Clinical Diagnosis

The diagnosis of sialuria may be suspected in infants or young children with the following:

• Mild facial coarsening

• Hypotonia

• Equivocal developmental delay

• Frequent upper respiratory infections

Note: The likelihood of sialuria is increased after exclusion of more prevalent disorders that share clinical features. See Differential Diagnosis.

Testing

Constitutive overproduction of free sialic acid is the metabolic defect of sialuria [Seppala et al 1999, Huizing 2005]. The sialic acids, a group of negatively charged sugars, are acetyl derivatives of the nine-carbon 3-deoxy-5-amino sugar acid called neuraminic acid. The sialic acid relevant here is N-acetyl-neuraminic acid (NANA). For laboratories offering biochemical testing, see .

Free sialic acid levels. Assay of free sialic acid in the urine requires expertise either in the spectrophotometric or the fluorometric thiobarbituric acid assay or in thin-layer chromatography; other methods may fail to detect mild-to-moderate elevation of free sialic acid. The biochemical detection method of Cardo et al [1985] is adequate for assay of free and bound sialic acid in urine and whole-cell homogenates [Leroy et al 2001]. Laboratory methodology for the assay of free sialic acid has been reviewed [Gopaul & Crook 2006].

Provided the free sialic acid storage disorders can be ruled out, the finding of excessive excretion of free sialic acid (elevated over 100-fold) in the urine suggests the diagnosis of sialuria.

Note: (1) In the spectrophotometric method, other substances may either decrease or increase the absorbance and thus lead to spurious results. (2) High-performance liquid chromatography and proton nuclear magnetic resonance spectroscopy (1H-NMR), performed on a research basis only, may be helpful in sorting out relevant differential diagnoses [Seppala et al 1999, Engelke et al 2004, Valianpour et al 2004].

On a research basis only, the combination of one- and two-dimensional correlation spectroscopy (COSY):

• Identifies a specific ¹H-NMR spectrum for urinary N-acetyl-neuraminic acid in sialuria, which can be distinguished from the spectrum associated with Salla disease [Engelke et al 2004];

• Distinguishes bound sialic acid from free sialic acid and hence distinguishes sialidosis from sialuria.

Cytoplasmic localization of free sialic acid. Establishing that the intracellular distribution of free sialic acid is cytoplasmic instead of lysosomal confirms the diagnosis. In sialuria, subcellular fractionation fails to find evidence of lysosomal accumulation of sialic acid that is characteristic of free sialic acid storage disorders [Aula & Gahl 2001]. Electron microscopic (EM) study of parenchymal cells or cultured fibroblasts shows no damage to the lysosomes, in spite of the high levels of free sialic acid found in the cytoplasm.

Assay of UDP-GlcNAc 2-epimerase activity. Assay of UDP-GlcNAc 2-epimerase enzyme activity in the presence and in the absence of 100µM cytidine monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) confirms the diagnosis. The activity of the wild-type enzyme is inhibited 95% by CMP-Neu5Ac, whereas the enzymatic activity of the mutant protein is barely, if at all, diminished by the natural inhibitor. These studies must be performed in a specialized laboratory.

Oligosaccharides. The urinary excretion of oligosaccharides is normal.

Molecular Genetic Testing

Gene. GNE is the only gene known to be associated with sialuria.

Clinical testing

• Sequence analysis. Six of the seven known persons with sialuria have been tested; all six had identifiable mutations in GNE [Ferreira et al 1999, Seppala et al 1999, Aula & Gahl 2001, Enns et al 2001, Leroy et al 2001, Huizing & Krasnewich 2009]. Mutations appear to reside exclusively in the short stretch of consecutive nucleotides in GNE that encodes the amino acids 263 to 266, which have an important role in the allosteric site of the gene product, UDP-N-acetylglucosamine 2-epimerase/N-acetyl mannosamine kinase (GNE/MNK). Of note, the borders of the putative allosteric site have not yet been determined [Huizing 2005].

Sequence analysis of select exons. Only exons 4 and 5 that include the allosteric domain are sequenced.

Table 1. Summary of Molecular Genetic Testing Used in Sialuria

View in own window

Gene Symbol	Test Method	Mutations Detected	Mutation Detection Frequency by Test Method 1	Test Availability
GNE	Sequence analysis	Sequence variants 2 including any in the allosteric domain	6/6 3	Clinical
	Sequence analysis of select exons 4	Missense mutations in exons 4 and 5 only, including the allosteric domain	6/6 3

Test Availability refers to availability in the GeneTests Laboratory Directory. GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.

1. The ability of the test method used to detect a mutation that is present in the indicated gene

2. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations.

3. Total number of persons known to have been tested to date

4. Exons sequenced may vary by laboratory.

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Testing Strategy

To confirm/establish the diagnosis in a proband. The following order of diagnostic testing is recommended, especially if more probable differential diagnoses have been ruled out:

1.

Assay of free sialic acid in urine

2.

Molecular genetic testing of GNE with special attention for changes in the allosteric site

Testing at-risk relatives of a proband to identify those who may be mildly affected requires prior identification of the disease-causing mutation in the family.

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutation in the family.

Note: It is the policy of GeneReviews to include clinical uses of testing available from laboratories listed in the GeneTests Laboratory Directory; inclusion does not necessarily reflect the endorsement of such uses by the author(s), editor(s), or reviewer(s).

Genetically Related (Allelic) Disorders

In contrast to the dominant gain-of-function effect of heterozygous mutations in the allosteric site observed in sialuria, homozygous or compound heterozygous GNE missense mutations are being increasingly recognized in adults with an autosomal recessive late-onset type myopathy that is distinct from sialuria (see Inclusion Body Myopathy 2).

The GNE-related myopathies have been known by several descriptive terms including hereditary inclusion body myopathy (hIBM), hereditary IBM quadriceps sparing type or h-IBM2 (OMIM 600737), and distal myopathy with rimmed vacuoles (DMRV) or Nonaka myopathy (OMIM 605820). Initially described and delineated as separate myopathies based on muscle pathology, these entities are now known to represent various stages in the natural course of this one disorder [Tomimitsu et al 2004, Huizing 2005, Huizing & Krasnewich 2009]. Initially observed most frequently in various populations in the Middle East [Argov et al 2003], GNE-related myopathies more recently have been reported in Japan (Nonaka myopathy) [Kayashima et al 2002, Nishino et al 2002, Tomimitsu et al 2004] and in several groups of European origin [Broccolini et al 2002, Vasconcelos et al 2002].

Hereditary inclusion body myopathy (hIBM) begins in the young adult with gait difficulties due to compromised foot dorsiflexion. Muscle weakness first apparent in the distal limb muscles, progresses in severity. In the early stages of the disorder the proximal limb muscles (quadriceps in the legs and deltoids, biceps, and triceps in the arms) appear to be spared. Weakness in these muscles appears in the later stages of the disorder. There is gradual reduction of muscle bulk in the limbs. Affected persons become wheelchair-bound. Intellectual functioning, sensation, and coordination remain intact even when the myopathy becomes more widespread and severe.

Diagnosis is based on the histopathologic findings of red rimmed vacuolar degeneration of muscle fibers; specific MRI T₁ documentation of quadriceps sparing, but fatty and fibrous replacement of the surrounding musculature; and molecular genetic testing. Creatine kinase (CK) in plasma may be mildly elevated in later clinical stages. Urinary excretion of sialic acid is normal [Argov & Mitrani-Rosenbaum 2008, Huizing & Krasnewich 2009]

Natural History

A phenotypic definition or natural history of sialuria must remain preliminary as only seven affected persons have been reported [Ferreira et al 1999, Leroy et al 2001]. Signs and symptoms are mild and can be transient.

Pregnancy is usually normal. Affected infants are rather small for gestational age. At birth, the OFC is normal; the facies appear rather flat and slightly coarse. Mild hepatomegaly occurs in the majority of children and prolonged neonatal jaundice can be observed. In early infancy, developmental delay is reported in most children and generalized hypotonia in some. Microcytic anemia in two infants was severe enough to require transfusion. Upper respiratory infections occur frequently into the second year of life. Sometimes they are associated with gastroenteritis, dehydration, and transient failure to thrive. Signs of dysostosis multiplex appear to be transient, but skeletal development is delayed at least in early childhood.

Developmental age or IQ is borderline low. Later in childhood, physical development is normal and intellectual development can be nearly normal. One child had febrile convulsions. In about half of the children who had seizures in childhood, the seizures were controlled with phenobarbital [Leroy et al 2001].

The phenotypic spectrum of sialuria is insufficiently known. Moreover, it may be either equivocally abnormal or indistinguishable from the normal variation in childhood development. It is likely that children with sialuria who have no significant medical problems in infancy and/or early childhood do not come to medical attention at all. The retrospective diagnosis of sialuria in the mother of a proband supports this contention [Leroy et al 2001].

Genotype-Phenotype Correlations

The direct correlation of genotype and phenotype is significant:

• In sialuria. In all persons who have been tested, the missense pathogenic GNE mutation was detected in the putative allosteric site (codons 263 or 266).

• In hereditary inclusion body myopathy (hIBM). Homozygous or compound heterozygous GNE mutations are observed outside the allosteric site in the epimerase or the kinase domain [Kayashima et al 2002, Argov et al 2003, Huizing et al 2004, Tomimitsu et al 2004, Huizing 2005] (see Genetically Related Disorders).

Penetrance

Penetrance can hardly be estimated clinically as the findings in this disorder are nonspecific, vary between affected persons, and can be transient and limited to early childhood. Moreover, the intellectual disability inconsistently associated with it is neither progressive nor significant.

In contrast, “biochemical” penetrance of the excessive urinary excretion of free sialic acid is probably complete in childhood. Excessive excretion of free sialic acid was also found in the mother of one proband, the only adult reported to have been tested so far. Hence any conclusion about “biochemical” penetrance in adults remains premature.

Nomenclature

Before the simple term "sialuria" was adopted, the disorder was known as French type sialuria, because the first person described with sialuria and those who reported him were French. This descriptive nomenclature was considered useful for differentiation from Finnish type sialuria, the initial designation of the clinically severe free sialic acid storage disorders, infantile free sialic acid storage disorder (ISSD), and Salla disease (see Differential Diagnosis), first described and most frequently observed in Finland.

Prevalence

Sialuria has been reported in only seven persons.

The prevalence of sialuria may be underestimated. Assay of urinary levels of free sialic acid is not a routine laboratory procedure. As a rule, it is performed only in infants or young children with progressive CNS disease for confirmation or exclusion of the free sialic acid storage disorders.

The prevalence of sialuria remains unknown and is probably higher than that estimated from the existing reports of symptomatic persons.

Evaluations Following Initial Diagnosis

To establish the extent of disease in a person diagnosed with sialuria, the following evaluations are recommended, if they have not already been completed. Note: The priority of these recommendations depends on the signs observed in the patient and/or noted by the parents:

• CBC with differential to evaluate for microcytic anemia

• Measurement of serum bilirubin concentration to evaluate for jaundice

• Skeletal survey to evaluate for dysostosis multiplex

• Developmental and neurologic assessment

• EEG when relevant

• Neuroimaging with the purpose of differentiating sialuria from neurodegenerative lysosomal storage disorders

Treatment of Manifestations

Persons with sialuria need symptomatic and supportive management, including treatment of anemia, prolonged jaundice, and convulsions. Barbiturates have been more effective in treating the occasional convulsion in early childhood than other antiepileptic drugs (AEDs).

Affected individuals benefit from early developmental intervention and appropriate educational programs.

Prevention of Secondary Complications

Appropriate antibiotics to prevent secondary bacterial super-infection in the upper/lower airways are indicated.

Surveillance

The following are appropriate:

• Clinical follow-up during and after infancy to confirm that CNS disease is not progressive (in contrast to free sialic acid storage disorders) and to document the gradual remission of signs and/or symptoms present in infancy

• Follow-up evaluations three to four times in infancy, twice in second year of life, and once every subsequent year

Testing of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes

Therapies Under Investigation

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

See Molecular Genetic Pathogenesis for therapy trials in “knock-in” mice yielding preliminary data for future possible therapies in humans.

Other

Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals.

Mode of Inheritance

Sialuria is inherited in an autosomal dominant manner.

Risk to Family Members

Parents of a proband

• Most persons diagnosed with sialuria do not have a parent known to be affected. However, molecular genetic testing has usually not been performed on both parents; thus, the actual percentage of persons who have inherited the mutation from a parent is unknown.

• A proband with sialuria most likely has the disorder as the result of a de novo mutation in the allosteric site of GNE. Five of the seven persons reported possibly represent simplex cases (i.e., a single affected individual in a family); however, study of urinary excretion of free sialic acid in their close relatives has not been reported.

• Recommendations for the evaluation of parents of a proband with an apparent de novo mutation include study of urinary excretion of free sialic acid of both parents and molecular genetic testing if the disease-causing mutation has been identified in the proband.

Sibs of a proband

• The risk to the sibs of the proband depends on the genetic status of the proband's parents.

• If a parent of the proband is affected or has a disease-causing mutation, the risk to the sibs of inheriting the mutation is 50%.

• If the disease-causing mutation found in the proband cannot be detected in the DNA of the either parent, the risk to sibs is low, but greater than that of the general population because although no instances of germline mosaicism have been reported, it remains a possibility.

Offspring of a proband. Each child of a person with sialuria has a 50% chance of inheriting the mutation.

Other family members of a proband

• The risk to other family members depends on the genetic status of the proband's parents.

• If a parent is affected or has a disease-causing mutation, his or her family members are at risk.

Related Genetic Counseling Issues

Considerations in families with an apparent de novo mutation. When neither parent of a proband with an autosomal dominant condition has the disease-causing mutation or clinical evidence of the disorder, it is likely that the proband has a de novo mutation. However, possible non-medical explanations including alternate paternity or maternity (e.g., with assisted reproduction) or undisclosed adoption could also be explored.

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal testing is before pregnancy.

• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk.

DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. See for a list of laboratories offering DNA banking.

Prenatal Testing

Molecular genetic testing. Prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approximately 15 to 18 weeks’ gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks’ gestation. The disease-causing allele of an affected family member must be identified before prenatal testing can be performed.

Note: Gestational age is expressed as menstrual weeks calculated either from

Interpretation of prenatal diagnosis testing is complicated by the current lack of information about the phenotype, particularly its long-term outcome. Results of prenatal testing cannot predict the age of onset, clinical course, or degree of disability.

Biochemical testing. No prenatal biochemical testing for sialuria has been performed; however, biochemical testing for prenatal diagnosis has been performed in free sialic acid storage disorders.

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutation has been identified. For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

Sialuria. The basic metabolic defect in sialuria is failed allosteric feedback inhibition of the bifunctional UDP-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase (EC 5.1.3.14) / N-acetylmannosamine (ManNAc) kinase (EC 2.7.1.60) (GNE/MNK), rate-limiting enzyme in the biosynthesis of sialic acid (Neu5Ac). The biologic inhibitory substance, CMP-Neu5Ac, the downstream product in this biosynthetic pathway, is formed in the cell nucleus per activation of Neu5Ac by CTP catalyzed by CMP-Neu5Ac synthase. It is subsequently transported into the Golgi apparatus assisted by a specific Golgi membrane protein. It serves in that location as a substrate for different sialyltransferases [Reinke et al 2009]. Feedback inhibition fails when CMP-sialic acid (CMP-neu5Ac) cannot bind to the small mutant allosteric site in GNE/MNK, itself a soluble protein of 722 amino acids found in the cytoplasm, mainly in the Golgi region, and also in the cell nucleus [Krause et al 2005] The allosteric site is still incompletely defined but comprises the consecutive amino acids 263 through 266 in the epimerase functional domain.

In each person with sialuria, the GNE disease-causing DNA mutation was found to be a missense mutation in one of the two nearly adjacent codons in exon 5, corresponding to the amino acids indicated. In each person, it was found only in the heterozygous state [Ferreira et al 1999, Seppala et al 1999, Aula & Gahl 2001, Enns et al 2001, Leroy et al 2001, Huizing & Krasnewich 2009]. The detection of this molecular defect provided the initial information that identified the allosteric site in the GNE/MNK enzyme and explains the main aspects of the pathogenesis of sialuria. Moreover, the finding that the regulatory mutation in all persons with sialuria is heterozygous establishes the autosomal dominant mode of inheritance. This is quite different from the homozygous or compound heterozygous mutations in either the epimerase or the kinase domain rendering GNE/MNK inactive and resulting in autosomal recessive hIBM (see Genetically Related Disorders).

The lack of feedback inhibition results in highly excessive production of free sialic acid and in its very elevated concentrations in the cellular cytoplasm, interstitial tissues, and body fluids, such as urine.

Defective allosteric inhibition is not an exceptional cause of human metabolic disease. It has been shown recently also for the glutamate dehydrogenase gene in infants with hyperinsulinism and hyperammonemia (see Familial Hyperinsulinism).

Hereditary inclusion body myopathy (hIBM). The metabolic effect of the GNE mutant genotypes, assayed as GNE-epimerase and MNK-kinase activity in several in vitro cultured cell types and in cell-free transcription-translation study systems, is reduction of activity to 30%-60% of normal control values. Any mutation in either the GNE domain outside the allosteric site or the MNK domain adversely affects not only the catalytic activity that depends on the protein domain directly affected by the mutation, but also the catalytic activity that normally results from the other protein domain. The expected decrease of intra- and pericellular sialic acid content in hIBM-derived muscle and other tissues has not been documented consistently, leaving unresolved the role of hyposialylation of specific glycosylated proteins, such as α-dystroglycan and neural cell adhesion molecules (NCAM), in the degeneration of muscle in hIBM.

The importance of this sialylation has, however, been highlighted by the recent demonstration in the transgenic mouse model with the Gne Asp176Val mutation: oral administration of ManNAc, a sialic acid metabolite, can prevent and/or postpone muscle weakness and atrophy and histopathologic features associated with hIBM [Malicdan et al 2009]. This work supports the contention that hIBM in humans is a potentially treatable myopathy. Oral ManNAc treatment has also been shown to rescue pups homozygous for the Met712Thr variant in another transgenic knock-in mouse model [Galeano et al 2007] from the severe glomerular lesions and concomitant proteinuria they had developed instead of myopathic findings. The treated animals survived considerably longer than untreated animals. The treatment was shown to increase sialylation of podocalyxin, the major podocyte sialoprotein.

Besides the obvious potential as a preclinical treatment in humans, the in vivo results of ManNAc treatment are useful also for the evaluation of hypotheses regarding the pathogenesis of loss of muscle cells in hIBM and the role of apoptosis [Argov & Mitrani-Rosenbaum 2008]. Hyposialylation of neprilysin (NEP), a metallopeptidase that normally cleaves amyloid-β known to accumulate in hIBM muscle, has been found to promote muscle degeneration also [Broccolini et al 2008]. The questions of how GNE (a gene product conserved in nature and of crucial importance from early embryonic stages of life) interacts with several proteins involved in the regulation of development [Reinke et al 2009] and its probable role in mitochondrial processes [Eisenberg et al 2008] deserve further exploration by the study of animal models.

At present the allelic heterogeneity of hIBM has been expanded significantly; currently 62 GNE mutations have been reported worldwide [Huizing & Krasnewich 2009]. The list includes three frequently observed mutations, considered to be founder mutations in various populations: p.Met712Thr, most predominant in persons of Persian-Jewish extraction [Eisenberg et al 2003]; p.Val572Leu, most often identified in persons of Japanese descent, but also encountered in other Asian populations [Kayashima et al 2002, Kim et al 2006]; and p.Asp176Val, also frequent in the Japanese [Nishino et al 2002]. Only 11 of 62 GNE alterations are either nonsense or frame shift-causing and expected to have a “null” or “amorph” phenotypic effect.

Genotypes homozygous for a null mutation or compound heterozygous with a null mutation have not yet been encountered in hIBM. Complete absence of GNE encoded enzyme activity is probably lethal, a conclusion also reached by the finding that Gne knock-out mouse models did not survive beyond the embryonic stage [Huizing & Krasnewich 2009].

Normal allelic variants. GNE consists of 14 exons, 13 of which are located closely together, whereas the recently discovered additional exon of 90 base pairs, named A1, resides 20 kb upstream of exon 1 as outlined in the references in Reinke et al [2009]. Four different mRNA splice variants are transcribed from GNE, resulting from alternative splicing of the exons A1, 1, and 2. Exon 1 is a non-coding exon. Hence, two of the splice variants encode a protein of 722 amino acids, hGNE1, as reported for the originally characterized GNE/MNK protein and referred to in most molecular biology studies. However, human GNE (GNE/MNK) exists in three different isoforms — hGNE1, hGNE2, and hGNE3 —the latter two possessing extended or deleted N-terminal regions, respectively. The isoform hGNE1 is ubiquitously expressed, but most intensively in liver and placenta. Lower concentrations are detectable in muscle, brain, kidney, and pancreas [Reinke et al 2009 and references therein].

It is of interest that as a monomer GNE/MNK has no catalytic activity. It requires di- and even multimerization of the nascent polypeptides in order to become fully active as a bifunctional enzyme [Huizing & Krasnewich 2009, Reinke et al 2009].

Pathologic allelic variants. In all persons with sialuria, one of three single missense mutations, p.Arg263Leu, p.Arg266Gln, or p.Arg266Trp, was found in only one allele of GNE (located in exon 5 and the epimerase domain of GNE/MNK) and associated with highly excessive urinary excretion of free sialic acid. This strongly suggested that the corresponding group of amino acids represents the allosteric site of the enzyme for retroinhibition by CMP-Neu5Ac acid binding [Seppala et al 1999]. In contrast to the sialic acid storage disorders, the clinical consequence has been mild and not associated with lysosomal retention of free sialic acid or by other histologically demonstrable cellular damage. The finding in the symptom-free mother of one of the probands confirms the mild clinical effect and proves the autosomal dominant inheritance of the disorder.

Homozygous or compound heterozygous mutations in either the epimerase domain or the kinase domain are associated with adult-onset autosomal recessive hereditary inclusion body myopathy (hIBM). The large majority of these mutations are of the missense type. See Genetically Related Disorders.

Normal gene product. Uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase (GNE) (EC 5.1.3.14)/ N-acetylmannosamine (ManNAc) kinase (MNK) (EC 2.7.1.60), a protein of 722 amino acids is a bifunctional enzyme that catalyzes the first rate-limiting step and the second step in the biosynthetic pathway of sialic acid [Seppala et al 1999, Aula & Gahl 2001, Huizing & Krasnewich 2009]. The first of these steps is inhibited by feedback from CMP-neu5Ac. The epimerase activity domain is found in the amino-terminal portion of the protein (amino acids 1 to ~378) and the kinase domain is found in the carboxy-terminal half (amino acids ~410 to 722) [Seppala et al 1999, Huizing 2005]. The allosteric site resides in exon 5 within the epimerase domain. The active site in either enzyme domain is still to be determined. GNE/MNK is a major determinant of cell surface glycoconjugate sialylation and a critical regulator of the function of specific cell-surface adhesion molecules. Bound N-acetyl-neuraminic acid (NANA) is widely distributed in normal tissues and is a constituent of glycoproteins and complex lipids such as gangliosides. In N-linked glycoproteins, NANA is consistently the terminal sugar in the oligosaccharide tree [Huizing & Krasnewich 2009].

Note: The codon numbers in this GeneReview correspond to reference sequence NP_005467.1 (sometimes referred to isoform 2), which contains a different 5' terminal exon compared to transcript variant 1, resulting in translation initiation from an in-frame downstream AUG and an isoform (2) with a shorter N-terminus compared to isoform 1. See Entrez Gene www.ncbi.nlm.nih.gov/gene/10020.

Abnormal gene product. The activity of the bifunctional and rate-limiting GNE enzyme is normal in sialuria fibroblasts, but no longer subject to retro-inhibition by the end-product CMP-sialic acid, when one and only one of the two GNE alleles has a missense mutation in the putative allosteric site in and probably near codons 263 and 266. Hence, there is significant and steady overproduction and vastly excessive urinary excretion of free sialic acid (neu5Ac).

Fortunately the apparently rare persons with sialuria have clinically only a mild disorder. Nevertheless, this mutation with allosteric autosomal dominant metabolic defect is of considerable importance in the study of the various physiologic roles of free sialic acid and of sialylation in tissues. Moreover, the metabolic trait has been shown to be important in the production of biologic molecules with therapeutic potential and in testing the feasibility of silencing mutation effects by RNA interference.

Over-expression of GNE in CHO-cells containing the pathologic variant p.Arg263Leu resulted in overproduction of sialic acid and in significant increase of polysialic acid bound to neural cell adhesion molecules (NCAM). Persons with sialuria may still have harmful consequences upon maintenance of cerebral and/or neural functions. One can but wonder whether inappropriate polysialylation of NCAM may not increase the risk of harmful effects in brain development, learning, and neural regeneration [Bork et al 2005]. The GNE sialuria mutation in the model CHO-cell expression system that produces recombinant human erythropoietin (rhEPO), a cytokine for erythrocyte precursors, expresses homogeneous highly sialylated EPO of the desired therapeutic value instead of the incompletely and heterogeneously sialylated expressed product [Bork et al 2007].

In vitro silencing of a GNE mutation by RNA interference (RNAi) with synthetic small interfering RNAs (siRNAs) in primary sialuria fibroblasts resulted in significantly decreased levels of free sialic acid. Feedback inhibition by CMP-neu5Ac was restored. This result is important as a principle demonstration of the possible therapeutic potential [Klootwijk et al 2008].

Loss of GNE activity itself is apparently often only partial as measured enzymatically in in vitro cell systems. It clearly interferes however with sialic acid production and precludes adequate sialylation of many glycoconjugates. That oral feeding of ManNAc to knock-in mutant mouse strains has postponed or prevented the expected hIBM has become the most robust argument in proving the importance of sialylation in adequate function and maintenance of muscular tissue (see Genetically Related Disorders).

Literature Cited

1. Argov Z, Eisenberg I, Grabov-Nardini G, Sadeh M, Wirguin I, Soffer D, Mitrani-Rosenbaum S. Hereditary inclusion body myopathy: the Middle Eastern genetic cluster. Neurology. 2003;60:1519–23. [PubMed: 12743242]
2. Argov Z, Mitrani-Rosenbaum S. The hereditary inclusion body myopathy enigma and its future therapy. Neurotherapeutics. 2008;5:633–7. [PubMed: 19019317]
3. Aula N, Jalanko A, Aula P, Peltonen L. Unraveling the molecular pathogenesis of free sialic acid storage disorders: altered targeting of mutant sialin. Mol Genet Metab. 2002;77(1-2):99–107. [PubMed: 12359136]
4. Aula N, Salomäki P, Timonen R, Verheijen F, Mancini G, Månsson JE, Aula P, Peltonen L. The spectrum of SLC17A5-gene mutations resulting in free sialic acid-storage diseases indicates some genotype-phenotype correlation. Am J Hum Genet. 2000;67:832–40. [PMC free article: PMC1287888] [PubMed: 10947946]
5. Aula P, Gahl WA. Disorders of free sialic acid storage. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. 8 ed. New York: McGraw-Hill; 2001:5109-20.
6. Bellini C, Hennekam RC, Fulcheri E, Rutigliani M, Morcaldi G, Boccardo F, Bonioli E. Etiology of nonimmune hydrops fetalis: a systematic review. Am J Med Genet A. 2009;149A:844–51. [PubMed: 19334091]
7. Bork K, Reutter W, Gerardy-Schahn R, Horstkorte R. The intracellular concentration of sialic acid regulates the polysialylation of the neural cell adhesion molecule. FEBS Lett. 2005;579:5079–83. [PubMed: 16137682]
8. Broccolini A, Gidaro T, De Cristofaro R, Morosetti R, Gliubizzi C, Ricci E, Tonali PA, Mirabella M. Hyposialylation of neprilysin possibly affects its expression and enzymatic activity in hereditary inclusion-body myopathy muscle. J Neurochem. 2008;105:971–81. [PubMed: 18182043]
9. Broccolini A, Pescatori M, D'Amico A, Sabino A, Silvestri G, Ricci E, Servidei S, Tonali PA, Mirabella M. An Italian family with autosomal recessive inclusion-body myopathy and mutations in the GNE gene. Neurology. 2002;59:1808–9. [PubMed: 12473780]
10. Cardo PP, Lombardo C, Gatti R. A simple detection of sialic acid storage disorders by urinary 'free' and 'total' sialic acid determinations. Clin Chim Acta. 1985;150:129–35. [PubMed: 3862469]
11. Cathey SS, Kudo M, Tiede S, Raas-Rothschild A, Braulke T, Beck M, Taylor HA, Canfield WM, Leroy JG, Neufeld EF, McKusick VA. Molecular order in mucolipidosis II and III nomenclature. Am J Med Genet A. 2008;146A:512–3. [PubMed: 18203164]
12. Cathey SS, Leroy JG, Wood T, Eaves K, Simensen RJ, Kudo M, Stevenson RE, Friez MJ. Phenotype and genotype in mucolipidoses II and III alpha/beta: a study of 61 probands. J Med Genet. 2010;47:38–48. [PubMed: 19617216]
13. D'Azzo A, Andria G, Strisciuglio P, Galjaard H Galactosialidosis. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. 8 ed. New York: McGraw-Hill; 2001:3811-26.
14. Eisenberg I, Grabov-Nardini G, Hochner H, Korner M, Sadeh M, Bertorini T, Bushby K, Castellan C, Felice K, Mendell J, Merlini L, Shilling C, Wirguin I, Argov Z, Mitrani-Rosenbaum S. Mutations spectrum of GNE in hereditary inclusion body myopathy sparing the quadriceps. Hum Mutat. 2003;21:99. [PubMed: 12497639]
15. Eisenberg I, Novershtern N, Itzhaki Z, Becker-Cohen M, Sadeh M, Willems PH, Friedman N, Koopman WJ, Mitrani-Rosenbaum S. Mitochondrial processes are impaired in hereditary inclusion body myopathy. Hum Mol Genet. 2008;17:3663–74. [PubMed: 18723858]
16. Engelke UF, Liebrand-van Sambeek ML, de Jong JG, Leroy JG, Morava E, Smeitink JA, Wevers RA. N-acetylated metabolites in urine: proton nuclear magnetic resonance spectroscopic study on patients with inborn errors of metabolism. Clin Chem. 2004;50:58–66. [PubMed: 14633929]
17. Enns GM, Seppala R, Musci TJ, Weisiger K, Ferrell LD, Wenger DA, Gahl WA, Packman S. Clinical course and biochemistry of sialuria. J Inherit Metab Dis. 2001;24:328–36. [PubMed: 11486897]
18. Ferreira H, Seppala R, Pinto R, Huizing M, Martins E, Braga AC, Gomes L, Krasnewich DM, Sa Miranda MC, Gahl WA. Sialuria in a Portuguese girl: clinical, biochemical, and molecular characteristics. Mol Genet Metab. 1999;67:131–7. [PubMed: 10356312]
19. Galeano B, Klootwijk R, Manoli I, Sun M, Ciccone C, Darvish D, Starost MF, Zerfas PM, Hoffmann VJ, Hoogstraten-Miller S, Krasnewich DM, Gahl WA, Huizing M. Mutation in the key enzyme of sialic acid biosynthesis causes severe glomerular proteinuria and is rescued by N-acetylmannosamine. J Clin Invest. 2007;117:1585–94. [PMC free article: PMC1878529] [PubMed: 17549255]
20. Gopaul KP, Crook MA. The inborn errors of sialic acid metabolism and their laboratory investigation. Clin Lab. 2006;52(3-4):155–69. [PubMed: 16584062]
21. Huizing M. Disease mechanisms associated with mutations of the GNE gene. Drug Discov today. 2005;2:519–27.
22. Huizing M, Krasnewich DM. Hereditary inclusion body myopathy: a decade of progress. Biochim Biophys Acta. 2009;1792:881–887. [PMC free article: PMC2748147] [PubMed: 19596068]
23. Huizing M, Rakocevic G, Sparks SE, Mamali I, Shatunov A, Goldfarb L, Krasnewich D, Gahl WA, Dalakas MC. Hypoglycosylation of alpha-dystroglycan in patients with hereditary IBM due to GNE mutations. Mol Genet Metab. 2004;81:196–202. [PubMed: 14972325]
24. Kayashima T, Matsuo H, Satoh A, Ohta T, Yoshiura K, Matsumoto N, Nakane Y, Niikawa N, Kishino T. Nonaka myopathy is caused by mutations in the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase gene (GNE). J Hum Genet. 2002;47:77–9. [PubMed: 11916006]
25. Kim BJ, Ki CS, Kim JW, Sung DH, Choi YC, Kim SH. Mutation analysis of the GNE gene in Korean patients with distal myopathy with rimmed vacuoles. J Hum Genet. 2006;51:137–40. [PubMed: 16372135]
26. Klootwijk RD, Savelkoul PJ, Ciccone C, Manoli I, Caplen NJ, Krasnewich DM, Gahl WA, Huizing M. Allele-specific silencing of the dominant disease allele in sialuria by RNA interference. FASEB J. 2008;22:3846–52. [PMC free article: PMC2574030] [PubMed: 18653764]
27. Kornfeld S, Sly WS. I-cell disease and pseudo-Hurler polydystrophy: disorders of lysosomal enzyme phosphorylation and localization. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. 8 ed. New York: McGraw-Hill; 2002:3469-82. Available at www.ommbid.com. Accessed 02-01-10.
28. Krause S, Hinderlich S, Amsili S, Horstkorte R, Wiendl H, Argov Z, Mitrani-Rosenbaum S, Lochmüller H. Localization of UDP-GlcNAc 2-epimerase/ManAc kinase (GNE) in the Golgi complex and the nucleus of mammalian cells. Exp Cell Res. 2005;304:365–79. [PubMed: 15748884]
29. Lemyre E, Russo P, Melançon SB, Gagné R, Potier M, Lambert M. Clinical spectrum of infantile free sialic acid storage disease. Am J Med Genet. 1999;82:385–91. [PubMed: 10069709]
30. Leroy JG Disorders of lysomal enzymes: clinical phenotypes. In: Royce PM, Steinmann B, eds. Connective Tissue and Its Heritable Disorders. 2 ed. New York: Wiley-Liss; 2002a:849-99.
31. Leroy JG. Oligosaccharidoses and allied disorders. In: Rimoin DL, Connor JM, Pyeritz RE, Korf BR, eds. Emery and Rimoin's Principles and Practice of Medical Genetics. 4 ed. London: Churchill Livingstone; 2002b:2677-711.
32. Leroy JG, Seppala R, Huizing M, Dacremont G, De Simpel H, Van Coster RN, Orvisky E, Krasnewich DM, Gahl WA. Dominant inheritance of sialuria, an inborn error of feedback inhibition. Am J Hum Genet. 2001;68:1419–27. [PMC free article: PMC1226128] [PubMed: 11326336]
33. Malicdan MC, Noguchi S, Hayashi YK, Nonaka I, Nishino I. Prophylactic treatment with sialic acid metabolites precludes the development of the myopathic phenotype in the DMRV-hIBM mouse model. Nat Med. 2009;15:690–5. [PubMed: 19448634]
34. Mochel F, Yang B, Barritault J, Thompson JN, Engelke UF, McNeill NH, Benko WS, Kaneski CR, Adams DR, Tsokos M, Abu-Asab M, Huizing M, Seguin F, Wevers RA, Ding J, Verheijen FW, Schiffmann R. Free sialic acid storage disease without sialuria. Ann Neurol. 2009;65:753–7. [PubMed: 19557856]
35. Neufeld EF, Muenzer J. The mucopolysaccharidoses. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. 8 ed. New York: McGraw-Hill; 2001:5109-20.
36. Nishino I, Noguchi S, Murayama K, Driss A, Sugie K, Oya Y, Nagata T, Chida K, Takahashi T, Takusa Y, Ohi T, Nishimiya J, Sunohara N, Ciafaloni E, Kawai M, Aoki M, Nonaka I. Distal myopathy with rimmed vacuoles is allelic to hereditary inclusion body myopathy. Neurology. 2002;59:1689–93. [PubMed: 12473753]
37. Reinke SO, Lehmer G, Hinderlich S, Reutter W. Regulation and pathophysiological implications of UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) as the key enzyme of sialic acid biosynthesis. Biol Chem. 2009;390:591–9. [PubMed: 19426133]
38. Seppala R, Lehto VP, Gahl WA. Mutations in the human UDP-N-acetylglucosamine 2-epimerase gene define the disease sialuria and the allosteric site of the enzyme. Am J Hum Genet. 1999;64:1563–9. [PMC free article: PMC1377899] [PubMed: 10330343]
39. Spranger J. Mucopolysaccharidoses. In: Rimoin DL, Connor JM, Pyeritz RE, Korf BR, eds. Emery and Rimoin's Principles and Practice of Medical Genetics. 4 ed. London: Churchill Livingstone; 2002:2666-76.
40. Stone DL, Sidransky E. Hydrops fetalis: lysosomal storage disorders in extremis. Adv Pediatr. 1999;46:409–40. [PubMed: 10645471]
41. Suzuki Y, Oshima A, Nanba E. B-galactosidase deficiency: GM1-gangliosidosis and Morquio B disease. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. 8 ed. New York: McGraw-Hill; 2001:3775-809.
42. Thomas GH. Disorders of glycoprotein degradation: a-mannosidosis, b-mannosidosis, fucosidosis and sialidosis. In: Scriver CR, Beaudet AL, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease. 8 ed. New York: McGraw-Hill; 2001:3507-33.
43. Tomimitsu H, Shimizu J, Ishikawa K, Ohkoshi N, Kanazawa I, Mizusawa H. Distal myopathy with rimmed vacuoles (DMRV): new GNE mutations and splice variant. Neurology. 2004;62:1607–10. [PubMed: 15136692]
44. Valianpour F, Abeling NG, Duran M, Huijmans JG, Kulik W. Quantification of free sialic acid in urine by HPLC-electrospray tandem mass spectrometry: a tool for the diagnosis of sialic acid storage disease. Clin Chem. 2004;50:403–9. [PubMed: 14684624]
45. Varho T, Jääskeläinen S, Tolonen U, Sonninen P, Vainionpää L, Aula P, Sillanpää M. Central and peripheral nervous system dysfunction in the clinical variation of Salla disease. Neurology. 2000;55:99–104. [PubMed: 10891913]
46. Varho TT, Alajoki LE, Posti KM, Korhonen TT, Renlund MG, Nyman SR, Sillanpää ML, Aula PP. Phenotypic spectrum of Salla disease, a free sialic acid storage disorder. Pediatr Neurol. 2002;26:267–73. [PubMed: 11992753]
47. Vasconcelos OM, Raju R, Dalakas MC. GNE mutations in an American family with quadriceps-sparing IBM and lack of mutations in s-IBM. Neurology. 2002;59:1776–9. [PubMed: 12473769]
48. Verheijen FW, Verbeek E, Aula N, Beerens CE, Havelaar AC, Joosse M, Peltonen L, Aula P, Galjaard H, van der Spek PJ, Mancini GM. A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases. Nat Genet. 1999;23:462–5. [PubMed: 10581036]

Suggested Reading

1. Varki A, Schauer R. Sialic acids. In: Varki A, Cummings RD, Esko JD, Freeze HH, Stanley P, Bertozzi CR, Hart GW, Etzlar ME, eds. Essentials of Glycobiology. 2 ed. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press; 2009:199-217.

Acknowledgments

The help and advice regarding recent progress in hIBM, provided by Drs. Marjan Huizing and Donna M Krasnewich, NIH, Bethesda, MD, is gratefully acknowledged.

Revision History

• 2 March 2010 (me) Comprehensive update posted live

• 27 February 2007 (jgl) Revision: clinical testing and prenatal diagnosis no longer available

• 10 March 2006 (me) Comprehensive update posted to live Web site

• 14 January 2004 (me) Review posted to live Web site

• 26 September 2003 (jgl) Original submission