Diagnosis Of Congenital Disorders Of Glycosylation Research Articles

Assessing carnosinase 1 activity for diagnosing congenital disorders of glycosylation

Diagnosing Congenital Disorders of Glycosylation (CDG) is challenging due to clinical heterogeneity and the limited sensitivity of the classic serum transferrin isoelectric focusing (IEF) or capillary zone electrophoresis test. This study investigates the potential of using carnosinase 1 (CN1) activity as a diagnostic marker for CDG patients. CN1 activity was measured photometrically in serum from 81 genetically confirmed CDG patients and healthy individuals. While the IEF transferrin method detected 77 patients, four remained undetected.In healthy individuals, serum CN1 activity ranged from 0.1 to 6.4 μmol/ml/h depending on age, with mean CN1 activities up to four-fold higher than in CDG patients. CDG patients´ CN1 activities never exceeded 2,04 μmol/ml/h. Using the 25th percentile to differentiate between groups, the test performance varied by age. For children over 10 years old, the sensitivity and specificity were 96 % and 83 %, respectively. For those under 10, sensitivity and specificity dropped to 71 % and to 64 %. However, CN1 activity successfully identified three of four patients with normal IEF patterns.Although mean CN1 activity in CDG patients is significantly lower than in healthy controls, the test's reliability for classic CDG diagnosis is limited, as the diagnosis is usually made at a young age. Nevertheless, it is a simple, cost-effective assay that can complement classic tests, especially in settings with limited access to complex methods or for patients with normal transferrin patterns but suspicious for CDG.

Landmark Evaluation of Transferrin Glycoforms: Pioneering Congenital Glycosylation Disorder Diagnosis in Cuba

Glycosylation, the most significant modification of proteins, plays an essential role in cellular processes. Congenital disorders of glycosylation (CDG) are multisystemic diseases resulting from mutations affecting glycosylation steps. Diagnosis of CDG relies on biochemical assays, such as isoelectric focusing. Despite its significance, CDG cases remain unreported in Cuba due to the lack of implemented diagnostic methods. This study aimed to evaluate transferrin glycoforms for diagnosing congenital N-glycosylation defects. We conducted a cross-sectional descriptive study using serum samples from 17 patients stored at the National Center for Medical Genetics of Cuba for suspected CDG. Results revealed abnormal glycosylation patterns in 7 patients, suggesting congenital N-glycosylation defects. We conclude that the implementation of transferrin glycoform analysis could pioneer a preliminary diagnosis of CDG in Cuba.

Genetic counseling for congenital disorders of glycosylation (CDG).

Congenital disorders of glycosylation (CDGs) are a genetically and clinically diverse group of disorders that arise as a result of defects within glycosylation synthetic pathways. CDGs are caused by pathogenic variants in many different genes in the glycosylation network. With over 160 different CDG types currently identified and a vast range of severity and presentations existing within and across those types, the road to a CDG diagnosis is often lengthy and complicated. The perils of this arduous CDG diagnostic odyssey are fraught with various genetic counseling uncertainties: (1) confusion about family planning, (2) queries about inheritance, (3) managing treatment, and (4) dealing with the uncertainty of rare diseases. Thus, the role of the genetic counselor is paramount in helping affected individuals and their families navigate these genetic counseling complexities. Case examples of common genetic counseling difficulties for CDGs are outlined, providing clinical applications of what CDG presentations, diagnostic processes, and common difficulties look like. Information on the nomenclature, incidence, prevalence, diagnostic testing, treatment, and management of CDGs are also discussed to provide a comprehensive summary of CDGs for genetic counselors, and subsequently to affected individuals and their families.

Congenital Disorders of Glycosylation: What Clinicians Need to Know?

Congenital disorders of glycosylation (CDG) are a group of clinically heterogeneous disorders characterized by defects in the synthesis of glycans and their attachment to proteins and lipids. This manuscript aims to provide a classification of the clinical presentation, diagnostic methods, and treatment of CDG based on the literature review and our own experience (referral center in Poland). A diagnostic algorithm for CDG was also proposed. Isoelectric focusing (IEF) of serum transferrin (Tf) is still the method of choice for diagnosing N-glycosylation disorders associated with sialic acid deficiency. Nowadays, high-performance liquid chromatography, capillary zone electrophoresis, and mass spectrometry techniques are used, although they are not routinely available. Since next-generation sequencing became more widely available, an improvement in diagnostics has been observed, with more patients and novel CDG subtypes being reported. Early and accurate diagnosis of CDG is crucial for timely implementation of appropriate therapies and improving clinical outcomes. However, causative treatment is available only for few CDG types.

Prevalence of Congenital Disorders of Glycosylation in Childhood Epilepsy and Effects of Anti-Epileptic Drugs on the Transferrin Isoelectric Focusing Test

Introduction: Childhood epilepsy is one of the most common neurological problems. The transferrin isoelectric focusing (TIEF) test is a screening test for congenital disorders of glycosylation (CDG). We identified abnormal TIEF test in children with epilepsy in our epilepsy genetics clinic. To determine if an abnormal TIEF test is associated with anti-epileptic medications or abnormal liver functions, we performed a retrospective cohort study. Methods: This study was performed between January 2012 and March 2020. Electronic patient charts were reviewed. Standard non-parametric statistical tests were applied using R statistical software. Fischer’s exact test was used for comparisons. Results: There were 206 patients. The TIEF test was abnormal in 11% (23 out of 206) of the patients. Nine patients were diagnosed with CDG: PMM2-CDG (n = 5), ALG3-CDG (n = 1), ALG11-CDG (n = 2), SLC35A2-CDG (n = 1). We report 51 different genetic diseases in 84 patients. Two groups, (1) abnormal TIEF test; (2) normal TIEF test, showed statistically significant differences for abnormal liver functions and for valproic acid treatment. Conclusion: The TIEF test guided CDG diagnosis in 2.9% of the patients. Due to the high prevalence of CDG (4.4%) in childhood epilepsy, the TIEF test might be included into the diagnostic investigations to allow earlier and cost-effective diagnosis.

Nonimmune hydrops fetalis and congenital disorders of glycosylation: A systematic literature review.

Numerous etiologies may lead to nonimmune hydrops fetalis (NIHF) including congenital disorders of glycosylation (CDG). Recognition of CDG in NIHF is challenging. This study reviews prenatal and neonatal characteristics of CDG presenting with NIHF. A systematic literature search was performed. Thirteen articles met the inclusion criteria. Twenty-one cases with NIHF associated with a CDG were reported. There were 17 live births, three pregnancy terminations, and one fetal demise. Timing of CDG diagnosis was reported mostly postnatally (90%; 10/11). Postnatal genetic testing was reported in 18 patients; three patients were diagnosed by isoelectric focusing of serum transferrin that showed a type 1 pattern. The genes reported for CDG with NIHF for 15 distinct families include: PMM2 in 47% (7/15), ALG9 in 20% (3/15), ALG8 in 13% (2/15), ALG1 in 7% (1/15), MGAT2 in 7% (1/15), and COG6 7% (1/15). In our review, 81% (17/21) reported facial dysmorphism, 52% (11/21) reported CNS abnormalities, most commonly cerebellar atrophy (64%; 7/11), and 38% (8/21) reported cardiovascular abnormalities, most commonly hypertrophic cardiomyopathy (63%; 5/8). Among live births, 71% (12/17) infants died at a median age of 34 days (range 1-185). Thrombocytopenia was reported in 53% (9/17) patients. Of those who survived past the neonatal period, 80% (4/5) had significant reported developmental delays. CDG should be on the differential diagnosis of NIHF in the presence of cerebellar atrophy, hypertrophic cardiomyopathy, or thrombocytopenia. Our review highlights the poor prognosis in infants with NIHF due to CDG and demonstrates the importance of identifying these disorders prenatally to guide providers in their counseling with families regarding pregnancy management. SYNOPSIS: Poor prognosis in fetuses and infants with nonimmune hydrops fetalis due to congenital disorders of glycosylation highlights the importance of prenatal diagnosis of this disorder.

The challenge of CDG diagnosis

Congenital disorders of glycosylation (CDG) are a rapidly growing family of genetic diseases that currently includes some 130 different types. CDG diagnosis is a challenge, not only because of this large number but also because of the huge clinical heterogeneity even within a number of CDG. In addition, the classical screening test, serum transferrin isoelectrofocusing, is only positive in about 60% of CDG, and can even become negative in some CDG particularly in PMM2-CDG, the most frequent N-glycosylation defect. In order to facilitate CDG diagnosis, we hereby provide some practical tools: (1) a list of clinical features strongly suggestive of a distinctive CDG; (2) a table of clinical, biochemical and laboratory findings reported in CDG, arranged per organ/system; (3) an overview of the affected organs/systems in each CDG; and (4) a diagnostic decision tree in face of a patient with a suspicion of CDG. Most important is to keep in mind a CDG in any unexplained syndrome, in particular when there is neurological involvement.This mini-review enumerates clinical and biochemical hallmarks of these diseases and the biochemical and genetic testing available, and provides an updated list and information on identified CDG. The main aim is to act as a CDG diagnosis simplified guide for healthcare professionals and, additionally, as an awareness and lobbying tool to help in the effectiveness and promptness of CDG diagnosis.

Complementarity of electrophoretic, mass spectrometric, and gene sequencing techniques for the diagnosis and characterization of congenital disorders of glycosylation.

Congenital disorders of glycosylation (CDG) are rare autosomal genetic diseases affecting the glycosylation of proteins and lipids. Since CDG-related clinical symptoms are classically extremely variable and nonspecific, a combination of electrophoretic, mass spectrometric, and gene sequencing techniques is often mandatory for obtaining a definitive CDG diagnosis, as well as identifying causative gene mutations and deciphering the underlying biochemical mechanisms. Here, we illustrate the potential of integrating data from capillary electrophoresis of transferrin, two-dimensional electrophoresis of N- and O-glycoproteins, mass spectrometry analyses of total serum N-linked glycans and mucin core1 O-glycosylated apolipoprotein C-III for the determination of various culprit CDG gene mutations. "Step-by-step" diagnosis pathways of four particular and new CDG cases, including MGAT2-CDG, ATP6V0A2-CDG, SLC35A2-CDG, and SLC35A3-CDG, are described as illustrative examples.

Electroclinical Features of Early-Onset Epileptic Encephalopathies in Congenital Disorders of Glycosylation (CDGs).

Congenital disorders of glycosylation (CDG) are a constantly growing group of genetic defects of glycoprotein and glycolipid glycan synthesis. CDGs are usually multisystem diseases, and in the majority of patients, there is an important neurological involvement comprising psychomotor disability, hypotonia, ataxia, seizures, stroke-like episodes, and peripheral neuropathy. To assess the incidence, among early-onset epileptic encephalopathies (EOEE), of patients with identified congenital disorders of glycosylation (CDG), we made a review of clinical, electrophysiological, and neuroimaging findings of 27 CDG patients focusing on seizure onset, semiology and frequency, response to antiepileptic drugs (AED), and early epileptic manifestations. Epilepsy was uncommon in PMM2-CDG (11%), while it was a main concern in other rare forms. We describe a series of patients with EOEE and genetically confirmed CDG (ALG3-CDG, ALG6-CDG, DPM2-CDG, ALG1-CDG). Epileptic seizures at onset included myoclonic and clonic fits and focal seizures. With time, patients developed recurrent and intractable seizures principally tonic-clonic seizures, infantile spasms, and myoclonic seizures. Electrophysiological correlates included focal and multifocal epileptic discharges, slowed background rhythm, and generalized epileptic activity including burst suppression pattern and status epilepticus. We propose a diagnostic flowchart for the early diagnosis of CDG in patients presenting with EOEE and suggest to perform serum transferrin IEF (or capillary zone electrophoresis) as a first-line screening in early-onset epilepsy.

Molecular diagnostic testing for congenital disorders of glycosylation (CDG): Detection rate for single gene testing and next generation sequencing panel testing

Congenital disorders of glycosylation (CDG) are comprised of over 60 disorders with the majority of defects residing within the N-glycosylation pathway. Approximately 20% of patients do not survive beyond five years of age due to widespread organ dysfunction. A diagnosis of CDG is based on abnormal glycosylation of transferrin but this method cannot identify the specific gene defect. For many individuals diagnosed with CDG the gene defect remains unknown. To improve the molecular diagnosis of CDG we developed molecular testing for 25 CDG genes including single gene testing and next generation sequencing (NGS) panel testing. From March 2010 through November 2012, a total of 94 samples were referred for single gene testing and 68 samples were referred for NGS panel testing. Disease causing mutations were identified in 24 patients resulting in a molecular diagnosis rate of 14.8%. Coverage of the 24 CDG genes using panel testing and whole exome sequencing (WES) was compared and it was determined that many exons of these genes were not adequately covered using a WES approach and a panel approach may be the preferred first option for CDG patients. A collaborative effort between physicians, researchers and diagnostic laboratories will be very important as NGS testing using panels and exome becomes more widespread. This technology will ultimately improve the molecular diagnosis of patients with CDG in hard to solve cases.

Perinatal and early infantile symptoms in congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are a rapidly growing family of inborn errors. Screening for CDG in suspected cases is usually performed in the first year of life by serum transferrin isoelectric focusing or mass spectrometry. Based on the transferrin analysis patients can be biochemically diagnosed with a type 1 or type 2 transferrin pattern, and labeled as CDG-I, or CDG-II. The diagnosis of CDG is frequently delayed due to the highly variable phenotype, some cases showing single organ involvement and others mimicking syndromes, like skeletal dysplasia, cutis laxa syndrome, or congenital muscle dystrophy. The aim of our study was to evaluate perinatal abnormalities and early discriminative symptoms in 58 patients consecutively diagnosed with diverse CDG-subtypes. Neonatal findings and clinical features in the first months of life were studied in 36 children with CDG-I and 22 with CDG-II. Maternal complications were found in five, small for gestational age in nine patients. Five children had abnormal neonatal screening results for hypothyroidism. Congenital microcephaly and neonatal seizures were common in CDG-II. Inverted nipples were uncommon with 5 out of 58 children. Dysmorphic features were mostly nonspecific, except for cutis laxa. Early complications included feeding problems, cardiomyopathy, thrombosis, and bleeding. Cases presenting in the neonatal period had the highest mortality rate. Survival in CDG patients is highly dependent on early intervention therapy. We recommend low threshold screening for glycosylation disorders in infants with neurologic symptoms, even in the absence of abnormal fat distribution. Growth retardation and neonatal bleeding increase suspicion for CDG.

DDOST Mutations Identified by Whole-Exome Sequencing Are Implicated in Congenital Disorders of Glycosylation

Congenital disorders of glycosylation (CDG) are inherited autosomal-recessive diseases that impair N-glycosylation. Approximately 20% of patients do not survive beyond the age of 5 years old as a result of widespread organ dysfunction. Although most patients receive a CDG diagnosis based on abnormal glycosylation of transferrin, this test cannot provide a genetic diagnosis; indeed, many patients with abnormal transferrin do not have mutations in any known CDG genes. Here, we combined biochemical analysis with whole-exome sequencing (WES) to identify the genetic defect in an untyped CDG patient, and we found a 22 bp deletion and a missense mutation in DDOST, whose product is a component of the oligosaccharyltransferase complex that transfers the glycan chain from a lipid carrier to nascent proteins in the endoplasmic reticulum lumen. Biochemical analysis with three biomarkers revealed that N-glycosylation was decreased in the patient's fibroblasts. Complementation with wild-type-DDOST cDNA in patient fibroblasts restored glycosylation, indicating that the mutations were pathological. Our results highlight the power of combining WES and biochemical studies, including a glyco-complementation system, for identifying and confirming the defective gene in an untyped CDG patient. This approach will be very useful for uncovering other types of CDG as well.

How to find and diagnose a CDG due to defective N‐glycosylation

The group of Congenital Disorders of Glycosylation (CDG) is expanding rapidly since the first clinical description of the N-glycosylation defect PMM2-CDG (CDG-Ia) in 1980 (Jaeken et al. 1980). Since then, more than 50 defects have been identified in protein N-glycosylation (some of them also associated with an O-glycosylation defect), in protein O-glycosylation only and in lipid glycosylation (GPI anchor and glycosphingolipid synthesis). Here, we provide a simple approach to the clinical, biochemical and genetic diagnosis of CDG due to a N-glycosylation defect (including combined N- and O-glycosylation defects).

The impact of mass spectrometry in the diagnosis of congenital disorders of glycosylation

Contribution of mass spectrometry (MS) in the diagnosis and characterization of congenital disorders of glycosylation (CDG) has long been known. CDG type I diseases are characterized by the under-occupancy of protein N-glycosylation sites. Electrospray (ESI) MS and matrix assisted laser desorption ionization (MALDI) MS are effective for underglycosylation analyses of intact serum Transferrin (Tf) in CDG-I patients by mass determination of individual component glycoforms. Thus, high-throughput methods developed to speed-up analytical times found increasing application in clinical testing for CDG detection. ESI MS recognizable glycoform profiles of serum Tf have been reported in CDG-I different from PMM2-CDG and in individual CDG-II defects. MALDI MS analysis of acidic and neutral N-linked glycans released from total plasma or targeted glycoproteins, is the mainstream tool to explore abnormal oligosaccharide structure and changes in the relative amount of individual oligosaccharides in CDG-II patients. Here we briefly review state-of-the-art and updates of MS-based applications for the diagnosis of CDG with special emphasis to detectable glycosylation profiles reported in different CDG types.

Cardiomyopathy in the congenital disorders of glycosylation (CDG): a case of late presentation and literature review

The congenital disorders of glycosylation (CDG) are a recently described group of inherited multisystem disorders characterized by defects predominantly of N- and O-glycosylation of proteins. Cardiomyopathy in CDG has previously been described in several subtypes; it is usually associated with high morbidity and mortality and the majority of cases present in the first 2 years of life. This is the first case with presentation in late childhood and the article reviews current literature. An 11-year-old female with a background of learning difficulties presented in cardiac failure secondary to severe dilated cardiomyopathy. Prior to the diagnosis of CDG, her condition deteriorated; she required mechanical support (Excor Berlin Heart) and was listed for cardiac transplant. Investigations included screening for glycosylation disorders, and isoelectric focusing of transferrin revealed an abnormal type 1 pattern. Analysis of phosphomannomutase and phosphomannose isomerase showed normal enzyme activity, excluding PMM2 (CDG Ia) and MPI (CDG Ib). Lipid-linked oligosaccharide and mutational studies have not yet defined the defect. Despite aggressive therapy there were persistent difficulties achieving adequate anticoagulation and she developed multiple life-threatening thrombotic complications. She was removed from the transplant list and died from overwhelming sepsis 5 weeks following admission. This case emphasizes the need to screen all children with an undiagnosed cardiomyopathy for CDG, regardless of age, and where possible to exclude CDG before the use of cardiac bridging devices. It highlights the many practical and ethical challenges that may be encountered where clinical knowledge and experience are still evolving.

Comparison between high performance liquid chromatography and capillary zone electrophoresis for the diagnosis of congenital disorders of glycosylation

Transferrin isoelectric focusing (IEF) is the most widely used method to screen for congenital disorders of glycosylation (CDG). Our aim was to compare high performance liquid chromatography (HPLC) and capillary zone electrophoresis (CZE) procedures for serum sialotransferrin analysis. 58 serum samples were processed both by CZE and HPLC: 35 were from paediatric controls, 18 from patients with an altered sialotransferrin IEF pattern and 5 were transferrin variant samples. HPLC analysis was performed with an anion-exchange column with spectrophotometric detection at 470 nm. CZE analysis was done using the commercial CEofix-CDT kit with spectrophotometric detection at 200 nm. Passing-Bablok regression analysis showed good agreement for tri-, tetra- and penta-sialotransferrin by both procedures. But for disialotransferrin, higher values were observed by the HPLC procedure. The HPLC and CZE methods allowed reproducible separation and analysis of single transferrin glycoforms with similar peak patterns. All patients presented values outside the range established in our control population either by HPLC or by CZE, even in patients with moderate forms of CDG that had been difficult to detect by IEF. In conclusion, measurement of sialotransferrin isoforms and interpretation using method-specific reference values may offer some advantages for the diagnosis of CDG as compared with the standard IEF procedure.

Biomarkers and diagnosis of congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are clinically and biochemically versatile disorders affecting the N- or O-glycosylation of proteins or lipids. The rapidly growing number of distinct CDG increasingly makes clear that a comprehensive screening for the vast majority of CDG is not possible on a simple basis. The prevailing methods for the screening and the diagnosis of CDG are presented, showing the potential and the limits of the diagnostic and screening tools used, orientated at suitability for daily use. Although valid and comprehensive diagnostic tests for CDG type I exist, there are limitations in the diagnosis of CDG type II, and many undiagnosed CDG type IIs must be expected. The apparent limitations of the methods used at present imply a necessity for new and broad diagnostic tools that comprise modern methodology to meet the requirements for screening tools.

Peters Plus Syndrome Is a New Congenital Disorder of Glycosylation and Involves Defective O-Glycosylation of Thrombospondin Type 1 Repeats

Peters Plus syndrome is an autosomal recessive disorder characterized by anterior eye chamber defects, disproportionate short stature, developmental delay, and cleft lip and/or palate. It is caused by splice site mutations in what was thought to be a beta1,3-galactosyltransferase-like gene (B3GALTL). Recently, we and others found this gene to encode a beta1,3-glucosyltransferase involved in the synthesis of the disaccharide Glc-beta1,3-Fuc-Omicron-that occurs on thrombospondin type 1 repeats of many biologically important proteins. No functional tests have been performed to date on the presumed glycosylation defect in Peters Plus syndrome. We have established a sensitive immunopurification-mass spectrometry method, using multiple reaction monitoring, to analyze Omicron-fucosyl glycans. It was used to compare the reporter protein properdin from Peters Plus patients with that from control heterozygous relatives. In properdin from patients, we could not detect the Glc-beta1,3-Fuc-Omicron-disaccharide, and we only found Fuc-Omicron-at all four Omicron-fucosylation sites. In contrast, properdin from heterozygous relatives and a healthy volunteer carried the Glc-beta1,3-Fuc-Omicron-disaccharide. These data firmly establish Peters Plus syndrome as a new congenital disorder of glycosylation.

N-Glycosylation Site Occupancy in Serum Glycoproteins Using Multiple Reaction Monitoring Liquid Chromatography-Mass Spectrometry

Congenital disorders of glycosylation (CDGs) are a family of N-linked glycosylation defects associated with severe clinical manifestations. In CDG type-I, deficiency of lipid-linked oligosaccharide assembly leads to the underoccupancy of N-glycosylation sites on glycoproteins. Although the level of residual glycosylation activity is known to correlate with the clinical phenotype linked to individual CDG mutations, it is not known whether the degree of N-glycosylation site occupancy by itself correlates with the severity of the disease. To quantify the extent of underglycosylation in healthy control and in CDG samples, we developed a quantitative method of N-glycosylation site occupancy based on multiple reaction monitoring LC-MS/MS. Using isotopically labeled standard peptides, we directly quantified the level of N-glycosylation site occupancy on selected serum proteins. In healthy control samples, we determined 98-100% occupancy for all N-glycosylation sites of transferrin and alpha(1)-antitrypsin. In CDG type-I samples, we observed a reduction in N-glycosylation site occupancy that correlated with the severity of the disease. In addition, we noticed a selective underglycosylation of N-glycosylation sites, indicating preferential glycosylation of acceptor sequons of a given glycoprotein. In transferrin, a preferred occupancy for the first N-glycosylation site was observed, and a decreasing preference for the first, third, and second N-glycosylation sites was observed in alpha(1)-antitrypsin. This multiple reaction monitoring LC-MS/MS method can be extended to multiple glycoproteins, thereby enabling a glycoproteomics survey of N-glycosylation site occupancies in biological samples.

Laboratory Diagnosis of Congenital Disorders of Glycosylation Type I by Analysis of Transferrin Glycoforms

Congenital disorders of glycosylation (CDG) are being recognized as a rapidly growing and complex group of disorders. The pathophysiology results from depressed synthesis or remodeling of oligosaccharide moieties of glycoproteins. The ultimate result is the formation of abnormal glycoproteins affecting their structure and metabolic functions. The most thoroughly studied subset of CDG are the type I defects affecting N-glycosylation. Causal mutations occur in at least 12 different genes which encode primarily monosaccharide transferases necessary for N-glycosylation in the endoplasmic reticulum. The broad clinical presentation of these glycosylation defects challenge clinicians to test for these defects in a variety of clinical settings. The first described CDG was a phosphomannomutase deficiency (CDG-Ia). The original method used to define the glycosylation defect was isoelectric focusing (IEF) of transferrin. More recently, the use of other charge separation methods and electrospray-mass spectrometry (ESI-MS) has proven valuable in detecting type I CDG defects. By mass resolution, the under-glycosylation of transferrin is characterized as the total absence of one or both N-linked oligosaccharide. Beyond providing a new understanding of the structure of transferrin in type I CDG patients, it is adaptable to high throughput serum analysis. The use of transferrin under-glycosylation to detect the type I CDG provides limited insight into the specific site of the defect in oligosaccharide assembly since its value is constrained to observation of the final product of glycoprotein synthesis. New analytical targets and tools are converging with the clinical need for diagnosis of CDG. Defining the biosynthetic sites responsible for specific CDG phenotypes is in progress, and ten more type I defects have been putatively identified. This review discusses current methods, such as IEF and targeted proteomics using mass spectrometry, that are used routinely to test for type I CDG disorders, along with some newer approaches to define the defective synthetic sites responsible for the type I CDG defects. All diagnostic endeavors are followed by the quest for a reliable treatment. The isolated success of CDG-Ib treatment will be described with the hope that this may expand to other type I CDG disorders.