Investigating causality in associations between smoking initiation and schizophrenia using Mendelian randomization

Area of Science:

• Psychiatry
• Genetics
• Epidemiology

Background:

• Smoking is highly prevalent among individuals with schizophrenia.
• The causal relationship between smoking and schizophrenia remains debated, with self-medication and risk factor hypotheses.

Purpose of the Study:

• To investigate the potential causal effect of smoking initiation on schizophrenia risk.
• To examine the potential causal effect of schizophrenia on smoking initiation.

Main Methods:

• Two-sample bi-directional Mendelian randomization.
• Utilized summary-level genome-wide association data from large consortia (Tobacco And Genetics, Psychiatric Genomics).
• Employed inverse-variance weighted fixed-effects and MR Egger regression for analysis and sensitivity testing.

Main Results:

• Initial analysis suggested a causal effect of smoking initiation on schizophrenia risk (OR 1.73), but this was attenuated upon further analysis (OR 1.03).
• Little evidence supported a causal effect of schizophrenia on smoking initiation (OR 1.01).
• Sensitivity analyses indicated no significant pleiotropy.

Conclusions:

• The study found limited evidence for a causal association between smoking initiation and schizophrenia in either direction.
• A causal effect of heavier, lifetime smoking exposure on schizophrenia risk cannot be ruled out.