ALG6-CDG in South Africa: Genotype-Phenotype Description of Five Novel Patients

Psychiatric assessment and therapy in an adolescent with ALG6-CDG: a six-month follow-up case report

ALG6-congenital disorder of glycosylation (ALG6-CDG) is a complex of rare inherited disorders caused by mutations in the ALG6 gene, which encodes the α-1,3-glucosyltransferase enzyme required for N-glycosylation. ALG6-CDG affects multiple systems and exhibits clinical heterogeneity. Besides developmental delays and neurological signs and symptoms, behavioral and psychological symptoms are also an important group of clinical features of ALG6-CDG. Here, we present the case of a 17-year-old Chinese girl with ALG6-CDG who first visited the psychiatric department with apathy, language reduction, and substupor symptoms. The psychiatric assessments and treatment processes performed are described and discussed in this report. During diagnostic process, we found a novel mutation, c.849delT, in ALG6 by whole-exome sequencing. The patient's symptoms improved with escitalopram and risperidone treatment. However, above a certain dosage, she was sensitive to extrapyramidal side effects. This study accumulates clinical experience for diagnosing and treating ALG6-CDG and improves our understanding of this rare genetic disorder.

Inherited Retinal Degeneration Caused by Dehydrodolichyl Diphosphate Synthase Mutation-Effect of an ALG6 Modifier Variant

Modern advances in disease genetics have uncovered numerous modifier genes that play a role in the severity of disease expression. One such class of genetic conditions is known as inherited retinal degenerations (IRDs), a collection of retinal degenerative disorders caused by mutations in over 300 genes. A single missense mutation (K42E) in the gene encoding the enzyme dehydrodolichyl diphosphate synthase (DHDDS), which is required for protein N-glycosylation in all cells and tissues, causes DHDDS-IRD (retinitis pigmentosa type 59 (RP59; OMIM #613861)). Apart from a retinal phenotype, however, DHDDS-IRD is surprisingly non-syndromic (i.e., without any systemic manifestations). To explore disease pathology, we selected five glycosylation-related genes for analysis that are suggested to have disease modifier variants. These genes encode glycosyltransferases (ALG6, ALG8), an ER resident protein (DDOST), a high-mannose oligosaccharyl transferase (MPDU1), and a protein N-glycosylation regulatory protein (TNKS). DNA samples from 11 confirmed DHDDS (K42E)-IRD patients were sequenced at the site of each candidate genetic modifier. Quantitative measures of retinal structure and function were performed across five decades of life by evaluating foveal photoreceptor thickness, visual acuity, foveal sensitivity, macular and extramacular rod sensitivity, and kinetic visual field extent. The ALG6 variant, (F304S), was correlated with greater macular cone disease severity and less peripheral rod disease severity. Thus, modifier gene polymorphisms may account for a significant portion of phenotypic variation observed in human genetic disease. However, the consequences of the polymorphisms may be counterintuitively complex in terms of rod and cone populations affected in different regions of the retina.

A Case of ALG6-CDG with Explosive Onset of Intractable Epilepsy During Infancy

ALG6-CDG is a rare, but second most common, type 1 congenital disorder of glycosylation (CDG) caused by a defect in the α-1-3-glucosyltransferase (ALG6) enzyme in the N-glycan assembly pathway. Many mutations have been identified and inherited in an autosomal recessive pattern. There are less than 100 ALG6-CDG cases reported, all sharing the phenotype of hypotonia and developmental delay. The majority (perhaps >70%) have seizures, but a minority have intractable epilepsy or epileptic encephalopathy. We report the clinical course, EEG findings, and neuroimaging of a child found to have compound heterozygous alleles c.257 + 5G > A and c.680G > A (p.G227E) who developed explosive onset of intractable epilepsy and epileptic encephalopathy. Initially, CDG was not suspected due to its rarity and lack of multi-organ system involvement, but rapid whole exam sequence (8-day turnaround) revealed the specific diagnosis quickly.

Structure and mechanism of the ER-based glucosyltransferase ALG6

In eukaryotic protein N-glycosylation, a series of glycosyltransferases catalyse the biosynthesis of a dolichylpyrophosphate-linked oligosaccharide before its transfer onto acceptor proteins¹. The final seven steps occur in the lumen of the endoplasmic reticulum (ER) and require dolichylphosphate-activated mannose and glucose as donor substrates². The responsible enzymes-ALG3, ALG9, ALG12, ALG6, ALG8 and ALG10-are glycosyltransferases of the C-superfamily (GT-Cs), which are loosely defined as containing membrane-spanning helices and processing an isoprenoid-linked carbohydrate donor substrate^3,4. Here we present the cryo-electron microscopy structure of yeast ALG6 at 3.0 Å resolution, which reveals a previously undescribed transmembrane protein fold. Comparison with reported GT-C structures suggests that GT-C enzymes contain a modular architecture with a conserved module and a variable module, each with distinct functional roles. We used synthetic analogues of dolichylphosphate-linked and dolichylpyrophosphate-linked sugars and enzymatic glycan extension to generate donor and acceptor substrates using purified enzymes of the ALG pathway to recapitulate the activity of ALG6 in vitro. A second cryo-electron microscopy structure of ALG6 bound to an analogue of dolichylphosphate-glucose at 3.9 Å resolution revealed the active site of the enzyme. Functional analysis of ALG6 variants identified a catalytic aspartate residue that probably acts as a general base. This residue is conserved in the GT-C superfamily. Our results define the architecture of ER-luminal GT-C enzymes and provide a structural basis for understanding their catalytic mechanisms.

Hyperkinetic movement disorders in congenital disorders of glycosylation

BACKGROUND AND PURPOSE

Congenital disorders of glycosylation (CDG) represent an increasing number of rare inherited metabolic diseases associated with abnormal glycan metabolism and disease onset in infancy or early childhood. Most CDG are multisystemic diseases mainly affecting the central nervous system. The aim of the current study was to investigate hyperkinetic movement disorders in patients affected by CDG and to characterize phenomenology based on CDG subtypes.

METHODS

Subjects were identified from a cohort of patients with CDG who were referred to the University Hospital of Catania, Italy. Patients were evaluated by neurologists with expertise in movement disorders and videotaped using a standardized protocol.

RESULTS

A variety of hyperkinetic movement disorders was detected in eight unrelated CDG patients. Involuntary movements were generally observed early in childhood, maintaining a clinical stability over time. Distribution ranged from a generalized, especially in younger subjects, to a segmental/multifocal involvement. In patients with phosphomannomutase 2 CDG, the principal movement disorders included dystonia and choreo-athetosis. In patients affected by other CDG types, the movement disorders ranged from pure generalized chorea to mixed movement disorders including dystonia and complex stereotypies.

CONCLUSIONS

Hyperkinetic movement disorder is a key clinical feature in patients with CDG. CDG should be considered in the differential diagnosis of childhood-onset dyskinesia, especially when associated with ataxia, developmental delay, intellectual disability, autism or seizure disorder.

Congenital disorders of glycosylation

Congenital disorders of glycosylation are a genetically and clinically heterogeneous group of >130 diseases caused by defects in various steps along glycan modification pathways. The vast majority of these monogenic diseases are autosomal recessive and have multi-systemic manifestations, mainly growth failure, developmental delay, facial dysmorphisms, and variable coagulation and endocrine abnormalities. Carbohydrate deficient transferrin (CDT) and protein-linked glycan analysis with mass spectrometry can diagnose some subtypes of congenital disorders of glycosylation (CDG), while many currently rely on massively parallel genomic sequencing for diagnosis. Early detection is important, as a few of these disorders are treatable. Molecular and biochemical techniques continue to further our understanding of this rapidly expanding group of clinically and genetically diverse disorders.

ALG6-CDG: a recognizable phenotype with epilepsy, proximal muscle weakness, ataxia and behavioral and limb anomalies

INTRODUCTION

Alpha-1,3-glucosyltransferase congenital disorder of glycosylation (ALG6-CDG) is a congenital disorder of glycosylation. The original patients were described with hypotonia, developmental disability, epilepsy, and increased bleeding tendency.

METHODS

Based on Euroglycan database registration, we approached referring clinicians and collected comprehensive data on 41 patients.

RESULTS

We found hypotonia and developmental delay in all ALG6-CDG patients and epilepsy, ataxia, proximal muscle weakness, and, in the majority of cases, failure to thrive. Nine patients developed intractable seizures. Coagulation anomalies were present in <50 % of cases, without spontaneous bleedings. Facial dysmorphism was rare, but seven patients showed missing phalanges and brachydactyly. Cyclic behavioral change, with autistic features and depressive episodes, was one of the most significant complaints. Eleven children died before the age of 4 years due to protein losing enteropathy (PLE), sepsis, or seizures. The oldest patient was a 40 year-old Dutch woman. The most common pathogenic protein alterations were p.A333V and p.I299Del, without any clear genotype-phenotype correlation.

DISCUSSION

ALG6-CDG has been now described in 89 patients, making it the second most common type of CDG. It has a recognizable phenotype and a primary neurologic presentation.

Defining the Phenotype and Assessing Severity in Phosphoglucomutase-1 Deficiency

OBJECTIVE

To define phenotypic groups and identify predictors of disease severity in patients with phosphoglucomutase-1 deficiency (PGM1-CDG).

STUDY DESIGN

We evaluated 27 patients with PGM1-CDG who were divided into 3 phenotypic groups, and group assignment was validated by a scoring system, the Tulane PGM1-CDG Rating Scale (TPCRS). This scale evaluates measurable clinical features of PGM1-CDG. We examined the relationship between genotype, enzyme activity, and TPCRS score by using regression analysis. Associations between the most common clinical features and disease severity were evaluated by principal component analysis.

RESULTS

We found a statistically significant stratification of the TPCRS scores among the phenotypic groups (P < .001). Regression analysis showed that there is no significant correlation between genotype, enzyme activity, and TPCRS score. Principal component analysis identified 5 variables that contributed to 54% variance in the cohort and are predictive of disease severity: congenital malformation, cardiac involvement, endocrine deficiency, myopathy, and growth.

CONCLUSIONS

We established a scoring algorithm to reliably evaluate disease severity in patients with PGM1-CDG on the basis of their clinical history and presentation. We also identified 5 clinical features that are predictors of disease severity; 2 of these features can be evaluated by physical examination, without the need for specific diagnostic testing and thus allow for rapid assessment and initiation of therapy.

Mitotic Intragenic Recombination: A Mechanism of Survival for Several Congenital Disorders of Glycosylation

Congenital disorders of glycosylation (CDGs) are disorders of abnormal protein glycosylation that affect multiple organ systems. Because most CDGs have been described in only a few individuals, our understanding of the associated phenotypes and the mechanisms of individual survival are limited. In the process of studying two siblings, aged 6 and 11 years, with MOGS-CDG and biallelic MOGS (mannosyl-oligosaccharide glucosidase) mutations (GenBank: NM_006302.2; c.[65C>A; 329G>A] p.[Ala22Glu; Arg110His]; c.[370C>T] p.[Gln124(∗)]), we noted that their survival was much longer than the previous report of MOGS-CDG, in a child who died at 74 days of age. Upon mutation analysis, we detected multiple MOGS genotypes including wild-type alleles in their cultured fibroblast and peripheral blood DNA. Further analysis of DNA from cultured fibroblasts of six individuals with compound heterozygous mutations of PMM2 (PMM2-CDG), MPI (MPI-CDG), ALG3 (ALG3-CDG), ALG12 (ALG12-CDG), DPAGT1 (DPAGT1-CDG), and ALG1 (ALG1-CDG) also identified multiple genotypes including wild-type alleles for each. Droplet digital PCR showed a ratio of nearly 1:1 wild-type to mutant alleles for most, but not all, mutations. This suggests that mitotic recombination contributes to the survival and the variable expressivity of individuals with compound heterozygous CDGs. This also provides an explanation for prior observations of a reduced frequency of homozygous mutations and might contribute to increased levels of residual enzyme activity in cultured fibroblasts of individuals with MPI- and PMM2-CDGs.