Jos Jansen

150 Appendices English summary Congenital Disorders of Glycosylation (CDG) are a group of inborn errors of metabolism that affect glycosylation. This group comprises over 100 different monogenic defects and most inherit in an autosomal recessive manner. Classically, patients present at early infancy with a multisystemic phenotype, almost always involving psychomotor functioning and dysmorphic features. As our ability to analyze a patients glycosylation profile increases due to advances in analytical methods, novel CDG are discovered regularly. In this thesis we described identification of two CDG subtypes in patients with a primarily hepatic phenotype. Additionally, we analyzed the glycosylation profile of patients with severe liver disease to see the effects of hepatic injury on glycosylation. In chapter 2 and 3 we described the discovery of CCDC115- and TMEM199- deficiency as two new CDG. We identified eight families with a similar hepatic phenotype and mild type 2 glycosylation defects. Regular workup, including whole exome sequencing, failed to identify a clear pathogenic variant. Based on previous knowledge that defects in subunits of the intracellular proton pump (the V-ATPase) cause a CDG, we set out to identify human orthologs of yeast proteins associated with V-ATPase and we identified CCDC115 and TMEM199 as orthologs of V-ATPase assembly factors. Both proteins have an unknown function in human. We provided evidence that pathogenic variants in both genes are causative for the patients phenotype. Additionally, by using immunofluorescence studies, we showed that both proteins are located in the ER-to-Golgi region in HeLa cells. Patients shared an overlapping phenotype with elevated serum transaminases, elevated alkaline phosphatase, low ceruloplasmin and hypercholesterolemia. Additionally, hepatosplenomegaly and steatosis were present. Severity of disease ranged from mildly elevated liverenzymes to liver failure warranting liver transplantation. Chapter 4 is a narrative review on the possible roles and function of both proteins. First, we hypothesized that CCDC115 and TMEM199 are assembly factors of the human V-ATPase. We propose that they work together to stabilize part of the V-ATPase V0-domain and possibly travel with the V0 domain to the Golgi apparatus.