Identification of mutations in the GNPTA (MGC4170) gene coding for GlcNAc-phosphotransferase α/β subunits in Korean patients with mucolipidosis type II or type IIIA

Outcomes after HSCT for mucolipidosis II (I-cell disease) caused by novel compound heterozygous GNPTAB mutations

Background

Mucolipidosis type II (MLII), or I-cell disease, is a rare lysosomal storage disease (LSD) caused by variants in the GNPTAB gene. MLII patients exhibit clinical phenotypes in the prenatal or neonatal stage, such as marked dysmorphic features, cardiac involvement, respiratory symptoms, dysostosis multiplex, severe growth abnormalities, and mental and motor developmental abnormalities. The median age at diagnosis for MLII is 0.7 years, the median survival is 5.0 years, and the median age at death is 1.8 years. No cure for MLII exists.

Methods

Sanger sequencing of the GNPTAB gene identified the compound heterozygous mutations c.673C > T in exon 7 and c.1090C > T in exon 9, which were novel double heterozygous mutations first reported in China. For the first time, we describe our experience in the use of HSCT for MLII. Our patient underwent HSCT with cells from a 9/10 human leukocyte antigen (HLA)-matched unrelated donor at 12 months of age. Myeloid neutrophil and platelet engraftment occurred on Days 10 and 11, respectively.

Results

The patient's limb muscle tension was significantly reduced, and his gross and fine motor skills were improved four months after transplantation. DST(Developmental Screen Test) results showed that the patient's fine motor skills and mental development were improved compared with before HSCT.

Conclusion

MLII is a very severe lysosomal storage disease, to date, only 3 cases have been reported on the use of HSCT to treat MLII. Our data show that HSCT is a potential way to prolong the life of patients and improve their quality of life. Due to the lack of comparable data and time, the exact benefit remains unclear in MLII patients. Longer-term follow-up and in-depth prospective studies are indispensable.

Whole exome sequencing uncovered highly penetrant recessive mutations for a spectrum of rare genetic pediatric diseases in Bangladesh

Collectively, rare genetic diseases affect a significant number of individuals worldwide. In this study, we have conducted whole-exome sequencing (WES) and identified underlying pathogenic or likely pathogenic variants in five children with rare genetic diseases. We present evidence for disease-causing autosomal recessive variants in a range of disease-associated genes such as DHH-associated 46,XY gonadal dysgenesis (GD) or 46,XY sex reversal 7, GNPTAB-associated mucolipidosis II alpha/beta (ML II), BBS1-associated Bardet-Biedl Syndrome (BBS), SURF1-associated Leigh Syndrome (LS) and AP4B1-associated spastic paraplegia-47 (SPG47) in unrelated affected members from Bangladesh. Our analysis pipeline detected three homozygous mutations, including a novel c. 863 G > C (p.Pro288Arg) variant in DHH, and two compound heterozygous variants, including two novel variants: c.2972dupT (p.Met991Ilefs*) in GNPTAB and c.229 G > C (p.Gly77Arg) in SURF1. All mutations were validated by Sanger sequencing. Collectively, this study adds to the genetic heterogeneity of rare genetic diseases and is the first report elucidating the genetic profile of (consanguineous and nonconsanguineous) rare genetic diseases in the Bangladesh population.

Mucolipidoses Overview: Past, Present, and Future

Mucolipidosis II and III (ML II/III) are caused by a deficiency of uridine-diphosphate N-acetylglucosamine: lysosomal-enzyme-N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase, EC2.7.8.17), which tags lysosomal enzymes with a mannose 6-phosphate (M6P) marker for transport to the lysosome. The process is performed by a sequential two-step process: first, GlcNAc-1-phosphotransferase catalyzes the transfer of GlcNAc-1-phosphate to the selected mannose residues on lysosomal enzymes in the cis-Golgi network. The second step removes GlcNAc from lysosomal enzymes by N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase (uncovering enzyme) and exposes the mannose 6-phosphate (M6P) residues in the trans-Golgi network, in which the enzymes are targeted to the lysosomes by M6Preceptors. A deficiency of GlcNAc-1-phosphotransferase causes the hypersecretion of lysosomal enzymes out of cells, resulting in a shortage of multiple lysosomal enzymes within lysosomes. Due to a lack of GlcNAc-1-phosphotransferase, the accumulation of cholesterol, phospholipids, glycosaminoglycans (GAGs), and other undegraded substrates occurs in the lysosomes. Clinically, ML II and ML III exhibit quite similar manifestations to mucopolysaccharidoses (MPSs), including specific skeletal deformities known as dysostosis multiplex and gingival hyperplasia. The life expectancy is less than 10 years in the severe type, and there is no definitive treatment for this disease. In this review, we have described the updated diagnosis and therapy on ML II/III.

The lysosomal storage disorders mucolipidosis type II, type III alpha/beta, and type III gamma: Update on GNPTAB and GNPTG mutations

Mutations in the GNPTAB and GNPTG genes cause mucolipidosis (ML) type II, type III alpha/beta, and type III gamma, which are autosomal recessively inherited lysosomal storage disorders. GNPTAB and GNPTG encode the α/β-precursor and the γ-subunit of N-acetylglucosamine (GlcNAc)-1-phosphotransferase, respectively, the key enzyme for the generation of mannose 6-phosphate targeting signals on lysosomal enzymes. Defective GlcNAc-1-phosphotransferase results in missorting of lysosomal enzymes and accumulation of non-degradable macromolecules in lysosomes, strongly impairing cellular function. MLII-affected patients have coarse facial features, cessation of statural growth and neuromotor development, severe skeletal abnormalities, organomegaly, and cardiorespiratory insufficiency leading to death in early childhood. MLIII alpha/beta and MLIII gamma are attenuated forms of the disease. Since the identification of the GNPTAB and GNPTG genes, 564 individuals affected by MLII or MLIII have been described in the literature. In this report, we provide an overview on 258 and 50 mutations in GNPTAB and GNPTG, respectively, including 58 novel GNPTAB and seven novel GNPTG variants. Comprehensive functional studies of GNPTAB missense mutations did not only gain insights into the composition and function of the GlcNAc-1-phosphotransferase, but also helped to define genotype-phenotype correlations to predict the clinical outcome in patients.

Identification of predominant GNPTAB gene mutations in Eastern Chinese patients with mucolipidosis II/III and a prenatal diagnosis of mucolipidosis II

Mucolipidosis II α/β, mucolipidosis III α/β, and mucolipidosis III γ are autosomal recessive disorders belonging to the family of lysosomal storage disorders caused by deficiency of the UDP-N-acetylglucosamine, a lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase) localized in the Golgi apparatus, which is essential for normal processing and packaging of soluble lysosomal enzymes with initiating the first step of tagging lysosomal enzymes with mannose-6-phosphate (M6P). Mucolipidosis II and III are caused by mutations in the GNPTAB and GNPTG genes, and patients with these diseases are characterized by short stature, skeletal abnormalities, and developmental delay. In this study we report 38 patients with mucolipidosis II and III enrolled in Eastern China during the past 8 years. The diagnosis was made based on clinical characteristics and measurement of plasma lysosomal enzyme activity. Sanger sequencing of GNPTAB and/or GNPTG for all patients and real-time quantitative PCR were performed to confirm the diagnosis. In addition, 11 cases of prenatal mucolipidosis II were diagnosed based on measurement of the enzyme activity in amniotic fluid supernatant and genetic testing of cultured amniotic cells. Based on molecular genetic tests, 30 patients were diagnosed with mucolipidosis II α/β, 6 were diagnosed with III α/β and 2 were diagnosed with III γ. Thirty-seven different GNPTAB gene mutations were identified in 29 patients with mucolipidosis II α/β and six patients with III α/β. These mutations included 22 new mutations (p.W44X, p.E279X, p.W416X, p.W463X, p.Q802X, p.Q882X, p.A34P, p.R334P, p.D408N, p.D534N, p.Y997C, p.D1018V, p.L1025S, p.L1033P, c.88_89delAC, c.890_891insT, c.1150_1151insTTA, c.1523delG, c.2473_2474insA, c.2980_2983delGCCT, c.3094delA, and deletion of exon 9). Four new GNPTG gene mutations were identified (c.13delC, p.Y81X, p.G126R and c.609+1delG) in two mucolipidosis III γ patients. Among the 11 cases of prenatal diagnosis, four were mucolipidosis II fetuses, three were heterozygous, and the remaining four were normal fetuses. This study expands the mutation spectrum of the GNPTAB and GNPTG genes and contributes to specific knowledge of mucolipidosis II/III in a population from Eastern China.

A GNPTAB nonsense variant is associated with feline mucolipidosis II (I-cell disease)

Background

Mucolipidosis II (ML II; I-cell disease) is caused by a deficiency of N-acetylglucosamine-1-phosphotransferase (GNPTAB; EC 2.7.8.17), which leads to a failure to internalize acid hydrolases into lysosomes for proper catabolism of various substances. This is an autosomal recessive lysosomal storage disease and causes severe progressive neuropathy and oculoskeletal dysfunction in humans (OMIM 252500). A naturally occurring disease model has been reported in juvenile domestic cats (OMIA 001248–9685) with clinical signs similar to human patients. We investigated the molecular genetic basis of ML II in a colony of affected cats by sequencing the coding and regulatory regions of GNPTAB from affected and clinically healthy related and unrelated domestic cats and compared the sequences to the published feline genome sequence (NCBI-RefSeq accession no. XM_003989173.4, Gene ID: 101100231).

Results

All affected cats were homozygous for a single base substitution (c.2644C > T) in exon 13 of GNPTAB. This variant results in a premature stop codon (p.Gln882*) which predicts severe truncation and complete dysfunction of the GNPTAB enzyme. About 140 GNPTAB variants have been described in human ML II patients, with 41.3% nonsense/missense mutations, nine occurring in the same gene region as in this feline model. Restriction fragment length polymorphism and allelic discrimination real-time polymerase chain reaction assays accurately differentiated between clear, asymptomatic carriers and homozygous affected cats.

Conclusion

Molecular genetic characterization advances this large animal model of ML II for use to further define the pathophysiology of the disease and evaluate novel therapeutic approaches for this fatal lysosomal storage disease in humans.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1728-1) contains supplementary material, which is available to authorized users.

GNPTAB c.2404C > T nonsense mutation in a patient with mucolipidosis III alpha/beta: a case report

Background

Mucolipidosis alpha/beta is an inborn error of metabolism characterized by deficiency of GlcNAc-1-phosphotransferase, in which essential alpha/beta subunits are encoded by the GNPTAB gene. The autosomal recessive condition is due to disruptions of hydrolase mannose 6-phosphate marker generation, defective lysosomal targeting and subsequent intracellular accumulation of non-degraded material. Clinical severity depends on residual GlcNAc-1-phosphotransferase activity, which distinguishes between the milder type III disease and the severe, neonatal onset type II disease.

Case presentation

We report the clinical, biochemical and genetic diagnosis of mucolipidosis III alpha/beta in a two-year-old Chinese boy who initially presented with poor weight gain, microcephaly and increased tone. He was confirmed to harbor the common splice site mutation c.2715 + 1G > A and the nonsense variant c.2404C > T (p.Q802*). Clinically, the patient had multiple phenotypic features typical of mucopolysaccharidosis including joint contractures, coarse facial features, kypho-lordosis, pectus carinatum and umbilical hernia. However, the relatively mild developmental delay compared to severe type I and type II mucopolysaccharidosis and the absence of macrocephaly raised the possibility of the less commonly diagnosed mucolipidosis alpha/beta. Critical roles of lysosomal enzyme activity assay, which showed elevated α-iduronidase, iduronate sulfatase, galactose-6-sulphate sulphatase, arylsulfatase B and α-hexosaminidase activities; and genetic study, which confirmed the parental origin of both mutations, were highlighted.

Conclusions

The recently reported nonsense variant c.2404C > T in the GNPTAB gene is further recognized and this contributes to the genotype-phenotype spectrum of mucolipidosis alpha/beta.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0679-5) contains supplementary material, which is available to authorized users.

Impact, Characterization, and Rescue of Pre-mRNA Splicing Mutations in Lysosomal Storage Disorders

Lysosomal storage disorders (LSDs) represent a group of more than 50 severe metabolic diseases caused by the deficiency of specific lysosomal hydrolases, activators, carriers, or lysosomal integral membrane proteins, leading to the abnormal accumulation of substrates within the lysosomes. Numerous mutations have been described in each disease-causing gene; among them, about 5-19% affect the pre-mRNA splicing process. In the last decade, several strategies to rescue/increase normal splicing of mutated transcripts have been developed and LSDs represent excellent candidates for this type of approach: (i) most of them are inherited in an autosomic recessive manner and patients affected by late-onset (LO) phenotypes often retain a fair amount of residual enzymatic activity; thus, even a small recovery of normal splicing may be beneficial in clinical settings; (ii) most LSDs still lack effective treatments or are currently treated with extremely expensive approaches; (iii) in few LSDs, a single splicing mutation accounts for up to 40-70% of pathogenic alleles. At present, numerous preclinical studies support the feasibility of reverting the pathological phenotype by partially rescuing splicing defects in LSDs. This review provides an overview of the impact of splicing mutations in LSDs and the related therapeutic approaches currently under investigation in these disorders.

Pharmaceutical Chaperones and Proteostasis Regulators in the Therapy of Lysosomal Storage Disorders: Current Perspective and Future Promises

Different approaches have been utilized or proposed for the treatment of lysosomal storage disorders (LSDs) including enzyme replacement and hematopoietic stem cell transplant therapies, both aiming to compensate for the enzymatic loss of the underlying mutated lysosomal enzymes. However, these approaches have their own limitations and therefore the vast majority of LSDs are either still untreatable or their treatments are inadequate. Missense mutations affecting enzyme stability, folding and cellular trafficking are common in LSDs resulting often in low protein half-life, premature degradation, aggregation and retention of the mutant proteins in the endoplasmic reticulum. Small molecular weight compounds such as pharmaceutical chaperones (PCs) and proteostasis regulators have been in recent years to be promising approaches for overcoming some of these protein processing defects. These compounds are thought to enhance lysosomal enzyme activity by specific binding to the mutated enzyme or by manipulating components of the proteostasis pathways promoting protein stability, folding and trafficking and thus enhancing and restoring some of the enzymatic activity of the mutated protein in lysosomes. Multiple compounds have already been approved for clinical use to treat multiple LSDs like migalastat in the treatment of Fabry disease and others are currently under research or in clinical trials such as Ambroxol hydrochloride and Pyrimethamine. In this review, we are presenting a general overview of LSDs, their molecular and cellular bases, and focusing on recent advances on targeting and manipulation proteostasis, including the use of PCs and proteostasis regulators, as therapeutic targets for some LSDs. In addition, we present the successes, limitations and future perspectives in this field.

I Cell Disease (Mucolipidosis II Alpha/Beta): From Screening to Molecular Diagnosis

Mucopolysaccharidosis (MPS) and Mucolipidosis (ML) share common phenotypes (coarse facial features, organomegaly, dysostosis multiplex) despite having different molecular basis. Thus, they pose great diagnostic challenge to treating clinicians. Differentiating between the two conditions requires a battery of tests from screening to molecular diagnosis. Besides discussing differential diagnosis of MPS like features with negative urinary Glycosaminoglycans (GAG), the authors also discuss the utility of p-nitrocatechol sulphate based chemical test as an important screening tool, besides establishing molecular basis in index case.

Clinical, biochemical and molecular characterization of Korean patients with mucolipidosis II/III and successful prenatal diagnosis

BACKGROUND

Mucolipidosis types II and III (ML II/III) are autosomal recessive disorders caused by a deficiency in the lysosomal enzyme N-acetylglucosamine-1-phosphotransferase. We investigated the molecular genetic characteristics of the GNPTAB gene, which codes for the alpha/beta subunits of a phosphotransferase, in Korean ML II/III patients. We included prenatal tests and evaluated the spectrum of mutations in East Asian populations with ML II/III through a literature review.

METHODS

Seven patients from six families were enrolled in the study including two prenatal tests using chorionic villi samples. A diagnosis of ML II/III was made based on clinical findings and increases in serum lysosomal enzyme levels. PCR and direct sequencing were performed to identify GNPTAB mutations.

RESULTS

We found 14 mutant alleles including seven known mutations of c.2189delT (p.Leu730fs*7), c.1090C > T (p.Arg364*), c.2681G > A (p.Trp894*), c.3565C > T (p.Arg1189*), c.310C > T (p.Gln104*), c.1071G > A (p.Trp357*) and c.2574_2575delGA (p.Asn859Glnfs*2). Four were novel variants of unknown significance: c.992A > G (p.Tyr331Cys), c.2666 T > A (p.Leu889*), c.637-6 T > G (p.Thr213Phefs*11), and c.471_472delTT (p.Tyr158Serfs*8). Family studies revealed the probands to be compound heterozygotes. The fetuses carried the same GNPTAB mutations as the mucolipidosis II/III probands in the prenatal diagnosis.

CONCLUSIONS

We identified GNPTAB mutations in all patients with ML II/III, but did not identify a hot spot in Korean patients. We successfully performed prenatal diagnosis using molecular investigation.

Bidirectional traffic between the Golgi and the endosomes - machineries and regulation

The bidirectional transport between the Golgi complex and the endocytic pathway has to be finely regulated in order to ensure the proper delivery of newly synthetized lysosomal enzymes and the return of sorting receptors from degradative compartments. The high complexity of these routes has led to experimental difficulties in properly dissecting and separating the different pathways. As a consequence, several models have been proposed during the past decades. However, recent advances in our understanding of endosomal dynamics have helped to unify these different views. We provide here an overview of the current insights into the transport routes between Golgi and endosomes in mammalian cells. The focus of the Commentary is on the key molecules involved in the trafficking pathways between these intracellular compartments, such as Rab proteins and sorting receptors, and their regulation. A proper understanding of the bidirectional traffic between the Golgi complex and the endolysosomal system is of uttermost importance, as several studies have demonstrated that mutations in the factors involved in these transport pathways result in various pathologies, in particular lysosome-associated diseases and diverse neurological disorders, such as Alzheimer's and Parkinson's disease.

Mutation Analysis of 16 Mucolipidosis II and III Alpha/Beta Chinese Children Revealed Genotype-Phenotype Correlations

Mucolipidosis II and III alpha/beta are autosomal recessive diseases caused by mutations in the GNPTAB gene which encodes the α and β subunits of the N-acetylglucosamine-1-phosphotransferase. Clinically, mucolipidosis II (MLII) is characterized by severe developmental delay, coarse facial features, skeletal deformities, and other systemic involvement. In contrast, MLIII alpha/beta is a much milder disorder, the symptoms of which include progressive joint stiffness, short stature, and scoliosis. To study the relationship between the genotypes and phenotypes of the MLII and MLIII alpha/beta patients, we analyzed the GNPTAB gene in 16 Chinese MLII and MLIII alpha/beta patients. We collected and analyzed the patients’ available clinical data and all showed clinical features typical of MLII or MLIII alpha/beta. Moreover, the activity of several lysosomal enzymes was measured in the plasma and finally the GNPTAB gene was sequenced. We detected 30 mutant alleles out of 32 alleles in our patients. These include 10 new mutations (c.99delC, c.118-1G>A, c.523_524delAAinsG, c.1212C>G, c.2213C>A, c.2345C>T, c.2356C>T, c.2455G>T, c.2821dupA, and c.3136-2A>G) and 5 previously reported mutations (c.1071G>A, c.1090C>T, c.2715+1G>A, c.2550_2554delGAAA, and c.3613C>T). The most frequent mutation was the splicing mutation c.2715+1G>A, which accounted for 28% of the mutations. The majority of the mutations reported in the Chinese patients (57%) were located on exon 13 or in its intronic flanking regions.

Lysosomal Storage Diseases-Regulating Neurodegeneration

Autophagy is a complex pathway regulated by numerous signaling events that recycles macromolecules and can be perturbed in lysosomal storage diseases (LSDs). The concept of LSDs, which are characterized by aberrant, excessive storage of cellular material in lysosomes, developed following the discovery of an enzyme deficiency as the cause of Pompe disease in 1963. Great strides have since been made in better understanding the biology of LSDs. Defective lysosomal storage typically occurs in many cell types, but the nervous system, including the central nervous system and peripheral nervous system, is particularly vulnerable to LSDs, being affected in two-thirds of LSDs. This review provides a summary of some of the better characterized LSDs and the pathways affected in these disorders.

AAV8-mediated expression of N-acetylglucosamine-1-phosphate transferase attenuates bone loss in a mouse model of mucolipidosis II

Mucolipidoses II and III (ML II and ML III) are lysosomal disorders in which the mannose 6-phosphate recognition marker is absent from lysosomal hydrolases and other glycoproteins due to mutations in GNPTAB, which encodes two of three subunits of the heterohexameric enzyme, N-acetylglucosamine-1-phosphotransferase. Both disorders are caused by the same gene, but ML II represents the more severe phenotype. Bone manifestations of ML II include hip dysplasia, scoliosis, rickets and osteogenesis imperfecta. In this study, we sought to determine whether a recombinant adeno-associated viral vector (AAV2/8-GNPTAB) could confer high and prolonged gene expression of GNPTAB and thereby influence the pathology in the cartilage and bone tissue of a GNPTAB knock out (KO) mouse model. The results demonstrated significant increases in bone mineral density and content in AAV2/8-GNPTAB-treated as compared to non-treated KO mice. We also showed that IL-6 (interleukin-6) expression in articular cartilage was reduced in AAV2/8-GNPTAB treated ML II mice. Together, these data suggest that AAV-mediated expression of GNPTAB in ML II mice can attenuate bone loss via inhibition of IL-6 production. This study emphasizes the value of the MLII KO mouse to recapitulate the clinical manifestations of the disease and highlights its amenability to therapy.

Mucolipidosis types II and III and non-syndromic stuttering are associated with different variants in the same genes

Homozygous mutations in GNPTAB and GNPTG are classically associated with mucolipidosis II (ML II) alpha/beta and mucolipidosis III (ML III) alpha/beta/gamma, which are rare lysosomal storage disorders characterized by multiple pathologies. Recently, variants in GNPTAB, GNPTG, and the functionally related NAGPA gene have been associated with non-syndromic persistent stuttering. In a worldwide sample of 1013 unrelated individuals with non-syndromic persistent stuttering we found 164 individuals who carried a rare non-synonymous coding variant in one of these three genes. We compared the frequency of these variants with those in population-matched controls and genomic databases, and their location with those reported in mucolipidosis. Stuttering subjects displayed an excess of non-synonymous coding variants compared to controls and individuals in the 1000 Genomes and Exome Sequencing Project databases. We identified a total of 81 different variants in our stuttering cases. Virtually all of these were missense substitutions, only one of which has been previously reported in mucolipidosis, a disease frequently associated with complete loss-of-function mutations. We hypothesize that rare non-synonymous coding variants in GNPTAB, GNPTG, and NAGPA may account for as much as 16% of persistent stuttering cases, and that variants in GNPTAB and GNPTG are at different sites and may in general, cause less severe effects on protein function than those in ML II alpha/beta and ML III alpha/beta/gamma.

Mucolipidosis II-related mutations inhibit the exit from the endoplasmic reticulum and proteolytic cleavage of GlcNAc-1-phosphotransferase precursor protein (GNPTAB)

Mucolipidosis (ML) II and MLIII alpha/beta are two pediatric lysosomal storage disorders caused by mutations in the GNPTAB gene, which encodes an α/β-subunit precursor protein of GlcNAc-1-phosphotransferase. Considerable variations in the onset and severity of the clinical phenotype in these diseases are observed. We report here on expression studies of two missense mutations c.242G>T (p.Trp81Leu) and c.2956C>T (p.Arg986Cys) and two frameshift mutations c.3503_3504delTC (p.Leu1168GlnfsX5) and c.3145insC (p.Gly1049ArgfsX16) present in severely affected MLII patients, as well as two missense mutations c.1196C>T (p.Ser399Phe) and c.3707A>T (p.Lys1236Met) reported in more mild affected individuals. We generated a novel α-subunit-specific monoclonal antibody, allowing the analysis of the expression, subcellular localization, and proteolytic activation of wild-type and mutant α/β-subunit precursor proteins by Western blotting and immunofluorescence microscopy. In general, we found that both missense and frameshift mutations that are associated with a severe clinical phenotype cause retention of the encoded protein in the endoplasmic reticulum and failure to cleave the α/β-subunit precursor protein are associated with a severe clinical phenotype with the exception of p.Ser399Phe found in MLIII alpha/beta. Our data provide new insights into structural requirements for localization and activity of GlcNAc-1-phosphotransferase that may help to explain the clinical phenotype of MLII patients.

Mucolipidosis II: first report from Saudi Arabia

BACKGROUND AND OBJECTIVES

Mucolipidosis II (MLII) is characterized by severe global developmental delay, coarse facial features, skeletal deformities, and other systemic involvement. It is caused by a deficiency in N-acetylglucosamine-1 phosphotransferase.

DESIGN AND SETTINGS

This is a case series study conducted at King Abdulaziz Medical City in Riyadh, Saudi Arabia, between 2008-2012.

PATIENTS AND METHODS

We described three unrelated Saudi children who presented with neonatal hyperparathyroidism, microcephaly, craniosynostosis, coarse facial features, cardiac involvement, and skeletal deformities.

RESULTS

The MLII diagnosis was confirmed by assaying enzyme activities in fibroblasts, which showed a severe reduction in hydrolyzed substrates compared to controls, and by identifying a pathogenic homozygous GNPTAB gene mutation. One of the children died at 2 months of age due to severe pulmonary hypertension, and the other two children were still alive at 12 months and 18 months of age, respectively. Both surviving children had severe global developmental delay at 2 months of age.

CONCLUSION

Clinicians should investigate any child presenting with neonatal hyperparathyroidism, craniosynostosis, skeletal deformities, and coarse facial features for MLII.

Two homozygous nonsense mutations of GNPTAB gene in two Chinese families with mucolipidosis II alpha/beta using targeted next-generation sequencing

Mucolipidosis II alpha/beta (ML II alpha/beta; I-cell disease) is a rare, inherited, metabolic disease and has often been clinically misdiagnosed. ML II alpha/beta results from a deficiency of the enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-PT), which causes the lysosomal enzymes to accumulate in plasma. We identified two new Chinese patients with ML II alpha/beta by lysosomal enzyme assay. Using targeted next-generation sequencing genetic analysis, we located two homozygous nonsense mutations in the GNPTAB gene, c.1071G>A (p.W357X) and c.1090C>T (p.R364X). These results were confirmed by Sanger sequencing. To our knowledge, the c.1071G>A mutation has not been previously reported. Our findings add to the number of reported cases of this rare illness and to the GNPTAB pathogenic mutation database. This work also demonstrates the application of lysosomal enzyme assay and targeted next-generation sequencing for the genetic screening analysis and diagnosis of ML II alpha/beta.

Mucolipidosis II and III alpha/beta in Brazil: analysis of the GNPTAB gene

Mucolipidosis II and III (MLII and MLIII) alpha/beta are rare autosomal recessive lysosomal storage diseases (LSDs) caused by pathogenic variations in the GNPTAB gene. GNPTAB gene codes for the α and β subunits of phosphotransferase, the enzyme responsible for synthesis of the mannose-6-phosphate (M6P) marker that directs lysosomal enzymes to the lysosome.

OBJECTIVES

The objective of this study is to identify sequence variations of the GNPTAB gene in Brazilian patients with MLII and MLIII alpha/beta.

METHOD

Sequencing of the GNPTAB gene was performed in samples of gDNA extracted from the peripheral blood of patients with MLII/III diagnosed at a national reference center for LSDs.

RESULTS

Twelve unrelated patients, from several regions of Brazil, were included in this study. Only one was born of consanguineous parents. All patients were found to carry at least one nonpathogenic variation. Nine causal sequence variations were found: c.242G>T (p.W81L); c.1123C>T (p.R375X); c.1196C>T (p.S399F); c.1208T>C (p.I403T); c.1514G>A (p.C505Y); c.1759C>T (p.R587X); c.2808A>G (p.Y937_M972del, novel mutation); c. 2269_2273delGAAAC (p.E757KfsX2, novel mutation); and c.3503_3504delTC (p.L1168QfsX5). Both pathogenic variations were identified in 8 of 12 patients; in four patients, only one pathogenic variation was identified. Mutation c.3503_3504delTC, located in exon 19, was the most frequent pathogenic variation found (n=11/24 alleles). The deleterious effect of the c.2808A>C mutation on splicing was confirmed by cDNA analysis.

DISCUSSION/CONCLUSIONS

Our findings confirm that the GNPTAB gene presents broad allelic heterogeneity and suggests that, in Brazilian ML II and III patients, screening for mutations should begin at exon 19 of the GNPTAB gene. Further analyses will be conducted on patients in whom both pathogenic mutations have not been found in this study.

A case of mucolipidosis II presenting with prenatal skeletal dysplasia and severe secondary hyperparathyroidism at birth

Mucolipidosis II (ML II) or inclusion cell disease (I-cell disease) is a rarely occurring autosomal recessive lysosomal enzyme-targeting disease. This disease is usually found to occur in individuals aged between 6 and 12 months, with a clinical phenotype resembling that of Hurler syndrome and radiological findings resembling those of dysostosis multiplex. However, we encountered a rare case of an infant with ML II who presented with prenatal skeletal dysplasia and typical clinical features of severe secondary hyperparathyroidism at birth. A female infant was born at 37⁺¹ weeks of gestation with a birth weight of 1,690 g (<3rd percentile). Prenatal ultrasonographic findings revealed intrauterine growth retardation and skeletal dysplasia. At birth, the patient had characteristic features of ML II, and skeletal radiographs revealed dysostosis multiplex, similar to rickets. In addition, the patient had high levels of alkaline phosphatase and parathyroid hormone, consistent with severe secondary neonatal hyperparathyroidism. The activities of β-D-hexosaminidase and α-N-acetylglucosaminidase were moderately decreased in the leukocytes but were 5- to 10-fold higher in the plasma. Examination of a placental biopsy specimen showed foamy vacuolar changes in trophoblasts and syncytiotrophoblasts. The diagnosis of ML II was confirmed via GNPTAB genetic testing, which revealed compound heterozygosity of c.3091C>T (p.Arg1031X) and c.3456_3459dupCAAC (p.Ile1154GlnfsX3), the latter being a novel mutation. The infant was treated with vitamin D supplements but expired because of asphyxia at the age of 2 months.

Autophagy in lysosomal storage disorders

Lysosomes are ubiquitous intracellular organelles that have an acidic internal pH, and play crucial roles in cellular clearance. Numerous functions depend on normal lysosomes, including the turnover of cellular constituents, cholesterol homeostasis, downregulation of surface receptors, inactivation of pathogenic organisms, repair of the plasma membrane and bone remodeling. Lysosomal storage disorders (LSDs) are characterized by progressive accumulation of undigested macromolecules within the cell due to lysosomal dysfunction. As a consequence, many tissues and organ systems are affected, including brain, viscera, bone and cartilage. The progressive nature of phenotype development is one of the hallmarks of LSDs. In recent years biochemical and cell biology studies of LSDs have revealed an ample spectrum of abnormalities in a variety of cellular functions. These include defects in signaling pathways, calcium homeostasis, lipid biosynthesis and degradation and intracellular trafficking. Lysosomes also play a fundamental role in the autophagic pathway by fusing with autophagosomes and digesting their content. Considering the highly integrated function of lysosomes and autophagosomes it was reasonable to expect that lysosomal storage in LSDs would have an impact upon autophagy. The goal of this review is to provide readers with an overview of recent findings that have been obtained through analysis of the autophagic pathway in several types of LSDs, supporting the idea that LSDs could be seen primarily as "autophagy disorders."

Mannose-6-phosphate pathway: a review on its role in lysosomal function and dysfunction

Lysosomal hydrolases are synthesized in the rough endoplasmic reticulum and specifically transported through the Golgi apparatus to the trans-Golgi network, from which transport vesicles bud to deliver them to the endosomal/lysosomal compartment. The explanation of how are the lysosomal enzymes accurately recognized and selected over many other proteins in the trans-Golgi network relies on being tagged with a unique marker: the mannose-6-phosphate (M6P) group, which is added exclusively to the N-linked oligosaccharides of lysosomal soluble hydrolases, as they pass through the cis-Golgi network. Generation of the M6P recognition marker depends on a reaction involving two different enzymes: UDP-N-acetylglucosamine 1-phosphotransferase and α-N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase. The M6P groups are then recognized by two independent transmembrane M6P receptors, present in the trans-Golgi network: the cation-independent M6P receptor and/or the cation-dependent M6P receptor. These proteins bind to lysosomal hydrolases on the lumenal side of the membrane and to adaptins in assembling clathrin coats on the cytosolic side. In this way, the M6P receptors help package the hydrolases into vesicles that bud from the trans-Golgi network to deliver their contents to endosomes that ultimately will develop into mature lysosomes, where recently-delivered hydrolases may start digesting the endocyted material. The above described process is known as the M6P-dependent pathway and is responsible for transporting most lysosomal enzymes. This review synthesizes the current knowledge on each of the major proteins involved in the M6P-dependent pathway. Impairments in this pathway will also be addressed, highlighting the lysosomal storage disorders associated to GlcNAc-1-phosphotransferase loss of function: mucolipidosis type II and III.

A key enzyme in the biogenesis of lysosomes is a protease that regulates cholesterol metabolism

Mucolipidosis II is a severe lysosomal storage disorder caused by defects in the α and β subunits of the hexameric N-acetylglucosamine-1-phosphotransferase complex essential for the formation of the mannose 6-phosphate targeting signal on lysosomal enzymes. Cleavage of the membrane-bound α/β-subunit precursor by an unknown protease is required for catalytic activity. Here we found that the α/β-subunit precursor is cleaved by the site-1 protease (S1P) that activates sterol regulatory element-binding proteins in response to cholesterol deprivation. S1P-deficient cells failed to activate the α/β-subunit precursor and exhibited a mucolipidosis II-like phenotype. Thus, S1P functions in the biogenesis of lysosomes, and lipid-independent phenotypes of S1P deficiency may be caused by lysosomal dysfunction.

Genetic advances in the study of speech and language disorders

Developmental speech and language disorders cover a wide range of childhood conditions with overlapping but heterogeneous phenotypes and underlying etiologies. This characteristic heterogeneity hinders accurate diagnosis, can complicate treatment strategies, and causes difficulties in the identification of causal factors. Nonetheless, over the last decade, genetic variants have been identified that may predispose certain individuals to different aspects of speech and language difficulties. In this review, we summarize advances in the genetic investigation of stuttering, speech-sound disorder (SSD), specific language impairment (SLI), and developmental verbal dyspraxia (DVD). We discuss how the identification and study of specific genes and pathways, including FOXP2, CNTNAP2, ATP2C2, CMIP, and lysosomal enzymes, may advance our understanding of the etiology of speech and language disorders and enable us to better understand the relationships between the different forms of impairment across the spectrum.

A novel single-chain antibody fragment for detection of mannose 6-phosphate-containing proteins: application in mucolipidosis type II patients and mice

Newly synthesized soluble lysosomal hydrolases require mannose 6-phosphate (Man6P) residues on their oligosaccharides for their transport to lysosomes. The formation of Man6P residues is catalyzed by the GlcNAc-1-phosphotransferase, which is defective in the lysosomal storage disorders mucolipidosis type II (ML II) and ML III. Both hypersecretion and reduced intracellular level of lysosomal enzymes as well as direct sequencing of GlcNAc-1-phosphotransferase genes are important diagnostic markers for ML II and ML III. A high-affinity Man6P-specific single-chain antibody fragment was generated, allowing the rapid indirect demonstration of defective GlcNAc-1-phosphotransferase. In media and extracts of cultured fibroblasts of healthy controls but not of ML II and ML III patients, several Man6P-containing proteins could be detected by anti-Man6P Western blotting. Immunoprecipitation of Man6P-containing proteins from conditioned media or mouse brain extracts followed by arylsulfatase A and cathepsin D Western blotting confirmed the specificity of the antibody fragment for lysosomal proteins. Application of the antibody fragment in immunohistochemistry of human brain slices from nonaffected patients showed strong neuronal immunoreactivity, which was not observed in cortical sections of an ML II patient. Finally, in brain extracts of a novel GlcNAc-1-phosphotransferase knock-in mouse no Man6P-containing proteins were detectable. Thus, the single-chain antibody fragment against Man6P was demonstrated to allow the specific, rapid, and convenient detection of Man6P-containing proteins and facilitates the diagnosis of ML II and ML III.

Inhibition of autophagosome formation restores mitochondrial function in mucolipidosis II and III skin fibroblasts

Mucolipidosis II and III are progressive lysosomal storage disorders caused by a deficiency of N-acetylglucosamine-1-phosphotransferase, leading to massive accumulation of undigested substrates in lysosomes (inclusion bodies) in skin fibroblast. In this study, we demonstrated accumulation of autolysosomes and increased levels of p62 and ubiquitin proteins in cultured fibroblasts. These autophagic elevations were milder in mucolipidosis III compared with mucolipidosis II. Mitochondrial structure was fragmented and activity was impaired in the affected cells, and 3-methyladenine, an inhibitor of autophagosome formation, restored these. These results show for the first time autophagic and mitochondrial dysfunctions in this disorder.

Mannose phosphorylation in health and disease

Lysosomal hydrolases catalyze the degradation of a variety of macromolecules including proteins, carbohydrates, nucleic acids and lipids. The biogenesis of lysosomes or lysosome-related organelles requires a continuous substitution of soluble acid hydrolases and lysosomal membrane proteins. The targeting of lysosomal hydrolases depends on mannose 6-phosphate residues (M6P) that are recognized by specific receptors mediating their transport to an endosomal/prelysosomal compartment. The key role in the formation of M6P residues plays the GlcNAc-1-phosphotransferase localized in the Golgi apparatus. Two genes have been identified recently encoding the type III alpha/beta-subunit precursor membrane protein and the soluble gamma-subunit of GlcNAc-1-phosphotransferase. Mutations in these genes result in two severe diseases, mucolipidosis type II (MLII) and III (MLIII), biochemically characterized by the missorting of multiple lysosomal hydrolases due to impaired formation of the M6P recognition marker, and general lysosomal dysfunction. This review gives an update on structural properties, localization and functions of the GlcNAc-1-phosphotransferase subunits and improvements of pre- and postnatal diagnosis of ML patients. Further, the generation of recombinant single-chain antibody fragments against M6P residues and of new mouse models of MLII and MLIII will have considerable impact to provide deeper insight into the cell biology of lysosomal dysfunctions and the pathomechanisms underlying these lysosomal disorders.

Compensatory expression of human N-acetylglucosaminyl-1-phosphotransferase subunits in mucolipidosis type III gamma

The N-Acetylglucosaminyl-1-phosphotransferase plays a key role in the generation of mannose 6-phosphate (M6P) recognition markersessential for efficient transport of lysosomal hydrolases to lysosomes. The phosphotransferase is composed of six subunits (alpha2, beta2, gamma2). The alpha- and beta-subunits are catalytically active and encoded by a single gene, GNPTAB, whereas the gamma-subunit encoded by GNPTG is proposed to recognize conformational structures common to lysosomal enzymes. Defects in GNPTG cause mucolipidosis type III gamma, which is characterized by missorting and cellular loss of lysosomal enzymes leading to lysosomal accumulation of storage material. Using plasmon resonance spectrometry, we showed that recombinant gamma-subunit failed to bind the lysosomal enzyme arylsulfatase A. Additionally, the overexpression of the gamma-subunit in COS7 cells did not result in hypersecretion of newly synthesized lysosomal enzymes expected for competition for binding sites of the endogenous phosphotransferase complex. Analysis of fibroblasts exhibiting a novel mutation in GNPTG (c.619insT, p.K207IfsX7) revealed that the expression of GNPTAB was increased whereas in gamma-subunit overexpressing cells the GNPTAB mRNA was reduced. The data suggest that the gamma-subunit is important for the balance of phosphotransferase subunits rather for general binding of lysosomal enzymes.

Identification and molecular characterization of six novel mutations in the UDP-N-acetylglucosamine-1-phosphotransferase gamma subunit (GNPTG) gene in patients with mucolipidosis III gamma

Mucolipidosis type III (MLIII) is an autosomal recessive disorder affecting lysosomal hydrolase trafficking. In a study of 10 patients from seven families with a clinical phenotype and enzymatic diagnosis of MLIII, six novel GNPTG gene mutations were identified. These included missense (p.T286M) and nonsense (p.W111X) mutations and a transition in the obligate AG-dinucleotide of the intron 8 acceptor splice site (c.610-2A>G). Three microdeletions were also identified, two of which (c.611delG and c.640_667del28) were located within the coding region whereas one (c.609+28_610-16del) was located entirely within intron 8. RT-PCR analysis of the c.610-2A>G transition demonstrated that the change altered splicing, leading to the production of two distinct aberrantly spliced forms, viz. the skipping of exon 9 (p.G204_K247del) or the retention of introns 8 and 9 (p.G204VfsX28). RT-PCR analysis, performed on a patient homozygous for the intronic deletion (c.609+28_610-16del), failed to detect any GNPTG RNA transcripts. To determine whether c.609+28_610-16del allele-derived transcripts were subject to nonsense-mediated mRNA decay (NMD), patient fibroblasts were incubated with the protein synthesis inhibitor anisomycin. An RT-PCR fragment retaining 43 bp of intron 8 was consistently detected suggesting that the 33-bp genomic deletion had elicited NMD. Quantitative real-time PCR and GNPTG western blot analysis confirmed that the homozygous microdeletion p.G204VfsX17 had elicited NMD resulting in failure to synthesize GNPTG protein. Analysis of the sequences surrounding the microdeletion breakpoints revealed either intrinsic repetitivity of the deleted region or short direct repeats adjacent to the breakpoint junctions. This is consistent with these repeats having mediated the microdeletions via replication slippage and supports the view that the mutational spectrum of the GNPTG gene is strongly influenced by the properties of the local DNA sequence environment.

Phenotype and genotype in mucolipidoses II and III alpha/beta: a study of 61 probands

BACKGROUND

Mucolipidoses II and III alpha/beta (ML II and ML III) are lysosomal disorders in which the essential mannose 6-phosphate recognition marker is not synthesised on to lysosomal hydrolases and other glycoproteins. The disorders are caused by mutations in GNPTAB, which encodes two of three subunits of the heterohexameric enzyme, N-acetylglucosamine-1-phosphotransferase.

OBJECTIVES

Clinical, biochemical and molecular findings in 61 probands (63 patients) are presented to provide a broad perspective of these mucolipidoses.

METHODS

GNPTAB was sequenced in all probands and/or parents. The activity of several lysosomal enzymes was measured in plasma, and GlcNAc-1-phosphotransferase was assayed in leucocytes. Thirty-six patients were studied in detail, allowing extensive clinical data to be abstracted.

RESULTS

ML II correlates with near-total absence of phosphotransferase activity resulting from homozygosity or compound heterozygosity for frameshift or nonsense mutations. Craniofacial and orthopaedic manifestations are evident at birth, skeletal findings become more obvious within the first year, and growth is severely impaired. Speech, ambulation and cognitive function are impaired. ML III retains a low level of phosphotransferase activity because of at least one missense or splice site mutation. The phenotype is milder, with minimal delays in milestones, the appearance of facial coarsening by early school age, and slowing of growth after the age of 4 years.

CONCLUSIONS

Fifty-one pathogenic changes in GNPTAB are presented, including 42 novel mutations. Ample clinical information improves criteria for delineation of ML II and ML III. Phenotype-genotype correlations suggested in more general terms in earlier reports on smaller groups of patients are specified and extended.

Comparative pathology of murine mucolipidosis types II and IIIC

UDP-GlcNAc: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase) is an alpha(2)beta(2)gamma(2) hexameric enzyme that catalyzes the first step in the synthesis of the mannose 6-phosphate targeting signal on lysosomal hydrolases. In humans, mutations in the gene encoding the alpha/beta subunit precursor give rise to mucolipidosis II (MLII), whereas mutations in the gene encoding the gamma subunit cause the less severe mucolipidosis IIIC (MLIIIC). In this study we describe the phenotypic, histologic, and serum lysosomal enzyme abnormalities in knockout mice lacking the gamma subunit and compare these findings to those of mice lacking the alpha/beta subunits and humans with MLII and MLIIIC. We found that both lines of mutant mice had elevated levels of serum lysosomal enzymes and cytoplasmic alterations in secretory cells of several exocrine glands; however, lesions in gamma-subunit deficient (Gnptg(-/-)) mice were milder and more restricted in distribution than in alpha/beta-subunit deficient (Gnptab(-/-)) mice. We found that onset, extent, and severity of lesions that developed in these two different knockouts correlated with measured lysosomal enzyme activity; with a more rapid, widespread, and severe storage disease phenotype developing in Gnptab(-/-) mice. In contrast to mice deficient in the alpha/beta subunits, the mice lacking the gamma subunits were of normal size, lacked cartilage defects, and did not develop retinal degeneration. The milder disease in the gamma-subunit deficient mice correlated with residual synthesis of the mannose 6-phosphate recognition marker. Of significance, neither strain of mutant mice developed cytoplasmic vacuolar inclusions in fibrocytes or mesenchymal cells (I-cells), the characteristic lesion associated with the prominent skeletal and connective tissue abnormalities in humans with MLII and MLIII. Instead, the predominant lesions in both lines of mice were found in the secretory epithelial cells of several exocrine glands, including the pancreas, and the parotid, submandibular salivary, nasal, lacrimal, bulbourethral, and gastric glands. The absence of retinal and chondrocyte lesions in Gnptg(-/-) mice might be attributed to residual beta-glucuronidase activity. We conclude that mice lacking either alpha/beta or gamma subunits displayed clinical and pathologic features that differed substantially from those reported in humans having mutations in orthologous genes.

Molecular analysis of the GlcNac-1-phosphotransferase

Modification of the carbohydrate chains of soluble lysosomal enzymes with mannose 6-phosphate residues is a prerequisite for their mannose 6-phosphate receptor-dependent transport to lysosomes. GlcNac-1-phosphotransferase localized in the Golgi apparatus represents a hexameric alpha(2)beta(2)gamma(2) subunit complex and plays a key role in the formation of the mannose 6-phosphate recognition marker. Defects in the GlcNac-1-phosphotransferase complex cause two diseases, mucolipidosis type II and III, which are characterized by missorting and cellular loss of lysosomal enzymes, and lysosomal accumulation of storage material. The recent identification of two genes, GNPTAB and GNPTG, encoding the three subunits of GlcNac-1-phosphotransferase leads to an improvement of both pre- and postnatal diagnosis of affected individuals, and permits the analysis of structural requirements for efficient formation of mannose 6-phosphate residues on lysosomal enzymes. The alpha/beta subunits precursor matures by proteolytic cleavage and contains the catalytic activity as well as the capability to recognize lysosomal enzymes. The role of the gamma-subunits for activity, stability and oligomerization of the GlcNac-1-phosphotransferase subunits is still unclear.

An Alu insertion in compound heterozygosity with a microduplication in GNPTAB gene underlies Mucolipidosis II

Mucolipidosis type II (ML II) is a fatal, autosomal recessive, lysosomal storage disorder characterized by severe clinical and radiologic features. ML II results from mutations in alpha and beta subunits, encoded by the GlcNAc-1-phosphotransferase gene (GNPTAB). Most of the 40 different GNPTAB mutations reported so far are insertions and deletions predicting diverse types of aberrant proteins. Alu mobile elements have however never been involved in these events up to now. The Italian ML II patient of our study showed an Alu retrotrasposition in GNPTAB exon 5. The Alu insertion mutation (NM_024312.3:c.555_556insHSU14569) generated a transcript with a skipping of the target exon 5 and a frameshift p.S122fs, causing a premature translation termination codon at position 123. This insertion mutation was found in compound heterozygosity with the frameshift p.S887KfsX33, resulting from a new mono-nucleotide duplication (c.2659dupA) that occurred in GNPTAB exon 13. A possible involvement of cis-splicing elements having an exonic location, such as exon enhancers (ESEs), is discussed as mechanism that led to the production of the aberrant mRNA splicing.

Murine UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase lacking the gamma-subunit retains substantial activity toward acid hydrolases

UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase) mediates the first step in the synthesis of the mannose 6-phosphate recognition marker on acid hydrolases. The transferase exists as an alpha(2)beta(2)gamma(2) hexameric complex with the alpha- and beta-subunits derived from a single precursor molecule. The catalytic function of the transferase is attributed to the alpha- and beta-subunits, whereas the gamma-subunit is believed to be involved in the recognition of a conformation-dependent protein determinant common to acid hydrolases. Using knock-out mice with mutations in either the alpha/beta gene or the gamma gene, we show that disruption of the alpha/beta gene completely abolishes phosphorylation of high mannose oligosaccharides on acid hydrolases whereas knock-out of the gamma gene results in only a partial loss of phosphorylation. These findings demonstrate that the alpha/beta-subunits, in addition to their catalytic function, have some ability to recognize acid hydrolases as specific substrates. This process is enhanced by the gamma-subunit.

Abnormal expressions of the subunits of the UDP-N-acetylglucosamine: lysosomal enzyme, N-acetylglucosamine-1-phosphotransferase, result in the formation of cytoplasmic vacuoles resembling those of the I-cells

Defects in the multimeric enzyme, UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GNPT), result in the diseases of mucolipidosis (ML). This enzyme generates the mannose 6-phosphate residues on newly synthesized lysosomal enzymes for the efficient receptor-mediated transport to lysosomes. The enzyme contains alpha/beta and gamma subunits. Mutations in the alpha/beta subunit result in the classical ML II and IIIA, while defects in the gamma subunit results in the clinically milder ML IIIC. I-cells, a distinct histological feature characterized by the presence of abnormal cytoplasmic vacuoles, are detected in many cell types, most noticeably, in ML II patients. In this study, we investigated the interactions of the alpha/beta and gamma subunits in the pathogenesis of I-cells. We noted low and deranged alpha/beta subunit expressions in human mucolipidosis cell lines. Unexpectedly, high gamma subunit expressions were also observed. In normal mouse fibroblasts, when alpha/beta subunit was suppressed, abnormal cytoplasmic vacuoles were induced, and up-regulation of the gamma subunit was also observed. On the other hand, suppressing the gamma subunit resulted in biphasic responses of the alpha/beta subunit, while abnormal cytoplasmic vacuoles were not formed, regardless of the expression levels of the alpha/beta subunit. Our data suggest reciprocal feedback mechanisms between alpha/beta and the gamma subunits. A fine balance of the expressions of these subunits may play an important role in the formation of I-cells in this group of lysosomal storage disorders.

When Mucolipidosis III meets Mucolipidosis II: GNPTA gene mutations in 24 patients

Mucolipidosis II (ML II) and Mucolipidosis type III (ML III) are autosomal recessive disorders of lysosomal hydrolases trafficking due to the deficiency of the multimeric enzyme, UDP-N-acetylglucosamine-1-phosphotransferase. The alpha/beta subunits encoded by the GNPTA gene is the catalytic subunit of the enzyme while the gamma recognition subunit is encoded by the GNPTAG gene. We report the molecular analysis of GNPTA in 21 families with ML II and 3 families with ML III. The ML II mutant genotypes included three splice-site mutations [IVS1-2A>G; IVS17+1G>A; IVS18+1G>A] in seven Palestinian, Israeli Arab-Muslims, and Turkish patients; a two base pair deletion [c.3503_4delTC] [corrected] in 11 patients from Israel, Turkey, and Ireland; two nonsense mutations [c.2533C>T (Q845X); c.3613C>T (R1205X)], in a Turkish and an Arab-Muslim patient from the Nablus area, respectively, and an insertion mutation [c.2916insT] in a patient from Nablus. The ML III mutant genotypes included a splice-site mutation [IVS17+6T>G] in two patients from Irish/Scottish origin who were compound heterozygous for a nonsense mutation [c.3565C>T (R1189X)] and the deletion mutation [c.3503_4delTC] [corrected] respectively. The third ML III patient from France was compound heterozygous for a missense mutation [c.1196C>T] and the same deletion [c.3503_4delTC] [corrected] found homozygous in 11 ML II patients. The 21 ML II patients were homozygous while the three ML III patients were compound heterozygous for mutations in GNPTA. The results of this study confirm that ML II or ML III phenotype is not due to the localization of the mutations, but rather to the severity of the mutations, ML II and ML III might be allelic, and ML III is genetically heterogeneous. We suggest that the diseases due to mutations in GNPTA represent a clinical continuum between ML III and ML II, and the classification of these diseases should be based on the age of onset, clinical symptoms, and severity.

Structural requirements for efficient processing and activation of recombinant human UDP-N-acetylglucosamine:lysosomal-enzyme-N-acetylglucosamine-1-phosphotransferase

Mannose 6-phosphate-modified N-glycans are the determinant for intracellular targeting of newly synthesized lysosomal hydrolases to the lysosome. The enzyme responsible for the initial step in the synthesis of mannose 6-phosphate is UDP-N-acetylglucosamine:lysosomal-enzyme-N-acetylglucosmine-1-phosphotransferase(GlcNAc-phosphotransferase). GlcNAc-phosphotransferase is a multisubunit enzyme with an alpha2beta2gamma2 arrangement that requires a detergent for solubilization. Recent cloning of cDNAs and genes encoding these subunits revealed that the alpha- and beta-subunits are encoded by a single gene as a precursor, whereas the gamma-subunit is encoded by a second gene. The hydropathy plots of the deduced amino acid sequences suggested that the alpha- and beta-subunits but not the gamma-subunit contain transmembrane domains. Access to these cDNAs allowed us to express a soluble form of human recombinant GlcNAc-phosphotransferase by removing the putative transmembrane and cytoplasmic domains from the alpha- and beta-subunits. Because this modification prevented precursor processing to mature alpha- and beta-subunits, the native cleavage sequence was replaced by a cleavage site for furin. When the modified alpha/beta-subunits (alpha'/beta'-subunits) precursor and wild type gamma-subunit cDNAs were co-expressed in 293T or CHO-K1 cells, a furin-like protease activity in these cells cleaved the precursor and produced an active and processed soluble GlcNAc-phosphotransferase with an alpha'2beta'2gamma2-subunits arrangement. Recombinant soluble GlcNAc-phosphotransferase exhibited specific activity and substrate preferences similar to the wild type bovine GlcNAc-phosphotransferase and was able to phosphorylate a lysosomal hydrolase, acid alpha-glucosidase in vitro.

Mucolipidosis II is caused by mutations in GNPTA encoding the alpha/beta GlcNAc-1-phosphotransferase

Mucolipidosis II (ML II) is a fatal lysosomal storage disorder resulting from defects in the multimeric GlcNAc-1-phosphotransferase responsible for the initial step in the generation of the mannose 6-phosphate (M6P) recognition marker. M6P residues on oligosaccharides of newly synthesized lysosomal enzymes are essential for efficient receptor-mediated transport to lysosomes. We used the recombinant GlcNAc-1-phosphotransferase gamma subunit as an affinity matrix to purify an unknown protein identified as the product of GNPTA (encoding GNPTA, previously known as MGC4170). The cDNA encodes a protein of 1,256 amino acids with two putative transmembrane domains and a complex preserved modular structure comprising at least six domains. The N-terminal domain of GNPTA, interrupted by a long insertion, shows similarities to bacterial capsule biosynthesis proteins. We identified seven mutations in GNPTA that lead to premature translational termination in six individuals with ML II. Retroviral transduction of fibroblasts from an individual with ML II resulted in the expression and localization of GNPTA in the Golgi apparatus, accompanied by the correction of hypersecretion of lysosomal enzymes. Our results provide evidence that GNPTA encodes a subunit of GlcNAc-1-phosphotransferase defective in individuals with ML II.