Hypothesis: lobe A (COG1–4)-CDG causes a more severe phenotype than lobe B (COG5–8)-CDG

Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation

The Golgi is the central organelle of the secretory pathway and it houses the majority of the glycosylation machinery, which includes glycosylation enzymes and sugar transporters. Correct compartmentalization of the glycosylation machinery is achieved by retrograde vesicular trafficking as the secretory cargo moves forward by cisternal maturation. The vesicular trafficking machinery which includes vesicular coats, small GTPases, tethers and SNAREs, play a major role in coordinating the Golgi trafficking thereby achieving Golgi homeostasis. Glycosylation is a template-independent process, so its fidelity heavily relies on appropriate localization of the glycosylation machinery and Golgi homeostasis. Mutations in the glycosylation enzymes, sugar transporters, Golgi ion channels and several vesicle tethering factors cause congenital disorders of glycosylation (CDG) which encompass a group of multisystem disorders with varying severities. Here, we focus on the Golgi vesicle tethering and fusion machinery, namely, multisubunit tethering complexes and SNAREs and their role in Golgi trafficking and glycosylation. This review is a comprehensive summary of all the identified CDG causing mutations of the Golgi trafficking machinery in humans.

Liver Involvement in Congenital Disorders of Glycosylation: A Systematic Review

An ever-increasing number of disturbances in glycosylation have been described to underlie certain unexplained liver diseases presenting either almost isolated or in a multi-organ context. We aimed to update previous literature screenings which had identified up to 23 forms of congenital disorders of glycosylation (CDG) with associated liver disease. We conducted a comprehensive literature search of three scientific electronic databases looking at articles published during the last 20 years (January 2000-October 2020). Eligible studies were case reports/series reporting liver involvement in CDG patients. Our systematic review led us to point out 41 forms of CDG where the liver is primarily affected (n = 7) or variably involved in a multisystem disease with mandatory neurological abnormalities (n = 34). Herein we summarize individual clinical and laboratory presentation characteristics of these 41 CDG and outline their main presentation and diagnostic cornerstones with the aid of two synoptic tables. Dietary supplementation strategies have hitherto been investigated only in seven of these CDG types with liver disease, with a wide range of results. In conclusion, the systematic review recognized a liver involvement in a somewhat larger number of CDG variants corresponding to about 30% of the total of CDG so far reported, and it is likely that the number may increase further. This information could assist in an earlier correct diagnosis and a possibly proper management of these disorders.

Genetic analysis and prenatal diagnosis in a Chinese with growth retardation, abnormal liver function, and microcephaly

Background

Congenital disorders of glycosylation (CDG) are a genetically heterogeneous group of disorders caused by defects in the synthesis and processing of glycoproteins. COG6‐CDG is a kind of disorder caused by conserved oligomeric golgi complex 6 (COG6) deficiency. To date, only 19 patients with COG6‐CDG have been reported.

Methods

We report a girl in a Chinese family with developmental delay, growth retardation, microcephaly, abnormal liver function, and hypohidrosis. Trio whole‐exome sequencing was performed for this patient and her parents, and the variants identified were validated by Sanger sequencing. Prenatal diagnosis was done for this family during a subsequent pregnancy. The literature review on these patients was performed by reviewing articles published in English and Chinese.

Results

Genetic sequencing identified two novel heterozygous mutations: c.428G>T (p.S143I) and c.1843C>T (p.Q615X) in the COG6 gene, inherited from her healthy parents, respectively. A total of 11 different mutations in COG6 have been reported previously, and mutations potentially affecting splicing are the most common. The main clinical features included development delay, facial dysmorphism, growth retardation, skin abnormalities (hypohidrosis), microcephaly, abnormal brain structure, liver involvement, and recurrent infections.

Conclusion

Our work broadens the mutation spectrum of COG6 gene and states the importance of whole‐exome sequencing in facilitating the definitive diagnosis of this disorder and prenatal diagnosis in a subsequent pregnancy.

Proteoglycan synthesis in conserved oligomeric Golgi subunit deficient HEK293T cells is affected differently, depending on the lacking subunit

The Conserved Oligomeric Golgi (COG) complex is an eight subunit protein complex associated with Golgi membranes. Genetic defects affecting individual COG subunits cause congenital disorders of glycosylation (CDGs), due to mislocalization of Golgi proteins involved in glycosylation mechanisms. While the resulting defects in N-and O-glycosylation have been extensively studied, no corresponding study of proteoglycan (PG) synthesis has been undertaken. We here show that glycosaminoglycan (GAG) modification of PGs is significantly reduced, regardless which COG subunit that is missing in HEK293T cells. Least reduction was observed for cells lacking COG1 and COG8 subunits, that bridge the A and B lobes of the complex. Lack of these subunits did not reduce GAG chain lengths of secreted PGs, which was reduced in cells lacking any other subunit (COG2-7). COG3 knock out (KO) cells had particularly reduced ability to polymerize GAG chains. For cell-associated GAGs, the mutant cell lines, except COG4 and COG7 KO, displayed longer GAG chains than wild-type cells, indicating that COG subunits play a role in cellular turnover of PGs. In light of the important roles PGs play in animal development, the effects KO of individual COG subunits have on GAG synthesis could explain the variable severity of COG associated CDGs.

COG1-congenital disorders of glycosylation: Milder presentation and review

Congenital disorders of glycosylation (CDG) are a heterogeneous group of genetic defects in glycoprotein and glycolipid glycan synthesis and attachment. A CDG subgroup are defects in the conserved oligomeric Golgi complex encoded by eight genes, COG1-COG8. Pathogenic variants in all genes except the COG3 gene have been reported. COG1-CDG has been reported in five patients. We report a male with neonatal seizures, dysmorphism, hepatitis and a type 2 serum transferrin isoelectrofocusing. Exome sequencing identified a homozygous COG1 variant (NM_018714.3: c.2665dup: p.[Arg889Profs*12]), which has been reported previously in one patient. We review the reported patients.

Disorder of sex development associated with a novel homozygous nonsense mutation in COG6 expands the phenotypic spectrum of COG6-CDG

Congenital disorders of glycosylation (CDG) are an expanding group of metabolic disorders that result from abnormal protein glycosylation. A special subgroup of CDG type II comprises defects in the Conserved Oligomeric Golgi Complex (COG). In order to further delineate the genotypic and phenotypic spectrum of COG complex defect, we describe a novel variant of COG6 gene found in homozygosity in a Moroccan patient with severe presentation of COG6-CDG (OMIM #614576). We compared the phenotype of our patient with other previously reported COG6-CDG cases. Common features in COG6-CDG are facial dysmorphism, growth retardation, microcephaly, developmental disability, liver or gastrointestinal disease, recurrent infections, hypohidrosis/hyperthermia. In addition to these phenotypic features, our patient exhibited a disorder of sexual differentiation, which has rarely been reported in COG6-CDG. We hypothesize that the severe COG6 gene mutation interferes with glycosylation of a disintegrin and metalloprotease family members, inhibiting the correct gonadal distal tip cells migration, fundamental for the genitalia morphogenesis. This report broadens the genetic and phenotypic spectrum of COG6-CDG and provides further supportive evidence that COG6-CDG can present as a disorder of sexual differentiation.

Golgi inCOGnito: From vesicle tethering to human disease

The Conserved Oligomeric Golgi (COG) complex, a multi-subunit vesicle tethering complex of the CATCHR (Complexes Associated with Tethering Containing Helical Rods) family, controls several aspects of cellular homeostasis by orchestrating retrograde vesicle traffic within the Golgi. The COG complex interacts with all key players regulating intra-Golgi trafficking, namely SNAREs, SNARE-interacting proteins, Rabs, coiled-coil tethers, and vesicular coats. In cells, COG deficiencies result in the accumulation of non-tethered COG-complex dependent (CCD) vesicles, dramatic morphological and functional abnormalities of the Golgi and endosomes, severe defects in N- and O- glycosylation, Golgi retrograde trafficking, sorting and protein secretion. In humans, COG mutations lead to severe multi-systemic diseases known as COG-Congenital Disorders of Glycosylation (COG-CDG). In this report, we review the current knowledge of the COG complex and analyze COG-related trafficking and glycosylation defects in COG-CDG patients.

Congenital disorders of glycosylation: Still "hot" in 2020

BACKGROUND

Congenital disorders of glycosylation (CDG) are inherited metabolic diseases caused by defects in the genes important for the process of protein and lipid glycosylation. With the ever growing number of the known subtypes and discoveries regarding the disease mechanisms and therapy development, it remains a very active field of study.

SCOPE OF REVIEW

This review brings an update on the CDG-related research since 2017, describing the novel gene defects, pathobiomechanisms, biomarkers and the patients' phenotypes. We also summarize the clinical guidelines for the most prevalent disorders and the current therapeutical options for the treatable CDG.

MAJOR CONCLUSIONS

In the majority of the 23 new CDG, neurological involvement is associated with other organ disease. Increasingly, different aspects of cellular metabolism (e.g., autophagy) are found to be perturbed in multiple CDG.

GENERAL SIGNIFICANCE

This work highlights the recent trends in the CDG field and comprehensively overviews the up-to-date clinical recommendations.

Neonatal presentation of COG6-CDG with prominent skin phenotype

Many of the genetic childhood disorders leading to death in the perinatal period follow autosomal recessive inheritance and bear specific challenges for genetic counseling and prenatal diagnostics. Often, affected children die before a genetic diagnosis can be established, thereby precluding targeted carrier testing in parents and prenatal or preimplantation genetic diagnosis in further pregnancies. The clinical phenotype of congenital disorders of glycosylation (CDG) is very heterogeneous and ranges from relatively mild symptoms to severe multisystem dysfunction and even a fatal course. A very rare subtype, COG6-CDG, is caused by deficiency of subunit 6 of the conserved oligomeric Golgi complex and is usually characterized by growth retardation, developmental delay, microcephaly, liver and gastrointestinal disease, joint contractures and episodic fever. It has been proposed that a distinctive feature of COG6-CDG can be ectodermal signs such as hypohidrosis/hyperthermia, hyperkeratosis and tooth anomalies. In a Greek family, who had lost two children in the neonatal period, with prominent skin features initially resembling restrictive dermopathy, severe arthrogryposis, respiratory insufficiency and a rapid fatal course trio whole-exome sequencing revealed the homozygous nonsense mutation c.511C>T, p.(Arg171*) in the COG6 gene. Skin manifestations such as dry skin and hyperkeratosis have been reported in only five out of the 21 reported COG6-CDG cases so far, including two patients with the c.511C>T variant in COG6 but with milder ectodermal symptoms. Our case adds to the phenotypic spectrum of COG6-CDG with prominent ectodermal manifestations at birth and underlines the importance of considering CDG among the possible causes for congenital syndromic genodermatoses.

Hypothesis: determining phenotypic specificity facilitates understanding of pathophysiology in rare genetic disorders

In the rapidly growing group of rare genetic disorders, data scarcity demands an intelligible use of available data, in order to improve understanding of underlying pathophysiology. We hypothesize, based on the principle that clinical similarities may be indicative of shared pathophysiology, that determining phenotypic specificity could provide unsuspected insights in pathophysiology of rare genetic disorders. We explored our hypothesis by studying subunit deficiencies of the conserved oligomeric Golgi (COG) complex, a subgroup of congenital disorders of glycosylation (CDG). In this systematic data assessment, all 45 reported patients with COG-CDG were included. The vocabulary of the Human Phenotype Ontology was used to annotate all phenotypic features and to assess occurrence in other genetic disorders. Gene occurrence ratios were calculated by dividing the frequency in the patient cohort over the number of associated genes, according to the Human Phenotype Ontology. Prioritisation based on phenotypic specificity was highly informative and captured phenotypic features commonly associated with glycosylation disorders. Moreover, it captured features not seen in any other glycosylation disorder, among which episodic fever, likely reflecting underappreciated other cellular functions of the COG complex. Interestingly, the COG complex was recently implicated in the autophagy pathway, as are more than half of the genes underlying disorders that present with episodic fever. This suggests that whereas many phenotypic features in these patients are caused by disrupted glycosylation, episodic fever might be caused by disrupted autophagy. Thus, we here demonstrate support for our hypothesis that determining phenotypic specificity could facilitate understanding of pathophysiology in rare genetic disorders.

De Novo Heterozygous POLR2A Variants Cause a Neurodevelopmental Syndrome with Profound Infantile-Onset Hypotonia

The RNA polymerase II complex (pol II) is responsible for transcription of all ∼21,000 human protein-encoding genes. Here, we describe sixteen individuals harboring de novo heterozygous variants in POLR2A, encoding RPB1, the largest subunit of pol II. An iterative approach combining structural evaluation and mass spectrometry analyses, the use of S. cerevisiae as a model system, and the assessment of cell viability in HeLa cells allowed us to classify eleven variants as probably disease-causing and four variants as possibly disease-causing. The significance of one variant remains unresolved. By quantification of phenotypic severity, we could distinguish mild and severe phenotypic consequences of the disease-causing variants. Missense variants expected to exert only mild structural effects led to a malfunctioning pol II enzyme, thereby inducing a dominant-negative effect on gene transcription. Intriguingly, individuals carrying these variants presented with a severe phenotype dominated by profound infantile-onset hypotonia and developmental delay. Conversely, individuals carrying variants expected to result in complete loss of function, thus reduced levels of functional pol II from the normal allele, exhibited the mildest phenotypes. We conclude that subtle variants that are central in functionally important domains of POLR2A cause a neurodevelopmental syndrome characterized by profound infantile-onset hypotonia and developmental delay through a dominant-negative effect on pol-II-mediated transcription of DNA.

Novel compound heterozygous COG5 mutations in a Chinese male patient with severe clinical symptoms and type IIi congenital disorder of glycosylation: A case report

In the current study, one case of COG5-CDG involving a Chinese male patient with severe neurological symptoms, who had previously been misdiagnosed with congenital gyrus malformation, is described. A clinical investigation was performed and targeted next-generation sequencing (NGS) was used to identify COG5 variants in the patient and his family. PCR and Sanger sequencing were performed for the verification of NGS results. The patient showed severe central and peripheral neurological symptoms, while only mild symptoms were reported in a previous reported case, in which different mutations were involved. The reported patient carried the frameshift mutation c.330delT (p.V111Lfs*22), and a missense mutation c.2324 C>T (p.P775L) in the COG5 gene. The c.330delT (p.V111Lfs*22) variant is a novel mutation, while c.2324 C>T (p.P775L) has previously been reported. Inheriting one variant from each of his parents, the current case report furthers the understanding of genotype-phenotype correlations in COG5-CDG.

Membrane detachment is not essential for COG complex function

COG is a multisubunit vesicle tethering complex in the Golgi. We demonstrate that both COG subcomplexes can be permanently attached to Golgi membranes and that major COG functions do not require cycling between the membrane and cytosol.

The conserved oligomeric Golgi (COG) complex is a vesicle tether of the “complexes associated with tethering containing helical rods” family, which functions on the cytoplasmic side of Golgi. It is currently unknown whether COG function, or function of any multisubunit vesicular tether, depends on cycling between the membrane and cytosol. Therefore, we permanently anchored key subunits of COG subcomplexes (COG4, COG7, and COG8) to Golgi membranes using transmembrane protein TMEM115 (TMEM-COG). All TMEM-COG subunits tested were Golgi localized, integrated into the COG complex, and stabilized membrane association of endogenous subunits. Interestingly, TMEM-COG4 and TMEM-COG7 equally rescued COG function in organization of Golgi markers, glycosylation, and abundance of COG-sensitive proteins. In contrast, TMEM-COG8 was not as effective, indicating that N-terminal attachment of COG8 interfered with overall COG structure and function, and none of the TMEM-COG subunits rescued the abnormal Golgi architecture caused by COG knockout. Collectively, these data indicate that both subcomplexes of the COG complex can perform most of COG function when permanently attached to membranes and that the cytosolic pool of COG is not completely essential to COG function.