SRD5A3 Is Required for Converting Polyprenol to Dolichol and Is Mutated in a Congenital Glycosylation Disorder

Congenital (genetic) disorders of glycosylation (CDG) are a rapidly growing disease family, with some 45 members reported since its first clinical description in 1980. Most of these are protein hypoglycosylation diseases, but recently three defects in lipid glycosylation have been identified. Most protein hypoglycosylation diseases are due to defects in the N-glycosylation pathway (16 diseases). The remaining ones affect the O-glycosylation pathway (8 diseases), both the N- and the O-glycosylation pathways, or other pathways (17 diseases). CDG can affect nearly all organs and systems, but there is often an important neurological component. The first-line screening for the N-glycosylation diseases is serum transferrin isoelectrofocusing (IEF), and for the O-glycosylation disorders apo CIII IEF. It has to be stressed that a normal test result does by no means exclude a CDG. In case of an abnormal result and as long as the basic defect has not been elucidated, the disease is labeled CDG-x (CDG-Ix when the transferrin IEF shows a type 1 pattern, and CDG-IIx when it shows a type 2 pattern).

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Congenital Disorders of Glycosylation

Over 100 human genetic disorders result from mutations in glycosylation-related genes. In 2013, a new glycosylation disorder was reported every 17 days. This trend will probably continue given that at least 2% of the human genome encodes glycan-biosynthesis and -recognition proteins. Established biosynthetic pathways provide many candidate genes, but finding unanticipated mutated genes will offer new insights into glycosylation. Simple glycobiomarkers can be used in narrowing the candidates identified by exome and genome sequencing, and those can be validated by glycosylation analysis of serum or cells from affected individuals. Model organisms will expand the understanding of these mutations' impact on glycosylation and pathology. Here, we highlight some recently discovered glycosylation disorders and the barriers, breakthroughs, and surprises they presented. We predict that some glycosylation disorders might occur with greater frequency than current estimates of their prevalence. Moreover, the prevalence of some disorders differs substantially between European and African Americans.

Solving glycosylation disorders: fundamental approaches reveal complicated pathways.

Alpha-dystroglycanopathies such as Walker Warburg syndrome represent an important subgroup of the muscular dystrophies that have been related to defective O-mannosylation of alpha-dystroglycan. In many patients, the underlying genetic etiology remains unsolved. Isolated muscular dystrophy has not been described in the congenital disorders of glycosylation (CDG) caused by N-linked protein glycosylation defects. Here, we present a genetic N-glycosylation disorder with muscular dystrophy in the group of CDG type I. Extensive biochemical investigations revealed a strongly reduced dolichol-phosphate-mannose (Dol-P-Man) synthase activity. Sequencing of the three DPM subunits and complementation of DPM3-deficient CHO2.38 cells showed a pathogenic p.L85S missense mutation in the strongly conserved coiled-coil domain of DPM3 that tethers catalytic DPM1 to the ER membrane. Cotransfection experiments in CHO cells showed a reduced binding capacity of DPM3(L85S) for DPM1. Investigation of the four Dol-P-Man-dependent glycosylation pathways in the ER revealed strongly reduced O-mannosylation of alpha-dystroglycan in a muscle biopsy, thereby explaining the clinical phenotype of muscular dystrophy. This mild Dol-P-Man biosynthesis defect due to DPM3 mutations is a cause for alpha-dystroglycanopathy, thereby bridging the congenital disorders of glycosylation with the dystroglycanopathies.

https://www.cell.com/ajhg/pdf/S0002-9297(09)00242-0.pdf

Deficiency of Dol-P-Man synthase subunit DPM3 bridges the congenital disorders of glycosylation with the dystroglycanopathies.

Congenital disorders of glycosylation comprise most of the nearly 70 genetic disorders known to be caused by impaired synthesis of glycoconjugates. The effects are expressed in most organ systems, and most involve the nervous system. Typical manifestations include structural abnormalities (eg, rapidly progressive cerebellar atrophy), myopathies (including congenital muscular dystrophies and limb-girdle dystrophies), strokes and stroke-like episodes, epileptic seizures, developmental delay, and demyelinating neuropathy. Patients can also have neurological symptoms associated with coagulopathies, immune dysfunction with or without infections, and cardiac, renal, or hepatic failure, which are common features of glycosylation disorders. The diagnosis of congenital disorder of glycosylation should be considered for any patient with multisystem disease and in those with more specific phenotypic features. Measurement of concentrations of selected glycoconjugates can be used to screen for many of these disorders, and molecular diagnosis is becoming more widely available in clinical practice. Disease-modifying treatments are available for only a few disorders, but all affected individuals benefit from early diagnosis and aggressive management.

/pdf/neurology-of-inherited-glycosylation-disorders-224e7spvwd.pdf

Neurology of inherited glycosylation disorders

The following study describes the discovery of a new inherited metabolic disorder, dolichol kinase (DK1) deficiency. DK1 is responsible for the final step of the de novo biosynthesis of dolichol phosphate. Dolichol phosphate is involved in several glycosylation reactions, such as N-glycosylation, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, and C- and O-mannosylation. We identified four patients who were homozygous for one of two mutations (c.295T→A [99Cys→Ser] or c.1322A→C [441Tyr→Ser]) in the corresponding hDK1 gene. The residual activity of mutant DK1 was 2%–4% when compared with control cells. The mutated alleles failed to complement the temperature-sensitive phenotype of DK1-deficient yeast cells, whereas the wild-type allele restored the normal growth phenotype. Affected patients present with a very severe clinical phenotype, with death in early infancy. Two of the patients died from dilative cardiomyopathy.

Anne Erlekotte

Papers

A defect in dolichol phosphate biosynthesis causes a new inherited disorder with death in early infancy.

Novel ALG8 mutations expand the clinical spectrum of congenital disorder of glycosylation type Ih