A new insight into PMM2 mutations in the French population.

Congenital disorder of Glycosylation type Ia is an autosomal recessive disorder, characterized by a central nervous system dysfunction and multiorgan failure associated with defective N-glycosylation and phosphomannomutase (PMM) deficiency related to mutations in the PMM2 gene (mRNA U85773.1, gene ID 5373). More than 75 different mutations have been previously described. In our study, 38 different mutations were found in 52 French families with CDG-Ia. Eleven mutations had not been previously published in CDG-Ia patients: eight missense and three splice mutations. We studied the PMM activity of eight novel recombinant mutant proteins in an E. coli expression system, comparing them with the wild type protein, c.422 G>A (R141H), and c.415 G>A (E139K) mutant proteins. We also studied the previously described c.590 C>A (E197A) found on the same allele as c.394 A>T (I132F). All mutant proteins studied except E197A had decreased activity and/or were thermolabile, and were pathogenic mutations. Haplotype studies revealed a founder effect for E139K mutation, only described in France and found in seven CDG-Ia families (7.6%). In contrast, at least two different haplotypes were observed for the R141H mutation in France, studied in 23 families. The R141H seems to be a combination of the "old" R141H mutation found all over Europe and a second "French" R141H, and could be substantially older than E139K.

From splitting GLUT1 deficiency syndromes to overlapping phenotypes.

INTRODUCTION: Glucose transporter type 1 deficiency syndrome (GLUT1DS) is a rare genetic disorder due to mutations or deletions in SLC2A1, resulting in impaired glucose uptake through the blood brain barrier. The classic phenotype includes pharmacoresistant epilepsy, intellectual deficiency, microcephaly and complex movement disorders, with hypoglycorrhachia, but milder phenotypes have been described (carbohydrate-responsive phenotype, dystonia and ataxia without epilepsy, paroxysmal exertion-induced dystonia). The aim of our study was to provide a comprehensive overview of GLUT1DS in a French cohort. METHODS: 265 patients were referred to the French national laboratory for molecular screening between July 2006 and January 2012. Mutations in SLC2A1 were detected in 58 patients, with detailed clinical data available in 24, including clinical features with a focus on their epileptic pattern and electroencephalographic findings, biochemical findings and neuroimaging findings. RESULTS: 53 point mutations and 5 deletions in SLC2A1 were identified. Most patients (87.5%) exhibited classic phenotype with intellectual deficiency (41.7%), epilepsy (75%) or movement disorder (29%) as initial symptoms at a medium age of 7.5 months, but diagnostic was delayed in most cases (median age at diagnostic 8 years 5 months). Sensitivity to fasting or exertion in combination with those 3 main symptoms were the main differences between mutated and negative patients (p < 0.001). Patients with myoclonic seizures (52%) evolved with more severe intellectual deficiency and movement disorders compared with those with Early Onset Absence Epilepsy (38%). Three patients evolved from a classic phenotype during early childhood to a movement disorder predominant phenotype at a late childhood/adulthood. CONCLUSIONS: Our data confirm that the classic phenotype is the most frequent in GLUT1DS. Myoclonic seizures are a distinctive feature of severe forms. However a great variability among patients and overlapping through life from milder classic phenotype to paroxysmal-prominent- movement-disorder phenotype are possible, thus making it difficult to identify definite genotype-phenotype correlations.

Expanding the Spectrum of PMM2-CDG Phenotype.

Congenital Disorders of Glycosylation (CDG) are a group of recently described inborn errors of metabolism affecting glycosylation. CDG are disorders that have been reported with a great variability in the clinical presentation, especially for the most common PMM2-CDG. The classical form is neurologic but severe forms with multisystem disorders and hydrops fetalis have been described. Here, we extend on the opposite end the clinical spectrum to an asymptomatic PMM2-CDG case. The case was the father of a child who died of neonatal galactosemia few days after birth. He presented without any clinical or biological signs, except a typical CDG 1 pattern in Western blot of glycoproteins associated with a deficient phosphomannomutase activity in blood leukocytes and compound heterozygosity in PMM2 gene. The sister of the father, who was also affected by PMM deficiency, presented with infertility and premature ovarian failure. Finally, the absence of any abnormal clinical or biological signs as for the case completes the clinical spectrum of PMM2-CDG at its extreme end, at the opposite of the supposed total lethality of the R141H homozygous status.

Identification of roles for peptide: N-glycanase and endo-beta-N-acetylglucosaminidase (Engase1p) during protein N-glycosylation in human HepG2 cells.

BACKGROUND: During mammalian protein N-glycosylation, 20% of all dolichol-linked oligosaccharides (LLO) appear as free oligosaccharides (fOS) bearing the di-N-acetylchitobiose (fOSGN2), or a single N-acetylglucosamine (fOSGN), moiety at their reducing termini. After sequential trimming by cytosolic endo beta-N-acetylglucosaminidase (ENGase) and Man2c1 mannosidase, cytosolic fOS are transported into lysosomes. Why mammalian cells generate such large quantities of fOS remains unexplored, but fOSGN2 could be liberated from LLO by oligosaccharyltransferase, or from glycoproteins by NGLY1-encoded Peptide-N-Glycanase (PNGase). Also, in addition to converting fOSGN2 to fOSGN, the ENGASE-encoded cytosolic ENGase of poorly defined function could potentially deglycosylate glycoproteins. Here, the roles of Ngly1p and Engase1p during fOS metabolism were investigated in HepG2 cells. METHODS/PRINCIPAL FINDINGS: During metabolic radiolabeling and chase incubations, RNAi-mediated Engase1p down regulation delays fOSGN2-to-fOSGN conversion, and it is shown that Engase1p and Man2c1p are necessary for efficient clearance of cytosolic fOS into lysosomes. Saccharomyces cerevisiae does not possess ENGase activity and expression of human Engase1p in the png1Delta deletion mutant, in which fOS are reduced by over 98%, partially restored fOS generation. In metabolically radiolabeled HepG2 cells evidence was obtained for a small but significant Engase1p-mediated generation of fOS in 1 h chase but not 30 min pulse incubations. Ngly1p down regulation revealed an Ngly1p-independent fOSGN2 pool comprising mainly Man(8)GlcNAc(2), corresponding to approximately 70% of total fOS, and an Ngly1p-dependent fOSGN2 pool enriched in Glc(1)Man(9)GlcNAc(2) and Man(9)GlcNAc(2) that corresponds to approximately 30% of total fOS. CONCLUSIONS/SIGNIFICANCE: As the generation of the bulk of fOS is unaffected by co-down regulation of Ngly1p and Engase1p, alternative quantitatively important mechanisms must underlie the liberation of these fOS from either LLO or glycoproteins during protein N-glycosylation. The fully mannosylated structures that occur in the Ngly1p-dependent fOSGN2 pool indicate an ERAD process that does not require N-glycan trimming.

PMM2 intronic branch-site mutations in CDG-Ia.

Congenital Disorders of Glycosylation (CDG, OMIM#212065)-Ia is an autosomal recessive disorder, characterized by central nervous system dysfunction and multiorgan failure associated with mutations in the PMM2 gene. We report two patients who are compound heterozygotes with respect to two new intronic mutations that affect a highly conserved adenosine in a consensus branch-site sequence. The mutations, one in intron 7: c.340 -23A > G (IVS7 -23A > G) and the other in intron 2: c.179 -25A > G (IVS2 -25A > G), are associated with the c.422G > A (R141H) and c.193 G > T (D65Y) mutations, respectively. The c.179 -25A > G and the c.340 -23A > G changes cause exon 3 and exon 8 to be lost at the RNA level, respectively. This kind of mutation can cause a problem in molecular diagnosis of CDG-Ia if intronic primers are not correctly chosen, and if molecular diagnosis is not performed at both the DNA and mRNA levels.