Congenital disorders of glycosylation presenting as epileptic encephalopathy with migrating partial seizures in infancy

A novel variant in ALG1 gene associated with congenital disorder of glycosylation: A case report and short literature review

BACKGROUND

The congenital disorder of glycosylation associated with ALG1 (ALG1-CDG) is a rare autosomal recessive disease. Due to the deficiency of β1,4 mannosyltransferase caused by pathogenic variants in ALG1 gene, the assembly and processing of glycans in the protein glycosylation pathway are impaired, resulting in a broad clinical spectrum with multi-organ involvement. To raise awareness of clinicians for its manifestations and genotype, we here reported a new patient with a novel variant in ALG1 gene and reviewed the literature to study the genotype-phenotype correlation.

METHOD

Clinical characteristics were collected, and clinical exome sequencing was used to identify the causative variants. MutationTaster, PyMol, and FoldX were used to predict the pathogenicity, changes in 3D model molecular structure of protein, and changes of free energy caused by novel variants.

RESULTS

The proband was a 13-month-old Chinese Han male characterized by epileptic seizures, psychomotor development delay, muscular hypotonia, liver and cardiac involvement. Clinical exome sequencing revealed the biallelic compound heterozygosity variants, a previously reported variant c.434G>A (p.G145N, paternal) and a novel variant c.314T>A (p.V105N, maternal). The literature review found that in severe phenotypes, the incidences of clinical manifestations were significantly higher than that in mild phenotypes, including congenital nephrotic syndrome, agammaglobulinemia, and severe hydrops. Homozygous c.773C>T was a strongly pathogenic variant associated with a severe phenotype. When heterozygous for c.773C>T, patients with another variant leading to substitution in amino acids within the strongly conserved regions (c.866A>T, c.1025A>C, c.1182C>G) may cause a more severe phenotype than those within less-conserved regions (c.434G>A, c.450C>G, c.765G>A, c.1287T>A). c.1129A>G, c.1076C>T, and c.1287T>A were more likely to be associated with a mild phenotype. The assessment of disease phenotypes requires a combination of genotype and clinical manifestations.

CONCLUSIONS

The case reported herein adds to the mutations identified in ALG1-CDG and a review of this literature expands the study of the phenotypic and genotypic spectrum of this disorder.

Congenital disorders of glycosylation and infantile epilepsy

Congenital disorders of glycosylation (CDG) are a group of rare inherited metabolic disorders caused by defects in various defects of protein or lipid glycosylation pathways. The symptoms and signs of CDG usually develop in infancy. Epilepsy is commonly observed in CDG individuals and is often a presenting symptom. These epilepsies can present across the lifespan, share features of refractoriness to antiseizure medications, and are often associated with comorbid developmental delay, psychomotor regression, intellectual disability, and behavioral problems. In this review, we discuss CDG and infantile epilepsy, focusing on an overview of clinical manifestations and electroencephalographic features. Finally, we propose a tiered approach that will permit a clinician to systematically investigate and identify CDG earlier, and furthermore, to provide genetic counseling for the family.

Neurological Consequences of Congenital Disorders of Glycosylation

The chapter is devoted to neurological aspects of congenital disorders of glycosylation (CDG). At the beginning, the various types of CDG with neurological presentation of symptoms are summarized. Then, the occurrence of various neurological constellation of abnormalities (for example: epilepsy, brain anomalies on neuroimaging, ataxia, stroke-like episodes, autistic features) in different CDG types are discussed followed by data on possible biomarkers and limited treatment options.

ILAE classification and definition of epilepsy syndromes with onset in neonates and infants: Position statement by the ILAE Task Force on Nosology and Definitions

The International League Against Epilepsy (ILAE) Task Force on Nosology and Definitions proposes a classification and definition of epilepsy syndromes in the neonate and infant with seizure onset up to 2 years of age. The incidence of epilepsy is high in this age group and epilepsy is frequently associated with significant comorbidities and mortality. The licensing of syndrome specific antiseizure medications following randomized controlled trials and the development of precision, gene-related therapies are two of the drivers defining the electroclinical phenotypes of syndromes with onset in infancy. The principal aim of this proposal, consistent with the 2017 ILAE Classification of the Epilepsies, is to support epilepsy diagnosis and emphasize the importance of classifying epilepsy in an individual both by syndrome and etiology. For each syndrome, we report epidemiology, clinical course, seizure types, electroencephalography (EEG), neuroimaging, genetics, and differential diagnosis. Syndromes are separated into self-limited syndromes, where there is likely to be spontaneous remission and developmental and epileptic encephalopathies, diseases where there is developmental impairment related to both the underlying etiology independent of epileptiform activity and the epileptic encephalopathy. The emerging class of etiology-specific epilepsy syndromes, where there is a specific etiology for the epilepsy that is associated with a clearly defined, relatively uniform, and distinct clinical phenotype in most affected individuals as well as consistent EEG, neuroimaging, and/or genetic correlates, is presented. The number of etiology-defined syndromes will continue to increase, and these newly described syndromes will in time be incorporated into this classification. The tables summarize mandatory features, cautionary alerts, and exclusionary features for the common syndromes. Guidance is given on the criteria for syndrome diagnosis in resource-limited regions where laboratory confirmation, including EEG, MRI, and genetic testing, might not be available.

ALG1-CDG Caused by Non-functional Alternative Splicing Involving a Novel Pathogenic Complex Allele

This study reports on a Mexican mestizo patient with a multi-systemic syndrome including neurological involvement and a type I serum transferrin profile. Clinical exome sequencing revealed complex alleles in ALG1, the encoding gene for the chitobiosyldiphosphodolichol beta-mannosyltransferase that participates in the formation of the dolichol-pyrophosphate-GlcNAc2Man5, a lipid-linked glycan intermediate during N-glycan synthesis. The identified complex alleles were NM_019109.5(ALG1): c.[208 + 16_208 + 19dup; 208 + 25G > T] and NM_019109.5(ALG1): c.[208 + 16_208 + 19dup; 1312C > T]. Although both alleles carried the benign variant c.208 + 16_208 + 19dup, one allele carried a known ALG1 pathogenic variant (c.1312C > T), while the other carried a new uncharacterized variant (c.208 + 25G > T) causing non-functional alternative splicing that, in conjunction with the benign variant, defines the pathogenic protein effect (p.N70S_S71ins9). The presence in the patient’s serum of the pathognomonic N-linked mannose-deprived tetrasaccharide marker for ALG1-CDG (Neu5Acα2,6Galβ1,4-GlcNAcβ1,4GlcNAc) further supported this diagnosis. This is the first report of an ALG1-CDG patient from Latin America.

ALG1-CDG: A Patient with a Mild Phenotype and Literature Review

ALG1-congenital disorder of glycosylation (ALG1-CDG) is an autosomal recessive multisystem disease. We here present a patient with a mild phenotype of ALG1-CDG. A 15-month-old female was referred with hypotonia, failure to thrive, and developmental delay. At 8 months of age, failure to thrive, feeding difficulties and developmental delay became apparent, and an epileptic seizure was observed at 11 months of age. Progressive deterioration and swallowing difficulty were observed. A brain MRI revealed a widening of the cerebrospinal fluid spaces and ventricular system, and decreased protein C, protein S and antithrombin III levels were identified. The isoelectric focusing showed a type 1 pattern. A homozygous c.1076C&#x3e;T (p.Ser359Leu) variant was found in the <i>ALG1</i> gene. CDG should be taken into consideration in patients presenting with unexplained multisystem involvement.

Metabolic Seizures

Metabolic diseases should always be considered when evaluating children presenting with seizures. This is because many metabolic disorders are potentially treatable and seizure control can be achieved when these diseases are appropriately treated. Seizures caused by underlying metabolic diseases (metabolic seizures) should be particularly considered in unexplained neonatal seizures, refractory seizures, seizures related to fasting or food intake, seizures associated with other systemic or neurologic features, parental consanguinity, and family history of epilepsy. Metabolic seizures can be caused by various amino acids metabolic disorders, disorders of energy metabolism, cofactor-related metabolic diseases, purine and pyrimidine metabolic diseases, congenital disorders of glycosylation, and lysosomal and peroxisomal disorders. Diagnosing metabolic seizures without delay is essential because the immediate initiation of appropriate therapy for many metabolic diseases can prevent or minimize complications.

ANALYSIS OF CARBOHYDRATES AND GLYCOCONJUGATES BY MATRIX-ASSISTED LASER DESORPTION/IONIZATION MASS SPECTROMETRY: AN UPDATE FOR 2015-2016

This review is the ninth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2016. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented over 30 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show no sign of deminishing. © 2020 Wiley Periodicals, Inc.

Expanding the phenotype, genotype and biochemical knowledge of ALG3-CDG

Congenital disorders of glycosylation (CDGs) are a continuously expanding group of monogenic disorders of glycoprotein and glycolipid biosynthesis that cause multisystem diseases. Individuals with ALG3-CDG frequently exhibit severe neurological involvement (epilepsy, microcephaly, and hypotonia), ocular anomalies, dysmorphic features, skeletal anomalies, and feeding difficulties. We present 10 unreported individuals diagnosed with ALG3-CDG based on molecular and biochemical testing with 11 novel variants in ALG3, bringing the total to 40 reported individuals. In addition to the typical multisystem disease seen in ALG3-CDG, we expand the symptomatology of ALG3-CDG to now include endocrine abnormalities, neural tube defects, mild aortic root dilatation, immunodeficiency, and renal anomalies. N-glycan analyses of these individuals showed combined deficiencies of hybrid glycans and glycan extension beyond Man₅ GlcNAc₂ consistent with their truncated lipid-linked precursor oligosaccharides. This spectrum of N-glycan changes is unique to ALG3-CDG. These expanded features of ALG3-CDG facilitate diagnosis and suggest that optimal management should include baseline endocrine, renal, cardiac, and immunological evaluation at the time of diagnosis and with ongoing monitoring.

ALG3-CDG: a patient with novel variants and review of the genetic and ophthalmic findings

Background

ALG3-CDG is a rare autosomal recessive disease. It is characterized by deficiency of alpha-1,3-mannosyltransferase caused by pathogenic variants in the ALG3 gene. Patients manifest with severe neurologic, cardiac, musculoskeletal and ophthalmic phenotype in combination with dysmorphic features, and almost half of them die before or during the neonatal period.

Case presentation

A 23 months-old girl presented with severe developmental delay, epilepsy, cortical atrophy, cerebellar vermis hypoplasia and ocular impairment. Facial dysmorphism, clubfeet and multiple joint contractures were observed already at birth. Transferrin isoelectric focusing revealed a type 1 pattern. Funduscopy showed hypopigmentation and optic disc pallor. Profound retinal ganglion cell loss and inner retinal layer thinning was documented on spectral-domain optical coherence tomography imaging. The presence of optic nerve hypoplasia was also supported by magnetic resonance imaging. A gene panel based next-generation sequencing and subsequent Sanger sequencing identified compound heterozygosity for two novel variants c.116del p.(Pro39Argfs*40) and c.1060 C > T p.(Arg354Cys) in ALG3.

Conclusions

Our study expands the spectrum of pathogenic variants identified in ALG3. Thirty-three variants in 43 subjects with ALG3-CDG have been reported. Literature review shows that visual impairment in ALG3-CDG is most commonly linked to optic nerve hypoplasia.

Clinical and genetic characteristics of epilepsy of infancy with migrating focal seizures in Chinese children

OBJECTIVE

Epilepsy of infancy with migrating focal seizures (EIMFS) is a rare and severe developmental epileptic encephalopathy. The aim of this study was to improve our understanding of EIMFS by using phenotype-genotype correlation.

METHODS

We recruited, performed clinical genetic testing, and summarized the clinical features and genetic characteristics in five patients with EIMFS in China.

RESULTS

The five recruited patients included 2 males and 3 females. The median age of seizure onset was 2 months (range, day 3 to 3 months). All patients exhibited the characteristics of clinically migrating focal motor (tonic or clonic) seizures. Typical migrating ictal electrical patterns were found in 1 patient; the remaining four patients presented with overlapping seizures with different areas of ictal onset in differing hemispheres. All the patients had the associated variants, including KCNT1, SCN1A, SCN2A, TBC1D24 and ALG1. All patients received two or more antiseizure medications, and 1 patient became seizure-free, 1 reported >75 % seizure reduction, 2 reported >50 % seizure reduction, and 1 patient showed no improvement. Varying degrees of psychomotor developmental delays were observed in all patients.

CONCLUSIONS

The course of EIMFS could be related to the type of gene variant present, and different genes may have specific clinical features. Larger cohorts are required to elucidate such potential phenotype-genotype correlations.

Congenital Disorders of Glycosylation from a Neurological Perspective

Most plasma proteins, cell membrane proteins and other proteins are glycoproteins with sugar chains attached to the polypeptide-glycans. Glycosylation is the main element of the post-translational transformation of most human proteins. Since glycosylation processes are necessary for many different biological processes, patients present a diverse spectrum of phenotypes and severity of symptoms. The most frequently observed neurological symptoms in congenital disorders of glycosylation (CDG) are: epilepsy, intellectual disability, myopathies, neuropathies and stroke-like episodes. Epilepsy is seen in many CDG subtypes and particularly present in the case of mutations in the following genes: ALG13, DOLK, DPAGT1, SLC35A2, ST3GAL3, PIGA, PIGW, ST3GAL5. On brain neuroimaging, atrophic changes of the cerebellum and cerebrum are frequently seen. Brain malformations particularly in the group of dystroglycanopathies are reported. Despite the growing number of CDG patients in the world and often neurological symptoms dominating in the clinical picture, the number of performed screening tests eg transferrin isoforms is systematically decreasing as broadened genetic testing is recently more favored. The aim of the review is the summary of selected neurological symptoms in CDG described in the literature in one paper. It is especially important for pediatric neurologists not experienced in the field of metabolic medicine. It may help to facilitate the diagnosis of this expanding group of disorders. Biochemically, this paper focuses on protein glycosylation abnormalities.

ALG13 participates in epileptogenesis via regulation of GABAA receptors in mouse models

ALG13 (asparagine-linked glycosylation 13) plays crucial roles in the process of N-linked glycosylation. Mutations of the ALG13 gene underlie congenital disorders of glycosylation type I (CDG-I), a rare human genetic disorder with defective glycosylation. Epilepsy is commonly observed in congenital disorders of glycosylation type I (CDG-I). In our study, we found that about 20% of adult ALG13KO knockout mice display spontaneous seizures, which were identified in a simultaneous video and intracranial EEG recording. However, the mechanisms of ALG13 by which deficiency leads to epilepsy are unknown. Whole-cell patch-clamp recordings demonstrated that ALG13KO mice show a marked decrease in gamma-aminobutyric acid A receptor (GABAAR)-mediated inhibitory synaptic transmission. Furthermore, treatment with low-dose diazepam (a positive allosteric modulator of GABAA receptors), which enhances GABAAR function, also markedly ameliorates severity of epileptic seizures in ALG13KO mice. Moreover, ALG13 may influenced the expression of GABAARα2 membrane and total protein by changing transcription level of GABAARα2. Furthermore, protein interactions between ALG13 and GABAARα2 were observed in the cortex of wild-type mice. Overall, these results reveal that ALG13 may be involved in the occurrence of epilepsy through the regulation of GABAAR function, and may provide new insight into epilepsy prevention and treatment.

BRAT1 encephalopathy: a recessive cause of epilepsy of infancy with migrating focal seizures

Epilepsy of infancy with migrating focal seizures (EIMFS), one of the most severe developmental and epileptic encephalopathy syndromes, is characterized by seizures that migrate from one hemisphere to the other. EIMFS is genetically heterogeneous with 33 genes. We report five patients with EIMFS caused by recessive BRAT1 variants, identified via next generation sequencing. Recessive pathogenic variants in BRAT1 cause the rigidity and multifocal seizure syndrome, lethal neonatal with hypertonia, microcephaly, and intractable multifocal seizures. The epileptology of BRAT1 encephalopathy has not been well described. All five patients were profoundly impaired with seizure onset in the first week of life and focal seizure migration between hemispheres. We show that BRAT1 is an important recessive cause of EIMFS with onset in the first week of life, profound impairment, and early death. Early recognition of this genetic aetiology will inform management and reproductive counselling.

Successful treatment of intractable epilepsy with ketogenic diet therapy in twins with ALG3-CDG

BACKGROUND

Congenital disorders of glycosylation (CDG) is a heterogeneous group of congenital metabolic diseases with multisystem clinical involvement. ALG3-CDG is a very rare subtype with only 24 cases reported so far.

CASE

Here, we report two siblings with dysmorphic features, growth retardation, microcephaly, intractable epilepsy, and hemangioma in the frontal, occipital and lumbosacral regions.

RESULTS

We studied two siblings by whole exome sequencing. A pathogenic variant in ALG3 (NM_005787.6: c.165C > T; p.Gly55=) that had been previously associated with congenital glycolysis defect type 1d was identified. Their intractable seizures were controlled by ketogenic diet.

CONCLUSION

Although prominent findings of growth retardation and microcephaly seen in our patients have been extensively reported before, presence of hemangioma is a novel finding that may be used as an indication for ALG3-CDG diagnosis. Our patients are the first reported cases whose intractable seizures were controlled with ketogenic diet. This report adds ketogenic diet as an option for treatment of intractable epilepsy in ALG3-CDG.

Research advances on neurite outgrowth inhibitor B receptor

Neurite outgrowth inhibitor‐B (Nogo‐B) is a membrane protein which is extensively expressed in multiple organs, especially in endothelial cells and vascular smooth muscle cells of blood vessels and belongs to the reticulon protein family. Notably, its specific receptor, Nogo‐B receptor (NgBR), encoded by NUS1, has been implicated in many crucial cellular processes, such as cholesterol trafficking, lipid metabolism, dolichol synthesis, protein N‐glycosylation, vascular remodelling, angiogenesis, tumorigenesis and neurodevelopment. In recent years, accumulating studies have demonstrated the statistically significant changes of NgBR expression levels in human diseases, including Niemann‐Pick type C disease, fatty liver, congenital disorders of glycosylation, persistent pulmonary hypertension of the newborn, invasive ductal breast carcinoma, malignant melanoma, non‐small cell lung carcinoma, paediatric epilepsy and Parkinson's disease. Besides, both the in vitro and in vivo studies have shown that NgBR overexpression or knockdown contribute to the alteration of various pathophysiological processes. Thus, there is a broad development potential in therapeutic strategies by modifying the expression levels of NgBR.

Metabolic Evaluation of Epilepsy: A Diagnostic Algorithm With Focus on Treatable Conditions

Although inborn errors of metabolism do not represent the most common cause of seizures, their early identification is of utmost importance, since many will require therapeutic measures beyond that of common anti-epileptic drugs, either in order to control seizures, or to decrease the risk of neurodegeneration. We translate the currently-known literature on metabolic etiologies of epilepsy (268 inborn errors of metabolism belonging to 21 categories, with 74 treatable errors), into a 2-tiered diagnostic algorithm, with the first-tier comprising accessible, affordable, and less invasive screening tests in urine and blood, with the potential to identify the majority of treatable conditions, while the second-tier tests are ordered based on individual clinical signs and symptoms. This resource aims to support the pediatrician, neurologist, biochemical, and clinical geneticists in early identification of treatable inborn errors of metabolism in a child with seizures, allowing for timely initiation of targeted therapy with the potential to improve outcomes.

DPAGT1 Deficiency with Encephalopathy (DPAGT1-CDG): Clinical and Genetic Description of 11 New Patients

Pathogenic mutations in DPAGT1 cause a rare type of a congenital disorder of glycosylation termed DPAGT1-CDG or, alternatively, a milder version with only myasthenia known as DPAGT1-CMS. Fourteen disease-causing mutations in 28 patients from 10 families have previously been reported to cause the systemic form, DPAGT1-CDG. We here report on another 11 patients from 8 families and add 10 new mutations. Most patients have a very severe disease course, where common findings are pronounced muscular hypotonia, intractable epilepsy, global developmental delay/intellectual disability, and early death. We also present data on three affected females that are young adults and have a somewhat milder, stable disease. Our findings expand both the molecular and clinical knowledge of previously published data but also widen the phenotypic spectrum of DPAGT1-CDG.

RFT1-CDG: Absence of Epilepsy and Deafness in Two Patients with Novel Pathogenic Variants

This report is on two novel patients with RFT1-CDG. Their phenotype is characterized by mild psychomotor disability, behavioral problems, ataxia, and mild dysmorphism. Neither of them shows signs of epilepsy, which was observed in all RFT1-CDG patients reported to date (n = 14). Also, deafness, which is often associated with this condition, was not observed in our patients. Molecular analysis of RFT1 showed biallelic missense variants including three novel ones: c.827G > A (p.G276D), c.73C > T (p.R25W), and c.208T > C (p.C70R).

Asparagine Synthetase deficiency-report of a novel mutation and review of literature

Asparagine synthetase deficiency is a rare inborn error of metabolism caused by a defect in ASNS, a gene encoding asparagine synthetase. It manifests with a severe neurological phenotype manifesting as severe developmental delay, congenital microcephaly, spasticity and refractory seizures. To date, nineteen patients from twelve unrelated families have been identified. Majority of the mutations are missense and nonsense mutations in homozygous or compound heterozygous state. We add another case from India which harbored a novel homozygous missense variation in exon 11 and compare the current case with previously reported cases.

Neurite Outgrowth Inhibitor B Receptor: A Versatile Receptor with Multiple Functions and Actions

Members of the reticulon protein family are predominantly distributed within the endoplasmic reticulum. The neurite outgrowth inhibitor (Nogo) has three subtypes, including Nogo-A (200 kDa), Nogo-B (55 kDa), and Nogo-C (25 kDa). Nogo-A and Nogo-C are potent Nogos that are predominantly expressed in the central nervous system. Nogo-B, the splice variant of reticulon-4, is expressed widely in multiple human organ systems, including the liver, lung, kidney, blood vessels, and inflammatory cells. Moreover, the Nogo-B receptor (NgBR) can interact with Nogo-B and can independently affect nervous system regeneration, the chemotaxis of endothelial cells, proliferation, and apoptosis. In recent years, it has been demonstrated that NgBR plays an important role in human pathophysiological processes, including lipid metabolism, angiogenesis, N-glycosylation, cell apoptosis, chemoresistance in human hepatocellular carcinoma, and epithelial-mesenchymal transition. The pathophysiologic effects of NgBR have garnered increased attention, and the detection and enhancement of NgBR expression may be a novel approach to monitor the development and to improve the prognosis of relevant human clinical diseases.

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.

Phenotypic and genotypic spectrum of congenital disorders of glycosylation type I and type II

BACKGROUND

Congenital disorders of glycosylation (CDG) are inborn defects of glycan metabolism. They are multisystem disorders. Analysis of transferrin isoforms is applied as a screening test for CDG type I (CDG-I) and type II (CDG-II). We performed a retrospective cohort study to determine spectrum of phenotype and genotype and prevalence of the different subtypes of CDG-I and CDG-II.

MATERIAL AND METHODS

All patients with CDG-I and CDG-II evaluated in our institution's Metabolic Genetics Clinics were included. Electronic and paper patient charts were reviewed. We set-up a high performance liquid chromatography transferrin isoelectric focusing (TIEF) method to measure transferrin isoforms in our Institution. We reviewed the literature for the rare CDG-I and CDG-II subtypes seen in our Institution.

RESULTS

Fifteen patients were included: 9 with PMM2-CDG and 6 with non-PMM2-CDG (one ALG3-CDG, one ALG9-CDG, two ALG11-CDG, one MPDU1-CDG and one ATP6V0A2-CDG). All patients with PMM2-CDG and 5 patients with non-PMM2-CDG showed abnormal TIEF suggestive of CDG-I or CDG-II pattern. In all patients, molecular diagnosis was confirmed either by single gene testing, targeted next generation sequencing for CDG genes, or by whole exome sequencing.

CONCLUSION

We report 15 new patients with CDG-I and CDG-II. Whole exome sequencing will likely identify more patients with normal TIEF and expand the phenotypic spectrum of CDG-I and CDG-II.

cis-Prenyltransferase: New Insights into Protein Glycosylation, Rubber Synthesis, and Human Diseases

cis-Prenyltransferases (cis-PTs) constitute a large family of enzymes conserved during evolution and present in all domains of life. cis-PTs catalyze consecutive condensation reactions of allylic diphosphate acceptor with isopentenyl diphosphate (IPP) in the cis (Z) configuration to generate linear polyprenyl diphosphate. The chain lengths of isoprenoid carbon skeletons vary widely from neryl pyrophosphate (C10) to natural rubber (C>10,000). The homo-dimeric bacterial enzyme, undecaprenyl diphosphate synthase (UPPS), has been structurally and mechanistically characterized in great detail and serves as a model for understanding the mode of action of eukaryotic cis-PTs. However, recent experiments have revealed that mammals, fungal, and long-chain plant cis-PTs are heteromeric enzymes composed of two distantly related subunits. In this review, the classification, function, and evolution of cis-PTs will be discussed with a special emphasis on the role of the newly described NgBR/Nus1 subunit and its plants' orthologs as essential, structural components of the cis-PTs activity.