First genome-wide significant gene identified for cannabis use disorder risk

What the researchers found

A variant (rs56372821) near the CHRNA2 gene reached genome-wide significance for cannabis use disorder risk (P = 9.31 x 10^-12) and replicated independently (P = 3.27 x 10^-3). Higher genetic loading for cognitive performance variants was associated with decreased CUD risk.

Why it matters

This is the first robustly replicated genetic variant for cannabis use disorder. Understanding the biological pathways involved could lead to targeted prevention or treatment strategies, and the cholinergic connection links cannabis addiction biology to nicotine pathways.

The numbers in context

Discovery: 2,387 cases, 48,985 controls. Replication: 5,501 cases, 301,041 controls. Lead variant P-value: 9.31 x 10^-12. Twin heritability estimates for CUD: 51-70%. Cannabis is the most frequently used illicit substance worldwide; ~1 in 10 users become dependent.

How the study worked

Genome-wide association study of 2,387 CUD cases and 48,985 controls, with replication in 5,501 cases and 301,041 controls. Gene expression analysis and polygenic scoring for cognitive traits were also performed.

What this study cannot tell us

GWAS identifies associations, not mechanisms. The identified variant explains only a small fraction of CUD heritability. European-ancestry populations were overrepresented, limiting generalizability. Environmental factors interacting with genetic risk were not captured.

How to read the evidence

Strong: large sample sizes with independent replication, genome-wide significance threshold, and gene expression validation.

When this study was published

Published in 2019 in Nature Neuroscience.

The bigger picture

Finding that a nicotinic receptor gene is involved in cannabis dependence risk suggests overlapping biology between cannabis and nicotine addiction. The cognitive performance connection also hints that the same genetic factors influencing thinking ability may protect against developing cannabis use disorder.

Questions still open

• How does CHRNA2 expression in the brain specifically increase vulnerability to cannabis dependence? Could medications targeting nicotinic receptors help treat cannabis use disorder? What additional genetic variants contribute to the remaining unexplained heritability?

Common questions

Read the original research

Genome-wide association study implicates CHRNA2 in cannabis use disorder.

Nature neuroscience, 22(7), 1066-1074

Citation

Demontis, Ditte; Rajagopal, Veera Manikandan; Thorgeirsson, Thorgeir E; Als, Thomas D; Grove, Jakob; Leppälä, Kalle; Gudbjartsson, Daniel F; Pallesen, Jonatan; Hjorthøj, Carsten; Reginsson, Gunnar W; Tyrfingsson, Thorarinn; Runarsdottir, Valgerdur; Qvist, Per; Christensen, Jane Hvarregaard; Bybjerg-Grauholm, Jonas; Bækvad-Hansen, Marie; Huckins, Laura M; Stahl, Eli A; Timmermann, Allan; Agerbo, Esben; Hougaard, David M; Werge, Thomas; Mors, Ole; Mortensen, Preben Bo; Nordentoft, Merete; Daly, Mark J; Stefansson, Hreinn; Stefansson, Kari; Nyegaard, Mette; Børglum, Anders D. (2019). Genome-wide association study implicates CHRNA2 in cannabis use disorder.. Nature neuroscience, 22(7), 1066-1074. https://doi.org/10.1038/s41593-019-0416-1

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