in clinical settings or have yet to be developed. Perhaps the major limitation of these techniques is the need for serial sampling over extended periods of at least 24 hours and in some cases longer. The development of an assay from a single blood draw would represent a major step forward, facilitating assessments of circadian disruption in a range of diseases.

An alternative strategy to existing assays is to use genomic microarrays to analyze circadian rhythms. Many studies in a number of organisms as well as multiple organs and tissues have found that substantial fractions of the genome (2-10%) are under robust circadian clock control. Importantly, these hundreds of rhythmic genes exhibit expression peaks at all times throughout the day, presumably reflecting their time-of-day specific functions. Using this as a foundation, Ueda and colleagues proposed an alternative strategy that would allow assessment of circadian time from a single blood draw allowing more routine assessments of circadian clock state. In brief, they identified the complement of rhythmic genes in livers of mice. They then selected a set of approximately 50 genes with unique peak times as "time-indicating genes." They then assessed the transcript levels of these time-indicating genes at a single time of day and found that they could accurately determine the time of day that the liver was taken based on the relative expression levels of the time-indicating genes. These studies provide proof-of-principle for the approach that we propose here. Establishing a molecular assay in humans for circadian rhythms from a single time point will allow us to identify circadian rhythm disorders, and to assess internal biological time to deliver therapies at their most efficacious time.