Anne Pisanté

COMT: A common susceptibility gene in bipolar disorder and schizophrenia

A variety of psychiatric illnesses, including schizophrenia and bipolar disorder, have been reported in patients with microdeletion on chromosome 22q11—a region which includes the catechol‐O‐methyltransferase (COMT) gene. The variety of psychiatric manifestations in patients with the 22q11 microdeletion and the role of COMT in the degradation of catecholamine neurotransmitters may thus suggest a general involvement of the COMT gene in psychiatric diseases. We have previously reported on a significant association between a COMT haplotype and schizophrenia. In this study, we attempt to test for association between bipolar disorder and the polymorphisms implicated in schizophrenia. The association between COMT and bipolar disorder was tested by examining the allele and haplotype found to be associated with schizophrenia. A significant association between bipolar disorder and COMT polymorphisms was found. The estimated relative risk is greater in women, a result consistent with our previous findings in schizophrenia. We suggest that polymorphisms in the COMT gene may influence susceptibility to both diseases—and probably also a wider range of behavioral traits.

Susceptibility to chronic pain following nerve injury is genetically affected by CACNG2

Chronic neuropathic pain is affected by specifics of the precipitating neural pathology, psychosocial factors, and by genetic predisposition. Little is known about the identity of predisposing genes. Using an integrative approach, we discovered that CACNG2 significantly affects susceptibility to chronic pain following nerve injury. CACNG2 encodes for stargazin, a protein intimately involved in the trafficking of glutamatergic AMPA receptors. The protein might also be a Ca(2+) channel subunit. CACNG2 has previously been implicated in epilepsy. Initially, using two fine-mapping strategies in a mouse model (recombinant progeny testing [RPT] and recombinant inbred segregation test [RIST]), we mapped a pain-related quantitative trait locus (QTL) (Pain1) into a 4.2-Mb interval on chromosome 15. This interval includes 155 genes. Subsequently, bioinformatics and whole-genome microarray expression analysis were used to narrow the list of candidates and ultimately to pinpoint Cacng2 as a likely candidate. Analysis of stargazer mice, a Cacng2 hypomorphic mutant, provided electrophysiological and behavioral evidence for the genes functional role in pain processing. Finally, we showed that human CACNG2 polymorphisms are associated with chronic pain in a cohort of cancer patients who underwent breast surgery. Our findings provide novel information on the genetic basis of neuropathic pain and new insights into pain physiology that may ultimately enable better treatments.

Genome-wide association study implicates NDST3 in schizophrenia and bipolar disorder

Schizophrenia and bipolar disorder are major psychiatric disorders with high heritability and overlapping genetic variance. Here we perform a genome-wide association study in an ethnically homogeneous cohort of 904 schizophrenia cases and 1,640 controls drawn from the Ashkenazi Jewish population. We identify a novel genome-wide significant risk locus at chromosome 4q26, demonstrating the potential advantages of this founder population for gene discovery. The top single-nucleotide polymorphism (SNP; rs11098403) demonstrates consistent effects across 11 replication and extension cohorts, totalling 23, 191 samples across multiple ethnicities, regardless of diagnosis (schizophrenia or bipolar disorder), resulting in Pmeta=9.49 × 10−12 (odds ratio (OR)=1.13, 95% confidence interval (CI): 1.08–1.17) across both disorders and Pmeta=2.67 × 10−8 (OR=1.15, 95% CI: 1.08–1.21) for schizophrenia alone. In addition, this intergenic SNP significantly predicts postmortem cerebellar gene expression of NDST3, which encodes an enzyme critical to heparan sulphate metabolism. Heparan sulphate binding is critical to neurite outgrowth, axon formation and synaptic processes thought to be aberrant in these disorders.

A survey of the 22q11 microdeletion in a large cohort of schizophrenia patients

The occurrence of a microdeletion at 22q11 has long been considered to constitute a risk factor for schizophrenia. Higher rates of 22q11 deletions have been reported in cohorts of patients with schizophrenia. In order to estimate the prevalence of the 22q11 deletion in schizophrenia patients more accurately, a screening for 22q11 deletions was conducted on a cohort of 634 schizophrenia patients, the largest sample size screened to date. Seven microsatellites and three SNPs were used to assess the deletion genotype. In cases where all markers were found to be homozygous (hemizygous), the individual was assumed to carry the deletion. The method used here is simple and efficient in comparison with hybridization technologies. Moreover, the rate of false positives is very low (P-value in the range of 10(-4) to 10(-3)). Approximately 1% of the patient cohort was found to carry 22q11 deletions.

pain1: A neuropathic pain QTL on mouse chromosome 15 in a C3H×C58 backcross

&NA; We have produced a backcross (BC) population of 267 mice from the parental strains C3H/HeN and C58/J. The mice were phenotyped for neuropathic pain using the neuroma model. Subsequently all BC mice were genotyped in a region of chromosome 15 that has been previously suggested to contain a quantitative trait locus (QTL) for this trait. We have confirmed the linkage of the QTL, named pain1, to the central region of chromosome 15. Our finding provides the necessary robustness to justify efforts towards identification of the underlying gene.

Sex‐specific variability and a ‘cage effect’ independently mask a neuropathic pain quantitative trait locus detected in a whole genome scan

Sex and environment may dramatically affect genetic studies, and thus should be carefully considered. Beginning with two inbred mouse strains with contrasting phenotype in the neuroma model of neuropathic pain (autotomy), we established a backcross population on which we conducted a genome‐wide scan. The backcross population was partially maintained in small social groups and partially in isolation. The genome scan detected one previously reported quantitative trait locus (QTL) on chromosome 15 (pain1), but no additional QTLs were found. Interestingly, group caging introduced phenotypic noise large enough to completely mask the genetic effect of the chromosome 15 QTL. The reason appears to be that group‐caging animals from the low‐autotomy strain together with animals from the high‐autotomy strain dramatically increases autotomy in the otherwise low‐autotomy mice (males or females). The converse, suppression of pain behaviour in the high‐autotomy strain when caged with the low‐autotomy strain was also observed, but only in females. Even in isolated mice, the genetic effect of the chromosome 15 QTL was significant only in females. To determine why, we evaluated autotomy levels of females in 12 different inbred stains of mice and compared them to previously reported levels for males. Strikingly larger environmental variation was observed in males than in females for this pain phenotype. The high baseline variance in males can explain the difficulty in detecting the genetic effect, which was readily seen in females. Our study emphasizes the importance of sex and environment in the genetic analysis of pain.

Type 2 diabetes susceptibility loci in the Ashkenazi Jewish population

Until last year, type 2 diabetes (T2D) susceptibility loci have hardly been identified, despite great effort. Recently, however, several whole-genome association (WGA) studies jointly uncovered 10 robustly replicated loci. Here, we examine these loci in the Ashkenazi Jewish (AJ) population in a sample of 1,131 cases versus 1,147 controls. Genetic predisposition to T2D in the AJ population was found similar to that established in the previous studies. One SNP, rs7754840 in the CDKAL1 gene, presented a significantly stronger effect in the AJ population as compared to the general Caucasian population. This may possibly be due to the increased homogeneity of the AJ population. The use of the SNPs considered in this study, to identify individuals at high (or low) risk to develop T2D, was found of limited value. Our study, however, strongly supports the robustness of WGA studies for the identification of genes affecting complex traits in general and T2D in particular.

pain2: A neuropathic pain QTL identified on rat chromosome 2

&NA; We aimed to locate a chronic pain‐associated QTL in the rat (Rattus norvegicus) based on previous findings of a QTL (pain1) on chromosome 15 of the mouse (Mus musculus). The work was based on rat selection lines HA (high autotomy) and LA (low autotomy) which show a contrasting pain phenotype in response to nerve injury in the neuroma model of neuropathic pain. An F2 segregating population was generated from HA and LA animals. Phenotyped F2 rats were genotyped on chromosome 7 and chromosome 2, regions that share a partial homology with mouse chromosome 15. Our interval mapping analysis revealed a LOD score value of 3.63 (corresponding to p = 0.005 after correcting for multiple testing using permutations) on rat chromosome 2, which is suggestive of the presence of a QTL affecting the predisposition to neuropathic pain. This QTL was mapped to the 14–26 cM interval of chromosome 2. Interestingly, this region is syntenic to mouse chromosome 13, rather than to the region of mouse chromosome 15 that contains pain1. This chromosomal position indicates that it is possibly a new QTL, and hence we name it pain2. Further work is needed to replicate and to uncover the underlying gene(s) in both species.

A variant in the reelin gene increases the risk of schizophrenia and schizoaffective disorder but not bipolar disorder.

Schizophrenia (SZ), schizoaffective disorder (SA), and bipolar disorder (BD) are among the most common severe psychiatric disorders. Although the diseases are defined as distinct by the Diagnostic and Statistical Manual of Mental Disorder fourth edition, they share noticeable behavioral similarities. On the basis of the pedigree observations, it has been argued for a possible shared genetic background, as articulated by Silberschmidt and Sponheim (2008) in their introduction. Consequently, it seems of value and importance to test genetic associations discovered for SZ, also in BD and SA samples. We have recently conducted a whole genome association study to identify genes involved in SZ, where the GG genotype of the rs7341475 single-nucleotide polymorphism in the RELN gene was found to increase the risk to develop SZ in women only (P = 8.8 10 – ) (Shifman et al., 2008). In this study, we now investigate the genotypic distribution of this variant among SA and BD patients, separately in men and women.

Further tests of the association between schizophrenia and single nucleotide polymorphism markers at the catechol-O-methyltransferase locus in an Askenazi Jewish population using microsatellite markers.

Association studies are now primarily being conducted with single nucleotide polymorphisms because they are present everywhere in the genome and can be genotyped in “high throughput” formats. Microsatellite markers have a higher degree of polymorphism than single nucleotide polymorphisms and have been widely used in both linkage and association studies of disease. Polymorphic microsatellite markers with several alleles can readily detect linkage disequilibrium but at any given locus there may be differences between single nucleotide polymorphisms and microsatellites in their power to detect linkage disequilibrium because of the evolutionary history of the locus, especially the rate at which both the single nucleotide polymorphisms and microsatellite polymorphisms have mutated and the number of disease mutations and their history. In the current study, we examined the efficiency of microsatellite markers in association analysis by looking at all existent microsatellite markers in the catechol-O-methyltransferase gene region and by genotyping these microsatellites in a large cohort of schizophrenia patients and healthy controls, a subset of a sample where catechol-O-methyltransferase and schizophrenia were found to be associated. We also estimated the levels of linkage disequilibrium between these microsatellites and the previously reported single nucleotide polymorphisms (within the catechol-O-methyltransferase gene) found to be associated with schizophrenia. A modest allelic association of P=0.041 was found between schizophrenia and the microsatellite marker D22S944, which was not significant, however, when corrected for all microsatellites tested. Nevertheless, significant linkage disequilibrium was found between this marker and the three single nucleotide polymorphisms within the catechol-O-methyltransferase gene that displayed association with the disease in the previously published research on this sample. Significant linkage disequilibrium was also observed between microsatellites up to approximately 300 kb distant from those single nucleotide polymorphisms. Although significant, the extent of linkage disequilibrium in terms of r2 was small (in the order of 0.01).