Possible association of nicastrin polymorphisms and Alzheimer disease in the Finnish population

Neuron-specific deletion of presenilin enhancer2 causes progressive astrogliosis and age-related neurodegeneration in the cortex independent of the Notch signaling

Introduction

Presenilin enhancer2 (Pen‐2) is an essential subunit of γ‐secretase, which is a key protease responsible for the cleavage of amyloid precursor protein (APP) and Notch. Mutations on Pen‐2 cause familial Alzheimer disease (AD). However, it remains unknown whether Pen‐2 regulates neuronal survival and neuroinflammation in the adult brain.

Methods

Forebrain neuron‐specific Pen‐2 conditional knockout (Pen‐2 cKO) mice were generated for this study. Pen‐2 cKO mice expressing Notch1 intracellular domain (NICD) conditionally in cortical neurons were also generated.

Results

Loss of Pen‐2 causes astrogliosis followed by age‐dependent cortical atrophy and neuronal loss. Loss of Pen‐2 results in microgliosis and enhanced inflammatory responses in the cortex. Expression of NICD in Pen‐2 cKO cortices ameliorates neither neurodegeneration nor neuroinflammation.

Conclusions

Pen‐2 is required for neuronal survival in the adult cerebral cortex. The Notch signaling may not be involved in neurodegeneration caused by loss of Pen‐2.

Variants regulating ZBTB4 are associated with age-at-onset of Alzheimer's disease

The identification of novel genetic modifiers of age-at-onset (AAO) of Alzheimer's disease (AD) could advance our understanding of AD and provide novel therapeutic targets. A previous genome scan for modifiers of AAO among families affected by early-onset AD caused by the PSEN2 N141I variant identified 2 loci with significant evidence for linkage: 1q23.3 and 17p13.2. Here, we describe the fine-mapping of these 2 linkage regions, and test for replication in 6 independent datasets. By fine-mapping these linkage signals in a single large family, we reduced the linkage regions to 11% their original size and nominated 54 candidate variants. Among the 11 variants associated with AAO of AD in a larger sample of Germans from Russia, the strongest evidence implicated promoter variants influencing NCSTN on 1q23.3 and ZBTB4 on 17p13.2. The association between ZBTB4 and AAO of AD was replicated by multiple variants in independent, trans-ethnic datasets. Our results show association between AAO of AD and both ZBTB4 and NCSTN. ZBTB4 is a transcriptional repressor that regulates the cell cycle, including the apoptotic response to amyloid beta, while NCSTN is part of the gamma secretase complex, known to influence amyloid beta production. These genes therefore suggest important roles for amyloid beta and cell cycle pathways in AAO of AD.

Functional and genetic analysis of haplotypic sequence variation at the nicastrin genomic locus

Nicastrin (NCSTN) is a component of the γ-secretase complex and therefore potentially a candidate risk gene for Alzheimer's disease. Here, we have developed a novel functional genomics methodology to express common locus haplotypes to assess functional differences. DNA recombination was used to engineer 5 bacterial artificial chromosomes (BACs) to each express a different haplotype of the NCSTN locus. Each NCSTN-BAC was delivered to knockout nicastrin (Ncstn(-/-)) cells and clonal NCSTN-BAC(+)/Ncstn(-/-) cell lines were created for functional analyses. We showed that all NCSTN-BAC haplotypes expressed nicastrin protein and rescued γ-secretase activity and amyloid beta (Aβ) production in NCSTN-BAC(+)/Ncstn(-/-) lines. We then showed that genetic variation at the NCSTN locus affected alternative splicing in human postmortem brain tissue. However, there was no robust functional difference between clonal cell lines rescued by each of the 5 different haplotypes. Finally, there was no statistically significant association of NCSTN with disease risk in the 4 cohorts. We therefore conclude that it is unlikely that common variation at the NCSTN locus is a risk factor for Alzheimer's disease.

Deep sequencing of the Nicastrin gene in pooled DNA, the identification of genetic variants that affect risk of Alzheimer's disease

Nicastrin is an obligatory component of the γ-secretase; the enzyme complex that leads to the production of Aβ fragments critically central to the pathogenesis of Alzheimer's disease (AD). Analyses of the effects of common variation in this gene on risk for late onset AD have been inconclusive. We investigated the effect of rare variation in the coding regions of the Nicastrin gene in a cohort of AD patients and matched controls using an innovative pooling approach and next generation sequencing. Five SNPs were identified and validated by individual genotyping from 311 cases and 360 controls. Association analysis identified a non-synonymous rare SNP (N417Y) with a statistically higher frequency in cases compared to controls in the Greek population (OR 3.994, CI 1.105–14.439, p = 0.035). This finding warrants further investigation in a larger cohort and adds weight to the hypothesis that rare variation explains some of genetic heritability still to be identified in Alzheimer's disease.

Evidence for three loci modifying age-at-onset of Alzheimer's disease in early-onset PSEN2 families

Families with early-onset Alzheimer's disease (AD) sharing a single PSEN2 mutation exhibit a wide range of age-at-onset, suggesting that modifier loci segregate within these families. While APOE is known to be an age-at-onset modifier, it does not explain all of this variation. We performed a genome scan within nine such families for loci influencing age-at-onset, while simultaneously controlling for variation in the primary PSEN2 mutation (N141I) and APOE. We found significant evidence of linkage between age-at-onset and chromosome 1q23.3 (P < 0.001) when analysis included all families, and to chromosomes 1q23.3 (P < 0.001), 17p13.2 (P = 0.0002), 7q33 (P = 0.017), and 11p14.2 (P = 0.017) in a single large pedigree. Simultaneous analysis of these four chromosomes maintained strong evidence of linkage to chromosomes 1q23.3 and 17p13.2 when all families were analyzed, and to chromosomes 1q23.3, 7q33, and 17p13.2 within the same single pedigree. Inclusion of major gene covariates proved essential to detect these linkage signals, as all linkage signals dissipated when PSEN2 and APOE were excluded from the model. The four chromosomal regions with evidence of linkage all coincide with previous linkage signals, associated SNPs, and/or candidate genes identified in independent AD study populations. This study establishes several candidate regions for further analysis and is consistent with an oligogenic model of AD risk and age-at-onset. More generally, this study also demonstrates the value of searching for modifier loci in existing datasets previously used to identify primary causal variants for complex disease traits.

Analysis of the nicastrin promoter rs10752637 polymorphism and its association with Alzheimer's disease

Alzheimer's disease (AD) is a common, degenerative form of dementia characterized by the accumulation of plaques containing amyloid beta-peptides (A beta). Nicastrin (NCSTN) is a type I trans-membrane glycoprotein and an essential component of gamma-secretase, a multiprotein complex required for the production of the mature form of A beta. Overexpression of wild-type NCSTN increases A beta production, indicating that the strict regulation of NCSTN expression may play a fundamental role in the pathogenesis of AD. In this study we investigated the effect of a single-nucleotide polymorphism (SNP; rs10752637), located in the promoter region of the NCSTN gene, on NCSTN promoter activity. First, the rs10752637 genotypes were determined in a Chinese population consisting of 462 patients with sporadic AD and 470 normal control subjects. The distributions of the rs10752637 genotypes and allele frequencies were significantly different between the AD and control groups, with the -922T allele significantly associated with the occurrence of AD. Reporter assays indicated that the rs10752637 -922T allele had a significantly increased promoter activity relative to the -922G allele. Furthermore, gel shift assays demonstrated that the -922T allele preferentially bound to components of nuclear extracts. Overall, our results indicate that the rs10752637 SNP can likely influence the expression of NCSTN, and that this may be an influencing factor during the pathogenesis of AD.

Association between promoter polymorphisms of the nicastrin gene and sporadic Alzheimer's disease in North Chinese Han population

Increasing evidences have shown that nicastrin (NCSTN) plays a crucial role in gamma-cleavage of the amyloid precursor protein (APP). Inhibition of NCSTN demonstrated an altered gamma-cleavage activity, suggesting its potential implication in developing Alzheimer's disease (AD). We detected the NCSTN gene promoter region in 359 sporadic AD (SAD) patients and 331 controls and found three promoter single nucleotide polymorphisms (SNPs): -1216C/A (rs2147471), -796T/G (rs10752637) and -436C/T (rs1324738). For -1216C/A, there were significant differences in the allele and genotype frequency between AD and control subjects (allele P=0.031, genotype P=0.017). The allele and genotype frequencies remained significant before and after APOEvarepsilon4 stratification. The -1216CC carriers increased 2-fold risk for the development of SAD compared to the carriers with -1216CA and AA genotypes (OR=2.049, 95%CI=1.410-2.976, P=0.000). For -796T/G, there were significant differences in the genotype frequency between SAD and control subjects (P=0.009). This trend is still obvious in the subjects without APOEvarepsilon4 allele. The -796GG carriers might decrease the risk compared to the carriers with -796TG and TT genotypes (OR=0.602, 95%CI=0.393-0.932, P=0.022). No significant difference was detected either in genotype or in allele frequencies between SAD and control for -436C/T, even after APOEvarepsilon4 stratification. The haplotype -1216A/-796G may be a protective factor for SAD (OR=0.795, 95%CI=0.636-0.995, P=0.045). Our investigation suggests that -1216C/A and -796T/G are probably related to the development of SAD.

A novel alternative splice variant of nicastrin and its implication in Alzheimer disease

Nicastrin interacts with gamma-secretase complex components predominantly via the N-terminal third of the transmembrane domain. The authentic transmembrane domain is critically required for the interaction with gamma-secretase complex components and for formation of an active gamma-secretase complex. In this study, we have identified a novel alternatively spliced transcript of nicastrin in human brain tissue. This transcript (NCSTN-DeltaE16) lacks exon 16 of nicastrin mRNA, which leads to deletion of 71 amino acids just upstream of its transmembrane domain. Its expression pattern was analyzed in the hippocampus of patients with pathologically diagnosed Alzheimer disease (cases) and non-Alzheimer dementia (controls). In patients with the APOE-epsilon4 allele, the frequency of Alzheimer disease appeared to be increased in the NCSTN-DeltaE16-positive group, but the association was not statistically significant. In conclusion, the expression of NCSTN-DeltaE16 transcript may confer some additional risk for developing Alzheimer disease beyond the risk due to ApoE-epsilon4 allele. Further investigation in larger scale population would be necessary to address its potential implication in Alzheimer disease.

Rat nicastrin gene: cDNA isolation, mRNA variants and expression pattern analysis

Nicastrin is a type 1 transmembrane glycoprotein that interacts with presenilin, Aph-1, and Pen-2 proteins to form a high molecular complex with gamma secretase activity. Then, nicastrin has a central role in presenilin-mediated processing of beta-amyloid precursor protein and in some aspects of Notch/glp-1 signaling in vivo. Here, we isolated a rat nicastrin cDNA and investigated gene expression in embryonic and adult rat tissues. The predicted amino acid sequence is comprised of 708 residues and showed a high degree of identity with other vertebrate orthologs. Besides full-length nicastrin mRNA, we identified an alternative spliced variant lacking the whole exon 3 and predicted to encode a 62-residue-long truncated protein. Full-length nicastrin mRNA was observed to be ubiquitously expressed, while the spliced variant was preferentially transcribed in the nervous system, whether in embryonic or adult neural tissues. Studies performed on primary cell cultures demonstrated that the short isoform was expressed in neurons, but not in astrocyte and microglial cells. Further experiments performed to verify the presence of the variant in neuroblastoma culture failed to show any truncated protein. Treatments by cyclohexamide showed the involvement of a quality control-based surveillance mechanism, which selectively degrades the exon 3-skipped isoform. In summary, this is the first report describing a novel skipped isoform of nicastrin which may suggest a new possible control mechanism based on the alternative splicing and nonsense-mediated mRNA decay to regulate brain protein expression and provide newer insights into potential implication in Alzheimer's disease.

Twenty years of the Alzheimer's disease amyloid hypothesis: a genetic perspective

From Alois Alzheimer's description of Auguste D.'s brain in 1907 to George Glenner's biochemical dissection of beta-amyloid in 1984, the "amyloid hypothesis" of Alzheimer's disease has continued to gain support over the past two decades, particularly from genetic studies. Here we assess the amyloid hypothesis based on both known and putative Alzheimer's disease genes.