Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry to Detect Diagnostic Glycopeptide Markers of Congenital Disorders of Glycosylation

Normal transferrin glycosylation does not rule out severe ALG1 deficiency

ALG1-CDG is a rare, clinically variable metabolic disease, caused by the defect of adding the first mannose (Man) to N-acetylglucosamine (GlcNAc2)-pyrophosphate (PP)-dolichol to the growing oligosaccharide chain, resulting in impaired N-glycosylation of proteins. N-glycosylation has a key role in functionality, stability, and half-life of most proteins. Therefore, congenital defects of glycosylation typically are multisystem disorders. Here we report a 3-year-old patient with severe neurological, cardiovascular, respiratory, musculoskeletal and gastrointestinal symptoms. ALG1-CDG was suggested based on exome sequencing and Western blot analysis. Despite her severe clinical manifestations and genetic diagnosis, serum transferrin glycoform analysis was normal. Western blot analysis of highly glycosylated proteins in fibroblasts revealed decreased intercellular adhesion molecule 1 (ICAM1), but normal lysosomal associated membrane protein 1 and 2 (LAMP1 and LAMP2) expression levels. Glycoproteomics in fibroblasts showed the presence of the abnormal tetrasacharide. Reviewing the literature, we found 86 reported ALG1-CDG patients, but only one with normal transferrin analysis. Based on our results we would like to highlight the importance of multiple approaches in diagnosing ALG1-CDG, as normal serum transferrin glycosylation or other biomarkers with normal expression levels can occur.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth 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 2020. 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. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

A case of ALG11-congenital disorders of glycosylation diagnosed by post-mortem whole exome sequencing

INTRODUCTION

Congenital disorders of glycosylation (CDG) are inherited inborn errors of metabolism due to abnormal protein and lipid glycosylation that present with multi-systemic manifestations. The heterogeneity of CDG poses a serious diagnostic challenge; therefore, whole-exome sequencing (WES), which plays an increasingly important role in the molecular diagnosis of CDG, is used for examining patients with CDG.

CASE REPORT

We report the case of a two-month-old male patient who developed developmental and epileptic encephalopathy (DEE) with intractable seizures and microcephaly. EEG demonstrated a suppression-burst (S-B) pattern, and MRI showed delayed myelination and progressive atrophic changes. Although CDG was clinically suspected, serum transferrin isoelectric focusing analysis appeared to be normal. The patient died by six years of age. Postmortem WES performed approximately 20 years after the patient's death revealed homozygous variants in ALG11 (NM_001004127.3: c.935A > C, p.Glu312Ala), and the patient was diagnosed with ALG11-CDG.

CONCLUSION

We present a case of the patient with ALG11-CDG diagnosed using post-mortem WES. The EEG revealed a S-B pattern that indicated severely drug-resistant DEE, which was associated with poor prognosis. If a CDG is suspected, WES should be considered.

Siblings with MAN1B1-CDG Showing Novel Biochemical Profiles

Congenital disorders of glycosylation (CDG), inherited metabolic diseases caused by defects in glycosylation, are characterized by a high frequency of intellectual disability (ID) and various clinical manifestations. Two siblings with ID, dysmorphic features, and epilepsy were examined using mass spectrometry of serum transferrin, which revealed a CDG type 2 pattern. Whole-exome sequencing showed that both patients were homozygous for a novel pathogenic variant of MAN1B1 (NM_016219.4:c.1837del) inherited from their healthy parents. We conducted a HPLC analysis of sialylated N-linked glycans released from total plasma proteins and characterized the α1,2-mannosidase I activity of the lymphocyte microsome fraction. The accumulation of monosialoglycans was observed in MAN1B1-deficient patients, indicating N-glycan-processing defects. The enzymatic activity of MAN1B1 was compromised in patient-derived lymphocytes. The present patients exhibited unique manifestations including early-onset epileptic encephalopathy and cerebral infarction. They also showed coagulation abnormalities and hypertransaminasemia. Neither sibling had truncal obesity, which is one of the characteristic features of MAN1B1-CDG.

Novel ALG12 variants and hydronephrosis in siblings with impaired N-glycosylation

BACKGROUND

ALG12-CDG is a rare autosomal recessive type I congenital disorder of glycosylation (CDG) due to pathogenic variants in ALG12 which encodes the dolichyl-P-mannose:Man-7-GlcNAc-2-PP-dolichyl-alpha-6-mannosyltransferase. Thirteen patients from unrelated 11 families have been reported, most of them result in broad multisystem manifestations with clinical variability. It is important to validate abnormal glycosylation to establish causal relationship.

CASE REPORT

Here, we report two siblings with novel compound heterozygous variants in ALG12: c.443T>C, p.(Leu148Pro) and c.412_413insCGT, p.(Gln137_Phe138insSer). Both patients showed global developmental delay, microcephaly, hypotonia, failure to thrive, facial dysmorphism, skeletal malformations and coagulation abnormalities, which are common in ALG12-CDG. In addition, one of our patients showed left hydronephrosis, which is a novel clinical feature in ALG12-CDG. Brain MRI showed hypoplasia of cerebrum, brain stem and cerebellar vermis in both patients. N-glycosylation defects of trypsin digested transferrin peptides were revealed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and electrospray ionization MS verified the lack of N-glycans in transferrin.

CONCLUSIONS

The present study can add hydronephrosis to phenotypic spectrum of ALG12-CDG. Since the symptoms of ALG12-CDG are quite diverse, the combination of whole-exome sequencing and transferrin glycopeptide analysis with MS, can help diagnosis of ALG12-CDG.

Treatment Options in Congenital Disorders of Glycosylation

Despite advances in the identification and diagnosis of congenital disorders of glycosylation (CDG), treatment options remain limited and are often constrained to symptomatic management of disease manifestations. However, recent years have seen significant advances in treatment and novel therapies aimed both at the causative defect and secondary disease manifestations have been transferred from bench to bedside. In this review, we aim to give a detailed overview of the available therapies and rising concepts to treat these ultra-rare diseases.

Apolipoprotein C-III O-glycoform profiling of 500 serum samples by matrix-assisted laser desorption/ionization mass spectrometry for diagnosis of congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are caused by defects in various genes governing glycoconjugate biosynthesis. Several responsible genes have been identified in the protein N-glycosylation process. Analyses of mucin-type core-1 O-glycoform of apolipoprotein C-III (apoCIII) have recently revealed combined N- and O-glycosylation defects. We applied matrix-assisted laser desorption/ionization mass spectrometry profiling of apoCIII glycoforms to 500 serum samples for CDG screening, and reference values were determined. The content of unglycosylated apoCIII was low in early infancy, indicating that the O-glycan occupancy should be assessed based on age-matched reference values. The samples from patients with mutations in the ALG1, ATP6V0A2, B4GALT1, COG2, GCS1, PGM1, SLC35A2, and TRAPPC11 genes were analyzed. B4GALT1- and TRAPPC11-CDG were accompanied by under-sialylation of O-glycans and are now recognized as combined N- and O-glycosylation disorders.