Congenital disorder of glycosylation type Ix: review of clinical spectrum and diagnostic steps

Single-center experience of congenital disorders of glycosylation syndrome screening in Tunisia: A retrospective study over a 15-year period (2007-2021)

BACKGROUND

We report the results gathered over 15 years of screening for congenital disorders of glycosylation syndrome (CDGS) in Tunisia according to clinical and biochemical characteristics.

METHODS

Our laboratory received 1055 analysis requests from various departments and hospitals, for children with a clinical suspicion of CDGS. The screening was carried out through separation of transferrin isoforms by capillary zone electrophoresis.

RESULTS

During the 15-year period, 23 patients were diagnosed with CDGS (19 patients with CDG-Ia, three patients with CDG-IIx, and one patient with CDG-X). These patients included 13 boys and 10 girls aged between 3 months and 13 years, comprising 2.18 % of the total 1055 patients screened. The incidence for CDGS was estimated to be 1:23,720 live births (4.21 per 100,000) in Tunisia. The main clinical symptoms related to clinical disease state in newborn and younger patients were psychomotor retardation (91 %), cerebellar atrophy (91 %), ataxia (61 %), strabismus (48 %), dysmorphic symptoms (52 %), retinitis pigmentosa, cataract (35 %), hypotonia (30 %), and other symptoms.

CONCLUSION

In Tunisia, CDGS still remains underdiagnosed or misdiagnosed. The resemblance to other diseases, especially neurological disorders, and physicians' unawareness of the existence of these diseases are the main reasons for the underdiagnosis. In routine diagnostics, the screening for CDGS by biochemical tests is mandatory to complete the clinical diagnosis.

A novel variant in ALG1 gene associated with congenital disorder of glycosylation: A case report and short literature review

BACKGROUND

The congenital disorder of glycosylation associated with ALG1 (ALG1-CDG) is a rare autosomal recessive disease. Due to the deficiency of β1,4 mannosyltransferase caused by pathogenic variants in ALG1 gene, the assembly and processing of glycans in the protein glycosylation pathway are impaired, resulting in a broad clinical spectrum with multi-organ involvement. To raise awareness of clinicians for its manifestations and genotype, we here reported a new patient with a novel variant in ALG1 gene and reviewed the literature to study the genotype-phenotype correlation.

METHOD

Clinical characteristics were collected, and clinical exome sequencing was used to identify the causative variants. MutationTaster, PyMol, and FoldX were used to predict the pathogenicity, changes in 3D model molecular structure of protein, and changes of free energy caused by novel variants.

RESULTS

The proband was a 13-month-old Chinese Han male characterized by epileptic seizures, psychomotor development delay, muscular hypotonia, liver and cardiac involvement. Clinical exome sequencing revealed the biallelic compound heterozygosity variants, a previously reported variant c.434G>A (p.G145N, paternal) and a novel variant c.314T>A (p.V105N, maternal). The literature review found that in severe phenotypes, the incidences of clinical manifestations were significantly higher than that in mild phenotypes, including congenital nephrotic syndrome, agammaglobulinemia, and severe hydrops. Homozygous c.773C>T was a strongly pathogenic variant associated with a severe phenotype. When heterozygous for c.773C>T, patients with another variant leading to substitution in amino acids within the strongly conserved regions (c.866A>T, c.1025A>C, c.1182C>G) may cause a more severe phenotype than those within less-conserved regions (c.434G>A, c.450C>G, c.765G>A, c.1287T>A). c.1129A>G, c.1076C>T, and c.1287T>A were more likely to be associated with a mild phenotype. The assessment of disease phenotypes requires a combination of genotype and clinical manifestations.

CONCLUSIONS

The case reported herein adds to the mutations identified in ALG1-CDG and a review of this literature expands the study of the phenotypic and genotypic spectrum of this disorder.

SRD5A3-CDG: 3D structure modeling, clinical spectrum, and computer-based dysmorphic facial recognition

Pathogenic variants in Steroid 5 alpha reductase type 3 (SRD5A3) cause rare inherited congenital disorder of glycosylation known as SRD5A3-CDG (MIM# 612379). To date, 43 affected individuals have been reported. Despite the development of various dysmorphic features in significant number of patients, facial recognition entity has not yet been established for SRD5A3-CDG. Herein, we reported a novel SRD5A3 missense pathogenic variant c.460 T > C p.(Ser154Pro). The 3D structural modeling of the SRD5A3 protein revealed additional transmembrane α-helices and predicted that the p.(Ser154Pro) variant is located in a potential active site and is capable of reducing its catalytic efficiency. Based on phenotypes of our patients and all published SRD5A3-CDG cases, we identified the most common clinical features as well as some recurrent dysmorphic features such as arched eyebrows, wide eyes, shallow nasal bridge, short nose, and large mouth. Based on facial digital 2D images, we successfully designed and validated a SRD5A3-CDG computer based dysmorphic facial analysis, which achieved 92.5% accuracy. The current work integrates genotypic, 3D structural modeling and phenotypic characteristics of CDG-SRD5A3 cases with the successful development of computer tool for accurate facial recognition of CDG-SRD5A3 complex cases to assist in the diagnosis of this particular disorder globally.

Review of clinical and molecular variability in autosomal recessive cutis laxa 2A

ATP6V0A2-related cutis laxa, also known as autosomal recessive cutis laxa type 2A (ARCL2A), is a subtype of hereditary cutis laxa originally characterized by skin, skeletal, and neurological involvement, and a combined defect of N-glycosylation and O-glycosylation. The associated clinical spectrum subsequently expanded to a less severe phenotype dominated by cutaneous involvement. At the moment, ARCL2A was described in a few case reports and series only. An Italian adult woman ARCL2A with a phenotype restricted to skin and the two novel c.3G>C and c.1101dup ATP6V0A2 variants has been reported. A systematic literature review allowed us to identify 69 additional individuals from 64 families. Available data were scrutinized in order to describe the clinical and molecular variability of ARCL2A. About 78.3% of known variants were predicted null alleles, while 11 were missense and 2 affected noncanonical splice sites. Age at ascertainment appeared as the unique phenotypic discriminator with earlier age more commonly associated with facial dysmorphism (p .02), high/cleft palate (p .005), intellectual disability/global developmental delay (p .013), and seizures (p .024). No specific genotype-phenotype correlations were identified. This work confirmed the existence of an attenuated phenotype associated with ATP6V0A2 biallelic variants and offers an updated critique to the clinical and molecular variability of ARCL2A.

Association of ABO blood groups and Rh factor with retinal and choroidal thickness

PURPOSE:

To evaluate if ABO blood group and Rh factor have an effect on retinal and choroidal thickness.

METHODS:

This study was designed prospectively. Retinal nerve fiber layer, retinal, and choroidal thicknesses were measured with spectral-domain optical coherence tomography. Retinal and choroidal thickness measurements (one subfoveal, three temporal, and three nasal) were obtained at 500-μm intervals up to 1500 μm with the caliper system.

RESULTS:

In this study, 109 male and 151 female, 260 individuals in total were included. There were 125 subjects in group A, 29 in group B, 34 in group AB, and 72 in group O. Rh factor was positive in 194 subjects and negative in 66. There was no significant difference between the groups regarding age (p = 0.667). The groups did not show any statistical difference in retinal nerve fiber layer thickness. There was significant difference found for mean retinal thickness at temporal 1000 μm when four groups were compared (p = 0.037). No statistically significant difference was detected for the remaining retinal and choroidal sectoral regions. The groups did not statistically significantly differ concerning Rh factor (p > 0.05).

CONCLUSION:

Although we found a significant difference in retinal thickness in the temporal retina between group B with group A and group O, we suggest that both blood group and Rh factor have no effect on retinal and choroidal thickness.

Congenital nephrotic syndrome in an infant with ALG1-congenital disorder of glycosylation

Congenital nephrotic syndrome (NS) in the newborn is most frequently related to mutations in genes specific for structural integrity of the glomerular basement membrane and associated filtration structures within the kidney, resulting in massive leakage of plasma proteins into the urine. Occurrence of congenital NS in a multi-system syndrome is less common. We describe the case of an infant with deteriorating neurological status, seizures, edema, and proteinuria who was found to have a mutation in gene ALG1 and a renal biopsy consistent with congenital NS. Furthermore, we briefly review rare existing case reports documenting congenital NS in patients with mutations in ALG1, and treatment strategies, including novel use of peritoneal dialysis.

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.

Homozygous Truncating Intragenic Duplication in TUSC3 Responsible for Rare Autosomal Recessive Nonsyndromic Intellectual Disability with No Clinical or Biochemical Metabolic Markers

Intellectual disability (ID), which affects around 2-3% of the general population, is classically divided into syndromic and nonsyndromic forms, with several modes of inheritance. Nonsyndromic autosomal recessive ID (NS-ARID) appears extremely heterogeneous with numerous genes identified to date, including inborn errors of metabolism. The TUSC3 gene encodes a subunit of the endoplasmic reticulum (ER)-bound oligosaccharyltransferase complex, which mediates a key step of N-glycosylation. To date, only five families with NS-ARID and TUSC3 mutations or rearrangements have been reported in the literature. All patients had speech delay, moderate-to-severe ID, and moderate facial dysmorphism. Microcephaly was noted in one third of patients, as was short stature. No patients had congenital malformation except one patient with unilateral cryptorchidism. Glycosylation analyses of patients' fibroblasts showed normal N-glycan synthesis and transfer. We present a review of the 19 patients previously described in the literature and report on a sixth consanguineous family including two affected sibs, with intellectual disability, unspecific dysmorphic features, and no additional malformations identified by high-resolution array-CGH. A homozygous truncating intragenic duplication of the TUSC3 gene leading to an aberrant transcript was detected in two siblings. This observation, which is the first reported case of TUSC3 homozygous duplication, confirms the implication of TUSC3 in NS-ARID and the power of the high-resolution array-CGH in identifying intragenic rearrangements of genes implicated in nonsyndromic ID and rare diseases.

Defining the phenotype in congenital disorder of glycosylation due to ALG1 mutations

Deficiency of β-1,4 mannosyltransferase (MT-1) congenital disorder of glycosylation (CDG), due to ALG1 gene mutations. Features in 9 patients reported previously consisted of prenatal growth retardation, pregnancy-induced maternal hypertension and fetal hydrops. Four patients died before 5 years of age, and survivors showed a severe psychomotor retardation. We report on 7 patients with psychomotor delay, microcephaly, strabismus and coagulation abnormalities, seizures and abnormal fat distribution. Four children had a stable clinical course, two had visual impairment, and 1 had hearing loss. Thrombotic and vascular events led to deterioration of the clinical outcome in 2 patients. Four novel ALG1 mutations were identified. Pathogenicity was determined in alg1 yeast mutants transformed with hALG1. Functional analyses showed all novel mutations representing hypomorphs associated with residual enzyme activity. We extend the phenotypic spectrum including the first description of deafness in MT1 deficiency, and report on mildly affected patients, surviving to adulthood. The dysmorphic features, including abnormal fat distribution and strabismus highly resemble CDG due to phosphomannomutase-2 deficiency (PMM2-CDG), the most common type of CDG. We suggest testing for ALG1 mutations in unsolved CDG patients with a type 1 transferrin isoelectric focusing pattern, especially with epilepsy, severe visual loss and hemorrhagic/thrombotic events.

Gene identification in the congenital disorders of glycosylation type I by whole-exome sequencing

Congenital disorders of glycosylation type I (CDG-I) form a growing group of recessive neurometabolic diseases. Identification of disease genes is compromised by the enormous heterogeneity in clinical symptoms and the large number of potential genes involved. Until now, gene identification included the sequential application of biochemical methods in blood samples and fibroblasts. In genetically unsolved cases, homozygosity mapping has been applied in consanguineous families. Altogether, this time-consuming diagnostic strategy led to the identification of defects in 17 different CDG-I genes. Here, we applied whole-exome sequencing (WES) in combination with the knowledge of the protein N-glycosylation pathway for gene identification in our remaining group of six unsolved CDG-I patients from unrelated non-consanguineous families. Exome variants were prioritized based on a list of 76 potential CDG-I candidate genes, leading to the rapid identification of one known and two novel CDG-I gene defects. These included the first X-linked CDG-I due to a de novo mutation in ALG13, and compound heterozygous mutations in DPAGT1, together the first two steps in dolichol-PP-glycan assembly, and mutations in PGM1 in two cases, involved in nucleotide sugar biosynthesis. The pathogenicity of the mutations was confirmed by showing the deficient activity of the corresponding enzymes in patient fibroblasts. Combined with these results, the gene defect has been identified in 98% of our CDG-I patients. Our results implicate the potential of WES to unravel disease genes in the CDG-I in newly diagnosed singleton families.

Two Argentinean Siblings with CDG-Ix: A Novel Type of Congenital Disorder of Glycosylation?

Congenital disorders of glycosylation (CDG) are genetic diseases caused by abnormal protein and lipid glycosylation. In this chapter, we report the clinical, biochemical, and molecular findings in two siblings with an unidentified CDG (CDG-Ix). They are the first and the third child of healthy consanguineous Argentinean parents. Patient 1 is now a 11-year-old girl, and patient 2 died at the age of 4 months. Their clinical picture involved liver dysfunction in the neonatal period, psychomotor retardation, microcephaly, seizures, axial hypotonia, feeding difficulties, and hepatomegaly. Patient 1 also developed strabismus and cataract. They showed a type 1 pattern of serum sialotransferrin. Enzymatic analysis for phosphomannomutase and phosphomannose isomerase in leukocytes and fibroblasts excluded PMM2-CDG and MPI-CDG. Lipid-linked oligosaccharide (LLO) analysis showed a normal profile. Therefore, this result could point to a deficiency in the dolichol metabolism. In this context, ALG8-CDG, DPAGT1-CDG, and SRD5A3-CDG were analyzed and no defects were identified. In conclusion, we could not identify the genetic deficiency in these patients yet. Further studies are underway to identify the basic defect in them, taking into account the new CDG types that have been recently described.

A newborn with unusual morphology: some practical aspects

Newborns with an unusual phenotype with or without malformations are common in the practice of every paediatrician. Determining whether the phenotype is a variation of normal or should be considered abnormal and, if the latter, also finding the cause can be extremely difficult. Here the main steps that should be followed in the diagnostic procedures are discussed. A careful family history and detailed physical examination remain the hallmarks of the investigations in all newborns. Very frequently clinical photographs will facilitate discussing patients with colleagues. Additional investigations usually include radiological examinations of all body parts that show abnormalities, and screening of the heart, kidneys, eyes and hearing. The studies with the highest yield are cytogenetic analyses which nowadays often involve microarray assays. In the near future, total exome sequencing will be available for diagnostic purposes which will have a major impact on the diagnostic process.

Plasma N-glycan profiling by mass spectrometry for congenital disorders of glycosylation type II

BACKGROUND

Determination of the genetic defect in patients with a congenital disorder of glycosylation (CDG) is challenging because of the wide clinical presentation, the large number of gene products involved, and the occurrence of secondary causes of underglycosylation. Transferrin isoelectric focusing has been the method of choice for CDG screening; however, improved methods are required for the molecular diagnosis of patients with CDG type II.

METHODS

Plasma samples with a typical transferrin isofocusing profile were analyzed. N-glycans were released from these samples by PNGase F [peptide-N4-(acetyl-β-glucosaminyl)-asparagine amidase] digestion, permethylated and purified, and measured on a MALDI linear ion trap mass spectrometer. A set of 38 glycans was used for quantitative comparison and to establish reference intervals for such glycan features as the number of antennae, the level of truncation, and fucosylation. Plasma N-glycans from control individuals, patients with known CDG type II defects, and patients with a secondary cause of underglycosylation were analyzed.

RESULTS

CDGs due to mannosyl (α-1,6-)-glycoprotein β-1,2-N-acetylglucosaminyltransferase (MGAT2), β-1,4-galactosyltransferase 1 (B4GALT1), and SLC35C1 (a GDP-fucose transporter) defects could be diagnosed directly from the N-glycan profile. CDGs due to defects in proteins involved in Golgi trafficking, such as subunit 7 of the conserved oligomeric Golgi complex (COG7) and subunit V0 a2 of the lysosomal H(+)-transporting ATPase (ATP6V0A2) caused a loss of triantennary N-glycans and an increase of truncated structures. Secondary causes with liver involvement were characterized by increased fucosylation, whereas the presence of plasma sialidase produced isolated undersialylation.

CONCLUSIONS

MALDI ion trap analysis of plasma N-glycans documents features that discriminate between primary and secondary causes of underglycosylation and should be applied as the first step in the diagnostic track of all patients with an unsolved CDG type II.

A novel cerebello-ocular syndrome with abnormal glycosylation due to abnormalities in dolichol metabolism

Cerebellar hypoplasia and slowly progressive ophthalmological symptoms are common features in patients with congenital disorders of glycosylation type I. In a group of patients with congenital disorders of glycosylation type I with unknown aetiology, we have previously described a distinct phenotype with severe, early visual impairment and variable eye malformations, including optic nerve hypoplasia, retinal coloboma, congenital cataract and glaucoma. Some of the symptoms overlapped with the phenotype in other congenital disorders of glycosylation type I subtypes, such as vermis hypoplasia, anaemia, ichtyosiform dermatitis, liver dysfunction and coagulation abnormalities. We recently identified pathogenic mutations in the SRD5A3 gene, encoding steroid 5α-reductase type 3, in a group of patients who presented with this particular phenotype and a common metabolic pattern. Here, we report on the clinical, genetic and metabolic features of 12 patients from nine families with cerebellar ataxia and congenital eye malformations diagnosed with SRD5A3-congenital disorders of glycosylation due to steroid 5α-reductase type 3 defect. This enzyme is necessary for the reduction of polyprenol to dolichol, the lipid anchor for N-glycosylation in the endoplasmic reticulum. Dolichol synthesis is an essential metabolic step in protein glycosylation. The current defect leads to a severely abnormal glycosylation state already in the early phase of the N-glycan biosynthesis pathway in the endoplasmic reticulum. We detected high expression of SRD5A3 in foetal brain tissue, especially in the cerebellum, consistent with the finding of the congenital cerebellar malformations. Based on the overlapping clinical, biochemical and genetic data in this large group of patients with congenital disorders of glycosylation, we define a novel syndrome of cerebellar ataxia associated with congenital eye malformations due to a defect in dolichol metabolism.

Next generation sequencing in a family with autosomal recessive Kahrizi syndrome (OMIM 612713) reveals a homozygous frameshift mutation in SRD5A3

As part of a large-scale, systematic effort to unravel the molecular causes of autosomal recessive mental retardation, we have previously described a novel syndrome consisting of mental retardation, coloboma, cataract and kyphosis (Kahrizi syndrome, OMIM 612713) and mapped the underlying gene to a 10.4-Mb interval near the centromere on chromosome 4. By combining array-based exon enrichment and next generation sequencing, we have now identified a homozygous frameshift mutation (c.203dupC; p.Phe69LeufsX2) in the gene for steroid 5α-reductase type 3 (SRD5A3) as the disease-causing change in this interval. Recent evidence indicates that this enzyme is required for the conversion of polyprenol to dolichol, a step that is essential for N-linked protein glycosylation. Independently, another group has recently observed SRD5A3 mutations in several families with a type 1 congenital disorder of glycosylation (CDG type Ix, OMIM 212067), mental retardation, cerebellar ataxia and eye disorders. Our results show that Kahrizi syndrome and this CDG Ix subtype are allelic disorders, and they illustrate the potential of next-generation sequencing strategies for the elucidation of single gene defects.

SRD5A3 is required for converting polyprenol to dolichol and is mutated in a congenital glycosylation disorder

N-linked glycosylation is the most frequent modification of secreted and membrane-bound proteins in eukaryotic cells, disruption of which is the basis of the congenital disorders of glycosylation (CDGs). We describe a new type of CDG caused by mutations in the steroid 5alpha-reductase type 3 (SRD5A3) gene. Patients have mental retardation and ophthalmologic and cerebellar defects. We found that SRD5A3 is necessary for the reduction of the alpha-isoprene unit of polyprenols to form dolichols, required for synthesis of dolichol-linked monosaccharides, and the oligosaccharide precursor used for N-glycosylation. The presence of residual dolichol in cells depleted for this enzyme suggests the existence of an unexpected alternative pathway for dolichol de novo biosynthesis. Our results thus suggest that SRD5A3 is likely to be the long-sought polyprenol reductase and reveal the genetic basis of one of the earliest steps in protein N-linked glycosylation.

Association of ABO blood groups with glaucoma in the Pakistani population

OBJECTIVE

To study the association of blood groups with different types of glaucoma including primary open-angle glaucoma (POAG), primary closed-angle glaucoma (PCAG), and pseudoexfoliative glaucoma (PEXG) in the Pakistani population.

STUDY DESIGN

The present study was a prospective case control study.

PARTICIPANTS

ABO and Rh blood groups were analyzed in 2046 controls and 477 glaucoma patients (220 POAG, 146 PCAG, and 111 PEXG).

METHODS

Hemagglutination patterns were used to determine the prevalence of the ABO and Rh blood groups in all the subjects. Logistic regression analysis was carried out to evaluate any association of the different blood groups with glaucoma.

RESULTS

In the present study, the percentage of blood groups A, B, AB, and O in patients was found to be 19%, 41%, 10%, and 30%, and in the control group, the values were 26%, 31%, 12%, and 31%, respectively. A significant positive association was found between the B blood group and glaucoma (p value < 0.05, odds ratio [OR] 1.5, and c2 15.8). Logistic regression analysis revealed that the blood group B was associated with all types of glaucoma with OR of 1.35 (95% CI 1.01-1.80; p = 0.04) for POAG, 1.71 (95% CI 1.21-2.40; p = 0.002) for PCAG, and 1.61 (95% CI 1.09-2.36; p = 0.016) for PEXG. POAG was also found to be associated with the Rh- allele (p < 0.05) with an OR of 4.05 (95% CI 2.98-5.51), as compared with controls.

CONCLUSIONS

In the Pakistani patient cohort, blood group B is associated with all types of glaucoma and the Rhallele is associated only with POAG.

Vacuolar H+-ATPase meets glycosylation in patients with cutis laxa

Glycosylation of proteins is one of the most important post-translational modifications. Defects in the glycan biosynthesis result in congenital malformation syndromes, also known as congenital disorders of glycosylation (CDG). Based on the iso-electric focusing patterns of plasma transferrin and apolipoprotein C-III a combined defect in N- and O-glycosylation was identified in patients with autosomal recessive cutis laxa type II (ARCL II). Disease-causing mutations were identified in the ATP6V0A2 gene, encoding the a2 subunit of the vacuolar H(+)-ATPase (V-ATPase). The V-ATPases are multi-subunit, ATP-dependent proton pumps located in membranes of cells and organels. In this article, we describe the structure, function and regulation of the V-ATPase and the phenotypes currently known to result from V-ATPase mutations. A clinical overview of cutis laxa syndromes is presented with a focus on ARCL II. Finally, the relationship between ATP6V0A2 mutations, the glycosylation defect and the ARCLII phenotype is discussed.

Glycosylation diseases: quo vadis?

About 250 to 500 glycogenes (genes that are directly involved in glycan assembly) are in the human genome representing about 1-2% of the total genome. Over 40 human congenital diseases associated with glycogene mutations have been described to date. It is almost certain that the causative glycogene mutations for many more congenital diseases remain to be discovered. Some glycogenes are involved in the synthesis of only a specific protein and/or a specific class of glycan whereas others play a role in the biosynthesis of more than one glycan class. Mutations in the latter type of glycogene result in complex clinical phenotypes that present difficult diagnostic problems to the clinician. In order to understand in biochemical terms the clinical signs and symptoms of a patient with a glycogene mutation, one must understand how the glycogene works. That requires, first of all, determination of the target protein or proteins of the glycogene followed by an understanding of the role, if any, of the glycogene-dependent glycan in the functions of the protein. Many glycogenes act on thousands of glycoproteins. There are unfortunately no general methods to identify all the potentially large number of glycogene target proteins and which of these proteins are responsible for the mutant phenotypes. Whereas biochemical methods have been highly successful in the discovery of glycogenes responsible for many congenital diseases, it has more recently been necessary to use other methods such as homozygosity mapping. Accurate diagnosis of many recently discovered diseases has become difficult and new diagnostic procedures must be developed. Last but not least is the lack of effective treatment for most of these children and of animal models that can be used to test new therapies.