The skeletal manifestations of the congenital disorders of glycosylation

Clinical and biochemical footprints of inherited metabolic diseases. XIII. Respiratory manifestations

At any age, respiratory manifestations are a major cause of increased morbidity and mortality of inherited metabolic diseases (IMDs). Type and severity are extremely variable, this depending on the type of the underlying disorder. Symptoms and signs originating from upper or lower airways and/or thoracic wall and/or respiratory muscles involvement can occur either at presentation or in the late clinical course. Acute respiratory symptoms can trigger metabolic decompensation which, in turn, makes airway symptoms worse, creating a vicious circle. We have identified 181 IMDs associated with various types of respiratory symptoms which were classified into seven groups according to the type of clinical manifestations affecting the respiratory system: (i) respiratory failure, (ii) restrictive lung disease, (iii) interstitial lung disease, (iv) lower airway disease, (v) upper airway obstruction, (vi) apnea, and (vii) other. We also provided a list of investigations to be performed based on the respiratory phenotypes and indicated the therapeutic strategies currently available for IMD-associated airway disease. This represents the thirteenth issue in a series of educational summaries providing a comprehensive and updated list of metabolic differential diagnoses according to system involvement.

Clinical, biochemical and genetic characteristics of MOGS-CDG: a rare congenital disorder of glycosylation

PURPOSE

To summarise the clinical, molecular and biochemical phenotype of mannosyl-oligosaccharide glucosidase-related congenital disorders of glycosylation (MOGS-CDG), which presents with variable clinical manifestations, and to analyse which clinical biochemical assay consistently supports diagnosis in individuals with bi-allelic variants in MOGS.

METHODS

Phenotypic characterisation was performed through an international and multicentre collaboration. Genetic testing was done by exome sequencing and targeted arrays. Biochemical assays on serum and urine were performed to delineate the biochemical signature of MOGS-CDG.

RESULTS

Clinical phenotyping revealed heterogeneity in MOGS-CDG, including neurological, immunological and skeletal phenotypes. Bi-allelic variants in MOGS were identified in 12 individuals from 11 families. The severity in each organ system was variable, without definite genotype correlation. Urine oligosaccharide analysis was consistently abnormal for all affected probands, whereas other biochemical analyses such as serum transferrin analysis was not consistently abnormal.

CONCLUSION

The clinical phenotype of MOGS-CDG includes multisystemic involvement with variable severity. Molecular analysis, combined with biochemical testing, is important for diagnosis. In MOGS-CDG, urine oligosaccharide analysis via matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry can be used as a reliable biochemical test for screening and confirmation of disease.

Component of oligomeric Golgi complex 1 deficiency leads to hypoglycemia: a case report and literature review

Background

Congenital disorders of glycosylation (CDG) are a group of metabolic diseases with clinical and genetic heterogeneity, and CDG-IIg is one of the rare reported types of CDG. The aim of this study is to report the clinical manifestations and gene-phenotype characteristics of a rare case of CDG caused by a COG1 gene mutation and review literatures of CDG disease.

Case presentation

The patient was male, and the main clinical symptoms were developmental retardation, convulsion, strabismus, and hypoglycemia, which is rarely reported in CDG-IIg. We treated the patient with glucose infusion and he was recovered from hypoglycemia. Genetic analysis showed that the patient carried the heterozygous intron mutation c.1070 + 3A > G (splicing) in the coding region of the COG1 gene that was inherited from the mother, and the heterozygous mutation c.2492G > A (p. Arg831Gln) in exon 10 of the COG1 gene that was inherited from the father. The genes interacting with COG1 were mainly involved in the transport and composition of the Golgi. The clinical data and laboratory results from a patient diagnosed with CDG-IIg were analyzed, and the causative gene mutation was identified by high-throughput sequencing. The genes and signal pathways related to COG1 were analyzed by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses.

Conclusions

The c.2492G > A (p. Arg831Gln) mutation in exon 10 of the COG1 gene may be a potential pathogenetic variant for CDG-IIg. Because of the various manifestations of CDG in clinical practice, multidisciplinary collaboration is important for the diagnosis and treatment of this disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12887-021-02922-7.

NANS-CDG: Delineation of the Genetic, Biochemical, and Clinical Spectrum

Background: NANS-CDG is a recently described congenital disorder of glycosylation caused by biallelic genetic variants in NANS, encoding an essential enzyme in de novo sialic acid synthesis. Sialic acid at the end of glycoconjugates plays a key role in biological processes such as brain and skeletal development. Here, we present an observational cohort study to delineate the genetic, biochemical, and clinical phenotype and assess possible correlations.

Methods: Medical and laboratory records were reviewed with retrospective extraction and analysis of genetic, biochemical, and clinical data (2016–2020).

Results: Nine NANS-CDG patients (nine families, six countries) referred to the Radboudumc CDG Center of Expertise were included. Phenotyping confirmed the hallmark features including intellectual developmental disorder (IDD) (n = 9/9; 100%), facial dysmorphisms (n = 9/9; 100%), neurologic impairment (n = 9/9; 100%), short stature (n = 8/9; 89%), skeletal dysplasia (n = 8/9; 89%), and short limbs (n = 8/9; 89%). Newly identified features include ophthalmological abnormalities (n = 6/9; 67%), an abnormal septum pellucidum (n = 6/9; 67%), (progressive) cerebral atrophy and ventricular dilatation (n = 5/9; 56%), gastrointestinal dysfunction (n = 5/9; 56%), thrombocytopenia (n = 5/9; 56%), and hypo–low-density lipoprotein cholesterol (n = 4/9; 44%). Biochemically, elevated urinary excretion of N-acetylmannosamine (ManNAc) is pathognomonic, the concentrations of which show a significant correlation with clinical severity. Genotypically, eight novel NANS variants were identified. Three severely affected patients harbored identical compound heterozygous pathogenic variants, one of whom was initiated on experimental prenatal and postnatal treatment with oral sialic acid. This patient showed markedly better psychomotor development than the other two genotypically identical males.

Conclusions: ManNAc screening should be considered in all patients with IDD, short stature with short limbs, facial dysmorphisms, neurologic impairment, and an abnormal septum pellucidum +/– congenital and neurodegenerative lesions on brain imaging, to establish a precise diagnosis and contribute to prognostication. Personalized management includes accurate genetic counseling and access to proper supports and tailored care for gastrointestinal symptoms, thrombocytopenia, and epilepsy, as well as rehabilitation services for cognitive and physical impairments. Motivated by the short-term positive effects of experimental treatment with oral sialic, we have initiated this intervention with protocolized follow-up of neurologic, systemic, and growth outcomes in four patients. Research is ongoing to unravel pathophysiology and identify novel therapeutic targets.

Novel form of rhizomelic skeletal dysplasia associated with a homozygous variant in GNPNAT1

BACKGROUND

Studies exploring molecular mechanisms underlying congenital skeletal disorders have revealed novel regulators of skeletal homeostasis and shown protein glycosylation to play an important role.

OBJECTIVE

To identify the genetic cause of rhizomelic skeletal dysplasia in a consanguineous Pakistani family.

METHODS

Clinical investigations were carried out for four affected individuals in the recruited family. Whole genome sequencing (WGS) was completed using DNA from two affected and two unaffected individuals from the family. Sequencing data were processed, filtered and analysed. In silico analyses were performed to predict the effects of the candidate variant on the protein structure and function. Small interfering RNAs (siRNAs) were used to study the effect of Gnpnat1 gene knockdown in primary rat chondrocytes.

RESULTS

The patients presented with short stature due to extreme shortening of the proximal segments of the limbs. Radiographs of one individual showed hip dysplasia and severe platyspondyly. WGS data analyses identified a homozygous missense variant c.226G>A; p.(Glu76Lys) in GNPNAT1, segregating with the disease. Glucosamine 6-phosphate N-acetyltransferase, encoded by the highly conserved gene GNPNAT1, is one of the enzymes required for synthesis of uridine diphosphate N-acetylglucosamine, which participates in protein glycosylation. Knockdown of Gnpnat1 by siRNAs decreased cellular proliferation and expression of chondrocyte differentiation markers collagen type 2 and alkaline phosphatase, indicating that Gnpnat1 is important for growth plate chondrocyte proliferation and differentiation.

CONCLUSIONS

This study describes a novel severe skeletal dysplasia associated with a biallelic, variant in GNPNAT1. Our data suggest that GNPNAT1 is important for growth plate chondrogenesis.

Pulmonary Manifestations of Endocrine and Metabolic Diseases in Children

Advances in technology, methodology, and deep phenotyping are increasingly driving the understanding of the pathologic basis of disease. Improvements in patient identification and treatment are impacting survival. This is true in endocrinology and inborn errors of metabolism, where disease-modifying therapies are developing. Inherent to this evolution is the increasing awareness of the respiratory manifestations of these rare diseases. This review updates clinicians, stratifying diseases spirometerically; pulmonary hypertension and diseases with a predisposition to recurrent pulmonary infection are discussed. This division is artificial; many diseases have multiple pathologic effects on respiration. This review does not cover the impact of obesity.

Experiences of parents with children with congenital disorders of glycosylation: What can we learn from them?

BACKGROUND

Congenital disorders of glycosylation are a group of rare metabolic, genetic diseases that cause severe cognitive and physical impairments. Owing to the rarity of this condition, the experiences of these parents are poorly understood.

OBJECTIVE

This study aimed to explore parents' experiences of caring for a child or young adult with congenital disorders of glycosylation.

METHODS

Semi-structured interviews were conducted with 33 parents from 11 countries by teleconference to assess their experience of parenting children with the stated condition. Through thematic analysis, combining deductive and inductive strategies, we identified common themes across the interviews regarding the initial stage of diagnosis and the current experience.

RESULTS

Parents reported many difficulties in managing the disease and its consequences, mainly related to the condition's management and the child's well-being, focusing less on their own burden and distress. Receiving and adapting to the diagnosis was described as a strenuous and highly emotional process, with parents facing a lack of medical knowledge and difficulty in accessing competent health providers. Regarding the experience of parenting a child with congenital disorders of glycosylation, participants' concerns focused on the child and were related to promoting the child's development and autonomy and finding adequate health and educational support. Participants identified several support strategies. Relevant patient associations provided critical informational, instrumental, and social support.

CONCLUSIONS

Results point to parents' need to receive support from informed healthcare and educational providers that recognize their unique challenges and multiple needs.

Biallelic TMEM251 variants in patients with severe skeletal dysplasia and extreme short stature

Skeletal dysplasias are a heterogeneous group of disorders ranging from mild to lethal skeletal defects. We investigated two unrelated families with individuals presenting with a severe skeletal disorder. In family NMD02, affected individuals had a dysostosis multiplex-like skeletal dysplasia and severe short stature (<-8.5 SD). They manifested increasingly coarse facial features, protruding abdomens, and progressive skeletal changes, reminiscent of mucopolysaccharidosis. The patients gradually lost mobility and the two oldest affected individuals died in their twenties. The affected child in family ID01 had coarse facial features and severe skeletal dysplasia with clinical features similar to mucopolysaccharidosis. She had short stature, craniosynostosis, kyphoscoliosis, and hip-joint subluxation. She died at the age of 5 years. Whole-exome sequencing identified two homozygous variants c.133C>T; p.(Arg45Trp) and c.215dupA; p.(Tyr72Ter), respectively, in the two families, affecting an evolutionary conserved gene TMEM251 (NM_001098621.1). Immunofluorescence and confocal studies using human osteosarcoma cells indicated that TMEM251 is localized to the Golgi complex. However, p.Arg45Trp mutant TMEM251 protein was targeted less efficiently and the localization was punctate. Tmem251 knockdown by small interfering RNA induced dedifferentiation of rat primary chondrocytes. Our work implicates TMEM251 in the pathogenesis of a novel disorder and suggests its potential function in chondrocyte differentiation.

Respiratory complications of metabolic disease in the paediatric population: A review of presentation, diagnosis and therapeutic options

Inborn errors of metabolism (IEMs) whilst individually rare, as a group constitute a field which is increasingly demands on pulmonologists. With the advent of new therapies such as enzyme replacement and gene therapy, early diagnosis and treatment of these conditions can impact on long term outcome, making their timely recognition and appropriate investigation increasingly important. Conversely, with improved treatment, survival of these patients is increasing, with the emergence of previously unknown respiratory phenotypes. It is thus important that pulmonologists are aware of and appropriately monitor and manage these complications. This review aims to highlight the respiratory manifestations which can occur. It isdivided into conditions resulting primarily in obstructive airway and lung disease, restrictive lung disease such as interstitial lung disease or pulmonary alveolar proteinosis and pulmonary hypertension, whilst acknowledging that some diseases have the potential to cause all three. The review focuses on general phenotypes of IEMs, their known respiratory complications and the basic metabolic investigations which should be performed where an IEM is suspected.

Keeping an eye on congenital disorders of O-glycosylation: A systematic literature review

Congenital disorders of glycosylation (CDG) are a rapidly growing family comprising >100 genetic diseases. Some 25 CDG are pure O-glycosylation defects. Even among this CDG subgroup, phenotypic diversity is broad, ranging from mild to severe poly-organ/system dysfunction. Ophthalmic manifestations are present in 60% of these CDG. The ophthalmic manifestations in N-glycosylation-deficient patients have been described elsewhere. The present review documents the spectrum and incidence of eye disorders in patients with pure O-glycosylation defects with the aim of assisting diagnosis and management and promoting research.

International clinical guidelines for the management of phosphomannomutase 2-congenital disorders of glycosylation: Diagnosis, treatment and follow up

Phosphomannomutase 2 (PMM2-CDG) is the most common congenital disorder of N-glycosylation and is caused by a deficient PMM2 activity. The clinical presentation and the onset of PMM2-CDG vary among affected individuals ranging from a severe antenatal presentation with multisystem involvement to mild adulthood presentation limited to minor neurological involvement. Management of affected patients requires a multidisciplinary approach. In this article, a systematic review of the literature on PMM2-CDG was conducted by a group of international experts in different aspects of CDG. Our managment guidelines were initiated based on the available evidence-based data and experts' opinions. This guideline mainly addresses the clinical evaluation of each system/organ involved in PMM2-CDG, and the recommended management approach. It is the first systematic review of current practices in PMM2-CDG and the first guidelines aiming at establishing a practical approach to the recognition, diagnosis and management of PMM2-CDG patients.

Glycans modify mesenchymal stem cell differentiation to impact on the function of resulting osteoblasts

Glycans are inherently heterogeneous, yet glycosylation is essential in eukaryotes, and glycans show characteristic cell type-dependent distributions. By using an immortalized human mesenchymal stromal cell (MSC) line model, we show that both N- and O-glycan processing in the Golgi functionally modulates early steps of osteogenic differentiation. We found that inhibiting O-glycan processing in the Golgi prior to the start of osteogenesis inhibited the mineralization capacity of the formed osteoblasts 3 weeks later. In contrast, inhibition of N-glycan processing in MSCs altered differentiation to enhance the mineralization capacity of the osteoblasts. The effect of N-glycans on MSC differentiation was mediated by the phosphoinositide-3-kinase (PI3K)/Akt pathway owing to reduced Akt phosphorylation. Interestingly, by inhibiting PI3K during the first 2 days of osteogenesis, we were able to phenocopy the effect of inhibiting N-glycan processing. Thus, glycan processing provides another layer of regulation that can modulate the functional outcome of differentiation. Glycan processing can thereby offer a novel set of targets for many therapeutically attractive processes.

Summary: Both N- and O-glycan processing modulate MSC differentiation early during osteogenesis to influence mineral formation. Inhibition of N-glycan processing increases mineralization.

A human case of SLC35A3-related skeletal dysplasia

Researchers have identified a subset of Holstein having a range of skeletal deformities, including vertebral anomalies, referred to as complex vertebral malformation due to mutations in the SLC35A3 gene. Here, we report the first case in humans of SLC35A3-related vertebral anomalies. Our patient had prenatally diagnosed anomalous vertebrae, including butterfly, and hemivertebrae throughout the spine, as well as cleft palate, micrognathia, patent foramen ovale, patent ductus arteriosus, posterior embryotoxon, short limbs, camptodactyly, talipes valgus, rocker bottom feet, and facial dysmorphism including proptosis, nevus flammeus, and a cupped left ear. Clinical exome sequencing revealed a novel missense homozygous mutation in SLC35A3. Follow-up biochemical analysis confirmed abnormal protein glycosylation, consistent with a defective Golgi UDP-GlcNAc transporter, validating the mutations. Congenital disorders of glycosylation, including SLC35A3-CDG, can present as a wide phenotypic spectrum, including skeletal dysplasia. Previously reported patients with SLC35A3-CDG have been described with syndromic autism, epilepsy, and arthrogryposis.

A Population-Based Study on Congenital Disorders of Protein N- and Combined with O-Glycosylation Experience in Clinical and Genetic Diagnosis

OBJECTIVE

To describe the clinical, biochemical, and genetic features of patients with congenital disorders of glycosylation (CDG) identified in Spain during the last 20 years.

STUDY DESIGN

Patients were selected among those presenting with multisystem disease of unknown etiology. The isoforms of transferrin and of ApoC3 and dolichols were analyzed in serum; phosphomannomutase and mannosephosphate isomerase activities were measured in fibroblasts. Conventional or massive parallel sequencing (customized panel or Illumina Clinical-Exome Sequencing TruSight One Gene Panel) was used to identify genes and mutations.

RESULTS

Ninety-seven patients were diagnosed with 18 different CDG. Eighty-nine patients had a type 1 transferrin profile; 8 patients had a type 2 transferrin profile, with 6 of them showing an alteration in the ApoC3 isoform profile. A total of 75% of the patients had PMM2-CDG presenting with a heterogeneous mutational spectrum. The remaining patients showed mutations in any of the following genes: MPI, PGM1, GFPT1, SRD5A3, DOLK, DPGAT1, ALG1, ALG6, RFT1, SSR4, B4GALT1, DPM1, COG6, COG7, COG8, ATP6V0A2, and CCDC115.

CONCLUSION

Based on literature and on this population-based study of CDG, a comprehensive scheme including reported clinical signs of CDG is offered, which will hopefully reduce the timeframe from clinical suspicion to genetic confirmation. The different defects of CDG identified in Spain have contributed to expand the knowledge of CDG worldwide. A predominance of PMM2 deficiency was detected, with 5 novel PMM2 mutations being described.

Immunological aspects of congenital disorders of glycosylation (CDG): a review

Congenital disorders of glycosylation (CDG) are a rapidly growing family of genetic diseases comprising more than 85 known distinct disorders. They show a great phenotypic variability ranging from multi-organ/system to mono-organ/system involvement with very mild to extremely severe expression. Immunological dysfunction has a significant impact on the phenotype in a minority of CDG. CDG with major immunological involvement are ALG12-CDG, MAGT1-CDG, MOGS-CDG, SLC35C1-CDG and PGM3-CDG. This review discusses the variety of immunological abnormalities reported in human CDG. Understanding the immunological aspects of CDG may contribute to a better management/treatment of these pathologies and possibly of more common diseases, such as inflammatory diseases.

Classical Galactosaemia and CDG, the N-Glycosylation Interface. A Review

Classical galactosaemia is a rare disorder of carbohydrate metabolism caused by galactose-1-phosphate uridyltransferase (GALT) deficiency (EC 2.7.7.12). The disease is life threatening if left untreated in neonates and the only available treatment option is a long-term galactose restricted diet. While this is lifesaving in the neonate, complications persist in treated individuals, and the cause of these, despite early initiation of treatment, and shared GALT genotypes remain poorly understood. Systemic abnormal glycosylation has been proposed to contribute substantially to the ongoing pathophysiology. The gross N-glycosylation assembly defects observed in the untreated neonate correct over time with treatment. However, N-glycosylation processing defects persist in treated children and adults.Congenital disorders of glycosylation (CDG) are a large group of over 100 inherited disorders affecting largely N- and O-glycosylation.In this review, we compare the clinical features observed in galactosaemia with a number of predominant CDG conditions.We also summarize the N-glycosylation abnormalities, which we have described in galactosaemia adult and paediatric patients, using an automated high-throughput HILIC-UPLC analysis of galactose incorporation into serum IgG with analysis of the corresponding N-glycan gene expression patterns and the affected pathways.

A mouse model of a human congenital disorder of glycosylation caused by loss of PMM2

The most common congenital disorder of glycosylation (CDG), phosphomannomutase 2 (PMM2)-CDG, is caused by mutations in PMM2 that limit availability of mannose precursors required for protein N-glycosylation. The disorder has no therapy and there are no models to test new treatments. We generated compound heterozygous mice with the R137H and F115L mutations in Pmm2 that correspond to the most prevalent alleles found in patients with PMM2-CDG. Many Pmm2^R137H/F115L mice died prenatally, while survivors had significantly stunted growth. These animals and cells derived from them showed protein glycosylation deficiencies similar to those found in patients with PMM2-CDG. Growth-related glycoproteins insulin-like growth factor (IGF) 1, IGF binding protein-3 and acid-labile subunit, along with antithrombin III, were all deficient in Pmm2^R137H/F115L mice, but their levels in heterozygous mice were comparable to wild-type (WT) littermates. These imbalances, resulting from defective glycosylation, are likely the cause of the stunted growth seen both in our model and in PMM2-CDG patients. Both Pmm2^R137H/F115L mouse and PMM2-CDG patient-derived fibroblasts displayed reductions in PMM activity, guanosine diphosphate mannose, lipid-linked oligosaccharide precursor and total cellular protein glycosylation, along with hypoglycosylation of a new endogenous biomarker, glycoprotein 130 (gp130). Over-expression of WT-PMM2 in patient-derived fibroblasts rescued all these defects, showing that restoration of mutant PMM2 activity is a viable therapeutic strategy. This functional mouse model of PMM2-CDG, in vitro assays and identification of the novel gp130 biomarker all shed light on the human disease, and moreover, provide the essential tools to test potential therapeutics for this untreatable disease.

MECHANISMS IN ENDOCRINOLOGY: Novel genetic causes of short stature

The fast technological development, particularly single nucleotide polymorphism array, array-comparative genomic hybridization, and whole exome sequencing, has led to the discovery of many novel genetic causes of growth failure. In this review we discuss a selection of these, according to a diagnostic classification centred on the epiphyseal growth plate. We successively discuss disorders in hormone signalling, paracrine factors, matrix molecules, intracellular pathways, and fundamental cellular processes, followed by chromosomal aberrations including copy number variants (CNVs) and imprinting disorders associated with short stature. Many novel causes of GH deficiency (GHD) as part of combined pituitary hormone deficiency have been uncovered. The most frequent genetic causes of isolated GHD are GH1 and GHRHR defects, but several novel causes have recently been found, such as GHSR, RNPC3, and IFT172 mutations. Besides well-defined causes of GH insensitivity (GHR, STAT5B, IGFALS, IGF1 defects), disorders of NFκB signalling, STAT3 and IGF2 have recently been discovered. Heterozygous IGF1R defects are a relatively frequent cause of prenatal and postnatal growth retardation. TRHA mutations cause a syndromic form of short stature with elevated T3/T4 ratio. Disorders of signalling of various paracrine factors (FGFs, BMPs, WNTs, PTHrP/IHH, and CNP/NPR2) or genetic defects affecting cartilage extracellular matrix usually cause disproportionate short stature. Heterozygous NPR2 or SHOX defects may be found in ∼3% of short children, and also rasopathies (e.g., Noonan syndrome) can be found in children without clear syndromic appearance. Numerous other syndromes associated with short stature are caused by genetic defects in fundamental cellular processes, chromosomal abnormalities, CNVs, and imprinting disorders.

ALG3-CDG: Report of two siblings with antenatal features carrying homozygous p.Gly96Arg mutation

Congenital disorders of glycosylation (CDG) are a group of inborn errors of metabolism presenting with heterogeneous multisystemic clinical manifestations. To date, more than 60 different types of CDG have been reported. ALG3-CDG is very rare, with only nine patients described so far. We report two affected siblings presenting prenatally with skeletal abnormalities associated with dysmorphic features, cerebellar vermis hypoplasia, corpus callosum agenesis, hepatic fibrosis and poor prognosis. This is the first detailed report of an affected fetus including clinical, radiographic and pathological findings. The patients showed some clinical features previously unreported in ALG3-CDG, such as bone dysplasia, cataract, corneal opacities, and pons hypoplasia. Both patients were homozygous for the previously unreported p.Gly96Arg mutation of the ALG3 gene. One patient showed chondrodysplasia punctata (CDP), which has not been previously reported in CDG. An exhaustive genetic and metabolic assessment, performed in order to rule out other possible causes of CDP, showed abnormally raised levels of anti-nuclear antibodies in the mother who, nevertheless, did not show any clinical sign of autoimmune disease during a 7 years follow-up. We speculate that the observed CDP may be explained by the maternal anti-nuclear antibodies; alternatively, a possible link to the underlying metabolic disorder cannot be ruled out. In conclusion, we report the clinical, pathological, biochemical and molecular characterization of two further patients affected by ALG3-CDG, expanding the phenotypic spectrum of this very rare disease.

A novel phenotype in N-glycosylation disorders: Gillessen-Kaesbach-Nishimura skeletal dysplasia due to pathogenic variants in ALG9

A rare lethal autosomal recessive syndrome with skeletal dysplasia, polycystic kidneys and multiple malformations was first described by Gillessen-Kaesbach et al and subsequently by Nishimura et al. The skeletal features uniformly comprise a round pelvis, mesomelic shortening of the upper limbs and defective ossification of the cervical spine. We studied two unrelated families including three affected fetuses with Gillessen-Kaesbach-Nishimura syndrome using whole-exome and Sanger sequencing, comparative genome hybridization and homozygosity mapping. All affected patients were shown to have a novel homozygous splice variant NM_024740.2: c.1173+2T>A in the ALG9 gene, encoding alpha-1,2-mannosyltransferase, involved in the formation of the lipid-linked oligosaccharide precursor of N-glycosylation. RNA analysis demonstrated skipping of exon 10, leading to shorter RNA. Mass spectrometric analysis showed an increase in monoglycosylated transferrin as compared with control tissues, confirming that this is a congenital disorder of glycosylation (CDG). Only three liveborn children with ALG9-CDG have been previously reported, all with missense variants. All three suffered from intellectual disability, muscular hypotonia, microcephaly and renal cysts, but none had skeletal dysplasia. Our study shows that some pathogenic variants in ALG9 can present as a lethal skeletal dysplasia with visceral malformations as the most severe phenotype. The skeletal features overlap with that previously reported for ALG3- and ALG12-CDG, suggesting that this subset of glycosylation disorders constitutes a new diagnostic group of skeletal dysplasias.

29 French adult patients with PMM2-congenital disorder of glycosylation: outcome of the classical pediatric phenotype and depiction of a late-onset phenotype

PMM2-CDG (formerly known as CDG Ia) a deficiency in phosphomannomutase, is the most frequent congenital disorder of glycosylation. The phenotype encompasses a wide range of neurological and non-neurological manifestations comprising cerebellar atrophy and intellectual deficiency. The phenotype of the disorder is well characterized in children but the long term course of the disease is unknown and the phenotype of late onset forms has not been comprehensively described. We thus retrospectively collected the clinical, biological and radiological data of 29 French PMM2-CDG patients aged 15 years or more with a proven molecular diagnosis (16 females and 13 males). In addition, thirteen of these patients were reexamined at the time of the study to obtain detailed information. 27 of the 29 patients had a typical PMM2-CDG phenotype, with infantile hypotonia, strabismus, developmental delay followed by intellectual deficiency, epilepsy, retinitis pigmentosa and/or visceral manifestations. The main health problems for these patients as teenagers and in adulthood were primary ovarian insufficiency, growth retardation, coagulation anomalies and thrombotic events, skeletal deformities and osteopenia/osteoporosis, retinitis pigmentosa, as well as peripheral neuropathy. Three patients had never walked and three lost their ability to walk. The two remaining patients had a late-onset phenotype unreported to date. All patients (n = 29) had stable cerebellar atrophy. Our findings are in line with those of previous adult PMM2-CDG cohorts and points to the need for a multidisciplinary approach to the follow up of PMM2-CDG patients to prevent late complications. Additionally, our findings add weight to the view that PMM2-CDG may be diagnosed in teenage/adult patients with cerebellar atrophy, even in the absence of intellectual deficiency or non-neurological involvement.

Congenital disorders of glycosylation and intellectual disability

The congenital disorders of glycosylation (CDG) are a rapidly growing group of inborn errors of metabolism that result from defects in the synthesis of glycans. Glycosylation is a major post-translational protein modification and an estimated 2% of the human genome encodes proteins for glycosylation. The molecular bases for the current 60 disorders, affecting approximately 800 individuals, have been identified, many in the last 5 years. CDG should be considered in any multi-system syndrome or single tissue disorder not explained by the identification of another disorder. The initial clinical presentation varies significantly among individuals, even between affected siblings. However, two thirds of the known CDGs are associated with intellectual disabilities and most affected individuals need support services throughout their lives. Additional disorders of glycosylation are likely to be characterized over time.

Human immunodeficiency virus type 1 enhancer-binding protein 3 is essential for the expression of asparagine-linked glycosylation 2 in the regulation of osteoblast and chondrocyte differentiation

Human immunodeficiency virus type 1 enhancer-binding protein 3 (Hivep3) suppresses osteoblast differentiation by inducing proteasomal degradation of the osteogenesis master regulator Runx2. In this study, we tested the possibility of cooperation of Hivep1, Hivep2, and Hivep3 in osteoblast and/or chondrocyte differentiation. Microarray analyses with ST-2 bone stroma cells demonstrated that expression of any known osteochondrogenesis-related genes was not commonly affected by the three Hivep siRNAs. Only Hivep3 siRNA promoted osteoblast differentiation in ST-2 cells, whereas all three siRNAs cooperatively suppressed differentiation in ATDC5 chondrocytes. We further used microarray analysis to identify genes commonly down-regulated in both MC3T3-E1 osteoblasts and ST-2 cells upon knockdown of Hivep3 and identified asparagine-linked glycosylation 2 (Alg2), which encodes a mannosyltransferase residing on the endoplasmic reticulum. The Hivep3 siRNA-mediated promotion of osteoblast differentiation was negated by forced Alg2 expression. Alg2 suppressed osteoblast differentiation and bone formation in cultured calvarial bone. Alg2 was immunoprecipitated with Runx2, whereas the combined transfection of Runx2 and Alg2 interfered with Runx2 nuclear localization, which resulted in suppression of Runx2 activity. Chondrocyte differentiation was promoted by Hivep3 overexpression, in concert with increased expression of Creb3l2, whose gene product is the endoplasmic reticulum stress transducer crucial for chondrogenesis. Alg2 silencing suppressed Creb3l2 expression and chondrogenesis of ATDC5 cells, whereas infection of Alg2-expressing virus promoted chondrocyte maturation in cultured cartilage rudiments. Thus, Alg2, as a downstream mediator of Hivep3, suppresses osteogenesis, whereas it promotes chondrogenesis. To our knowledge, this study is the first to link a mannosyltransferase gene to osteochondrogenesis.

Diagnosis of ALG12-CDG by exome sequencing in a case of severe skeletal dysplasia

Congenital Disorder of Glycosylation type Ig (ALG12-CDG) is part of a group of autosomal recessive conditions caused by deficiency of proteins involved in the assembly of dolichol-oligosaccharides used for protein N-glycosylation. In ALG12-CDG, the enzyme affected is encoded by the ALG12 gene. Affected individuals present clinically with neurodevelopmental delay, growth retardation, immune deficiency, male genital hypoplasia, and cardiomyopathy. A total of six individuals have been reported in the literature. Here, we present an infant with rhizomelic short stature, talipes equinovarus, platyspondyly, and joint dislocations. The infant had marked under-ossification of the pubic bones. Exome sequencing was performed and two deletions, each resulting in a frameshift, were found in ALG12. A review of the literature revealed two infants with ALG12-CDG and a severe skeletal dysplasia, including under-ossification of cervical vertebrae, pubic bones, and knees; in addition to talipes equinovarus and rhizomelic short stature. The phenotype of the individual we describe resembles pseudodiastrophic dysplasia and we discuss similarities and differences between ALG12-CDG and pseudodiastrophic dysplasia. The differential diagnosis in selected undiagnosed skeletal dysplasias should include CDGs.

Human diseases associated with form and function of the Golgi complex

The Golgi complex lies at the heart of the secretory pathway and is responsible for modifying proteins and lipids, as well as sorting newly synthesized molecules to their correct destination. As a consequence of these important roles, any changes in its proteome can negatively affect its function and in turn lead to disease. Recently, a number of proteins have been identified, which when either depleted or mutated, result in diseases that affect various organ systems. Here we describe how these proteins have been linked to the Golgi complex, and specifically how they affect either the morphology, membrane traffic or glycosylation ability of this organelle.

A zebrafish model of PMM2-CDG reveals altered neurogenesis and a substrate-accumulation mechanism for N-linked glycosylation deficiency

PMM2-CDG patients have phosphomannomutase (Pmm2) deficiency, with developmental and N-linked glycosylation defects attributed to depletion of mannose-1-phosphate and downstream lipid-linked oligosaccharides (LLOs). This, the first PMM2-CDG zebrafish model, shows, unexpectedly, that accumulation of the Pmm2 substrate mannose-6-phosphate explains LLO deficiency.

Congenital disorder of glycosylation (PMM2-CDG) results from mutations in pmm2, which encodes the phosphomannomutase (Pmm) that converts mannose-6-phosphate (M6P) to mannose-1-phosphate (M1P). Patients have wide-spectrum clinical abnormalities associated with impaired protein N-glycosylation. Although it has been widely proposed that Pmm2 deficiency depletes M1P, a precursor of GDP-mannose, and consequently suppresses lipid-linked oligosaccharide (LLO) levels needed for N-glycosylation, these deficiencies have not been demonstrated in patients or any animal model. Here we report a morpholino-based PMM2-CDG model in zebrafish. Morphant embryos had developmental abnormalities consistent with PMM2-CDG patients, including craniofacial defects and impaired motility associated with altered motor neurogenesis within the spinal cord. Significantly, global N-linked glycosylation and LLO levels were reduced in pmm2 morphants. Although M1P and GDP-mannose were below reliable detection/quantification limits, Pmm2 depletion unexpectedly caused accumulation of M6P, shown earlier to promote LLO cleavage in vitro. In pmm2 morphants, the free glycan by-products of LLO cleavage increased nearly twofold. Suppression of the M6P-synthesizing enzyme mannose phosphate isomerase within the pmm2 background normalized M6P levels and certain aspects of the craniofacial phenotype and abrogated pmm2-dependent LLO cleavage. In summary, we report the first zebrafish model of PMM2-CDG and uncover novel cellular insights not possible with other systems, including an M6P accumulation mechanism for underglycosylation.

Bone Dysplasia as a Key Feature in Three Patients with a Novel Congenital Disorder of Glycosylation (CDG) Type II Due to a Deep Intronic Splice Mutation in TMEM165

Three patients belonging to two families presented with a psychomotor-dysmorphism syndrome including postnatal growth deficiency and major spondylo-, epi-, and metaphyseal skeletal involvement. Other features were muscular hypotrophy, fat excess, partial growth hormone deficiency, and, in two of the three patients, episodes of unexplained fever. Additional investigations showed mild to moderate increases of serum transaminases (particularly of aspartate transaminase (AST)), creatine kinase (CK), and lactate dehydrogenase (LDH), as well as decreased coagulation factors VIII, IX, XI, and protein C. Diagnostic work-up revealed a type 2 serum transferrin isoelectrofocusing (IEF) pattern and a cathodal shift on apolipoprotein C-III IEF pointing to a combined N- and O-glycosylation defect. Known glycosylation disorders with similar N-glycan structures lacking galactose and sialic acid were excluded. Through a combination of homozygosity mapping and expression profiling, a deep intronic homozygous mutation (c.792 + 182G>A) was found in TMEM165 (TPARL) in the three patients. TMEM165 is a gene of unknown function, possibly involved in Golgi proton/calcium transport. Here we present a detailed clinical description of the three patients with this mutation. The TMEM165 deficiency represents a novel type of CDG (TMEM165-CDG). This disorder enlarges the group of CDG caused by deficiencies in proteins that are not specifically involved in glycosylation but that have functions in the organization and homeostasis of the intracellular compartments and the secretory pathway, like COG-CDG and ATP6V0A2-CDG.

Congenital disorders of glycosylation: sweet news

PURPOSE OF REVIEW

Congenital disorders of glycosylation (CDG) have grown enormously since the discovery of the first protein glycosylation defect in 1980, presenting with a broad clinical spectrum. Expansion in number and complexity of the CDG group has even necessitated a new nomenclature. By 2011, the CDG group includes lipid glycosylation disorders and other related processes and almost 50 distinct disorders.

RECENT FINDINGS

Current research has not only expanded the spectrum of CDG types, but has also given novel insight into those previously described. The discovery of genetic defects in the conserved oligomeric Golgi complex, affecting protein glycosylation and processing through the secretory pathway, raised the concept of 'secondary' glycosylation disorders. The number of lipid glycosylation disorders, linking lipid synthesis to CDG, that were previously regarded as rare, is also increasing rapidly. In other areas of research, the bridge between muscular dystrophies and metabolic disorders is being further reinforced with the discovery of additional defects in the DPM-CDG subgroup, a CDG characterized by significant muscle involvement.

SUMMARY

It is of great importance that clinicians stay up-to-date on the field of CDG and consider it in their differential diagnosis of unknown syndromal presentations. Nevertheless, many advances have yet to be made, including information on the natural course of CDG. The lack of treatment for nearly all CDG types is striking, and the field must continue to push for innovative therapies. Clinicians and researchers must work together to describe the natural course and, most importantly, collaborate to find new therapies.

Quantification of human serum procollagen C-proteinase enhancer (hsPCPE) glycopattern

BACKGROUND

Procollagen C-proteinase enhancer 1 (PCPE1), a glycoprotein secreted from differentiating osteoblast, enhances the rate-limiting step of collagen type I fibrillar formation. It is expressed and secreted by cells that produce collagen type I and has the potential to be a marker for bone pathologies.

METHODS

We developed an assay to quantify PCPE glycopattern based on isoelectric focusing (IEF) and detection with a bio-imaging camera (coefficient of variation within and between assays, 15% and 20%, respectively).

RESULTS

PCPE was quantified in 39 serum samples from healthy subjects (17 females and 22 males). The concentration in the serum was 305(274) ng/ml, median(IQR). The level of the PCPE isoforms and their relative distribution were altered in patients with bone disorders.

CONCLUSIONS

The data generated by our system, support our hypothesis that combined data on PCPE concentration and isoforms may be useful for the diagnosis and follow-up of bone diseases. Further research, on larger cohorts of both normal subjects and patients, must be done.

Golgi glycosylation and human inherited diseases

The Golgi factory receives custom glycosylates and dispatches its cargo to the correct cellular locations. The process requires importing donor substrates, moving the cargo, and recycling machinery. Correctly glycosylated cargo reflects the Golgi's quality and efficiency. Genetic disorders in the specific equipment (enzymes), donors (nucleotide sugar transporters), or equipment recycling/reorganization components (COG, SEC, golgins) can all affect glycosylation. Dozens of human glycosylation disorders fit these categories. Many other genes, with or without familiar names, well-annotated pedigrees, or likely homologies will join the ranks of glycosylation disorders. Their broad and unpredictable case-by-case phenotypes cross the traditional medical specialty boundaries. The gene functions in patients may be elusive, but their common feature may include altered glycosylation that provide clues to Golgi function. This article focuses on a group of human disorders that affect protein or lipid glycosylation. Readers may find it useful to generalize some of these patient-based, translational observations to their own research.

Metabolic manipulation of glycosylation disorders in humans and animal models

In the last decade, over 40 inherited human glycosylation disorders were identified. Most patients have hypomorphic, rather than null alleles. The phenotypic spectrum is broad and most of the disorders affect embryonic and early post-natal development; a few appear in adult life. Some deficiencies can be treated with simple dietary sugar (monosaccharide) supplements. Here we focus on four glycosylation disorders that have been treated with supplements in patients or in model systems, primarily the mouse. Surprisingly, small differences in the amount of exogenous sugar have a major impact on the diseases in specific cells or organs while others are unaffected. The underlying mechanisms are mostly unknown, but changes in the contributions of the de novo, salvage and dietary pathways may contribute to the beneficial outcome. Clearly, the metabolic chart is not flat; all arrows are not equally robust at all points of time and space. This metabolic perspective may help explain some of these observations and guide the development of other vertebrate models of glycosylation disorders that can respond to dietary manipulation.

The clinical spectrum of phosphomannomutase 2 deficiency (CDG-Ia)

Congenital disorders of glycosylation are a clinically and genetically heterogeneous group of disorders resulting from abnormal glycosylation of various glycoconjugates. The first description of congenital disorders of glycosylation was published in the early 80s and once screening tests for glycosylation disorders (CDGs) became readily available, CDG-Ia became the most frequently diagnosed CDG subtype. CDG-Ia is pan-ethnic and the spectrum of the clinical manifestations is still evolving: it spans from severe hydrops fetalis and fetal loss to a (nearly) normal phenotype. However, the most common presentation in infancy is of a multisystem disorder with central nervous system involvement.

Congenital disorder of glycosylation type Id (CDG Id): phenotypic, biochemical and molecular characterization of a new patient

Congenital disorders of glycosylation (CDG) are a family of multisystem inherited disorders caused by defects in the biosynthesis of N- or O-glycans. Among the many different subtypes of CDG, the defect of a mannosyltransferase encoded by the human ALG3 gene (chromosome 3q27) is known to cause CDG Id. Six patients with CDG Id have been described in the literature so far. We further delineate the clinical, biochemical, neuroradiological and molecular features of CDG Id by reporting an additional patient bearing a novel missense mutation in the ALG3 gene. All patients with CDG Id display a slowly progressive encephalopathy with microcephaly, severe psychomotor retardation and epileptic seizures. They also share some typical dysmorphic features but they do not present the multisystem involvement observed in other CDG syndromes or any biological marker abnormalities. Unusually marked osteopenia is a feature in some patients and may remain undiagnosed until revealed by pathological fractures. Serum transferrin screening for CDG should be extended to all patients with encephalopathy of unknown origin, even in the absence of multisystem involvement.