Distinguishing bipolar from unipolar depression: the importance of clinical symptoms and illness features.

BACKGROUND: Distinguishing bipolar disorder (BP) from major depressive disorder (MDD) has important relevance for prognosis and treatment. Prior studies have identified clinical features that differ between these two diseases but have been limited by heterogeneity and lack of replication. We sought to identify depression-related features that distinguish BP from MDD in large samples with replication. METHOD: Using a large, opportunistically ascertained collection of subjects with BP and MDD we selected 34 depression-related clinical features to test across the diagnostic categories in an initial discovery dataset consisting of 1228 subjects (386 BPI, 158 BPII and 684 MDD). Features significantly associated with BP were tested in an independent sample of 1000 BPI cases and 1000 MDD cases for classifying ability in receiver operating characteristic (ROC) analysis. RESULTS: Seven clinical features showed significant association with BPI compared with MDD: delusions, psychomotor retardation, incapacitation, greater number of mixed symptoms, greater number of episodes, shorter episode length, and a history of experiencing a high after depression treatment. ROC analyses of a model including these seven factors showed significant evidence for discrimination between BPI and MDD in an independent dataset (area under the curve = 0.83). Only two features (number of mixed symptoms, and feeling high after an antidepressant) showed an association with BPII versus MDD. CONCLUSIONS: Our study suggests that clinical features distinguishing depression in BPI versus MDD have important classification potential for clinical practice, and should also be incorporated as 'baseline' features in the evaluation of novel diagnostic biomarkers.

Type I interferon signaling genes in recurrent major depression: increased expression detected by whole-blood RNA sequencing.

A study of genome-wide gene expression in major depressive disorder (MDD) was undertaken in a large population-based sample to determine whether altered expression levels of genes and pathways could provide insights into biological mechanisms that are relevant to this disorder. Gene expression studies have the potential to detect changes that may be because of differences in common or rare genomic sequence variation, environmental factors or their interaction. We recruited a European ancestry sample of 463 individuals with recurrent MDD and 459 controls, obtained self-report and semi-structured interview data about psychiatric and medical history and other environmental variables, sequenced RNA from whole blood and genotyped a genome-wide panel of common single-nucleotide polymorphisms. We used analytical methods to identify MDD-related genes and pathways using all of these sources of information. In analyses of association between MDD and expression levels of 13 857 single autosomal genes, accounting for multiple technical, physiological and environmental covariates, a significant excess of low P-values was observed, but there was no significant single-gene association after genome-wide correction. Pathway-based analyses of expression data detected significant association of MDD with increased expression of genes in the interferon α/ß signaling pathway. This finding could not be explained by potentially confounding diseases and medications (including antidepressants) or by computationally estimated proportions of white blood cell types. Although cause-effect relationships cannot be determined from these data, the results support the hypothesis that altered immune signaling has a role in the pathogenesis, manifestation, and/or the persistence and progression of MDD.

A genome-wide association study of attempted suicide.

The heritable component to attempted and completed suicide is partly related to psychiatric disorders and also partly independent of them. Although attempted suicide linkage regions have been identified on 2p11-12 and 6q25-26, there are likely many more such loci, the discovery of which will require a much higher resolution approach, such as the genome-wide association study (GWAS). With this in mind, we conducted an attempted suicide GWAS that compared the single-nucleotide polymorphism (SNP) genotypes of 1201 bipolar (BP) subjects with a history of suicide attempts to the genotypes of 1497 BP subjects without a history of suicide attempts. In all, 2507 SNPs with evidence for association at P<0.001 were identified. These associated SNPs were subsequently tested for association in a large and independent BP sample set. None of these SNPs were significantly associated in the replication sample after correcting for multiple testing, but the combined analysis of the two sample sets produced an association signal on 2p25 (rs300774) at the threshold of genome-wide significance (P=5.07 × 10(-8)). The associated SNPs on 2p25 fall in a large linkage disequilibrium block containing the ACP1 (acid phosphatase 1) gene, a gene whose expression is significantly elevated in BP subjects who have completed suicide. Furthermore, the ACP1 protein is a tyrosine phosphatase that influences Wnt signaling, a pathway regulated by lithium, making ACP1 a functional candidate for involvement in the phenotype. Larger GWAS sample sets will be required to confirm the signal on 2p25 and to identify additional genetic risk factors increasing susceptibility for attempted suicide.

Co-morbid anxiety disorders in bipolar disorder and major depression: familial aggregation and clinical characteristics of co-morbid panic disorder, social phobia, specific phobia and obsessive-compulsive disorder.

BACKGROUND: Co-morbidity of mood and anxiety disorders is common and often associated with greater illness severity. This study investigates clinical correlates and familiality of four anxiety disorders in a large sample of bipolar disorder (BP) and major depressive disorder (MDD) pedigrees. METHOD: The sample comprised 566 BP families with 1416 affected subjects and 675 MDD families with 1726 affected subjects. Clinical characteristics and familiality of panic disorder, social phobia, specific phobia and obsessive-compulsive disorder (OCD) were examined in BP and MDD pedigrees with multivariate modeling using generalized estimating equations. RESULTS: Co-morbidity between mood and anxiety disorders was associated with several markers of clinical severity, including earlier age of onset, greater number of depressive episodes and higher prevalence of attempted suicide, when compared with mood disorder without co-morbid anxiety. Familial aggregation was found with co-morbid panic and OCD in both BP and MDD pedigrees. Specific phobia showed familial aggregation in both MDD and BP families, although the findings in BP were just short of statistical significance after adjusting for other anxiety co-morbidities. We found no evidence for familiality of social phobia. CONCLUSIONS: Our findings suggest that co-morbidity of MDD and BP with specific anxiety disorders (OCD, panic disorder and specific phobia) is at least partly due to familial factors, which may be of relevance to both phenotypic and genetic studies of co-morbidity.

Family-based association of FKBP5 in bipolar disorder.

The FKBP5 gene product forms part of a complex with the glucocorticoid receptor and can modulate cortisol-binding affinity. Variations in the gene have been associated with increased recurrence of depression and with rapid response to antidepressant treatment. We sought to determine whether common FKBP5 variants confer risk for bipolar disorder. We genotyped seven tag single-nucleotide polymorphisms (SNPs) in FKBP5, plus two SNPs previously associated with illness, in 317 families with 554 bipolar offspring, derived primarily from two studies. Single marker and haplotypic analyses were carried out with FBAT and EATDT employing the standard bipolar phenotype. Association analyses were also conducted using 11 disease-related variables as covariates. Under an additive genetic model, rs4713902 showed significant overtransmission of the major allele (P=0.0001), which was consistent across the two sample sets (P=0.004 and 0.006). rs7757037 showed evidence of association that was strongest under the dominant model (P=0.001). This result was consistent across the two datasets (P=0.017 and 0.019). The dominant model yielded modest evidence for association (P<0.05) for three additional markers. Covariate-based analyses suggested that genetic variation within FKBP5 may influence attempted suicide and number of depressive episodes in bipolar subjects. Our results are consistent with the well-established relationship between the hypothalamic-pituitary-adrenal (HPA) axis, which mediates the stress response through regulation of cortisol, and mood disorders. Ongoing whole-genome association studies in bipolar disorder and major depression should further clarify the role of FKBP5 and other HPA genes in these illnesses.

Sex-specific association of the Reelin gene with bipolar disorder.

The Reelin gene (RELN) encodes a secretory glycoprotein critical for brain development and synaptic plasticity. Post-mortem studies have shown lower Reelin protein levels in the brains of patients with schizophrenia and bipolar disorder (BP) compared with controls. In a recent genome-wide association study of schizophrenia, the strongest association was found in a marker within RELN, although this association was seen only in women. In this study, we investigated whether genetic variation in RELN is associated with BP in a large family sample. We genotyped 75 tagSNPs and 6 coding SNPs in 1,188 individuals from 318 nuclear families, including 554 affected offspring. Quality control measures, transmission-disequilibrium tests (TDTs), and empirical simulations were performed in PLINK. We found a significant overtransmission of the C allele of rs362719 to BP offspring (OR = 1.47, P = 5.9 x 10(-4)); this withstood empirical correction for testing of multiple markers (empirical P = 0.048). In a hypothesis-driven secondary analysis, we found that the association with rs362719 was almost entirely accounted for by overtransmission of the putative risk allele to affected females (OR(Female) = 1.79, P = 8.9 x 10(-5) vs. OR(Male) = 1.12, P = 0.63). These results provide preliminary evidence that genetic variation in RELN is associated with susceptibility to BP and, in particular, to BP in females. However, our findings should be interpreted with caution until further replication and functional assays provide convergent support.

Linkage of bipolar disorder to chromosome 18q and the validity of bipolar II disorder.

BACKGROUND: An analysis of the relationship between clinical features and allele sharing could clarify the issue of genetic linkage between bipolar affective disorder (BPAD) and chromosome 18q, contributing to the definition of genetically valid clinical subtypes. METHODS: Relatives ascertained through a proband who had bipolar I disorder (BPI) were interviewed by a psychiatrist, assigned an all-sources diagnosis, and genotyped with 32 markers on 18q21-23. Exploratory findings from the first 28 families (n = 247) were tested prospectively in an additional 30 families (n = 259), and the effect of confirmed findings on the linkage evidence was assessed. RESULTS: In exploratory analyses, paternal allele sharing on 18q21 was significantly (P =.03) associated with a diagnostic subtype, and was greatest in pairs where both siblings had bipolar II disorder (BPII). Prospective analysis confirmed the finding that BPII-BPII sibling pairs showed significantly (P =.016) greater paternal allele sharing. Paternal allele sharing across 18q21-23 was also significantly greater in families with at least one BPII-BPII sibling pair. In these families, multipoint affected sibling-pair linkage analysis produced a peak paternal lod score of 4.67 (1-lod confidence interval, 12 centimorgans [cM]) vs 1.53 (1-lod confidence interval, 44 cM) in all families. CONCLUSIONS: Affected sibling pairs with BPII discriminated between families who showed evidence of linkage to 18q, and families who did not. Families with a BPII sibling pair produced an increased lod score and improved linkage resolution. These findings, limited by the small number of BPII-BPII sibling pairs, strengthen the evidence of genetic linkage between BPAD and chromosome 18q, and provide preliminary support for BPII as a genetically valid subtype of BPAD.

Panic disorder with familial bipolar disorder.

If bipolar disorder is genetically heterogeneous, it may be possible to discern clinically heterogeneous familial subtypes based on differential risk for psychiatric comorbidity, for example panic disorder. We evaluated 528 members of 57 families ascertained for a genetic linkage study of bipolar disorder. Families were assorted according to the panic disorder diagnosis of the bipolar proband; the rates of panic and other disorders in relatives were compared. Eighty-eight percent of the 41 subjects with panic disorder had bipolar disorder. Panic disorder was diagnosed in 18% of family members with bipolar disorder. Ten of 57 bipolar probands had panic disorder. Their bipolar first-degree relatives had a significantly higher prevalence of panic disorder, bipolar II, cyclothymia, and dysthymia, but had lower prevalence of substance abuse than the relatives of the bipolar probands without panic disorder. These findings suggest the testable hypothesis that comorbid panic disorder is a marker of genetic heterogeneity in bipolar disorder.

NEDD4L on human chromosome 18q21 has multiple forms of transcripts and is a homologue of the mouse Nedd4-2 gene.

The validation of full-length cDNA represents a crucial step in gene identification and subsequent functional analysis. In searching for candidate genes for bipolar disorder on chromosome 18q21, a novel gene homologous to NEDD4 (Neural precursor cells expressed developmentally down-regulated) was identified using exon trapping and cDNA cloning. This novel gene is termed NEDD4L (Human Gene Nomenclature Committee symbol). Typical NEDD4 orthologues that contain a C2 (Ca(2+)/lipid-binding) and a HECT (Homologous to the E6-AP Carboxyl Terminus) ubiquitin-protein ligase domain, and multiple WW domains have been shown to regulate the epithelial sodium channel (ENaC). In mice, Nedd4 has two distinct isoforms termed Nedd4-1 that belongs to the typical NEDD4 class, and Nedd4-2 that is homologous to Nedd4-1 but lacks the C2 domain. NEDD4L contains the WW and HECT domains seen in the NEDD4 gene family, but lacks the C2 domain in the N-terminus. BLAST database search showed that the deduced polypeptide of NEDD4L has 97 and 62% sequence identity to mouse Nedd4-2 and human NEDD4, respectively. Multiple forms of transcripts of NEDD4L have been isolated, which differ in transcription start and termination sites together with the presence or absence of an alternative spliced exon. Northern blot analysis showed a 3.4 kb mRNA species was specifically expressed in heart and skeletal muscle, while a 3.2 kb band and/or an additional 3.6 kb band is seen in other tissues tested. Striking homology of NEDD4L to mouse Nedd4-2 suggests it is the human homologue of mouse Nedd4-2. Its position in a region of linkage for autosomal dominant orthostatic hypotensive disorder and its potential role in regulating ENaC make NEDD4L a candidate gene for this disorder.

Attempted suicide and alcoholism in bipolar disorder: clinical and familial relationships.

OBJECTIVE: This study examined the clinical and familial relationships between comorbid alcoholism and attempted suicide in affectively ill relatives of probands with bipolar I disorder. METHOD: In 71 families ascertained for a genetic linkage study, 337 subjects with major affective disorder were assessed by using the Schedule for Affective Disorders and Schizophrenia-Lifetime Version. RESULTS: Subjects with bipolar disorder and alcoholism had a 38.4% lifetime rate of attempted suicide, whereas those without alcoholism had a 21.7% rate. Attempted suicide among subjects with bipolar disorder and alcoholism clustered in a subset of seven families. Families with alcoholic and suicidal probands had a 40.7% rate of attempted suicide in first-degree relatives with bipolar disorder, whereas other families had a 19.0% rate. CONCLUSIONS: Comorbid alcoholism was associated with a higher rate of attempted suicide among family members with bipolar disorder. Attempted suicide and alcoholism clustered in a subset of families. These relationships may have a genetic origin and may be mediated by intoxication, mixed states, and/or temperamental instability.

Bipolar disorder and panic disorder in families: an analysis of chromosome 18 data.

OBJECTIVE: The authors performed an analysis of their published chromosome 18 linkage data on 28 families in which there was bipolar disorder to test the potential of comorbid panic disorder to define a genetic subtype of bipolar disorder. METHOD: Families ascertained through probands with bipolar I disorder were stratified into three groups based on a history of panic disorder, panic attacks, or no panic attacks in the probands. Multipoint nonparametric linkage analysis was performed on data from bipolar I and II family members in each group. RESULTS: Linkage scores for five consecutive 18q marker loci were highest in the families of the probands with panic disorder and lowest for the families of the probands without panic attacks. CONCLUSIONS: This study supports the authors' previously reported clinical hypothesis of a genetic subtype of bipolar disorder identified by comorbid panic disorder. The hypothesis merits prospective testing.

The familial aggregation of psychotic symptoms in bipolar disorder pedigrees.

OBJECTIVE: Symptomatic overlap between affective disorders and schizophrenia has long been noted. More recently, family and linkage studies have provided some evidence for overlapping genetic susceptibility between bipolar disorder and schizophrenia. If shared genes are responsible for the psychotic manifestations of both disorders, these genes may result in clustering of psychotic symptoms in some bipolar disorder pedigrees. The authors tested this hypothesis in families ascertained for a genetic study of bipolar disorder. METHOD: Rates of psychotic symptoms-defined as hallucinations or delusions-during affective episodes were compared in families of 47 psychotic and 18 nonpsychotic probands with bipolar I disorder. The analysis included 202 first-degree relatives with major affective disorder. RESULTS: Significantly more families of psychotic probands than families of nonpsychotic probands (64% versus 28%) contained at least one relative who had affective disorder with psychotic symptoms. Significantly more affectively ill relatives of psychotic probands than of nonpsychotic probands (34% versus 11%) had psychotic symptoms. An analysis of clustering of psychotic subjects across all families revealed significant familial aggregation. Clustering of psychosis was also apparent when only bipolar I disorder was considered the affected phenotype. CONCLUSIONS: Psychotic bipolar disorder may delineate a subtype of value for genetic and biological investigations. Families with this subtype should be used to search for linkage in chromosomal regions 10p12-13, 13q32, 18p11.2, and 22q11-13, where susceptibility genes common to bipolar disorder and schizophrenia may reside. Putative schizophrenia-associated biological markers, such as abnormal evoked response, oculomotor, and neuroimaging measures, could similarly be explored in such families.

Initial genome screen for bipolar disorder in the NIMH genetics initiative pedigrees: chromosomes 2, 11, 13, 14, and X.

We report on an initial genome screen of 540 individuals from 97 families collected as part of the NIMH Genetics Initiative Bipolar Group. Among the individuals studied, 232 were diagnosed with bipolar (BP) I, 72 with BPII, 88 with major depressive disorder-recurrent type (UPR), and 32 with schizoaffective disorder, bipolar type (SA/BP). A total of 53 markers on chromosomes 2, 11, 13, 14, and X (average spacing: 11.5 cM) were studied at Johns Hopkins University. Tests for linkage were performed using nonparametric affected sib-pair and whole pedigree methods with three definitions of affected status. Three regions of interest were identified (13q14-32, Xp22, and Xq26-28). On chromosomes 2, 11, and 14, a disease locus with relative risk lambda(i) = 1.5 could be excluded in <10% of the genetic distance studied, while a locus conferring lambda(i) = 3 or greater could be excluded across at least 96%. The autosomal region that could not be excluded even with lambda(i) = 5 was near 13q14-32. In this region, two-point affected sib-pair analyses revealed a pair of consecutive loci with excess sharing (P < 0.05) and a multipoint affected sib-pair LOD score of 1.12. On the X chromosome, nonparametric multipoint affected sib-pair analyses revealed peak total LOD scores of 0.94 on Xp22 and 1.34 on Xq26-28. A locus linked to the markers in Xp22 would have lambda(i) = 3.6 in affected brother-brother pairs, while a locus linked to the markers in Xq26-28 would have lambda(i) > 1.9 in affected sister-sister pairs. The results on 13q14-32, Xp22, and Xq26-28 suggest areas of interest for further studies.

Genome-wide linkage scan of 98 bipolar pedigrees and analysis of clinical covariates.

Despite compelling evidence that genetic factors contribute to bipolar disorder (BP), attempts to identify susceptibility genes have met with limited success. This may be due to the genetic heterogeneity of the disorder. We sought to identify susceptibility loci for BP in a genome-wide linkage scan with and without clinical covariates that might reflect the underlying heterogeneity of the disorder. We genotyped 428 subjects in 98 BP families at the Center for Inherited Disease Research with 402 microsatellite markers. We first carried out a non-parametric linkage analysis with MERLIN, and then reanalyzed the data with LODPAL to incorporate clinical covariates for age at onset (AAO), psychosis and comorbid anxiety. We sought to further examine the top findings in the covariate analysis in an independent sample of 64 previously collected BP families. In the non-parametric linkage analysis, three loci were nominally significant under a narrow diagnostic model and seven other loci were nominally significant under a broader model. The top findings were on chromosomes 2q24 and 3q28. The covariate analyses yielded additional evidence for linkage on 3q28 with AAO in the primary and independent samples. Although none of the linked loci were genome-wide significant, their congruence with prior results and, for the covariate analyses, their identification in two separate samples increases the likelihood that they are true positives and deserve further investigation. These findings further demonstrate the value of considering clinical features that may reflect the underlying heterogeneity of disease in order to facilitate gene mapping.

Genetics of manic depressive illness.

Manic depressive illness is a common and frequently debilitating familial psychiatric disorder. Efforts to understand the mechanisms of inheritance have been hindered by the complexity of the phenotype, which may range from benign mood swings to chronic psychosis, and by apparently nonmendelian modes of transmission. Early reports of linkage to chromosomal loci have fallen into doubt; however they have helped encourage the development of more sophisticated methods for analyzing complex phenotypes. Using such methods, linkage of manic depressive illness to loci on chromosome 18 has been reported and apparently replicated, and work is proceeding to identify genes associated with what is probably a genetically heterogeneous set of disorders. As molecular mechanisms of inheritance are elucidated, it will be important to consider the ethical implications of genetic testing in a clinically and genetically complex disorder such as manic depressive illness.

Linkage of bipolar affective disorder on chromosome 8q24: follow-up and parametric analysis.

Our group first reported a linkage finding for bipolar (BP) disorder on chromosome 8q24 in a study of 50 multiplex pedigrees, with an HLOD score reaching 2.39. Recently, Cichon et al reported an LOD score of 3.62 in the same region using two-point parametric analysis. Subsequently, we published the results of a genome scan for linkage to BP disorder using a sample extended to 65 pedigrees in which chromosome 8q24 provided the best finding, an NPL score of 3.13, approaching the accepted score for suggestive linkage. We have now fine mapped this region of chromosome 8 in our 65 pedigrees by the addition of 19 microsatellite markers reaching a marker density of 0.8 cM and an information content of 0.84. After the addition of the new data, the original NPL score slightly increased to 3.25. Two-point parametric analysis using the model employed by Cichon et al obtained an LOD score of 3.32 for marker D8S256 at theta=0.14 exceeding the proposed threshold for genomewide significance. After adjusting the parameters in accordance with the 'common disease-common variant' hypothesis, multipoint parametric analysis resulted in an HLOD of 2.49 (alpha=0.78) between D8S529 and D8S256, and defined a 1-LOD interval corresponding to a 2.3 Mb region. No allelic association with the disease was observed for our set of microsatellite markers. Biologically, plausible candidate genes in this region include thyroglobulin, KCNQ3 coding for a voltage-gated potassium channel and the gene for brain adenyl-cyclase (ADCY8).

Linkage of bipolar affective disorder to chromosome 18 markers in a new pedigree series.

Several groups have reported evidence suggesting linkage of bipolar affective disorder (BPAD) to chromosome 18. We have reported data from 28 pedigrees that showed linkage to marker loci on 18p and to loci 40 cM distant on 18q. Most of the linkage evidence derived from families with affected phenotypes in only the paternal lineage and from marker alleles transmitted on the paternal chromosome. We now report results from a series of 30 new pedigrees (259 individuals) genotyped for 13 polymorphic markers spanning chromosome 18. Subjects were interviewed by a psychiatrist and were diagnosed by highly reliable methods. Genotypes were generated with automated technology and were scored blind to phenotype. Affected sib pairs showed excess allele sharing at the 18q markers D18S541 and D18S38. A parent-of-origin effect was observed, but it was not consistently paternal. No robust evidence of linkage was detected for markers elsewhere on chromosome 18. Multipoint nonparametric linkage analysis in the new sample combined with the original sample of families supports linkage on chromosome 18q, but the susceptibility gene is not well localized.

Trapping and sequence analysis of 1138 putative exons from human chromosome 18.

In a search for novel genes on chromosome 18 (HC18), on which several regions have been linked to bipolar disorder, we applied exon trapping to HC18-specific cosmids. Among the 1138 exons trapped, 1052 of them have been mapped to HC18, and the remaining 86 have not been localized. No exons were localized to genomic regions other than HC18. BLAST database search revealed that 190 exons were identical to 98 Unigenes on HC18; 98 identical to additional 82 clusters of ESTs not present in the HC18 Unigene set; 39 homologous to genes from human and other species (e<10(-3)); and the remaining 811 exons had no significant homology to transcripts in public databases. The mapped exons were compared to the 867 annotated genes on HC18 in the Celera databases; 216 exons were identical to 104 Celera 'genes' and the remaining 836 exons were not found in the Celera databases. On average, there were two exons for a matched transcript (known genes and ESTs). Therefore, the 850 novel exons may represent hundreds of novel genes on chromosome 18.

Allelic distribution of CTG18.1 in Caucasian populations: association studies in bipolar disorder, schizophrenia, and ataxia.

CTG18.1 is a highly polymorphic and unstable CTG repeat within an intron of the SEF2-1 gene. We tested the CTG18.1 repeat length in affective disorder, schizophrenia, and nonspecific ataxia; these diseases all have shown clinical evidence for anticipation. There was no difference in the allele frequencies comparing the controls and disease groups. The most common allele contains 11 CAGs (35%) followed by alleles with 14-17 CAGs (35%). There was no difference in the distribution of the alleles in the cases vs controls for ataxia (P = 0.11), affective disorders (P = 0.21), or schizophrenia (P = 0.26). The frequency of unstable CTG18.1 alleles was approximately 3% in a population of N. European descent and is not related to the phenotypes tested.

A novel, heritable, expanding CTG repeat in an intron of the SEF2-1 gene on chromosome 18q21.1.

There are currently 13 diseases known to be caused by unstable triplet repeat mutations; however, there are some instances (as with FRAXF and FRA16) when these mutations appear to be asymptomatic. In a search for polymorphic CTG repeats as candidate genes for bipolar disorder, we screened a genomic human chromosome 18-specific library and identified a 1.6 kb clone (7,6A) with a CTG24 repeat that maps to 18q21.1. The CTG repeat locus, termed CTG18.1, is located within an intron of human SEF2-1, a gene encoding a basic hellx-loop-hellx DNA binding protein involved in transcriptional regulation. The CTGn repeat is highly polymorphic and very enlarged alleles, consistent with expansions of up to CTG2100, were identified. PCR and Southern blot analysis in pedigrees ascertained for a Johns Hopkins University bipolar disorder linkage study and in CEPH reference pedigrees revealed a tripartite distribution of CTG18.1 alleles with stable alleles (CTG10-CTG37), moderately enlarged and unstable alleles (CTG53-CTG250), and very enlarged, unstable alleles (CTG800-CTG2100). Moderately enlarged alleles were not associated with an abnormal phenotype and have a combined enlarged allele frequency of 3% in the CEPH and bipolar populations. Very enlarged alleles, detectable only by Southern blot analysis of genomic digests, have thus far been found in only three individuals from our bipolar pedigrees, and to date, have not been found in any of the CEPH reference pedigrees. These enlarged alleles may arise, at least in part, via somatic mutation.