No evidence for a pseudoautosomal locus for schizophrenia. Linkage analysis of multiply affected families

The XY gene hypothesis of psychosis: origins and current status

Sex differences in psychosis and their interaction with laterality (systematic departures from 50:50 left-right symmetry across the antero-posterior neural axis) are reviewed in the context of the X-Y gene hypothesis. Aspects of laterality (handedness/cerebral asymmetry/the torque) predict (1) verbal and non-verbal ability in childhood and across adult life and (2) anatomical, physiological, and linguistic variation relating to psychosis. Neuropsychological and MRI evidence from individuals with sex chromosome aneuploidies indicates that laterality is associated with an X-Y homologous gene pair. Within each mammalian species the complement of such X-Y gene pairs reflects their potential to account for taxon-specific sexual dimorphisms. As a consequence of the mechanism of meiotic suppression of unpaired chromosomes such X-Y gene pairs generate epigenetic variation around a species defining motif that is carried to the zygote with potential to initiate embryonic gene expression in XX or XY format. The Protocadherin11XY (PCDH11XY) gene pair in Xq21.3/Yp11.2 in probable coordination with a gene or genes within PAR2 (the second pseudo-autosomal region) is the prime candidate in relation to cerebral asymmetry and psychosis in Homo sapiens. The lately-described pattern of sequence variation associated with psychosis on the autosomes may reflect a component of the human genome's adjustment to selective pressures generated by the sexually dimorphic mate recognition system.

Sex differences in the genetic risk for schizophrenia: history of the evidence for sex-specific and sex-dependent effects

Although there is a long history to examinations of sex differences in the familial (and specifically, genetic) transmission of schizophrenia, there have been few investigators who have systematically and rigorously studied this issue. This is true even in light of population and clinical studies identifying significant sex differences in incidence, expression, neuroanatomic and functional brain abnormalities, and course of schizophrenia. This review highlights the history of work in this arena from studies of family transmission patterns, linkage and twin studies to the current molecular genetic strategies of large genome-wide association studies. Taken as a whole, the evidence supports the presence of genetic risks of which some are sex-specific (i.e., presence in one sex and not the other) or sex-dependent (i.e., quantitative differences in risk between the sexes). Thus, a concerted effort to systematically investigate these questions is warranted and, as we argue here, necessary in order to fully understand the etiology of schizophrenia.

Mode of transmission of schizophrenia

OBJECTIVE

Although the evidences for the phenomenon of "anticipation" and parental "imprinting" have been shown in schizophrenia, they are inconclusive. The purpose of this study was to test these hypotheses by examining three successive generations.

METHOD

58 schizophrenic patients who had their maternal or paternal parent or grandparent, or both, affected with schizophrenia or related disorders were analyzed. Chi-square test was used to assess the association of the sex of the parent with more than one of the affected proband families. The differences in the age of onset of the illness between the successive three generations was calculated using the t-test.

RESULTS

In comparison to mothers' affected families, a large proportion of the father side affected families had more than one of their offspring affected with the illness. The age of onset in probands was lower in comparison to that of those on the parental side and the difference was more significant when the paternal side was affected. Interestingly, when the age of onset in the grandparents was compared with either of the parental sides of the probands no difference emerged, indicating lack of support from this study for the theory of anticipation. At any rate, the age of onset of probands was significantly lower in comparison to that of the paternal grandfather side. Further, skipping of a generation in the process of transmission was noted in some families.

CONCLUSIONS

It is hard to ignore our findings that suggest paternal side transmission.

Genes and phenotypes of the human Y chromosome

By 1959 it was recognized that the gene (or genes) responsible for initiating the human male phenotype were carried on the Y chromosome. But in subsequent years, few phenotypes were associated with the Y chromosome. Recently, using molecular techniques combined with classical genetics, the Y chromosome has been the focus of intensive and productive investigation. Some of the findings are unexpected and have extended our understanding of the functions of the human Y chromosome. The notion that the Y chromosome is largely devoid of genes is changing. At the present, over 20 Y chromosome genes or pseudogenes have been identified or cloned, a number that is rapidly increasing. A high proportion of Y chromosome sequences have been found to be related to X chromosome sequences: the assembly of a complete physical map of the Y chromosome euchromatic region (believed to carry all of the genes) has shown 25% of the region studied to have homology to the X chromosome.3 Several X-homologous genes are located in the X and Y chromosome pairing regions, an area predicted to have shared homology. Surprisingly, some of the Y-encoded genes that lie outside of the X and Y pairing region share high sequence similarity, and in at least one case, functional identity, with genes on the X chromosome.

Linkage and associated studies of schizophrenia

Genetic epidemiology has provided consistent evidence over many years that schizophrenia has a genetic component, and that this genetic component is complex, polygenic, and involves epistatic interaction between loci. Molecular genetics studies have, however, so far failed to identify any DNA variant that can be demonstrated to contribute to either liability to schizophrenia or to any identifiable part of the underlying pathology. Replication studies of positive findings have been difficult to interpret for a variety of reasons. First, few have reproduced the initial findings, which may be due either to random variation between two samples in the genetic inputs involved, or to a lack of power to replicate an effect at a given alpha level. Where positive data have been found in replication studies, the positioning of the locus has been unreliable, leading no closer to positional cloning of genes involved. However, an assessment of all the linkage studies performed over the past ten years does suggest a number of regions where positive results are found numerous times. These include regions on chromosomes 1, 2, 4, 5, 6, 7, 8, 9, 10, 13, 15, 18, 22 and the X. All of these data are critically reviewed and their locations compared. Reasons for the difficulty in obtaining consistent results and possible strategies for overcoming them are discussed. Am. J. Med. Genet. (Semin. Med. Genet.) 97:23-44, 2000.

Localization of genes modulating the predisposition to schizophrenia: a revision

The genetics of schizophrenia or bipolar affective disorder has advanced greatly at the molecular level since the introduction of probes for the localization of specific genes. Research on gene candidates for susceptibility to schizophrenia can broadly be divided into two types, i.e., linkage studies, where a gene is found near a specific DNA marker on a specific chromosome, and association studies, when a condition is associated with a specific allele of a specific gene. This review covers a decade of publications in this area, from the 1988 works of Bassett et al. and Sherrington et al. on a gene localized on the long arm of chromosome 5 at the 5q11-13 loci, to the 1997 work of Lin et al. pointing to the 13q14.1-q32 loci of chromosome 13 and to the 1998 work of Wright et al. on an HLA DRB1 gene locus on chromosome 6 at 6p21-3. The most replicated loci were those in the long arm of chromosome 22 (22q12-q13.1) and on the short arm of chromosome 6 (6p24-22). In this critical review of the molecular genetic studies involved in the localization of genes which modulate the predisposition to schizophrenia the high variability in the results obtained by different workers suggests that multiple loci are involved in the predisposition to this illness.

Familial clustering of symptoms and disruptive behaviors in multiplex families with attention-deficit/hyperactivity disorder

OBJECTIVE

To examine familial clustering of attention-deficit/hyperactivity disorder (ADHD), ADHD subtypes, symptoms, and oppositional behaviors in affected sibling pairs (ASPs) and their parents.

METHOD

One hundred thirty-two ASPs, ranging in age from 5 to 25 years and ascertained through clinic and volunteer referrals, were examined for DSM-IV ADHD subtypes, oppositional defiant disorder (ODD), and conduct disorder (CD) with the Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime version (K-SADS-PL). Two hundred fifty-six parents in these families were assessed by means of the SADS-Lifetime version, Modified for the Study of Anxiety Disorders, Updated for DSM-IV (SADS-LA-IV), and the Behavioral Disorders supplement of the K-SADS-PL to determine ADHD, ODD, and CD.

RESULTS

Fifty-five percent of families ascertained through an ASP have at least one parent with a lifetime diagnosis of ADHD. The frequency of ADHD in at least one parent was higher in families with at least one affected girl (63%) than in families with only affected boys (45%) (p = .02). There was no evidence that affected siblings or parents within ASP families showed similar patterns of ADHD symptoms, such as ADHD subtype classification. In contrast, CD significantly clustered in ASP families.

CONCLUSIONS

The sex difference in prevalence of ADHD among ASPs is consistent with a model of inheritance in which girls require a greater loading of familial influences to develop ADHD. The lack of familial clustering of ADHD symptoms within ASP families suggests that hyperactive and inattentive symptoms reflect common familial underpinnings and not unique familial effects. In contrast, CD seems to reflect unique familial underpinnings distinct from those underlying ADHD.

Multipoint linkage analysis of the pseudoautosomal regions, using affected sibling pairs

Affected sibling pairs are often the design of choice in linkage-analysis studies with the goal of identifying the genes that increase susceptibility to complex diseases. Methods for multipoint analysis based on sibling amount of sharing that is identical by descent are widely available, for both autosomal and X-linked markers. Such methods have the advantage of making few assumptions about the mode of inheritance of the disease. However, with this approach, data from the pseudoautosomal regions on the X chromosome pose special challenges. Same-sex sibling pairs will share, in that region of the genome, more genetic material identical by descent, with and without the presence of a disease-susceptibility gene. This increased sharing will be more pronounced for markers closely linked to the sex-specific region. For the same reason, opposite-sex sibling pairs will share fewer alleles identical by descent. Failure to take this inequality in sharing into account may result in a false declaration of linkage if the study sample contains an excess of sex-concordant pairs, or a linkage may be missed when an excess of sex-discordant pairs is present. We propose a method to take into account this expected increase/decrease in sharing when markers in the pseudoautosomal region are analyzed. For quantitative traits, we demonstrate, using the Haseman-Elston method, (1) the same inflation in type I error, in the absence of an appropriate correction, and (2) the inadequacy of permutation tests to estimate levels of significance when all phenotypic values are permuted, irrespective of gender. The proposed method is illustrated with a genome screen on 350 sibling pairs affected with type I diabetes.

Investigation of Turner syndrome in schizophrenia

Both Turner syndrome and schizophrenia are relatively infrequent conditions. Consequently, individuals having both illnesses are rare. Previous reviews of sex chromosome abnormalities in schizophrenia have focused primarily on the presence of supernumerary X-chromosomes. After identifying two female patients with schizophrenia and Turner syndrome, we reevaluated the available literature that survey female schizophrenics for the presence of chromosomal abnormalities. Eleven patients with Turner syndrome were identified among 6,483 females with schizophrenia in non-case-report studies. These survey results indicate that Turner syndrome occurs approximately three-fold more frequently in schizophrenic females than in the general female population (P < 0.02). Including 6 other case reports and our 2 cases, a total of 19 females with both schizophrenia and Turner syndrome were reported. Interestingly, whereas most Turner syndrome patients have the 45,X karyotype, the majority (18/19) of women with both illnesses have a mosaic karyotype (P < 0.0002). Given the potential role of genes on the X-chromosome in the pathogenesis of schizophrenia, the study of unique populations with abnormalities in this chromosome, such as women with Turner syndrome, may offer clues into this illness.

The genetics of schizophrenia: past, present, and future concepts

Although a genetic susceptibility for schizophrenia has been long established and even noted by Kraepelin in 1907, the mechanisms for its inheritance remains unknown. No candidates have proven to be correct and while many weakly positive chromosomal linkages have been reported, none have yet been consistently replicated. The following review examines the present status of these findings. The conclusion is that the field must move on to finding a consistently replicable mutation segregating with schizophrenia in families, before any of the present linkage results can be resolved.

A linkage study of schizophrenia to markers within Xp11 near the MAOB gene

A sex chromosome locus for psychosis has been considered on the basis of some sex differences in genetic risk and expression of illness, and an association with X-chromosome anomalies. Previous molecular genetic studies produced weak evidence for linkage of schizophrenia to the proximal short arm of the X-chromosome, while some other regions were not ruled out. Here we report an attempt to expand the Xp findings in: (i) a multicenter collaboration focusing on 92 families with a maternal pattern of inheritance (Study I), and (ii) an independent sample of 34 families unselected for parental mode of transmission (Study II). In the multicenter study, a parametric analysis resulted in positive lod scores (highest of 1.97 for dominant and 1.19 for recessive inheritance at a theta of 0.20) for locus DXS7, with scores below 0.50 for other markers in this region (MAOB, DXS228, and ARAF1). Significant allele sharing among affected sibling pairs was present at DXS7. In the second study, positive lod scores were observed at MAOB (highest of 2.16 at a theta of 0.05 for dominant and 1.64 at a theta of 0.00 for recessive models) and ALAS2 (the highest of 1.36 at a theta of 0.05 for a recessive model), with significant allele sharing (P = 0.003 and 0.01, respectively) at these two loci. These five markers are mapped within a small region of Xp11. Thus, although substantial regions of the X-chromosome have been investigated without evidence for linkage being found, a locus predisposing to schizophrenia in the proximal short arm of the X-chromosome is not excluded.

Failure to support a pseudoautosomal locus for schizophrenia

Evidence for a pseudoautosomal locus for schizophrenia was sought through an assessment of the psychiatric status of the uncles and aunts (n = 30) of schizophrenic patients. The proportion of affected paternal uncles did not differ from that of affected paternal aunts. Likewise, the proportion of affected maternal aunts did not differ from that of affected maternal uncles. The research design used in this study failed to support the pseudoautosomal hypothesis of schizophrenia.

The molecular genetics of schizophrenia

There is overwhelming evidence for a significant genetic contribution to the etiology of schizophrenia. Molecular genetic techniques are now sufficiently advanced to be applied to complex genetic disorders with uncertain phenotypes, such as schizophrenia. In this article we first briefly discuss certain pertinent background issues: the evidence that schizophrenia has a heritable basis, the possible modes of inheritance involved, and how best to define schizophrenia in the light of this evidence; we then review the current status of research in the field. Large collaborative studies are currently underway that pave the way for the detection of genes of both major and minor effects. We may now be seeing the first consistently replicated results from chromosome 6 and 22 and from candidate genes, such as the dopamine D3 receptor gene.

Evidence against unusual sex concordance and pseudoautosomal inheritance in the catatonic subtype of schizophrenia

The study is based on sibships with multiply afflicted members derived from a family study of consecutively admitted probands with catatonic schizophrenia. As shown recently, the clinical subtype of periodic catatonia, as defined by Leonhard, is compatible with a major gene effect and genetic anticipation; that is, the age of illness onset of the probands is significantly earlier than that of their parents. In the present study, 83 probands with the clinical subtype of periodic catatonia had 26 afflicted siblings that were distributed among 23 families. We analyzed sex-concordance and pseudoautosomal inheritance patterns. Stratifying the 26 afflicted siblings by sibship size and by the proband's sex, we did not find unusual sex-concordance rates in sibships afflicted with periodic catatonia. Further, there was no association between sex concordance and maternal or paternal origin of the disease. Thus, our results provide strong evidence against pseudoautosomal inheritance or sex-linked transmission in affected sibships in the obviously familial schizophrenic subtype of periodic catatonia.

No evidence for linkage between the X-chromosome marker DXS7 and schizophrenia

DeLisi et al. (1994b) have examined the X and Y chromosomes for linkage to schizophrenia in 126 small families and report a small positive LOD score for the marker DXS7, adjacent to the MAO locus at Xp11.4-11.3. Because of this, we have examined the DXS7 for linkage to schizophrenia using 17 pedigrees in which male-to-male transmission of schizophrenia was absent. Alleles at DXS7 were genotyped using the PCR and LOD scores calculated using five models of inheritance, including classical dominant recessive and intermediate models. LOD scores were substantially negative for all models examined and analysis for linkage heterogeneity using the LOD2 method showed no significance. Analysis by the nonparametric affected sib-pair method likewise indicated no linkage. We conclude that DXS7 is not a major locus for schizophrenia in our collection of pedigrees.

Schizophrenia and the androgen receptor gene: report of a sibship showing co-segregation with Reifenstein syndrome but no evidence for linkage in 23 multiply affected families

Crow et al. [1993: Am J Med Genet (Neuropsychiatr Genet) 48:159-160] have reported excess sharing of alleles by male sibling pairs with schizophrenia, at a triplet repeat marker within the androgen receptor gene, indicating that mutations at or near this gene may be a risk factor for males. In this report, we describe a pair of male siblings concordant for both schizophrenia and Reifenstein syndrome, which is caused by a mutation in this gene. This provides support for the hypothesis that the androgen receptor may contribute to liability to develop schizophrenia. Because of this, we have examined a collection of 23 pedigrees multiply affected by schizophrenia for linkage to the androgen receptor. We have found no evidence for linkage by both the LOD score and affected sibling-pair methods, under a range of genetic models with a broad and narrow definition of phenotype, and when families with male-to-male transmission are excluded. However, because of the small number of informative male-male pairs in our sample, we cannot confirm or refute the excess allele sharing for males reported by Crow.

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.

Genes and psychosis: old wine in new bottles?

Despite initial setbacks, linkage studies with DNA markers continue to occupy center stage in psychiatric research. Advances in molecular and statistical techniques have revived the search for disease genes, leading to a new harvest of findings. Most interest in recent years has focused on potential linkages between schizophrenia and chromosomes X-Y (the pseudoautosomal region) and 22q12-13.1, and between bipolar affective disorder and chromosomes 18 (pericentromeric region) and 21q22.3. This article provides a critical evaluation of theses studies, with implications for future research. Concerns over earlier linkage trials make this scrutiny current and topical.

Schizophrenia: a genome scan targets chromosomes 3p and 8p as potential sites of susceptibility genes

Using a systematically ascertained sample of 57 families, each having 2 or more members with a consensus diagnosis of schizophrenia (DSM-III-R criteria), we have carried out linkage studies of 520 loci, covering approximately 70% of the genome for susceptibility loci for schizophrenia. A two-stage strategy based on lod score thresholds from simulation studies of our sample identified regions for further exploration. In each region, a dense map of highly informative dinucleotide repeat polymorphisms (heterozygosity greater than .70) was analyzed using dominant, recessive, and "affected only" models and nonparametric sib pair identity-by-descent methods. For one region, 8p22-p21, affected sib-pair analyses gave a P value = .0001, corresponding to a lod score approximately equal to 3.00. For 8p22-p21, the maximum two-point lod score occurred using the "affected only" recessive model (ZMAX = 2.35; theta M = theta F); allowing for a constant sex difference in recombination fractions found in reference pedigrees, ZMAX = 2.78 (theta M/theta F = 3). For a second region, 3p26-p24, the maximum two-point lod score was 2.34 ("affected only" dominant model), and the affected sib-pair P value was .01. These two regions are worthy of further exploration as potential sites of susceptibility genes for schizophrenia.

Report from the Maryland Epidemiology Schizophrenia Linkage Study: no evidence for linkage between schizophrenia and a number of candidate and other genomic regions using a complex dominant model

Our collaborative group has undertaken a linkage study of schizophrenia, using a systematic sample of patients admitted to Maryland hospitals. An initial sample of 39 families, each having two or more affecteds, was available for genotyping candidate genes, candidate regions, and highly polymorphic markers randomly distributed throughout the genome. We used a single complex dominant model (with a disease gene frequency of 0.005 and age-dependent penetrance for affected phenotype: for under 35, penetrance = .45; for 35 and older, penetrance = .85). We report here 130 markers, which met the exclusion criteria of LOD score < -2.00 at theta > 0.01 in at least 10 informative families, and no evidence for heterogeneity. We also report here markers that were tested as candidates for linkage to the schizophrenic phenotype. They were selected based on the following criteria: a) proximity to reported chromosomal rearrangements (both 5q and 11q), b) suggestions of linkage from other families (5q), or c) presence of a candidate gene (5q, 11q, 3q: Dopamine receptors 1, 2, and 3, respectively). We also tested for mutations of codon 717 in exon 17 of the amyloid precursor protein (APP) gene and were unable to detect the C to T substitution in our schizophrenic group.

The molecular genetics of schizophrenia

There is strong evidence for a genetic component in schizophrenia but its precise nature remains unclear. Positional cloning and studies of potential candidate genes offer prospects for progress. The diagnosis of schizophrenia can now be made reliably but questions remain over the most valid phenotypic definition. To deal with this and uncertainties regarding mode of transmission a 'polydiagnostic' approach is advisable. A wealth of new DNA markers has enhanced the potential for linkage studies which have so far focused on large multiply-affected families. Multi-centre collaborative studies that are currently under way are likely to identify genes of major effect but other strategies are required if it turns out that most cases result from the combined effect of multiple genes.

Search for linkage to schizophrenia on the X and Y chromosomes

Markers for X chromosome loci were used in linkage studies of a large group of small families (n = 126) with at least two schizophrenic members in one sibship. Based on the hypothesis that a gene for schizophrenia could be X-Y linked, with homologous loci on both X and Y, our analyses included all families regardless of the pattern of familial inheritance. Lod scores were computed with both standard X-linked and a novel X-Y model, and sib-pair analyses were performed for all markers examining the sharing of maternal alleles. Small positive lod scores were obtained for loci pericentromeric, from Xp11.4 to Xq12. Lod scores were also computed separately in families selected for evidence of maternal inheritance and absence of male to male transmission of psychosis. The lod score for linkage to the locus DXS7 reached a maximum of 1.83 at 0.08% recombination, assuming dominant inheritance on the X chromosome in these families (n = 34). Further investigation of the X-Y homologous gene hypothesis focussing on this region is warranted.

Pseudoautosomal region in schizophrenia: linkage analysis of seven loci by sib-pair and lod-score methods

In a previous study, we reported a nonrandom segregation between schizophrenia and the pseudoautosomal locus DXYS14 in a sample of 33 sibships. That study has been extended by the addition of 16 new sibships from 16 different families. Data from six other loci of the pseudoautosomal region and of the immediately adjacent part of the X specific region have also been analyzed. Two methods of linkage analysis were used: the affected sibling pair (ASP) method and the lod-score method. Lod-score analyses were performed on the basis of three different models--A, B, and C--all shown to be consistent with the epidemiological data on schizophrenia. No clear evidence for linkage was obtained with any of these models. However, whatever the genetic model and the disease classification, maximum lod scores were positive with most of the markers, with the highest scores generally being obtained for the DXYS14 locus. When the ASP method was used, the earlier finding of nonrandom segregation between schizophrenia and the DXYS14 locus was still supported in this larger data set, at an increased level of statistical significance. Findings of ASP analyses were not significant for the other loci. Thus, findings obtained from analyses using the ASP method, but not the lod-score method, were consistent with the pseudoautosomal hypothesis for schizophrenia.

Sequential strategy to identify a susceptibility gene for schizophrenia: report of potential linkage on chromosome 22q12-q13.1: Part 1

To identify genes responsible for the susceptibility for schizophrenia, and to test the hypothesis that schizophrenia is etiologically heterogeneous, we have studied 39 multiplex families from a systematic sample of schizophrenic patients. Using a complex autosomal dominant model, which considers only those with a diagnosis of schizophrenia or schizoaffective disorder as affected, a random search of the genome for detection of linkage was undertaken. Pairwise linkage analyses suggest a potential linkage (LRH = 34.7 or maximum lod score = 1.54) for one region (22q12-q13.1). Reanalyses, varying parameters in the dominant model, maximized the LRH at 660.7 (maximum lod score 2.82). This finding is of sufficient interest to warrant further investigation through collaborative studies.

An examination of linkage of schizophrenia and schizoaffective disorder to the pseudoautosomal region (Xp22.3)

We investigated linkage between schizophrenia and the loci DXYS14, DXYS17, and MIC2 within the pseudoautosomal region in 85 families with two or more siblings suffering from schizophrenia or schizoaffective disorder. A maximum lod score of 2.44 was reached at MIC2, with a dominant model of inheritance at a recombination fraction of 0.367 in females and 0.046 in males (a F:M sex ratio > 1, i.e. opposite to that expected with a pseudoautosomal locus). Evidence consistent with linkage (P = 0.01) was also obtained with a sibling pair analysis at the MIC2 locus. These data do not support (although they do not definitively exclude) a locus within the pseudoautosomal region; they are consistent with the presence of a gene that predisposes to schizophrenia in the sex-specific regions of the X and Y chromosomes.

Non-concordance by gender for schizophrenia and related disorders in sibships

We analysed gender-concordance rates among 29 prospectively sampled schizophrenic probands and their 39 affected and 71 unaffected siblings. We did not find any unusual concordance rates. We found no same-gender concordance particularly in siblings affected by schizophrenia and related disorders. We considered unaffected siblings in an additional attempt to make valid and unbiased comparisons between genders, but this reduced the number of informative sibships to 20. We stratified the siblings of probands by sibship and by the proband's gender in order to check gender distribution within families. The data do not support hypotheses that schizophrenia is pseudo-autosomal or male-female chromosomally transmitted.

Pseudoautosomal region in schizophrenia: sex concordance of the affected sibpairs and the association study with DNA markers

To test a hypothesis that the pseudoautosomal region of the sex chromosomes contributes to the pathogenesis of schizophrenia, we carried out the following studies: First, the sex concordant rates of 77 schizophrenic sibpairs were examined. Secondly, 46 schizophrenic patients and 150 healthy controls were tested for association with DXYS17, DXYS20, DXYS28, and MIC2 in the pseudoautosomal region. Sex concordant rates in sibpairs with schizophrenia were not higher than would be expected by chance. No significant associations were found between four DNA markers we tested and schizophrenia. These results did not support the hypothesis; however, linkage disequilibrium can only be detected if the marker and trait loci are located close enough. Linkage analyses in multiplex families need to be carried out before ruling out this region as a location for a gene for schizophrenia.

Failure to find linkage between schizophrenia and genetic markers on chromosome 21

We sought evidence for the involvement of mutations in the amyloid precursor protein gene (APP) in the pathogenesis of schizophrenia in two ways. First, linkage analysis was performed in a sample of 24 families multiply affected with schizophrenia. The genotypes were studied for GT12 (D21S210), a highly polymorphic microsatellite marker at the APP locus. Second, we used single strand conformation analysis (SSCA) to screen for mutations in exon 17 of APP in one affected member from each family and in a sample of 44 unrelated patients. In addition, we looked for linkage between schizophrenia and a series of highly polymorphic markers situated at approximately 20cM intervals along the long arm of chromosome 21. We were unable to find evidence for linkage to GT12 or the other markers studied. SSCA did not reveal any mutations in exon 17 of AP. We conclude that mutations within APP are an unlikely cause of schizophrenia. Moreover, this study provides no evidence for a major gene for schizophrenia on chromosome 21, and linkage can be excluded from much of this region under some genetic models.

Male siblings with schizophrenia share alleles at the androgen receptor above chance expectation

In families that included two or more siblings with schizophrenia or schizo-affective disorder male-male pairs were found to share alleles at the androgen receptor (AR) gene (in Xq11.2-q12) above chance expectation (p < 0.003); female-female and mixed sex pairs showed no such tendency. The findings are compatible with X-Y linkage or with an X-linked contribution to liability in males.