TMEM199 Deficiency Is a Disorder of Golgi Homeostasis Characterized by Elevated Aminotransferases, Alkaline Phosphatase, and Cholesterol and Abnormal Glycosylation

Abstract

Congenital disorders of glycosylation (CDGs) form a genetically and clinically heterogeneous group of diseases with aberrant protein glycosylation as a hallmark. A subgroup of CDGs can be attributed to disturbed Golgi homeostasis. However, identification of pathogenic variants is seriously complicated by the large number of proteins involved. As part of a strategy to identify human homologs of yeast proteins that are known to be involved in Golgi homeostasis, we identified uncharacterized transmembrane protein 199 (TMEM199, previously called C17orf32) as a human homolog of yeast V-ATPase assembly factor Vph2p (also known as Vma12p). Subsequently, we analyzed raw exome-sequencing data from families affected by genetically unsolved CDGs and identified four individuals with different mutations in TMEM199. The adolescent individuals presented with a mild phenotype of hepatic steatosis, elevated aminotransferases and alkaline phosphatase, and hypercholesterolemia, as well as low serum ceruloplasmin. Affected individuals showed abnormal N- and mucin-type O-glycosylation, and mass spectrometry indicated reduced incorporation of galactose and sialic acid, as seen in other Golgi homeostasis defects. Metabolic labeling of sialic acids in fibroblasts confirmed deficient Golgi glycosylation, which was restored by lentiviral transduction with wild-type TMEM199. V5-tagged TMEM199 localized with ERGIC and COPI markers in HeLa cells, and electron microscopy of a liver biopsy showed dilated organelles suggestive of the endoplasmic reticulum and Golgi apparatus. In conclusion, we have identified TMEM199 as a protein involved in Golgi homeostasis and show that TMEM199 deficiency results in a hepatic phenotype with abnormal glycosylation.

Genome-wide CRISPR screen identifies host dependency factors for influenza A virus infection

“…We focused on genes highly ranked in our screen but not previously investigated in the context of IAV infection for functional follow-up experiments. Three of our top ranked hits from the CRISPR screens, WDR7, CCDC115 and TMEM199, have been reported as putative V-type ATPase-associated cofactors 41,42,70,71 , but their functions in mammalian cells and especially in the context of viral infections are poorly understood. Here, we provide evidence that all three genes are required for efficient V-type ATPase assembly and IAV entry.…”

New mutation of the ceruloplasmin gene in the case of a neurologically asymptomatic patient with microcytic anaemia, obesity and supposed Wilson’s disease

“…An inherent reduction in CP is typical of Wilson's disease (due to mutations in the ATP7B gene, which encodes a copper-transporting P-type ATPase, with subsequent accumulation of copper in affected tissues) and of Menkes disease (due to mutations in the ATB7A gene causing a disorder of intestinal copper uptake) [10]. TMEM199-CDG is a rare genetic liver disease with abnormal glycosylation, chronically elevated serum transaminases, steatosis and low serum ceruloplasmin [12,13].…”

An Unexplained Congenital Disorder of Glycosylation-II in a Child with Neurohepatic Involvement, Hypercholesterolemia and Hypoceruloplasminemia

“…It is usually attributed to a decrease in the cholesterol binding proteins apo A and apo B. Hypercholesterolemia is an exceptional feature in CDG. It has recently been reported in CCDC115-CDG (Jansen et al 2016a) and TMEM199-CDG (Calvo et al 2008;Jansen et al 2016b), combined N-and O-glycosylation disorders of Golgi homeostasis. No explanation was provided for the hypercholesterolemia in these patients.…”