Jos Jansen

1 Introduction to hepatic congenital glycosylation disorders 19 Research question 1: can we find the pathogenic gene(s) in eight families with abnormal glycosylation In chapter 2 and chapter 3 I describe how we used comparative genomics to identify the pathogenic variants in families with CDG. Available knowledge on Golgi proteins from saccharomyces cerevisiae , including the yeast Vacuolar H+- ATPase (V-ATPase), was used. The V-ATPase is an intracellular proton pump responsible for the pH gradient of the secretory pathway. This pump was particularly interesting because in human, mutations in a gene encoding one of the core subunits ( ATP6V0A2 ) leads to abnormal glycosylation due to a Golgi homeostasis defect. This model was used with success to identify homozygous and compound heterozygous missense mutations in CCDC115 and TMEM199 in twelve patients from eight families. For TMEM199 , these mutations were incompletely covered by exome sequencing. Yeast homologs of CCDC115 and TMEM199, Vma22p and Vph2p, are ER-dedicated assembly factors of a subdomain of the V-ATPase. These two chapters cover the identification and confirmation of mutations in both genes. For the first time we show that both proteins are localized in the early secretory pathway in human. Additionally, the patient’s phenotype, mostly hepatic, and glycosylation profile are described. Research question 2: can we correlate the phenotype of CCDC115 and TMEM199 deficiency to more common diseases Patients with mutations in CCDC115 or TMEM199 all have liver injury. In chapter 2 and chapter 3 we show that both proteins are located in the ERGIC organelle. This led to the hypothesis that both proteins are involved in trafficking of new proteins from the ER to the Golgi and back. Abnormal glycosylation can be hypothesized to be caused by an imbalance in Golgi homeostasis. However, the effect of disturbed ER-to-Golgi trafficking on the origin of steatosis remains unclear. In addition to CCDC115 and TMEM199 deficiency our group also identified ATP6AP1 mutations in patients with similar symptoms. ATP6AP1 is also a V-ATPase assembly factor. In chapter 4 we wrote a narrative on the possible pathomechanism of V-ATPase assembly factor deficiencies. We hypothesized a model where disturbed trafficking via COPI vesicles results in hepatic steatosis.