Normal transferrin patterns in congenital disorders of glycosylation with Golgi homeostasis disruption: apolipoprotein C-III at the rescue!

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2021-2022

The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates is a well-established technique and this review is the 12th update of the original article published in 1999 and brings coverage of the literature to the end of 2022. As with previous review, this review also includes a few papers that describe methods appropriate to analysis by MALDI, such as sample preparation, even though the ionization method is not MALDI. The review follows the same format as previous reviews. It is divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of computer software for structural identification. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other general areas such as medicine, industrial processes, natural products and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. MALDI is still an ideal technique for carbohydrate analysis, particularly in its ability to produce single ions from each analyte and advancements in the technique and range of applications show little sign of diminishing.

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.

Biochemical diagnosis of congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are one of the fastest growing groups of inborn errors of metabolism, comprising over 160 described diseases to this day. CDG are characterized by a dysfunctional glycosylation process, with molecular defects localized in the cytosol, the endoplasmic reticulum, or the Golgi apparatus. Depending on the CDG, N-glycosylation, O-glycosylation and/or glycosaminoglycan synthesis can be affected. Various proteins, lipids, and glycosylphosphatidylinositol anchors bear glycan chains, with potential impacts on their folding, targeting, secretion, stability, and thus, functionality. Therefore, glycosylation defects can have diverse and serious clinical consequences. CDG patients often present with a non-specific, multisystemic syndrome including neurological involvement, growth delay, hepatopathy and coagulopathy. As CDG are rare diseases, and typically lack distinctive clinical signs, biochemical and genetic testing bear particularly important and complementary diagnostic roles. Here, after a brief introduction on glycosylation and CDG, we review historical and recent findings on CDG biomarkers and associated analytical techniques, with a particular emphasis on those with relevant use in the specialized clinical chemistry laboratory. We provide the reader with insights and methods which may help them properly assist the clinician in navigating the maze of glycosylation disorders.

Identification of a novel intronic variant of ATP6V0A2 in a Han-Chinese family with cutis laxa

BACKGROUND

Cutis laxa is a connective tissue disease caused by abnormal synthesis or secretion of skin elastic fibers, leading to skin flabby and saggy in various body parts. It can be divided into congenital cutis laxa and acquired cutis laxa, and inherited cutis laxa syndromes is more common in clinic.

METHODS

In this study, we reported a case of a Han-Chinese male newborn with ATP6V0A2 gene variant leading to cutis laxa. The proband was identified by whole-exome sequencing to determine the novel variant, and their parents were verified by Sanger sequencing. Bioinformatics analysis and minigene assay were used to verify the effect of this variant on splicing function.

RESULTS

The main manifestations of the proband are skin laxity, abnormal facial features, and enlargement of the anterior fontanelle. Whole-exome sequencing showed that the newborn carried a non-canonical splicing-site variant c.117 + 5G > T, p. (?) in ATP6V0A2 gene. Sanger sequencing showed that both parents of the proband carried the heterozygous variant. The results of bioinformatics analysis and minigene assay displayed that the variant site affected the splicing function of pre-mRNA of the ATP6V0A2 gene.

CONCLUSIONS

In this study, it was identified that ATP6V0A2 gene c. 117 + 5G > T may be the cause of the disease. The non-canonical splicing variants of ATP6V0A2 gene were rarely reported in the past, and this variant expanded the variants spectrum of the gene. The functional study of minigene assay plays a certain role in improving the level of evidence for the pathogenicity of splicing variants, which lays a foundation for prenatal counseling and follow-up gene therapy.

Higher frequency of TMEM199-CDG in the southern mediterranean area is associated with c.92G>C (p.Arg31Pro) mutation

Congenital disorders of glycosylation (CDG) are genetic multisystem diseases, characterized by defective glycoconjugate synthesis. A small number of CDG with isolated liver damage have been described, such as TMEM199-CDG, a non-encephalopathic liver disorder with Wilson disease-like phenotype. Only eight patients with TMEM199-CDG have been described including seven Europeans (originating from Greece and Italy) and one Chinese. Three patients from southern Italy (Campania) shared the same known missense mutation pathogenetic variant NM_152464.3:c. 92G > C (p.Arg31Pro), also found in a Greek patient. Here we report a new patient from southern Italy (Sicily), with a homozygous c.92G > C p.(Arg31Pro) variant in TMEM199. The patient's phenotype is characterized by mild non-progressive hepatopathy with a normal hepatic echo structure. A persistent increase in serum transaminases, total and low-density lipoprotein cholesterol and low serum ceruloplasmin and copper levels and normal urinary copper excretion were observed. Matrix-assisted laser desorption/ionization mass spectrometry analyses showed abnormal N- and O- protein glycosylation, indicative of a Golgi processing defect and supporting the function of TMEM199 in maintaining Golgi homeostasis. TMEM199-CDG is an ultra-rare CDG relatively frequent in the southern Mediterranean area (7 in 9 patients, 77%). It is mainly associated with the c.92G > C (p.Arg31Pro) pathogenetic allele globally reported in 4 out of 7 families (57%), including one from Greece and three unrelated families from southern Italy. The almost uniform clinical phenotype described in patients with TMEM199-CDG appears to reflect a higher prevalence of the same variant in patients from the southern Mediterranean area.

Electrospray Ionization Mass Spectrometry of Apolipoprotein CIII to Evaluate O-glycan Site Occupancy and Sialylation in Congenital Disorders of Glycosylation

Congenital disorders of glycosylation (CDG) are inherited metabolic diseases that affect the synthesis of glycoconjugates. Defects in mucin-type O-glycosylation occur independently or in combination with N-glycosylation disorders, and the profiling of the O-glycans of apolipoprotein CIII (apoCIII) by mass spectrometry (MS) can be used to support a diagnosis. The biomarkers are site occupancy and sialylation levels, which are indicated by the content of non-glycosylated apoCIII0a isoform and by the ratio of monosialylated apoCIII1 to disialylated apoCIII2 isoforms, respectively. In this report, electrospray ionization (ESI) quadrupole MS of apoCIII was used to identify these biomarkers. Among the instrumental parameters, the declustering potential (DP) induced the fragmentation of the O-glycan moiety including the Thr–GalNAc linkage, resulting in an increase in apoCIII0a ions. This incurs the risk of creating a false positive for reduced site occupancy. The apoCIII1/apoCIII2 ratio was substantially unchanged despite some dissociation of sialic acids. Therefore, appropriate DP settings are especially important when transferrin, which requires a higher DP, for N-glycosylation disorders is analyzed simultaneously with apoCIII in a single ESI MS measurement. Finally, a reference range of diagnostic biomarkers and mass spectra of apoCIII obtained from patients with SLC35A1-, TRAPPC11-, and ATP6V0A2-CDG are presented.