Congenital disorders of glycosylation (CDG) are a heterogeneous group of metabolic diseases caused by defects in glycan biosynthesis, predominantly affecting the N-glycosylation pathway. Pathogenic variants in the DDOST gene, encoding a non-catalytic subunit of the oligosaccharyltransferase (OST) complex, underlie an ultra-rare CDG subtype with only three patients reported to date, complicating diagnosis and genotype-phenotype correlations. We describe a female patient with developmental delay, hypotonia, and dysmorphic features. Genetic analyses included chromosomal microarray, FMR1 testing, and trio-based next-generation sequencing with a neurodevelopmental disorder panel. Variants were assessed using ACMG criteria, population databases, segregation, and in silico analysis. We also reviewed published DDOST-CDG cases to compare clinical and molecular findings. Compound heterozygous DDOST variants were identified: (i) a maternally inherited 413-kb deletion encompassing exons 3-11, predicted to abolish gene function, and (ii) a paternally inherited in-frame deletion (p.Lys435del), removing a highly conserved residue in the luminal C-terminal domain. Both were absent from population databases and classified as pathogenic or likely pathogenic, consistent with autosomal recessive inheritance. Metabolic testing confirmed a type I transferrin isoform pattern, supporting a congenital disorder of glycosylation. This report expands the genetic and phenotypic spectrum of DDOST-CDG by describing the first structural variant. Review of published cases highlights hypotonia and motor delay as consistent features, while other traits such as strabismus, feeding difficulties, or hepatic dysfunction appear variable. Our findings underscore the clinical heterogeneity of DDOST-CDG and the complementary role of metabolic and genomic testing.

DDOST is an important subunit of N-glycosylated oligosaccharyltransferase and is closely related to protein N-glycosylation. Some studies have reported that abnormal expression of DDOST is associated with congenital disorders of glycosylation, solid tumours and other diseases. To better understand the progress of research on DDOST in diseases, we herein provide a comprehensive review of the basic functions of DDOST, interactions molecules, DDOST-congenital disorders of glycosylation (DDOST-CDG) and solid tumours. Our review findings will lay a foundation for researchers to better understand the functions of DDOST and to investigate its specific mechanisms of action.

Congenital disorders of glycosylation (CDG) are a group of heterogeneous inherited metabolic disorders affecting posttranslational protein modification. DDOST-CDG, caused by biallelic pathogenic variants in DDOST which encodes dolichyl-diphospho-oligosaccharide-protein glycosyltransferase, a subunit of N-glycosylation oligosaccharyltransferase (OST) complex, is an ultra-rare condition that has been described in two patients only. The main clinical features in the two reported patients include profound developmental delay, failure to thrive, and hypotonia. In addition, both patients had abnormal transferrin glycosylation. Here, we report an 18-year-old male who presented with moderate developmental delay, progressive opsoclonus, myoclonus, ataxia, tremor, and dystonia. Biochemical studies by carbohydrate deficient transferrin analysis showed a type I CDG pattern. Exome sequencing identified compound heterozygous variants in DDOST: a maternally inherited variant, c.1142dupT (p.Leu381Phefs*11), and a paternally inherited variant, c.661 T > C (p.Ser221Pro). Plasma N-glycan profiling showed mildly increased small high mannose glycans including Man0-5 GlcNAc2, a pattern consistent with what was previously reported in DDOST-CDG or defects in other subunits of OST complex. Western blot analysis on patient's fibroblasts revealed decreased expression of DDOST and reduced intracellular N-glycosylation, as evident by the biomarkers ICAM-1 and LAMP2. Our study highlights the clinical variability, expands the clinical and biochemical phenotypes, and describes new genotype, which all are essential for diagnosing and managing patients with DDOST-CDG.

Congenital disorders of glycosylation (CDGs) are inherited metabolic diseases affecting protein and lipid glycosylation. DDOST-CDG is a rare, newly identified type of CDGs, with only one case reported so far. In this study, we report a Chinese patient with a homozygous pathogenic variant in DDOST (c.1187G>A) and who presented with feeding difficulty, lactose intolerance, facial dysmorphism, failure to thrive, strabismus, high myopia, astigmatism, hypotonia, developmental delay and situs inversus totalis. Serum transferrin isoelectrofocusing demonstrated defective glycosylation in our patient. This finding further identifies DDOST as a genetic cause of CDGs and expands the clinical phenotype of DDOST-CDG.