Role in cartilage development and osteoarthritis (OA):
Mouse CHST11 has been shown to be required for cartilage growth plate morphogenesis (Klüppel et al., 2005). Loss of CHST11 caused chondrodysplasia with severely shortened long bones, caused by shortened and thickened cartilage growth plates with disorganized and hypo-cellular cartilage growth plates with fibrillated ECM and an overall loss of chondroitin sulfate. Increased apoptosis of mutant chondrocytes was observed, and TGFbeta and BMP signaling was disturbed in mutant growth plates (Klüppel et al., 2005). Many of these cartilage deficiencies are characteristic of the degenerative alterations observed in OA, a degenerative disease characterized by loss of matrix GAGs and cartilage integrity. Increased CHST11 expression has been observed in OA (Zeggini et al., 2012). Combined, these data suggest a requirement for tightly controlled regulation of CHST11 expression in the development and maintenance of healthy cartilage.

Role in HSV infection:
Herpes simplex virus (HSV) envelope glycoproteins utilize cell-surface GAGs to efficiently bind to and infect host cells. The gC HSV envelope protein has been suggested to bind cell-surface CS-E-proteoglycans with high affinity, and treatment with exogenous CS-E could potently inhibit HSV infectivity, thus identifying CS-E chains of cell-surface proteoglycans as key receptors for HSV entry into a host cell. Deficiency in CHST11 expression leads to drastically reduced susceptibility to HSV infection in L-cell fibroblasts, presumably through the absence of CHST11-mediated CS-E synthesis (Uyama et al., 2006).

Role in malaria:
Malaria is caused by the parasites of the species Plasmodium, and is transmitted through infected mosquitos. High affinity adherence of P. falciparum-infected erythrocytes to endothelial cells is mediated by the CHST11 product C4S on endothelial cell-surface proteoglycans (Rogerson et al., 1995; Cooke et al., 1996; Pouvelle et al., 1997; Beeson et al., 1998).

Role in Costello syndrome:
Costello syndrome is a pediatric genetic disorder linked to oncogenic germline mutations in the HRAS gene (Gripp, 2005; Quezada and Gripp, 2007; Rauen, 2007; Gripp and Lin, 2012). The disease is characterized by multiple developmental abnormalities as well as predisposition to malignancies (White et al., 2005; Quezada and Gripp, 2007; Rauen et al., 2008). Reduction in CHST11 mRNA and protein expression, as well as loss of C4S has been identified in primary fibroblasts derived from Costello syndrome patients (Hinek et al., 2005; Klüppel et al., 2012). Oncogenic HRAS in normal fibroblasts can repress CHST11 expression, while interference with oncogenic HRAS signaling in these cells elevated CHST11 expression, thus identifying CHST11 as a negatively regulated target gene of HRAS signaling (Klüppel et al., 2012). Forced expression of CHST11 in Costello fibroblasts rescued the proliferation and elastogenesis defects associated with oncogenic HRAS signaling in these cells (Klüppel et al., 2012). These results indicate that reduced CHST11 expression and a subsequent chondroitin sulfation imbalance mediate the effects of oncogenic HRAS signaling in the pathogenesis of Costello syndrome.

Role in cancer:
Changes in CS levels and chondroitin sulfation balance have been described during tumor progression (Ricciardelli et al., 1997; Suwiwat et al., 2004; Theocharis et al., 2006; Sakko et al., 2008; Teng et al., 2008; Svensson et al., 2011; Vallen et al., 2012). Experimental elimination of chondroitin sulfate in orthotopic breast cancer mouse models lead to increased metastasis, demonstrating a critical role of chondroitin epitopes in tumor progression in vivo (Prinz et al., 2011). The CHST11 gene was highly expressed in aggressive breast cancer cells, but significantly less so in less aggressive breast cancer cell lines (Cooney et al., 2011). Moreover, a positive correlation was observed between the expression levels of CHST11 and P-selectin-mediated adherence of breast cancer cells to endothelial cells (Cooney et al., 2011). Increased expression of the CHST11 gene has been observed in multiple myeloma (Bret et al., 2009). One case report of a patient with B-cell chronic lymphocytic leukemia (B-CLL), a chromosomal translocation with breakpoints in the IGH locus on chromosome 14, and the CHST11 locus on chromosome 12 [t(12;14)(q23;q32)] was identified. The translocation breakpoint mapped to intron 2 of the CHST11 locus, and resulted in the expression of two truncated forms of CHST11 (Schmidt et al., 2004).

Role in Wnt-β-catenin signaling:
Studies were performed in mutant sog9 L-cell fibroblasts, which lack the expression of both EXT1 (Extosis-1, required for heparan sulfate biosynthesis) and CHST11 genes (Nadanaka et al., 2008). Mutant cells had a significant decrease in Wnt3a-stimulated β-catenin accumulation, which could be rescued by stably expressing CHST11, but not EXT-1 (Nadanaka et al., 2008). In addition, this study showed that the specific chondroitin sulfate form CS-E, but not the other chondroitin sulfate forms, was able to bind Wnt3a ligand with high affinity. Addition of CS-E to normal L-cells reduced β-catenin levels, much like what was seen in the sog9 mutant L-cells lacking CHST11 expression (Nadanaka et al., 2008). Together, this data suggested that the CHST11, through its ability to produce CS-E containing proteoglycans, might play a role in the Wnt/β-catenin signaling pathway. The investigators of this study suggested a model in which CHST11 expression increases the level of CS-E containing proteoglycans, which can then bind Wnt3a, and facilitate the binding of Wnt ligands to its receptor complex, thus increasing the efficiency of ligand-receptor interactions. In a follow-up study, Nadanaka et al. (2011) show that L-cells stably expressing the Wnt3a ligand had a reduction in CHST11 gene expression, and subsequently a change in sulfation balance with a higher concentration of chondroitin sulfate products with low affinity for Wnt3a ligand binding (Nadanaka et al., 2011). This allows the Wnt3a ligand to freely diffuse across L-cell fibroblast cultures. Forced expression of CHST11 was suggested to inhibit the diffusion of Wnt3a ligand in L-cell fibroblast cultures, because of the increase in production of CS-E containing proteoglycans (Nadanaka et al., 2011). These and previous studies suggested that CHST11 expression is able to inhibit Wnt3a diffusion and sustained signaling, but CHST11 gene expression is negatively regulated by active Wnt/β-catenin signaling (Nadanaka et al., 2011). We reported the identification of the CHST11 product CS-E as an inhibitor of specific molecular and biological outcomes of Wnt3a signaling in NIH3T3 fibroblasts (Willis and Klüppel, 2012). CS-E could decrease Wnt3a signaling through negative regulation of LRP6 receptor activation. However, this inhibitory effect of CS-E only affected Wnt3a-mediated induction, but not repression, of target gene expression (Willis and Klüppel, 2012). We went on to identify a critical Wnt3a signaling threshold that differentially affects target gene induction versus repression. This Wnt3a signaling threshold also differentially controlled the effects on proliferation and serum starvation-induced apoptosis (Willis and Klüppel, 2012). These data established the feasibility to manipulate the chondroitin sulfate biosynthesis machinery, in particular CHST11, to selectively inhibit Wnt/β-catenin transcriptional programs and biological outcomes through the exploitation of intrinsic signaling thresholds (Willis and Klüppel, 2012).