Nathalie Brouwers

APOE and Alzheimer disease: a major gene with semi-dominant inheritance

Apolipoprotein E (APOE) dependent lifetime risks (LTRs) for Alzheimer Disease (AD) are currently not accurately known and odds ratios alone are insufficient to assess these risks. We calculated AD LTR in 7351 cases and 10 132 controls from Caucasian ancestry using Rochester (USA) incidence data. At the age of 85 the LTR of AD without reference to APOE genotype was 11% in males and 14% in females. At the same age, this risk ranged from 51% for APOE44 male carriers to 60% for APOE44 female carriers, and from 23% for APOE34 male carriers to 30% for APOE34 female carriers, consistent with semi-dominant inheritance of a moderately penetrant gene. Using PAQUID (France) incidence data, estimates were globally similar except that at age 85 the LTRs reached 68 and 35% for APOE 44 and APOE 34 female carriers, respectively. These risks are more similar to those of major genes in Mendelian diseases, such as BRCA1 in breast cancer, than those of low-risk common alleles identified by recent GWAS in complex diseases. In addition, stratification of our data by age groups clearly demonstrates that APOE4 is a risk factor not only for late-onset but for early-onset AD as well. Together, these results urge a reappraisal of the impact of APOE in Alzheimer disease.

Serum biomarker for progranulin‐associated frontotemporal lobar degeneration

Mutations that lead to a loss of progranulin (PGRN) explain a considerable portion of the occurrence of frontotemporal lobar degeneration. We tested a biomarker allowing rapid detection of a loss of PGRN.

Potent amyloidogenicity and pathogenicity of Aβ43

The amyloid-β peptide Aβ42 is known to be a primary amyloidogenic and pathogenic agent in Alzheimers disease. However, the role of Aβ43, which is found just as frequently in the brains of affected individuals, remains unresolved. We generated knock-in mice containing a pathogenic presenilin-1 R278I mutation that causes overproduction of Aβ43. Homozygosity was embryonic lethal, indicating that the mutation involves a loss of function. Crossing amyloid precursor protein transgenic mice with heterozygous mutant mice resulted in elevated Aβ43, impairment of short-term memory and acceleration of amyloid-β pathology, which accompanied pronounced accumulation of Aβ43 in plaque cores similar in biochemical composition to those observed in the brains of affected individuals. Consistently, Aβ43 showed a higher propensity to aggregate and was more neurotoxic than Aβ42. Other pathogenic presenilin mutations also caused overproduction of Aβ43 in a manner correlating with Aβ42 and with the age of disease onset. These findings indicate that Aβ43, an overlooked species, is potently amyloidogenic, neurotoxic and abundant in vivo.

Molecular genetics of Alzheimer's disease: an update.

Alzheimers disease (AD) is a complex disorder of the central nervous system (CNS). Molecular genetic research has provided a wealth of information regarding the genetic etiology of this devastating disease. Identification and functional characterization of autosomal dominant mutations in the amyloid precursor protein gene (APP) and the presenilin genes 1 and 2 (PSEN1 and PSEN2) have contributed substantially to our understanding of the biological mechanisms leading towards CNS neurodegeneration in AD. Nonetheless, a large part of the genetic etiology remains unresolved, especially that of more common, sporadic forms of AD. While substantial efforts were invested in the identification of genetic risk factors underlying sporadic AD, using carefully designed genetic association studies in large patient-control groups, the only firmly established risk factor remains the ε4 allele of the apolipoprotein E gene (APOE). Nevertheless, one can expect that with the current availability of high-throughput genotyping platforms and dense maps of single-nucleotide polymorphisms (SNPs), large-scale genetic studies will eventually generate additional knowledge about the genetic risk profile for AD. This review provides an overview of the current understanding in the field of AD genetics, covering both the rare monogenic forms as well as recent developments in the search for novel AD susceptibility genes.

Progranulin genetic variability contributes to amyotrophic lateral sclerosis.

Objectives: Null mutations in progranulin (PGRN) cause ubiquitin-positive frontotemporal dementia (FTD) linked to chromosome 17q21 (FTDU-17). Here we examined PGRN genetic variability in amyotrophic lateral sclerosis (ALS), a neurodegenerative motor neuron disease that overlaps with FTD at a clinical, pathologic, and epidemiologic level. Methods: We sequenced all exons, exon-intron boundaries, and 5′ and 3′ regulatory regions of PGRN in a Belgian sample of 230 patients with ALS. The frequency of observed genetic variants was determined in 436 healthy control individuals. The contribution of eight frequent polymorphisms to ALS risk, onset age, and survival was assessed in an association study in the Belgian sample and a replication series of 308 Dutch patients with ALS and 345 Dutch controls. Results: In patients with ALS we identified 11 mutations, 5 of which were predicted to affect PGRN protein sequence or levels (four missense mutations and one 5′ regulatory variant). Moreover, common variants (rs9897526, rs34424835, and rs850713) and haplotypes were significantly associated with a reduction in age at onset and a shorter survival after onset of ALS in both the Belgian and the Dutch studies. Conclusion: PGRN acts as a modifier of the course of disease in patients with amyotrophic lateral sclerosis, through earlier onset and shorter survival.

Alzheimer risk associated with a copy number variation in the complement receptor 1 increasing C3b/C4b binding sites

Two multicentre genome-wide association (GWA) studies provided substantial evidence, implicating the complement receptor 1 gene (CR1) in Alzheimer disease (AD) genetic etiology. CR1 encodes a large transmembrane receptor with a crucial role in the immune complement cascade. We performed a genetic follow-up of the GWA CR1 association in a Flanders–Belgian cohort (n=1883), and investigated the effect of single-nucleotide polymorphisms (SNPs) located in the CR1 locus on AD risk and cerebrospinal fluid (CSF) biomarker levels. We obtained significant association (Padj<0.03; odds ratio (OR)=1.24 (95% confidence interval (CI): 1.02–1.51)) for one CR1 risk haplotype, and haplotype association was strongest in individuals carrying apolipoprotein E (APOE) ɛ4 alleles (Padj<0.006; OR=1.50 (95% CI: 1.08–2.09)). Also, four SNPs correlated with increased CSF amyloid Aβ1−42 levels, suggesting a role for the CR1 protein in Aβ metabolism. Moreover, we quantified a low-copy repeat (LCR)-associated copy number variation (CNV) in CR1, producing different CR1 isoforms, CR1-F and CR1-S, and obtained significant association in carriers of CR1-S. We replicated the CR1 CNV association finding in a French cohort (n=2003) and calculated in the combined cohorts, an OR of 1.32; 95% CI: 1.10–1.59 (P=0.0025). Our data showed that the common AD risk association may well be explained by the presence of CR1-S increasing the number of C3b/C4b and cofactor activity sites and AD risk with 30% in CR1-S carriers. How precisely the different functional role of CR1-S in the immune complement cascade contributes to AD pathogenesis will need additional functional studies.

Rescue of Progranulin Deficiency Associated with Frontotemporal Lobar Degeneration by Alkalizing Reagents and Inhibition of Vacuolar ATPase

Numerous loss-of-function mutations in the progranulin (GRN) gene cause frontotemporal lobar degeneration with ubiquitin and TAR–DNA binding protein 43-positive inclusions by reduced production and secretion of GRN. Consistent with the observation that GRN has neurotrophic properties, pharmacological stimulation of GRN production is a promising approach to rescue GRN haploinsufficiency and prevent disease progression. We therefore searched for compounds capable of selectively increasing GRN levels. Here, we demonstrate that four independent and highly selective inhibitors of vacuolar ATPase (bafilomycin A1, concanamycin A, archazolid B, and apicularen A) significantly elevate intracellular and secreted GRN. Furthermore, clinically used alkalizing drugs, including chloroquine, bepridil, and amiodarone, similarly stimulate GRN production. Elevation of GRN levels occurs via a translational mechanism independent of lysosomal degradation, autophagy, or endocytosis. Importantly, alkalizing reagents rescue GRN deficiency in organotypic cortical slice cultures from a mouse model for GRN deficiency and in primary cells derived from human patients with GRN loss-of-function mutations. Thus, alkalizing reagents, specifically those already used in humans for other applications, and vacuolar ATPase inhibitors may be therapeutically used to prevent GRN-dependent neurodegeneration.

SORL1 is genetically associated with increased risk for late-onset Alzheimer disease in the Belgian population

SORL1 has recently been identified as a major genetic contributor to increased risk for late‐onset Alzheimer disease (AD). Here we aimed at replicating this finding in a large, well‐characterized group of 550 Belgian late‐onset AD patients and 637 healthy control individuals using a gene‐wide genotyping approach across the SORL1 locus. We observed significant associations, both for individual SNPs (SNPs 6, 8, 9, 10 and 27; p‐values ranging from 0.001 to 0.040) and 3‐SNP haplotypes (SNPs 5‐6‐7 and SNPs 25‐26‐27; p‐values ranging from 0.008 to 0.035). Moreover, the associations at SNP 8, 9 and 10 represented a direct replication of the initial association data. Two signals in distinct regions of the gene were shown to be mutually independent, supporting allelic heterogeneity at the SORL1 locus in the Belgian population. Our findings confirm that genetic variants in SORL1 may be important risk factors for late‐onset AD.

Amyloid precursor protein mutation E682K at the alternative β‐secretase cleavage β′‐site increases Aβ generation

BACE1 cleaves the amyloid precursor protein (APP) at the β‐cleavage site (Met671–Asp672) to initiate the generation of amyloid peptide Aβ. BACE1 is also known to cleave APP at a much less well‐characterized β′‐cleavage site (Tyr681–Glu682). We describe here the identification of a novel APP mutation E682K located at this β′‐site in an early onset Alzheimers disease (AD) case. Functional analysis revealed that this E682K mutation blocked the β′‐site and shifted cleavage of APP to the β‐site, causing increased Aβ production. This work demonstrates the functional importance of APP processing at the β′‐site and shows how disruption of the balance between β‐ and β′‐site cleavage may enhance the amyloidogenic processing and consequentially risk for AD. Increasing exon‐ and exome‐based sequencing efforts will identify many more putative pathogenic mutations without conclusive segregation‐based evidence in a single family. Our study shows how functional analysis of such mutations allows to determine the potential pathogenic nature of these mutations. We propose to classify the E682K mutation as probable pathogenic awaiting further independent confirmation of its association with AD in other patients.

Genetic association of CR1 with Alzheimer's disease: A tentative disease mechanism

CR1 is a novel Alzheimers disease (AD) gene identified by genome-wide association studies (GWAS). Recently, we showed that AD risk could be explained by an 18-kilobase insertion responsible for the complement component (3b/4b) receptor 1 (CR1)-S isoform. We investigated the relevance of the CR1 isoforms to AD in a Canadian dataset. Also, we genotyped rs4844610 tagging the GWAS-significant CR1 single nucleotide polymorphisms. Individuals with F/S genotype had a 1.8 times increased risk for AD compared with F/F genotype (p-adjusted = 0.003), while rs4844610 was only marginally significant (p-adjusted = 0.024). The analyses of brain samples demonstrated that the CR1-S isoform is expressed at lower protein levels than CR1-F (p < 0.0001) hence likely associated with increased complement activation. Intriguingly, our neuropathological results show that the pattern of CR1 expression in neurons is different between the F/F and F/S genotypes (filiform vs. vesicular-like profiles). Furthermore, double labeling studies supported a differential distribution of CR1 in neurons (endoplasmic reticulum intermediate compartment vs. lysosomes). These observations indicate that the CR1-S and CR1-F isoforms could be processed in different ways in neurons. In conclusion, our results support that the CR1-S isoform explains the GWAS signals and open a novel prospect for the investigation of CR1-related disease mechanisms.