Association study of nicotinic-receptor variants and major depressive disorder

Unanswered questions in the regulation and function of the duplicated α7 nicotinic receptor gene CHRFAM7A

The α7 nicotinic receptor (α7 nAChR) is an important entry point for Ca²⁺ into the cell, which has broad and important effects on gene expression and function. The gene (CHRNA7), mapping to chromosome (15q14), has been genetically linked to a large number of diseases, many of which involve defects in cognition. While numerous mutations in CHRNA7 are associated with mental illness and inflammation, an important control point may be the function of a recently discovered partial duplication CHRNA7, CHRFAM7A, that negatively regulates the function of the α7 receptor, through the formation of heteropentamers; other functions cannot be excluded. The deregulation of this human specific gene (CHRFAM7A) has been linked to neurodevelopmental, neurodegenerative, and inflammatory disorders and has important copy number variations. Much effort is being made to understand its function and regulation both in healthy and pathological conditions. However, many questions remain to be answered regarding its functional role, its regulation, and its role in the etiogenesis of neurological and inflammatory disorders. Missing knowledge on the pharmacology of the heteroreceptor has limited the discovery of new molecules capable of modulating its activity. Here we review the state of the art on the role of CHRFAM7A, highlighting unanswered questions to be addressed. A possible therapeutic approach based on genome editing protocols is also discussed.

The Human-Restricted Isoform of the α7 nAChR, CHRFAM7A: A Double-Edged Sword in Neurological and Inflammatory Disorders

CHRFAM7A is a relatively recent and exclusively human gene arising from the partial duplication of exons 5 to 10 of the α7 neuronal nicotinic acetylcholine receptor subunit (α7 nAChR) encoding gene, CHRNA7. CHRNA7 is related to several disorders that involve cognitive deficits, including neuropsychiatric, neurodegenerative, and inflammatory disorders. In extra-neuronal tissues, α7nAChR plays an important role in proliferation, differentiation, migration, adhesion, cell contact, apoptosis, angiogenesis, and tumor progression, as well as in the modulation of the inflammatory response through the “cholinergic anti-inflammatory pathway”. CHRFAM7A translates the dupα7 protein in a multitude of cell lines and heterologous systems, while maintaining processing and trafficking that are very similar to the full-length form. It does not form functional ion channel receptors alone. In the presence of CHRNA7 gene products, dupα7 can assemble and form heteromeric receptors that, in order to be functional, should include at least two α7 subunits to form the agonist binding site. When incorporated into the receptor, in vitro and in vivo data showed that dupα7 negatively modulated α7 activity, probably due to a reduction in the number of ACh binding sites. Very recent data in the literature report that the presence of the duplicated gene may be responsible for the translational gap in several human diseases. Here, we will review the studies that have been conducted on CHRFAM7A in different pathologies, with the intent of providing evidence regarding when and how the expression of this duplicated gene may be beneficial or detrimental in the pathogenesis, and eventually in the therapeutic response, to CHRNA7-related neurological and non-neurological diseases.

Kynurenine 3-monooxygenase deficiency induces depression-like behavior via enhanced antagonism of α7 nicotinic acetylcholine receptors by kynurenic acid

Tryptophan (TRP) is metabolized via the kynurenine (KYN) pathway, which is related to the pathogenesis of major depressive disorder (MDD). Kynurenine 3-monooxygenase (KMO) is a pivotal enzyme in the metabolism of KYN to 3-hydroxykynurenine. In rodents, KMO deficiency induces a depression-like behavior and increases the levels of kynurenic acid (KA), a KYN metabolite formed by kynurenine aminotransferases (KATs). KA antagonizes α7 nicotinic acetylcholine receptor (α7nAChR). Here, we investigated the involvement of KA in depression-like behavior in KMO knockout (KO) mice. KYN, KA, and anthranilic acid but not TRP or 3-hydroxyanthranilic acid were elevated in the prefrontal cortex of KMO KO mice. The mRNA levels of KAT1 and α7nAChR but not KAT2-4, α4nAChR, or β2nAChR were elevated in the prefrontal cortex of KMO KO mice. Nicotine blocked increase in locomotor activity, decrease in social interaction time, and prolonged immobility in a forced swimming test, but it did not decrease sucrose preference in the KMO KO mice. Methyllycaconitine (an α7nAChR antagonist) antagonized the effect of nicotine on decreased social interaction time and prolonged immobility in the forced swimming test, but not increased locomotor activity. Galantamine (an α7nAChR allosteric agonist) blocked the increased locomotor activity and prolonged immobility in the forced swimming test, but not the decreased social interaction time in the KMO KO mice. In conclusion, elevation of KA levels contributes to depression-like behaviors in KMO KO mice by α7nAChR antagonism. The ameliorating effects of nicotine and galantamine on depression-like behaviors in KMO KO mice are associated with the activation of α7nAChR.

Antidepressant activity in mice elicited by 3-furan-2-yl-N-p-tolyl-acrylamide, a positive allosteric modulator of the α7 nicotinic acetylcholine receptor

The objective of the current study is to determine whether 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2), a positive allosteric modulator of α7 nicotinic receptors (AChRs), produces antidepressant-like behavior in mice, and reactivates desensitized α7 AChRs expressed in CH3-α7 cells. Mice from both sexes were injected (i.p.) with PAM-2 (1.0mg/kg) on a daily basis for three weeks. Forced swim tests (FSTs) were performed on Day 1 and Day 7 to determine the acute and subchronic effects of PAM-2, respectively, and on Days 14 and 21 to determine its chronic activity. To examine the residual effects after drug treatment, a withdrawal period of two more weeks was continued with FSTs performed on Day 28 and 35. Our results indicate that: (1) PAM-2 does not induce acute antidepressant effects in male or female mice, (2) PAM-2 induces antidepressant effects in mice from both sexes after one (subchronic) and two (chronic) weeks, whereas at the third week (chronic), the antidepressant effect is decreased in male and increased in female mice. Since PAM-2 does not influence the locomotor activity of mice, the observed antidepressant activity is not driven by nonspecific motor-stimulant actions, (3) the residual antidepressant effect mediated by PAM-2 after one week of treatment cessation is observed only in female mice, and finally the Ca(2+) influx results indicate that (4) PAM-2 can reactivate desensitized α7 AChRs. Our results clearly indicate that PAM-2 elicits antidepressant activity, probably by enhancing the activity of the endogenous neurotransmitter acetylcholine on α7 AChRs, without inducing receptor desensitization, and that this activity is gender-dependent. This is the first time that an antidepressant activity is described for an α7 PAM, supporting further studies as potential therapeutic medications for depressive states.

Association study of the 2-bp deletion polymorphism in exon 6 of the CHRFAM7A gene with idiopathic generalized epilepsy

There is evidence of linkage between the 15q13-q14 locus, containing the gene encoding the α7 subunit (CHRNA7) of the neuronal nicotinic acetylcholine receptor (nAChR) and its partially duplicated isoform (CHRFAM7A), and epilepsy. Additionally, a 2-bp deletion polymorphism (c.497-498delTG; rs67158670) in CHRFAM7A, resulting in a frame shift and truncation of the protein product, is associated with some neurological diseases. This study was designed to explore the possibility of an association of the c.497-498delTG polymorphism of CHRFAM7A with idiopathic generalized epilepsies (IGEs) in Polish children and young patients. The study included 197 IGE patients and 258 unrelated healthy individuals. The frequency of the CHRFAM7A c.497-498delTG polymorphism was determined in each group using heteroduplex analysis. An association between the c.497-498delTG polymorphism of CHRFAM7A and IGE was evidenced. It was demonstrated that the frequency of the CHRFAM7A 2-bp deletion carriers was significantly lower in the IGE patients than in the control group. The observed frequency of 2-bp deletion carriers was high in IGE subjects (64%), but significantly higher in control subjects (76%). Carriers of at least one copy of the -2 bp allele had halved their risk of IGE susceptibility (delTG/delTG and delTG/wild-type versus wild-type/wild-type: odds ratio=0.55; 95% confidence intervals=0.365-0.827; p=0.004). Moreover, it has been demonstrated that this polymorphic variant is associated with the c.524-12_524-11insGTT variation (rs10649395) in intron 7 of CHRFAM7A. Our study substantiates the involvement of the α7 subunit of nAChR in the pathophysiology of IGEs and indicates that the CHRFAM7A c.497-498TG deletion or a nearby polymorphism may play a role in the pathogenesis of IGE. Further work should concentrate on ascertaining the exact mechanism of this polymorphism's effect and its relationship with IGE.

Hippocampal α7 nicotinic acetylcholine receptor levels in patients with schizophrenia, bipolar disorder, or major depressive disorder

BACKGROUND

The α7 nicotinic acetylcholine receptor (nAChR) is involved in cognitive function and synaptic plasticity. Consequently, changes in α7 nAChR function have been implicated in a variety of mental disorders, especially schizophrenia. However, there is little knowledge regarding the levels of the α7 nAChR in patients with bipolar disorder.

METHODS

We performed [(125)I]-bungarotoxin autoradiography to selectively visualize and measure α7 nAChRs on postmortem sections of the temporal lobe from patients with schizophrenia, bipolar disorder, or major depressive disorder, as well as control subjects. Radioligand binding was determined in the dentate gyrus, CA3, and CA1 subfields of the hippocampus and the perirhinal cortex.

RESULTS

Bungarotoxin binding was significantly increased in the CA1 and perirhinal cortex of patients with bipolar disorder compared to control subjects, whereas in patients with schizophrenia or major depressive disorder the level of binding did not significantly differ from control subjects in any region measured.

CONCLUSIONS

These data are consistent with the reported genetic associations linking the α7 nAChR to the pathology of bipolar disorder, and may suggest a dysfunction of α7 nAChR-dependent signalling in bipolar disorder. We could not reproduce the previously reported decrease in hippocampal bungarotoxin binding in schizophrenia.

Poor replication of candidate genes for major depressive disorder using genome-wide association data

Data from the Genetic Association Information Network (GAIN) genome-wide association study (GWAS) in major depressive disorder (MDD) were used to explore previously reported candidate gene and single-nucleotide polymorphism (SNP) associations in MDD. A systematic literature search of candidate genes associated with MDD in case-control studies was performed before the results of the GAIN MDD study became available. Measured and imputed candidate SNPs and genes were tested in the GAIN MDD study encompassing 1738 cases and 1802 controls. Imputation was used to increase the number of SNPs from the GWAS and to improve coverage of SNPs in the candidate genes selected. Tests were carried out for individual SNPs and the entire gene using different statistical approaches, with permutation analysis as the final arbiter. In all, 78 papers reporting on 57 genes were identified, from which 92 SNPs could be mapped. In the GAIN MDD study, two SNPs were associated with MDD: C5orf20 (rs12520799; P=0.038; odds ratio (OR) AT=1.10, 95% CI 0.95-1.29; OR TT=1.21, 95% confidence interval (CI) 1.01-1.47) and NPY (rs16139; P=0.034; OR C allele=0.73, 95% CI 0.55-0.97), constituting a direct replication of previously identified SNPs. At the gene level, TNF (rs76917; OR T=1.35, 95% CI 1.13-1.63; P=0.0034) was identified as the only gene for which the association with MDD remained significant after correction for multiple testing. For SLC6A2 (norepinephrine transporter (NET)) significantly more SNPs (19 out of 100; P=0.039) than expected were associated while accounting for the linkage disequilibrium (LD) structure. Thus, we found support for involvement in MDD for only four genes. However, given the number of candidate SNPs and genes that were tested, even these significant may well be false positives. The poor replication may point to publication bias and false-positive findings in previous candidate gene studies, and may also be related to heterogeneity of the MDD phenotype as well as contextual genetic or environmental factors.

Electrophysiological perspectives on the therapeutic use of nicotinic acetylcholine receptor partial agonists

Partial agonist therapies rely variously on two hypotheses: the partial agonists have their effects through chronic low-level receptor activation or the partial agonists work by decreasing the effects of endogenous or exogenous full agonists. The relative significance of these activities probably depends on whether acute or chronic effects are considered. We studied nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus laevis oocytes to test a model for the acute interactions between acetylcholine (ACh) and weak partial agonists. Data were best-fit to a basic competition model that included an additional factor for noncompetitive inhibition. Partial agonist effects were compared with the nAChR antagonist bupropion in prolonged bath application experiments that were designed to mimic prolonged drug exposure typical of therapeutic drug delivery. A primary effect of prolonged application of nicotine was to decrease the response of all nAChR subtypes to acute applications of ACh. In addition, nicotine, cytisine, and varenicline produced detectable steady-state activation of α4β2* [(α4)(2)(β2)(3), (α4)(3)(β2)(2), and (α4)(2)(β2)(2)α5)] receptor subtypes that was not seen with other test compounds. Partial agonists produced no detectable steady-state activation of α7 nAChR, but seemed to show small potentiation of ACh-evoked responses; however, "run-up" of α7 ACh responses was also sometimes observed under control conditions. Potential off-target effects of the partial agonists therefore included the modulation of α7 responses by α4β2 partial agonists and decreases in α4β2* responses by α7-selective agonists. These data indicate the dual effects expected for α4β2* partial agonists and provide models and insights for utility of partial agonists in therapeutic development.

A 2-base pair deletion polymorphism in the partial duplication of the alpha7 nicotinic acetylcholine gene (CHRFAM7A) on chromosome 15q14 is associated with schizophrenia

Multiple genetic linkage studies support the hypothesis that the 15q13-14 chromosomal region contributes to the etiology of schizophrenia. Among the putative candidate genes in this area are the alpha7 nicotinic acetylcholine receptor gene (CHRNA7) and its partial duplication, CHRFAM7A. A large chromosomal segment including the CHRFAM7A gene locus, but not the CHRNA7 locus, is deleted in some individuals. The CHRFAM7A gene contains a polymorphism consisting of a 2 base pair (2 bp) deletion at position 497-498 bp of exon 6. We employed PCR-based methods to quantify the copy number of CHRFAM7A and the presence of the 2 bp polymorphism in a large, multi-ethnic population. The 2 bp polymorphism was associated with schizophrenia in African Americans (genotype p=0.005, allele p=0.015), and in Caucasians (genotype p=0.015, allele p=0.009). We conclude that the presence of the 2 bp polymorphism at the CHRFAM7A locus may have a functional significance in schizophrenia.

Convergent functional genomics of genome-wide association data for bipolar disorder: comprehensive identification of candidate genes, pathways and mechanisms

Given the mounting convergent evidence implicating many more genes in complex disorders such as bipolar disorder than the small number identified unambiguously by the first-generation Genome-Wide Association studies (GWAS) to date, there is a strong need for improvements in methodology. One strategy is to include in the next generation GWAS larger numbers of subjects, and/or to pool independent studies into meta-analyses. We propose and provide proof of principle for the use of a complementary approach, convergent functional genomics (CFG), as a way of mining the existing GWAS datasets for signals that are there already, but did not reach significance using a genetics-only approach. With the CFG approach, the integration of genetics with genomics, of human and animal model data, and of multiple independent lines of evidence converging on the same genes offers a way of extracting signal from noise and prioritizing candidates. In essence our analysis is the most comprehensive integration of genetics and functional genomics to date in the field of bipolar disorder, yielding a series of novel (such as Klf12, Aldh1a1, A2bp1, Ak3l1, Rorb, Rora) and previously known (such as Bdnf, Arntl, Gsk3b, Disc1, Nrg1, Htr2a) candidate genes, blood biomarkers, as well as a comprehensive identification of pathways and mechanisms. These become prime targets for hypothesis driven follow-up studies, new drug development and personalized medicine approaches.

Meta-analyses of genetic studies on major depressive disorder

The genetic basis of major depressive disorder (MDD) has been investigated extensively, but the identification of MDD genes has been hampered by conflicting results from underpowered studies. We review all MDD case-control genetic association studies published before June 2007 and perform meta-analyses for polymorphisms that had been investigated in at least three studies. The study selection and data extraction were performed in duplicate by two independent investigators. The 183 papers that met our criteria studied 393 polymorphisms in 102 genes. Twenty-two polymorphisms (6%) were investigated in at least three studies. Seven polymorphisms had been evaluated in previous meta-analyses, 5 of these had new data available. Hence, we performed meta-analyses for 20 polymorphisms in 18 genes. Pooled odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Statistically significant associations were found for the APOE varepsilon2 (OR, 0.51), GNB3 825T (OR, 1.38), MTHFR 677T (OR, 1.20), SLC6A4 44 bp Ins/Del S (OR, 1.11) alleles and the SLC6A3 40 bpVNTR 9/10 genotype (OR, 2.06). To date, there is statistically significant evidence for six MDD susceptibility genes (APOE, DRD4, GNB3, MTHFR, SLC6A3 and SLC6A4).

No evidence for association between 19 cholinergic genes and bipolar disorder

Cholinergic dysfunction has been proposed for the pathogenesis of bipolar disorder (BD), and we have therefore performed a systematic association study of cholinergic system genes in BD (including schizoaffective disorder bipolar type). We genotyped 93 single nucleotide polymorphisms (SNPs) in 19 genes (CHAT, CHRM1-5, CHRNA1-7, CHRNA9, CHRNA10, and CHRNB1-4) in two series of samples: the National Institute of Mental Health (NIMH) Genetics Initiative pedigrees with 474 samples from 152 families, and the Clinical Neurogenetics (CNG) pedigrees with 83 samples from 22 multiplex families. Sib-transmission/disequilibrium test (sib_TDT) analysis showed nominally significant transmission bias for four SNPs (CHRNA2: rs7017417, P = 0.024; CHRNA5: rs514743, P = 0.031; CHRNB1: rs2302762, P = 0.049; CHRNB4: rs1948, P = 0.031). Haploview analyses showed nominally significant transmission bias of several haplotypes in CHRNA2, CHRNA7, CHRNB1, and CHRNB4, respectively. However, none of these associations reached gene-wide significance after correction by permutation. Alcohol dependence (including alcohol abuse) was not a significant covariate in the present genetic association analysis. Thus, it is unlikely that these 19 cholinergic genes play a major role in the pre-disposition to BD in these pedigrees.

Links between depression and substance abuse in adolescents: neurobiological mechanisms

Adolescence is a high-risk period for development of both depressive and substance use disorders. These two disorders frequently co-occur in adolescents and are associated with significant morbidity and mortality. Given the added economic and psychosocial burden associated with the comorbid condition, identification of risk factors associated with their co-occurrence is of great public health importance. Research with adult animals and humans has indicated several common neurobiological systems that link depressive and addictive disorders. Given the ongoing maturation of these systems throughout adolescence and early adult life, it is not clear how these neurobiological processes influence development and progression of both disorders. A better understanding of the pathophysiological mechanisms leading to the onset and course of these disorders during adolescence will be helpful in developing more effective preventive and treatment strategies, and thereby allow these youth to reach their full potential as adults.

Proposed multigenic Composite Inheritance in major depression

Various rationale have been considered in the familial inheritance pattern of major depression ranging from simple one-gene Mendelian inheritance to pseudo-additive gene action. We instead predict broad genetic expressivity patterns in the progeny of parents where at least one parent has recurrent major depression. In keeping with this idea, we feel that recurrent major depression could involve an expression imbalance of "normal" genes either exclusively or along with allelic variation(s). The patterns of pathology are theoretically conceptualized as qualitative and quantitative, meaning that expressivity of the genetic pattern in these children may range from minimal to complete even among siblings. Thus, prediction of the particular genetic pattern expressed by a particular child might prove difficult. The complex inheritance pattern that we propose is referred to as Composite Inheritance. Composite Inheritance considers that both the up- and down-regulation of luxury genes and housekeeping genes are involved in this dichotomous qualitative inheritance pattern and also the wide quantitative expressivity. The luxury genes include such genes as those coding for the neurotransmitter transporters and receptors. The housekeeping genes found to date include those that code for proteins involved in gene transcription, secondary signaling systems, fatty acid metabolism and transport, and intracellular calcium homeostasis. Other luxury and housekeeping genes no doubt remain to be discovered. Our current research utilizes an empirical approach involving advanced genomics and specialized pattern recognition mathematics in families having at least one parent with recurrent major depression. The goal of our research is to develop a pattern recognition system of genetic expressivity in major depression to which prevention and early intervention may be tailored.

Multidisciplinary approach to childhood epilepsy: exploring the scientific rationale and practical aspects of implementation

The management of childhood epilepsy requires attention to more than seizure control because children with epilepsy often suffer from comorbidities that lead to an increased frequency of psychiatric disease, learning difficulties, and other problems of psychosocial development. These comorbidities can stem in part from the same genetic traits that determine seizure susceptibility. Thus, mutations affecting potassium, calcium, and sodium channels have been linked with epilepsy syndromes and affective and behavioral abnormalities. It is important to consider the effect of antiepilepsy drugs on comorbid conditions and the effect on seizures of drugs used to treat comorbidities. A number of antiepilepsy drugs are available that have minimal adverse cognitive effects, and some can have positive effects on mood and behavior. Epilepsy in a child is a condition that affects and is affected by the entire family situation. In addition to appropriate neuropsychologic evaluation, optimal management of childhood epilepsy also can require the involvement of the social worker, advanced practice nurse, and educational specialist. Many elements of the multidisciplinary team approach can be instituted by the child neurologist in community practice and at large, specialized epilepsy centers.

Association study of the human partially duplicated α7 nicotinic acetylcholine receptor genetic variant with bipolar disorder

The human alpha7 nicotinic acetylcholine receptor subunit (CHRNA7) gene cluster maps to the chromosome 15q13-q14 and is implicated as a candidate gene for bipolar disorder (BPD) by genetic linkage study. A -2 bp deletion polymorphism has been found in the duplicated CHRNA7 (CHRNA7-like) gene, which is located 1 Mb apart from CHRNA7. We tested the hypothesis that the allelic variant, 2 bp deletion (-2 bp), confers susceptibility to BPD or is related to the psychotic features of BPD. We genotyped the -2 bp polymorphism in 77 patients with BPD and 135 normal controls. The distribution of -2 bp genotypes showed a moderately significant difference between the BPD patients and controls (P=0.044). Three BPD patients carried more than two alleles of the -2 bp deletion genotype, while this genotype was not found in the control group. The -2 bp polymorphism was not associated with age of onset or psychotic features in BPD patients. The results of this study suggest that the -2 bp polymorphism or a nearby polymorphism may play a role in the pathogenesis of BPD. Determination of the functional impact of the -2 bp variant in the nervous system and, in particular, the effect of harboring more than two alleles of the -2 bp deletion needs further exploration.

Effect of nicotine and nicotinic receptors on anxiety and depression

Nicotine has been shown to have effects on anxiety and depression in both human and animal studies. These studies suggest that nicotinic acetylcholine receptors (nAChRs) can modulate the function of pathways involved in stress response, anxiety and depression in the normal brain, and that smoking can result in alterations of anxiety level and mood. The effects of nicotine are complex however, and nicotine treatment can be either anxiolytic or anxiogenic depending on the anxiety model tested, the route of nicotine administration and the time course of administration. The paradoxical effects of nicotine on emotionality are likely due to the broad expression of nAChRs throughout the brain, the large number of nAChR subtypes that have been identified and the ability of nicotine treatment to both activate and desensitize nAChRs. Activation of nAChRs has been shown to modulate many systems associated with stress response including stress hormone pathways, monoaminergic transmission and release of classical neurotransmitters throughout the brain. Local administration studies in animals have identified brain areas that may be involved in the anxiogenic and anxiolytic actions of nicotine including the lateral septum, the dorsal raphe nuclei, the mesolimbic dopamine system and the hippocampus. The ensemble of studies to date suggest that under certain conditions nicotine can act as an anxiolytic and an antidepressant, but that following chronic use, adaptations to nicotine can occur resulting in increased anxiety and depression following withdrawal.