Congenital disorders of glycosylation (CDG) are a group of hereditary metabolic diseases characterized by abnormal glycosylation of proteins and lipids. Often, multisystem disorders with central nervous system involvement and a large variety of clinical symptoms occur. The main characteristics are developmental delay, seizures, and ataxia. In this paper we report the clinical and biochemical characteristics of a 5-year-old girl with a defective galactosylation of N-glycans, resulting in developmental delay, muscular hypotonia, epileptic seizures, inverted nipples, and visual impairment. Next generation sequencing revealed a de novo mutation (c.797G > T, p.G266V) in the X-chromosomal gene SLC35A2 (solute carrier family 35, UDP-galactose transporter, member A2; MIM 300896). While this mutation was found heterozygous, random X-inactivation of the normal allele will lead to loss of normal SLC35A2 activity in respective cells. The functional relevance of the mutation was demonstrated by complementation of UGT-deficient MDCK-RCA(r) and CHO-Lec8 cells by normal UGT-expression construct but not by the mutant version. The effect of dietary galactose supplementation on glycosylation was investigated, showing a nearly complete normalization of transferrin glycosylation.

Pathogenic de novo variants in the X-linked gene SLC35A2 encoding the major Golgi-localized UDP-galactose transporter required for proper protein and lipid glycosylation cause a rare type of congenital disorder of glycosylation known as SLC35A2congenital disorders of glycosylation (CDG; formerly CDG-IIm). To date, 29 unique de novo NG ET AL. | 909

Background: Knowledge regarding UDP-N-acetylglucosamine transporter (NGT; SLC35A3) is incomplete due to the lack of NGT-deficient model cell lines. Results: The siRNA approach showed that NGT silencing reduces branching of complex N-glycans and keratan sulfate synthesis. Conclusion: NGT function may be coupled to the specific glycosylation pathway(s) of particular macromolecules. Significance: Our results add to the understanding of glycosylation, one of the basic posttranslational modifications.

a b s t r a c t UDP-galactose transporter (UGT; SLC35A2) and UDP-N-acetylglucosamine transporter (NGT; SLC35A3) are evolutionarily related. We hypothesize that their role in glycosylation may be coupled through heterologous complex formation. Coimmunoprecipitation analysis and FLIM-FRET measurements performed on living cells showed that NGT and UGT form complexes when overexpressed in MDCK-RCA r cells. We also postulate that the interaction of NGT and UGT may explain the dual localization of UGT2. For the first time we demonstrated in vivo homodimerization of the NGT nucleotide sugar transporter. In conclusion, we suggest that NGT and UGT function in glycosylation is combined via their mutual interaction. Structured summary of protein interactions:NGT physically interacts with UGT2 by anti tag coimmunoprecipitation (View Interaction: 1, 2) NGT physically interacts with UGT1 by anti tag coimmunoprecipitation (View interaction) UGT2 physically interacts with NGT by fluorescent resonance energy transfer (View interaction) NGT physically interacts with NGT by fluorescent resonance energy transfer (View interaction) UGT1 physically interacts with UGT2 by anti tag coimmunoprecipitation (View interaction) UGT1 physically interacts with NGT by fluorescent resonance energy transfer (View interaction)