Association between 9p21-23 Locus and Frailty in a Community-Dwelling Greek Population: Results from the Hellenic Longitudinal Investigation of Ageing and Diet

BACKGROUND

Frailty is a complex geriatric syndrome arising from a combination of genetic and environmental factors and is associated with adverse health outcomes and mortality. A recent study reported an association between variants of the 9p21-23 locus, associated with a number of age-related disorders, including Alzheimer's disease (AD), and frailty. Frailty has been associated with increased risk of developing AD and it has been proposed that frailty burden may modify AD clinical presentation. In view of the overlapping genetic architecture between the two disorders, it is noteworthy to conduct studies to uncover risk variants that contribute to both AD and frailty. The purpose of this study is to test the reproducibility of the association of 9p21-23 locus with frailty in a population that is ethnically different from previous work and in the context of multidimensional definitions of frailty that will allow us to examine the potential impact to domains pertaining to AD pathology.

METHODS

We operationalized frailty according two definitions and the corresponding instruments, the Frailty Index (FI) and the Tilburg Frailty Indicator (TFI) and we determined genotypes of eight alleles previously identified as risk increasing for frailty in 1172 community-dwelling older participants (57% females) from the HELIAD study with a mean age of 74 years old. We cross-sectionally investigated the association between risk alleles and frailty, as well as with specific components of each definition using linear regression analyses adjusted for age, sex and years of education.

RESULTS

Compared to non-carriers, carriers of rs7038172 C risk allele, were associated with a higher FI Score (β=0.089, p=0.002). Similarly, we found a positive association between the presence of at least one rs7038172 C variant and TFI score (β=0.053, p=0.04). Moreover, the rs7038172 variant was associated, irrespectively of dementia status, with the memory and psychological domain of FI and TFI, respectively.

CONCLUSION

Our study confirms the association of the rs7038172 C allele with the frailty syndrome in a Greek population and in the context of multidimensional definitions of frailty. Furthermore, we report novel associations between this allele and the memory domain of FI and the psychological domain of TFI, that includes memory problems on its components. Given that frailty burden has been shown to modify the AD clinical presentation, it is likely that rs7038172 C allele may accelerate the transition of AD or frailty to dementia Overall, our study corroborates the role of the 9p21-23 region in frailty development and draw potential links with AD pathology.

17q21.31 duplication causes prominent tau-related dementia with increased MAPT expression

To assess the role of rare copy number variations in Alzheimer's disease (AD), we conducted a case-control study using whole-exome sequencing data from 522 early-onset cases and 584 controls. The most recurrent rearrangement was a 17q21.31 microduplication, overlapping the CRHR1, MAPT, STH and KANSL1 genes that was found in four cases, including one de novo rearrangement, and was absent in controls. The increased MAPT gene dosage led to a 1.6-1.9-fold expression of the MAPT messenger RNA. Clinical signs, neuroimaging and cerebrospinal fluid biomarker profiles were consistent with an AD diagnosis in MAPT duplication carriers. However, amyloid positon emission tomography (PET) imaging, performed in three patients, was negative. Analysis of an additional case with neuropathological examination confirmed that the MAPT duplication causes a complex tauopathy, including prominent neurofibrillary tangle pathology in the medial temporal lobe without amyloid-β deposits. 17q21.31 duplication is the genetic basis of a novel entity marked by prominent tauopathy, leading to early-onset dementia with an AD clinical phenotype. This entity could account for a proportion of probable AD cases with negative amyloid PET imaging recently identified in large clinical series.

SORL1 rare variants: a major risk factor for familial early-onset Alzheimer's disease

The SORL1 protein plays a protective role against the secretion of the amyloid β peptide, a key event in the pathogeny of Alzheimer's disease. We assessed the impact of SORL1 rare variants in early-onset Alzheimer's disease (EOAD) in a case-control setting. We conducted a whole exome analysis among 484 French EOAD patients and 498 ethnically matched controls. After collapsing rare variants (minor allele frequency ≤1%), we detected an enrichment of disruptive and predicted damaging missense SORL1 variants in cases (odds radio (OR)=5.03, 95% confidence interval (CI)=(2.02-14.99), P=7.49.10(-5)). This enrichment was even stronger when restricting the analysis to the 205 cases with a positive family history (OR=8.86, 95% CI=(3.35-27.31), P=3.82.10(-7)). We conclude that predicted damaging rare SORL1 variants are a strong risk factor for EOAD and that the association signal is mainly driven by cases with positive family history.

A genome-wide association meta-analysis of plasma Aβ peptides concentrations in the elderly

Amyloid beta (Aβ) peptides are the major components of senile plaques, one of the main pathological hallmarks of Alzheimer disease (AD). However, Aβ peptides' functions are not fully understood and seem to be highly pleiotropic. We hypothesized that plasma Aβ peptides concentrations could be a suitable endophenotype for a genome-wide association study (GWAS) designed to (i) identify novel genetic factors involved in amyloid precursor protein metabolism and (ii) highlight relevant Aβ-related physiological and pathophysiological processes. Hence, we performed a genome-wide association meta-analysis of four studies totaling 3 528 healthy individuals of European descent and for whom plasma Aβ1-40 and Aβ1-42 peptides levels had been quantified. Although we did not observe any genome-wide significant locus, we identified 18 suggestive loci (P<1 × 10(-)(5)). Enrichment-pathway analyses revealed canonical pathways mainly involved in neuronal functions, for example, axonal guidance signaling. We also assessed the biological impact of the gene most strongly associated with plasma Aβ1-42 levels (cortexin 3, CTXN3) on APP metabolism in vitro and found that the gene protein was able to modulate Aβ1-42 secretion. In conclusion, our study results suggest that plasma Aβ peptides levels are valid endophenotypes in GWASs and can be used to characterize the metabolism and functions of APP and its metabolites.

Meta-analysis of 74,046 individuals identifies 11 new susceptibility loci for Alzheimer's disease

Eleven susceptibility loci for late-onset Alzheimer's disease (LOAD) were identified by previous studies; however, a large portion of the genetic risk for this disease remains unexplained. We conducted a large, two-stage meta-analysis of genome-wide association studies (GWAS) in individuals of European ancestry. In stage 1, we used genotyped and imputed data (7,055,881 SNPs) to perform meta-analysis on 4 previously published GWAS data sets consisting of 17,008 Alzheimer's disease cases and 37,154 controls. In stage 2, 11,632 SNPs were genotyped and tested for association in an independent set of 8,572 Alzheimer's disease cases and 11,312 controls. In addition to the APOE locus (encoding apolipoprotein E), 19 loci reached genome-wide significance (P < 5 × 10(-8)) in the combined stage 1 and stage 2 analysis, of which 11 are newly associated with Alzheimer's disease.