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

Bipolar disorder and polymorphisms of glutathione S-transferases M1 (GSTM1) and T1 (GSTT1)

Glutathione S-transferases are ubiquitous multifunctional enzymes, which play a key role in cellular detoxification. The present case-control study was performed in Shiraz, Iran to investigate the association between polymorphisms of glutathione S-transferases M1 (GSTM1) and T1 (GSTT1) and susceptibility to bipolar disorder (BPD). A total of 228 BPD patients participated in the study. In addition, 236 healthy blood donors, who frequency matched with the patients according to age and gender, were also studied as a control group. Statistical analysis revealed that polymorphisms of neither GSTM1 (OR=0.73, 95% CI: 0.50-1.05) nor GSTT1 (OR=0.98, 95% CI: 0.65-1.47) were associated with risk of BPD. Patients were stratified according to their age of onset into early onset (below 19 years old) and late onset (more than 19 years old) groups. Among the early onset group, the GSTM1 null genotype decreases the risk of BPD (OR=0.43, 95% CI: 0.24-0.79). Further analysis showed that a combination of "GSTM1 positive genotype and GSTT1 null genotype" versus "positive genotypes of GSTM1 and GSTT1" increased the risk of BPD (OR=2.28, 95% CI: 1.07-4.85). However, there was no significant association between the study polymorphisms and risk of BPD among the late onset group. The present finding indicated that GSTM1 and GSTT1 are candidate polymorphisms for susceptibility to BDP among adolescents.

The phenotypes of bipolar disorder: relevance for genetic investigations

The search for susceptibility genes for bipolar disorder (BD) depends on appropriate definitions of the phenotype. In this paper, we review data on diagnosis and clinical features of BD that could be used in genetic studies to better characterize patients or to define homogeneous subgroups. Clinical symptoms, long-term course, comorbid conditions, and response to prophylactic treatment may define groups associated with more or less specific loci. One such group is characterized by symptoms of psychosis and linkage to 13q and 22q. A second group includes mainly bipolar II patients with comorbid panic disorder, rapid mood switching, and evidence of chromosome 18 linkage. A third group comprises typical BD with an episodic course and favourable response to lithium prophylaxis. Reproducibility of cognitive deficits across studies raises the possibility of using cognitive profiles as endophenotypes of BD, with deficits in verbal explicit memory and executive function commonly reported. Brain imaging provides a more ambiguous data set consistent with heterogeneity of the illness.

Epigenetics and bipolar disorder: New opportunities and challenges

Despite significant effort, understanding of the molecular causes and mechanisms of bipolar disorder (BD) remains a major challenge. Numerous molecular genetic linkage and association studies have been conducted over the last two decades; however, the data are quite inconsistent or even controversial. This article develops an argument that molecular studies of BD would benefit significantly from adding an epigenetic (epiG) perspective. EpiG factors refer to modifications of DNA and chromatin that "orchestrate" the activity of the genome, including regulation of gene expression. EpiG mechanisms are consistent with various non-Mendelian features of BD such as the relatively high degree of discordance in monozygotic (MZ) twins, the critical age group for susceptibility to the disease, clinical differences in males and females, and fluctuation of the disease course, including interchanges of manic and depressive phases, among others. Apart from the phenomenological consistency, molecular epiG peculiarities may shed new light on the understanding of controversial molecular genetic findings. The relevance of epigenetics for the molecular studies of BD is demonstrated using the examples of genetic studies of BD on chromosome 11p and the X chromosome. A spectrum of epiG mechanisms such as genomic imprinting, tissue-specific effects, paramutagenesis, and epiG polymorphism, as well as epiG regulation of X chromosome inactivation, is introduced. All this serves the goal of demonstrating that epiG factors cannot be ignored anymore in complex phenotypes such as BD, and systematic large-scale epiG studies of BD have to be initiated.

JI, Craddock N, DePaulo JR, Baron M, Gershon ES, Ekholm J, Cichon S, Turecki G, Claes S, Kelsoe JR, Schofield PR, Badenhop RF, Morissette J, Coon H, Blackwood D, McInnes LA, Foroud T, Edenberg HJ, Reich T, Rice JP, Goate A, McInnis MG, McMahon FJ, Badner JA, Goldin LR, Bennett P, Willour VL, Zandi PP, Liu J, Gilliam C, Juo SH, Berrettini WH, Yoshikawa T, Peltonen L, Lönnqvist J, Nöthen MM, Schumacher J, Windemuth C, Rietschel M, Propping P, Maier W, Alda M, Grof P, Rouleau GA, Del-Favero J, Van Broeckhoven C, Mendlewicz J, Adolfsson R, Spence MA, Luebbert H, Adams LJ, Donald JA, Mitchell PB, Barden N, Shink E, Byerley W, Muir W, Visscher PM, Macgregor S, Gurling H, Kalsi G, McQuillin A, Escamilla MA, Reus VI, Leon P, Freimer NB, Ewald H, Kruse TA, Mors O, Radhakrishna U, Blouin JL, Antonarakis SE, Akarsu N. Genome scan meta-analysis of schizophrenia and bipolar disorder, part III: Bipolar disorder

Genome scans of bipolar disorder (BPD) have not produced consistent evidence for linkage. The rank-based genome scan meta-analysis (GSMA) method was applied to 18 BPD genome scan data sets in an effort to identify regions with significant support for linkage in the combined data. The two primary analyses considered available linkage data for "very narrow" (i.e., BP-I and schizoaffective disorder-BP) and "narrow" (i.e., adding BP-II disorder) disease models, with the ranks weighted for sample size. A "broad" model (i.e., adding recurrent major depression) and unweighted analyses were also performed. No region achieved genomewide statistical significance by several simulation-based criteria. The most significant P values (<.01) were observed on chromosomes 9p22.3-21.1 (very narrow), 10q11.21-22.1 (very narrow), and 14q24.1-32.12 (narrow). Nominally significant P values were observed in adjacent bins on chromosomes 9p and 18p-q, across all three disease models on chromosomes 14q and 18p-q, and across two models on chromosome 8q. Relatively few BPD pedigrees have been studied under narrow disease models relative to the schizophrenia GSMA data set, which produced more significant results. There was no overlap of the highest-ranked regions for the two disorders. The present results for the very narrow model are promising but suggest that more and larger data sets are needed. Alternatively, linkage might be detected in certain populations or subsets of pedigrees. The narrow and broad data sets had considerable power, according to simulation studies, but did not produce more highly significant evidence for linkage. We note that meta-analysis can sometimes provide support for linkage but cannot disprove linkage in any candidate region.

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.

Genome scan for susceptibility loci for schizophrenia and bipolar disorder

BACKGROUND

Despite the widely accepted view that schizophrenia and bipolar disorder represent independent illnesses and modes of inheritance, some data in the literature suggest that the diseases may share some genetic susceptibility. The objective of our analyses was to search for vulnerability loci for the two disorders.

METHODS

A genomewide map of 388 microsatellite DNA markers was genotyped in five schizophrenia and three bipolar disorder Austrian families. Linkage analyses was used to compute the usual parametric logarithm of the likelihood of linkage (LOD) scores and nonparametric linkage analysis (NPL scores Z(all)) was used to assess the pattern of allele sharing at each marker locus relative to the presence of the disease (GENEHUNTER). Affected status was defined as severe affective disorder or schizophrenia.

RESULTS

Across the genome, p values associated with NPL scores resulted in evidence (i.e., p <.0007) for linkage at marker D3S1265 on chromosome 3q (NPL score Z (all) = 3.74, p =.0003). Two other markers (on 3q and 6q) showed p values of <.01.

CONCLUSIONS

We detected a potential susceptibility locus for bipolar disorder and schizophrenia on chromosome 3q, which has not been reported previously. The possibility of a false positive result has to be taken into account. Our data suggest shared loci for schizophrenia and bipolar affective disorders and are consistent with the continuum model of psychosis.

Molecular linkage studies of bipolar disorders

OBJECTIVES

To review the reports of linkage findings for bipolar disorder.

METHODS

Literature review of published linkage findings in bipolar disorder.

RESULTS

There are several regions of the human genome that have been implicated repeatedly by independent investigators. These include 4p16, 12q24, 18q22, 18p11, 21q21 and 22q11. Two of these regions (18p11 and 22q11) are also implicated in genome scans of schizophrenia, suggesting that these two distinct nosological categories may share some genetic susceptibility. This hypothesis can only be tested when the underlying genes are identified.

Search for bipolar disorder susceptibility loci: the application of a modified genome scan concentrating on gene-rich regions

Conducting genome wide screens for evidence of genetic linkage has become a well-established method for identifying regions of the human genome harboring susceptibility loci for complex disorders. For bipolar disorder, a number of such studies have been performed, and several regions of the genome have potentially been implicated in the disorder. The classic design for a genome screen involves examining polymorphic genetic markers spaced at regular intervals throughout the genome, typically every 10 cM, for evidence of linkage. An alternative design, based on the observation that genes do not appear to be evenly distributed, was proposed, enabling the number of markers examined in a genome wide screen to be reduced. This article describes the application of such a modified screen to a collection of 48 Irish families with bipolar disorder, comprising a total of 82 affected sib-pairs. From the results obtained a number of regions are highlighted for further study. One of these regions (17q11.1-q12) coincides with the location of a candidate gene, the serotonin transporter, whereas others concur with the findings of published studies. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:728-732, 2000.

Genetics of psychiatric disease

Genetic epidemiologic studies reveal that relatives of bipolar (BIP) probands are at increased risk for recurrent unipolar (RUP), BIP, and schizoaffective (SA) disorders, while relatives of schizophrenia (SZ) probands are at increased risk for SZ, SA, and RUP disorders. The overlap in familial risk may reflect shared genetic susceptibility. Recent genetic linkage studies have defined confirmed susceptibility loci for BIP disorder for multiple regions of the human genome, including 4p16, 12q24, 18p11.2, 18q22, 21q21, 22q11-13, and Xq26. Studies of SZ kindreds have yielded robust evidence for susceptibility at 18p11.2 and 22q11-13, both of which are implicated in susceptibility to BIP disorder. Similarly, confirmed SZ vulnerability loci have been mapped for 6p24, 8p and 13q32. Strong statistical evidence for a 13q32 BIP susceptibility locus has been reported. Thus, both family and molecular studies of these disorders suggest shared genetic susceptibility. These two group of disorders may not be so distinct as current nosology suggests.

WFS1 gene mutation search in depressive patients: detection of five missense polymorphisms but no association with depression or bipolar affective disorder

BACKGROUND

Wolfram syndrome (WFS) is an autosomal recessive neurodegenerative disorder. Recently, the WFS1 gene was isolated, and approximately 80% of the mutations responsible for WFS were found in exon 8 of WFS1. It has been noted that heterozygous carriers of the WFS gene are 26-fold more likely to be hospitalized for depression, and it has been estimated that approximately 25% of all people hospitalized for depression may carry the WFS gene(s).

METHODS

We searched for mutations in exon 8 of WFS1 in 30 depressive patients with a history of hospitalization and whose age at onset was under 40 years. We also examined 47 bipolar affective patients and 62 control subjects for an association.

RESULTS

A were detected. Four of the six were novel. No nonsense or frameshift mutation was detected. Genotypic and allelic distributions were similar between the depressive patients and the controls. No association with bipolar affective disorder was suggested.

LIMITATIONS

Because of the small sample size, the probability of finding at least one patient with WFS-responsible mutation(s) was 70% if depression is associated with WFS1 mutation(s) in 5% of patients.

CONCLUSION

It is not likely that WFS1 mutations are responsible for as much as 25% of depressive illness.

Peek-a-boo fragile site at 16d associated with Tourette syndrome, bipolar disorder, autistic disorder, and mental retardation

Five patients with a fragile site at 16q22-23 and neuropsychiatric disorders are reported. Three of five had Tourette disorder, three had mental retardation, two had bipolar disorder, and one had autistic disorder. During our attempts to study the fragile sites in more detail we were unable to reproduce the fragile sites found several years earlier. The potential relationship between the fragile sites and the neuropsychiatric disorders in these patients is discussed. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:69-73, 2000.

Elevated levels of tyrosine hydroxylase in the locus coeruleus in major depression

BACKGROUND

Levels of tyrosine hydroxylase (TH) are regulated in the noradrenergic locus coeruleus (LC) in response to changes in the activity of LC neurons and in response to changes in brain levels of norepinephrine. To study the potential role of central noradrenergic neurons in the pathobiology of major depression, TH protein was measured in the LC from postmortem brains of 13 subjects with a diagnosis of major depression and 13 age-matched control subjects having no Axis I psychiatric diagnosis. Most of the major depressive subjects died as a result of suicide.

METHODS

Protein from sections cut through multiple rostro-caudal levels of LC was transferred to Immobilon-P membrane, immunoblotted for TH, and quantified autoradiographically.

RESULTS

The distribution of TH-immunoreactivity (TH-ir) along the rostro-caudal axis of the LC was uneven and was paralleled by a similar uneven distribution of neuromelanin-containing cells in both major depressive and psychiatrically normal control subjects. Amounts of TH-ir in the rostral, middle and caudal levels of the LC from major depressive subjects were significantly higher than that of matched control subjects. There were no significant differences in the number of noradrenergic cells at any particular level of the LC comparing major depressive subjects to control subjects.

CONCLUSIONS

Elevated expression of TH in the LC in major depression implies a premortem overactivity of these neurons, or a deficiency of the cognate transmitter, norepinephrine.

Tyrosine hydroxylase polymorphism and phenotypic heterogeneity in bipolar affective disorder: a multicenter association study

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the metabolism of catecholamines, is considered a candidate gene in bipolar affective disorder (BPAD) and has been the subject of numerous linkage and association studies. Taken together, most results do not support a major gene effect for the TH gene in BPAD. Genetic and phenotypic heterogeneity may partially explain the difficulty of confirming the exact role of this gene using both association and linkage methods. Four hundred one BPAD patients and 401 unrelated matched controls were recruited within a European collaborative project (BIOMED1 project in the area of brain research, European Community grant number CT 92-1217, project leader: J. Mendlewicz) involving 14 centers for a case-control association study with a tetranucleotide polymorphism in the TH gene. Patients and controls were carefully matched for geographical origin. Phenotypic heterogeneity was considered and subgroup analyses were performed with relevant variables: age at onset, family history, and diagnostic stability. No association was observed in the total sample or for subgroups according to age at onset (n = 172), family history alone (n = 159), or high degree of diagnostic stability and a positive family history (n = 131). The results of this association study do not confirm the possible implication of TH polymorphism in the susceptibility to BPAD.

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.

Progress and pitfalls: bipolar molecular linkage studies

This paper reviews the history of molecular genetic linkage studies of bipolar disorder. The topic is introduced with a brief discussion of various genetic concepts, including linkage, lod scores and non-parametric statistics. It is emphasized that criteria for declaring linkage must include independent confirmation by a second group of investigators. Given that the inherited susceptibility for bipolar disorder is most likely explained by multiple genes of small effect, simulations indicate that universal confirmation of valid linkages cannot be expected. With this background, several valid linkages of BP disorder to genomic regions are reviewed. These valid linkages include 18p11, 18q22, 21q21, Xq26 and 4pter. The issue of anticipation and expanding triplet repeats is discussed. Finally, there is a brief section on recommendations for future genetic linkage studies of bipolar disorder.

Mapping and characterization of novel (CAG)n repeat cDNAs from adult human brain derived by the oligo capture method

The expansion of a (CAG)n trinucleotide repeat has been associated with at least eight neurological disorders in which the repeats code for polyglutamine in the protein. To identify additional genes that possess (CAG)n repeats, single-stranded cDNA clones derived from adult human brain were screened using biotinylated oligonucleotide (CAG)8, and the hybridizing complexes were isolated with strepavidin-coated paramagnetic beads. A total of 119 cDNA clones were isolated and initially characterized by end sequencing. BLAST homology searches were used to reduce redundancies with overlapping clones and to eliminate those that show sequence identity with previously published cDNAs with triplet repeats. Only cDNA clones with more than five CAG repeats were pursued for analysis. A total of 19 novel cDNAs were further characterized by determining chromosomal assignments using the Stanford G3 and Genebridge radiation-reduced hybrid mapping panels. Transcript sizes and tissue expression patterns were determined by Northern blot analysis. Two of 19 clones showed specific or high expression in brain. These cDNAs are ideal candidate genes for other neurodegenerative disorders, such as spinocerebellar ataxia types 5 and 7, and may also be implicated in psychiatric diseases such as bipolar affected disorder and schizophrenia.

Lack of evidence for a major locus for bipolar disorder in the pericentromeric region of chromosome 18 in Irish pedigrees

Seven families, multiply affected by bipolar mood disorder, have been collected from the Irish population and have been genotyped with microsatellite markers from the pericentromeric region of chromosome 18, a region that has been implicated as a site for a susceptibility gene for this relative common psychiatric disorder. The families significantly excluded linkage of bipolar disorder to this region under various models. Although the data provided no evidence of linkage heterogeneity among families, the number of families investigated may be too small to exclude completely the possibility of linkage in a small number of families.

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.

Initial genome scan of the NIMH genetics initiative bipolar pedigrees: chromosomes 4, 7, 9, 18, 19, 20, and 21q

An initial genome scan was performed on 540 individuals from 97 families segregating bipolar disorder, collected through the National Institutes of Mental Health Genetics Initiative. We report here affected-sib-pair (ASP) data on 126 marker loci (approximately 68,000 genotypes) mapping to chromosomes 4, 7, 9, 18, 19, 20, and 21q, under three affection status models. Modest increases in identical-by-descent (IBD) allele sharing were found at the following loci: D4S2397 and D4S391 (P < 0.05) on 4p, D4S1647 (P < 0.05) on 4q, D7S1802 and D7S1869 (low P = 0.01) on 7p, D9S302 (P = 0.004) on 9q, and D20S604 on 20p and D20S173 on 20q (P < 0.05). In addition, five markers on 7q displayed increased IBD sharing (P = 0.046-0.002). Additional ASP analyses on chromosomes 18 and 21q marker data were performed using disease phenotype models defined previously. On chromosome 18, only D18S40 on 18p and D18S70 on 18q yielded a slight elevation in allele sharing (P = 0.02), implying that the reported linkages in these regions were not confirmed. On chromosome 21q, a cluster of markers within an approximately 9 cM interval: D21S1254, D21S65, D21S1440, and D21S1255 exhibited excess allele sharing (P = 0.041-0.008). Multilocus data on overlapping marker quartets, from D21S1265 to D21S1255, which were consistent with increased IBD sharing (P < 0.01, with a low of 0.0009), overlapped a broad interval of excess allele sharing reported previously, increasing support for a susceptibility locus for bipolar disorder on 21q.

Linkage studies suggest a possible locus for bipolar disorder near the velo-cardio-facial syndrome region on chromosome 22

Velo-cardio-facial syndrome (VCFS) is a congenital anomaly characterized by multiple dysmorphisms, cleft palate, cardiac anomalies, and learning disabilities, that results from a microdeletion of chromosome 22q11. An increased prevalence of psychiatric illness has been observed, with both schizophrenia and bipolar disorder commonly being diagnosed. For these reasons, the VCFS region is an interesting candidate region for bipolar disorder. We examined this region in 17 bipolar families from three populations: 13 families from the general North American population (University of California, San Diego/University of British Columbia, UCSD/UBC), three larger families from New York, and a portion of Old Order Amish pedigree 110. Three microsatellite markers spanning 13 cM around the VCFS region were genotyped in all the families. A maximum lod score of 2.51 was obtained in the UCSD/UBC families under a dominant model at D22S303. In the combined family set, maximum lod scores of 1.68 and 1.28 were obtained at this marker under dominant and recessive models, respectively. Four additional markers were subsequently typed in selected positive families, and yielded positive lods at 6 of 7 markers spanning 18 cM in this region. Nonparametric, multipoint analyses using the affected pedigree member (APM) method also yielded suggestive evidence for linkage in both the UCSD/UBC family set (P = 0.0024) and in the combined families (P = 0.017). Affected sibpair analyses were similarly positive in the UCSD/UBC families (P = 0.017), and in the combined families (P = 0.004). These results are suggestive of a possible locus for bipolar disorder near the VCFS region on chromosome 22.

Association study of bipolar disorder with candidate genes involved in catecholamine neurotransmission: DRD2, DRD3, DAT1, and TH genes

Despite strong evidence for genetic involvement in the etiology of affective disorders (from twin adoption and family studies), linkage and association methodologies are still exploring the nature of genetic factors in these diseases. Interesting testable hypotheses have been described, including candidate genes involved in catecholamine neurotransmission. We studied 69 bipolar patients and 69 matched controls (for age, sex, and geographical origin) for association and linkage disequilibrium with DNA markers at the following genes: the tyrosine hydroxylase gene, dopamine transporter gene, and dopamine D2 and D3 receptor genes. Association and linkage disequilibrium were excluded between bipolar affective disorder and these four candidate genes in our sample.

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.

A locus for bipolar affective disorder on chromosome 4p

The main clinical feature of bipolar affective disorder is a change of mood to depression or elation. Unipolar disorder, also termed major depressive disorder, describes the occurrence of depression alone without episodes of elevated mood. Little is understood about the underlying causes of these common and severe illnesses which have estimated lifetime prevalences in the region of 0.8% for bipolar and 6% for unipolar disorder. Strong support for a genetic aetiology is found in the familial nature of the condition, the increased concordance of monozygotic over dizygotic twins and adoption studies showing increased rates of illness in children of affected parents. However, linkage studies have met with mixed success. An initial report of linkage on the short arm of chromosome 11 (ref. 4) was revised and remains unreplicated. Reports proposing cosegregation of genes found on the X chromosome with bipolar illness have not been supported by others. More recently bipolar disorder has been reported to be linked with markers on chromosomes 18, 21, 16 and a region on the X chromosome different from those previously suggested. We have carried out a linkage study in twelve bipolar families. In a single family a genome search employing 193 markers indicated linkage on chromosome 4p where the marker D4S394 generated a two-point lod score of 4.1 under a dominant model of inheritance. Three point analyses with neighbouring markers gave a maximum lod score of 4.8. Eleven other bipolar families were typed using D4S394 and in all families combined there was evidence of linkage with heterogeneity with a maximum two-point lod score of 4.1 (theta = 0, alpha = 0.35).

Linkage analysis between pericentrometric markers on chromosome 18 and bipolar disorder: a replication test

Replication was attempted of a recent report on linkage between bipolar affective disorder and pericentrometric loci on chromosome 18. Linkage to these markers was excluded in a sample of five extended multiplex families using lod-score and affected-pedigree-member methods. In one family, however, the lod score exceeded 1.0. Although the proposed susceptibility genes are unlikely to have a major impact on the occurrence of bipolar disorder, they might modify the genetic risk in a minority of familial cases.

Adrenocorticotropin receptor/melanocortin receptor-2 maps within a reported susceptibility region for bipolar illness on chromosome 18

We have examined the possible linkage of adrenocorticotropin receptor/melanocortin receptor-2 (ACTHR/MC-2) to a reported putative susceptibility locus for bipolar illness (BP) in 20 affected pedigrees. Initially, allelic variants of the gene were identified by polymerase chain reaction-single stranded conformation polymorphism (PCR-SSCP) and the gene was genetically mapped using both the Centre d'Etudes du Polymorphisme Humain (CEPH) pedigrees and the BP pedigrees used in this study. We found that the ACTHR/MC-2 gene maps between D18S53 and D18S66. These loci span a region of chromosome 18 which, in a previous study [Berrettini et al.: Proc Natl Acad Sci USA 91:5918-5921, 1994) revealed a putative predisposing locus to BP through nonparametric methods of linkage analysis. Linkage of ACTHR/MC-2 to BP was not demonstrable under parametric and nonparametric methods of analyses, although affected sib-pair (ASP) method revealed an increase in allele sharing among ill individuals, P = 0.023. Since this receptor is within a potential linkage region, ACTHR/MC-2 could be considered a candidate gene for BP.