Association analysis of the partially duplicated alpha7 nicotinic acetylcholine receptor genetic variant and Alzheimer's disease

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.

Phase II crossover trial of varenicline in mild-to-moderate Alzheimer's disease

BACKGROUND

Evidence supports a role of α4β2 receptors in Alzheimer's disease (AD).

METHODS

This Korean, multicenter, double-blind, two-period (6 weeks each), crossover study randomized participants to the order in which they received varenicline (1 mg twice daily) and placebo. Assessments included AD Assessment Scale-Cognitive Subscale (ADAS-Cog) 75, Neuropsychiatric Inventory (NPI), adverse events (AEs) and Columbia-Suicide Severity Rating Scale (C-SSRS).

RESULTS

For varenicline versus placebo (n = 66 randomized), there was no significant difference in the week 6 least square (LS) mean ADAS-Cog 75 total score (primary endpoint; 18.07 vs. 18.49; p = 0.3873) and a slight worsening in the week 6 LS mean NPI (3.82 vs. 2.55; p = 0.0468), primarily driven by decreased appetite/eating. Common treatment-related AEs were nausea (23.3; 3.4%), vomiting (15.0%; 0) and decreased appetite (15.0; 6.8%).

CONCLUSIONS

Varenicline did not improve cognition, behavior or global change in this population. The most frequent varenicline-associated AEs were gastrointestinal; psychiatric AEs were rare and similar between the groups.

Chaperoning α7 neuronal nicotinic acetylcholine receptors

The α7 subtype of nicotinic acetylcholine receptors (AChRs) is one of the most abundant members of the Cys-loop family of receptors present in the central nervous system. It participates in various physiological processes and has received much attention as a potential therapeutic target for a variety of pathologies. The importance of understanding the mechanisms controlling AChR assembly and cell-surface delivery lies in the fact that these two processes are key to determining the functional pool of receptors actively engaged in synaptic transmission. Here we review recent studies showing that RIC-3, a protein originally identified in the worm Caenorhabditis elegans, modulates the expression of α7 AChRs in a subtype-specific manner. Potentiation of AChR expression by post-transcriptional events is also critically assessed.

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.

In vitro Paclitaxel and Radiation Effects on the Cell Types Responsible for Vascular Stenosis: A Preliminary Analysis

Hemodialysis vascular access dysfunction as a result of venous neointimal hyperplasia in dialysis access grafts and fistulae is currently a huge clinical problem. The aim of this study was to assess the effects of paclitaxel and radiation, both singly and in combination on the proliferation of cell types present within the lesion of venous neointimal hyperplasia (vascular smooth muscle cells, fibroblasts and endothelial cells within the neointimal microvessels). Vascular smooth muscle cells, fibroblasts and endothelial cells were plated onto 96-well plates and exposed to different concentrations and doses of paclitaxel and radiation, respectively (both individually and in combination). Growth inhibition was assessed with an MTT assay. Both paclitaxel and radiation resulted in significant growth inhibition of all three cell types. However, even small doses of paclitaxel appeared to attenuate the antiproliferative effect of radiation on these cell types. Further experiments to elucidate the mechanism behind these findings could result in a better understanding of combination antiproliferative therapies.

Hemodialysis vascular access dysfunction: a cellular and molecular viewpoint

Hemodialysis vascular access dysfunction is a major cause of morbidity and hospitalization in the hemodialysis population. The major cause of hemodialysis vascular access dysfunction is venous stenosis as a result of neointimal hyperplasia. Despite the magnitude of the clinical problem, however, there has been a paucity of novel therapeutic interventions in this field. This is in marked contrast to a recent plethora of targeted interventions for the treatment of arterial neointimal hyperplasia after coronary angioplasty. The reasons for this are two-fold. First there has been a relative lack of cellular and molecular research that focuses on venous neointimal hyperplasia in the specific setting of hemodialysis vascular access. Second, there have been inadequate efforts by the nephrology community to translate the recent advances in molecular and interventional cardiology into therapies for hemodialysis vascular access. This review therefore (1) briefly examines the different forms of hemodialysis vascular access that are available, (2) describes the pathology and pathogenesis of hemodialysis vascular access dysfunction in both polytetrafluoroethylene grafts and native arteriovenous fistulae, (3) reviews recent concepts about the pathogenesis of vascular stenosis that could potentially be applied in the setting of hemodialysis vascular access dysfunction, (4) summarizes novel experimental and clinical therapies that could potentially be used in the setting of hemodialysis vascular access dysfunction, and, finally, (5) offers some broad guidelines for future innovative translational and clinical research in this area that hopefully will reduce the huge clinical morbidity and economic costs that are associated with this condition.

Novel therapies for hemodialysis vascular access dysfunction: fact or fiction!

Hemodialysis vascular access dysfunction is a major cause of morbidity in the hemodialysis population and contributes significantly to the overall cost of end-stage renal disease programs. At a histological level, most hemodialysis vascular access dysfunction (in both native arteriovenous fistulae and PTFE dialysis access grafts) is due to venous stenosis and thrombosis, secondary to venous neointimal hyperplasia. However, despite a wealth of experimental and clinical data on the use of novel therapeutic interventions that target neointimal hyperplasia in the setting of coronary artery disease, there are unfortunately no effective therapeutic interventions for hemodialysis vascular access dysfunction at the present time. This is particularly unfortunate, since neointimal hyperplasia in the setting of hemodialysis vascular access fistulae and grafts could be the ideal clinical model to test novel therapeutic interventions for neointimal hyperplasia.

Association analysis for the muscarinic M1 receptor genetic polymorphisms and Alzheimer's disease

In the ongoing search to reveal the pathophysiology of Alzheimer's disease (AD), the cholinergic system is important due to its role in cognitive function and its significance with respect to the results of postmortem pathology and animal model studies. For this investigation, we tested the hypothesis that the allelic variant (C267A) of the cholinergic receptor muscarinic 1 (CHRM1) confers susceptibility to AD or is related to its age of onset, in a sample population of 232 AD patients and 169 normal controls. The distribution of the CHRM1 genotypes (p = 0.919) and alleles (p = 0.327) did not differ significantly comparing AD patients and controls, even after stratification according to apolipoprotein E genotype. The onset age was not significantly different comparing the CHRM1 genotype groups. Our negative findings suggest that it is unlikely that the CHRM1 C267A polymorphism plays a substantial role in conferring susceptibility to AD. We propose that other genetic variations of CHRM1, relating either to AD or to the therapeutic response for AD, may need further investigation.

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.

Amyloid beta(1-42) peptide alters the gating of human and mouse alpha-bungarotoxin-sensitive nicotinic receptors

The beta-amyloid(1-42) peptide (Abeta(1-42)), a major constituent of the Alzheimer's disease amyloid plaque, specifically binds to the neuronal alpha-bungarotoxin (alpha-BuTx)-sensitive alpha7 nicotinic acetylcholine receptor (alpha7 nAChR). Accordingly, Abeta1-42 interferes with the function of alpha7 nAChRs in chick and rodent neurons. To gain insights into the human disease, we studied the action of Abeta(1-42) on human alpha7 nAChRs expressed in Xenopus oocytes. In voltage-clamped oocytes expressing the wild-type receptor, Abeta(1-42) blocked ACh-evoked currents. The block was non-competitive, required over 100 s to develop and was partially reversible. In oocytes expressing the mutant L248T receptor, Abeta(1-42) activated methyllycaconitine-sensitive currents in a dose-dependent manner. Peptide-evoked unitary events, recorded in outside-out patches, showed single-channel conductances and open duration comparable to ACh-evoked events. Abeta(1-42) had no effect on the currents evoked by glutamate, GABA or glycine in oocytes expressing human or mouse receptors for these transmitters. Muscle nAChRs are also alpha-BuTx-sensitive and we therefore investigated whether they respond to Abeta(1-42). In human kidney BOSC 23 cells expressing the fetal or adult mouse muscle nAChRs, Abeta(1-42) blocked ACh-evoked whole-cell currents, accelerating their decay. Outside-out single-channel recordings showed that the block was due to a reduced channel open probability and enhanced block upon ACh application. We also report that the inverse peptide Abeta(42-1), but not Abeta(40-1), partially mimicked the effects of the physiological Abeta(1-42) peptide. Possible implications for degenerative neuronal and muscular diseases are discussed.

Association analysis for the genetic variants of the NMDA receptor subunit 2b and Alzheimer's disease

N-methyl-D-aspartate (NMDA) receptor dysfunction has been implicated in the pathogenesis of Alzheimer's disease (AD). The NMDA receptor is composed of several subunits, of which the receptor 2b subunit (NR2b) is of particular significance for AD. Abundant in the hippocampus of normal subjects, reductions in NR2b have been demonstrated in the hippocampus and entorhinal cortex of AD patients. In this study, we tested the hypothesis that the allelic variant (C2664T) of the NR2b confers susceptibility to AD using a sample population of 132 AD patients and 114 normal controls. The distribution of the NR2b genotypes (p = 0.600) and alleles (p = 0.652) did not differ significantly between AD patients and controls, however, suggesting that it is unlikely that the NR2b C2664T polymorphism plays a substantial role in conferring susceptibility to AD. We propose that other genetic variations of the NMDA subunits, relating either to AD or to the therapeutic response for NMDA partial agonists, may need further investigation.