There is evidence of a link between cannabis use and schizophrenia, but it is unclear whether the drug causes this disorder or vice versa. A new study released Monday, which relies in part on genetic data from 23andMe volunteers, might provide some clarity about this relationship. It turns out that people who are genetically at risk of schizophrenia also start smoking, suggesting that the disorder may cause cannabis use in some people.
The current study, published in Nature Neuroscience, is a continuation of earlier efforts to study the genetic variations that cause people to consume cannabis, a project known as the International Cannabis Consortium. The authors of the study, which includes some researchers from the DNA testing company 23andMe, studied anonymised genetic data from past or ongoing studies, such as the UK Biobank, as well as those who provided their DNA for research purposes 23andMe registered? Overall, they viewed more than 1
A person's genetic code may be slightly different in many ways from other people, but the most common variant is a single nucleotide polymorphism or SNP. An SNP is a tiny change in the building blocks that make up DNA (and RNA), known as nucleotides. For example, in a particular section of DNA, most people might have adenine (A), one of the four nucleobases that make up a nucleotide, but others might instead have cytosine (C).
In the study, the researchers found eight of these SNPs that were associated with lifelong cannabis use. Overall, they calculated that these differences accounted for 11 percent of the difference in whether someone reported smoking or not.
Using various assays, they also found 35 genes in 16 different sections of the genome associated with cannabis use. Many of these genes appear to be associated with other habits, personality traits, and mental health conditions, particularly the CADM2 gene. Variations in CADM2, the authors said, have already been associated with more risk, more alcohol consumption, and personality traits such as extraversion. They also found a genetic overlap with schizophrenia.
"That's not a big surprise, because previous studies have often shown that cannabis use and schizophrenia are associated," author Jacqueline Vink, a researcher at Radboud University in London, told The Netherlands, said in a statement. "However, we also investigated whether this association is causal."
They attempted to find a possible cause-and-effect relationship by using a method called Mendelian randomization. With this technique, geneticists can ask if the known genes for one thing (schizophrenia, in this study) predispose you directly to another thing (marijuana). In this case, they found evidence that people who are genetically prone to schizophrenia were more likely to use the pot, possibly to manage their condition, the authors said.
This result is particularly important because we still do not really understand how cannabis and schizophrenia are related. Other studies have shown that the use of kippers themselves increases the risk of schizophrenia, especially if they are started at an early age by people who are already at risk of mental illness. The authors carefully point out that their only study does not refute this theory, but it suggests how the other genetic studies show that the relationship is complicated.
The researchers now want to investigate if there are specific genes predicting a more frequent or stronger use of cannabis.