Genetic Association Study of the Alpha 7 Nicotinic Receptor (CHRNA7) with the Development of Schizophrenia and Bipolar Disorder in Korean Population

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.

Genetic differences between bipolar disorder subtypes: A systematic review focused in bipolar disorder type II

BACKGROUND

The identification of bipolar disorder (BD) type II patients has both treatment and prognostic implications. Better understanding of its underlying genetics may yield useful diagnostic tools.

METHODS

A systematic review on BDII genetics was done using articles published in 2009-2019, following PRISMA recommendations.

RESULTS

The most studied polymorphism was BDNF Val66Met with several gene-gene interactions within the dopaminergic system. Associations were reported within the monoaminergic systems (DRD3, ADH1B and SLC6A4), calcium (CACNB2 and CACNG2) and cAMP (PDE1DA, PDE4B and DISC1) signal transduction pathways and the immune system (TNFα, IFNδ and IL-10). Chromosomes 2, 3 and 10 were associated with BDII and polygenic risk scores distinguished between BD subtypes and with major depressive disorder.

CONCLUSIONS

Research on BDII stems from BDI findings, however with a stronger contribution of gene-gene interactions and low-effect alleles on known neuroplasticity and monoaminergic system genes. Genome studies point to transdiagnostic backgrounds, with wider associations across bipolar spectrum disorders. Findings able to accurately differentiate BDII remain elusive, dependent on better phenotypic characterization and new research methods.

Gene Polymorphisms Affecting the Pharmacokinetics and Pharmacodynamics of Donepezil Efficacy

Donepezil (DNP) is the first-line drug used for Alzheimer's disease (AD). However, the therapeutic response rate of patients to DNP varies from 20 to 60%. The main reason for the large differences in the clinical efficacy of DNP therapy is genetic factors, some of which affect pharmacokinetics (PK), while others affect pharmacodynamics (PD). Thus, much emphasis has been placed on the investigation of an association between PK- and PD-related gene polymorphisms and therapeutic response to DNP, but a consistent view does not yet exist. In this review, we summarize recent findings regarding genetic factors influencing the clinical efficacy of DNP, including substantial differences in individual responses as a consequence of polymorphisms in Cytochrome P450 (CYP) 2D6, CY3A4, CY3A5, APOE, ABCA1, ABCB1, ESR1, BCHE, PON-1, CHRNA7, and CHAT. We also discuss possible strategies for the evaluation of the clinical efficacy of DNP, with a specific focus on possible biomarkers of PK/PD parameters, and provide perspectives and limitations within the field, which will also be beneficial for understanding the multiple mechanisms of DNP therapy in AD.

Genes Involved in Neurodevelopment, Neuroplasticity, and Bipolar Disorder: CACNA1C, CHRNA1, and MAPK1

BACKGROUND

Bipolar disorder (BPD) is a common and severe mental disorder. The involvement of genetic factors in the pathophysiology of BPD is well known. In the present study, we tested the association of several single-nucleotide polymorphisms (SNPs) within 3 strong candidate genes (CACNA1C, CHRNA7, and MAPK1) with BPD. These genes are involved in monoamine-related pathways, as well as in dendrite development, neuronal survival, synaptic plasticity, and memory/learning.

METHODS

One hundred and thirty-two subjects diagnosed with BPD and 326 healthy controls of Korean ancestry were genotyped for 40 SNPs within CACNA1C, CHRNA17, and MAPK1. Distribution of alleles and block of haplotypes within each gene were compared in cases and controls. Interactions between variants in different loci were also tested.

RESULTS

Significant differences in the distribution of alleles between the cases and controls were detected for rs1016388 within CACNA1C, rs1514250, rs2337980, rs6494223, rs3826029 and rs4779565 within CHRNA7, and rs8136867 within MAPK1. Haplotype analyses also confirmed an involvement of variations within these genes in BPD. Finally, exploratory epistatic analyses demonstrated potential interactive effects, especially regarding variations in CACNA1C and CHRNA7.

LIMITATIONS

Limited sample size and risk of false-positive findings.

DISCUSSION

Our data suggest a possible role of these 3 genes in BPD. Alterations of 1 or more common brain pathways (e.g., neurodevelopment and neuroplasticity, calcium signaling) may explain the obtained results.

Is Chromosome 15q13.3 Duplication Involving CHRNA7 Associated With Oral Clefts?

Copy number variants have been associated with intellectual disability, multiple congenital anomalies and craniofacial disorders. It has been reported that microduplication of 15q13.3 is associated with autism, cognitive impairment, seizures, and attention-deficit hyperactivity disorder. Here, the author identified microduplications in the 15q13.3 region in 4 cases from 3 Chinese families using chromosomal microarray analysis-single nucleotide polymorphism array (CMA-SNP). These 4 cases include 2 fetuses from 2 unrelated families and a father and a daughter from a third family. The identified microduplications of 15q13.3 are approximately 400 kb in size, encompassing just 1 gene, cholinergic receptor, neuronal nicotinic, alpha polypeptide 7 (CHRNA7). Three-fourths of the probands exhibit oral clefts, which has not been previously reported in cases with this duplication genotype. Therefore, in this study, the author describes for the first time the common feature of oral clefts in patients carrying a microduplication of 15q13.3 encompassing the CHRNA7 gene, which sheds light on the correlation between CHRNA7 and cleft palate.

Therapeutic Potential of α7 Nicotinic Acetylcholine Receptors

Progress in the fields of neuroscience and molecular biology has identified the forebrain cholinergic system as being important in many higher order brain functions. Further analysis of the genes encoding the nicotinic acetylcholine receptors (nAChRs) has highlighted, in particular, the role of α7 nAChRs in these higher order brain functions as evidenced by their peculiar physiologic and pharmacological properties. As this receptor has gained the attention of scientists from academia and industry, our knowledge of its roles in various brain and bodily functions has increased immensely. We have also seen the development of small molecules that have further refined our understanding of the roles of α7 nAChRs, and these molecules have begun to be tested in clinical trials for several indications. Although a large body of data has confirmed a role of α7 nAChRs in cognition, the translation of small molecules affecting α7 nAChRs into therapeutics has to date only progressed to the stage of testing in clinical trials. Notably, however, most recent human genetic and biochemical studies are further underscoring the crucial role of α7 nAChRs and associated genes in multiple organ systems and disease states. The aim of this review is to discuss our current knowledge of α7 nAChRs and their relevance as a target in specific functional systems and disease states.

Effect of APOE and CHRNA7 genotypes on the cognitive response to cholinesterase inhibitor treatment at different stages of Alzheimer's disease

The loss of cholinergic transmission is considered to be an important cause of Alzheimer's disease (AD). Treatment with acetyl cholinesterase inhibitors (ChEIs) shows benefits; however, great heterogeneity has been observed in patient responses. We evaluated apolipoprotein E (APOE) and α7 nicotinic receptor (CHRNA7) single-nucleotide polymorphisms (SNPs) and associated these SNPs with pharmacological responses to ChEIs in a Brazilian population with AD. We studied 177 outpatients using ChEIs, and they were classified as responders and nonresponders according to variation in Mini-Mental State Examination (MMSE) status. The analysis of APOE genotypes showed that patients with the ε4 allele had a worse response than those without the ε4 allele. We observed an association between the CHRNA7 T allele and a better response to treatment with ChEIs in patients with mild AD (MMSE ≥ 20). The SNP rs6494223 of CHRNA7 as well as APOEε4 could be useful for understanding the response to ChEI treatment in patients with AD.

Meta-analyses of 10 polymorphisms associated with the risk of schizophrenia

Schizophrenia (SCZ) is a severe complex psychiatric disorder that generates problems for the associated family and society and causes disability with regards to work for patients. The aim of the present study was to assess the contribution of 10 genetic polymorphisms to SCZ susceptibility. Meta-analyses were conducted using the data without a limitation for time or language. A total of 27 studies with 7 genes and 10 polymorphisms were selected for the meta-analyses. Two polymorphisms were found to be significantly associated with SCZ. SNAP25 rs3746544 was shown to increase the SCZ risk by 18% [P=0.01; odds ratio (OR), 1.18; 95% confidence interval (CI), 1.05-1.34] and GRIK3 rs6691840 was found to increase the risk by 30% (P=0.008; OR, 1.30; 95% CI, 1.07-1.58). Significant results were found under the dominant (P=0.001; OR, 1.36; 95% CI, 1.13-1.65) and additive (P=0.02; OR, 1.45; 95% CI, 1.06-1.98) model for the SNAP25 rs3746544 polymorphism and under the additive model for the GRIK3 rs6691840 polymorphism (P=0.03; OR, 1.73; 95% CI, 1.04-2.85). There were no significant results observed for the other eight polymorphisms, which were CCKAR rs1800857, CHRNA7 rs904952, CHRNA7 rs6494223, CHRNA7 rs2337506, DBH Ins>Del, FEZ1 rs559668, FEZ1 rs597570 and GCLM rs2301022. In conclusion, the present meta-analyses indicated that the SNAP25 rs3746544 and GRIK3 rs6691840 polymorphisms were risk factors of SCZ, which may provide valuable information for the clinical diagnosis of SCZ.

Endogenous kynurenic acid regulates extracellular GABA levels in the rat prefrontal cortex

The tryptophan metabolite kynurenic acid (KYNA) is an endogenous antagonist of the α7 nicotinic acetylcholine receptor (α7nAChR) and, at higher concentrations, inhibits ionotropic glutamate receptors. Increases in KYNA levels are seen in brain and cerebrospinal fluid in individuals with schizophrenia (SZ) and may be causally related to cognitive deficits in SZ and other psychiatric diseases. As dysfunction of circuits involving GABAergic neurons in the prefrontal cortex (PFC) likely plays a role in the cognitive impairments seen in these disorders, we examined the effects of KYNA on extracellular GABA in this brain area. Applied to awake rats for 2 h by reverse dialysis, KYNA concentration-dependently and reversibly reduced extracellular GABA levels, with 300 nM KYNA causing a nadir of ∼45% of baseline concentrations. This effect was not duplicated by reverse dialysis of the selective glycineB receptor antagonist 7-Cl-KYNA (100 nM) or the AMPA/kainate receptor antagonist CNQX (100 μM), and was prevented by co-application of galantamine (5 μM), a positive allosteric modulator of the α7nAChR. Conversely, inhibition of endogenous KYNA formation by reverse dialysis of (S)-4-(ethylsulfonyl)benzoylalanine (ESBA; 5 mM) reversibly increased GABA levels in the PFC, reaching a peak of ∼160% of baseline concentrations. Co-infusion of 30 nM KYNA neutralized this effect. Taken together, these results demonstrate a role for endogenous KYNA in the bi-directional control of GABAergic neurotransmission in the PFC. Pharmacological manipulation of KYNA may therefore be useful in the treatment of GABAergic impairments in SZ and other brain disorders involving the PFC.

No effect of polymorphisms in the non-duplicated region of the CHRNA7 gene on sensory gating P50 ratios in patients with schizophrenia and bipolar disorder

Previous research has reported that bipolar disorder and schizophrenic patients evidence sensory gating deficits. The use of intermediate phenotypes may facilitate genetic studies. Four single nucleotide polymorphisms (SNPs) located on the non-duplicated region of the alpha-7 nicotinic receptor gene (CHRNA7) were genotyped in 95 healthy subjects, 127 bipolar disorder and 153 schizophrenic patients. We evaluated the association of these polymorphisms with P50 evoked potential measures. Our results do not support a role for the candidate gene in this neurophysiological disturbance.

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.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.