Manic depressive illness not linked to factor IX region in an independent series of pedigrees

Molecular genetics of affective disorders

Evidence for familial aggregation in Affective Disorders (AD) has been provided in classical studies. Linkage and association genetic studies have been proposed to detect genetic factors implicated in AD. However, findings from molecular genetic studies remain inconclusive. Nevertheless, current research is focusing on the phenotypes, both sub- and endophenotypes. In addition, recent advances in technology, such as microarrays, provide new tools in psychiatric genetics. These different approaches offer a new optimism era in the search of genetic factors in AD.

Genetic linkage in bipolar disorder

Bipolar disorder is an etiologically complex syndrome that is clearly heritable. Multiple genes, working singly or in concert, are likely to cause susceptibility to bipolar disorder. Bipolar disorder genetics has progressed rapidly in the last few decades. However, specific causal genetic mutations for bipolar disorder have not been identified. Both candidate gene studies and complete genome screens have been conducted. They have provided compelling evidence for several potential bipolar disorder susceptibility loci in several regions of the genome. The strongest evidence suggests that bipolar disorder susceptibility loci may lie in one or more genomic regions on chromosomes 18, 4, and 21. Other regions of interest, including those on chromosomes 5 and 8, are also under investigation. New approaches, such as the use of genetically isolated populations and the use of endophenotypes for bipolar disorder, hold promise for continued advancement in the search to identify specific bipolar disorder genes.

Genome scan of a second wave of NIMH genetics initiative bipolar pedigrees: chromosomes 2, 11, 13, 14, and X

As part of the on-going NIMH Genetics Initiative on Bipolar Disorder, we have ascertained 153 multiplex bipolar pedigrees and genotyped them in two waves. We report here the genome scan results for chromosomes 2, 11, 13, 14, and X in the second wave of 56 families. A total of 354 individuals were genotyped and included in the current analyses, including 5 with schizoaffective/bipolar (SA/BP), 139 with bipolar I disorder (BPI), 41 with bipolar II disorder (BPII), and 43 with recurrent unipolar depression (RUP). Linkage analyses were carried out with multi-point parametric and non-parametric affected relative pair methods using three different definitions of the affected phenotype: (model 1) SA/BP and BPI; (model 2) SA/BP, BPI, and BPII; and (model 3) SA/BP, BPI, BPII, and RUP. The best findings were on 11p15.5 (NPL = 2.96, P = 0.002) and Xp11.3 (NPL = 2.19, P = 0.01). These findings did not reach conventional criteria for significance, but they were located near regions that have been identified in previous genetic studies of bipolar disorder. The relatively modest but consistent findings across studies may suggest that these loci harbor susceptibility genes of modest effect in a subset of families. Large samples such as that being collected by the NIMH Initiative will be necessary to examine the heterogeneity and identify these susceptibility genes.

The human genome project and its impact on psychiatry

There has been substantial evidence for more than three decades that the major psychiatric illnesses such as schizophrenia, bipolar disorder, autism, and alcoholism have a strong genetic basis. During the past 15 years considerable effort has been expended in trying to establish the genetic loci associated with susceptibility to these and other mental disorders using principally linkage analysis. Despite this, only a handful of specific genes have been identified, and it is now generally recognized that further advances along these lines will require the analysis of literally hundreds of affected individuals and their families. Fortunately, the emergence in the past three years of a number of new approaches and more effective tools has given new hope to those engaged in the search for the underlying genetic and environmental factors involved in causing these illnesses, which collectively are among the most serious in all societies. Chief among these new tools is the availability of the entire human genome sequence and the prospect that within the next several years the entire complement of human genes will be known and the functions of most of their protein products elucidated. In the meantime the search for susceptibility loci is being facilitated by the availability of single nucleotide polymorphisms (SNPs) and by the beginning of haplotype mapping, which tracks the distribution of clusters of SNPs that segregate as a group. Together with high throughput DNA sequencing, microarrays for whole genome scanning, advances in proteomics, and the development of more sophisticated computer programs for analyzing sequence and association data, these advances hold promise of greatly accelerating the search for the genetic basis of most mental illnesses while, at the same time, providing molecular targets for the development of new and more effective therapies.

Excess of allele1 for alpha3 subunit GABA receptor gene (GABRA3) in bipolar patients: a multicentric association study

The available data from preclinical and pharmacological studies on the role of gamma amino butyric acid (GABA) support the hypothesis that a dysfunction in brain GABAergic system activity contributes to the vulnerability to bipolar affective disorders (BPAD). Moreover, the localization of the alpha3 subunit GABA receptor GABRA3 gene on the Xq28, a region of interest in certain forms of bipolar illness, suggests that GABRA3 may be a candidate gene in BPAD. In the present study, we tested the genetic contribution of the GABRA3 dinucleotide polymorphism in a European multicentric case-control sample, matched for sex and ethnogeographical origin. Allele and genotype (in females) frequencies were compared in 185 BPAD patients and 370 controls. A significant increase of genotype 1-1 was observed in BPAD females compared to controls (P=0.0004). Furthermore, when considering recessivity of allele 1 (females with genotype 1-1 and males carrying allele 1), results were even more significant (P= 0.00002). Our findings suggest that the GABRA3 polymorphism may confer susceptibility to or may be in linkage disequilibrium with another gene involved in the genetic etiology of BPAD.

Association between a polymorphism in the pseudoautosomal X-linked gene SYBL1 and bipolar affective disorder

In the past decade, several chromosomal regions have been analyzed for linkage with bipolar affective disorder (BPAD). There have been conflicting results regarding the involvement of X-chromosomal regions in harboring susceptibility genes for BPAD. Recently, a new candidate gene (SYBL1) for BPAD has been described on Xq28. SYBL1, which maps to the Xq pseudoautosomal region (PAR), encodes a member of the synaptobrevin family of proteins involved in synaptic vesicle docking, exocytosis, and membrane transport. A subsequent case-control association study, including 110 US-American patients with BPAD and 119 unrelated controls, investigated a potential etiological role of a novel polymorphism (G-->C transversion) in a regulatory region of the SYBL1 gene. In this analysis, the C allele showed a statistical trend to be more frequent in males with BPAD than in respective controls (P=0.06). This finding prompted us to verify whether a similar effect was also present in a larger German sample of 164 unrelated patients with BPAD (148 patients with BP I disorder, 16 patients with BP II disorder) and 267 controls. We observed a significantly increased frequency of genotypes homozygous for the C allele in females with BPAD in comparison with controls (P=0.017). Thus, our data strengthen the role of the SYBL1 gene as a candidate gene for BPAD.

Molecular genetic and family studies in affective disorders: state of the art

The identification of genes responsible for mood disorders will contribute to significant advances in the awareness of diagnosis (diagnostic process and early recognition), pathophysiology, epidemiology and treatment issues. During the past two decades, the search for genes for mood disorders has mainly contributed to better understand and confirm the genetic complexities inherent to these disorders. The large amount of results available and the difficulty to digest them corroborate this observation. The major contribution of these findings should be integrated in the context of the world-wide efforts to identify the thousands of genes of the human genome. Some of these genes may be identified within the next decade. Several consistent hypotheses are currently being tested and will, hopefully, speed up the process of narrowing the important regions when the complete genome map will be available. The most promising chromosomal regions have been localized on chromosomes 4, 5, 11, 12, 18, 21 and X. A number of candidate genes have also been investigated, some of these are directly linked to neurobiological hypotheses of the aetiology of affective disorders. In parallel, specific hypotheses have been implicated, such as anticipation and dynamic mutations. Further research should concentrate on these hypotheses and confirm positive findings through interdisciplinary and multicenter projects.

Analysis of the pseudoautosomal X-linked gene SYBL1in bipolar affective disorder: description of a new candidate allele for psychiatric disorders

The absence of father-to-son transmission has been observed in a subset of families with bipolar disorder (BPD), suggestive of a susceptibility gene on the sex-linked portion of the X chromosome. This is supported by some genetic linkage studies that have provided evidence for a susceptibility locus near Xq28. We have analyzed one candidate gene on Xq28, SYBL1, which maps to the Xq pseudoautosomal region (PAR). SYBL1 encodes a member of the synaptobrevin family of proteins that is involved in synaptic vesicle docking and membrane transport. Genes in the PAR generally escape X-chromosome inactivation and have an active homolog on the Y chromosome, which would result in an increase in same-sex concordance in paternal transmitted traits. However, SYBL1 is neither expressed on the Y chromosome nor the inactive X chromosome and would therefore be expected to show typical sex-linked transmission. We have screened SYBL1 for mutations that could be tested as candidate alleles in the development of BPD. Following single-strand conformation polymorphism (SSCP) analysis and DNA sequencing, four single nucleotide polymorphisms were detected: a silent mutation at codon 108, two intron mutations without any obvious biological significance, and a G-->C transversion in the polypyrimidine tract at the 3' splice acceptor site preceding exon 8. This polymorphism, which creates a perfect 16/16 stretch of pyrimidines, was analyzed in 110 patients with BPD not selected for sex-linked transmission and 119 control subjects. The results show a statistical trend toward an increase in the frequency of the C allele in males with BPD but not females. Males: chi(2) = 3.46, 1 df, p =.06; Females: chi(2) =.20, 1 df, p =.66.

Searching high and low: a review of the genetics of bipolar disorder

OBJECTIVES

To review the methodologies and findings in the genetics of bipolar disorder (BPD), and to suggest future directions for research.

METHODS

Reports of family, twin, adoption, linkage, association, cytogenetic, and animal model studies, and segregation analyses in English, were identified from multiple MEDLINE searches. Hand searches were carried out in bibliographies from review articles.

RESULTS

Family, twin, and adoption studies have provided strong evidence for a genetic etiology in BPD. Early reports of linkage of BPD to DNA markers at several chromosomal sites have not proven robust, perhaps because of the complex nature of BPD inheritance. However, linkage findings in the 1990s, on chromosomes 18, 21q, 12q, and 4p, have provided leads that are being pursued through both genetic and physical mapping. No gene has yet been definitively implicated in BPD.

CONCLUSIONS

Strategies for increasing the power to detect BPD genes include: (1) dividing the phenotype into genetically meaningful subtypes to decrease heterogeneity: and (2) ascertaining a very large family sample--a multicenter study now in progress will collect 700 bipolar I sibling pairs. BPD may result from several genes acting in concert so that new multilocus statistical methods could enhance the capacity to detect loci involved. Family-based association studies using a very large number of newly identified single nucleotide polymorphisms (SNPs) may allow for more efficient screening of the genome. As the Human Genome Project approaches its goal of isolating all genes by 2003, the data generated is likely to speed identification of candidate BPD genes.

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.

Genomic survey of bipolar illness in the NIMH genetics initiative pedigrees: a preliminary report

Four sites collaborated with the NIMH to develop a resource for the genetic study of bipolar (BP) illness. Common methods of ascertainment and assessment were developed in 1989. A series of families with a bipolar I (BPI) proband and at least one BPI or schizoaffective, bipolar type (SA/BP) first-degree relative has been studied. We now report initial data from a genomic survey with an average intermarker interval of 10 cM on 540 subjects from 97 families. This is the largest commonly ascertained and assessed linkage sample for bipolar illness reported to date; it includes 232 subjects with BPI, 32 SA/BP, 72 bipolar II (BPII), and 88 unipolar, recurrent (UPR). Nonparametric methods of analysis were employed, with all sites using affected sib pair analysis. The strongest findings thus far appear to be on chromosomes 1, 6, 7, 10, 16, and 22. Support has also been found for some previously reported linkages, including 21q and possibly Xq26. All these areas (as well as others) will be followed up with additional markers and further analyses. No locus tested thus far meets stringent criteria for an initial finding of significant linkage.

Molecular Genetics of Mental Disorders with Particular Reference to Affective Disorders

The present article reviews the recent molecular genetic findings in affective disorders. Results of linkage and association studies are discussed in regard to the main limitations of these approaches in psychiatric disorders. On the whole, linkage and association studies contributed to the localisation of some potential vulnerability genes for Bipolar affective disorder on chromosomes 18, 5, 11, 4, 21 and X. The hypothesis of anticipation in affective disorders is also considered in light of interesting results with trinucleotide repeat mutations.

Molecular genetics of bipolar disorder

Bipolar (BP) disorder is a severe mood disorder affecting about 1% of the population. Even though the traditional twin, family, and adoption studies have demonstrated that it is highly heritable, the specific vulnerability genes have so far escaped identification. The early years of molecular genetic studies in BP disorder were hampered by the complexities in the inheritance and phenotype of BP disorder, the poor marker maps and the low informativeness of DNA markers available at that time. The new developments in molecular genetics and statistical analysis methods for complex disorders have provided researchers with better tools to cope with these difficulties. During the past few years, several potential susceptibility loci have been reported in chromosomes 18, 21 and X, and the possible role of trinucleotide repeat expansions in the aetiology of BP disorder has been developed. It seems that the molecular genetics of BP disorder are entering a new era of rapid developments.

A systematic evaluation of linkage studies in bipolar disorder

Genetic factors have long been implicated in the aetiology of bipolar disorder (BD). During the past two decades several linkage studies have been carried out with the aim of identifying major genes. However, remarkable discrepancies in results both between and within studies have constituted a major problem. In order to elucidate some of these conflicts, we assessed the published literature on linkage studies of bipolar disorder, focusing on methodological issues. Studies published between January 1980 and December 1994 were identified by computerized literature searches and subsequent scanning of review articles, and the reference lists of the articles primarily identified. A set of defined inclusion and exclusion criteria was used to select studies for assessment. A total of 31 variables were determined, and pre-defined codes were assigned in a structured manner. More than 200 citations were reviewed, and 60 articles were included in this study. Descriptive statistical analyses of the variables, as well as associations between variables, are presented. The findings are discussed with regard to the possibility that, beyond the genetic complexity of the disorder itself, there are several other similarly complicated study design issues which should be more carefully observed. Moreover, the need for standardization of basic criteria to use and report clinical and analytical parameters employed in linkage studies is strongly suggested.

Linkage analysis of families with bipolar illness and chromosome 18 markers

Linkage of bipolar (BP) illness with chromosome 18 markers located at 18p11 was recently reported. A possible role for chromosome 18 in the etiology of BP illness was implicated previously by the finding in three unrelated patients of a ring chromosome with breakpoints and deleted segments at 18pter-p11 and 18q23-qter. To test the potential importance of a gene defect on chromosome 18 in our material, we examined linkage with chromosome 18 markers in two families with multiple patients with BP illness or BP spectrum disorders. fourteen simple tandem repeat polymorphisms were used located in the chromosomal region 18p11 to 18q23 and separated by distances of approximately 10 cM on the genetic map. In one family linkage to chromosome 18 could not be excluded. Linkage and segregation analysis in the family suggests that the 12-cM region between D18S51 and D18S61 located at 18q21.33-q23 may contain a candidate gene for BP illness.

Evidence of a predisposing locus to bipolar disorder on Xq24-q27.1 in an extended Finnish pedigree

An X-chromosomal predisposing locus to manic-depressive illness has been suggested since 1969 on the basis of the cosegregation of this trait in some families with phenotypic markers, such as color blindness, the glucose-6-phosphate dehydrogenase deficiency, and the coagulation factor IX deficiency. However, the conclusive evidence and the exact location of the putative X-chromosomal locus have remained controversial. We report here a linkage between DNA markers near the coagulation factor IX gene and bipolar disorder in an extended pedigree rising from the genetically isolated population of Finland. A distinct chromosomal haplotype covering a 20-cM region on Xq24-q27.1 could be demonstrated to segregate with bipolar disorder. These findings should encourage research groups to study extended family materials with Xq24-q27.1 markers to finally resolve the question of the X-chromosomal linkage of bipolar disorder.

Linkage analysis of bipolar illness with X-chromosome DNA markers: a susceptibility gene in Xq27-q28 cannot be excluded

Transmission studies have supported the presence of a susceptibility gene for bipolar (BP) illness on the X-chromosome. Initial linkage studies with color blindness (CB), glucose-6-phosphate dehydrogenase (G6PD) deficiency, and the blood coagulation factor IX (F9) have suggested that a gene for BP illness is located in the Xq27-q28 region. We tested linkage with several DNA markers located in Xq27-q28 in 2 families, MAD3 and MAD4, that previously were linked to F9 and 7 newly ascertained families of BP probands. Linkage was also examined with the gene encoding the alpha 3 subunit of the gamma-amino butyric acid receptor (GABRA3), a candidate gene for BP illness located in this region. The genetic data were analyzed with the LOD score method using age-dependent penetrance of an autosomal dominant disease gene and narrow and broad clinical models. In MAD3 and MAD4 the multipoint LOD score data suggested a localization of a BPI gene again near F9. In the 7 new families the overall linkage data excluded the Xq27-q28 region. However, if the families were grouped according to their proband's phenotype BPI or BPII, a susceptibility gene for BPI disorder at the DXS52-F8 cluster could not be excluded.

Genetic linkage mapping for a susceptibility locus to bipolar illness: chromosomes 2, 3, 4, 7, 9, 10p, 11p, 22, and Xpter

We are conducting a genome search for a predisposing locus to bipolar (manic-depressive) illness by genotyping 21 moderate-sized pedigrees. We report linkage data derived from screening marker loci on chromosomes 2, 3, 4, 7, 9, 10p, 11p, 22, and the pseudoautosomal region at Xpter. To analyze for linkage, two-point marker to illness lod scores were calculated under a dominant model with either 85% or 50% maximum penetrance and a recessive model with 85% maximum penetrance, and two affection status models. Under the dominant high penetrance model the cumulative lod scores in the pedigree series were less than -2 at theta = 0.01 in 134 of 142 loci examined, indicating that if the disease is genetically homogeneous linkage could be excluded in these marker regions. Similar results were obtained using the other genetic models. Heterogeneity analysis was conducted when indicated, but no evidence for linkage was found. In the course of mapping we found a positive total lod score greater than +3 at the D7S78 locus at theta = 0.01 under a dominant, 50% penetrance model. The lod scores for additional markers within the D7S78 region failed to support the initial finding, implying that this was a spurious positive. Analysis with affected pedigree member method for COL1A2 and D7S78 showed no significance for linkage but for PLANH1, at the weighting functions f(p) = 1 and f(p) = 1/sqrt(p) borderline P values of 0.036 and 0.047 were obtained. We also detected new polymorphisms at the mineralocorticoid receptor (MLR) and calmodulin II (CALMII) genes. These genes were genetically mapped and under affection status model 2 and a dominant, high penetrance mode of transmission the lod scores of < -2 at theta = 0.01 were found.

A linkage study of distal chromosome 5q and bipolar disorder

There are well-established abnormalities of hypothalamic-pituitary-adrenal (HPA) axis and beta 2 adrenergic receptor function in affective disorders. The genes for the glucocorticoid receptor (GRL) and the beta 2 adrenergic receptor (ADRB2) have been cloned and mapped to distal chromosome 5q. In this study, we have examined polymorphisms of these two candidate genes and other nearby markers for linkage to bipolar disorder in Amish pedigree 110 and three large Icelandic pedigrees. These loci were tested for linkage in two-point and multipoint analyses using a model of autosomal dominant transmission with age-dependent reduced penetrance. Two-point analyses revealed a maximum LOD score of 1.14 at theta = 0.20 from GRL. Linkage could be excluded to ADRB2, as well as to three nearby anonymous markers, D5S207, D5S70, and D5S119. Analyses of another anonymous marker, D5S36, were inconclusive. Multipoint analyses excluded linkage to a 55 cM region including the interval between D5S207 and D5S36 and flanking regions, with the exception of a 7 cM interval between GRL and ADRB2. Despite the intriguing positive LOD score obtained with GRL, linkage to bipolar disorder could not be demonstrated in the region examined.

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.

Suggestion of linkage between manic-depressive illness and the enzyme phosphoglycolate phosphatase (PGP) on chromosome 16p

Two large Danish pedigrees with manic-depressive illness (MDI) were ascertained through bipolar probands. The pedigrees include bipolar as well as unipolar cases. An autosomal dominant mode of inheritance with incomplete penetrance was assumed. Linkage relationships between MDI and 37 autosomal serum, enzyme and blood group markers were investigated. For phosphoglycolate phosphatase, a maximum lod score of 2.20 at 0% recombination was found for the largest family. The other family was not informative. This may suggest assignment of a major gene for MDI to chromosome 16p13.

Approaches to the genetics of affective disorders

Affective (mood) disorders are common. There are several methodological impediments to genetic studies of affective disorders, including uncertainties about the best definition of disease phenotype, difficulties in the assessment of lifetime diagnosis and variable age of onset of illness. Despite these difficulties, family, twin and adoption studies provide compelling evidence for the existence of important genetic factors in determining susceptibility to affective disorders. However, the mode of inheritance is unknown. Simple mendelian inheritance may occur in some families but cannot explain the majority of cases. With the advent of polymorphic DNA markers, linkage and association studies have become more useful methods for the genetic analysis of complex disorders such as affective illness. No consistent finding has yet emerged, although chromosomal region 11p15 (and to a lesser extent Xq28) are of continuing interest. In addition to further study of these regions it will also be necessary to look for susceptibility loci in other parts of the genome. Large samples will almost certainly be required. If susceptibility loci of major effect exist then linkage approaches will find them. However, if there are only loci of small effect, then association approaches will be necessary. At present, it seems prudent to pursue both linkage and association approaches together.

Minireview: Molecular genetics in affective illness

Genetic transmission in manic depressive illness (MDI) has been explored in twins, adoption, association, and linkage studies. The X-linked transmission hypothesis has been tested by using several markers on chromosome X: Xg blood group, colour blindness, glucose-6-phosphate dehydrogenase (G6PD), factor IX (haemophilia B), and DNA probes such as DXS15, DXS52, F8C, ST14. The hypothesis of autosomal transmission has been tested by association studies with the O blood group located on chromosome 9, as well as linkage studies on chromosome 6 with the Human Leucocyte Antigens (HLA) haplotypes and on Chromosome 11 with DNA markers for the following genes: D2 dopamine receptor, tyrosinase, C-Harvey-Ras-A (HRAS) oncogene, insuline (ins), and tyrosine hydroxylase (TH). Although linkage studies support the hypothesis of a major locus for the transmission of MDI in the Xq27-28 region, several factors are limiting the results, and are discussed in the present review.

Diminished support for linkage between manic depressive illness and X-chromosome markers in three Israeli pedigrees

The hypothesis that chromosomal region Xq27-28 harbours a gene for manic-depression has been a focus of interest in human genetics. X-linked inheritance of manic depressive illness has been re-examined in 3 multigeneration Israeli kindreds. Extension and re-evaluation of pedigree data, including new individuals, diagnostic follow-up, and analysis with DNA markers, shows greatly diminished support for linkage to Xq28. The peak lod scores in two of the pedigrees have dropped several lod units to clearly negative values at the RCP-F8-G6PD gene cluster. On the other hand, positive lod scores (Zmax = 2.09) are sustained in another pedigree at the same map location. None of the pedigrees show linkage to more proximal markers, including the Xq27 locus DXS98. Our analysis underscores the uncertainties in studying complex disorders.

Advances and retreats in the molecular genetics of major mental illness

With the last two decades, the importance of genetic factors in the aetiology of major mental illness has been firmly re-established and psychiatric research has now firmly embraced the era of molecular genetics. Despite a number of false starts in the study of schizophrenia and affective disorder, there have been successes in unmasking some of the aetiological secrets of Alzheimer's disease. We will give an overview of the rationale behind these studies and the major findings to date.

Marker genotyping errors in old data on X-linkage in bipolar illness

Investigations of linkage markers of the X-chromosome colorblindness region in bipolar manic-depressive illness (BP) have yielded inconsistent results, with linkage accepted in some and rejected in other studies. Although genetic heterogeneity has been proposed as the reason for differences, other possibilities exist, including systematic procedural errors. Statistical evidence for linkage between the markers, Xg and colorblindness, is present in a series of papers on bipolar illness reported in 1972-1975. The linkage implied by this reanalysis is spurious, since the two markers are at opposite ends of the X chromosome. The presumptive reason for this spurious linkage is that it is a result of systematic genotyping errors. The support provided by these data to the X-linkage hypothesis in BP illness is thus diminished. That is, the linkage to illness may depend on systematic errors in marker genotyping. In general, the possible causes of inconsistency between linkage reports may be divided into statistical and systematic causes. Statistical causes would generally consist of chance differences in sampling, such as might occur under genetic heterogeneity. If this occurs, the reports rejecting linkage may be false negatives, or the reports detecting linkage may be false-positive results. Systematic causes of differences among reports could include systematic errors (or variations) in procedures, including ascertainment, diagnosis, genotyping, or analysis. Consistency of the marker map in a particular study with the known marker map is one test for systematic errors in genotyping.