Shared Genetic Liability across Systems of Psychiatric and Physical Illness

Epidemiological literature describes widespread psychiatric–physical comorbidity. However, the extent to which observed bivariate risk is transdiagnostic remains uncertain. We apply Genomic SEM and introduce Genomic E-SEM, an exploratory factor-analytic extension of Genomic SEM, here applied to 73 physical outcomes (~1.9 million cases) across eight medical domains. We then modeled latent physical illness factors alongside previously identified psychiatric factors. Thought/psychotic and compulsive factors exhibited few associations with physical illness factors. Conversely, internalizing, neurodevelopmental, and substance-use factors showed substantial genome-wide genetic correlations with all physical illness systems. Multivariate GWAS of a transdiagnostic physical illness factor, defined by 21 physical disorders, identified 27 genomic risk loci and exhibited substantial predictive validity in subsequent PheWAS. This transdiagnostic factor again exhibited substantial genome-wide genetic correlations with internalizing, neurodevelopmental, and substance-use factors. Our results reveal pervasive risk sharing between specific groups of psychiatric and physical conditions and suggest the need for additional nosological frameworks. Using large-scale genetics and Genomic SEM/E-SEM, the study shows broad shared genetic risk between many physical illnesses and internalizing, neurodevelopmental, and substance-use disorders, revealing a transdiagnostic illness factor and cross-cutting disease pathways.