Jos Jansen

3 67 TMEM199 Deficiency is characterized by elevated liverenzymes, hypercholesterolemia and abnormal glycosylation The introduction of exome sequencing in genetics has had an enormous impact on the identification of Mendelian disorders.(1) However, genetic variants can appear to be falsely positive, and a considerable percentage of cases still remains genetically unsolved.(2,3) The availability of functional data improves candidate gene selection in the case of well-known biochemical pathways.(4) However, this is more complicated for complex pathways with many unknown proteins, such as the secretory pathway, which is composed of the endoplasmic reticulum (ER), the Golgi apparatus, and secretory vesicles. Multiple monogenic disorders have been associatedwith abnormal membrane trafficking, resulting in diverse clinical phenotypes.(5) Protein glycosylation also occurs in the secretory pathway: assembly and quality control of the glycoprotein occurs in the ER and further glycan modification in the Golgi.(6) Congenital Disorders of glycosylation (CDG) form a group of > 100 monogenic diseases affecting glycosylation.(7) In a subgroup of CDG, abnormal glycosylation of serum proteins is caused by a disturbance of Golgi homeostasis. Examples are abnormal retrograde Golgi transport of glycosyltransferases (conserved oligomeric Golgi [COG] complex defects), abnormal ion transport (TMEM165 deficiency [MIM: 614727]), and abnormal Golgi pH regulation (ATP6V0A2 deficiency [MIM: 219200]).(8–12) ATP6V0A2 is a subunit of the vacuolar H+-ATPase (V-ATPase), the proton pump responsible for acidification of the secretory pathway, among other functions. (13) Identificationofmore genes involved in this emerging groupof Golgi homeostasis defects will generate biological insights into organelle homeostasis and vesicular transport. However, this is challenging given the large number of proteins involved, many of these with unknown functions. We set out to identify human Golgi-related proteins by using a bioinformatics search for human homologs of yeast proteins with a known role in ER-to-Golgi transport and/or Golgi glycosylation, including the V-ATPase. Yeast-to-human comparative genomics via a profile-based method, Position-Specific Iterated (PSI)-BLAST,(14) revealed transmembrane protein 199 (TMEM199, also known as C17orf32 [GenBank: NP_689677.1]) as a human homolog for yeast Vph2p (also known as Vma12p [GenBank: NP_012803]). Both proteins were identified as each other’s best hit in the third iteration with an E-value of 3e-08 and a reciprocal E-value of 6e-18, suggesting that both proteins are orthologs.(15) Vph2p localizes to the ER and has a confirmed role as a V-ATPase assembly factor.(16,17) Subsequently, we searched for TMEM199 mutations in individuals with genetically unsolved Golgi glycosylation defects.