Reviewing Articles: Identifying the Genetic Predispositions for Cannabis Use Disorders

The Brisbane Longitudinal Twin Study: Pathways to Cannabis Use, Abuse, and Dependence Project – Current Status, Preliminary Results, and Future Directions
Authors: Gillespie, Henders, Davenport, et al. (2013)
Journal: Twin Research and Human Genetics
In this article, the authors describe their approach and preliminary findings from an ongoing longitudinal study of twin and non-twin siblings focused on the potential genetic loci of cannabis use disorders (CUDs). Very little is known about the specific genetic influences on the development of CUDs, with only two genome-wide association studies (GWAS) on the CUD phenotype having been completed to date (Agrawal et al., 2011; Verweij et al., 2012). 
To fill this gap, the authors initiated the Pathways to Cannabis Use, Abuse, and Dependence project using resources from the Brisbane Longitudinal Twin Study (BLTS), which was begun in 1992 to study the development of melanocytic naevi (i.e., “moles”). The BLTS has currently recruited approximately 3,000 subjects, composed of adolescent and young adult twins (monozygotic and dizygotic) and non-twin siblings, and sampled from primary and secondary schools in Brisbane, Australia. The sample is primarily of European-descent, with an average age of 22 years. A combination of computer assisted telephone interview (CATI) and online survey methods were used to collect data on cannabis and other substance use disorders conducted among those likely to have already passed through the peak periods of risk (ages 18-30). Other measures include longitudinal data on neuroticism and psychiatric symptoms, and cross-sectional (soon to be longitudinal) data on cognition, brain imaging, legal and illegal drug use, psychiatric diagnoses, and life events/social support. 
Around 60% of males and 49% of females in the sample had ever used cannabis, with similar proportion using nicotine, and nearly all (>98%) having had used alcohol. Over half of male cannabis users (58%) had a desire to stop or quit using cannabis, with a substantial proportion also endorsing using more than intended (48%) and craving (42%). Females experienced a similar pattern of features, though of reduced proportions. There was strong correlations between monozygotic twins for withdrawal (0.96) and craving (0.75) symptoms. There was substantial prevalence of depression and anxiety in the sample (27% and 18%, respectively). Future plans include modeling empirically based CUD phenotypes based on DSM-VI/V criteria, with subsequent tests for quantitative trait loci for CUD using GWA.

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