Cannabis is the most commonly used illicit drug with particularly high rates of use among young people. The question of whether cannabis is harmful remains the subject of heated debate. The rapidly changing attitudes towards the legalization of cannabis for medical and recreational purposes add urgency to the need for scientific answers. Adolescence is of particular concern because use in this period has been associated with altered brain development, long-term health effects, and dramatically higher odds for meeting DSM-IV criteria for cannabis use dependence.

Two particular factors limit inferences from current research on the health effects of adolescent cannabis use. First, as most findings involve cross-sectional studies, it is unclear whether reported associations reflect the effect of cannabis or preexisting differences between users and nonusers. Thus, we need prospective, longitudinal studies that begin in adolescence and continue into adulthood, and where cannabis use data has been collected before and after initiation. Second, current markers for cannabis exposure in peripheral tissues only indicate whether it has recently been used. Instead, there is a need for biomarkers that reflect its biological impact and can predict the risk for cannabis dependence and negative health outcomes. Methylation marks are ideal for this purpose as they involve the stable methyl-cytosine bond that can be measured cost-effectively in easy to collect genomic (histone-free) DNA.

To better understand cannabis use and its health effects, we propose the use of already collected samples from the Great Smoky Mountains Study (GSMS). The GSMS started 25 years ago with children aged 9 to 13 and is still ongoing. Both cannabis use information and blood samples were obtained at two-year intervals for the same subjects. Using data and blood samples from before and after initiation, this allows us to control for preexisting differences between user groups and link cannabis induced biological changes to health outcomes later in life. To develop new biomarkers, we will first assay the methylation status of all 28 million common CpGs and study the impact of regular cannabis use on the methylome. Next, we will use the methylation data to predict cannabis use trajectories as well as effects of cannabis on short and long-term health outcomes. The most promising methylation sites will be followed up using a targeted assay in samples from the Avon Longitudinal Study of Children and Parents (ALSPAC).

Successful completion of this research will yield replicable methylation signatures of regular cannabis use as well as biomarkers that predict risk of dependence and health outcomes. Our results will inform public health policies and become part of algorithms that can be used to improve prognosis, diagnosis, and treatment.