New DNA markers with increased informativeness show diminished support for a chromosome 5q11-13 schizophrenia susceptibility locus and exclude linkage in two new cohorts of British and Icelandic families.

Genetic linkage of schizophrenia to markers at 5q11.2-13.3 had been reported previously in five Icelandic and two British families, but attempts at replication in independent samples have been unsuccessful. We report here an update on the diagnoses and results of linkage analyses using newer highly polymorphic microsatellite markers at or near the loci D5S76 and D5S39 in the original sample of pedigrees and in two new family samples from Iceland and from Britain. The new results show a reduction in evidence for linkage in the original sample and evidence against linkage in the two new family samples. Although it is possible that a rare locus is present, perhaps in the region 5p14.1-13.1 rather than 5q11.2-13.3, it appears most likely that the original positive lod scores represent an exaggeration of the 'true' lod scores due to random effects and that the small lod scores we now obtain could have arisen by chance.

No evidence for linkage of schizophrenia to DXS7 at chromosome Xp11.

There have been claims that a gene on the X chromosome may contribute to susceptibility to schizophrenia. Crow (1988) initially proposed that such a gene might lie in the pseudoautosomal region, but when evidence that weakened this hypothesis accumulated, he proposed that a susceptibility locus might be present elsewhere on the sex chromosomes instead. DeLisi et al. (1994) found a small nonsignificant positive lod score between the marker DXS7 and schizophrenia, but other failed to replicate this finding. Another study reported by Crow and DeLisi's group was also weakly positive for this marker (Dann et al., 1997). This locus was then investigated in a collaborative study by Laval et al. (1997), which produced a nonparametric lod score of 2.44. Using a sample of 17 pedigrees from Britain and Iceland, we have also tested the hypothesis of linkage between DXS7 and schizophrenia. The 17 families were selected from a larger sample on the basis of an absence of male-to-male transmission for schizophrenia. These families were originally selected for having multiple cases of schizophrenia within them and for having no cases of bipolar affective disorder. We genotyped subjects for a marker at DXS7 and performed classical lod score and model-free linkage analysis using broad and narrow definitions of affection with schizophrenia. We found strongly negative lod scores and no evidence for linkage using model-free analysis. Therefore, this study does not support the hypothesis of linkage of schizophrenia to DXS7, and the evidence for a susceptibility locus on this part of the X chromosome is weakened.

No evidence of expansion of CAG or GAA repeats in schizophrenia families and monozygotic twins.

Many diseases caused by trinucleotide expansion exhibit increased severity and decreased age of onset (genetic anticipation) in successive generations. Apparent evidence of genetic anticipation in schizophrenia has led to a search for trinucleotide repeat expansions. We have used several techniques, including Southern blot hybridization, repeat expansion detection (RED) and locus-specific PCR to search for expanded CAG/CTG repeats in 12 families from the United Kingdom and 11 from Iceland that are multiplex for schizophrenia and demonstrate anticipation. The unstable DNA theory could also explain discordance of phenotype for schizophrenia in pairs of monozygotic twins, where the affected twin has a greater number of repeats than the unaffected twin. We used these techniques to look for evidence of different CAG/CTG repeat size in 27 pairs of monozygotic twins who are either concordant or discordant for schizophrenia. We have found no evidence of an increase in CAG/CTG repeat size for affected members in the families, or for the affected twins in the MZ twin sample. Southern hybridization and RED analysis were also performed for the twin and family samples to look for evidence of expansion of GAA/TTC repeats. However, no evidence of expansion was found in either sample. Whilst these results suggest that these repeats are not involved in the etiology of schizophrenia, the techniques used for detecting repeat expansions have limits to their sensitivity. The involvement of other trinucleotide repeats or other expandable repeat sequences cannot be ruled out.

Linkage analysis of manic depression (bipolar affective disorder) in Icelandic and British kindreds using markers on the short arm of chromosome 18.

Attempts were made to follow up results of a previous linkage study which suggested that a locus-modifying susceptibility to bipolar and related unipolar affective disorder might be present in the pericentromeric region of the short arm of chromosome 18. Twenty-three multiply affected pedigrees collected from Iceland and the UK were genotyped using three highly polymorphic microsatellite markers at D18S37, D18S40 and D18S44 which span the region implicated. Lod score analyses under the assumption of heterogeneity and non-parametric linkage analyses were performed. The total lod scores obtained were strongly negative, and analysis allowing for heterogeneity did not suggest that any subgroup of the families was linked. Model-free linkage analysis using extended relative pair analysis and MFLINK also failed to detect any evidence for linkage. Our study provides no support for the presence of a locus-modifying genetic susceptibility to bipolar affective disorder in the pericentromeric region of chromosome 18q11. Further analyses in independent samples should help to reveal whether our negative results are due to locus heterogeneity or whether the original results were false-positive.

Two-locus admixture linkage analysis of bipolar and unipolar affective disorder supports the presence of susceptibility loci on chromosomes 11p15 and 21q22.

Following a report of a linkage study that yielded evidence for a susceptibility locus for bipolar affective disorder on the long arm of chromosome 21, we studied 23 multiply affected pedigrees collected from Iceland and the UK, using the markers PFKL, D21S171, and D21S49. Counting only bipolar cases as affected, a two-point LOD of 1.28 was obtained using D21S171 (theta = 0.01, alpha = 0.35), with three Icelandic families producing LODs of 0.63, 0.62, and 1.74 (all at theta = 0.0). Affected sib pair analysis demonstrated increased allele sharing at D21S171 (P = 0.001) when unipolar cases were also considered affected. The same set of pedigrees had previously been typed for a tyrosine hydroxylase gene (TH) polymorphism at 11p15 and had shown some moderate evidence for linkage. When information from TH and the 21q markers was combined in a two-locus admixture analysis, an overall admixture LOD of 3.87 was obtained using the bipolar affection model. Thus the data are compatible with the hypothesis that a locus at or near TH influences susceptibility in some pedigrees, while a locus near D21S171 is active in others. Similar analyses in other datasets should be carried out to confirm or refute our tentative finding.

Exclusion of linkage between schizophrenia and the gene encoding a neutral amino acid glutamate/aspartate transporter, SLC1A5.

An abnormality in glutamatergic function has been hypothesized as being of etiological importance in schizophrenia. Twenty-three multiplex English and Icelandic schizophrenia families were genotyped with a polymorphic dinucleotide repeat sequence in the 3'-untranslated region of the glutamate/aspartate transporter gene called SLC1A5. Using the lod and a model-free method of linkage analysis (MFLINK), no evidence of linkage between SLC1A5 and schizophrenia was found. Our results do not support the hypothesis that SLC1A5 gene mutations or allelic variants provide a major gene contribution to the etiology of schizophrenia. However, because of the likelihood of heterogeneity of linkage in schizophrenia, there is a case for testing other pedigrees for linkage to the SLC1A5 locus. The SLC1A5 locus is one of a complex family of genes encoding neutral amino acid transporter proteins and the genetic relation between these other loci and schizophrenia has not yet been established.

Test of Xq26.3-28 linkage in bipolar and unipolar affective disorder in families selected for absence of male to male transmission.

BACKGROUND: There have been several reports of linkage between genetic markers on the X chromosome at Xq26.3-28 and bipolar affective disorder in family samples obtained from distinct ethnic and geographical origins. As part of a genome search in a series of 23 UK and Icelandic families, specifically selected for their large size and power to resolve the issue of linkage heterogeneity, we have tested the hypothesis that there is a locus for a genetic subtype of bipolar affective disorder which is linked to this region. METHOD: In families selected on the basis of absent male to male transmission for affective disorder, we performed two-point and FASTMAP multipoint linkage analyses with markers spanning the region between the genetic loci DXS102 and F8. RESULTS: We found negative lod scores for several models of affection status in families selected under stringent and relaxed criteria for the absence of male to male transmission. CONCLUSIONS: In the family sample we have obtained, our study provides no support for the presence of a locus increasing genetic susceptibility to bipolar affective disorder in this region of the X chromosome. It is likely that our finding reflects heterogeneity of linkage for bipolar and genetically related unipolar disorder that exists in specific ethnic populations. Alternatively the X-linked subtype of the disorder may have been present only in a few of our small families resulting in loss of power to detect the Xq26.3-28 linked subtype.

Lack of evidence for close linkage of the glutamate GluR6 receptor gene with schizophrenia.

OBJECTIVE: Previous research has consistently implicated genetic factors in the pathogenesis of schizophrenia. It has been hypothesized that an abnormality in glutamatergic function is of etiologic importance in schizophrenia, and therefore the glutamate receptor family of genes are potential susceptibility loci for schizophrenia. To test this hypothesis the authors sought to detect linkage between the GluR6 glutamate receptor gene and schizophrenia. METHOD: Twenty-three English and Icelandic families containing multiple cases of schizophrenia were genotyped with a microsatellite trinucleotide repeat polymorphism localized at the GluR6 glutamate receptor locus. Lod scores, model-free linkage analysis, and extended relative pair analysis were used to test for linkage. RESULTS: No statistically significant evidence of linkage between GluR6 and schizophrenia was found. CONCLUSIONS: The results do not support the hypothesis that GluR6 allelic variants provide a major gene contribution to the etiology of schizophrenia in a large proportion of these pedigrees.

Further tests for linkage of bipolar affective disorder to the tyrosine hydroxylase gene locus on chromosome 11p15 in a new series of multiplex British affective disorder pedigrees.

OBJECTIVE: This study was undertaken to confirm or refute previous reports that link bipolar affective disorder to polymorphic DNA markers at or near the gene for tyrosine hydroxylase. METHOD: A previous linkage analysis, which used a tetranucleotide repeat polymorphism at the tyrosine hydroxylase locus, of six Icelandic families was extended to include a new series of 17 multiply affected British families. RESULTS: Overall lod scores under the assumption of locus heterogeneity were between 1.20 and 1.40 at zero recombination with tyrosine hydroxylase, and these scores persisted across three affective disorder models. CONCLUSIONS: These results provide some support for linking affective disorder to this genetic region and suggest that additional linkage and association studies should be conducted to determine whether tyrosine hydroxylase or a nearby locus contributes to susceptibility to bipolar affective disorder in some families.

Linkage study of the D5 dopamine receptor gene (DRD5) in multiplex Icelandic and English schizophrenia pedigrees.

OBJECTIVE: The authors investigated the possibility that genetic variation or mutation of the dopamine D5 receptor gene might modify susceptibility to schizophrenia. METHOD: Twenty-three Icelandic and English pedigrees containing multiple cases of schizophrenia were genotyped by using a highly informative microsatellite for the D5 dopamine receptor gene DRD5. RESULTS: By means of three different affection models, negative lod scores were obtained under assumptions of autosomal dominant and recessive inheritance. There was no evidence for locus heterogeneity. Nonparametric extended relative pair analysis also produced negative results. CONCLUSIONS: These data indicate that mutations of the D5 dopamine receptor gene are not a major cause of schizophrenia in these pedigrees. Because of the probable existence of locus heterogeneity, the D5 receptor gene may be of etiologic importance in other families with schizophrenia.

Investigation by linkage analysis of the XY pseudoautosomal region in the genetic susceptibility to schizophrenia.

BACKGROUND: A susceptibility locus for schizophrenia in the pseudoautosomal region has been proposed on the basis of a possible excess of sex chromosome aneuploidies among patients with schizophrenia and an increased sex concordance in affected sib pairs. Several studies investigating this hypothesis have produced conflicting evidence. METHOD: In a series of Icelandic and British families, we used lod score and sib pair linkage analyses with markers for the MIC2 and DXYS14 loci on the pseudoautosomal XY region. RESULTS: Lod and sib pair linkage analysis with these markers produced strongly negative scores. Heterogeneity testing also produced negative results. CONCLUSION: We conclude that the present study provides no support for the involvement of either the pseudoautosomal region or the nearby region of the sex chromosomes in the aetiology of schizophrenia.

Linkage analysis of chromosome 22q12-13 in a United Kingdom/Icelandic sample of 23 multiplex schizophrenia families.

A possible linkage to a genetic subtype of schizophrenia and related disorders has been reported on the long arm of chromosome 22 at q12-13. (Pulver et al., 1994: Am J Med Genet 54:36-43; Coon et al., 1994: Am J Med Genet 54:72-79; Pulver et al., 1994: Am J Med Genet 54:44-50). However formal statistical tests in a combined sample could not reject homogeneity and prove that there was a linked subgroup of families. We have studied 23 schizophrenia pedigrees to test whether some multiplex schizophrenia families may be linked to the microsatellite markers D22S274 and D22S283 which span the 22q12-13 region. Two point followed by multipoint lod and non-parametric linkage analyses under the assumption of heterogeneity provided no evidence for linkage over the relevant region.

Exclusion of linkage of schizophrenia to the gene for the dopamine D2 receptor (DRD2) and chromosome 11q translocation sites.

There have been previous reports of a 1q43;11q21 translocation cosegregating with schizophrenia and a 9p22;11q22.3 translocation cosegregating with manic depression. In addition, the genes for the dopamine D2 receptor and for tyrosinase both map to chromosome 11q. Three 11q DNA markers were used to investigate 23 pedigrees containing multiple cases of schizophrenia. Strongly negative lod scores were obtained, providing evidence against linkage over a 70 cM region which included both translocation sites and both candidate genes.

A linkage study of affective disorder with DNA markers for the ABO-AK1-ORM linkage group near the dopamine beta hydroxylase gene.

Combining data from a number of studies has provided evidence for a susceptibility allele for affective disorder near to the ABO-AK1-ORM region on chromosome 9q34. The dopamine beta hydroxylase gene locus is also at 9q34. Five multigenerational families with bipolar and unipolar affective disorder were analyzed for linkage with highly polymorphic microsatellite markers from the candidate region. The segregation of the illness in these families was compatible with an autosomal dominant susceptibility allele. Linkage analyses using conservative parameters seemed to provide strong evidence against a major susceptibility allele in this region including the candidate gene dopamine beta hydroxylase in these families.

The Marfan syndrome gene locus as a favoured locus for susceptibility to schizophrenia.

Marfan syndrome (MS) is a rare autosomal dominant disorder of connective tissue with manifestations in the cardiovascular, ocular and skeletal systems. Genetic linkage analysis with random probes has mapped the MS locus to 15q21.1. There have been several reports of Marfan syndrome co-segregating with schizophrenia within families, which suggest that a common genetic factor may be shared between schizophrenia susceptibility and MS. This could be due to a cytogenetic abnormality affecting both genetic loci or due to co-segregation of two disease loci near each other on the same chromosome. We tested this hypothesis by using genetic linkage analysis with multiplex families. Using three genetic markers spanning the MS locus, we were unable to find evidence of linkage with schizophrenia across the Marfan syndrome locus on chromosome 15.

Linkage between tyrosine hydroxylase gene and affective disorder cannot be excluded in two of six pedigrees.

Genetic linkage between manic depression and DNA markers on the short arm of chromosome 11 was first reported in 1987 but not supported by further analyses. However, genetic markers at the tyrosine hydroxylase (TH) gene located within this region have been reported to show allelic association with the affective disorder phenotype. We present the results of a linkage analysis using polymorphic DNA segments within the TH gene and the nearby dopamine D4 receptor (DRD4) gene in 6 families multiply affected with affective disorder. Small positive Lod scores were obtained in 2 of 6 pedigrees with the TH polymorphism which may be indicative of genetic heterogeneity.