Progress toward discovery of susceptibility genes for bipolar manic-depressive illness and schizophrenia

The inconsistency in linkage results that has bedeviled psychiatric genetics has been observed to occur regularly in common diseases with complex inheritance. Nonetheless, in two such instances--noninsulin-dependent diabetes mellitus (NIDDM) and inflammatory bowel disease (IBD)--susceptibility genes have been discovered based on the follow-ups of linkage findings. In bipolar illness disorder (BPD) and schizophrenia (SZ), there are some linkage reports with replication of other studies similar to the situation in NIDDM and IBD before the successful positional cloning efforts. Two of the regions with linkage reports, BPD and SZ, on the long arms of chromosomes 13 and 22, show linkage to the same markers in both diseases. This lends some plausibility to the hypothesis of some shared genetic predispositions for both disorders. Cytogenetic evidence offers another positional approach to susceptibility genes. The velocardiofacial syndrome is associated with deletions very close to the linkage region on chromosome 22, and with psychiatric manifestations of both BPD and SZ. Endophenotypes of SZ, previously demonstrated to the be heritable, have been found to have chromosomal linkage in at least one study. These include eye-tracking abnormalities linked to 6p, and an abnormality of the P50 cortical evoked potential linked to chromosome 15. Variants in specific genes have been associated with susceptibility to the psychiatric illnesses. These genetic findings may contribute to etiologic subcategorization of BPD and SZ, and the development of new treatment approaches. A table of genetic terms is included for review