Support for a chromosome 18p locus conferring susceptibility to functional psychoses in families with schizophrenia, by association and linkage analysis

Do schizophrenia and affective disorder share susceptibility genes?

Schizophrenia and affective disorders are relatively common neuropsychiatric diseases with a complex genetic etiology. A multigenic inheritance with variable influence of unknown environmental factors may be involved. Family studies have demonstrated the existence of both phenotypes in the same kindreds, and in certain cases, a transition from one phenotype to another occurs. In addition, intermediate phenotypes such as schizoaffective disorders are found in families with schizophrenia and affective illness. Recent genome and chromosomal scans appear to support these epidemiologic data, since susceptibility regions for both schizophrenia and affective disorders have been found to overlap, on chromosomes 10p13-p12, 13q32, 18p and 22q11-q13. These studies were performed in independently ascertained family samples with index patients afflicted either with schizophrenia or bipolar disorder. Taken together, these findings imply shared loci for schizophrenia and affective disorders among those required for the full expression of the phenotype. Identification and molecular characterization of the genetic components conferring risk to both disorders would impact positively on diagnosis, prevention, and treatment.

A two-stage genome scan for schizophrenia susceptibility genes in 196 affected sibling pairs

We undertook a systematic search for linkage in 196 affected sibling pairs (ASPs) with DSMIV schizophrenia. In stage 1 we typed 97 ASPs with 229 microsatellite markers at an average inter-marker distance of 17.26 cM. Multipoint affected sib pair analysis identified seven regions with a maximum lod score (MLS) at or above the level associated with a nominal pointwise significance of 5%, on chromosomes 2q, 4p, 10q, 15q, 18p, 20q and Xcen. In stage 2 we genotyped a further 54 markers in 196 ASPs together with parents and unaffected siblings. This allowed the regions identified in stage 1 to be typed at an average spacing of 5.15 cM, while the region of interest on chromosome 2 was typed to 9.55 cM. Analysis was performed on the whole data set. Simulation studies suggested that we would expect one multipoint MLS of 1.5 per genome scan in the absence of linkage. An MLS of 3 would be expected only once in every 20 genome scans and thus corresponds to a genome-wide significance of 0.05. We obtained three multipoint MLSs >1.5 and, on this basis, the results on chromosomes 4p, 18q and Xcen can be considered suggestive. However, none approached a genome-wide significance of 0. 05. The power of this study was >0.95 to detect a susceptibility locus of lambda(s)= 3 with a genome-wide significance of 0.05, but only 0.70 to detect a locus of lambda(s)= 2. Our results suggest that common genes of major effect (lambda(s)> 3) are unlikely to exist for schizophrenia.

Molecular linkage studies of bipolar disorder

Linkage studies have defined at least five bipolar (BP) disorder susceptibility loci that meet suggested guidelines for initial identification and subsequent confirmation. These loci, found on 18p11, 18q22, 21q21, 4p16, and Xq26, are targets for BP candidate gene investigations. Molecular dissection of expressed sequences for these regions is likely to yield specific BP susceptibility alleles in most cases, in all probability, these BP susceptibility alleles will be common in the general population, and, individually, will be neither necessary nor sufficient for manifestation syndrome. Additive or multiplicative oligogenic models involving several susceptibility loci appear most reasonable at present, it is hoped thai these BP susceptibility genes will increase understanding of many mysteries surrounding these disorders, including drug response, cycling patterns, age-of-onset, and modes of transmission.

Searching for genes in Schizophrenia

Decades of research into the etiology of schizophrenia on a phenotypic level, i.e. studies of neuroanatomy, neuropathology, neurophysiology and other areas such as immunology have yielded only fragmentary results. A contribution of genetic factors, has been consistently shown, however, beginning with E. Kraepelin's pioneering studies at the turn of the century. Evidence has accumulated from family-, twin-, and adoption studies. Identical twins have a 48% risk of developing schizophrenia if one of them is affected. In contrast, a 17 % risk is reported for nonidentical twins. These rates are similar to other complex genetic disorders such as diabetes, hypertension and asthma. Advances in the genetic analysis of complex traits as well as progress in the Human Genome Project should provide a basis for uncovering the molecular causes of schizophrenic disorders and for investigating the neuropathology of this individually and socially devastating neuropsychiatrie disorder. There is no doubt, that discovery of the genetic variation associated with the illness would help in identifying specific targets for development of more effective, targeted treatments.

A high-density genome scan detects evidence for a bipolar-disorder susceptibility locus on 13q32 and other potential loci on 1q32 and 18p11.2

Bipolar disorder is a severe mental illness characterized by mood swings of elation and depression. Family, twin, and adoption studies suggest a complex genetic etiology that may involve multiple susceptibility genes and an environmental component. To identify chromosomal loci contributing to vulnerability, we have conducted a genome-wide scan on approximately 396 individuals from 22 multiplex pedigrees by using 607 microsatellite markers. Multipoint nonparametric analysis detected the strongest evidence for linkage at 13q32 with a maximal logarithm of odds (lod) score of 3.5 (P = 0. 000028) under a phenotype model that included bipolar I, bipolar II with major depression, schizoaffective disorder, and recurrent unipolar disorder. Suggestive linkage was found on 1q31-q32 (lod = 2. 67; P = 0.00022) and 18p11.2 (lod = 2.32; P = 0.00054). Recent reports have linked schizophrenia to 13q32 and 18p11.2. Our genome scan identified other interesting regions, 7q31 (lod = 2.08; P = 0. 00099) and 22q11-q13 (lod = 2.1; P = 0.00094), and also confirmed reported linkages on 4p16, 12q23-q24, and 21q22. By comprehensive screening of the entire genome, we detected unreported loci for bipolar disorder, found support for proposed linkages, and gained evidence for the overlap of susceptibility regions for bipolar disorder and schizophrenia.