Gene-Gene Interaction between LCE and CLEC16A Increases the Risk of Psoriasis in a Chinese Population.

Dear Editor: Psoriasis is a common cutaneous disease characterized by inflammation and abnormal epidermal proliferation. Currently, some inflammatory cells, such as dendritic cells, macrophages, neutrophils, and keratinocytes, and several cytokines are believed to play important roles in the pathogenesis of psoriasis1. Our previous genome-wide association study (GWAS) provided convincing evidence for the LCE gene cluster being a susceptibility factor for psoriasis and showed that CLEC16A was significantly associated with development of psoriasis (SNP rs193756, odds ratio [OR]=0.8, p=0.0004)2, although Prs193756 was found to be >10-8. Moreover, CLEC16A has previously been found to be linked to multiple sclerosis, and patients with this gene were at a higher risk of developing psoriasis3. The LCE gene cluster encodes epidermal barrier proteins, and perturbation of expression of these genes is associated with psoriasis4. Bergboer et al.5 found that the expression of LCE proteins was regulated by a combination of proinflammatory cytokines. In fact, CLEC16A is shown to be highly expressed in inflammatory cells such as dendritic cells, macrophages, B-lymphocytes, and natural killer cells6. In addition, the interaction between LCE and HLA-C and among the MHC locus, LCE, and IL12B have also been studied in large samples of diverse ethnic populations7,8. Therefore, the postulated common pathway of LCE and CLEC16A between inflammatory response and epidermal barrier prompted us to examine the combined contribution of both these genes to susceptibility to psoriasis. The study subjects (5,101 psoriasis patients and 6,183 healthy controls) were the same as those in our previous GWAS2. Diagnosis, clinical assessment, and recruitment of subjects have previously been described2. Cases and controls were matched on age and gender. Collection of blood samples and information were carried out after obtaining written informed consent from the participants. The study was approved by the ethics committee of Anhui Medical University and was conducted according to the principles of the Declaration of Helsinki. Our previous GWAS identified four novel SNPs (rs4112788, rs4845454, rs4085613, and rs1886734) within the LCE gene cluster for psoriasis, and all four SNPs are located within the same linkage disequilibrium (LD) block and in almost complete LD (pairwise r2>0.95). Furthermore de Cid et al.9 found LCE3C_LCE3B-del was tagged by rs4112788 (r2=0.93), which was also strongly associated with psoriasis. Thus, we selected SNP rs4112788 for this study. Genotyping data of LCE rs4112788 and CLEC16A rs193756 were directly extracted from the previous study2. For quality control, we excluded the samples with no call for rs4112788 and rs193756. The association between dichotomous variables was determined based on the OR, and 95% confidence intervals were estimated by the Cornfield method or the exact method. p-values were estimated by chi-square or Fisher exact tests. Interrelations were analyzed by multiple logistic regression analysis. All statistical analyses were performed using the SPSS 12.0 (SPSS Inc., Chicago, IL, USA). Associations of psoriasis with LCE rs4112788 and CLEC16A rs193756 and the interaction between these two SNPs are shown in Table 1 for the combined series. As expected, LCE rs4112788-CC patients were at an increased risk to develop psoriasis (OR=1.76, p=5.90×10-5); however, the presence of CLEC16A rs193756 was not significantly associated with psoriasis (p>0.05). Furthermore, under four different interactive models of LCE and CLEC16A, we observed that the subjects carrying the LCE rs4112788-CC and the CLEC16A rs193756-GG had a significant higher possibility of development of psoriasis (OR=1.57, p=0.0053), suggesting the existence of a gene-gene interaction between these two genes. Table 1 Disease association analysis of LCE and CLEC16A and their interaction We discovered the association with prompt significance (Prs193756=0.0004>10-8) between CLEC16A and psoriasis in our previous GWAS; however, the presence of CLEC16A rs193756 did not show a significant association with psoriasis in this study. Moreover, we observed that the subjects carrying the LCE rs4112788-CC and the CLEC16A rs193756-GG had a significantly higher chance of development of psoriasis. CLEC16A maybe a minor gene associated with psoriasis development, triggering psoriasis through interaction with LCE. A reconstructed skin model showed that a combination of the psoriasis-associated proinflammatory cytokines induced the expression of LCE proteins, and the LCE proteins may play a vital role in repairing the epidermal barrier in response to skin injury4. LCE rs4112788-CC may be associated with reduced LCE expression. Todd et al.10 have suggested that CLEC16A may be a functional element involved in maintaining immune homeostasis by the immunoreceptor tyrosine-based activation motif, and the CLEC16A protein is shown to be highly expressed on inflammatory cells. CLEC16A rs193756-GG may be linked to enhanced expression of CLEC16A which enhances inflammatory reaction. We speculated that CLEC16A rs193-756-GG first altered inflammatory cells and responses in the injured skin via several proinflammatory cytokines, some of which in turn induced the expression of LCE proteins. On the other hand, LCE rs4112788-CC resulted in defective LCE proteins, because of which the epidermal barrier could not be repaired. In addition, it may be that defective LCE first rendered the injured epidermal barrier unrepairable and then CLEC16A acted to induce a more aggressive inflammatory response. In short, the combined actions of LCE and CLEC16A may lead to development of psoriasis or may aggravate the disease. It was perhaps not surprising that LCE would interact with CLEC16A between repairing epidermal barrier and inducing an inflammatory response in the development of psoriasis. Our findings provide a new way to search for new susceptibility genes and suggest that the interaction between the epidermal barrier and the inflammatory response system may be involved in the pathogenesis of psoriasis, although further studies are needed to confirm this.