Transferrin isoelectric focusing for the investigation of congenital disorders of glycosylation: analysis of a ten-year experience in a Brazilian center

A complement C4-derived glycopeptide is a biomarker for PMM2-CDG

BACKGROUNDDiagnosis of PMM2-CDG, the most common congenital disorder of glycosylation (CDG), relies on measuring carbohydrate-deficient transferrin (CDT) and genetic testing. CDT tests have false negatives and may normalize with age. Site-specific changes in protein N-glycosylation have not been reported in sera in PMM2-CDG.METHODSUsing multistep mass spectrometry-based N-glycoproteomics, we analyzed sera from 72 individuals to discover and validate glycopeptide alterations. We performed comprehensive tandem mass tag-based discovery experiments in well-characterized patients and controls. Next, we developed a method for rapid profiling of additional samples. Finally, targeted mass spectrometry was used for validation in an independent set of samples in a blinded fashion.RESULTSOf the 3,342 N-glycopeptides identified, patients exhibited decrease in complex-type N-glycans and increase in truncated, mannose-rich, and hybrid species. We identified a glycopeptide from complement C4 carrying the glycan Man5GlcNAc2, which was not detected in controls, in 5 patients with normal CDT results, including 1 after liver transplant and 2 with a known genetic variant associated with mild disease, indicating greater sensitivity than CDT. It was detected by targeted analysis in 2 individuals with variants of uncertain significance in PMM2.CONCLUSIONComplement C4-derived Man5GlcNAc2 glycopeptide could be a biomarker for accurate diagnosis and therapeutic monitoring of patients with PMM2-CDG and other CDGs.FUNDINGU54NS115198 (Frontiers in Congenital Disorders of Glycosylation: NINDS; NCATS; Eunice Kennedy Shriver NICHD; Rare Disorders Consortium Disease Network); K08NS118119 (NINDS); Minnesota Partnership for Biotechnology and Medical Genomics; Rocket Fund; R01DK099551 (NIDDK); Mayo Clinic DERIVE Office; Mayo Clinic Center for Biomedical Discovery; IA/CRC/20/1/600002 (Center for Rare Disease Diagnosis, Research and Training; DBT/Wellcome Trust India Alliance).

Coagulation abnormalities in a prospective cohort of 50 patients with PMM2-congenital disorder of glycosylation

BACKGROUND

Given the lack of reliable data on the prevalence of bleeding abnormalities and thrombotic episodes in PMM2-CDG patients, and whether coagulation abnormalities change over time, we prospectively collected and reviewed natural history data. Patients with PMM2-CDG often have abnormal coagulation studies due to glycosylation abnormalities but the frequency of complications resulting from these has not been prospectively studied.

METHODS

We studied fifty individuals enrolled in the Frontiers in Congenital Disorders of Glycosylation Consortium (FCDGC) natural history study with molecularly confirmed diagnosis of PMM2-CDG. We collected data on prothrombin time (PT), international normalized ratio (INR), activated partial thromboplastin time (aPTT), platelets, factor IX activity (FIX), factor XI activity (FXI), protein C activity (PC), protein S activity (PS) and antithrombin activity (AT).

RESULTS

Prothrombotic and antithrombotic factor activities were frequently abnormal in PMM2-CDG patients, including AT, PC, PT, INR, and FXI. AT deficiency was the most common abnormality in 83.3% of patients. AT activity was below 50% in 62.5% of all patients (normal range 80-130%). Interestingly, 16% of the cohort experienced symptoms of spontaneous bleeding and 10% had thrombosis. Stroke-like episodes (SLE) were reported in 18% of patients in our cohort. Based on the linear growth models, on average, patients did not show significant change in AT (n = 48; t(23.8) = 1.75, p = 0.09), FIX (n = 36; t(61) = 1.60, p = 0.12), FXI (n = 39; t(22.8) = 1.88, p = 0.07), PS (n = 25; t(28.8) = 1.08, p = 0.29), PC (n = 38; t(68) = 1.61, p = 0.11), INR (n = 44; t(184) = -1.06, p = 0.29), or PT (n = 43; t(192) = -0.69, p = 0.49) over time. AT activity positively correlated with FIX activity. PS activity was significantly lower in males.

CONCLUSION

Based on our natural history data and previous literature, we conclude that caution should be exercised when the AT levels are lower than 65%, as most thrombotic events occur in patients with AT below this level. All five, male PMM2-CDG patients in our cohort who developed thrombosis had abnormal AT levels, ranging between 19% and 63%. Thrombosis was associated with infection in all cases. We did not find significant change in AT levels over time. Several PMM2-CDG patients had an increased bleeding tendency. More long-term follow-up is necessary on coagulation abnormalities and the associated clinical symptoms to provide guidelines for therapy, patient management, and appropriate counseling.

SYNOPSIS

Most PMM2-CDG patients display chronic coagulation abnormalities without significant improvement, associated with a frequency of 16% clinical bleeding abnormalities, and 10% thrombotic episodes in patients with severe antithrombin deficiency.

Missing heritability of Wilson disease: a search for the uncharacterized mutations

Wilson disease (WD), a copper metabolism disorder caused by mutations in ATP7B, manifests heterogeneous clinical features. Interestingly, in a fraction of clinically diagnosed WD patients, mutations in ATP7B appears to be missing. In this review we discuss the plausible explanations of this missing heritability and propose a workflow that can identify the hidden mutations. Mutation analyses of WD generally includes targeted sequencing of ATP7B exons, exon-intron boundaries, and rarely, the proximal promoter region. We propose that variants in the distal cis-regulatory elements and/or deep intronic variants that impact splicing might well represent the hidden mutations. Heterozygous del/ins that remain refractory to conventional PCR-sequencing method may also represent such mutations. In this review, we also hypothesize that mutations in the key copper metabolism genes, like, ATOX1, COMMD1, and SLC31A1, could possibly lead to a WD-like phenotype. In fact, WD does present overlapping symptoms with other rare genetic disorders; hence, the possibility of a misdiagnosis and thus adding to missing heritability cannot be excluded. In this regard, it seems that whole-genome analysis will provide a comprehensive and rapid molecular diagnosis of WD. However, considering the associated cost for such a strategy, we propose an alternative customized screening schema of WD which include targeted sequencing of ATP7B locus as well as other key copper metabolism genes. Success of such a schema has been tested in a pilot study.

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.

Non-functional alternative splicing caused by a Latino pathogenic variant in a case of PMM2-CDG

We report on a Mexican mestizo with a multisystemic syndrome including neurological involvement and a type I serum transferrin isoelectric focusing (Tf IEF) pattern. Diagnosis of PMM2-CDG was obtained by clinical exome sequencing (CES) that revealed compound heterozygous variants in PMM2, the encoding gene for the phosphomannomutase 2 (PMM2). This enzyme catalyzes the conversion of mannose-6-P to mannose-1-P required for the synthesis of GDP-Man and Dol-P-Man, donor substrates for glycosylation reactions. The identified variants were c.422G>A (R141H) and c.178G>T, the former being the most frequent PMM2 pathogenic mutation and the latter a previously uncharacterized variant restricted to the Latino population with conflicting interpretations of pathogenicity and that we here report causes leaky non-functional alternative splicing (p.V60Cfs*3).

Galactose in human metabolism, glycosylation and congenital metabolic diseases: Time for a closer look

Galactose is an essential carbohydrate for cellular metabolism, as it contributes to energy production and storage in several human tissues while also being a precursor for glycosylation. Galactosylated glycoconjugates, such as glycoproteins, keratan sulfate-containing proteoglycans and glycolipids, exert a plethora of biological functions, including structural support, cellular adhesion, intracellular signaling and many more. The biological relevance of galactose is further entailed by the number of pathogenic conditions consequent to defects in galactosylation and galactose homeostasis. The growing number of rare congenital disorders involving galactose along with its recent therapeutical applications are drawing increasing attention to galactose metabolism. In this review, we aim to draw a comprehensive overview of the biological functions of galactose in human cells, including its metabolism and its role in glycosylation, and to provide a systematic description of all known congenital metabolic disorders resulting from alterations of its homeostasis.

Congenital disorders of glycosylation: Prevalence, incidence and mutational spectrum in the Polish population

Introduction

The incidence and prevalence of congenital disorders of glycosylation (CDG) have not been well established. The aim of the study was to evaluate the prevalence, incidence and genotypes of CDG patients diagnosed during the last 23 years in Poland (1997 – 30th October 2020).

Material and methods

The diagnosis was based on serum Tf IEF which is performed at The Children's Memorial Health Institute (CMHI) in Warsaw. Based on demographic data, the prevalence of CDG among the Polish population in 2020 as well as the birth prevalence of CDG from 1990 to 2020 were estimated.

Results

39 patients (from 35 families) with molecularly confirmed CDG were diagnosed, including 17 (44%) patients (from 16 families) with PMM2-CDG. The c.422G > A, p.Arg141His and c.691G > A, p.Val231Met pathogenic missense variants were the most common identified PMM2 variants. Eleven other patients were diagnosed with CDG based on serum Tf IEF analysis only; the molecular analysis is pending. Ten CDG patients died, including 6 with PMM2-CDG, 1 with PGM1-CDG and 1 with DPAGT1-CDG. The prevalence of CDG in the Polish population was estimated at approximately 1 per million while that of PMM2 at 0.4 per million. The annual incidence of CDG was estimated at 0.013 per 100,000 people in 2020.

Conclusions

A low frequence of CDG in our study could be underestimated.

Identification through exome sequencing of the first PMM2-CDG individual of Mexican mestizo origin

Congenital Disorders of Glycosylation (CDG) are scarcely reported from Latin America. We here report on a Mexican mestizo with a multi-systemic syndrome including neurological involvement and a type I transferrin (Tf) isoelectric focusing (IEF) pattern. Clinical exome sequencing (CES) showed known compound missense variants in PMM2 c.422G > A (p.R141H) and c.395 T > C (p.I132T), coding for the phosphomanomutase 2 (PMM2). PMM2 catalyzes the conversion of mannose-6-P to mannose-1-P required for the synthesis of GDP-Man and Dol-P-Man, donor substrates for glycosylation reactions. This is the third reported Mexican CDG patient and the first with PMM2-CDG. PMM2 has been recently identified as one of the top 10 genes carrying pathogenic variants in a Mexican population cohort.