Objective: The genetic complexity of schizophrenia may be compounded by the diagnostic imprecision inherent in distinguishing schizophrenia from closely related mood and substance use disorders. Further complexity may arise from studying genetically and/or environmentally diverse ethnic groups. Reported here are the ascertainment, demographic features and clinical characteristics, of a diagnostically and ethnically homogeneous schizophrenia pedigree sample from Tamil Nadu, India. Also reported is the theoretical power to detect genetic linkage in the subset of affected sibling pairs. Method: Affected sibling pair and trio pedigrees were identified by caste/ethnicity. Affected probands and siblings fulfilled DSM-IV criteria for schizophrenia or schizoaffective disorder. Results: The present sample consisted of 159 affected sibling pairs and 187 parent–offspring trios originating primarily from the Tamil Brahmin caste, but also incorporating pedigrees from genetically similar, geographically proximal caste groups. Consistent with previous studies in Tamil Nadu, a very low prevalence of affective psychoses such as schizoaffective disorder, was observed, with most affected individuals having schizophrenia (499/504). Also observed were extremely low rates of nicotine (12.4%), alcohol (1.1%) and illicit drug use (0%). Most affected individuals exhibited negative symptoms (>90%) and a severe, chronic course. All participants lived in the same geographic and climatic region and most affected individuals resided with close family members, increasing uniformity of the sociocultural environment. In affected sibling pairs, power to detect linkage to small-effect risk loci was modest, but this homogeneous sample may be enriched for loci of larger effect. Conclusions: This Indian schizophrenia sample exhibits diagnostic and ethnic homogeneity with high consistency of sociocultural environmental features. These characteristics should assist efforts to identify risk genes for schizophrenia.
Australian and New Zealand Journal of Psychiatry Vol. 43, Issue 6, p. 561-570