Metabolic adaptation of human skin fibroblasts to ER stress caused by glycosylation defect in PMM2-CDG.

PMM2-CDG is the most prevalent type of congenital disorders of glycosylation (CDG). It is caused by pathogenic variants in the gene encoding phosphomannomutase 2 (PMM2), which converts mannose-6-phosphate to mannose-1-phosphate and thus activates this saccharide for further glycosylation processes. Defective glycosylation can lead to an abnormal accumulation of unfolded proteins in endoplasmic reticulum (ER) and cause its stress. The ER is a key compartment for glycosylation, and its connection and communication with mitochondria has been described extensively in literature. Their crosstalk is important for cell proliferation, calcium homeostasis, apoptosis, mitochondrial fission regulation, bioenergetics, autophagy, lipid metabolism, inflammasome formation and unfolded protein response. Therefore, in the present study we posed a question, whether defective glycosylation leads to bioenergetic disruption. Our data reveal possible chronic stress in ER and activated unfolded protein response via PERK pathway in PMM2-CDG fibroblasts. Presumably, it leads to bioenergetic reorganization and increased assembly of respiratory chain complexes into supercomplexes together with suppressed glycolysis in PMM2-CDG patient cells. These changes cause alterations in Krebs cycle, which is tightly connected to electron transport system in mitochondria. In summary, we present data showing metabolic adaptation of cells to glycosylation defect caused by various pathogenic variants in PMM2.

A new role for dolichol isoform profile in the diagnostics of CDG disorders.

Dolichol is a membrane lipid which carries monosaccharides and glycans for N-linked protein glycosylation occurring in the endoplasmic reticulum. Recently, some types of congenital disorders of glycosylation (CDG) have been described as consequences of defects in dolichol biosynthesis and metabolism, yet these types of CDG are not detectable by standard screening methods. The aim of this project was to evaluate the potential of dolichol as a biomarker of CDG. Biological material for this study consisted of urine samples from 75 controls, 6 patients with CDG and 43 patients with suspicion of CDG; samples of the frontal cortex, liver, muscle and heart tissues from 2 patients with mutation in the NUS1 gene and controls. Molecular species profiles of dolichol were analyzed by liquid chromatography combined with tandem mass spectrometry. In the control group, a significant correlation between the ratio of dolichol 18 to dolichol 19 (Dol18/Dol19) and age was found in urine. We established a reference range for Dol18/Dol19 from urine samples. The ratio of Dol18/Dol19 was significantly higher in both urine and tissue samples from patients with mutation in NUS1 in comparison to controls. Our results show a novel diagnostic option for patients with rare congenital disorders of glycosylation.

Review of SRD5A3 Disease-Causing Sequence Variants and Ocular Findings in Steroid 5α-Reductase Type 3 Congenital Disorder of Glycosylation, and a Detailed New Case.

Steroid 5α-reductase type 3 congenital disorder of glycosylation (SRD5A3-CDG) is a severe metabolic disease manifesting as muscle hypotonia, developmental delay, cerebellar ataxia and ocular symptoms; typically, nystagmus and optic disc pallor. Recently, early onset retinal dystrophy has been reported as an additional feature. In this study, we summarize ocular phenotypes and SRD5A3 variants reported to be associated with SRD5A3-CDG. We also describe in detail the ophthalmic findings in a 12-year-old Czech child harbouring a novel homozygous variant, c.436G>A, p.(Glu146Lys) in SRD5A3. The patient was reviewed for congenital nystagmus and bilateral optic neuropathy diagnosed at 13 months of age. Examination by spectral domain optical coherence tomography and fundus autofluorescence imaging showed clear signs of retinal dystrophy not recognized until our investigation. Best corrected visual acuity was decreased to 0.15 and 0.16 in the right and left eye, respectively, with a myopic refractive error of -3.0 dioptre sphere (DS) / -2.5 dioptre cylinder (DC) in the right and -3.0 DS / -3.0 DC in the left eye. The proband also had optic head nerve drusen, which have not been previously observed in this syndrome.