Creation of an additional glycosylation site as a mechanism for type I antithrombin deficiency.

We report the identification of a new mutation resulting in type I antithrombin (AT) deficiency and the mechanism by which the deficiency arose. The single base substitution of G to A at nucleotide 2709 was identified in a proband with a family history of venous thrombosis. The mutation results in a substitution of 82 Ser by Asn, creating a new glycosylation site. Expression studies were then carried out, to confirm Asn-linked glycosylation occurred at this consensus site and that this resulted in the AT deficient phenotype. Cell-free translations using rabbit reticulocyte lysate in the presence of microsomes demonstrated that the 82 Asn variant was post-translationally processed efficiently. The 82 Asn variant protein was of a higher molecular weight than normal AT. consistent with the addition of a fifth glycan chain. Incubation of translation product with endoglycosidase H, confirmed that the higher molecular weight product had resulted from additional carbohydrate. Expression of the 82 Asn variant in COS-7 cells resulted in intracellular accumulation, with a low level of secretion of the protein into culture supernatant, consistent with type I AT deficiency. The addition of an extra carbohydrate side chain to residue 82 of antithrombin may block post-translational folding. trapping the variant intracellulary.     

Characterisation of blood coagulation factor XI T475I

PCR-SSCP and DNA sequence analysis of a factor XI (FXI) deficient patient (FXI:C 39 U/dL; FXI:Ag 27 U/dL) identified a C to T transition in exon 12 of the FXI gene (F11 c.1521C>T) that predicts the substitution of Thr475 by Ile (FXI T475I) within the serine protease domain of FXI. This mutation destroys a consensus sequence for N-linked glycosylation, N473-Y-T475, known to be utilized in vivo. The FXIT475I variant was generated by site-directed mutagenesis, together with other variants that could help explain the phenotype, and recombinant FXI variants were expressed in Chinese hamster ovary cells. FXI:Ag expression was analysed by Western blot analysis, ELISA and immunocytochemical staining. Wild-type FXI:Ag was secreted at high levels, however the mutant (FXI T475I) was secreted very poorly. Substitution of Thr475 by Ala, Pro, Lys or Arg (all of which abolish the glycosylation consensus sequence) also severely reduced the level of secreted FXI:Ag suggesting that glycosylation at Asn473 is required for folding or secretion. Concordant with this hypothesis the conservative substitution of Thr475 by Ser (which preserves the glycosylation consensus sequence) had no effect on FXI secretion. Thr/Ser475 is highly conserved in serine protease domains but the glycosylation site (Asn473) is not. Surprisingly, substitution of Asn473 by Ala (which removes the N-linked glycosylation site) had no effect on the levels of FXI:Ag secreted. In conclusion, although the FXI-T475I mutation destroys an N-linked glycosylation consensus sequence, the cause of failure to secrete FXI is not the loss of a glycosylation site but rather a direct effect of the substitution of this highly conserved residue.     

Modulation of clearance of recombinant serum albumin by either glycosylation or truncation

Albumin is an abundant non-glycosylated plasma protein with a slow clearance profile. It has been employed as a fusion partner in efforts to slow the clearance of small antithrombotic proteins like hirudin. In the present study, the in vivo clearance of recombinant rabbit serum albumin (rRSA), of mutant rRSAs containing consensus sequences for N-linked glycosylation (D494N and V14T variants), and of mutant mini-proteins truncated at albumin domain boundaries (rRSAs 1-185, 1-377, or 378-584) was examined. Mean terminal catabolic half-lives (t_0.5cat) in rabbits for plasma-derived RSA, rRSA, and the V14T variant did not differ significantly (range 4.32–4.76 days). In contrast, mean t_0.5cat was reduced to 2.87 days for the D494N variant and to less than 0.071 days for all mini-proteins. The mini-proteins were found in the urine in tissue distribution experiments, suggesting a renal route of clearance. Our results suggest that all three internally repeated albumin domains are required to maintain the slow in vivo clearance profile of albumin, and that albumin glycosylation can be associated with an acceleration of clearance. This information could be used to design fusion proteins, including those with antithrombotic properties, with predictably altered in vivo half-lives less than that of serum albumin.     

Gradually glycosylated protein C mutants (Arg178Gln and Cys331Arg) are degraded by proteasome after mannose trimming

Proteins that fail to attain their correct three-dimensional structure are retained in the endoplasmic reticulum (ER) and eventually degraded within the cells. We investigated the degradation of mutant proteins, using naturally occurring protein C (PC) mutants (Arg178Gln and Cys331Arg) which lead to congenital deficiencies. Chinese hamster ovary (CHO) cells were transfected with normal or mutant expression vectors. The introduction of mutation at Asn329 of an unusual sequence Asn-X-Cys for N-linked glycosylation revealed that the mutation at Cys331, which may preclude a formation of disulfide bond with Cys345, resulted in no addition of N-linked oligosaccharides at Asn329. PC mutants with 4 glycosylation sites were gradually glycosylated in the ER, and the fourth glycosylation site is less accessible for glycosylation as reported for PC in plasma. The half lives of PC178 and PC331 mutants were about 5 and 4 h, respectively. PC mutants were degraded, but the degradation was inhibited by inhibitors for proteasome. Mannose trimming of N-linked oligosaccharides after glucose removal targeted PC mutants for degradation by proteasomes. And also the inhibition of glucose trimming immediately led to mannose trimming, resulting in the accelerated degradation of PC mutants. These degradations were inhibited by mannosidase I inhibitor, kifunensine. These results indicate that the initiation of mannose trimming by mannosidase I leads to the proteasomemediated degradation of glucose-trimmed or untrimmed PC mutants.     

Role of each Asn-linked glycan in the anticoagulant activity of human protein C inhibitor

The N-glycosylation site mutants of human protein C inhibitor (PCI; N230S, N243Q, N319Q, N230S/N243Q, and N230S/N319Q) were prepared by amino acid replacement of the asparagine residue with a serine or glutamine residue using site-directed mutagenesis and expressed in the baculovirus/insect cell expression system. To examine the importance of each Asn-linked glycan in the activity of PCI, we compared wtPCI with the mutants of N-glycosylation site(s) in terms of the procoagulant protease-inhibitory and anticoagulant activities. The inhibitory activities of N230S, N319Q, and N230S/N319Q toward human thrombin and plasma kallikrein were significantly increased compared with wtPCI, but those of N243Q and N230S/N243Q were reduced. The inhibitory activity of N230S toward human plasma coagulation was significantly increased compared with wtPCI, and that of N230S/N319Q was also significantly increased compared with N319Q. Furthermore, the procoagulant protease-inhibitory and anticoagulant activities of N230S/N319Q (glycosylated on Asn243 only) compared favorably with those of N230S, and both of the mutants possessed highest activities in the purified mutants. These results suggest that the Asn243-linked glycan in PCI molecule possesses critical roles for its anticoagulant activity in the circulation, and the Asn230-linked glycan down-regulates the activity of PCI.     

N-glycosylation of acid-sensing ion channel 1a regulates its trafficking and acidosis-induced spine remodeling

Acid-sensing ion channel-1a (ASIC1a) is a potential therapeutic target for multiple neurological diseases. We studied here ASIC1a glycosylation and trafficking, two poorly understood processes pivotal in determining the functional outcome of an ion channel. We found that most ASIC1a in the mouse brain was fully glycosylated. Inhibiting glycosylation with tunicamycin reduced ASIC1a surface trafficking, dendritic targeting, and acid-activated current density. N-glycosylation of the two glycosylation sites, Asn393 and Asn366, has differential effects on ASIC1a biogenesis. Maturation of Asn393 increased ASIC1a surface and dendritic trafficking, pH sensitivity, and current density. In contrast, glycosylation of Asn366 was dispensable for ASIC1a function and may be a rate-limiting step in ASIC1a biogenesis. In addition, we revealed that acidosis reduced the density and length of dendritic spines in a time- and ASIC1a-dependent manner. ASIC1a N366Q, which showed increased glycosylation and dendritic targeting, potentiated acidosis-induced spine loss. Conversely, ASIC1a N393Q, which had diminished dendritic targeting and inhibited ASIC1a current dominant-negatively, had the opposite effect. These data tie N-glycosylation of ASIC1a with its trafficking. More importantly, by revealing a site-specific effect of acidosis on dendritic spines, our findings suggest that these processes have an important role in regulating synaptic plasticity and determining long-term consequences in diseases that generate acidosis.     

The circulatory half-lives of alpha-profibrin and alpha-fibrin monomer,and comparisons with other fibrin(ogen) derivatives

Previous studies showed that alpha-fibrin monomer (lacking both A-fibrinopeptides, FPA) is normally cleared from the circulation before it assembles into a clot. Recent studies indicate that substantial quantities of an intermediate, alpha-profibrin lacking only one of the two FPA are produced in the course of conversion of human fibrinogen to fibrin. Since clearance of the alpha-fibrin monomer is saturable and receptor mediated, the extent to which alpha-profibrin or other fibrin(ogen) derivatives might compete for monomer uptake was deemed important. We compared plasma decay of injected human alpha-fibrin, fibrinogen, and alpha-profibrin in rabbits using rabbit anti-human fibrinogen for assays. The circulatory half-life of human alpha-fibrin monomer was short (t(1/2) = 2.3 h) and followed a simple exponential decay curve, as anticipated from clearance of rabbit alpha-fibrin. It was absorbed as fast as it permeated the extravascular space with no redistribution. Human fibrinogen had a long half-life (t(1/2) = 39.5 h), calculated from the double exponential plasma decay curves (redistribution + catabolism) observed over 28 h. The alpha-profibrin had an intermediary half-life (t(1/2) = 11 h) determined from double exponential decay curves. Since redistribution accompanied the slow clearance of alpha-profibrin, its binding by the fibrin receptor(s) must be weak, probably too weak to compete with the clearance of alpha-fibrin monomer. The initial production of alpha-fibrin monomer is only partially dependent on prior formation of alpha-profibrin, as recently shown. Thus, it is the slow clearance and the weak competition from alpha-profibrin that underlie the occurrence of substantial levels of alpha-profibrin unaccompanied by detectable levels of alpha-fibrin monomer in many subjects with vascular disease.     

The influence of carbohydrate structure on the clearance of recombinant tissue-type plasminogen activator

Modification of the carbohydrate structures of recombinant tissue-type plasminogen activator (rt-PA) can increase or decrease its rate of clearance in rabbits. When rt-PA was treated with sodium periodate to oxidize carbohydrate residues, the rate of clearance was decreased from 9.6 +/- 1.9 ml min-1 kg-1 to 3.5 +/- 0.6 ml min-1 kg-1 (mean +/- SD, n = 5). A similar change in the clearance of rt-PA was introduced by the use of endo-beta-N-acetyl-glucosaminidase H (Endo-H), which selectively removes high mannose asparagine-linked oligosaccharides; the clearance of Endo-H-treated rt-PA was 5.0 +/- 0.5 ml min-1 kg-1. A mutant of rt-PA was produced with an amino acid substitution at position 117 (Asn replaced with Gln) to remove a potential glycosylation site that normally contains a high mannose structure. The clearance of this material was also decreased, similar to the periodate and Endo-H-treated rt-PA. Conversely, when rt-PA was produced in the CHO 15B cell line, which can produce only high mannose oligosaccharide structures on glycoproteins, the clearance was increased by a factor of 1.8. These results demonstrate that the removal of rt-PA from the blood depends significantly upon the nature of its oligosaccharide structures.     

Glycans of myelin proteins

Human P0 is the main myelin glycoprotein of the peripheral nervous system. It can bind six different glycans, all linked to Asn⁹³, the unique glycosylation site. Other myelin glycoproteins, also with a single glycosylation site (PMP22 at Asn³⁶, MOG at Asn³¹), bind only one glycan. The MAG has 10 glycosylation sites; the glycoprotein OMgp has 11 glycosylation sites. Aside from P0, no comprehensive data are available on other myelin glycoproteins. Here we review and analyze all published data on the physicochemical structure of the glycans linked to P0, PMP22, MOG, and MAG. Most data concern bovine P0, whose glycan moieties have an MW ranging from 1,294.56 Da (GP3) to 2,279.94 Da (GP5). The pI of glycosylated P0 protein varies from pH 9.32 to 9.46. The most charged glycan is MS2 containing three sulfate groups and one glucuronic acid; whereas the least charged one is the BA2 residue. All glycans contain one fucose and one galactose. The most mannose rich are the glycans MS2 and GP4, each of them has four mannoses; OPPE1 contains five N‐acetylglucosamines and one sulfated glucuronic acid; GP4 contains one sialic acid. Furthermore, human P0 variants causing both gain and loss of glycosylation have been described and cause peripheral neuropathies with variable clinical severity. In particular, the substitution T⁹⁵→M is a very common in Europe and is associated with a late‐onset axonal neuropathy. Although peripheral myelin is made up largely of glycoproteins, mutations altering glycosylation have been described only in P0. This attractive avenue of research requires further study. © 2014 Wiley Periodicals, Inc.     

The GIGYF2 variants are not associated with Parkinson's disease in the mainland Chinese population

In order to determine the prevalence of GIGYF2 (Grb10-Interacting GYF Protein 2) variants in the Chinese population and to better understand the association between GIGYF2 and Parkinson's disease (PD), we conducted the genetic screening of GIGYF2 in the Chinese population. Twelve exonic variants were identified in 52 familial PD probands and 56 healthy controls. Non-synonymous point variants (Thr25Ala, Asn457Th and Pro460Th) were analyzed in 510 PD patients and 481 healthy controls of Chinese Han ethnicity. The insertion and deletion variants in Exon 25 (Ins Q 1212, Ins QQ 1217, Del Q 1210, Del Q 1216 and Del PPQ1217_1219) are not related to the onset of familial PD. Our data indicate the GIGYF2 variants are not associated with PD in the mainland Chinese Population.     

An important role for the activation peptide domain in controlling factor IX levels in the blood of haemophilia B mice

The factors responsible for the removal of injected factor IX (fIX) from the blood of individuals with haemophilia B are only partly understood, and may include binding to endothelial or subendothelial sites, passive extravasation related to size or charge, or interactions requiring fIX activation. To investigate these issues, we have produced and characterised recombinant fIX proteins with amino acid changes: delta155-177, an internal deletion which removes most of the activation peptide while retaining the activation cleavage sites; S365A, which inactivates the serine protease activity of fIXa; and K5A, previously shown to eliminate fIX binding of endothelial/subendothelial collagen IV. All proteins were expressed in stably transfected HEK 293 cells, purified by immunoaffinity chromatography, and compared to the wild type HEK 293-derived protein (fIX (WT)). Mutant fIX proteins K5A and delta155-177 exhibited 72 and 202% of the specific activity of fIX (WT), respectively; S365A was without activity. Following intravenous injection in haemophilia B (fIX knockout) mice, recoveries did not differ for fIX (WT) and delta155-177, but were higher for K5A and S365A.The terminal catabolic half-life of delta155-177, alone among the mutants, was increased, by 45% versus fIX (WT). Nine hours post-injection, the observed areas under the clearance curve (AUCs) of delta155-177 and K5, but not S365A, were elevated 2-fold. delta155-177 was equally effective as fIX (WT) in reducing blood loss following tail vein transection in haemophilia B mice. Our results suggest that deletion of the multiple sites of fIX post-translational modification found within the activation peptide eliminated important fIX clearance motifs.     

Clearance of recombinant tissue factor pathway inhibitor (TFPI) in rabbits.

The pharmacokinetics of recombinant tissue factor pathway inhibitor (TFPI) after an intravenous bolus injection was studied in rabbits. Clearance of TFPI was followed by measurement of the radioactivity of the 125I-labelled compound in the whole plasma or the trichloroacetic acid precipitate and by quantitation of the functional TFPI activity of the unlabelled compound using a tissue factor-induced coagulation assay. When iodinated TFPI was used, the ratios of the trichloroacetic acid precipitable counts vs. that of the whole plasma was about 1 in the first 10 min after TFPI injection, but this ratio gradually decreased to less than 0.5 after 2 h. This result suggested that the iodinated TFPI in the plasma was partially degraded after prolonged circulation in the animal. When unlabelled TFPI was used, the clearance of TFPI activity from the plasma exhibited bi-exponential elimination kinetics with a rapid alpha phase half-life (t1/2 alpha) of 2.3 min, and a terminal beta phase half-life (t1/2 beta) of 79 min. The plasma clearance was 4.2 ml kg-1 min-1. The tissue distribution of intravenously administered 125I-TFPI in the rabbit was studied using whole-body autoradiography. At 3 min after dosing, significant levels of TFPI were apparent in the liver, kidney, and other highly blood perfused tissues. Significant levels of 125I-TFPI-derived radioactivity were also apparent in the liver and kidney at 30 min after intravenous administration. The localization within the liver demonstrated a mottled appearance, suggesting regions of higher uptake within the liver. In the kidney, the outer cortex consistently revealed the highest activity.     

Recombinant variants of tissue-type plasminogen activator containing amino acid substitutions in the finger domain.

Tissue-type plasminogen activator (t-PA) is a fibrin-specific agent which has been used to treat acute myocardial infarction. In an attempt to clarify the determinants for its rapid clearance in vivo and high affinity for fibrin clots, we produced five variants containing amino acid substitutions in the finger domain, at amino acid residues 7-9, 10-14, 15-19, 28-33, and 37-42. All the variants had a prolonged half-life and a decreased affinity for fibrin of various degrees. The 37-42 variant demonstrated about a 6-fold longer half-life with a lower affinity for fibrin. Human plasma clot lysis assay estimated the fibrinolytic activity of the 37-42 variant to be 1.4-fold less effective than that of the wild-type rt-PA. In a rabbit jugular vein clot lysis model, doses of 1.0 and 0.15 mg/kg were required for about 70% lysis in the wild-type and 37-42 variant, respectively. Fibrinogen was degraded only when the wild-type rt-PA was administered at a dose of 1.0 mg/kg. These findings suggest that the 37-42 variant can be employed at a lower dosage and that it is a more fibrin-specific thrombolytic agent than the wild-type rt-PA.     

An Autism-Associated Mutation Impairs Neuroligin-4 Glycosylation and Enhances Excitatory Synaptic Transmission in Human Neurons

Neuroligins (NLGNs) are a class of postsynaptic cell adhesion molecules that interact with presynaptic neurexins (NRXNs) and regulate synapse function. NLGN4 is a member of the NLGN family and consists of a unique amino acid sequence in humans that is not evolutionarily well conserved in rodents. The human-specific NLGN4 gene has been reported to be mutated in many patients with autism and other neurodevelopmental disorders. However, it remained unclear how these mutations might alter the molecular properties of NLGN4 and affect synaptic transmission in human neurons. Here, we describe a severely autistic male patient carrying a single amino acid substitution (R101Q) in the NLGN4 gene. When expressed in HEK293 cells, the R101Q mutation in NLGN4 did not affect its binding affinity for NRXNs or its capacity to form homodimers. This mutation, however, impaired the maturation of NLGN4 protein by inhibiting N-linked glycosylation at an adjacent residue (N102), which is conserved in all NLGNs. As a result, the R101Q substitution significantly decreased the surface trafficking of NLGN4 and increased its retention in the endoplasmic reticulum and Golgi apparatus. In human neurons derived from male stem cell lines, the R101Q mutation also similarly reduced the synaptic localization of NLGN4, resulting in a loss-of-function phenotype. This mutation-induced trafficking defect substantially diminished the ability of NLGN4 to form excitatory synapses and modulate their functional properties. Viewed together, our findings suggest that the R101Q mutation is pathogenic for NLGN4 and can lead to synaptic dysfunction in autism.     

Biochemical importance of glycosylation of plasminogen activator inhibitor-1

The serpin plasminogen activator inhibitor-1 (PAI-1) is a potential target for anti-thrombotic and anti-cancer therapy. PAI-1 has 3 potential sites for N-linked glycosylation. We demonstrate here that PAI-1 expressed recombinantly or naturally by human cell lines display a heterogeneous glycosylation pattern of the sites at N209 and N265, while that at N329 is not utilised. The IC(50)-values for inactivation of PAI-1 by 4 monoclonal antibodies differed strongly between glycosylated PAI-1 and non-glycosylated PAI-1 expressed in E. coli. For 3 antibodies, an overlap of the epitopes with the glycosylation sites could be excluded as explanation for the differential reactivity. The latency transition of non-glycosylated, but not of glycosylated PAI-1, was strongly accelerated by a non-ionic detergent. The different biochemical properties of glycosylated and non-glycosylated PAI-1 depended specifically on glycosylation of either one or the other of the utilised sites. The PAI-1-binding protein vitronectin reversed the changes associated with the lack of glycosylation at one of the sites. Our results stress the importance of the source of PAI-1 when studying the mechanisms of action of PAI-1-inactivating compounds of potential clinical importance.     

Recombinant Kunitz protease inhibitory domain of the amyloid beta-protein precursor as an anticoagulant in venovenous extracorporeal circulation in rabbits

Investigations were performed to characterize a recombinant Kunitz protease inhibitory domain of the amyloid beta-protein precursor (rKPI) as anticoagulants. After a single intravenous infusion of wild type rKPI into dogs, its elimination fit a two compartment model with a t1/2alpha and t1/2beta of 5 and 77 min, respectively. Further investigations determined if a variant form of rKPI with 178-fold more potent anti-factor Xa activity (rKPI-DD135, Ki = 0.9 nM) could serve as an anticoagulant in a rabbit model of extracorporeal circulation using a venovenous shunt. A prospective investigation was initiated to compare standard heparin (n = 8) at 400 U/kg with different infusion concentrations of rKPI-DD135. After a single intravenous infusion of 1.89 mg/kg of rKPI-DD135 followed by a constant infusion at 0.003 (n = 3), 0.03 (n = 7), or 0.3 (n = 5) mg/kg/min, the anti-factor Xa activity of the animals' plasma rapidly reaches a steady state for the two lower infusion concentrations of the agent. All infusions of rKPI-DD135 prolong the activated clotting time with less variation than that seen with heparin administration. rKPI-DD135 anticoagulation does not prevent a drop in the platelet counts. Fibrinogen levels decrease only slightly when the circuit is anticoagulated with rKPI-DD135. rKPI-DD135 markedly prolongs the APTT, has little effect on the PT, and reduces plasma prekallikrein and plasminogen activation. The 0.3 mg/kg/min infusion concentration of rKPI-DD135 results in reduced deposition of 111Indium-labeled platelets on the circuit when compared to heparin. Last, after a steady state level is achieved, 60% of the plasma anti-factor Xa activity of rKPI-DD135 is eliminated within 60 min after stopping the infusion. These data show the rKPI-DD135 can provide single agent anticoagulation in a rabbit extracorporeal circuit. Development of short acting factor Xa inhibitors may be useful anticoagulants for cardiopulmonary bypass.     

Elimination of antithrombin III concentrate in healthy pregnant and preeclamptic women with an acquired antithrombin III deficiency

The activity elimination half-life of heat-treated antithrombin III (AT III) concentrate was studied in 5 healthy pregnant and 5 preeclamptic women with a documented AT III deficiency. Healthy pregnant women received 1500 units over 20 minutes. Serial blood specimens were obtained over the next 12 hours. The mean (+/- SEM) activity elimination half-life of AT III was 29.4h +/- 3.4h. Preeclamptic subjects had a mean baseline AT III activity of 70.5 +/- 2% (range 61 to 75%). Their activity eliminator half-life after 3000 units of AT III concentrate was 8.5 +/- 1.2h. There was a direct relationship between the pre-concentrate AT III activity level and the AT III activity elimination half-life (r = 0.79, p = 0.01) for all subjects. Based upon parameters calculated from the first infusion, the AT III activity of preeclamptic subjects was maintained by a constant infusion at approximately 100% for 96h. At the conclusion of the infusion, the activity elimination half-life was again measured. A dramatic increase in the activity elimination half-life was demonstrated (433.6h). We conclude that the activity elimination half-life of AT III concentrate is increased during normal pregnancy and further increased in preeclamptic women with an acquired deficiency.     

Pharmacokinetic properties of recombinant factor VIII compared with a monoclonally purified concentrate (Hemofil M). The Recombinate Study Group.

A recombinant FVIII preparation, Recombinate, was compared with a high-purity plasma-derived concentrate, Hemofil M, in 47 hemophilia A patients in a cross-over evaluation of pharmacokinetic properties. The recombinant material showed a significantly lower clearance, volume of distribution, and higher in vivo recovery, but a similar half-life to the plasma-based product. In a comparison with reported data from other standard concentrates, the recombinant preparation exhibited potentially better pharmacokinetic properties in that its clearance was slower and its half-life was longer. We conclude that the recombinant DNA method of preparation does not adversely affect the biological and pharmacological characteristics of the factor VIII molecule.     

Congenital disorders of glycosylation: the rapidly growing tip of the iceberg.

In addition to many other organs, the brain is affected in 10 of the 11 known congenital disorders of N-linked glycosylation, mostly to a severe degree. Because a large number of enzymes, transporters and other proteins are involved in glycosylation (both N-linked and O-linked), it is expected that the great majority of congenital disorders of glycosylation (CDG) are yet to be identified. Many neurological patients with a CDG escape diagnosis for that reason, but also because existing screening methods fail to detect all patients with a known CDG. These disorders should be looked for in any patient, regardless of age, with an unexplained neurological disorder.     

Myopathy associated with homozygous PYROXD1 pathogenic variants detected by genome sequencing

Biallelic pathogenic variants in the gene PYROXD1 have recently been described to cause early-onset autosomal recessive myopathy. Myopathy associated with PYROXD1 pathogenic variants is rare and reported in only 17 individuals. Known pathogenic variants in PYROXD1 include missense, insertion and essential splice-site variants. Here we describe a consanguineous family of individuals affected with late-onset myopathy and homozygous PYROXD1 missense variants (NM_024854.5:c.464A>G [p.Asn155Ser]) expanding our understanding of the possible disease phenotypes of PYROXD1-associated myopathy.