To identify potential functional risk variants and genes at each
associated locus, we first annotated a list of prioritized variants from the 24
associated loci (excluding
APOE) (
n = 1,873).
This variant list combined variants in LD with the sentinel variants
(
r2 ≥ 0.5) using INFERNO
160 (link) LD expansion
(
n = 1,339) and variants with suggestive significance
(
P < 10
−5) and LD
(
r2 ≥ 0.5) with the sentinel variants for
the 24 associated loci (excluding
APOE) (
n =
1,421 variants). We then identified variants with regulatory potential in this
set of variants using four programs that incorporate various annotations to
identify likely regulatory variants: RegulomeDB
56 (link), HaploReg v.4.1 (refs.
57 (link),161 (link)), GWAS4D
59 (link), and the Ensembl Regulatory Build
58 (link). We used the ChromHMM (core 15-state
model) as ‘source epigenomes’ for the HaploReg analyses. We used
immune (Monocytes-CD14
+, GM12878 lymphoblastoid, HSMM myoblast) and
brain (NH-A astroctyes) for the Ensembl Regulatory Build analyses. We then used
the list of 1,873 prioritized variants to search for genes functionally linked
via eQTLs in LOAD relevant tissues including various brain and blood tissue
types, including all immune-related cell types, most specifically myeloid cells
(macrophages and monocytes) and B-lymphoid cells, which are cell types
implicated in LOAD and neurodegeneration by a number of recent studies
14 (link),45 (link),162 (link),163 (link). While their specificity may be lower
for identifying Alzheimer’s disease risk eQTLs, we included whole blood
cell studies in our Alzheimer’s disease–relevant tissue class due
to their high correlation of eQTLs with Alzheimer’s
disease–relevant tissues (70% with brain
164 (link); 51–70% for monocytes and
lymphoblastoid cell lines
165 (link))
and their large sample sizes that allow for increased discovery power. See the
Supplementary Notefor details on the eQTL databases and studies searched, and
Supplementary Table 13 for sample
sizes of each database/study.
Formal co-localization testing of our summary Stage 1 results was
conducted using (1) COLOC
166 (link)via INFERNO and (2) Summary Mendelian Randomization (SMR)-Heidi
analysis
167 (link). The
approximate Bayes factor (ABF), which was used to assess significance in the
INFERNO COLOC analysis, is a summary measure that provides an alternative to the
P value for the identification of associations as
significant. SMR-Heidi analysis, which employs a heterogeneity test (HEIDI test)
to distinguish pleiotropy or causality (a single genetic variant affecting both
gene expression and the trait) from linkage (two distinct genetic variants in
LD, one affecting gene expression and one affecting trait), was also employed
for co-localization analysis. Genes located less than 1 Mb from the GWAS
sentinel variants that pass a 5% Benjamini–Hochberg FDR-corrected SMR
P-value significance threshold and a HEIDI
P-value > 0.05 threshold were considered
significant. The Westra eQTL
168 (link) summary data and Consortium for the Architecture of Gene
Expression (CAGE) eQTL summary data were used for analysis. These datasets,
conducted in whole blood, are large eQTL studies (Westra: discovery phase
n = 5,311, replication phase
n = 2,775;
CAGE:
n = 2,765), and while there is some overlap in samples
between the two datasets, CAGE provides finer coverage. The ADGC reference panel
dataset referenced above for GCTA COJO analysis was used for LD
calculations.
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