Provisional mapping of quantitative trait loci modulating the acoustic startle response and prepulse inhibition of acoustic startle

Prepulse inhibition (PPI) of the acoustic startle response (ASR) is a form of sensorimotor gating, defined as an inhibition of the startle response when a low intensity stimulus, the prepulse, precedes the startling stimulus. Deficits in PPI have been reported in schizophrenia and other psychiatric/neurological disorders, and correlate with symptom severity in schizophrenia, suggesting that deficient PPI per se or abnormalities in neural circuits regulating PPI may cause some symptoms of schizophrenia. If so, then genes conferring reduced PPI may contribute toward genetic vulnerability to schizophrenia. Studies with selectively bred rodent strains indicate that PPI is under genetic control; however, the identity of the relevant genes is unknown. The current study used recombinant congenic mouse strains derived from C57BL/6J and A/J parents to assess genetic variability in PPI and in ASR and to identify provisional quantitative trait loci (QTLs) modulating these phenotypes. Significant between-strain differences in ASR and in PPI at each of several prepulse intensities (75, 80, 85, 90, 95 dB) were found. Correlations between PPI at the various prepulse intensities were highly significant, suggesting appreciable overlap in genetic regulation of PPI across prepulse intensities. Five QTLs (chromosomes 3, 5, 7, 16) associated with PPI across all prepulse intensities, but not with ASR, were identified. Two additional QTLs (chromosomes 2, 11) associated with both PPI and ASR were found. Fifteen QTLs were associated with ASR alone. Data on genotypes of informative congenic strains were used to support probable involvement of loci modulating PPI and to narrow the probable chromosomal location of QTLs. If confirmed, these QTLs may suggest candidate genes directing novel mechanisms for regulation of PPI

Genetics of schizophrenia: from animal models to clinical studies

Genetic epidemiological studies strongly suggest that additive and interactive genes, each with small effects, mediate the genetic vulnerability for schizophrenia. With the human genome working draft at hand, candidate gene (and ultimately large-scale genome-wide) association studies are gaining renewed interest in the effort to unravel the complex genetics of schizophrenia. In the absence of an unequivocally established biological theory for schizophrenia, identifying candidate genes to be tested in an association paradigm remains a challenging task. We maintain that it is possible to use animal models to map genes or loci involved in behavioural traits that are relevant to schizophrenia. The human genes (or syntenic loci) homologous to those identified in mice can subsequently be tested in patients with schizophrenia who have been carefully phenotyped for traits "isomorphic" to the ones modelled in mice. If confirmed in humans, these genes may be further analyzed in the animal model to identify their role and the biological network they are involved in. To tackle the complex and intimidating problem of the genetics of schizophrenia, it may be necessary to go from animal models to human studies and vice versa; this strategy has been proven to be efficient in less complicated, though complex, human diseases.

Neuropsychological impairments in neuroleptic-responder vs. -nonresponder schizophrenic patients and healthy volunteers

To determine whether two groups of schizophrenic patients representing the two extremes of the neuroleptic response-spectrum (consistent responders vs. consistent nonresponders) differ with respect to their neuropsychological profile. Neuroleptic-responder (R; n=36) and -nonresponder (NR; n=39) schizophrenic patients were recruited according to a priori defined criteria of responsiveness to typical neuroleptics. Seven neuropsychological domains were assessed and compared between groups: attention-vigilance, abstraction-flexibility, spatial organization, visual-motor processing, visual memory, verbal abilities, and verbal memory and learning. All measures were standardized using the scores of 36 healthy volunteers. NR schizophrenic patients performed worse in all neuropsychological domains compared to normal controls and R schizophrenic patients. However, only performances on visual memory, verbal abilities, and verbal memory and learning were significantly poorer in NR compared to R patients. Only the latter domain significantly differentiated NR patients from the other two groups. R patients performed at an intermediate level in all domains. This report of differences in neuropsychological profile between neuroleptic-responder and -nonresponder schizophrenic patients adds to the growing evidence supporting the value of distinguishing schizophrenic patients on the basis of their therapeutic response to neuroleptics.

TPH and suicidal behavior: a study in suicide completers

An association between the gene that codes for tryptophan hydroxylase (TPH)-the rate-limiting enzyme in the synthesis of serotonin-and suicidal behavior has been investigated with some detail in samples of living subjects who attempted suicide. In this study, we investigated TPH and suicide completion, the most severe form of suicidal behavior. A relatively large sample of suicide completers (n = 101) was genotyped at three TPH loci (two polymorphisms in the promoter region, A-6526G and G-5806T, and one in intron 7, A218C) and compared to psychiatrically normal living controls (n = 129). Although no significant differences were found between groups for genetic variation at single loci, haplotype analysis revealed that one haplotype (-6526G -5806T 218C) was significantly more frequent among suicide cases than in normal controls (chi(2) = 11.30, df = 2, P = 0.0008; OR = 2.0 CI: 1.30-3.6). Further analyses suggested that this haplotype is particularly more frequent among subjects who committed suicide using violent methods. Similar results were observed in recent haplotype analyses in suicide attempters, which found that the equivalent of haplotype -6526G -5806T 218C was more frequent in impulsive attempters (Rotondo et al, Mol Psychiatry 1999; 4: 360-368). Our results replicate in suicide completers previous data observed in suicide attempters. These and other results continue to point to the substantial role that the gene that codes for TPH may play in the neurobiology of suicidal behavior.

Association and linkage studies of candidate genes involved in GABAergic neurotransmission in lithium-responsive bipolar disorder

OBJECTIVE

To test for genetic linkage and association with GABAergic candidate genes in lithium-responsive bipolar disorder.

DESIGN

Polymorphisms located in genes that code for GABRA3, GABRA5 and GABRB3 subunits of the GABAA receptor were investigated using association and linkage strategies.

PARTICIPANTS

A total of 138 patients with bipolar 1 disorder with a clear response to lithium prophylaxis, selected from specialized lithium clinics in Canada and Europe that are part of the International Group for the Study of Lithium-Treated Patients, and 108 psychiatrically healthy controls. Families of 24 probands were suitable for linkage analysis.

OUTCOME MEASURES

The association between the candidate genes and patients with bipolar disorder versus that of controls and genetic linkage within families.

RESULTS

There was no significant association or linkage found between lithium-responsive bipolar disorder and the GABAergic candidate genes investigated.

CONCLUSIONS

This study does not support a major role for the GABAergic candidate genes tested in lithium-responsive bipolar disorder.

DRD3 and DAT1 genes in schizophrenia: an association study

OBJECTIVE

To investigate the role of the dopamine receptor 3 (DRD3) and transporter 1 (DAT1) genes in schizophrenia or in modulating its phenotype.

METHODS

a Ser9Gly polymorphism in codon 9 of the DRD3 and a VNTR polymorphism in the DAT1genes were examined in two groups of schizophrenic patients, one of excellent neuroleptic responders (N=42) and one of nonresponders (N=64). A group of healthy volunteers screened for major psychiatric disorders was also included (N=89). In addition, age at onset of psychotic symptoms, attention performance and family loading for schizophrenia spectrum disorders were compared between patients with different genotypes in the DRD3 and DAT1 genes.

RESULTS

No significant differences in the allelic distribution of the DRD3 and DAT1 polymorphisms were detected between schizophrenic patients and controls. A trend toward an excess of DRD3 genotype Gly/Gly was observed in neuroleptic nonresponder schizophrenic patients compared to controls (chi(2)=3. 30, df=1, p=0.07). No significant differences in age at onset of psychotic symptoms, attention task performance or family loading for schizophrenia spectrum disorders were observed between groups with different DRD3 and DAT1 genotypes.

CONCLUSION

These results do not support the role of either of these genes in increasing susceptibility to schizophrenia or in modulating its phenotype in the studied population.

Association between the methylenetetrahydrofolate reductase 677C-->T missense mutation and schizophrenia

The schizophrenia phenotype is heterogeneous with respect to clinical presentation, long-term response to medication, and outcome, possibly reflecting genetic heterogeneity and/or the presence of modifier genes. Compared to non-responders, schizophrenic patients who are responders to neuroleptic medications are characterized by a high female/male ratio, a better long-term outcome and more frequently disturbed dopamine neurotransmission. In this study, we compared two groups of schizophrenic patients selected on the basis of their long-term response to neuroleptics (excellent responders and non-responders) and a group of healthy volunteers, with regard to a missense mutation (677C-->T) in the methylenetetrahydrofolate reductase (MTHFR) gene. This polymorphism was chosen because it is functional and was previously associated with schizophrenia. The present study revealed a significant association between schizophrenia and allele T of this gene. This association was entirely due to an over-representation of allele T in responder patients compared to controls; nonresponder patients did not differ from controls. Genotype TT was more frequent in responder patients compared to controls, thus replicating the findings of Arinami et al. These results strongly suggest that the MTHFR gene is involved in the pathogenesis of schizophrenia characterized by a rapid and sustained therapeutic response to typical neuroleptics and/or a good long-term prognosis/favorable therapeutic outcome.

Association and linkage studies of CRH and PENK genes in bipolar disorder: a collaborative IGSLI study

Corticotropin-releasing hormone (CRH) and proenkephalin (PENK) are hypothalamic peptides involved in the stress response and hypothalamic-pituitary axis regulation. Previous research has implicated these peptides in the pathogenesis of affective disorders. In this study we investigated two polymorphisms located in the genes that code for CRH and PENK by means of association and linkage analyses. A total of 138 bipolar patients and 108 controls were included in the association study. In addition, 24 families were available for linkage analysis, including six families of probands with documented periodic positivity of dexamethasone suppression tests (DST) during remission. We found no association of bipolar disorder with either gene. Similarly, we did not find any evidence of linkage (P = 0.56 for CRH and 0.52 for PENK) in the entire sample or in the subsample of families of DST positive probands. In conclusion, our study does not support the hypothesis that genes coding for CRH or PENK contribute to the genetic susceptibility to bipolar disorder. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 96:178-181, 2000.

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.

Analysis of 14 CAG repeat-containing genes in schizophrenia

Recently, it has been suggested that trinucleotide repeat-containing genes may be involved in the etiology of schizophrenia. This study was aimed at investigating putative associations between allelic variants or expansions of CAG repeat-containing genes (CAGrCG) and schizophrenia or its variability with respect to responsiveness to conventional neuroleptics. CAG repeat allelic variants of 14 expressed sequences were compared among three groups of subjects: neuroleptic-responder (R; n = 43) and neuroleptic-nonresponder (NR; n = 63) schizophrenic patients, and a control group (C; n = 122). No CAG expansions, in the range of those observed in neurodegenerative diseases, were identified in these 14 expressed sequences. The sizes of CAG repeat for the hGT1 gene were marginally different among the three groups of subjects (Kruskal-Wallis H (2, 456) = 10.48, Bonferroni corrected P = 0.047). Comparisons among the different groups indicated that neuroleptic responders have shorter alleles compared to controls (Mann-Whitney adjusted Z = -3.23, P = 0.0012). NR patients were not different from controls. These preliminary results suggest that the hGT1 gene, or a gene in its vicinity, may be involved in the etiology of schizophrenia or in modifying the disease phenotype with regard to outcome and/or neuroleptic responsiveness. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:694-699, 1999.

Neural correlates of eye tracking deficits in first-degree relatives of schizophrenic patients: a positron emission tomography study

BACKGROUND

Schizophrenia is thought to arise from the interaction of genetically mediated and environmentally triggered abnormalities in brain function. Reduced frontal activation, reported in schizophrenic patients, may be one expression of genetic risk. The present study investigated whether frontal activation in relatives of schizophrenic patients would be related to eye tracking deficits (ETD), which are considered a behavioral marker of risk for schizophrenia.

METHODS

Subjects were first-degree relatives of schizophrenic patients (n = 17) and controls (n = 11). Relatives were divided into those with normal and abnormal pursuit based on qualitative ratings. Subjects were scanned using positron emission tomography and the H(2)15O bolus subtraction technique while performing smooth pursuit and fixation. Brain areas more active in pursuit than fixation were identified in the 3 groups. Correlations were used to investigate the relationship between activation of pursuit regions and pursuit gain in the relatives.

RESULTS

Controls significantly activated frontal eye fields (FEFs) and posterior areas, including the motion processing area, V5, and cuneus. The 2 groups of relatives activated the same posterior regions as controls, but differed from each other in activation of FEFs. Relatives with normal tracking activated right dorsal FEFs while relatives with ETD did not. Individual subtractions revealed that 90% of controls and 100% of the relatives with normal tracking activated FEFs during pursuit compared with 42% of relatives with ETD (P = .009). Pursuit gain was significantly and selectively associated with percent activation of right dorsal FEFs (r = 0.74).

CONCLUSIONS

Subtle frontal dysfunction seems to be a pathophysiological substrate of ETD in relatives of schizophrenic patients, and may be one aspect of genetically mediated differences in brain function relevant to schizophrenia.

Prediction of level of serotonin 2A receptor binding by serotonin receptor 2A genetic variation in postmortem brain samples from subjects who did or did not commit suicide

OBJECTIVE

Postmortem studies have indicated that suicide victims have greater serotonin receptor 2A (5-HTR2A) binding in prefrontal brain regions. However, there remains some controversy regarding the biological specificity of these findings. The authors hypothesized that the variance observed in brain 5-HTR2A binding is genetically mediated, at least in part.

METHOD

Postmortem data from 56 subjects who had committed suicide and 126 normal comparison subjects were studied; brain tissue was available from 11 subjects who committed suicide and 11 comparison subjects. Homogenate binding assays were carried out with [3H]ketanserin. Variation at the 5-HTR2A gene (HTR2A) was investigated by means of two polymorphisms: T102C and A-1438G.

RESULTS

5-HTR2A binding was greater in the prefrontal cortex of the subjects who committed suicide. In addition, the findings suggest that HTR2A variation significantly affects 5-HTR2A binding. However, no interaction between suicidal behavior and this locus was observed.

CONCLUSIONS

These results confirm previous reports of greater 5-HTR2A binding in subjects who committed suicide; they also provide preliminary evidence suggesting that the number of 5-HTR2A receptors is genetically mediated.

Lithium responsive bipolar disorder, unilineality, and chromosome 18: A linkage study

Over the last three years several studies have investigated the hypothesis of linkage between bipolar disorder and markers on chromosome 18. Although independent groups have reported positive results, it is still not clear how these should be interpreted, as linkage spans a considerably large segment of the chromosome. In this study we have investigated linkage with chromosome 18 markers in 19 families of lithium-responsive bipolar patients, as a way to select a more homogeneous population. In addition, we have investigated whether there is evidence of a parent-of-origin effect as suggested by previous studies. Eleven markers spanning the whole chromosome were typed and linkage analysis was carried out using parametric and nonparametric methods. Analysis of the whole sample provided nonsignificant linkage results. However, when the sample included only unilineal families, and was further stratified according to parental origin, two chromosomal regions provided modestly positive lod scores. Maximum lod scores of 1.04 (P = 0.001) at D18S53 and 0.87 (P = 0.045) at D18S61 were observed for maternal and paternal pedigrees, respectively. Nonparametric analysis yielded similar results. In conclusion, our results are congruent with previous reports that suggest an advantage of unilineal pedigrees in linkage analysis of bipolar disorder and cannot rule out a parent-of-origin effect in this genomic region.

Lack of association between the hSKCa3 channel gene CAG polymorphism and schizophrenia

Genetic anticipation, a phenomenon characterized by increased severity of symptoms and earlier age at onset of a disease in successive generations, is believed to be present in schizophrenia. In several neurodegenerative diseases showing anticipation, the mutation causing the disease is an expanded trinucleotide repeat. Therefore, genes containing trinucleotide repeats prone to expansion have become a suitable family of candidate genes in schizophrenia. A human calcium-activated potassium channel gene (hSKCa3), possibly mapping to chromosome 22q11-13, a region previously linked to schizophrenia, was recently described. This gene contains two contiguous expressed CAG repeat stretches. Recently, long allelic variants of one of these CAG repeats were found to be overrepresented in schizophrenic patients compared to normal controls. In this study we attempted to replicate this result and to study the relationship between the length of this CAG repeat on the one hand and the severity and age at onset of the disease on the other hand. No association with the disease or correlation with the severity of schizophrenia was identified. In addition, hSKCa3 was mapped to chromosome 1. Our results do not support the involvement of this particular CAG repeat-containing gene in schizophrenia.

T102C polymorphism in the 5HT2A gene and schizophrenia: relation to phenotype and drug response variability

Although genes play a major role in the etiology of schizophrenia, no major genes involved in this disease have been identified. However, several genes with small effect have been reported, though inconsistently, to increase the risk for schizophrenia. Recently, the 5HT2A 2 allele (T102C polymorphism) was reported to be over-represented in patients with schizophrenia. Other reports have found an excess of allele 2(C) only in schizophrenic patients who are resistant to clozapine, not in those who respond to clozapine. In this study, the 5HT2A receptor allele 2 frequencies were compared between 2 groups of patients with schizophrenia (39 responders and 63 nonresponders) based on long-term outcome and response to typical neuroleptics. A control group of 90 healthy volunteers screened for mental disorders was also included. Genotype 2/2 tended to be more frequent in patients with schizophrenia with poor long-term outcome and poor response to typical neuroleptics (Bonferroni corrected p = 0.09). This difference was significant in men (Bonferroni corrected p = 0.054) but not in women. In addition, the age at first contact with psychiatric care was significantly younger in the patients with schizophrenia with genotype 2/2 than in patients with genotype 1/1. These result suggest that the 5HT2A-receptor gene may play a role in a subset of schizophrenia characterized by poor long-term outcome and poor response to neuroleptics.

MAOA: association and linkage studies with lithium responsive bipolar disorder

A number of association studies have investigated the role of the monoamine oxidase A (MAOA) gene in the susceptibility to bipolar disorder. Although some studies have reported positive findings, there remains some controversy, because results from different studies have not been consistent. A common explanation for inconsistencies between studies is genetic heterogeneity. We have focused on lithium responsive bipolar disorder as a way to reduce heterogeneity. In this study, we investigated the role of MAOA in lithium responsive bipolar patients using association and linkage study designs. The investigation used 138 patients and 108 normal controls. In addition, 25 families were also studied. Our results were not supportive of a major role of MAOA in the predisposition to bipolar disorder.

Polyglutamine-containing proteins in schizophrenia

Genetic anticipation, manifested by increased severity and earlier age-at-onset of the disease over successive generations, is reported in schizophrenia. The molecular basis of anticipation in several neurodegenerative diseases is unstable coding CAG repeat expansions. Anticipation was reported in schizophrenia. Recently, studies suggested that enlarged CAG/CTG repeats are over represented in schizophrenic patients compared to normal controls. Together, these observations suggest that unstable CAG repeats may play a role in the etiology of schizophrenia. The purpose of this study is to test for the presence of polyglutamine-expanded tracts, encoded by CAG repeats, in total protein extracts derived from lymphoblastoid cell lines of schizophrenic patients. Proteins from schizophrenic patients (n = 59) and normal controls (n = 73) were separated by means of SDS-polyacrylamide gel electrophoresis, wet blotted onto nitrocellulose membrane and probed with a monoclonal antibody (mab 1C2) recognizing expanded polyglutamine arrays. Three abnormal bands corresponding to protein(s) of molecular weight of approximately 50 kDa were identified in two unrelated schizophrenic patients and in a sibling of one of these patients. None of the normal controls tested positive for this abnormal band. These results suggest that expanded polyglutamine-containing proteins, though rare, may play a role in the pathogenesis of schizophrenia.

Evidence for a role of phospholipase C-gamma1 in the pathogenesis of bipolar disorder

Several studies have indicated that patients with bipolar disorder (BD) who respond well to lithium prophylaxis constitute a biologically distinct subgroup. Lithium is thought to stabilize mood by acting at the phosphoinositide cycle. We have investigated a polymorphism located in the gene (PLCG1) that codes for a gamma-1 isozyme of phospholipase (PLC), an enzyme that plays an important role in the phosphoinositide second messenger system. A population-based association study and a family-based linkage study were carried out on patients who were considered excellent responders to lithium prophylaxis. Response to lithium was evaluated prospectively with an average follow-up of 14.4 +/- 6.8 years. The PLCG1 polymorphism was investigated in 136 excellent lithium responders and 163 controls. In addition, the segregation of this marker was studied in 32 families ascertained through lithium-responsive bipolar probands. The allele distributions between lithium-responsive bipolar patients and controls were different, with a higher frequency of one of the PLCG1 polymorphisms in patients (chi2 = 8.09; empirical P = 0.033). This polymorphism, however, confers only a small risk (OR = 1.88, CI 1.19-3.00). Linkage studies with the same marker yielded modest support for the involvement of this gene in the pathogenesis of BD when unilineal families were considered (Max LOD = 1.45; empirical P = 0.004), but not in the whole sample. Our results provide preliminary evidence that a PLC isozyme may confer susceptibility to bipolar disorder, probably accounting for a fraction of the total genetic variance. Whether this polymorphism is implicated in the pathogenesis of BD or in the mechanism of lithium response remains to be determined.