Comprehensive description of the phenotype of the first case of congenital disorder of glycosylation due to RFT1 deficiency (CDG In)

Congenital Disorders of Glycosylation from a Neurological Perspective

Most plasma proteins, cell membrane proteins and other proteins are glycoproteins with sugar chains attached to the polypeptide-glycans. Glycosylation is the main element of the post-translational transformation of most human proteins. Since glycosylation processes are necessary for many different biological processes, patients present a diverse spectrum of phenotypes and severity of symptoms. The most frequently observed neurological symptoms in congenital disorders of glycosylation (CDG) are: epilepsy, intellectual disability, myopathies, neuropathies and stroke-like episodes. Epilepsy is seen in many CDG subtypes and particularly present in the case of mutations in the following genes: ALG13, DOLK, DPAGT1, SLC35A2, ST3GAL3, PIGA, PIGW, ST3GAL5. On brain neuroimaging, atrophic changes of the cerebellum and cerebrum are frequently seen. Brain malformations particularly in the group of dystroglycanopathies are reported. Despite the growing number of CDG patients in the world and often neurological symptoms dominating in the clinical picture, the number of performed screening tests eg transferrin isoforms is systematically decreasing as broadened genetic testing is recently more favored. The aim of the review is the summary of selected neurological symptoms in CDG described in the literature in one paper. It is especially important for pediatric neurologists not experienced in the field of metabolic medicine. It may help to facilitate the diagnosis of this expanding group of disorders. Biochemically, this paper focuses on protein glycosylation abnormalities.

A family with floppy neonates with severe respiratory insufficiency: A lethal phenotype of RFT1-CDG due to a novel mutation

Congenital disorders of glycosylation (CDG) are a rapidly expanding group of inborn errors of metabolism with around 100 types described so far. Because of the limited number of reported cases in each type except PMM2-CDG, the complete clinical picture of other types is not known. RFT1-CDG is a rare type, with ten cases reported in the literature. Our patient presented as a floppy neonate with severe respiratory insufficiency and ventilator dependence in the newborn period. He had fetal growth restriction, facial dysmorphism, high arched palate, bilateral cryptorchidism, hypoplastic pons and cerebellum and probable hearing impairment. He succumbed to the illness on day 24 of life. There was a similar history of two previous sibling deaths in the early neonatal period due to respiratory insufficiency and history of multiple neonatal and infant deaths in the extended family. Transferrin iso-electric focusing was normal. Clinical exome sequencing revealed a novel homozygous missense mutation (c.1018 G > A) in RFT1 gene [NM_052859; c.1018G > A; p.G340S; ENST00000296292] and the parents were heterozygous for the same (ClinVar SVC000778540). The pathogenic variants so far reported are all missense variants affecting the luminal loops; whereas the variant in our case is in the trans-membrane helical domain. A strong family history of neonatal deaths and similar presentations in the previous 2 siblings suggests the homogenous phenotype of this mutation. Severe respiratory insuffiency and ventilator dependence shows the lethality of the disease phenotype and incompatibility with survival beyond the neonatal period.

RFT1-CDG: Absence of Epilepsy and Deafness in Two Patients with Novel Pathogenic Variants

This report is on two novel patients with RFT1-CDG. Their phenotype is characterized by mild psychomotor disability, behavioral problems, ataxia, and mild dysmorphism. Neither of them shows signs of epilepsy, which was observed in all RFT1-CDG patients reported to date (n = 14). Also, deafness, which is often associated with this condition, was not observed in our patients. Molecular analysis of RFT1 showed biallelic missense variants including three novel ones: c.827G > A (p.G276D), c.73C > T (p.R25W), and c.208T > C (p.C70R).

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.

RFT1-CDG in adult siblings with novel mutations

RFT1-CDG is a rare N-glycosylation disorder. Only 6 children with RFT1-CDG have been described, all with failure to thrive, feeding problems, hypotonia, developmental delay, epilepsy, decreased vision, deafness and thrombotic complications. We report on two young adult siblings with RFT1-CDG, compound heterozygotes for the novel missense mutations c.1222A>G (p.M408V) and c.1325G>A (p.R442Q) in RFT1 gene. Similar to the previously described patients, these siblings have profound intellectual disability but no feeding problems or failure to thrive. Their epilepsy is well controlled and coagulopathy is mild without clinical consequences. In addition, visual acuity is normal in both patients and hearing impairment is present only in one. Our findings extend the phenotype associated with RFT1-CDG.

Congenital protein hypoglycosylation diseases

Glycosylation is an essential process by which sugars are attached to proteins and lipids. Complete lack of glycosylation is not compatible with life. Because of the widespread function of glycosylation, inherited disorders of glycosylation are multisystemic. Since the identification of the first defect on N-linked glycosylation in the 1980s, there are over 40 different congenital protein hypoglycosylation diseases. This review will include defects of N-linked glycosylation, O-linked glycosylation and disorders of combined N- and O-linked glycosylation.

RFT1 deficiency in three novel CDG patients

The medical significance of N-glycosylation is underlined by a group of inherited human disorders called Congenital Disorders of Glycosylation (CDG). One key step in the biosynthesis of the Glc(3)Man(9)GlcNAc(2)-PP-dolichol precursor, essential for N-glycosylation, is the translocation of Man(5)GlcNAc(2)-PP-dolichol across the endoplasmic reticulum membrane. This step is facilitated by the RFT1 protein. Recently, the first RFT1-deficient CDG (RFT1-CDG) patient was identified and presented a severe N-glycosylation disorder. In the present study, we describe three novel CDG patients with an RFT1 deficiency. The first patient was homozygous for the earlier reported RFT1 missense mutation (c.199C>T; p.R67C), whereas the two other patients were homozygous for the missense mutation c.454A>G (p.K152E) and c.892G>A (p.E298 K), respectively. The pathogenic character of the novel mutations was illustrated by the accumulation of Man(5)GlcNAc(2)-PP-dolichol and by reduced recombinant DNase 1 secretion. Both the glycosylation pattern and recombinant DNase 1 secretion could be normalized by expression of normal RFT1 cDNA in the patients' fibroblasts. The clinical phenotype of these patients comprised typical CDG symptoms in addition to sensorineural deafness, rarely reported in CDG patients. The identification of additional RFT1-deficient patients allowed to delineate the main clinical picture of RFT1-CDG and confirmed the crucial role of RFT1 in Man(5)GlcNAc(2)-PP-dolichol translocation.

RFT1-CDG: deafness as a novel feature of congenital disorders of glycosylation

Congenital disorders of glycosylation (CDG) are genetic diseases due to defects in the synthesis of glycans and in the attachment of glycans to lipids and proteins. Actually, some 42 CDG are known including defects in protein N-glycosylation, in protein O-glycosylation, in lipid glycosylation, and in multiple and other glycosylation pathways. Most CDG are multisystem diseases and a large number of signs and symptoms have already been reported in CDG. An exception to this is deafness. This symptom has not been observed as a consistent feature in CDG. In 2008, a novel defect was identified in protein N-glycosylation, namely in RFT1. This is a defect in the assembly of N-glycans. RFT1 is involved in the transfer of Man(5)GlcNAc(2)-PP-Dol from the cytoplasmic to the luminal side of the endoplasmic reticulum. According to the novel nomenclature (non-italicized gene symbol followed by -CDG) this defect is named RFT1-CDG. Recently, three other patients with RFT1-CDG have been reported and here we report two novel patients. Remarkably, all six patients with RFT1-CDG show sensorineural deafness as part of a severe neurological syndrome. We conclude that RFT1-CDG is the first 'deafness-CDG'. CDG should be included in the work-up of congenital, particularly syndromic, hearing loss.

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.