Mucolipidosis II and III alpha/beta: mutation analysis of 40 Japanese patients showed genotype–phenotype correlation

Structure of a truncated human GlcNAc-1-phosphotransferase variant reveals the basis for its hyperactivity

Mutations that cause loss of function of GlcNAc-1-phosphotransferase (PTase) lead to the lysosomal storage disorder mucolipidosis II. PTase is the key enzyme of the mannose 6-phosphate (M6P) targeting system that is responsible for tagging lysosomal hydrolases with the M6P moiety for their delivery to the lysosome. We had previously generated a truncated hyperactive form of PTase termed S1S3 which was shown to notably increase the phosphorylation level of secreted lysosomal enzymes and enhance their uptake by cells. Here, we report the 3.4 Å cryo-EM structure of soluble S1S3 lacking both transmembrane domains and cytosolic tails. The structure reveals a high degree of conservation of the catalytic core to full-length PTase. In this dimeric structure, the EF hand of one protomer is observed interacting with the conserved region 4 of the other. In addition, we present a high-quality EM 3D map of the UDP-GlcNAc bound form of the full-length soluble protein showing the key molecular interactions between the nucleotide sugar donor and side chain amino acids of the protein. Finally, although the domain organization of S1S3 is very similar to that of the Drosophila melanogaster (fruit fly) PTase homolog, we establish that the latter does not act on lysosomal hydrolases.

Early diagnostic clues of mucolipidosis type II: Significance of radiological findings

Mucolipidosis type-II (ML-II) is an ultra-rare disorder caused by deficiency of N-acetylglucosaminyl-1-phosphotransferase enzyme due to biallelic pathogenic variants in GNPTAB gene. There are a few known about the natural history of ML-II. In this study, we presented the natural course of 24 patients diagnosed with ML-II. Mean age at diagnosis was 9.3 ± 5.7 months. All patients had coarse face, developmental delay, and hypotonia. The mean survival time was 3.01 ± 1.4 years. The oldest patient was 6.5 years old. Twelve patients died due to lung infection and respiratory failure. We observed early and significant radiological findings of ML-II were different from typical dysostosis multiplex such as femoral cloaking, rickets-like changes, and talocalcaneal stippling. These are significant findings observed in the fetal or newborn period which is considered to be highly characteristic of ML-II and disappears in the first year. Cloaking, rickets-like changes, and stippling were not observed in patients older than three months of age and this suggests that these findings disappear within the first year. These radiological features can be used as important clues for diagnosis. We detected eight different pathogenic variants in GNPTAB gene, three of them were novel.

Hepatomegaly and Splenomegaly: An Approach to the Diagnosis of Lysosomal Storage Diseases

Clinical findings of hepatomegaly and splenomegaly, the abnormal enlargement of the liver and spleen, respectively, should prompt a broad differential diagnosis that includes metabolic, congestive, neoplastic, infectious, toxic, and inflammatory conditions. Among the metabolic diseases, lysosomal storage diseases (LSDs) are a group of rare and ultrarare conditions with a collective incidence of 1 in 5000 live births. LSDs are caused by genetic variants affecting the lysosomal enzymes, transporters, or integral membrane proteins. As a result, abnormal metabolites accumulate in the organelle, leading to dysfunction. Therapeutic advances, including early diagnosis and disease-targeted management, have improved the life expectancy and quality of life of people affected by certain LSDs. To access these new interventions, LSDs must be considered in patients presenting with hepatomegaly and splenomegaly throughout the lifespan. This review article navigates the diagnostic approach for individuals with hepatosplenomegaly particularly focusing on LSDs. We provide hints in the history, physical exam, laboratories, and imaging that may identify LSDs. Additionally, we discuss molecular testing, arguably the preferred confirmatory test (over biopsy), accompanied by enzymatic testing when feasible.

GCAF(TMEM251) regulates lysosome biogenesis by activating the mannose-6-phosphate pathway

The mannose-6-phosphate (M6P) biosynthetic pathway for lysosome biogenesis has been studied for decades and is considered a well-understood topic. However, whether this pathway is regulated remains an open question. In a genome-wide CRISPR/Cas9 knockout screen, we discover TMEM251 as the first regulator of the M6P modification. Deleting TMEM251 causes mistargeting of most lysosomal enzymes due to their loss of M6P modification and accumulation of numerous undigested materials. We further demonstrate that TMEM251 localizes to the Golgi and is required for the cleavage and activity of GNPT, the enzyme that catalyzes M6P modification. In zebrafish, TMEM251 deletion leads to severe developmental defects including heart edema and skeletal dysplasia, which phenocopies Mucolipidosis Type II. Our discovery provides a mechanism for the newly discovered human disease caused by TMEM251 mutations. We name TMEM251 as GNPTAB cleavage and activity factor (GCAF) and its related disease as Mucolipidosis Type V.

Detection of Structural Variants by NGS: Revealing Missing Alleles in Lysosomal Storage Diseases

Lysosomal storage diseases (LSDs) are a heterogeneous group of rare multisystem metabolic disorders occurring mostly in infancy and childhood, characterized by a gradual accumulation of non-degraded substrates inside the cells. Although biochemical enzymatic assays are considered the gold standard for diagnosis of symptomatic patients, genotyping is a requirement for inclusion in enzyme replacement programs and is a prerequisite for carrier tests in relatives and DNA-based prenatal diagnosis. The emerging next-generation sequencing (NGS) technologies are now offering a powerful diagnostic tool for genotyping LSDs patients by providing faster, cheaper, and higher-resolution testing options, and are allowing to unravel, in a single integrated workflow SNVs, small insertions and deletions (indels), as well as major structural variations (SVs) responsible for the pathology. Here, we summarize the current knowledge about the most recurrent and private SVs involving LSDs-related genes, review advantages and drawbacks related to the use of the NGS in the SVs detection, and discuss the challenges to bring this type of analysis in clinical diagnostics.

Mucolipidosis Ⅱ and III with neurological symptoms due to spinal cord compression

In mucopolysaccharidoses (MPS), spinal cord compression (SCC) resulting from glycosaminoglycan (GAG) accumulation is a critical complication that can cause significant neurological and respiratory morbidities. However, clinically similar disorders such as mucolipidosis types II and III (ML) with SCC have been scarcely reported. Herein, we report four patients with ML who had SCC. Brain MRI revealed progressive spinal canal stenosis and SCC. In addition, T2-weighted high signal changes in the cervical cord were detected in two cases. Severe cases of SCC were detected as early as 1 year of age. All cases had respiratory problems. One case showed severe hypoxia and another, severe sleep apnea. In two cases, respiratory insufficiency and tetraplegia rapidly progressed as SCC progressed. Then, the patients became bedridden and needed artificial ventilation. In addition, two of the four patients died of respiratory failure. The autopsy of one patient revealed a compressed cervical cord and marked dura mater thickening due to GAG accumulation. These findings suggest that the accumulation of substrates in the dura mater caused SCC in the patients with ML. Our cases indicate that SCC is expected to be a common and critical complication of ML and MPS. MRI evaluation of cervical involvements and careful clinical observation are required in patients with ML.

Placental pathology in an unsuspected case of mucolipidosis type II with secondary hyperparathyroidism in a premature infant

Mucolipidosis type II (MLII, MIM 252500) is a lysosomal storage disorders caused by defects in GNPTAB gene which encodes alpha and beta subunits of N-acetylglucosamine (GlcNAc)-1-phosphotransferase. Neonatal presentation includes coarse facial features, restricted postnatal growth, generalized hypotonia, gingival hypertrophy and multiple skeletal anomalies. Here we present a case of a 26-week gestational age preterm infant with MLII who did not exhibit the typical facial features at birth; however, the diagnosis was suggested from abnormal placental pathology showing trophoblastic lipidosis and initial skeletal abnormalities from chest radiograph revealing generalized diffuse severe bone demineralizing disease and multiple fractures. Biochemical testing revealed elevation of plasma lysosomal enzymes. Homozygous pathogenic variant, designated c.3505_3504del, was discovered from GNPTAB sequencing. Her course was complicated by respiratory distress, secondary hyperparathyroidism, abdominal distention and feeding difficulties. Urine mucopolysaccharides analysis revealed mild elevation of total and individual glycosaminoglycan species in a non-specific pattern. To our knowledge, our case is the most premature example of mucolipidosis type II that has ever been reported to date. This report highlights the importance of placental pathological studies in the diagnosis of lysosomal storage disorders.

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.

A Rare Manifestation of Right Ventricular Dysfunction in an Adult Patient With Mucolipidosis Type III α/β

Mucolipidosis type III α/β is an autosomal recessive lysosomal storage disease, caused by the deficient activity of UDP-N-acetyl glucosamine-1-phosphotransferase. The resultant intralysosomal accumulation of partly degraded mucopolysaccharides and sphingolipids causes multiple-organ damage, including the heart. The most documented cardiac manifestation is the thickening and insufficiency of mitral and aortic valves, but there are very few reports about the myocardial involvement. We report a case with mucolipidosis type III α/β complicated by marked dilatation and dysfunction of the right ventricle, which is quite rare and further broadens the clinical spectrum of the disease.

Identification and characterization of 30 novel pathogenic variations in 69 unrelated Indian patients with Mucolipidosis Type II and Type III

Mucolipidosis (ML) (OMIM 607840 & 607838) is a rare autosomal recessive inherited disorder that occurs due to the deficiency of golgi enzyme uridine diphosphate (UDP)- N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase) responsible for tagging mannose-6-phosphate for proper trafficking of lysosomal enzymes to lysosomes. Variants in GlcNAc-phosphotransferase (GNPTAB (α, β subunits) and GNPTG (γ subunits) are known to result in impaired targeting of lysosomal enzymes leading to Mucolipidosis (ML) Type II or Type III. We analyzed 69 Indian families of MLII/III for clinical features and molecular spectrum and performed in silico analysis for novel variants. We identified 38 pathogenic variants in GNPTAB and 5 pathogenic variants in GNPTG genes including missense, frame shift, deletion, duplication and splice site variations. A total of 26 novel variants were identified in GNPTAB and 4 in GNPTG gene. In silico studies using mutation prediction software like SIFT, Polyphen2 and protein structure analysis further confirmed the pathogenic nature of the novel sequence variants detected in our study. Except for a common variant c.3503_3504delTC in early onset MLII, we could not establish any other significant genotype and phenotype correlation. This is one of the largest studies reported till date on Mucolipidosis II/III in order to identify mutation spectrum and any recurrent mutations specific to the Indian ethnic population. The mutational spectrum information in Indian patients will be useful in better genetic counselling, carrier detection and prenatal diagnosis for patients with ML II/III.

Development of an Antisense Oligonucleotide-Mediated Exon Skipping Therapeutic Strategy for Mucolipidosis II: Validation at RNA Level

Lysosomal storage disorders (LSDs) are a group of rare inherited metabolic diseases caused by the malfunction of the lysosomal system, which results in the accumulation of undergraded substrates inside the lysosomes and leads to severe and progressive pathology. Despite there currently being a broad understanding of the molecular defects behind LSDs, curative therapies have been approved for only few of these diseases, whereas existing treatments are still mostly symptomatic with several limitations. Mucolipidosis type II alpha/beta (ML II) is one of most severe LSDs, which is caused by the total deficiency of the GlcNAc-1-phosphotransferase, a key enzyme for the formation of specific targeting signals on lysosomal hydrolases to lysosomes. GlcNAc-1-phosphotransferase is a multimeric enzyme complex encoded by two genes: GNPTAB and GNPTG. One of the most frequent ML II causal mutation is a dinucleotide deletion on exon 19 of GNPTAB (c.3503_3504del) that leads to the generation of a truncated protein, loss of GlcNAc-1-phosphotransferase activity, and missorting of multiple lysosomal enzymes. Presently, there is no therapy available for ML II. In this study, we explored the possibility of an innovative therapeutic strategy for ML II based on the use of antisense oligonucleotides (AOs) capable to induce the skipping of GNPTAB exon 19 harboring the most common disease-causing mutation, c.3503_3504del. The approach confirmed the ability of specific AOs for RNA splicing modulation, thus paving the way for future studies on the therapeutic potential of this strategy.

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.

Mucolipidosis type III, a series of adult patients

BACKGROUND

Mucolipidosis type III α/β or γ (MLIII) are rare autosomal recessive diseases, in which reduced activity of the enzyme UDP-N-acetyl glucosamine-1-phosphotransferase (GlcNAc-PTase) leads to intra-lysosomal accumulation of different substrates. Publications on the natural history of MLIII, especially the milder forms, are scarce. This study provides a detailed description of the disease characteristics and its natural course in adult patients with MLIII.

METHODS

In this retrospective chart study, the clinical, biochemical and molecular findings in adult patients with a confirmed diagnosis of MLIII from three treatment centres were collected.

RESULTS

Thirteen patients with MLIII were included in this study. Four patients (31%) were initially misdiagnosed with a type of mucopolysaccharidosis (MPS). Four patients (31%) had mild cognitive impairment. Six patients (46%) needed help with activities of daily living (ADL) or were wheelchair-dependent. All patients had dysostosis multiplex and progressive secondary osteoarthritis, characterised by cartilage destruction and bone lesions in multiple joints. All patients underwent multiple orthopaedic surgical interventions as early as the second or third decades of life, of which total hip replacement (THR) was the most common procedure (61% of patients). Carpal tunnel syndrome (CTS) was found in 12 patients (92%) and in eight patients (61%), CTS release was performed.

CONCLUSIONS

Severe skeletal abnormalities, resulting from abnormal bone development and severe progressive osteoarthritis, are the hallmark of MLIII, necessitating surgical orthopaedic interventions early in life. Future therapies for this disease should focus on improving cartilage and bone quality, preventing skeletal complications and improving mobility.

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.

Variants in GNPTAB, GNPTG and NAGPA genes are associated with stutterers

Non-syndromic stuttering is a neurodevelopmental disorder characterized by disruptions in normal flow of speech in the form of repetition, prolongation and involuntary halts. Previously, mutations with more severe effects on GNPTAB and GNPTG have been reported to cause Mucolipidosisll (ML-ll) and Mucolipidosislll (ML-lll), two lysosomal storage disorders with multiple pathologies. We used homozygosity mapping and Sanger sequencing to investigate variants of the three genes in 25 Iranian families with at least two first degree related non-syndromic stutterers. Bioinformatic evaluation and Segregation analysis of the found variants helped us define probable consequences. We also compared our findings with those related to Mucolipidosis. 14 variations were found in the three genes 3 of which, including a novel variant within intronic region of GNPTG and a heterozygous 2-bp deletion in coding region of GNPTAB, co-segregated with stuttering in the families they were found. Bioinformatics analysis predicted all three variants causing deleterious effects on gene functioning. Our findings support the role of these three variants in non-syndromic stuttering. This finding may challenge the current belief that variations causing stuttering are at different sites and have less severe consequences than genetic changes that cause ML-ll and ML-lll.

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.

Knockout of Lysosomal Enzyme-Targeting Gene Causes Abnormalities in Mouse Pup Isolation Calls

Humans lacking a working copy of the GNPTAB gene suffer from the metabolic disease Mucolipidosis type II (MLII). MLII symptoms include mental retardation, skeletal deformities and cartilage defects as well as a speech delay with most subjects unable to utter single words (Otomo et al., 2009; Cathey et al., 2010; Leroy et al., 2012). Here we asked whether mice lacking a copy of Gnptab gene exhibited vocal abnormities. We recorded ultrasonic vocalizations from 5 to 8 day old mice separated from their mother and littermates. Although Gnptab-/- pups emitted a similar number of calls, several features of the calls were different from their wild type littermates. Gnptab-/- mice showed a decrease in the length of calls, an increase in the intra-bout pause duration, significantly fewer pitch jumps with smaller mean size, and an increase in the number of isolated calls. In addition, Gnptab-/- mice vocalizations had less power, particularly in the higher frequencies. Gnptab+/- mouse vocalizations did not appear to be affected. We then attempted to classify these recordings using these features to determine the genotype of the animal. We were able to correctly identify 87% of the recordings as either Gnptab-/- or Gnptab+/+ pup, significantly better than chance, demonstrating that genotype is a strong predictor of vocalization phenotype. These data show that deletion of genes in the lysosomal enzyme targeting pathway affect mouse pup isolation calls.

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.

Clinical and laboratory outcomes after umbilical cord blood transplantation in a patient with mucolipidosis II alpha/beta

Mucolipidosis (ML) II alpha/beta is an autosomal recessive disease caused by reduced enzyme activity of N-acetylglucosamine-1-phosphotransferase. Clinical symptoms of ML II are severe psychomotor delay and dysostosis multiplex; death usually occurs by 5-8 years of age from cardiopulmonary complications. Allogeneic hematopoietic stem cell transplantation (HSCT) has been attempted for ML; however, few reports have documented the detailed outcomes of HSCT for ML. A 26-month-old girl received a human leukocyte antigen 3/6-allele-matched transplant from cord blood. The preparative regimen consisted of fludarabine, cyclophosphamide, 6-Gy total body irradiation, and rabbit antithymocyte globulin. Although comparing before and after cord blood transplantation results, we observed that lysosomal enzyme activities in the plasma decreased by approximately 20-40%. Low serum levels of immunoglobulin A, G2, and G4 were also observed before HSCT; however, these values normalized after transplantation. Despite undergoing HSCT, she was treated twice for bacterial pneumonia with acute respiratory distress syndrome at ages 37 and 38 months. Although HSCT effects on the clinical manifestations were limited, laboratory data including plasma lysosomal enzyme activities and serum levels of immunoglobulin showed improvement.

Targeted Next-Generation Sequencing for Clinical Diagnosis of 561 Mendelian Diseases

Background

Targeted next-generation sequencing (NGS) is a cost-effective approach for rapid and accurate detection of genetic mutations in patients with suspected genetic disorders, which can facilitate effective diagnosis.

Methodology/Principal Findings

We designed a capture array to mainly capture all the coding sequence (CDS) of 2,181 genes associated with 561 Mendelian diseases and conducted NGS to detect mutations. The accuracy of NGS was 99.95%, which was obtained by comparing the genotypes of selected loci between our method and SNP Array in four samples from normal human adults. We also tested the stability of the method using a sample from normal human adults. The results showed that an average of 97.79% and 96.72% of single-nucleotide variants (SNVs) in the sample could be detected stably in a batch and different batches respectively. In addition, the method could detect various types of mutations. Some disease-causing mutations were detected in 69 clinical cases, including 62 SNVs, 14 insertions and deletions (Indels), 1 copy number variant (CNV), 1 microdeletion and 2 microduplications of chromosomes, of which 35 mutations were novel. Mutations were confirmed by Sanger sequencing or real-time polymerase chain reaction (PCR).

Conclusions/Significance

Results of the evaluation showed that targeted NGS enabled to detect disease-causing mutations with high accuracy, stability, speed and throughput. Thus, the technology can be used for the clinical diagnosis of 561 Mendelian diseases.

Analyses of disease-related GNPTAB mutations define a novel GlcNAc-1-phosphotransferase interaction domain and an alternative site-1 protease cleavage site

Mucolipidosis II (MLII) and III alpha/beta are autosomal-recessive diseases of childhood caused by mutations in GNPTAB encoding the α/β-subunit precursor protein of the GlcNAc-1-phosphotransferase complex. This enzyme modifies lysosomal hydrolases with mannose 6-phosphate targeting signals. Upon arrival in the Golgi apparatus, the newly synthesized α/β-subunit precursor is catalytically activated by site-1 protease (S1P). Here we performed comprehensive expression studies of GNPTAB mutations, including two novel mutations T644M and T1223del, identified in Brazilian MLII/MLIII alpha/beta patients. We show that the frameshift E757KfsX1 and the non-sense R587X mutations result in the retention of enzymatically inactive truncated precursor proteins in the endoplasmic reticulum (ER) due to loss of cytosolic ER exit motifs consistent with a severe clinical phenotype in homozygosity. The luminal missense mutations, C505Y, G575R and T644M, partially impaired ER exit and proteolytic activation in accordance with less severe MLIII alpha/beta disease symptoms. Analogous to the previously characterized S399F mutant, we found that the missense mutation I403T led to retention in the ER and loss of catalytic activity. Substitution of further conserved residues in stealth domain 2 (I346 and W357) revealed similar biochemical properties and allowed us to define a putative binding site for accessory proteins required for ER exit of α/β-subunit precursors. Interestingly, the analysis of the Y937_M972del mutant revealed partial Golgi localization and formation of abnormal inactive β-subunits generated by S1P which correlate with a clinical MLII phenotype. Expression analyses of mutations identified in patients underline genotype–phenotype correlations in MLII/MLIII alpha/beta and provide novel insights into structural requirements of proper GlcNAc-1-phosphotransferase activity.

Mannose 6 phosphorylation of lysosomal enzymes controls B cell functions

Antigen processing and presentation and cytotoxic targeting depend on the activities of several lysosomal enzymes that require mannose 6-phosphate (M6P) sorting signals for efficient intracellular transport and localization. In this paper, we show that mice deficient in the formation of M6P residues exhibit significant loss of cathepsin proteases in B cells, leading to lysosomal dysfunction with accumulation of storage material, impaired antigen processing and presentation, and subsequent defects in B cell maturation and antibody production. The targeting of lysosomal and granular enzymes lacking M6P residues is less affected in dendritic cells and T cells and sufficient for maintenance of degradative and lytic functions. M6P deficiency also impairs serum immunoglobulin levels and antibody responses to vaccination in patients. Our data demonstrate the critical role of M6P-dependent transport routes for B cell functions in vivo and humoral immunity in mice and human.

Analysis of mucolipidosis II/III GNPTAB missense mutations identifies domains of UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase involved in catalytic function and lysosomal enzyme recognition

UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase tags newly synthesized lysosomal enzymes with mannose 6-phosphate recognition markers, which are required for their targeting to the endolysosomal system. GNPTAB encodes the α and β subunits of GlcNAc-1-phosphotransferase, and mutations in this gene cause the lysosomal storage disorders mucolipidosis II and III αβ. Prior investigation of missense mutations in GNPTAB uncovered amino acids in the N-terminal region and within the DMAP domain involved in Golgi retention of GlcNAc-1-phosphotransferase and its ability to specifically recognize lysosomal hydrolases, respectively. Here, we undertook a comprehensive analysis of the remaining missense mutations in GNPTAB reported in mucolipidosis II and III αβ patients using cell- and zebrafish-based approaches. We show that the Stealth domain harbors the catalytic site, as some mutations in these regions greatly impaired the activity of the enzyme without affecting its Golgi localization and proteolytic processing. We also demonstrate a role for the Notch repeat 1 in lysosomal hydrolase recognition, as missense mutations in conserved cysteine residues in this domain do not affect the catalytic activity but impair mannose phosphorylation of certain lysosomal hydrolases. Rescue experiments using mRNA bearing Notch repeat 1 mutations in GNPTAB-deficient zebrafish revealed selective effects on hydrolase recognition that differ from the DMAP mutation. Finally, the mutant R587P, located in the spacer between Notch 2 and DMAP, was partially rescued by overexpression of the γ subunit, suggesting a role for this region in γ subunit binding. These studies provide new insight into the functions of the different domains of the α and β subunits.

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.

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.

A role for inherited metabolic deficits in persistent developmental stuttering

Stuttering is a common but poorly understood speech disorder. Consistent evidence for the involvement of genetic factors in stuttering has motivated studies aimed at identifying causative genetic variants that could shed light on the underlying molecular and cellular deficits in this disorder. Such studies have begun to identify causative genes. The purpose of this review is to summarize the gene discoveries to date, and to cover the subsequent functional studies that are beginning to provide insights into how these gene mutations might cause stuttering. Surprisingly, the first variant genes to be associated with stuttering are those encoding the lysosomal targeting system, GNPTAB, GNPTG, and NAGPA. Although mutations in NAGPA have not been associated with a disorder in humans, mutations in GNPTAB and GNPTG cause mucolipidosis types II and III, which are rare autosomal recessive lysosomal storage disorders, associated with pathology of bone, connective tissue, liver, spleen, and brain. Analysis of mutations in these genes has so far identified predominantly missense mutations in stuttering, in contrast to the truncating and other mutations that result in very low GNPTAB/G enzyme activity and are historically associated with mucolipidosis. Genetic evidence for the role of lysosomal targeting mutations in stuttering has now been buttressed by biochemical studies of the mutant enzymes found in this disorder. While data on the GlcNAc-phosphotransferase encoded by GNPTAB/G remains limited and only suggestive, a study of the enzyme encoded by NAGPA has shown that the mutations found in stuttering reduce the overall cellular activity of this enzyme by about half, and that they result in deficits in intracellular processing and trafficking that lead to a reduced cellular half life. How these deficits result in the presumed speech-specific neuropathology associated with stuttering is not yet known. However these findings have opened several new lines of inquiry, including studies in mice carrying human stuttering mutations, that represent promising approaches to this disorder.

Hypoyelination in I-cell disease; MRI, MR spectroscopy and neuropathological correlation

MRI of a female patient with genetically diagnosed I-cell disease at 2weeks, 4 and 8months revealed delayed myelination or hypomyelination with decreased choline on MR spectroscopy. Brain autopsy was performed 2h after death at 14-month-old. Immunoreactivities for myelin basic protein and proteolipid proteins, markers for mature myelin sheath, were reduced in the myelinated fibers and oligodendrocytes in the white matter, indicating the hypomyelination in the central nervous system. I-Cell disease should be added to the list of delayed or hypomyelination conditions, and this neuroimaging finding could be a key for differentiating I-cell disease from the clinically similar disorder of Hurler syndrome characterized by perivascular lacunation.

Unusual pulmonary findings in mucolipidosis II

We report undescribed pulmonary findings in a child with mucolipidosis II (ML-II). Children with ML-II bear significant pulmonary morbidity that may include extensive pulmonary fibrosis, persistent hemosiderosis as well as pulmonary airway excrescences as they reach preschool age.

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.

Genistein reduces heparan sulfate accumulation in human mucolipidosis II skin fibroblasts

Genistein, a soy isoflavone, reduces glycosaminoglycan synthesis and its effect on mucopolysaccharidoses has been tested. In this report, we examined the effect of genistein in human mucolipidosis II skin fibroblasts in vitro. Heparan sulfate was accumulated within both cells and in extracellular spaces in mucolipidosis II. Genistein reduced the amount of heparan sulfate in cultured cells dose dependently and also inhibited cell growth dose dependently.

Delineating the role of alterations in lipid metabolism to the pathogenesis of inherited skeletal and cardiac muscle disorders: Thematic Review Series: Genetics of Human Lipid Diseases

As the specific composition of lipids is essential for the maintenance of membrane integrity, enzyme function, ion channels, and membrane receptors, an alteration in lipid composition or metabolism may be one of the crucial changes occurring during skeletal and cardiac myopathies. Although the inheritance (autosomal dominant, autosomal recessive, and X-linked traits) and underlying/defining mutations causing these myopathies are known, the contribution of lipid homeostasis in the progression of these diseases needs to be established. The purpose of this review is to present the current knowledge relating to lipid changes in inherited skeletal muscle disorders, such as Duchenne/Becker muscular dystrophy, myotonic muscular dystrophy, limb-girdle myopathic dystrophies, desminopathies, rostrocaudal muscular dystrophy, and Dunnigan-type familial lipodystrophy. The lipid modifications in familial hypertrophic and dilated cardiomyopathies, as well as Barth syndrome and several other cardiac disorders associated with abnormal lipid storage, are discussed. Information on lipid alterations occurring in these myopathies will aid in the design of improved methods of screening and therapy in children and young adults with or without a family history of genetic diseases.

Alu-Alu Recombination Underlying the First Large Genomic Deletion in GlcNAc-Phosphotransferase Alpha/Beta (GNPTAB) Gene in a MLII Alpha/Beta Patient

Mucolipidosis type II α/β is a severe, autosomal recessive lysosomal storage disorder, caused by a defect in the GNPTAB gene that codes for the α/β subunits of the GlcNAc-phosphotransferase. To date, over 100 different mutations have been identified in MLII α/β patients, but no large deletions have been reported. Here we present the first case of a large homozygous intragenic GNPTAB gene deletion (c.3435-386_3602 + 343del897) encompassing exon 19, identified in a ML II α/β patient. Long-range PCR and sequencing methodologies were used to refine the characterization of this rearrangement, leading to the identification of a 21 bp repetitive motif in introns 18 and 19. Further analysis revealed that both the 5' and 3' breakpoints were located within highly homologous Alu elements (Alu-Sz in intron 18 and Alu-Sq2, in intron 19), suggesting that this deletion has probably resulted from Alu-Alu unequal homologous recombination. RT-PCR methods were used to further evaluate the consequences of the alteration for the processing of the mutant pre mRNA GNPTAB, revealing the production of three abnormal transcripts: one without exon 19 (p.Lys1146_Trp1201del); another with an additional loss of exon 20 (p.Arg1145Serfs*2), and a third in which exon 19 was substituted by a pseudoexon inclusion consisting of a 62 bp fragment from intron 18 (p.Arg1145Serfs*16). Interestingly, this 62 bp fragment corresponds to the Alu-Sz element integrated in intron 18.This represents the first description of a large deletion identified in the GNPTAB gene and contributes to enrich the knowledge on the molecular mechanisms underlying causative mutations in ML II.

Lysosomal storage causes cellular dysfunction in mucolipidosis II skin fibroblasts

Mucolipidosis II (ML-II) is a fatal inherited metabolic disease caused by deficiency of GlcNAc-phosphotransferase, which plays a role in generating the mannose 6-phosphate recognition marker on lysosomal enzymes. In ML-II, many lysosomal acid hydrolases are mistargeted out of cells, and lysosomes become filled with undigested substrates, which explains inclusion cell disease as an alternative name for this disease. In this study, we revealed various cellular phenotypes in ML-II skin fibroblasts. We quantitated phospholipid and cholesterol within cells and showed ~2-fold accumulation in ML-II as compared with normal cells. Lysosomal pH of ML-II cells was higher than that of normal cells (5.29 ± 0.08 versus 4.79 ± 0.10, p < 0.001). The proliferated lysosomes in ML-II cells were accumulated ~3-fold in amount as compared with normal cells. Intracellular logistics including endocytosis and mannose 6-phosphate receptor recycling were impaired in ML-II cells. To confirm whether these ML-II cellular phenotypes derive from deficient lysosomal acid hydrolases within lysosomes, we performed supplementation of lysosomal enzymes using a partially purified total enzyme mixture, which was derived from the conditioned culture medium of normal skin fibroblasts after NH(4)Cl treatment. This supplementation corrected all of the previously described ML-II phenotypes. In addition, the autophagic and mitochondrial impairment that we have previously reported improved, and inclusion bodies disappeared on electron micrography following total lysosomal enzyme supplementation. Our results indicate that various cellular phenotypes in ML-II are caused by the deficiency of many lysosomal enzymes and massive accumulation of undigested substrates.

Genetic approaches to understanding the causes of stuttering

Stuttering is a common but poorly understood speech disorder. Evidence accumulated over the past several decades has indicated that genetic factors are involved, and genetic linkage studies have begun to identify specific chromosomal loci at which causative genes are likely to reside. A detailed investigation of one such region on chromosome 12 has identified mutations in the GNPTAB gene that are associated with stuttering in large families and in the general population. Subsequent studies identified mutations in the functionally related GNPTG and NAGPA genes. Mutations in these genes disrupt the lysosomal targeting pathway that generates the Mannose 6-phosphate signal, which directs a diverse group of enzymes to their target location in the lysosome of the cell. While mutations in these three genes can be identified in less than 10% of cases of familial stuttering, this knowledge allows a variety of new studies that can help identify the neuropathology that underlies this disorder.

Elevated Bone Turnover in an Infantile Patient with Mucolipidosis II; No Association with Hyperparathyroidism

This present report concerns an infantile patient with mucolipidosis II, who showed transient cortical bone hyperostosis followed by severe osteopenia. The diagnosis of mucolipidosis II was made based on the leakage of lysosomal enzymes in serum and conditioned media of the patient's skin fibroblasts, low activity of lysosomal enzymes of the fibroblasts and mutation of c.2086_2089insC (p.L697fs) and c.3565C>T (p.R1189X) in the GNPTAB gene. Bone X-ray analysis demonstrated a periosteal reaction and elevated bone resorption at the age of 2 mo. Bone markers, including alkaline phosphatase, osteocalcin and urine deoxypyridinoline, also indicated a high turnover of bone metabolism; however, no apparent rickets-like changes and no increased levels of PTH were observed. Elevated bone resorption is possibly associated with the leakage of lysosomal enzyme from osteoclasts into bone matrices. Bone formation gradually reduced, and increased bone resorption persisted. This led to severe osteopenia at the age of 6 mo. Characteristic bone findings may contribute to early diagnosis of mucolipidosis II, but their pathogenesis remains to be clarified.

Pathology of the first autopsy case diagnosed as mucolipidosis type III α/β suggesting autophagic dysfunction

Mucolipidosis type III (MLIII) is a mild form of Mucolipidosis type II (MLII, I-cell disease) of late onset, of which almost no pathological study has been reported, as it is a very rare disease. We encountered the case of a 23-year-old man of Japanese and Caucasian mixed parentage diagnosed with MLIII by enzyme assay and genotyping. He died suddenly due to severe dilated cardiomyopathy. On the day after his death, autopsy was performed, and accumulation of Luxol Fast Blue (LFB) positive material was found to be most severe in the neuronal cells of dorsal root ganglions (DRG). Electromicroscopic DRG revealed the neuronal cytoplasm was filled with a zebra-body-like membranous matrix. We tried immunohistochemistry to investigate the mechanism of such accumulation in the DRG that resulted in double positive anti-ubiquitin antibody (FK-2) and anti-LC3 antibody (as specific marker for autophagy) staining, and speculated activating of autophagosome pathway, and 'zebra-body' should be suspected as dysfunctional autophagosome. We also detected foamy cell proliferation in the dura mater, Auerbach's plexus (peripheral nervous system), podocytes of almost all glomeruli, cartilage tissue in lumbar discs, and in cardiac muscle. We tried FK-2 and anti-LC3 antibody staining also for the podocytes, the area with the most marked proliferation of foamy cells, but the result was negative. This led us to speculate that these pathological findings, namely, accumulation of LFB-positive material and foamy fibroblast proliferation, might be the forms of dysfunctional autophagosome at various stages of development. This pathological study of MLIII supports the theory that MLIII is a mild type of MLII because of the close similarity of their pathological findings.

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.

Mutations in the lysosomal enzyme-targeting pathway and persistent stuttering

BACKGROUND

Stuttering is a disorder of unknown cause characterized by repetitions, prolongations, and interruptions in the flow of speech. Genetic factors have been implicated in this disorder, and previous studies of stuttering have identified linkage to markers on chromosome 12.

METHODS

We analyzed the chromosome 12q23.3 genomic region in consanguineous Pakistani families, some members of which had nonsyndromic stuttering and in unrelated case and control subjects from Pakistan and North America.

RESULTS

We identified a missense mutation in the N-acetylglucosamine-1-phosphate transferase gene (GNPTAB), which encodes the alpha and beta catalytic subunits of GlcNAc-phosphotransferase (GNPT [EC 2.7.8.15]), that was associated with stuttering in a large, consanguineous Pakistani family. This mutation occurred in the affected members of approximately 10% of Pakistani families studied, but it occurred only once in 192 chromosomes from unaffected, unrelated Pakistani control subjects and was not observed in 552 chromosomes from unaffected, unrelated North American control subjects. This and three other mutations in GNPTAB occurred in unrelated subjects with stuttering but not in control subjects. We also identified three mutations in the GNPTG gene, which encodes the gamma subunit of GNPT, in affected subjects of Asian and European descent but not in control subjects. Furthermore, we identified three mutations in the NAGPA gene, which encodes the so-called uncovering enzyme, in other affected subjects but not in control subjects. These genes encode enzymes that generate the mannose-6-phosphate signal, which directs a diverse group of hydrolases to the lysosome. Deficits in this system are associated with the mucolipidoses, rare lysosomal storage disorders that are most commonly associated with bone, connective tissue, and neurologic symptoms.

CONCLUSIONS

Susceptibility to nonsyndromic stuttering is associated with variations in genes governing lysosomal metabolism.

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.

Glycosylation- and phosphorylation-dependent intracellular transport of lysosomal hydrolases

Lysosomes contain more than 50 soluble hydrolases that are targeted to lysosomes in a mannose 6-phosphate (Man6P)-dependent manner. The phosphorylation of man- nose residues on high mannose-type oligosaccharides of newly synthesized lysosomal enzymes is catalyzed by two multimeric enzymes, GlcNAc-1-phosphotransferase and GlcNAc-1-phosphodiester-alpha-N-acetylglucosaminidase, allowing the binding to two distinct Man6P receptors in the Golgi apparatus. Inherited defects in the GlcNAc-1-phosphotransferase complex result in missorting and cellular loss of lysosomal enzymes, and the subsequent lysosomal dysfunction causes the lysosomal storage disorders mucolipidosis types II and III. Biosynthetic studies and the availability of Man6P receptor-deficient mouse models have provided new insights into the structural requirements for preferential binding of subsets of lysosomal enzymes to Man6P receptors as well as the identification of alternative targeting pathways.

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.