Genetic risk factors in ankylosing spondylitis: Insights into etiology and disease pathogenesis

Abstract

Ankylosing spondylitis (AS) is a complex inflammatory condition within the spectrum of spondyloarthropathies (SpAs). HLA-B*27, IL-17, IL-23, and complex pathways drive AS progression. Microbiota, ERAPs, and TLRs also contribute to AS. This editorial delves into AS genetics, unraveling its molecular underpinnings. Since its discovery in 1973, HLA-B*27's association with AS has been extensively studied. About 90%–95% of AS patients carry HLA-B*27, yet only 1%–2% develop AS. This highly polymorphic allele includes subtypes like HLA-B*27:02, B*27:04, and B*27:05, showing variable associations across populations. Hypotheses include the “arthritogenic peptide” and “molecular mimicry,” suggesting unique peptide presentation or microbial triggers.1, 2 Killer cell immunoglobulin-like receptors (KIRs) expressed by NK and T cells regulate immunity via inhibitory and activating signals and are key players in AS by engaging HLA-B*27. KIR-HLA variations may impact SpAs. Balancing activating and inhibitory KIR-HLA genotypes might influence AS susceptibility. KIR3DS1 allele frequency is elevated with HLA-B*27, while KIR3DL1, its inhibitory counterpart, is reduced in AS. Both interact with Bw4 epitope-bearing HLA-B alleles. B272 homodimers bind KIR3DL1, KIR3DL2, and LILRB2, impacting inflammatory responses (Figure 1A).3 KIR3DL2 ligation inhibits IFN-γ production and boosts IL-17, linking NK cells and CD4+ T cells to AS.4 While KIR3DS1's ligand is not yet known, AS patients show elevated KIR3DL2+ CD4+ T cells producing IL-17, linking them to disease progression. This intricate interplay underscores KIR-HLA's role in shaping AS immunopathology. Crucial antigenic peptides, vital for immune recognition, undergo a refining via endoplasmic reticulum aminopeptidases (ERAP1 and ERAP2). ERAP1 fine-tunes peptides to 8–10 amino acids, while ERAP2 prefers shorter variants (7-8-mers).5, 6 ERAPs also influence cytokine receptor shedding, cytokine signaling, angiogenesis, and macrophage activation.7 Their intricate association with AS involves HLA-B*27, evident in both positive and negative cases. ERAP1, combined with HLA-B*27, significantly influences familial AS risk. ERAP1 SNPs like rs30187, rs2287987, rs10050860, rs17482078, and rs27044 alter peptide interactions with HLA-B*27, shaping T-cell responses.8 Some ERAP1 variants offer protection via peptide trimming.9 Some AS patients display ERAP1 promoter hypermethylation, affecting gene expression and AS pathogenesis. Altered ERAP1 or ERAP2 functions lead to abnormal peptide processing, skewing T-cell responses when presented by HLA B27–β2m complexes (Figure 1B). Furthermore, ERAP1 dysfunction can impact HLA-B*27 misfolding, triggering the unfolded protein response (UPR) and upregulating pro-inflammatory genes.10, 11 HLA-B27 tends to misfold and form homodimers, a process influenced by cysteine residues at positions 67 (C67), 101 (C101), 164 (C164), and 325 (C325). Misfolding of HLA-B27 contributes to ER stress and cytokine production, influencing immune responses (Figure 1C).12 The “arthritogenic” peptide hypothesis suggests that HLA-B*27 can bind unique arthritogenic peptides, presenting them to self-reactive CD8+ T cells, leading to chronic inflammation (Figure 1D). However, the specific arthritogenic peptides triggering AS remain unidentified. These cascading effects collectively contribute to the to the onset and persistence of inflammation in AS. Recent GWAS studies emphasize the central role of the IL-17/IL-23 pathway in AS. HLA-B*27 misfolding triggers IL-23 production via UPR activation, leading to Th17 cell differentiation marked by the expression of the transcription factor RORγt. Th17 cells release IL-17, inducing pro-inflammatory mediators, including IL-1β from the gut microbiota. Amyloid A from the terminal ileum further fuels Th17 cell differentiation, influencing AS. Elevated IL-17 and IL-23 levels in AS patients' plasma correlate with the disease.13 IL23R gene variations impact AS susceptibility, with specific SNPs like rs11209026 offering protection.14 Additional loci (IL-1R1, IL-2R, IL-6R, IL-12B, IL-27, CARD9, RUNX3, STAT3, TYK2) have also been found in association with AS pathogenesis.15 The gut microbiota profoundly affects human health and immunity, potentially contributing to AS. Altered gut microbes and mucosal immunity genes are linked to AS susceptibility. Dysbiosis, including elevated Klebsiella pneumoniae and Bacteroides vulgatus, might lead to inflammation. Intestinal barrier compromise triggers immune responses, overproducing cytokines. Disturbed mucosal layers may induce stress and activate IL-23-responsive cells, inducing chronic inflammation. Increased gut permeability and bacterial by-products were detected in the synovial joints of AS patients (Figure 1E).16 Understanding the gut microbiome's role and mechanisms in AS could uncover diagnostic markers and therapies. The IL-17/IL-23 pathway's complexity affects bone dynamics, with IL-17 promoting osteoclast activity and IL-22 stimulating osteoproliferation. Elevated IL-17 and IL-23 levels in AS patients underscore their disease connection. IL-17 inhibits bone regeneration, while IL-22, generated by Th17 cells in response to IL-23, encourages bone growth. Consequently, the IL-23/IL-17 pathway leads to a paradoxical interplay of bone erosion and neogenesis in AS (Figure 2). TNF-α, a pivotal pro-inflammatory cytokine, orchestrates immune responses through its transmembrane (tmTNF-α) and soluble (sTNF-α) forms, binding to TNFR1 and TNFR2. Their diverse effects range from survival to cell death, governed by NF-κB signaling and caspase activation. While TNFR1 signaling triggers pro-inflammatory reactions, TNFR2 promotes an anti-inflammatory response. In AS, TNF-α's involvement is evident with elevated levels in sacroiliac joints and circulating soluble TNF receptors (sTNF-R1, sTNF-R2). TNF-α's intricate role in bone homeostasis includes promoting erosion and inhibiting formation through Wnt signaling inhibitors, sclerostin (SOST), and dickkopf protein (DKK1).17 This contributes to the hallmark features of AS, inhibiting osteoblast differentiation and activating osteoclastogenesis via the TNFR1-mediated NF-κB pathway.18 Recent strides in AS treatment involve TNF inhibitors, reducing inflammation and disease severity effectively.19 Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4) or CD152, found on activated T cells, regulates activation, proliferation, and cytokine production. It hinders early T-cell activation by blocking CD28 interaction. Elevated soluble CTLA-4 (sCTLA-4) in SpA patients, due to alternative splicing, correlates with disease activity. sCTLA-4 impacts both activating and inhibitory pathways, influencing SpA immune regulation. Notably, a significant association was reported between CTLA-4 polymorphisms (+49A/G, −318C/T, CT60A/G) and AS susceptibility.20, 21 Toll-like receptors (TLRs) are essential pattern recognition receptors (PRRs) that detect pathogen-associated molecular patterns (PAMPs), triggering immune responses. TLRs influence AS by activating NF-κB and inducing cytokine release like IL-12, IFN-γ, and TNF-α. Elevated TLR4, TLR5, TNF-α, IL-6, and CRP levels in AS patients were observed. TNF-α inhibitors reduce TLR4 and TLR5, enhancing TLR3.22 Polymorphisms in the TLR2 and TLR9 genes were linked to SpA development. TLR4 and TLR5 expression differ among populations. Caspase Recruitment Domain Family, Member 8 (CARD8) activates pro-inflammatory caspases and inhibits NF-κB, crucial in inflammatory diseases. AS susceptibility was linked to two SNPs: rs2043211 (exon) and rs7253718 (intron). rs2043211 introduces a premature stop codon, yielding a truncated CARD8 protein that fails to suppress NF-κB, leading to overproduction of pro-IL-1β and NLRP3 inflammasome activation.23 Reelin, an extracellular matrix protein encoded by the RELN gene, binds to receptors (VLDL-R, ApoER2, α3β1 integrin), activating Dab1 and downstream signals. This promotes Pla2g7 gene expression, leading to platelet-activating factor-acetylhydrolase (PAF-AH) production, which degrades platelet-activating factor (PAF). PAF fuels inflammation and cartilage degeneration.24 Elevated reelin levels in the synovial fluid of arthritic patients are noted. A novel RELN gene variant (c.7456A > G; p.Ser2486Gly, rs760019076) linked to AS has been identified. The variant might hinder reelin secretion, reducing PLA2G7 gene expression and altering macrophage pathways, instigating inflammation.25 This discovery sheds light on AS's intricate mechanisms and potential therapeutic targets. Calcium-Sensing Receptor (CaSR), a pivotal receptor of the C G protein-coupled family, regulates systemic Ca2+ balance via G proteins and β-arrestin signaling. It governs parathyroid hormone secretion and renal tubular Ca2+ reabsorption, impacting skeletal homeostasis.26 Aberrant CaSR upregulation in AS patient tissues is linked to new bone formation. Inflammatory cytokines amplify CaSR gene transcription in osteoblasts through the NF-κB/p65 and JAK/Stat3 pathways, activating PLCγ signaling and promoting AS-related bone growth.27 The selective antagonist NPS-2143 mitigates CaSR expression, reducing new bone formation and disease severity in AS models. CaSR deletion hampers osteogenic differentiation in inflammatory contexts, revealing its integral role in AS pathology.27 Anthrax Toxin Receptor 2 (ANTXR2), also known as CMG2, is strongly associated with AS in Caucasians via GWAS. This transmembrane protein binds laminins and type IV collagen during capillary morphogenesis. Its precise role in AS remains unclear, but the SNPs rs4333130, rs4389526, and rs12504282 show suggestive AS association in Caucasians. In Han Chinese, SNPs rs4690127, rs6823031, and rs4333130 in the ANTXR2 gene correlate with AS. Genetic variations in ANTXR2 downregulate its expression, possibly increasing capillary permeability and inducing inflammation in AS.28 G protein-coupled receptors (GPRs) play diverse roles in cell signaling. GPR35, GPR37, and GPR65 are associated with AS. GPR65 (TDAG8) influences thymocyte apoptosis and MMP3 expression, both linked to AS.29 GPR35 modulates immune cells and potential indoleamine 2,3-dioxygenase (IDO) communication, impacting inflammation. GPR37 SNP (rs2402752) is associated with AS.30 Though their exact roles in AS remain unclear, these GPRs contribute to the intricate disease landscape. Epigenetic changes, including DNA methylation and microRNAs (miRNAs), play roles in AS. DNA methylation alterations, like DNMT1 downregulation, affect immune cells in AS patients. The BCL11B gene, which impacts IL2 activation, is hypermethylated and suppressed in AS patients.31 miRNAs like miR-16, let-7i, miR-124, and miR-221 are upregulated in the T cells of AS patients. miRNAs affect TLR-4, TNF-α, osteogenesis-related proteins, and bone loss. Other miRNAs, including miR-29a, miR-20a, miR-300, miR-185, and miR-155, also contribute to AS pathogenesis.32 Transcription factors (TFs) influence gene expression by affecting regulatory elements like enhancers and promoters. Runt-related transcription factor 3 (RUNX3) impacts T-cell development and is linked to AS. Variants in RUNX3 are associated with AS susceptibility. Alterations in regulatory SNPs, including rs11249206 and rs760805, affect RUNX3 expression. SNP rs4648889 is tied to AS risk and hampers RUNX3 expression via IRF4 binding. Other TFs like EOMES, IL7R, ZMIZ1, and TBX21 also play roles in AS development. The transcription factor T-bet encoded by TBX21 is influenced by the risk allele rs11657479, linked to T-bet overexpression in AS.33 In a mouse model, T-bet loss protected against AS. The intricate interplay of genetic factors, immune responses, and environmental influences in AS underscores the complexity of this rheumatic disorder. The insights gained from unraveling the genetic mechanisms provide a deeper understanding of AS pathogenesis, paving the way for targeted therapies and improved patient care. As we continue to delve into the genetic landscape of AS, further discoveries promise to shape the future of diagnosis, treatment, and management of this challenging condition. AS: Data curation, Wrote the original draft. MCB: Writing—review & editing. RB: Resources, Writing—review & editing. LB: Conceptualization, Resources, Supervision, Data curation, writing—review & editing. We are grateful to the Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa Vidyapeetham, for their infrastructural and financial support. None. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

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