ALG13-Congenital Disorder of Glycosylation (ALG13-CDG): Updated clinical and molecular review and clinical management guidelines.

ALG13-Congenital Disorder of Glycosylation (CDG), is a rare X-linked CDG caused by pathogenic variants in ALG13 (OMIM 300776) that affects the N-linked glycosylation pathway. Affected individuals present with a predominantly neurological manifestation during infancy. Epileptic spasms are a common presenting symptom of ALG13-CDG. Other common phenotypes include developmental delay, seizures, intellectual disability, microcephaly, and hypotonia. Current management of ALG13-CDG is targeted to address patients' symptoms. To date, less than 100 individuals have been reported with ALG13-CDG. In this article, an international group of experts in CDG reviewed all reported individuals affected with ALG13-CDG and suggested diagnostic and management guidelines for ALG13-CDG. The guidelines are based on the best available data and expert opinion. Neurological symptoms dominate the phenotype of ALG13-CDG where epileptic spasm is confirmed to be the most common presenting symptom of ALG13-CDG in association with hypotonia and developmental delay. We propose that ACTH/prednisolone treatment should be trialed first, followed by vigabatrin, however ketogenic diet has been shown to have promising results in ALG13-CDG. In order to optimize medical management, we also suggest early cardiac, gastrointestinal, skeletal, and behavioral assessments in affected patients.

Manifestations of X-linked pyruvate dehydrogenase complex deficiency in female PDHA1 carriers.

BACKGROUND AND PURPOSE: Pyruvate dehydrogenase complex deficiency is in up to 90% caused by pathogenic variants in the X-linked PDHA1 gene. We aimed to investigate female relatives of index patients with PDHA1-related disease to (i) describe the prevalence of female PDHA1 carriers, (ii) determine whether they had symptoms and signs, and (iii) delineate the associated phenotype. METHODS: In a national population-based study, we identified 37 patients with pathogenic variants in PDHA1. Sanger sequencing for the presence of the pathogenic variant was performed in their mothers and female relatives. The identified female carriers were clinically assessed, and their medical records were reviewed. RESULTS: The proportion carrying a de novo variant was 86%. We identified seven female PDHA1 carriers from five families. Five of them exhibited clinical features of the disease and were previously undiagnosed; all had signs of peripheral axonal neuropathy, four presented with strokelike episodes including two with Leigh-like lesions, and three had facial stigmata. CONCLUSIONS: PDHA1-related disease is underrecognized in heterozygous female carriers. Peripheral axonal neuropathy, strokelike and Leigh-like changes, and facial dysmorphism should raise suspicion of the disorder. Genetic analysis and clinical examination of potential female carriers are important for genetic counseling and have implications for treatment.

Seizure freedom but not epilepsy surgery is associated with fewer neuropsychiatric difficulties in patients with tuberous sclerosis.

BACKGROUND: Drug-resistant epilepsy (DRE) in selected individuals with the rare tuberous sclerosis complex (TSC) may benefit from resective epilepsy surgery. Furthermore, associated neuropsychiatric disorders (TAND) are common in patients with TSC; however, long-term data on how surgery affects neuropsychiatric comorbidities are sparse. MATERIALS AND METHODS: Two retrospective approaches were used to identify children with TSC and DRE with onset at < 18 years of age. The study group (surgical) was identified through the Swedish National Epilepsy Surgery Registry (n = 17), a registry with complete national coverage since 1990 and prospective patient enrolment since 1995. The reference group (non-surgical) was identified by searching medical records retrieved from the tertiary hospital of Southern Sweden (n = 52). Eligible participants were invited to complete the validated TAND lifetime checklist. Those who did not complete the checklist, never had DRE, or were aged < 7 years old were excluded from the study. The reference group was balanced with the study group for putative confounders, in the following hierarchical order: DRE at the survey, age at seizure onset, age at follow-up, and sex. RESULTS: After the balancing procedure, both groups comprised 13 participants. The median time from epilepsy onset to the survey was 18.5 (range: 7.75-40.25) and 16.0 (7.33-33.5) years in the study and reference groups, respectively. The median time from surgery to the survey was 13 years (range: 4-22). No significant differences were found in behavioural problems, autism spectrum disorder diagnosis or symptoms, or intellectual disability between the groups, regardless of surgery. Seizure-free individuals (n = 11) performed better in social skills (p = 0.016), intellectual skills (p = 0.029), and overall TAND scores (p = 0.005) than the non-seizure-free group (n = 15). CONCLUSION: This is the first study to evaluate TAND comorbidities during the long-term follow-up after epilepsy surgery in patients with TSC. We found no evidence of the adverse effects of TAND comorbidities after tuberectomy. However, a larger study that allows for a better adjustment for confounders is needed. Following previous studies, seizure-free individuals had fewer symptoms within most TAND domains compared with the group with uncontrolled epilepsy, indicating less severe symptomatology.

Clinical characteristics of children with Borrelia-related peripheral facial palsy and utility of cerebrospinal fluid testing.

AIM: To describe the aetiology and clinical characteristics of acute peripheral facial palsy (PFP) in children and investigate the utility of the European Federation of Neurological Societies (EFNS) criteria for diagnosing Borrelia-related PFP (BPFP) based on cerebrospinal fluid (CSF) testing and the Centers for Disease Control and Prevention (CDC) criteria based on serology. METHODS: We retrospectively identified children aged <18 years diagnosed with acute PFP between 2014 and 2020. We used the EFNS criteria as the gold standard and the CDC criteria for diagnosing BPFP. RESULTS: Out of 257 children with PFP, 93 (36%) fulfilled the EFNS or CDC criteria for BPFP. We found a discrepancy between the EFNS criteria with CSF testing and the CDC without CSF testing in 27 (14%) of the 190 children with available data. Of the 37 children with PFP and ≥2 symptoms of fever, fatigue, nausea/vomiting or meningeal symptoms, 31 (84%) fulfilled the EFNS criteria for BPFP. CONCLUSION: Borrelia is a common cause of PFF in children, and its prevalence is higher in children with systemic symptoms. Also, CSF testing did not have decisive management implications in most cases. Therefore, clinical evaluation and Borrelia serology could be the initial steps in the diagnosis of PFP in children.

Overuse of EEG and ECG in children with breath-holding spells and its implication for the management of the spells.

AIM: Breath-holding spells (BHS) are common in children, but evidence-based clinical guidelines are lacking. We investigated a large population-based cohort of BHS patients, to propose a refined description of typical BHS and guidelines for its management. METHODS: In a cross-sectional retrospective study, patients diagnosed with BHS in Southern Sweden 2004-2018 were recruited. Disease characteristics and diagnostic data were collected from patient medical records. RESULTS: In total, 519 patients, mean age at diagnosis 19.8 ± 13.8 months with equal gender distribution, were included. In 48.3%, BHS had already been diagnosed after one spell. During spells, 78.0% of patients were unresponsive. For 71.5%, atonic, tonic, tonic-clonic or myoclonic seizures were reported, and 78.0% of patients had a spell lasting less than 1 min. Electroencephalography was conducted in 30.4% and Electrocardiography in 45.1%. Six children (3.8%) had a pathological electroencephalogram, four of which had concomitant epilepsy and only 0.9% of children had electrocardiogram findings suggesting pathology, none showing long QT syndrome. CONCLUSION: Children with BHS were frequently subjected to unnecessary diagnostic interventions. We characterise a typical presentation of BHS and propose a management-algorithm, which is expected to reduce unnecessary usage of electroencephalography and electrocardiography.

Neurological aspects of human glycosylation disorders.

This review presents principles of glycosylation, describes the relevant glycosylation pathways and their related disorders, and highlights some of the neurological aspects and issues that continue to challenge researchers. More than 100 rare human genetic disorders that result from deficiencies in the different glycosylation pathways are known today. Most of these disorders impact the central and/or peripheral nervous systems. Patients typically have developmental delays/intellectual disabilities, hypotonia, seizures, neuropathy, and metabolic abnormalities in multiple organ systems. Among these disorders there is great clinical diversity because all cell types differentially glycosylate proteins and lipids. The patients have hundreds of misglycosylated products, which afflict a myriad of processes, including cell signaling, cell-cell interaction, and cell migration. This vast complexity in glycan composition and function, along with the limited availability of analytic tools, has impeded the identification of key glycosylated molecules that cause pathologies. To date, few critical target proteins have been pinpointed.

Genotypic and phenotypic spectrum of infantile liver failure due to pathogenic TRMU variants.

PURPOSE: This study aimed to define the genotypic and phenotypic spectrum of reversible acute liver failure (ALF) of infancy resulting from biallelic pathogenic TRMU variants and determine the role of cysteine supplementation in its treatment. METHODS: Individuals with biallelic (likely) pathogenic variants in TRMU were studied within an international retrospective collection of de-identified patient data. RESULTS: In 62 individuals, including 30 previously unreported cases, we described 47 (likely) pathogenic TRMU variants, of which 17 were novel, and 1 intragenic deletion. Of these 62 individuals, 42 were alive at a median age of 6.8 (0.6-22) years after a median follow-up of 3.6 (0.1-22) years. The most frequent finding, occurring in all but 2 individuals, was liver involvement. ALF occurred only in the first year of life and was reported in 43 of 62 individuals; 11 of whom received liver transplantation. Loss-of-function TRMU variants were associated with poor survival. Supplementation with at least 1 cysteine source, typically N-acetylcysteine, improved survival significantly. Neurodevelopmental delay was observed in 11 individuals and persisted in 4 of the survivors, but we were unable to determine whether this was a primary or a secondary consequence of TRMU deficiency. CONCLUSION: In most patients, TRMU-associated ALF was a transient, reversible disease and cysteine supplementation improved survival.

Thrombosis risk with estrogen use for puberty induction in congenital disorders of glycosylation.

Congenital disorders of glycosylation are a group of rare related disorders causing multisystem dysfunction, including ovarian failure in females that requires early estrogen replacement. Glycosylation defects also disrupt normal synthesis of several coagulation factors, increasing thrombotic risks and complicating hormone replacement. This series describes four females with different types of CDG who developed venous thromboses while on transdermal estrogen replacement. The authors highlight the knowledge gaps around anticoagulation for this population and propose further investigations.

De novo DHDDS variants cause a neurodevelopmental and neurodegenerative disorder with myoclonus.

Subcellular membrane systems are highly enriched in dolichol, whose role in organelle homeostasis and endosomal-lysosomal pathway remains largely unclear besides being involved in protein glycosylation. DHDDS encodes for the catalytic subunit (DHDDS) of the enzyme cis-prenyltransferase (cis-PTase), involved in dolichol biosynthesis and dolichol-dependent protein glycosylation in the endoplasmic reticulum. An autosomal recessive form of retinitis pigmentosa (retinitis pigmentosa 59) has been associated with a recurrent DHDDS variant. Moreover, two recurring de novo substitutions were detected in a few cases presenting with neurodevelopmental disorder, epilepsy and movement disorder. We evaluated a large cohort of patients (n = 25) with de novo pathogenic variants in DHDDS and provided the first systematic description of the clinical features and long-term outcome of this new neurodevelopmental and neurodegenerative disorder. The functional impact of the identified variants was explored by yeast complementation system and enzymatic assay. Patients presented during infancy or childhood with a variable association of neurodevelopmental disorder, generalized epilepsy, action myoclonus/cortical tremor and ataxia. Later in the disease course, they experienced a slow neurological decline with the emergence of hyperkinetic and/or hypokinetic movement disorder, cognitive deterioration and psychiatric disturbances. Storage of lipidic material and altered lysosomes were detected in myelinated fibres and fibroblasts, suggesting a dysfunction of the lysosomal enzymatic scavenger machinery. Serum glycoprotein hypoglycosylation was not detected and, in contrast to retinitis pigmentosa and other congenital disorders of glycosylation involving dolichol metabolism, the urinary dolichol D18/D19 ratio was normal. Mapping the disease-causing variants into the protein structure revealed that most of them clustered around the active site of the DHDDS subunit. Functional studies using yeast complementation assay and in vitro activity measurements confirmed that these changes affected the catalytic activity of the cis-PTase and showed growth defect in yeast complementation system as compared with the wild-type enzyme and retinitis pigmentosa-associated protein. In conclusion, we characterized a distinctive neurodegenerative disorder due to de novo DHDDS variants, which clinically belongs to the spectrum of genetic progressive encephalopathies with myoclonus. Clinical and biochemical data from this cohort depicted a condition at the intersection of congenital disorders of glycosylation and inherited storage diseases with several features akin to of progressive myoclonus epilepsy such as neuronal ceroid lipofuscinosis and other lysosomal disorders.

Childhood tuberous sclerosis complex in southern Sweden: a paradigm shift in diagnosis and treatment.

AIM: To investigate the complete clinical spectrum of individuals with paediatric tuberous sclerosis complex in southern Sweden and explore changes over time. METHODS: In this retrospective observational study, 52 individuals aged up to 18 years at the study start were followed-up at regional hospitals and centres for habilitation from 2000 to 2020. RESULTS: Cardiac rhabdomyoma was detected prenatally/neonatally in 69.2% of the subjects born during the latest ten years of the study period. Epilepsy was diagnosed in 82.7% of subjects, and 10 (19%) were treated with everolimus, mainly (80%) for a neurological indication. Renal cysts were detected in 53%, angiomyolipomas in 47%, astrocytic hamartomas in 28% of the individuals. There was a paucity of standardized follow-up of cardiac, renal, and ophthalmological manifestations and no structured transition to adult care. CONCLUSION: Our in-depth analysis shows a clear shift towards an earlier diagnosis of tuberous sclerosis complex in the latter part of the study period, where more than 60% of cases showed evidence of this condition already in utero due to the presence of a cardiac rhabdomyoma. This allows for preventive treatment of epilepsy with vigabatrin and early intervention with everolimus for potential mitigation of other symptoms of tuberous sclerosis complex.

Novel imaging findings in pyruvate dehydrogenase complex (PDHc) deficiency-Results from a nationwide population-based study.

The vast clinical and radiological spectrum of pyruvate dehydrogenase complex (PDHc) deficiency continues to pose challenges both in diagnostics and disease monitoring. Prompt diagnosis is important to enable early initiation of ketogenic diet. The patients were recruited from an ongoing population-based study in Sweden. All patients with a genetically confirmed diagnosis who had been investigated with an MRI of the brain were included. Repeated investigations were assessed to study the evolution of the MRI changes. Sixty-two MRI investigations had been performed in 34 patients (23 females). The genetic cause was mutations in PDHA1 in 29, PDHX and DLAT in 2 each, and PDHB in 1. The lesions were prenatal developmental in 16, prenatal clastic in 18, and postnatal clastic in 15 individuals. Leigh-like lesions with predominant involvement of globus pallidus were present in 12, while leukoencephalopathy was present in 6 and stroke-like lesions in 3 individuals. A combination of prenatal developmental and clastic lesions was present in 15 individuals. In addition, one male with PDHA1 also had postnatal clastic lesions. The most common lesions found in our study were agenesis or hypoplasia of corpus callosum, ventriculomegaly, or Leigh-like lesions. Furthermore, we describe a broad spectrum of other MRI changes that include leukoencephalopathy and stroke-like lesions. We argue that a novel important clue, suggesting the possibility of PDHc deficiency on MRI scans, is the simultaneous presence of multiple lesions on MRI that have occurred during different phases of brain development.

Paediatric emergency departments should manage young febrile and afebrile infants the same if they have a fever before presenting.

AIM: Our aim was to evaluate the risk of bacterial meningitis, bacteremia, and urinary tract infection (UTI) in infants ≤60 days who presented to paediatric emergency departments (PEDs) after having fever at home. We also investigated any differences between infants who were afebrile or febrile on presentation. METHODS: This was a multicenter retrospective study of infants ≤60 days presented to four Swedish PEDs during 2014-2020 with reported fever at home. We used relative risks (RR) to compare the prevalence of UTI, bacteremia, and bacterial meningitis between the infants who were afebrile and the infants who were still febrile when they presented to the PED. RESULTS: The cohort comprised 1926 infants, and 702 (36%) were afebrile on presentation. The prevalence of UTI in the afebrile and febrile infants was 6.1% [95% confidence interval (CI) 4.5-8.2] versus 14.2% (95% CI 12.3-16.2), corresponding to an RR of 0.43 (95% CI 0.31-0.59). In infants ≤28 days, the RR for meningitis was 1.05 (95% CI 0.18-6.23) for afebrile versus febrile infants. Five times more febrile infants underwent a lumbar puncture. CONCLUSION: Infants who were afebrile on presentation underwent fewer lumbar punctures, but they had similar rates of bacterial meningitis to febrile infants. Different management approaches are not justified.

Nitrous oxide in labour predicted newborn screening total homocysteine and is a potential risk factor for infant vitamin B12 deficiency.

AIM: Risk factors for vitamin B12 deficiency in infants are not fully understood. The aim of the study was to assess predictors of total homocysteine and methylmalonic acid analysed in newborn screening dried blood spots. METHODS: In a Norwegian case control study, we analysed total homocysteine and methylmalonic acid in newborn screening dried blood spots of 86 infants clinically diagnosed with vitamin B12 deficiency during 2012-2018. Results were compared to 252 healthy infants and 400 dried blood spot controls. Medical records were reviewed, and mothers completed questionnaires. RESULTS: Both total homocysteine and methylmalonic acid were significantly higher on newborn screening dried blood spots in infants later clinically diagnosed with vitamin B12 deficiency than controls. Multiple regression analysis showed that the dose of nitrous oxide during labour was the strongest predictor for total homocysteine level in newborn screening dried blood spots for all infants, with larger effect in infants later clinically diagnosed with vitamin B12 deficiency than controls. CONCLUSION: Nitrous oxide dose during labour was a predictor for total homocysteine and may impact the interpretation of total homocysteine analysis in newborn screening. Nitrous oxide is suggested as a contributing risk factor for infants prone to develop vitamin B12 deficiency.

Age- and sex-specific prevalence of serious bacterial infections in febrile infants ≤60 days, in Sweden.

AIM: The aim of the study was to describe age- and sex-specific prevalence of serious bacterial infections (SBI: urinary tract infection, bacteraemia, meningitis) among febrile infants ≤60 days in Sweden. METHODS: This is a retrospective study in 4 Pediatric Emergency Departments from 2014 to 2017, in previously healthy, full-term infants ≤60 days with fever without a source. RESULTS: Of the 1,701 included infants, 214 (12.6%; 95% CI, 11.1-14.3) had an SBI. Urinary tract infection (UTI) was diagnosed in 196 (11.5%; 95% CI, 10.0-13.1) patients. In the ≤28 and 29-60 days age-groups, meningitis prevalence was 0.9% (95% CI, 0.3-2.0) and 0.3% (95% CI, 0.1-0.8), whereas bacteraemia prevalence was 3.2% (95% CI, 1.9-4.9) and 0.6% (95% CI, 0.2-1.3). The SBI prevalence was higher in boys 16.0% (95% CI, 13.8-18.5) than girls 8.0% (95% CI, 6.2-10.2; p<0.001), due to 2-fold higher UTI risk. The prevalence of meningitis in boys was 0.3% (95% CI, 0.1- 0.9) vs. 0.7% (95% CI, 0.2-1.6) in girls and of bacteraemia 1.8% (95% CI, 1.0-2.8) vs. 1.0% (95% CI, 0.4-2.0), respectively. CONCLUSIONS: The total SBI prevalence was 12.6%, and UTI represented the vast majority. The prevalence of bacteraemia and meningitis was low, particularly in the 29-60 days age group, without significant difference between boys and girls.

Early infantile epileptic encephalopathy due to biallelic pathogenic variants in PIGQ: Report of seven new subjects and review of the literature.

We investigated seven children from six families to expand the phenotypic spectrum associated with an early infantile epileptic encephalopathy caused by biallelic pathogenic variants in the phosphatidylinositol glycan anchor biosynthesis class Q (PIGQ) gene. The affected children were all identified by clinical or research exome sequencing. Clinical data, including EEGs and MRIs, was comprehensively reviewed and flow cytometry and transfection experiments were performed to investigate PIGQ function. Pathogenic biallelic PIGQ variants were associated with increased mortality. Epileptic seizures, axial hypotonia, developmental delay and multiple congenital anomalies were consistently observed. Seizure onset occurred between 2.5 months and 7 months of age and varied from treatable seizures to recurrent episodes of status epilepticus. Gastrointestinal issues were common and severe, two affected individuals had midgut volvulus requiring surgical correction. Cardiac anomalies including arrythmias were observed. Flow cytometry using granulocytes and fibroblasts from affected individuals showed reduced expression of glycosylphosphatidylinositol (GPI)-anchored proteins. Transfection of wildtype PIGQ cDNA into patient fibroblasts rescued this phenotype. We expand the phenotypic spectrum of PIGQ-related disease and provide the first functional evidence in human cells of defective GPI-anchoring due to pathogenic variants in PIGQ.

Predominant and novel de novo variants in 29 individuals with ALG13 deficiency: Clinical description, biomarker status, biochemical analysis, and treatment suggestions.

Asparagine-linked glycosylation 13 homolog (ALG13) encodes a nonredundant, highly conserved, X-linked uridine diphosphate (UDP)-N-acetylglucosaminyltransferase required for the synthesis of lipid linked oligosaccharide precursor and proper N-linked glycosylation. De novo variants in ALG13 underlie a form of early infantile epileptic encephalopathy known as EIEE36, but given its essential role in glycosylation, it is also considered a congenital disorder of glycosylation (CDG), ALG13-CDG. Twenty-four previously reported ALG13-CDG cases had de novo variants, but surprisingly, unlike most forms of CDG, ALG13-CDG did not show the anticipated glycosylation defects, typically detected by altered transferrin glycosylation. Structural homology modeling of two recurrent de novo variants, p.A81T and p.N107S, suggests both are likely to impact the function of ALG13. Using a corresponding ALG13-deficient yeast strain, we show that expressing yeast ALG13 with either of the highly conserved hotspot variants rescues the observed growth defect, but not its glycosylation abnormality. We present molecular and clinical data on 29 previously unreported individuals with de novo variants in ALG13. This more than doubles the number of known cases. A key finding is that a vast majority of the individuals presents with West syndrome, a feature shared with other CDG types. Among these, the initial epileptic spasms best responded to adrenocorticotropic hormone or prednisolone, while clobazam and felbamate showed promise for continued epilepsy treatment. A ketogenic diet seems to play an important role in the treatment of these individuals.

TANGO2 deficiency as a cause of neurodevelopmental delay with indirect effects on mitochondrial energy metabolism.

Exome sequencing has recently identified mutations in the gene TANGO2 (transport and Golgi organization 2) as a cause of developmental delay associated with recurrent crises involving rhabdomyolysis, cardiac arrhythmias, and metabolic derangements. The disease is not well understood, in part as the cellular function and subcellular localization of the TANGO2 protein remain unknown. Furthermore, the clinical syndrome with its heterogeneity of symptoms, signs, and laboratory findings is still being defined. Here, we describe 11 new cases of TANGO2-related disease, confirming and further expanding the previously described clinical phenotype. Patients were homozygous or compound heterozygous for previously described exonic deletions or new frameshift, splice site, and missense mutations. All patients showed developmental delay with ataxia, dysarthria, intellectual disability, or signs of spastic diplegia. Of importance, we identify two subjects (aged 12 and 17 years) who have never experienced any overt episode of the catabolism-induced metabolic crises typical for the disease. Mitochondrial complex II activity was mildly reduced in patients investigated in association with crises but normal in other patients. In one deceased patient, post-mortem autopsy revealed heterotopic neurons in the cerebral white matter, indicating a possible role for TANGO2 in neuronal migration. Furthermore, we have addressed the subcellular localization of several alternative isoforms of TANGO2, none of which were mitochondrial but instead appeared to have a primarily cytoplasmic localization. Previously described aberrations in Golgi morphology were not observed in cultured skin fibroblasts.

ALG1-CDG: Clinical and Molecular Characterization of 39 Unreported Patients.

Congenital disorders of glycosylation (CDG) arise from pathogenic mutations in over 100 genes leading to impaired protein or lipid glycosylation. ALG1 encodes a ß1,4 mannosyltransferase that catalyzes the addition of the first of nine mannose moieties to form a dolichol-lipid linked oligosaccharide intermediate required for proper N-linked glycosylation. ALG1 mutations cause a rare autosomal recessive disorder termed ALG1-CDG. To date 13 mutations in 18 patients from 14 families have been described with varying degrees of clinical severity. We identified and characterized 39 previously unreported cases of ALG1-CDG from 32 families and add 26 new mutations. Pathogenicity of each mutation was confirmed based on its inability to rescue impaired growth or hypoglycosylation of a standard biomarker in an alg1-deficient yeast strain. Using this approach we could not establish a rank order comparison of biomarker glycosylation and patient phenotype, but we identified mutations with a lethal outcome in the first two years of life. The recently identified protein-linked xeno-tetrasaccharide biomarker, NeuAc-Gal-GlcNAc2 , was seen in all 27 patients tested. Our study triples the number of known patients and expands the molecular and clinical correlates of this disorder.

Serum transferrin carrying the xeno-tetrasaccharide NeuAc-Gal-GlcNAc2 is a biomarker of ALG1-CDG.

ALG1-CDG (formerly CDG-Ik) is a subtype of congenital disorders of glycosylation (CDG) where the genetic defect disrupts the synthesis of the lipid-linked oligosaccharide precursor required for N-glycosylation. The initial step in the investigation for these disorders involves the demonstration of hypoglycosylated serum transferrin (TF). There are no specific biomarkers of this CDG subtype known to date. An LC/MS approach was used to analyze sera from patients with ALG1-CDG, PMM2-CDG, suspected CDG, and individuals with alcohol abuse. We show mass spectrometric data combined with data from enzymatic digestions that suggest the presence of a tetrasaccharide consisting of two N-acetylglucosamines, one galactose, and one sialic acid, appearing on serum TF, is a biomarker of this particular CDG subtype. This is the first time analysis of serum TF can suggest a specific CDG type I subtype and we suggest this tetrasaccharide be used in the clinic to guide the ALG1-CDG diagnostic process.