People with schizophrenia have a genetic propensity to smoke, a key risk factor for heart disease, finds a study.

Schizophrenia is known to increase the risk of cardiovascular disease (CVD).

The study, published in The American Journal of Psychiatry, revealed genetic overlap between schizophrenia and CVD risk factors, particularly body mass index (BMI) and smoking. 

The genetic overlap between schizophrenia and smoking behaviour means that people with schizophrenia may be more affected by nicotine's addictive properties, the researchers said.

"In particular, patients with schizophrenia experience greater reinforcing effects of nicotine and more severe withdrawal symptoms during abstinence," said Linn Rodevand, from the Norwegian Center for Mental Disorders Research at the University of Oslo.

The research team analysed recent genome-wide association study (GWAS) results to estimate the number of shared genetic variants and pinpoint specific shared locations.

Extensive genetic overlap was found between schizophrenia and CVD risk factors, particularly smoking initiation and BMI. 

Several specific shared locations were also found between schizophrenia and waist-to-hip ratio, systolic and diastolic blood pressure, type 2 diabetes, lipids, and coronary artery disease.

In line with previous evidence of a higher prevalence of low BMI before the onset of schizophrenia, the study results also indicate that people with schizophrenia are genetically predisposed to lower BMI. 

However, obesity is also more common in individuals with schizophrenia than those in the general population.

The findings indicate that factors other than common genetic variants play an important role in weight gain in schizophrenia, including adverse effects of antipsychotics and symptoms, depression, and socioeconomic challenges that contribute to unhealthy lifestyles.

In addition, genetic factors likely play an important role in antipsychotic-induced weight gain. The overlapping locations between schizophrenia and lipids, blood pressure, waist-to-hip ratio, type 2 diabetes, and coronary artery disease had mixed effect directions. 

This means that half of the genetic variants influencing schizophrenia were associated with increased cardiovascular disease risk, while the other half were associated with reduced cardiovascular disease risk, the researchers said.

This may suggest that subgroups of people with schizophrenia vary in their genetic vulnerability to CVD, which can underlie some of the differences in CVD comorbidity, the team said.