Aims

In a long-lasting effort to collect clinicopathological data and biomaterials from AD patients for genetic studies, we identified a multigenerational Flanders-Belgian family with autosomal dominant EOAD. We collected DNA, biomaterials and clinicopathological data in 2 consecutive generations: 5 AD patients (onset 64.2±13.2y), 2 with definite neuropathological diagnosis and 9 unaffected or at-risk individuals.

Methods

WES data of 4 patients. Selection of shared rare non-synonymous variants. Validation, co-segregation and frequency assessment using massive parallel-targeted sequencings (MPS). Candidate genes re-sequencing using MPS in a Belgian cohort of 320 EOAD (onset 57.8±5.9y) and 1166 late-onset (LO)AD (onset 77.1±6.5y) patients and a control cohort of 934 persons (inclusion 70.45±9.2y). Variants pathogenicity evaluated by NF-κB luciferase assay. Aβ₄₂ phagocytosis assay performed in iPSC-derived microglia in presence of conditioned media.

Results

TLR9 p.E317D (novel) co-segregated with EOAD in the family and was absent from the controls and public databases of variants. Resequencing identified additional rare variant carriers: 7 in EOAD (7/320, 2.19%) and 19 in LOAD (19/1166, 1.63%) patients and 8 in controls (8/934, 0.86%), while p.E317D was absent. TLR9 is a DNA-sensing receptor expressed in immune cells, activated by unmethylated CpG oligonucleodides leading to innate immune cells activation and mounting acquired responses. The NF-κB luciferase assay showed that p.E317D caused 50% reduction in receptor activation. We also demonstrated that wild-type TLR9 activation in cultured lymphocytes induced Aβ₄₂ phagocytosis when added to human iPSC-derived microglia.

Conclusions

We suggest that TLR9 mutations reduce receptor activation in the periphery with a consequent effect in brain.

Aims

ABCA7 was initially associated with Alzheimer’s disease (AD) in genome-wide association studies. Genetic screening of ABCA7 suggested a role for rare premature termination codon (PTC) mutations. We observed loss of ABCA7 protein in PTC mutation carriers, although with a high variability due to differences in transcript rescue. Additionally, rare missense mutations are present in ABCA7 with an unknown effect on the protein. We aimed to investigate the contribution of rare variants (MAF≤1%), single missense mutations or compound heterozygous mutations in ABCA7 in Belgian AD and control cohorts.

Methods

Targeted resequencing of the ABCA7 coding region or whole exome sequencing in 1365 Belgian AD patients and 976 controls. Allele-specific PCR, haplotype sharing analysis and long-read sequencing to determine cis/trans configuration of compound heterozygous mutations.

Results

We identified missense mutations in 7.3% (99/1365) of the patients and in 4.8% (47/976) of the controls. We observed co-segregation of p.G1820S with AD in a family with autosomal dominant inheritance (Fig.1). We observed compound heterozygous mutations in 10 patients (0.73%, Fig.2) and five controls (0.51%). We confirmed trans configuration in four patients and cis configuration in four other patients. In the remaining two patients, the configuration remains unknown. In the five controls, we confirmed trans configuration in one and cis configuration in the other four.

Conclusions

We observed an enrichment of rare single ABCA7 missense mutations and compound heterozygous carriers in patients. Functional studies of missense mutations are necessary to determine their potential pathogenic effect. In addition, trans compound heterozygous mutations in ABCA7 might modify penetrance and pathogenicity.

Aims

Pathogenic LOF and missense mutations in the TBK1 gene are associated with frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Here, we report on the prevalence and phenotype of TBK1 mutation carriers in the Flanders-Belgian population.

Methods

We screened the Flanders-Belgian FTD (n = 678), ALS (n = 220) and FTD-ALS (n = 46) cohorts for mutations in the TBK1 gene. We identified 19 carriers of pathogenic mutations. We sampled and screened family members. In total, we have a TBK1 carrier cohort of 47 individuals. We collected data on clinical characteristics, biomarkers of disease and neuropathology.

Results

Overall, the TBK1 mutation frequency was 2.0%, with 1.3% in FTD, 3.6% in ALS and 4.3% in FTD-ALS patients. Among the 47 carriers, 30 are affected: FTD (n = 10, 33.3%), ALS (n = 10, 33.3%), unspecified dementia (n = 6, 20.0%), FTD-ALS (n = 2, 6.7%), mild cognitive impairment (n = 1, 3.3%) and Alzheimer’s disease (n = 1, 3.3%). In the FTD group, the behavioral variant FTD (bvFTD) was the most common phenotype but non-fluent variant primary progressive aphasia (nvf-PPA) was also reported. Mean onset age and disease duration were 63.2 and 6.1 years (ranges 41-86 and 0-24 years). Neuropathology confirmed FTLD-TDP type B.

Conclusions

Pathogenic mutations in TBK1 are a frequent cause of FTD, ALS and particularly of FTD plus ALS in the Flanders-Belgian population. The most common phenotypes were FTD (mostly bvFTD, less common nfv-PPA), ALS and unspecified dementia. Brain autopsy revealed FTLD-TDP type B neuropathology.

Aims

In a cohort of Flanders Belgian Alzheimer’s disease (AD) patients, we identified 11 unrelated index patients carrying the presenilin 1 (PSEN1) missense mutation, p.Cys263Phe, plus three affected relatives of family DR1633 (n=14). We aimed to delineate a clinicopathological phenotype, and next compared this to genotype–phenotype data of AD patients carrying other causal gene mutations i.e. PSEN1 (n=25), PSEN2 (n=1), and APP (n=5).

Methods

Reviewing medical records of mutation carriers to obtain clinicopathological data for defining genotype-phenotype data.

Results

Mean onset age of the PSEN1 p.Cys263Phe carriers was 62.3±4.5 years (range 53-69), with a disease duration of 9.0±4.0 years (range 4-13). We observed a positive familial history in 90% of carriers and in family DR1633 co-segregation of AD was found with an autosomal dominant inheritance pattern. Amnestic presentation was present in all carriers, however, three patients also showed important frontal symptoms. Neuroimaging (n=10) displayed diffuse (sub)cortical atrophy, with evident hippocampal atrophy in three carriers. We observed severe signs of small vessel disease in four patients. Cerebrospinal fluid AD biomarkers were characteristic of AD in all. Neuropathology in two patients demonstrated severe levels of AD hallmarks plus cerebral amyloid angiopathy (CAA).

Carriers of PSEN1 p.Cys263Phe mutation had a later age at onset (62.3 years) than other PSEN1 carriers (50.8 years) or other causal gene mutation carriers (51.1 years).

Conclusions

PSEN1 p.Cys263Phe carriers presented with early-onset AD. Severe levels of AD neuropathology were seen with high levels of CAA. Disease onset of PSEN1 p.Cys263Phe carriers was later than other causal gene mutation carriers.

Aims

Alzheimer’s disease (AD) biomarkers show promise in aiding diagnosis and prediction by representing several neurodegenerative processes, such as synaptic dysfunction, neuronal inflammation and injury. Biomarkers could also aid in discovery of AD related genes and inform which biological mechanisms underlie a genetic risk effect. We therefore performed an exome-wide rare variant analysis of six AD biomarkers (β-amyloid, total/phosphorylated tau, NFL, YKL-40, and Neurogranin) to discover genes associated with these indicators and test whether they mediate genetic effects on AD symptoms.

Methods

We performed the exome-wide analysis in two studies: the European Medical Information Framework AD study and the AD Neuroimaging Initiative. Whole exome sequencing and biomarker information data was available for 505 participants. We applied a principal component (PC) analysis to derive combinations of biomarkers, which represent statistically independent biological processes. We then tested whether rare variants in 13,799 protein-coding genes associate with the PCs using a Meta-SKAT test. We also tested whether the PCs are intermediary to gene effects on AD symptoms with a SMUT test.

Results

One PC loaded on NFL and YKL40, both indicators of neuronal injury. Three genes were associated with this PC: IFFO1, PLEKHM1, and DTNB. Mediation tests suggested, that these genes also affect AD symptoms via NFL and YKL40. We also observed an association between a PC loading on Neurogranin and GABBR2 and CASZ1, but no mediation effects.

Conclusions

The results suggest that mutations in IFFO1, PLEKHM1, and DTNB heighten susceptibility to neuronal injury, potentially by altering cytoskeleton structure, resulting in an elevated AD risk.