Jos Jansen

A 151 Appendices Next, we explored the possible link between CCDC115- and TMEM199 deficiency with hepatic steatosis, a feature seen in all patients. We proposed a model involving COP-coated vesicles, which shuttle proteins to and from the Golgi apparatus and lipid droplets. Ineffective or absent CCDC115 or TMEM199 functioning could negatively influence this transport through yet unknown processes. Evidence for this hypothesis is based on (1) the localization of CCDC115 and TMEM199 in the ER-to-Golgi region and their co-localization with COP-coated vesicle proteins, (2) a general detrimental effect on Golgi-located glycosylation and (3) the steatotic phenotype of the patients. In chapter 5 we set out to screen a cohort of ~1000 patients with liver disease, including patients that needed a liver transplantation and patients that visited the hepatology outpatients clinic. We wanted to see if the glycosylation pattern of transferrin was different in liver disease compared to CDG. A secondary aim was to identify new CDG patients. We show that isoelectric focusing of transferrin is frequently abnormal in patients with liver disease, with 26% of the samples resembling a mild CDG type 2 pattern. Subsequent analysis of these samples with high-resolution mass spectrometry failed to identify clear CDG profiles. We confirmed that hyperfucosylation was associated with severe liver disease. Based on these experiments we suggest caution when interpreting isoelectric focusing results in patients with liver disease and suspected CDG. We did not identify glycosylation profiles that were suggestive of CDG. In conclusion, in this thesis we showed that mutations in either CCDC115 or TMEM199 resulted in a glycosylation disorder and a primarily hepatic phenotype. We proposed that this is due to defective ER-to-Golgi trafficking. Additionally, we showed that severe liver disease can hamper CDG diagnostics and provided evidence for hyperfucosylation as a distinguishing factor. Further research should focus on elucidation of the pathophysiology of both proteins, which hopefully leads to effective treatment options for patients in the near future.