Genome-wide association studies (GWAS) have helped identify numerous gene variants that contribute to the risk of autoimmunity. Despite the vast catalog of causal candidate genes generated by GWAS, the functional contribution to disease of most autoimmunity-associated gene variations remains to be defined (Hu and Daly, 2012). Notably, several genetic loci stand out for having been very broadly associated with autoimmunity. Among these, variations within CLEC16A at chromosomal position 16p13 have been associated with no less than 10 diseases, including type 1 diabetes, multiple sclerosis, systemic lupus erythematosus, celiac disease, Crohn's disease, Addison's disease, primary biliary cirrhosis, rheumatoid arthritis, juvenile idiopathic arthritis and alopecia areata (Dubois et al., 2010; Gateva et al., 2009; Hakonarson et al., 2007; Hischfield et al., 2012; IMSGC, 2009; Jagielska et al., 2012; Marquez et al., 2009; Martinez et al., 2010; Skinningsrud et al., 2008; Skinningsrud et al., 2010; Todd et al., 2007; WTCCC, 2007). The association of CLEC16A variation with multiple autoimmune disorders thus implicates this gene in an as yet undefined but likely fundamental aspect of immune regulation.
CLEC16A encodes a large protein of 1053 amino acids that contains several putative functional domains, including a C-type lectin domain which led to its classification as C-type lectin domain family 16A (Berge et al., 2013). At the time CLEC16A was associated first with type 1 diabetes (Hakonarson et al., 2007; Todd et al., 2007; WTCCC, 2007) and then with multiple sclerosis (IMSGC, 2009), this gene formerly known as KIAA0350 had neither been classified nor was anything known of its function. The first data relating to CLEC16A's cellular function came from studies of its Drosophila ortholog, termed Ema, that was shown to participate in endosomal maturation (Kim et al., 2010) and autophagy (Kim et al., 2012). A role in autophagy was recently confirmed in mouse, where beta cell-specific deletion of Clec16a impaired mitophagy (Soleimanpour et al., 2014). In their study of mice with Clec16a-deficient beta cells, Soleimanpour and colleagues suggested that variation in CLEC16A function in the pancreas may be causal for this gene's association with type 1 diabetes. These investigators postulated that a defect in insulin secretion secondary to disrupted autophagy would predispose beta cells to the autoimmune destruction that causes type 1 diabetes. However, this hypothesis does not provide an explanation for CLEC16A's association with a broad array of immune-mediated diseases (Dubois et al., 2010; Gateva et al., 2009; Hakonarson et al., 2007; Hischfield et al., 2012; IMSGC, 2009; Jagielska et al., 2012; Marquez et al., 2009; Martinez et al., 2010; Skinningsrud et al., 2008; Skinningsrud et al., 2010; Todd et al., 2007; WTCCC, 2007). The functional link between CLEC16A variation and autoimmunity therefore remains to be convincingly explained.
The data presented herein indicate that Clec16a variation impacts thymic selection, owing to a role in thymic epithelial cell autophagy, thus implicating CLEC16A in a fundamental aspect of immune tolerance. Our findings thereby provide a functional link between CLEC16A variation and the immune dysregulation that broadly underlies the risk of autoimmune disease.