ABSTRACT
OBJECTIVE: Synonymous variants can lead to disease; nevertheless, the majority of sequencing studies conducted in Alzheimer disease (AD) only assessed coding variation. METHODS: To detect synonymous variants modulating AD risk, we conducted a whole-genome sequencing study on 67 Caribbean Hispanic (CH) families multiply affected by AD. Identified disease-associated variants were further assessed in an independent cohort of CHs, expression quantitative trait locus (eQTL) data, brain autopsy data, and functional experiments. RESULTS: Rare synonymous variants in 4 genes (CDH23, SLC9A3R1, RHBDD2, and ITIH2) segregated with AD status in multiplex families and had a significantly higher frequency in these families compared with reference populations of similar ancestry. In comparison to subjects without dementia, expression of CDH23 (ß = 0.53, p = 0.006) and SLC9A3R1 (ß = 0.50, p = 0.02) was increased, and expression of RHBDD2 (ß = -0.70, p = 0.02) decreased in individuals with AD at death. In line with this finding, increased expression of CDH23 (ß = 0.26 ± 0.08, p = 4.9E-4) and decreased expression of RHBDD2 (ß = -0.60 ± 0.12, p = 5.5E-7) were related to brain amyloid load (p = 0.0025). SLC9A3R1 expression was associated with burden of TDP43 pathology (ß = 0.58 ± 0.17, p = 5.9E-4). Using eQTL data, the CDH23 variant was in linkage disequilibrium with variants modulating CDH23 expression levels (top single nucleotide polymorphism: rs11000035, p = 4.85E-6, D' = 1.0). Using minigene splicing assays, the CDH23 and SLC9A3R1 variants affected splicing efficiency. CONCLUSIONS: These findings suggest that CDH23, SLC9A3R1, RHBDD2, and possibly ITIH2, which are involved in synaptic function, the glutamatergic system, and innate immunity, contribute to AD etiology. In addition, this study supports the notion that synonymous variants contribute to AD risk and that comprehensive scrutinization of this type of genetic variation is warranted and critical.
ABSTRACT
A key characteristic of Alzheimer's disease and other tauopathies is the progressive accumulation of neurofibrillary tangles mainly composed of hyperphosphorylated tau protein. In the present study, we use transgenic Drosophila melanogaster as a model to analyze in vivo the effect of expressing pseudophosphorylated tau (S199E/T212E/T231E/S262E tau) on pathological human tau (PH-tau) and on the FTDP-17 mutant R406W (PH-tauR406W). We used two different inducers that produced different levels of tau expression. The expression of these forms of tau did not significantly affect the lifespan of the flies. Flies expressing PH-tau displayed a clear locomotor dysfunction compared to those expressing normal tau regardless of the level of expression. At lower level of expression, this pathological phenotype was found to be age-dependent. At 35 days old, PH-tau flies showed the strongest locomotor impairment compare to flies expressing human tau or control flies (46%, 18% and 18% of flies remained on the bottom of the vials, respectively). At higher levels of expression, PH-tau flies showed these defects at seven days of age and the dysfunction also became significant for flies expressing tauR406W and PH-tauR406W. Whole brain immunochemistry analysis revealed that PH-tau as well as PH-tauR406W appeared to have abnormal mushroom body structures, critical structures involved in olfactory learning and memory in Drosophila. Severe olfactory learning deficits were induced by the expression of PH-tau. Taken together, our findings demonstrate that PH-tau induced a toxic effect in Drosophila, as flies develop both an abnormal motor deficit, associated with disruption of the mushroom body neurons, and impaired olfactory learning.
