Conclusion
To our knowledge, this is the first description of abnormal glycosylation of lipids in the TMEM165 defect and the first report of successful dietary treatment in TMEM165 deficiency. We recommend the use of oral d-galactose therapy in TMEM165-CDG.
Objective
Galactose has been shown to be beneficial in other CDG types with abnormal galactosylation. The aim of this study was to characterize the effects of galactose supplementation on Golgi glycosylation in TMEM165-depleted HEK293 cells, as well as in 2 patients with TMEM165-CDG and in their cultured skin fibroblast cells. Design and setting: Glycosylation was assessed by mass spectrometry, western blot analysis, and transferrin isoelectrofocusing. Patients and interventions: Both unrelated patients with TMEM165-CDG with the same deep intronic homozygous mutation (c.792+182G>A) were allocated to receive d-galactose in a daily dose of 1 g/kg.
Results
We analyzed N-linked glycans and glycolipids in knockout TMEM165 HEK293 cells, revealing severe hypogalactosylation and GalNAc transfer defects. Although these defects were completely corrected by the addition of Mn2+, we demonstrated that the observed N-glycosylation defect could also be overcome by galactose supplementation. We then demonstrated that oral galactose supplementation in patients with TMEM165-deficient CDG improved biochemical and clinical parameters, including a substantial increase in the negatively charged transferrin isoforms, and a decrease in hypogalactosylated total N-glycan structures, endocrine function, and coagulation parameters.