Genetic factors are likely to influence clinical variation in schizophrenia, but it is unclear which variables are most suitable as phenotypes and which molecular genetic loci are involved. We evaluated clinical variable phenotypes and applied suitable phenotypes in genome-wide covariate linkage analysis. We ascertained 170 affected relative pairs (168 sibling-pairs and two avuncular pairs) with DSM-IV schizophrenia or schizoaffective disorder from the United Kingdom. We defined psychotic symptom dimensions, age at onset (AAO), and illness course using the OPCRIT checklist. We evaluated phenotypes using within sibling-pair correlations and applied suitable phenotypes in multipoint covariate linkage analysis based on 372 microsatellite markers at $10 cM intervals. The statistical significance of linkage results was assessed by simulation. The positive and disorganized symptom dimensions, AAO, and illness course qualified as suitable phenotypes. There were no genome-wide significant linkage results. There was suggestive evidence of linkage for the positive dimension on chromosomes 2q32, 10q26, and 20q12; the disorganized dimension on 8p21 and 17q21; and illness course on 2q33 and 22q11. The linkage peak for disorganization on 17q21 remained suggestive after correction for multiple testing. To our knowledge, this is the first study to integrate phenotype evaluation and genome-wide covariate linkage analysis for symptom dimensions and illness history variables in sibling-pairs with schizophrenia. The significant within-pair correlations strengthen the evidence that some clinical variables within schizophrenia are suitable phenotypes for molecular genetic investigations. At present there are no genome-wide significant linkage results for these phenotypes, but a number of suggestive findings warrant further investigation.
- age at onset