Association of the programmed cell death 1 (PDCD1) gene polymorphism with ankylosing spondylitis in the Korean population

Ankylosing spondylitis (AS) is an important chronic inflammatory disease with an incidence range of between 0.5% and 1.0% [1, 2]. Many genetic and environmental factors have been suggested to have some role in the development of AS [3]. However, the pathogenesis of AS is still unclear. Human leukocyte antigen (HLA) B27 makes up only a small proportion of the overall risk for spondyloarthritis. Fewer than 5% of HLA-B27-positive people in the general population develop these diseases [4]. In contrast, 20% of the HLA-B27-positive relatives of AS patients are affected. Family studies have suggested that HLA-B27 contributes to about 37% of the overall genetic risk for spondyloarthritis [5‐7], which suggests the involvement of other genes in the development of AS. Another new susceptible gene therefore needs to be identified.

Recently, the PDCD1 (programmed cell death 1) gene polymorphism was reported to be associated with systemic lupus erythematosus (SLE) [8, 9], lupus nephritis [10, 11] and seronegative rheumatoid arthritis (RA) [12]. It was also suggested that lupus nephritis and the seronegative RA susceptibility was associated with the PD-1.3 (position 7,146) A allele in a northern Sweden population.

The PD-1 molecule is a negative regulator of T cells [13] that belongs to the immunoglobulin receptor superfamily. It encodes a 55 kDa type 1 transmembrane inhibitory immunoreceptor and is responsible for the negative regulation of T-cell activation and peripheral tolerance [14]. Nishimura and Honjo [15] reported that PD-1 expression was observed only in activated T and B cells as well as in the early lymphoid precursors. PD-1 is actively expressed on the cell surface during the activation of T and B cells. The cytoplasmic immunoreceptor tyrosine-based inhibitory motif of PD-1 was activated by an interaction between PD-1 and its corresponding ligands, PDL-1 (B7-H1) and PDL-2 (B7-DC) [15, 16], which induces the inhibitory signal to inhibit the proliferation of T and B cells to maintain peripheral tolerance [14‐16].

The human gene encoding PD-1, PDCD1, is localized on 2q37.3, which is a susceptibility locus for SLE [10]. Allele T of a single-nucleotide polymorphism (SNP) corresponding to PD-1.5 C/T (dbSNP rs#cluster id rs2227981) was found to be associated with the development of RA (odds ratio (OR) 1.94, 95% confidence interval (CI) 1.25 to 3.01, p < 0.0025) but not SLE in Chinese patients living in Taiwan [17].

So far more than 30 SNPs have been identified. Among the 30 SNPs, 7 (namely PD-1.1, PD-1.2, PD-1.3, PD-1.4, PD-1.5, PD-1.6 and PD-1.9) were examined in SLE [12]. Two SNPs (PD-1.5 C/T (rs2227981) and PD-1.9 T/C/(rs2227982)) were selected because they occur in an exon, which affects protein synthesis. The change in PD-1.9 T/C causes a change in their synthesized amino acid from valine to alanine. Our study was therefore focused on determining whether PD-1.9 T/C SNP affects the development of AS.

Overall, PDCD1 is a strong candidate for susceptibility to autoimmune disease. This study therefore investigated whether or not PDCD1 is associated with AS in the Korean population.

The results show that a functionally important polymorphism in PDCD1 is associated with a subset of AS. A polymorphism in this gene has previously been shown to be associated with SLE [8, 9] and RA [12]. These results suggest that this gene may commonly have some role in the development of autoimmune disease. PDCD1 has a key role in the immune response as follows.

The ligands for PD-1 have been identified as PD-L1 (B7-H1), which is expressed in all haemopoietic cells as well as in many non-haemopoietic tissues, and PD-L2 (B7-DC), which is expressed primarily on dendritic cells and macrophages [14, 16, 18, 19]. This molecule has a role in the negative co-stimulated pathway involving the CD28 homologue PD-1 receptor [20]. PD-1 is expressed by activated, but not unstimulated, T cells, B cells and myeloid cells, which is in contrast with the predominantly T-cell-restricted expression of CD28 and CTLA-4 [21‐23]. PD-1^-/- mice display a variety of autoimmune pathologies, including dilated cardiomyopathy (in BALB/c mice) and a lupus-like syndrome (in B6 mice). This indicates that PD-1 has a key role in the maintenance of peripheral tolerance to self-antigens, which is analogous to that of CTLA-4 [13, 24]. The development of AS can therefore be affected by PD-1 expression or the function of the PD-1 molecule, and is associated with the continuous activation of immune cells such as CD4⁺ T cells or dendritic cells. HLA B27, which is the most important genetic factor for AS, is a class I antigen and reacts with CD 8⁺ T cells. However, most autoimmune diseases are associated with CD4⁺ T cells. Recently, misfolding HLA B27 was reported to stimulate CD4⁺ T cells or natural killer cells in HLA B27 transgenic rats [25]. In contrast, it was suggested that HLA B27 itself also could act as an autoantigen, because it is recognized by the T-cell receptors on CD4⁺ T cells [26] or by its presentation by HLA class II molecules. This suggests the importance of the CD4⁺ T-cell activation pathway and termination of its activation in AS. It is therefore possible that AS is associated with the function of the PD-1 molecule.

The PD-1 SNP has been examined in several studies but only in SLE and RA patients. Ferreiros-Vidal and colleagues [9] studied PD-1.1, PD-1.2, PD-1.3, PD-1.5, PD-1.6 and PD-1.9 SNPs in Spanish patients with SLE. They reported that the PD-1.9 minor allele was too rare and reported an incidence of less than 1.1% in 186 patients with SLE and 0% in the control group of 186 individuals. As a result, there were no more studies on PD-1.9 SNP. However, the results in the present study suggest that the PD-1.9 T allele is more frequent in a Korean population sample than in a Spanish population sample (21% in patients with AS and 6.9% in controls). The reason for this difference is that previous studies were confined to females because of the high female predominance of SLE. It should be noted that AS develops predominantly in males. Our results showed no gender difference in the frequency of PD-1.5 or PD-1.9. The PD-1.5 SNP has been studied extensively in several populations. The PD-1.5 C allele was found to be more frequent in Korean populations than in other populations (Spanish, Swedish, European and American Mexican). The data are shown in Table 4. Table 5 shows the data for another Asian group study, a Chinese population for RA [11]. In that study the control group was 33% female; in contrast, 41% of the control group in the present study were female.

The limitation of our study was that the control group was mismatched to the disease group in sex and age. With regard to sex, the polymorphism of this gene is not affected by the sex difference in our control group. With regard to age, with increasing age in the control group the possibility of being in the disease-free group increased because AS develops at a relatively young age.

The PD-1.9 T allele was found to be a susceptibility factor for AS; however, 1.5 SNP was not a factor. The PD-1.9 T allele substitutes valine for alanine 215 during PD-1 protein synthesis. However, other subgroup analyses according to gender, combined uveitis, the involvement of a peripheral joint and the existence of HLA B27 showed no significant differences. Therefore, further study will be needed to determine whether this amino acid change alters the protein structure and affects its function.

PDCD1 was known as another genetic risk factor in RA or SLE patients. However, there was no known study on this gene with regard to AS. We have examined the association between this autoimmune-associated gene and AS and concluded that the gene is associated with susceptibility to AS, although only a small sample was used. We believe that these results are sufficient to suggest another genetic susceptibility (PDCD1) for AS.