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Anaya JM, Shoenfeld Y, Rojas-Villarraga A, et al., editors. Autoimmunity: From Bench to Bedside [Internet]. Bogota (Colombia): El Rosario University Press; 2013 Jul 18.
Introduction
Autoimmune diseases (ADs) are responsible for a substantial amount of disability and morbidity worldwide. Although the epidemiology varies according to individual conditions, collectively autoimmune prevalence is at least 5% in the general population and is one of the major causes of premature mortality in young and middle-aged women (1).
ADs encompass a broad range of phenotypic manifestations and severity. The pathogenesis is considered to be multifactorial, and several of the features suggest shared etiologic factors (see Chapter 14). Most ADs are characterized by female predominance, and many are associated with the production of autoantibodies. A variety of pathogenic mechanisms are ultimately triggered during the progression of ADs, and dysregulations involving major cell signaling pathways and inflammatory responses are consistent features in most ADs (2,3). ADs can be categorized into two types of disorders. First, systemic ADs, such as systemic lupus erythematosus (SLE), in which the loss of immune tolerance is directed towards systemic antigens and disease manifestations can occur at a variety of different sites in the body. Second, organ-specific ADs, in which the immune response is predominantly or exclusively directed towards tissue-specific elements [e.g., type 1 diabetes (T1D) targets the pancreas, autoimmune thyroid disease (AITD) attacks the thyroid gland].
The inheritance pattern of ADs is polygenic; this indicates that multiple genes are involved in defining their development and outcome. Although the exact number of risk loci is not known for many ADs, the numbers are likely in the 100s for each. The human leukocyte antigen (HLA) region typically confers the strongest association with ADs discovered to date. However, other genes, both located within and outside of the HLA region, are risk factors for these diseases.
Recent advances in genomics have led to increased understanding of the molecular underpinnings of complex diseases. For instance, the technological and manufacturing advances made in the early 2000s led to the use of microarrays to conduct large-scale, mostly unbiased surveys of the human genome such as genome-wide association studies (GWAS) (BOX 1) to determine which genetic factors influence the onset and development disease. However, due to their multifactorial and polygenic nature and accompanied by a differential penetrance, genetic heterogeneity among populations, and the influence of environmental factors (4,5), untangling the genetic determinants defining their outcome and onset has proven to be extremely challenging. Data showing the existence of different ADs within a single family or within the same individual suggest a combination of genetic defects that may predispose individuals to different ADs sharing common pathogenic pathways (6,7). This chapter reviews the reported non-HLA shared genes, based on the plausibility of their functional or biological mechanisms, that affect the susceptibility to the most common ADs.
Shared genetic variants among ADs
GWAS has opened up a new horizon on the genetics of complex diseases by defining the genomic regions harboring disease risk alleles that display association at convincing levels of statistical significance (BOX 1) (8,9). In this regard, cohort studies have identified associations with common genetic markers across the entire human genome that satisfy stringent statistical criteria and have been replicated across multiple cohorts (Figure 1).
Perhaps nowhere have such studies been more fruitful than in ADs: more than 150 genetic loci have been shown to be associated with one or more autoimmune disorders (Figure 2). Although in most cases, the precise causal alleles or genes driving these associations have not been identified, some associated loci can be implicated with particular functional pathways including the intracellular signaling networks that drive the activation of T and B cells, signaling by cytokines and cytokine receptors, and pathways that mediate innate immunity and microbial responses (Table 1).
The results of several GWAS show that genetic variations in multiple genes are associated with each AD and that the associations are modest. The majority of GWAS have focused on case series of European ancestry, but studies of other populations show that some associations are observed across populations, and such associations point to pathways that may be particularly important in disease pathogenesis. Multiple ADs appear to have overlapping genetic associations and risk alleles, which suggested that common pathogenic mechanisms underpinning the diseases must be present (5,10-13). In addition, there is evidence that loci predisposing to one disease can have effects on the risk of a second disease (14) although the risk allele for one disease may not be the same for the second (15).
Each new genetic finding can suggest multiple hypotheses that need to be incorporated into an overall scheme of pathogenesis. Ongoing research points to some expected shared biology (16). However, the relative risk of each locus may differ between diseases. Before we can fully understand the relationships between ADs, we must first identify all risk loci for each disease, an effort which is still underway. Compelling and interesting observations have emerged implicating several genes with shared etiology that comprise the well-described examples for the major histocompatibility complex (MHC) (17) and non-MHC genes such as, PTPN22, which is associated with T1D, RA, SLE and AITD but not multiple sclerosis (MS) (18). Other examples include CTLA4, STAT4, TNFAIP3, and SH2B3, etc.
Many of the recently identified AD loci are involved in pathways related to B cell or T cell activation and differentiation, innate immunity, and regulation of cytokine signaling (7,19). This expected commonality has motivated several meta-analyses across pairs of diseases to establish their shared genetic basis. Approaches in such diseases as celiac disease (CD) and rheumatoid arthritis (RA) (20), T1D (14), and inflammatory bowel disease (IBD) (21) have revealed overlapping loci. However, these observations could potentially underestimate the actual extent of commonality as the lack of statistical power would inflate type 1 error for the associations found to genome-wide significance [i.e., 5x10-8]. In addition, modeling and simulation analyses estimate that, in several diseases where GWAS have been successful, further loci with low effect sizes remain to be discovered (22). This problem is exacerbated when considering independent discoveries across diseases since power would be multiplicative across studies. Therefore, estimates of the true extent of genetic sharing are probably underestimated by either simple overlaps or pairwise meta-analysis.
The main objective in genetic mapping studies is to identify the genetic variants and/or haplotypes affecting genetic susceptibility. Any associated marker in a locus may simply be genetically tagging the real causal variant, thus, statistical significance is not enough evidence to infer causality of variants. Another goal is to identify specific causal genes in regions when the associated region for a given trait encompasses multiple ADs. This requires additional refinement before inferences on genetic mechanisms and etiological causes of disease can be made (23). As an approach to be able to underpin these goals, part of the AD community developed shared resources, e.g., the ImmunoChip, a common platform for fine mapping AD-associated loci (24,25) as well as computational approaches to select the likeliest candidate genes from regions of association (26-28). Moreover, across the studied ADs, the application of a range of bioinformatics algorithms has generated plausible hypotheses about the causal genes and tissues underlying disease pathogenesis, motivated and functional experiments designed to test these hypotheses (29-31). In order to confront this critical aim of identifying pathways either shared across diseases or unique to specific ones, it is essential to develop network models for commonalities and to consider how genetic association data might be used to distinguish such models.
Currently, genomic technologies have been particularly attractive for acquiring, defining, and building network maps as they are amenable to automation. A limiting factor in annotating these networks is the careful acquisition of samples (i.e., rigorous standards in statistical experimental design, cell isolation, flow sorting, and sample preparation) required to gain cell-specific annotation. Parallel high-throughput gene expression studies are currently under way. Other types of information regarding the state of the DNA [e.g., DNase I hypersensitivity (32), nucleotide methylation status by sequencing (33), chromatin immunoprecipitation and sequencing (34), etc] are equally valuable and helpful are for capturing different types of information that could be used for deciphering the pathogenicity of the associated disease pathways.
Along with this extensive amount of information, the issue of interpretation comes into play. For example, several correlated alleles appear to confer risk of some diseases but are protective in others (15,35) and it remains unclear how this evidence should be incorporated into a pathway view of disease. Under a shared pathway architecture causal variants in each disease perturb a limited number of cellular processes. Thus pathway enrichment analysis (36,37) and functional genomics datasets can be queried to reveal pathway components that are preferentially encoded in associated loci. These approaches have shown that this cumulative burden hypothesis is true (28), which substantiates the argument that susceptibility alleles accumulate and perturb pathways to influence risk (38). These approaches can be expanded to incorporate genetic data from multiple diseases in much the same fashion (39).
Available genetic information points out that nearly half of the loci of an individual disease identified through GWAS have also been identified as influencing the risk of more than one additional disease. The argues for a genetic basis to co-morbidity. Moreover, there are examples of several variants with differing risk profiles depending on the disease. Support for the idea of common patterns of association and shared biological processes is obtained by loci clustered over a pattern of diseases, as they affect and harbor genes encoding for interacting proteins at a much higher rate than by chance. These results suggest that multi-phenotype mapping will identify the molecular mechanisms underlying co-morbid, immune-mediated inflammatory and autoimmune diseases.
Shared genetic factors
Extensive clinical and epidemiologic observations have shown that multiple ADs can occur in the same individual or in closely related family members. This clustering of multiple diseases appears more frequently than would be expected if the phenotypes were independent. Familial approaches have documented the clustering of certain ADs among the relatives of individuals who have RA, MS, SLE, T1D and other diseases (42-48) (see chapter 17).
The role of common variants versus rare variants in mediating susceptibility to common diseases has reemerged as a controversial topic (49,50). The controversy arises predominantly from the fact that the sum of the risk attributable to all of the loci identified can only account for a small fraction of the genetic heritability exhibited by many common diseases. This is often referred to as “missing heritability.” This has led to the possibility that rare variants with strong functional effects actually contribute significantly to the overall heritability of common diseases, which still needs to be validated and completely supported. In this regard, until recently rare variants were not regularly included in GWAS arrays, and several studies using re-sequencing of only the exons of each protein coding gene, called the exome, have identified a few rare mutations or copy number variations that contribute to the pathogenesis of ADs (51-57).
Recent reviews have summarized emerging work that identifies both genetic loci that are shared across the spectrum of ADs and the biological pathways whose involvement is implicated by these shared loci (3,19,51,58). The extent to which immune-related signaling and/or other pathways are implicated for each of these disorders varies and suggests that most of these pathways contribute or play a role to a variable degree in most of these disorders. The latter underscores the fact that most of these loci can be mapped to a few shared biological pathways.
Table 1 illustrates some of the susceptibility loci/alleles identified by genetic studies in ADs thus far, either by candidate gene and/or GWAS approaches. These associated loci have been organized on the functional role that their top positional candidate plays in the immune system. Although the adaptive immune system has long been a focus of attention, innate immune mechanisms are now viewed as main players in the pathogenesis of autoimmune-related disorders. Even further, the concept of quantitative thresholds for immune-cell signaling has emerged in the past decade as a potential way of understanding how multiple genetic factors of relatively small effect sizes may combine to create a state of susceptibility to autoimmune activation. The new genetic findings also emphasize that the identification of the environmental components that interact with host genetic factors will be critical in developing a deeper understanding of autoimmunity as well as new approaches to prevention, diagnosis, and treatment.
The immune system is in a constant struggle to maintain a balance between immunity (i.e., elimination of foreign pathogens) and tolerance (i.e., lack of response to self-tissue). When these regulatory immune response mechanisms fail, inflammatory destruction might occur. The following section describes evidence for genes associated with and between ADs (Table 1). Putative immune functions for each gene are given, and each is classified below into a few broad categories for clarity.
Genes modulating adaptive immunity
Variations in the properties of the antigen receptor pathways of T and B cells have long been postulated to be key elements in genetic predisposition to ADs. The activation of autoreactive T and B lymphocyte clones is the essence of autoimmunity. As shown in Table 1, several AD susceptibility genes that impact the adaptive immune system have been identified thus far. These susceptibility alleles are anticipated to modulate the development, activation, and regulation of a variety of immune cell lineages.
Genes impacting T cell activation and signaling
The differentiation of T cells into functional subsets is a major element in the diversification and regulation of adaptive immune responses. Defects in this process or imbalances between the populations have been linked to the development of ADs. The differentiation of naïve T cells into functional subsets is controlled by a variety of cell intrinsic (i.e., receptor specificity) and extrinsic (i.e., cytokines and cell–cell interactions) factors. The activation signals generated by the innate immune system significantly impact T cell differentiation and, as discussed above, variations in signals from the innate immune system can significantly impact T cell function. Additionally, T cell intrinsic molecular pathways that interpret signals from the innate immune system can also impact T cell differentiation.
PTPN22 (Protein tyrosine phosphatase, non-receptor type 22) encodes the protein tyrosine phosphatase, LYP, that functions as a negative regulator of T and B cell responses by dephosphorylating key downstream signaling molecules [e.g., lymphocyte-specific protein tyrosine kinase (LCK), protein-tyrosine kinase fyn (FYN) and ζ-chain (TCR)-associated protein kinase 70 kDa (ZAP70)] (59). A missense mutation at rs2476601 results in a change from arginine to tryptophan at position 620 (i.e., R620W). The functional consequences of this substitution appear to be a decreased ability to bind C-terminal Src tyrosine kinase, an important negative regulator of lymphocyte-specific protein tyrosine kinase (60,61). The consequences of this substitution are controversial, with decreased activation through both the BCR and TCR reported by some (62-64) and increased TCR signaling reported by others (65). The 620W allele has been associated with risk for SLE, T1D (60), and RA (61), whereas in IBD it is protective (66). In addition, there is evidence that this allele is not associated with other ADs, such as CD (67) and MS (18). Likewise, a rare missense substitution (R263Q) in PTPN22 was shown to reduce phosphatase activity and was associated with protection from SLE (68).
CTLA4 (cytotoxic T-lymphocyte-associated protein 4) is a key negative regulatory molecule that impacts antigen-driven activation of T cells. CTLA4 polymorphisms are reported to be associated with T1D (69), IBD (70), RA (71), CD (72), MS (73), and SLE (74). This gene maps to a region in strong LD with CD28 and ICOS, both of which also have important regulatory roles in adaptive immunity. A recent study reported the association of both genes with RA (75). However, the strong LD in the CD28/CTLA4/ICOS gene cluster makes it difficult to identify the true causal locus. In many respects, it is probably most accurate to interpret the association of SNPs from this LD block as representing the functional consequences of carrying a CD28/CTLA4/ICOS haplotype rather than focusing completely on one member of the cluster until more data becomes available to further dissect this association.
SH2B3 (SH2B adaptor protein 3), which encodes for the negative regulator of T cell receptor signaling, LNK (lymphocyte adaptor protein), harbors a non-synonymous variant, particularly associated with T1D (76) and CD (77) but also SLE. Mice genetically deficient for SH2B3 are hypersensitive to stimulation with multiple cytokines (78). SH2B3 also functions as a regulator of T cell signaling as overexpression of SH2B3 inhibits the activation of nuclear factor of activated T cells (NFAT) following TCR stimulation in vitro (79). Variants that affect SH2B3 function could, therefore, alter the signaling thresholds through many different receptors on cells of both the lymphoid and myeloid lineage. A non-synonymous SNP that leads to a R262W substitution in SH2B3 is associated with SLE, T1D, and CD (14,80) and occurs in the pleckstrin homology domain, which is known to be important for targeting the protein to the plasma membrane (14,81,82).
Similarly, TAGAP (a T cell activation GTPase-activating protein) is associated with CD (80), T1D (14), and RA (83). ICOSLG, which encodes a B7-related peptide involved in T cell activation and differentiation and binds to ICOS expressed on the surface of T cells, has been associated with IBD, CD, and T1D (84-86). All of these genes modulate antigen-driven T cell activation in a manner that potentiates susceptibility to multiple ADs.
CD226 is a type I transmembrane receptor of the immunoglobulin superfamily, mainly expressed on the surface of lymphocytes (87). CD226 induces protein kinase C (PKC) and Src family kinase Fyn upon binding to its ligands (88) and is involved in T cell activation and differentiation (89). A missense variant in the gene (Gly307Ser) is associated with T1D, MS, AITD, RA, Wegener’s granulomatosis, and SLE (82,90-93). The risk allele reduces the expression of CD226 in T and NK T cells (93).
The TNFSF4 [tumor necrosis factor (ligand) superfamily, member 4] binds to the surface of antigen-presenting cells by its receptor (TNFRSF4). TNFSF4-mediated signals inhibit IL-10-driven expression of regulatory T cell functions and in vitro production of IL-17 (94). Genetic variants in the promoter of TNFSF4 define a haplotype associated with SLE (95). In addition, the risk haplotype led to the increased expression of the TNFSF4 transcript as well as cell surface protein expression of TNFSF4 when compared to non-risk alleles in lymphoblastoid cell lines (96). Moreover, TNFSF4 and TNFRSF9 are involved in T cell activation and are implicated in CD (86).
Susceptibility genes impacting B cell activation
Human genetic diversity approaches have identified a variety of polymorphisms in genes potentially impacting B cell activation. Among these, BANK1 (B cell scaffold protein with ankyrin repeats 1) encodes a gene that is active in the transmission of BCR signaling. BANK1 promotes the protein tyrosine kinase (LYN)-mediated tyrosine phosphorylation of IP(3)R (inositol 1,4,5-trisphosphate receptor), thus affecting B cell receptor-induced calcium mobilization from intracellular calcium reservoirs (97). Potentially functional BANK1 variations are reported to be associated with SLE in Scandinavian (98), European, American, and Chinese populations (99-101).
Multiple members of the Src family of tyrosine kinases have polymorphisms associated with susceptibility to AD. LYN is associated with antigen receptor signaling in B cells and is associated with SLE (102,103). BLK (B lymphoid tyrosine kinase) encodes another Src family tyrosine kinase that also plays a role in B cell signal transduction. The variants rs2248932 and/or rs13277113 associate BLK with SLE (103,104), RA (105), ankylosing spondylitis (AS) (106), and scleroderma (SSc) (107). The risk allele of the SNP, rs13277113, is associated with low mRNA expression levels of BLK and high mRNA expression levels of FAM167A (104). Interestingly, BLK expression levels are downregulated by epigenetic factors like exposure to high amounts of interferon and infection with Epstein-Barr virus (108).
CD40 (CD40 molecule, TNF receptor superfamily member 5) plays a crucial role in the activation and differentiation of B lymphocytes. CD40 is associated with susceptibility to RA (109), MS, and IBD (110). KIF5A and PIP4K2C are in tight LD and are located on chr.12q13. PIP4K2C encodes a phosphatidylinositol-5-phosphate 4-kinase that is expressed in B cells and is predicted to be involved in phosphatidylinositol signaling. KIF5A encodes a member of the kinesin family with no obvious role in the immune system. The KIF5A/PIP4K2C LD block is a susceptibility locus for RA (109), T1D (11), and MS (111), although the functional mechanisms involved are unclear. ETS1 is a transcription factor in the ETS family that regulates differentiation of terminal B cells and Th17 cells (112). Distinct common variants within ETS1 have been associated with SLE and CD (86,113).
Finally, IL10 (interleukin-10) is an important immunoregulatory cytokine that downregulates the immune response by reducing T cell function and MHC Class II expression in antigen-presenting cells. IL-10 levels were found to be dysregulated in SLE patients (114). Specific variations in IL10 are inconsistently associated with SLE (115). However, this locus has been identified as a risk factor for IBD (116) and T1D (117).
Genes affecting Th1 and Th2 cells
The contribution of different subsets of T-helper cells to the pathogenesis of inflammatory diseases is the subject of much research and debate (118). CD4+ Th1 cells drive the cell-mediated immune responses that lead to tissue damage and particular IgG responses thought to play a role in many inflammatory diseases, whereas CD4+ Th2 cells drive the production of certain antibodies (i.e., IgE) that predominantly underlie allergic responses (119,120).
IL18RAP, STAT1-STAT4, STAT3, and IL12A impact the differentiation of Th1 and Th2 cells, and all have polymorphisms associated with ADs. rs917997 is correlated with alterations in expression of IL18RAP and is associated with susceptibility to IBD. Interestingly, the same SNP is associated with T1D, yet the same risk allele is protective for IBD (35). rs7574865 from STAT4 is associated with RA (121), SLE (103), T1D (122), and IBD (123). STAT1, which is adjacent to and in moderate LD with STAT4, is associated with MS (124) and SLE (125). Likewise, STAT3 is associated with IBD (126) and MS (127).
IL12B encodes the p40 subunit of the heterodimeric cytokines IL-12 and IL-23. IL-12 is composed of a bundle of four alpha helices. It is a heterodimeric cytokine encoded by two separate genes, IL-12A (p35) and IL-12B (p40). IL12B variants are associated with T1D (128), MS (129), psoriasis (PSO) (130), and IBD (131). IL-13 and IL-4 are located directly adjacent to each other in a common LD block. IL-13 is involved in Th2 effector functions and can inhibit Th17 differentiation, while IL-4 is critical for Th2 differentiation. An IL-13 promoter variant is associated with PSO, and the protective allele results in enhanced IL-13 expression in Th2 cells (132).
PRKCQ (protein kinase C, theta) is predominantly expressed in hematopoietic cells and is involved in TCR regulation (133). Mice deficient in PRKCQ show impaired differentiation of T-helper subsets, particularly in Th2- and Th17-mediated inflammatory responses (134). rs4750316 is located in an intergenic region on chr.10p15 proximal to PRKCQ and is associated with RA (135). On the other hand, rs947474, in the same gene is reported to be independently associated with T1D (136). CD58 and CD6 encode co-stimulatory molecules involved in T cell receptor signaling and differentiation. These molecules are associated with MS and RA (83).
Genes affecting Th17 cells
In recent years, the characterization of novel subsets of T-helper cells, most notably regulator T cells (Treg; see below) and Th17 cells, has led to a major paradigm shift in T cell biology, and these T-helper cell subsets have been shown to play an important role in the pathogenesis of ADs. Elevated IL-17 levels are reported in the serum of patients with active SLE (137), MS (138), and RA (139). Moreover, IL-17-producing T Cells are observed in inflammatory infiltrates of SLE, MS, and RA patients. In PSO, activated IFN-γ-producing T cells and IL-17 are detected in the skin and play an important role in disease pathogenesis. A key factor in the survival and expansion of Th17 cells is stimulation by IL-23 (140). A missense variant, rs11209026 (R381Q), in the receptor for IL-23 strongly associates with IBD (141), PSO (142), AS (143), and AITD (144). Lastly, IL-17-producing T cells express CCR6, which encodes a member of the G protein-coupled chemokine receptor family and has emerged as a predisposing genetic factor for IBD in a recent meta-analysis of GWAS (126). Similarly, a functionally disruptive 32bp indel in CCR5 associates with protection from T1D, RA, and CD (126). Polymorphisms in the CCR3 and CCR4 chemokine receptors are also associated with CD (86).
Genes affecting regulatory T cells (Treg cells)
Treg cells are involved in the maintenance of immunological tolerance and lymphoid homeostasis (145). Their deficiency can result from decreased production of the IL-2 (146). IL-2 is often absent in RA synovium (147), and severe autoimmunity develops in knockout mice deficient for IL-2, IL-2RA, and IL-2RB, presumably due to defective regulatory T cell production (148). Genetic variants within the LD block encompassing the KIAA1109/Tenr/IL-2/IL-21 genes have been associated with CD (149), T1D (82), RA (150), IBD (151), PSO (152), and SLE (153). Given the strong LD, it is difficult to define a true candidate gene; however, IL-2 and IL-21 appear to be the plausible casual genes based on their functions in adaptive immunity (154).
IL2RA variants are associated with RA (155), T1D (117) and MS (156), and IBD (157), presenting allelic heterogeneity among diseases. Likewise, IL2RB is associated with RA (158) and T1D (136). Finally, IL7RA variations mediate reduced splicing of the transmembrane domain, decreasing soluble forms of IL-7RA, and are associated with IBD (159) and MS (160). IL7RA along with IL7 and IL2, helps maintain a healthy T effector cell:Treg cell balance (161). All of these variations are postulated to modulate AD susceptibility through their impact on Treg cell differentiation and maintenance.
TNF receptor superfamily genes associated with ADs
GWAS approaches have led to the association of several TNF superfamily members with ADs. TNFSF15, expressed predominantly on endothelial cells, has been shown to be upregulated on macrophages and CD4+/CD8+ lymphocytes of the intestinal lamina propria of IBD patients (162), with a SNP haplotype associated with in multiple populations (163). TNFRSF14, another member of TNF-receptor superfamily, interacts with TRAF proteins (164) and may indirectly affect NF-kB-mediated inflammation. This gene is associated with RA and CD (165). Moreover, TNFRSF6B, encoding a decoy receptor that prevents FasL-induced apoptosis, is associated with IBD (85). Increases in TNFRSF6B protein are observed in IBD (166) and RA (167), suggesting an impact in susceptibility. TNFRSF1A harbors a variant leading to an amino acid substitution, R92Q, which has been identified as a susceptibility locus for MS that correlates strongly with episodic multi systemic inflammation (168,169).
TNFAIP3 encodes the protein A20, an ubiquitin-modifying protein that is strongly induced in cells following TNF stimulation. This molecule is an inhibitor of NF-kB and plays a key role in limiting the severity of inflammatory processes (170). Several TNFAIP3 variations are associated with RA, IBD, SLE, PSO, and CD (13,165,171). A recent meta-analysis confirmed the strongest association with severity of SLE (172,173).
Innate immunity genes
The innate immune system plays a crucial role in driving the activation of the immune response. Functions of the innate immune system include: (a) detection of infectious agents; (b) initiation of both local and systemic inflammatory responses by the production of cytokines; (c) attraction of effector cells into infected tissues via the production of chemokines; and (d) initiation of the adaptive immune response. There exists the possibility that exposure of a dysregulated innate immune system to environmental stimuli might initiate the development of autoimmune phenomena. Several studies have implicated viral or microbial infections with the initiation of ADs, although these studies are inconsistent (174). Studies in mice have established an important role for commensal gut flora in the development of arthritis (175) and IBD (176), and, more recently, in modulating susceptibility to T1D in the NOD mouse (177). The precise interactions involved are still unclear, although signaling through toll-like receptor (TLR) pathways appears to be crucial. Further analysis of the genes and molecular pathways within the innate immune system identified by GWAS studies may provide important insights into the events that trigger autoimmunity in genetically predisposed individuals.
The shared autoimmune loci in innate immunity are: TNIP1 [(associated with psoriasis (178)], IRF8 [(associated with MS (168) and SLE (179)], TYK2 [associated with MS (181) and T1D (182)] and TNFAIP3 discussed elsewhere in this chapter. A missense allele in IFIH1 is associated with risk for T1D (182), AITD (183), and SLE (115). IFIH1 (also called MDA5) is a cytoplasmic RNA sensor that promotes IFN-I production when activated by viruses. As the innate immune response to viral infection is hypothesized to play a role in the pathogenesis of multiple autoimmune diseases, the inappropriate activation of nucleic acid sensors such as TLRs and IFIH1 may contribute to a general predisposition towards autoimmunity (184). IFIH1 is associated with T1D, SLE, and AITD (56). A missense allele (R32Q) of CFB, an activator of the alternative complement pathway, is associated with protection from age-related macular degeneration (AMD) and SLE (185). MIF is an immunoregulatory cytokine that functions in both innate and adaptive immunity and is expressed by many immune cells such as monocytes, macrophages, and T and B lymphocytes. Polymorphisms in MIF have been associated with multiple autoimmune and inflammatory diseases [(reviewed in (186)].
Genes impacting the pathogen recognition receptor (PRR) pathways
The detection of microbial infections is predominantly mediated by the recognition of pathogen-associated molecular patterns (PAMPs) by the TLR and the NOD-like receptor (NLR) PRR families. Signaling through either the TLR or NLR pathways results in the secretion of pro-inflammatory cytokines such as IL-6, TNFα, IL1, and the Type I (interferon alpha/beta) and Type II (interferon gamma) interferons (IFN). Levels of these cytokines (and many others) are commonly increased in the active stages of ADs and are generally considered to play an important role in a variety of elements of disease pathogenesis.
Genetic polymorphisms throughout the TLR and NLR pathways are associated with susceptibility to ADs. Variations in TLR7/8, TLR4, and NOD2 are associated with ADs (85,86,187). Polymorphisms in TLR7/8 are associated with SLE. Genetic studies of IBD have associated a missense variant in TLR4 with early onset of disease (85). Multiple variations in the NOD2 gene are associated with susceptibility to IBD (85). Finally, multiple variants of the CLEC16A (C-type lectin domain family 16 member A) gene, which is predicted to play a role in sugar binding, are associated with T1D (188) and MS (189). All of these variations occur in PRR family receptors and presumably impact susceptibility to AID by modulating the ability of the innate immune system to identify environmental stimuli.
Innate system transcription factors associated with autoimmune susceptibility
Signaling through PRR pathways leads to the activation of multiple cellular functions and the secretion of a variety of cytokines and chemokines. These receptors are expressed in a variety of immune cell lineages, and the consequences of their signaling are dependent in part on the specific cells involved. The nine members of the interferon regulatory factor (IRF) family of transcription factors play crucial roles in mediating transcriptional activation following signaling through the TLR pathway. Genetic variations in IRF4, IRF5, IRF7, and IRF8 are associated with susceptibility to various ADs (179). IRF5 is strongly associated with SLE (190), RA (191), Sjögren’s syndrome (SS) (192), IBD (193), MS (194), and primary biliary cirrhosis (PBC) (195).
As noted above, interferon pathways mediate both autoimmunity and viral defense (196). Genetic associations exist that lead to the increased expression of IRF5 in SLE (197) and RA (155). IRF5 broadly regulates innate immune responses through toll-like receptors (198) and type 1 interferons.
NF-kB, a central regulator of immune responses and inflammation, is a key player in innate immune responses. Though variations in NF-kB have not been associated with ADs, a variety of NF-kB-interacting and regulatory molecules do affect susceptibility. c-Rel, involved in T and B Cell activation, is associated with CD (86) and RA (115). UBE2L3 (ubiquitin-conjugating enzyme E2L 3 isoform1) encodes a protein involved in ubiquitination, and its overexpression leads to quicker degradation of the NF-kB precursor and a diminished innate immune response. This gene is associated with SLE (199), CD (86), RA (155), and IBD (200).
Apoptosis/autophagy/immune-complex clearance
Further implicating a primary role for innate immunity, variants of autophagy genes (e.g., ATG16L1) that target intracellular components, including microbes, to lysosomes have also been associated with IBD (201). Another autophagy gene, LRRK2 (leucine-rich repeat kinase 2), has been identified as an IBD susceptibility locus (126). The associated variant (G2019S) is known to alter neurite length and autophagic vacuole size (202). GWAS reported an association between autophagy ATG5 (APG5 autophagy 5-like) gene and SLE. The associated SNP (rs6568431) lies in an intergenic region between PRDM1 (PR domain containing 1, also known as BLIMP1) and ATG5. Since both genes are viable candidate genes for modulating SLE, the causative gene for this disease association remains to be elucidated (103). MST1 encodes macrophage-stimulating protein (MSP) that regulates macrophage chemotaxis and activation and affects innate immune responses to several bacterial ligands. This gene is associated with IBD (203) along with AITD, RA, SLE, and T1D. Finally, CRP (pentraxin C-reactive protein) is a complement activator and an important innate immune modulator involved in the clearance of cellular debris and apoptotic bodies. Variants in the promoter of CRP have been associated with SLE and CRP levels in SLE patients (204).
Increases in the prevalence of IBD during the past century may well reflect corresponding changes in the composition of the intestinal microbiota resulting from changes in hygiene, such as eradication of intestinal parasites (205). However, the role of the intestinal microbiome is probably not limited to the intestinal immune system. Mouse models of autoimmune diabetes suggest that host differences in the capacity to sense intestinal microbes (177), and the specific composition of the intestinal microbiota modulate susceptibility to diabetes. The dynamic cross-talk between the host immune response and the intestinal microbiota will be an important focus of future research.
Rare variants associated with AD susceptibility
A current listing of rare alleles with a major effect on ADs obtained both in classic studies and more recently via targeted resequencing studies is provided in Table 1. Classic prototypes of rare alleles with major effects on autoimmunity are the deficiencies in components of the complement system. Deficiencies in C2, C4A, C4B, and C1q result in highly penetrant susceptibility to SLE (206). The mechanisms by which these deficiencies mediate a dramatic increase in AD susceptibility are unknown.
Copy number variations (CNV) have also been associated with strong relative risk effects in ADs. CNV of both C4 and FCGR3B, coding for FcγRIIIb, are associated with susceptibility to SLE (207). Other examples of CNV effects associated with AD susceptibility are the beta-defensins. These are small, secreted, antimicrobial peptides, that are encoded by DEFB genes, which show association between higher genomic copy number and the risk of PSO (53).
Next-generation sequencing technologies are currently being used to disentangle rare variants (i.e., frequency <1%). For this purpose, recent analyses in 480 T1D patients and 480 controls identified four rare variants in IFIH1 associated with the disease (56). Likewise, TREX1 encodes for a major 3’ - 5’ DNA exonuclease that may be responsible for a proofreading function during genome replication. TREX1 deficiency has been associated with Aicardi-Goutières syndrome (208), characterized by high type 1 IFN levels. Several rare variants are associated with SLE and SS. The precise mechanisms by which these deficiencies increase disease risk remain unknown.
Translational autoimmunity
Drastic advances on human “-omics” are giving rise to new possibilities in medicine and terms such as clinical bioinformatics (209) and translational bioinformatics (210). All these options lead to one common premise: ways of mining meaningful information from the vast amount of -omics data being generated. In this sense, application of comprehensive molecular information to clinical settings is been referred to as “genomic medicine” (211) with the ultimate goal to nurture, improve, and develop personalized medicine (PM) approaches. A genomic medicine approach will always require participation at a multidisciplinary research expertise level. These valuable advances will strengthen the overall approach to population and individual genetic susceptibility (212) and promise the ability to manage each patient as a biological individual. This will lead to changing paradigms and increasing efficiency and will foster a PM approach as an added plug-in capable of assisting in better prevention, detection, and classification of disease.
Last but not least, the protection of human participants, whether patients, unaffected family members, or unrelated population controls, has to be ensured. These individuals are the key component of genomic research, and their legal rights need to be protected if we wish to continue on with genomic science and to eventually apply genomic-based medicine for the good of humankind. More importantly, we shall not forget the sometimes understated premise that, “...we should not only be interested in the human genome but also in the human beings that carry it” (213).
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