A triplet repeat on 17q accounts for most expansions detected by the repeat-expansion-detection technique

Unstable repeat expansion in major psychiatric disorders: two decades on, is dynamic DNA back on the menu?

For a period in the mid-1990s, soon after the discovery of the involvement of trinucleotide repeat expansions in fragile-X syndrome (both A and E), Huntington's disease, myotonic dystrophy, and a number of hereditary ataxias, there was a clear sense that this new disease mechanism might provide answers for psychiatric disorders. Given the then failures to replicate initial genetic linkage findings for schizophrenia (SCZ) and bipolar disorder (BD), a greater emphasis was placed on the role of complex and non-Mendelian mechanisms, and repeat instability appeared to have the potential to provide adequate explanations for numerous apparently non-Mendelian features such as anticipation, incomplete penetrance, sporadic occurrence, and nonconcordance of monozygotic twins. Initial molecular studies using a ligation-based amplification method (repeat expansion detection) appeared to support the involvement of CAG•CTG repeat expansion in SCZ and BD. However, subsequent studies that dissected the large repeats responsible for much of the positive signal showed that there were three main loci where CAG•CTG repeat expansion was occurring (on 13q21.33, 17q21.33-q22, and 18q21.2). None of the expansions at these loci appeared to segregate with SCZ or BD, and research into repeat expansions in psychiatric illness petered out in the early 2000s. The 13q expansion occurs within a noncoding RNA and appears to be associated with spinocerebellar ataxia 8 (SCA8), but with a still unexplained dichotomy in penetrance - either very high or very low. The 17q expansion occurs within an intron of the carbonic anhydrase-like gene, CA10. The 18q expansion is located within an intron of the TCF4 gene. Mutations in TCF4 are a known cause of Pitt-Hopkins syndrome. Also, pertinently, genome-wide association studies have shown a well-replicated association between TCF4 and SCZ. Two decades on, in 2016, it appears to be an appropriate juncture to reflect on what we have learned, and, with the arrival of newer technologies, whether there is any mileage to be made in revisiting the unstable DNA hypothesis for psychiatric illness.

Molecular linkage studies of bipolar disorder

Linkage studies have defined at least five bipolar (BP) disorder susceptibility loci that meet suggested guidelines for initial identification and subsequent confirmation. These loci, found on 18p11, 18q22, 21q21, 4p16, and Xq26, are targets for BP candidate gene investigations. Molecular dissection of expressed sequences for these regions is likely to yield specific BP susceptibility alleles in most cases, in all probability, these BP susceptibility alleles will be common in the general population, and, individually, will be neither necessary nor sufficient for manifestation syndrome. Additive or multiplicative oligogenic models involving several susceptibility loci appear most reasonable at present, it is hoped thai these BP susceptibility genes will increase understanding of many mysteries surrounding these disorders, including drug response, cycling patterns, age-of-onset, and modes of transmission.

The unstable trinucleotide repeat story of major psychosis

New hopes for cloning susceptibility genes for schizophrenia and bipolar affective disorder followed the discovery of a novel type of DNA mutation, namely unstable DNA. One class of unstable DNA, trinucleotide repeat expansion, is the causal mutation in myotonic dystrophy, fragile X mental retardation, Huntington disease and a number of other rare Mendelian neurological disorders. This finding has led researchers in psychiatric genetics to search for unstable DNA sites as susceptibility factors for schizophrenia and bipolar affective disorder. Increased severity and decreased age at onset of disease in successive generations, known as genetic anticipation, was reported for undifferentiated psychiatric diseases and for myotonic dystrophy early in the twentieth century, but was initially dismissed as the consequence of ascertainment bias. Because unstable DNA was demonstrated to be a molecular substrate for genetic anticipation in the majority of trinucleotide repeat diseases including myotonic dystrophy, many recent studies looking for genetic anticipation have been performed for schizophrenia and bipolar affective disorder with surprisingly consistent positive results. These studies are reviewed, with particular emphasis placed on relevant sampling and statistical considerations, and concerns are raised regarding the interpretation of such studies. In parallel, molecular genetic investigations looking for evidence of trinucleotide repeat expansion in both schizophrenia and bipolar disorder are reviewed. Initial studies of genome-wide trinucleotide repeats using the repeat expansion detection technique suggested possible association of large CAG/CTG repeat tracts with schizophrenia and bipolar affective disorder. More recently, three loci have been identified that contain large, unstable CAG/CTG repeats that occur frequently in the population and seem to account for the majority of large products identified using the repeat expansion detection method. These repeats localize to an intron in transcription factor gene SEF2-1B at 18q21, a site named ERDA1 on 17q21 with no associated coding region, and the 3' end of a gene on 13q21, SCA8, that is believed to be responsible for a form of spinocerebellar ataxia. At present no strong evidence exists that large repeat alleles at either SEF2-1B or ERDA1 are involved in the etiology of schizophrenia or bipolar disorder. Preliminary evidence suggests that large repeat alleles at SCA8 that are non-penetrant for ataxia may be a susceptibility factor for major psychosis. A fourth, but much more infrequently unstable CAG/CTG repeat has been identified within the 5' untranslated region of the gene, MAB21L1, on 13q13. A fifth CAG/CTG repeat locus has been identified within the coding region of an ion transporter, KCNN3 (hSKCa3), on 1q21. Although neither large alleles nor instability have been observed at KCNN3, this repeat locus has been extensively analyzed in association and family studies of major psychosis, with conflicting findings. Studies of polyglutamine containing genes in major psychosis have also shown some intriguing results. These findings, reviewed here, suggest that, although a major role for unstable trinucleotides in psychosis is unlikely, involvement at a more modest level in a minority of cases cannot be excluded, and warrants further investigation.

Early development and unstable genes in schizophrenia: preliminary results

BACKGROUND

Trinucleotide repeats have been associated with schizophrenia, but the evidence, based on cross-sectional clinical information, is equivocal.

AIMS

To examine the relationship between genomic CAG/CTG repeat size and premorbid development in schizophrenia.

METHOD

Early development and premorbid functioning of 22 patients with DSM-IV diagnosis of schizophrenia were assessed by parental interviews. Repeat expansion detection (RED) technique was used to measure genomic CAG/CTG repeat size, and PCR for CAG repeat size at the ERDA-1 and CTG 18.1 loci.

RESULTS

There was an inverse association between CAG/CTG size and perinatal complications. Patients with speech and motor developmental delay had larger repeats. The results were not due to expansion in the ERDA-1 and CTG 18.1 genes.

CONCLUSIONS

CAG/CTG repeat expansion is associated with speech and motor developmental delay in schizophrenia. We propose that the developmental model may be useful for research into the genetics of schizophrenia.

European combined analysis of the CTG18.1 and the ERDA1 CAG/CTG repeats in bipolar disorder

Several groups have reported association between large CAG/CTG repeats in the genome and BP disorder using the Repeat Expansion Detection (RED) method. Molecular interpretation studies demonstrated that around 90% of the large CAG/CTG repeats detected by RED can by explained by repeat size at either the CTG18.1 or ERDA-1 locus. In this study we report the findings on a large European BP case-control sample analysed for these two frequently expanded repeats. The frequency of expanded alleles (>40 repeats) at the CTG18.1 locus was significantly higher in the subgroup of patients with a more severe phenotype BPI and a positive first degree family history than in a group of matched controls (9% vs 5%). No difference in ERDA-1 expansion frequency was seen between BP cases and matched controls. We conclude that the ERDA-1 locus is not related to the BP phenotype while expanded alleles at the CTG18.1 locus cannot be excluded as a vulnerability factor for BP disorder.

Association and linkage studies between bipolar affective disorder and the polymorphic CAG/CTG repeat loci ERDA1, SEF2-1B, MAB21L and KCNN3

Several reports have suggested the presence of anticipation in bipolar affective disorder (BPAD). In addition, independent studies using the RED (repeat expansion detection) have shown association between BPAD and longer CAG/CTG repeats. Therefore loci with large CAG/CTG repeats are plausible candidates in the inheritance of BPAD. The present study assesses the length of the repeats in four loci: the ERDA-1 locus which is known to account for most of the long CAG repeats detected by RED, the SEF2-1b locus which is placed in a region where positive linkage results have been reported and the loci MAB21L and KCNN3 as functional candidate genes. A Brazilian case-control sample with 115 unrelated BPAD patients and 196 healthy control subjects and 14 multiply affected bipolar families was investigated. With the case-control design the distribution of alleles between the two groups did not approach statistical significance. The extended transmission disequilibrium test (ETDT) performed in our families did not show evidence for linkage disequilibrium. Parametric and non-parametric linkage analysis also did not provide support for linkage between any of the four loci and BPAD. Our data do not support the hypothesis that variation at the polymorphic CAG/CTG repeat loci ERDA-1, SEF2-1b, MAB21L or KCNN3 influence susceptibility to BPAD in our sample.

CAG repeats of CTG18.1 and KCNN3 in Korean patients with bipolar affective disorder

BACKGROUND

Trinucleotide repetition combined with variable penetrance of expression could be responsible for the complex transmission pattern observed in bipolar affective disorder (BPAD). The purpose of this study was to investigate the association of excess longer allele of KCNN3 and CTG18.1 in the patients with BPAD.

METHODS

CAG/CTG repeat distribution in KCNN3, CTG 18.1 and ERDA1 was examined and the copy number of ligation product in repeat expansion detection (RED) was measured in Korean bipolar patients in comparison to ethnically matched healthy controls.

RESULTS

No significant difference was found in the allele distribution of those repeats between bipolar patients and controls. Ligation product size in RED was not increased in bipolar patients. However, the copy number of ligation product in RED was highly correlated with CAG/CTG copies of ERDA1 (P=0.0001), partly with CTG 18.1 (P=0.04), but not with KCNN3.

CONCLUSIONS

A longer CAG repeat alleles of KCNN3 or CTG 18.1 may not be a risk factor for BPAD in Korean population and the copy number of ligation product in RED in the patients with BPAD is influenced by the longer allele of CAG/CTG of ERDA1 or CTG 18.1.

Anticipation, imprinting, trinucleotide repeat expansions and psychoses

1. Since 1991, approximately 20 trinucleotide repeat expansion type neurodegenerative disorders have been reported. They are clinically characterized by anticipation, i.e., worsening severity or earlier age at onset with each succeeding generation for an inherited disease, and imprinting, i.e., a process whereby specific genes are differentially marked during parental gametogenesis, resulting in the differential expression of these genes in the embryo and adult. 2. The phenomenon of anticipation in psychoses has been pointed out since the 19th century; however, it was ignored because no one knew the genetic mechanism underlying this type of inheritance pattern at the time, and because of several possible biases. 3. The discovery of trinucleotide repeat expansion diseases has reawakened interest in the phenomenon of anticipation in psychiatric diseases. Anticipation has been confirmed in schizophrenia, mood disorders, and anxiety disorders in much more sophisticated manners, although still not perfectly. 4. Molecular approaches as well as clinical ones have been taken to reveal the involvement of trinucleotide repeat expansion mechanism in psychoses by means of direct analyses of candidate genes, RED and DIRECT. Most efforts have been made for CAG type trinucleotide repeats. So far, direct analyses have failed to reveal pathogenic gene(s). There were several positive RED data at first, however, nowadays there seems to be a tendency of much more negative results. The DIRECT results did not support trinucleotide repeat expansions mechanism in psychoses either. One plausable explanation for the 'false positive' result is the presence of CAG trinucleotide repeats which are highly polymorphic but not associated with an obvious abnormal phenotype. Screening for trinucleotide repeats other than ones of the CAG type remained to be performed.

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.

Polyglutamine coding genes in bipolar disorder: lack of association with selected candidate loci

BACKGROUND

Several studies have suggested that expanded trinucleotide repeats, particularly CAG, may have a role in the etiology of BD. Results obtained with the repeat expansion detection technique (RED) have indicated that bipolar patients have an excess of expanded CAG repeats. However, it is not clear which loci account for this difference.

METHODS

Using lithium-responsive bipolar patients in order to reduce heterogeneity, we investigated five loci that are expressed in the brain and contain translated CAG repeats. A sample of 138 cases and 108 controls was studied. Genotypes were coded quantitatively or qualitatively and repeat distributions were compared.

RESULTS

No difference was found in allele distribution between cases and controls for any of the loci studied. In one locus - L10378 - patients had a tendency to present shorter alleles (28.1 versus 27.9 repeats; t=2.55, df=205, P=0.011), however, this difference disappeared after correction for multiple testing.

LIMITATIONS

The study has limitations common to most candidate gene association studies, that is, limited number of loci investigated and limited power to detect loci that account for a small proportion of the total genetic variability.

CONCLUSIONS

Our results suggest that the loci investigated have no major role in the genetic predisposition to bipolar disorder.

An unstable trinucleotide-repeat region on chromosome 13 implicated in spinocerebellar ataxia: a common expansion locus

Larger CAG/CTG trinucleotide-repeat tracts in individuals affected with schizophrenia (SCZ) and bipolar affective disorder (BPAD) in comparison with control individuals have previously been reported, implying a possible etiological role for trinucleotide repeats in these diseases. Two unstable CAG/CTG repeats, SEF2-1B and ERDA1, have recently been cloned, and studies indicate that the majority of individuals with large repeats as detected by repeat-expansion detection (RED) have large repeat alleles at these loci. These repeats do not show association of large alleles with either BPAD or SCZ. Using RED, we have identified a BPAD individual with a very large CAG/CTG repeat that is not due to expansion at SEF2-1B or ERDA1. From this individual's DNA, we have cloned a highly polymorphic trinucleotide repeat consisting of (CTA)n (CTG)n, which is very long ( approximately 1,800 bp) in this patient. The repeat region localizes to chromosome 13q21, within 1.2 cM of fragile site FRA13C. Repeat alleles in our sample were unstable in 13 (5.6%) of 231 meioses. Large alleles (>100 repeats) were observed in 14 (1. 25%) of 1,120 patients with psychosis, borderline personality disorder, or juvenile-onset depression and in 5 (.7%) of 710 healthy controls. Very large alleles were also detected for Centre d'Etude Polymorphisme Humaine (CEPH) reference family 1334. This triplet expansion has recently been reported to be the cause of spinocerebellar ataxia type 8 (SCA8); however, none of our large alleles above the disease threshold occurred in individuals either affected by SCA or with known family history of SCA. The high frequency of large alleles at this locus is inconsistent with the much rarer occurrence of SCA8. Thus, it seems unlikely that expansion alone causes SCA8; other genetic mechanisms may be necessary to explain SCA8 etiology.

No evidence for long CAG/CTG repeats in families with spastic paraplegia linked to chromosome 2p21-24

Autosomal dominant familial spastic paraplegia (AD-FSP) is a genetically heterogeneous, neurodegenerative disorder characterized by spasticity and progressive weakness in the lower limbs. Anticipation has been suggested to occur and an association between expanded CAG/CTG repeats and AD-FSP linked to the SPG4 locus (2p21-p24) has been described. In this study, 42 affected individuals from six SPG4 families were screened for expanded CAG/CTG repeats using the repeat expansion detection (RED) method. Large RED products (range 180-240 nucleotides) corresponding in size to repeats at the ERDA1 locus were detected in eight patients and at the CTG 18.1 locus in one patient. The large ERDA1 repeats did not segregate with the disorder within families. Mean age at onset and index of severity were not significantly different between patients with or without expanded RED products. Furthermore, no abnormal proteins were found by Western blot in 15 selected patient samples as compared with controls, using the 1C2 antibody, which detects long polyglutamine stretches. Thus, in contrast to previous reports, our study provides evidence against the hypothesis that a large translated CAG repeat expansion is the basis of SPG4. We propose that mechanisms other than large pathogenic CAG/CTG repeats may account for the disease in the SPG4 families tested here.

Childhood-onset schizophrenia: rare but worth studying

Childhood-onset schizophrenia (defined by an onset of psychosis by age 12) is a rare and severe form of the disorder that is clinically and neurobiologically continuous with the adult-onset disorder. There is growing evidence for more salient risk or etiologic factors, particularly familial, in this possibly more homogeneous patient population. For the 49 patients with very early onset schizophrenia studied to date at the National Institute of Mental Health, there were more severe premorbid neuro-developmental abnormalities, a higher rate of cytogenetic anomalies, and a seemingly higher rate of familial schizophrenia and spectrum disorders than later onset cases. There was no evidence for increased obstetric complications or environmental stress. These data, while preliminary, suggest that a very early age of onset of schizophrenia may be secondary to greater familial vulnerability. Consequently, genetic studies of these patients may be particularly informative and may provide important etiologic information.

Molecular interpretation of expanded RED products in bipolar disorder by CAG/CTG repeats located at chromosomes 17q and 18q

Previously we provided evidence that the anticipation observed in bipolar (BP) disorder may be explained by expanded CAG/CTG triplet repeats. Data were generated with the repeat expansion detection (RED) method in a BP case-control sample showing a significant association of BP disorder with expanded CAG/CTG repeats (RED products of 120 bp). In this study we demonstrated that 86% of the RED expansions could be accounted for by the ERDA1 and CTG18.1 CAG/CTG repeats located respectively on chromosomes 17 and 18. Further, significantly different allele distributions were observed for ERDA1, with a larger proportion of BP patients (34.7%) carrying one or two expanded ERDA1 alleles (CAG/CTG repeats >40) than controls (19.2%) (P = 0.032). Also, a negative correlation was observed for ERDA1 between CAG/CTG length and age at onset in affected offspring of eight BP families. Although interesting, these data should be interpreted with caution since the ERDA1 association did not remain significant after correcting for multiple testing. Also, no linkage was observed between BP disorder and expanded ERDA1 alleles in the families.

Clinical characteristics of bipolar disorder subjects with large CAG/CTG repeat DNA

BACKGROUND

Unstable DNA has been implicated in a variety of neuropsychiatric disorders, with increasing severity of disease associated with larger DNA repeats. We examined the unstable DNA hypothesis for bipolar disorder by looking for increased severity of symptoms and earlier age of onset amongst bipolar individuals with large CAG/CTG repeats.

METHODS

From a sample of 91 bipolar subjects, eight with large CAG/CTG (> or = 270bp) trinucleotide repeats were matched to eight bipolar individuals with small repeats (< or = 150bp). Medical charts were reviewed for age of onset and a number of severity indicators. Candidate CAG/CTG expansions on chromosomes 17 and 18 were also genotyped.

RESULTS

No obvious differences were noted for the clinical indices, however seven out of eight individuals with large Repeat Expansion Detection (RED) products had expansions at the CTG18.1 locus, while four out of eight had large repeats at ERDA1. Both of these sites are unlikely to be related to disease.

LIMITATIONS

Our total sample size is small and less than 9% have large repeats.

CONCLUSIONS

The lack of increased severity or earlier age of onset amongst bipolar subjects with large CAG/CTG repeats suggests these repeats are unlikely to have a major etiological role in bipolar disorder.

Molecular linkage studies of bipolar disorder

Linkage studies have defined at least five bipolar (BP) disorder susceptibility loci that meet suggested guidelines for initial identification and subsequent confirmation. These loci, found on 18p11, 18q22, 21q21, 4p16, and Xq26, are targets for BP candidate gene investigations. Molecular dissection of expressed sequences for these regions is likely to yield specific BP susceptibility alleles in most cases, in all probability, these BP susceptibility alleles will be common in the general population, and, individually, will be neither necessary nor sufficient for manifestation syndrome. Additive or multiplicative oligogenic models involving several susceptibility loci appear most reasonable at present, it is hoped thai these BP susceptibility genes will increase understanding of many mysteries surrounding these disorders, including drug response, cycling patterns, age-of-onset, and modes of transmission.

Genome-wide scan for CAG/CTG repeat expansions in Pimas with early onset of type 2 diabetes mellitus

The expansion of polymorphic CAG/CTG repeats in specific genes causes several neurodegenerative disorders and in many instances the length of the disease-causing repeat correlates with the onset age and/or severity of symptoms. Type 2 diabetes mellitus has features in common with diseases resulting from trinucleotide repeat expansion, including a variable age of disease onset and penetrance. We have investigated whether CAG/CTG repeat expansion contributes to the genetic etiology of type 2 diabetes in the Pima Indians, a population with the highest reported prevalence of this disease. Using the Repeat Expansion Detection (RED) method, we determined the size range in nondiabetic Pimas to be between (CAG)20 and (CAG)130 (mean repeat length = 195 bp), which is significantly larger than the mean size reported in Caucasians (150 bp). We compared the distribution of CAG/CTG repeat lengths among 40 Pimas with an early onset of type 2 diabetes (<22 years) and 38 nondiabetic subjects (>55 years). A 240-bp CAG/CTG RED product was found more frequently in early onset diabetics relative to nondiabetic controls (26% vs 11%), whereas a 210-bp band was more prominent in unaffected subjects (29% vs 13%); however, these differences were not statistically significant. In one Pima kindred, we also identified large RED products (>/=360 bp) that displayed intergenerational instability among family members. However, these expansions were not associated with diabetes or any other clinical abnormalities in the carriers. We conclude that this unstable CAG/CTG repeat may represent a novel locus, consisting of large, but apparently nonpathogenic, unstable sequences.

Transmission disequilibrium analysis of a triplet repeat within the hKCa3 gene using family trios with schizophrenia

hKCa3 is a neuronal small conductance calcium-activated potassium channel which contains a polyglutamine tract, encoded by a polymorphic CAG repeat in the gene. Since an association between longer alleles of the CAG repeat and schizophrenia has been reported, we performed haplotype-based haplotype relative risk (HHRR) and transmission disequilibrium (TDT) in 97 family trios with schizophrenia from SW China. We found no evidence for an excess of longer CAG repeats in the patients, and the ETDT test was not significant for either allele-wise (P = 0.31) or genotype-wise analysis (P = 0.18). However, there was a deficit of transmission of the (CAG)20 repeat allele to affected offspring when this allele was considered individually by TDT (P = 0.012; not corrected for multiple testing). These data do not support a role for larger alleles at the hKCa3 locus in psychosis in Chinese subjects.