In this study, we performed a comprehensive assessment of the shared genetic architecture between AD and immune-mediated diseases by analyzing large-scale GWAS summary data using different but complementary genetic approaches in order to shed light into their shared underlying molecular biology mechanisms. The findings show extensive genetic overlap between AD and immune-mediated diseases regardless of genetic correlation. We identified several shared and novel loci using the conjFDR approach and identified several pathways and immunological signatures enriched in the brain and immune system.
In our analyses, LDSR did not show significant genome-wide genetic correlation between AD and each immune-mediated trait. Still, the evidence of significant genetic overlap verified by MiXeR in tandem with nonsignificant genetic correlation reflects shared genetic etiologies with mixed effect directions, a suggestion which is corroborated by the local genetic correlation performed using LAVA; this balanced mixture of concordant and discordant shared loci distributed across the genome indicates that some genetic variants can increase the risk of one disorder while decreasing the risk of the other. In addition, all MR analyses, with the exception of perhaps asthma, gave us no support for a significant causal relationship between AD and immune-mediated diseases, indicating that pleiotropic and common biological pathways may be a better explanation for their association. A similar study showed no significant causal relationship between AD and immune-mediated diseases with the exception of multiple sclerosis and Sjogren’s syndrome47. In our study, a total of 70 unique shared genomic loci were identified between AD and immune-mediated diseases by the conjFDR analyses employing GWAS data, being enriched in biological pathways related to cell adhesion and the immune system. They were mostly enriched in the lymphatic system, with a signal being seem in the central nervous system; the top enriched cell types were also related to the immune and nervous systems, including neurons and microglia.
Two SNPs were suggestive of deleteriousness, rs12790721 and rs646327. The first is an intron variant and eQTL of GRAMD1B (11q24.1) in blood, and the second is an exonic variant of FUT2 (19q13.33 ). Both genes are protein-coding genes, for the proteins aster-B and galactoside alpha-(1, 2)-fucosyltransferase 2, respectively. GRAMD1B is expressed in the brain, in astrocytes, oligodendrocytes precursor cells, and in inhibitory and excitatory neurons, mainly maintaining synaptic function, and is also expressed in the immune system. It is predicted to be involved in cellular response to cholesterol and cholesterol homeostasis. Aster-B, as a novel regulator of mitochondrial cholesterol and fatty acid transport, and excess mitochondrial cholesterol, has also been a biomarker in AD. Because abnormal mitochondrial cholesterol is a common phenotype in AD, Aster-B has been suggested as a target for the development of therapeutics for AD48. Regarding FUT2, fucosylated host glycoproteins or glycolipids mediate interaction with intestinal microbiota, influencing its composition49–51. There is evidence suggesting a link between gut microbiota dysbiosis and the pathogenesis of AD, with the gut microbiota modulating neuroinflammation indirectly by impacting microglia and having effects on synaptic neurotransmission dysfunction, with a cross-talk between peripheral and central inflammation through microbiota-mediated microglial alterations52, 53.
In addition to deleteriousness, seven lead SNPs were considered to have important regulatory functionality, mapped to the genes ADAMTS4 (UTR3 region), HBEGF (intergenic), WNT3 (intronic), TSPAN14 (intronic), DHODH (intronic), ABCB9 (intronic) and TNIP1 (intronic), all of each are protein-coding genes. For instance, the protein coded by ADAMTS4 is expressed in the cytoplasm in several tissues, being most abundant in the central nervous system. Its RNA is mostly expressed in oligodendrocytes, and is related to myelination. The enzyme coded by ADAMTS4 is responsible for amyloid deposition in AD54, 55. HBEGF has been associated with AD, with its overexpression resulting in increased APP protein level56. It is expressed in the cortex and hippocampus, mostly in neurons and astrocytes, and also in the immune system. Genetic deletion of HBEGF cause cognitive dysfunction, which is reversed by NMDA receptor agonists57. Heparin-binding epidermal growth factor-like growth factor (HB-EGF), coded by HBEGF, also restores neurogenesis in the hippocampus of aged mice58. It also contributes to the proliferation of glial cells and to the survival of dopaminergic neurons59. DHODH, it is detected in the immune system and is related to mitochondrial function and cellular homeostasis and also to alterations in reactive oxygen species levels60. TNIP1 is related to inflammatory response, including neuroinflammation and microglia activation, in addition to regulating nuclear factor kappa-B activation61, being related to AD62. Finally, in the loci to eQTL analyses, several SNPs were found associated with eQTL functionality in brain tissue and in the immune system. For instance, NDUFS2 gene is a protein-coding gene that is associated with mitochondrial complex I alterations. A transcriptome-wide association study of AD using brain tissue found that NDUFS2 is one of the putative causal genes in AD63, 64.
Six genes were found to be shared between AD and at least two immune-related diseases: ADAMTS4, JAZF1, Y_RNA, C1orf106, AC195454.1, and RP11-95M15.1. As discussed above, ADAMTS4 is responsible for amyloid deposition in AD54, 55. JAZF1 is a protein-coding gene which functions as a transcriptional repressor; it is involved in glucose and lipid metabolisms65. Y_RNA is a small non-coding RNA gene. It has been implicated in cellular processes such as DNA replication and RNA quality control. Y_RNA has been found in extracellular vesicles (EV) from multiple different cell lines, and EV-associated Y-RNA may be involved in a range of immune-related processes, including inflammation and immune suppression, being regulated in immune cells by Toll-like receptor (TLR) signaling66. Dysregulation of Y-RNA also has been found to cause alternative splicing in neurons of individuals with AD67. C1orf106, also known as CALHM6, encodes the protein calcium homeostasis modulator, which may play an important role in several immune inflammatory responses. It is upregulated by interferon-gamma and tumor necrosis factor alpha68. AC195454.1 is a long non-coding gene with unknown functional category. It is overexpressed in the brain and has been implicated in SLE69, 70. Finally, RP11-95M15.1, also a long non-coding gene, has been associated with PSC71.
One of the major strengths of our study is the employment of diverse albeit complementary statistical genetic approaches, enabling an extensive analysis of the genetic associations between AD and immune-mediated diseases. In addition, the fact that we included generally well-powered GWAS suggests that our results are mostly not due to small sample sizes. Another strength is that we included only diagnosed cases of AD and not by proxy subjects. Having said that, our study has inherent limitations that should be considered alongside the present findings. The analyses were restricted to participants of European ancestry; thus, our findings may not be generalizable to populations of other ancestries. In addition, since we used the most comprehensive GWAS data available, replication analyses in independent samples were not possible. Also, AD is not a highly polygenic disease, which might have underestimate the number of variants found in the MiXeR analyses. Further, the AIC values from the MiXeR analysis for the negative control HCM were all negative, showing poor fit and that the results of this particular analysis are unreliable. Finally, the gene mapping strategy was based on statistical analyses, and should be validated with experimental studies.
In summary, our study provides genetic insights into the observed epidemiological relationship between AD and immune-mediated diseases, exposing shared genetic susceptibility extensively distributed across the genome. Our results support a significant genetic association between AD and immune-mediated diseases with mixed effect directions, with some genes increasing the risk of one disease but decreasing the risk of the other. Furthermore, we pinpoint shared loci and genes between AD and immune-mediated diseases that have the potential to be intended for further research, such as GRAMD1B, FUT2, ADAMTS4, HBEGF, WNT3, TSPAN14, DHODH, ABCB9 and TNIP1, all of which are protein-coding, and thus show promise as potential drug targets, and showed deleteriousness or regulatory functionality. We also highlight the significance of the immune system as a shared mechanism, albeit a non-causal one, in AD and immune-mediated diseases, providing further support for the immunoinflammatory hypothesis in AD. Overall, our results provide an atlas of the shared genetic architecture of AD and immune-mediated diseases, and their long-noticed comorbidity association, with implications for personalized interventions for the prevention and treatment of AD and immune-mediated diseases.