Excitement and confusion on chromosome 6q: the challenges of neuropsychiatric genetics in microcosm

Abstract

The search for genes that are implicated in the pathogenesis of schizophrenia (SCZ), bipolar disorder (BPD) and other complex neuropsychiatric phenotypes has yielded a plethora of positive findings, but has also engendered a substantial degree of confusion. Exciting findings include positive linkage results in a number of chromosomal regions and the identification of several genes that have been associated with SCZ and to a lesser extent with BPD. Confusing aspects include the difference between studies in localization of linkage peaks in the same chromosomal regions, raising the possibility that these regions may harbor more than one gene, the fact that positive linkage findings as well as associated genes appear in several cases to be shared by more than one disorder, and the failure to identify thus far the precise pathogenic variants in associated genes. Recent findings of linkage and association studies on chromosome 6q illustrate the current status of neuropsychiatric genetics in intriguing microcosm. Positive findings from linkage and association studies are reviewed in order to identify approaches that may help to settle apparent contradictions and allow an interpretation of the results that may prove useful in application to findings from other chromosomal regions. Not only SCZ and BPD but also other psychiatric and neurological phenotypes are considered. Taking a topographic approach, we identify five foci of positive findings on chromosome 6q and suggest that each may harbor gene(s) that confer susceptibility to SCZ or BPD or may modify their onset or clinical course. We further suggest that in searching for these genes the possibility that they may be implicated in more than one disorder should be taken into account. We also discuss the potential contribution of rare genetic variants identified in homogeneous, isolated populations to the subsequent identification of common variants in the same gene that contribute to disease susceptibility in outbred populations. Keywords: schizophrenia; bipolar disorder; chromosome 6q; molecular genetics; linkageThe last three decades have seen considerable advances in the field of psychiatric genetics. First, family twin and adoption studies conclusively established that genes play a major role in the etiology of many psychiatric disorders. Then, an increasing number of groups embarked on linkage studies aiming at localizing these genes. With ever improving technology, whole genome scans became the standard vehicle for these endeavors. Studies became more and more sophisticated, involving larger samples of carefully characterized affected individuals, using denser maps of markers and employing increasingly powerful statistical tools. Over the years, linkage studies have repeatedly implicated several chromosomal regions as linked to major psychiatric disorders, particularly schizophrenia (SCZ) and bipolar disorder (BPD) (for reviews, see Riley and McGuffin, 1 Berrettini,2 Kohn and Lerer 3 and Mathews and Reus 4 ). Recent meta-analyses of linkage studies have establi...

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