There’s evidence of a connection between cannabis use and schizophrenia, but it’s unclear whether the drug leads to the disorder, or vice versa. A new study published today, which relies partly on genetic data from 23andMe volunteers, might offer a little clarity on that link. It found that people genetically at risk of schizophrenia are also more likely to start smoking marijuana, suggesting the disorder itself might cause cannabis use in some people.
The current study, published in Nature Neuroscience, is a continuation of previous efforts to sketch out the genetic variations that make people more likely to start using cannabis, a project known as the International Cannabis Consortium. The study authors, which include some researchers from DNA test company 23andMe, studied anonymised genetic data taken from previous or ongoing studies, such as the UK Biobank, as well as from people who have permitted their DNA to be used for research, such as those who signed up for genetic testing from 23andMe. Overall, they looked at more than 180,000 people, making this the largest study of its kind, according to the authors.
A person’s genetic code can differ slightly from someone else’s in lots of ways, but the most common variation is called a single-nucleotide polymorphism, or SNP. A SNP is a minute change in the building blocks that make up DNA (and RNA), known as nucleotides. So in one specific section of DNA, for example, most people might have adenine (A), one of the four nucleobases that make up a nucleotide, but others might have cytosine (C) instead.
In the study, the researchers found eight of these SNPs that were associated with lifetime cannabis use. Taken as a whole, they calculated, these variations accounted for 11 per cent of the difference in whether someone reported smoking marijuana or not.
Using different tests, they also found 35 genes in 16 different sections across the genome that were associated with cannabis use. Many of these genes seemed to be associated with other habits, personality traits, and mental health conditions, particularly the gene CADM2. Variations in CADM2, the authors noted, have already been linked to taking more risks, greater alcohol use, and personality traits such as extroversion. They also found a genetic overlap with schizophrenia.
“That is not a big surprise, because previous studies have often shown that cannabis use and schizophrenia are associated with each other,” lead author Jacqueline Vink, a researcher at Radboud University in the Netherlands, said in a statement. “However, we also studied whether this association is causal.”
They attempted to find a possible cause-and-effect relationship using a method called Mendelian randomisation. This technique lets geneticists ask whether having the known genes for one thing (schizophrenia, in this study) directly predisposes you to another thing (using marijuana). In this case, they found evidence that being genetically vulnerable to schizophrenia made people more likely to use marijuana, possibly as a way to cope with their condition, according to the authors.
This finding in particular is important because we still don’t really understand how cannabis and schizophrenia are tied to one another. Other research has found that marijuana use itself raises the risk of schizophrenia, especially if begun at an early age by people already at risk of mental illness. The authors are careful to point out their single study doesn’t disprove that theory, but it does suggest, as other genetic studies have, that the relationship is complicated.
The researchers next plan to study if there are specific genes that can predict more frequent or heavier use of cannabis. [Nature Neuroscience]