The current results support and extend previous work disentangling the biological pathways contributing to Alzheimer’s disease risk and pathogenesis. Consistent with previous findings, we found that a global AD PRS was significantly associated with diagnostic status, amyloid and tau positivity, and hippocampal volume (9–13, 15, 17). The global AD PRS captures the combined effects of multiple separable influences on disease risk and therefore is not useful for teasing apart genetically-mediated etiological differences among individuals. Several studies have examined the relationship of AD diagnosis or AD-related biomarkers with pathway-specific PRS calculated from GWAS-significant SNPs (36, 37, 39). Here, we generated pathway-specific PRSs from clusters of gene sets and SNPs with association strength p-values falling below the threshold of GWAS significance. Breaking down the global effects of polygenic factors into more refined genetic pathway-associated subset of polygenes has been shown to provide useful information above-and-beyond variants with more pronounced effects arising from GWAS for certain conditions, including AD and MCI (8, 9, 14).
As expected, most of the pathway PRSs associated with AD diagnostic status in the current study correspond to pathways that have consistently been uncovered in previous GWAS. These pathways include amyloid precursor processing, immune and microglial response, endocytosis, cholesterol transport, lipid-protein complex and amyloid clearance (4, 6, 7, 21). Several pathways of focus in our study, however, had less support from other GWAS, but have nonetheless been linked to AD in yet other studies. For example, the regulation cell junction and regulation protein tyrosine pathway-specific PRS we considered may capture effects of genes involved in synaptic functioning and cell signaling, consistent with studies suggesting this pathway is involved in AD-related cognitive decline (62, 63). In addition, mitochondrial function has been proposed as playing a key role in the development of AD (64, 65), and mitophagy in particular may have widespread impacts on age-related disease, including Alzheimer’s disease (66). Our results are also consistent with studies that have examined the association of pathway-specific PRS with AD diagnosis, including Aβ clearance, cholesterol transport, immune response, and endocytosis (19, 36, 37, 39)
Amyloid positivity was significantly associated with pathway-specific PRS for endocytosis and fibril regulation, response to misfolded proteins, and regulation of protein tyrosine. The pathways associated with these PRSs are involved in the production, trafficking and clearance of Aβ peptides, as well as their aggregation into fibrils, which has biological plausibility. The endocytic pathway plays a key role in the amyloidogenic processing of APP as it is internalized to the intracellular space followed by cleavage into Aβ in the early endosome (67, 68). Tyrosine kinases may be involved in both the trafficking of APP and upregulating BACE activity (69, 70). In addition, genes encompassed by the response misfolded protein PRS that we find associated with AD pathology, include molecular chaperones (e.g., CLU) and the ubiquitin-proteasome system, which mediate degradation of abnormal and misfolded proteins (71–73). A previous study examining pathway-PRS found that PRS related to Aβ clearance and cholesterol metabolism were also related to CSF and PET measures of amyloid; however, these scores only included GWAS-significant variants and the effects were primarily driven by APOE (37).
We also found that tau positivity was significantly associated with pathway PRSs for protein localization, immune signaling, and mitophagy. The protein localization pathway includes tau protein kinase activity, which may relate to abnormal hyperphosphorylation of tau (74). Sun et al. (75) also found that a PRS reflecting the tau kinase activity was associated with CSF and PET measures of tau. Tau binds to microtubules to provide stabilization, but detaches when phosphorylated which can reduce axonal integrity and disrupt protein transport along the cytoskeleton (76). Additionally, this unbound phosphorylated tau can aggregate into neurofibrillary tangles (77). Regarding the PRS reflecting immune signaling, although inflammation can be secondary to abnormal tau, there is also evidence for an upstream role of glial activation and neuroinflammation in driving the accumulation and spread of tau (78, 79). This is consistent with a previous finding that a PRS constructed from only GWAS significant variants related to immune response was associated with CSF tau (37). As with inflammation, disruptions to mitophagy may be secondary to disease processes, but there is evidence for upstream roles of mitochondrial function, including mitophagy, in the development of AD pathology (65, 80). Hippocampal volume was also associated with pathway PRSs for protein localization and mitophagy. Shared pathways with tau positivity and may reflect the tighter linkage between neurodegeneration and tau compared to amyloid (81, 82).
Our results are generally consistent with those of a recent GWAS on CSF measures of amyloid and tau (83). The authors found little overlap in the loci associated with amyloid and tau aside from APOE. In contrast, there was overlap between loci associated with tau and ventricular volume, which can be used as an indicator of neurodegeneration. The authors also examined associations of AD-associated variants with CSF amyloid and tau, and a cluster analysis pursued by them identified patterns that were broadly similar to our own. For example, variants associated with CSF amyloid were associated with amyloid processing, endocytosis, and tyrosine kinase, whereas variants associated with CSF tau were linked to the immune system. Deriving PRSs based on GWAS of AD biomarkers represents a promising approach to index risk in specific pathways, but sample sizes for such studies are relatively small. PRSs based on pathway analysis of diagnosis-based GWAS are therefore a useful alternative that can leverage large case-control datasets to provide converging information.
Although we focused on APOE-independent sources of AD risk, it is important to note that the number of APOE-ε4 alleles had a comparable or even stronger (in the case of amyloid) effect on the outcomes as the polygenic component indexed with a PRS. APOE belonged to gene sets that were part of several cluster, including endocytosis and fibril regulation, protein localization, cholesterol transport, and amyloid protein processing. The variants falling within the APOE region were excluded from our PRSs, but this does suggest APOE can exert an impact through multiple routes. Stratifying based on APOE-ε4 carrier status suggested stronger effects of pathway PRSs on tau positivity in non-carriers. It may be that smaller polygenic effects are obscured in the presence of a larger APOE-related signal. Alternatively, APOE-ε4 may be sufficient to increase risk for tau pathology whereas those lacking this risk allele require additional sources of risk to develop abnormal levels of tau.
There are several additional items worth mentioning to put our analyses into context. First, we find evidence that the effect size of the global PRS is much larger than any pathway-specific PRS effect sizes in our analyses. While pathway-specific PRSs may be beneficial for understanding disease etiology, they do not appear to add predictive power when considered in the aggregate over-and-above the global PRS. Second, we mapped SNPs to genes using the standard position-based approach available in MAGMA. However, many GWAS SNPs are located in non-coding regions and may be associated with disease risk through their gene regulatory effects (84). Thus, approaches that make use of information such as chromatin interactions (85) or expression quantitative trait loci (86) to determine which gene a variant present in a non-coding region affects may prove useful in developing pathway-specific PRS. Third, genes/SNPs may be part of multiple pathway clusters, so the pathway PRSs are not entirely independent with each other. Fourth, we added interactions with diagnosis to all biomarker models to determine whether PRS associations differed by group. However, no interaction terms were significant after correcting for multiple comparisons and there did not appear to be a consistent pattern among the few interactions that reached nominal significant. Finally, this analysis required choices of parameters at various steps, such as the p-value thresholds used to filter variants and gene sets, method used to construct PRS, and even which gene sets were used. We believe our choices represent a reasonable attempt to capture the broad sources of polygenic influence on AD risk while minimizing unrelated signal. However, alternative approaches may be equally valid and should be determined by the context of a given analysis.