Expression and characterization of 14 GLB1 mutant alleles found in GM1-gangliosidosis and Morquio B patients

Lysosomal Dysfunction: Connecting the Dots in the Landscape of Human Diseases

Lysosomes are the main organelles responsible for the degradation of macromolecules in eukaryotic cells. Beyond their fundamental role in degradation, lysosomes are involved in different physiological processes such as autophagy, nutrient sensing, and intracellular signaling. In some circumstances, lysosomal abnormalities underlie several human pathologies with different etiologies known as known as lysosomal storage disorders (LSDs). These disorders can result from deficiencies in primary lysosomal enzymes, dysfunction of lysosomal enzyme activators, alterations in modifiers that impact lysosomal function, or changes in membrane-associated proteins, among other factors. The clinical phenotype observed in affected patients hinges on the type and location of the accumulating substrate, influenced by genetic mutations and residual enzyme activity. In this context, the scientific community is dedicated to exploring potential therapeutic approaches, striving not only to extend lifespan but also to enhance the overall quality of life for individuals afflicted with LSDs. This review provides insights into lysosomal dysfunction from a molecular perspective, particularly in the context of human diseases, and highlights recent advancements and breakthroughs in this field.

Gene therapy approaches for GM1 gangliosidosis: Focus on animal and cellular studies

One of the most important inherited metabolic disorders is GM1 gangliosidosis, which is a progressive neurological disorder. The main cause of this disease is a genetic defect in the enzyme β-galactosidase due to a mutation in the glb1 gene. Lack of this enzyme in cells (especially neurons) leads to the accumulation of ganglioside substrate in nerve tissues, followed by three clinical forms of GM1 disease (neonatal, juvenile, and adult variants). Genetically, many mutations occur in the exons of the glb1 gene, such as exons 2, 6, 15, and 16, so the most common ones reported in scientific studies include missense/nonsense mutations. Therefore, many studies have examined the genotype-phenotype relationships of this disease and subsequently using gene therapy techniques have been able to reduce the complications of the disease and alleviate the signs and symptoms of the disease. In this regard, the present article reviews the general features of GM1 gangliosidosis and its mutations, as well as gene therapy studies and animal and human models of the disease.

Are GMI gangliosidosis and Morquio type B two different disorders or part of one phenotypic spectrum?

Monosialotetrahexosylganglioside (GMI) gangliosidosis and Morquio type B (MorB) are two lysosomal storage disorders (LSDs) caused by the same enzyme deficiency, β-galactosidase (βgal). GMI gangliosidosis, associated with GMI ganglioside accumulation, is a neurodegenerative condition characterized by psychomotor regression, visceromegaly, cherry red spot, and facial and skeletal abnormalities. MorB is characterized by prominent and severe skeletal deformities due to keratan sulfate (KS) accumulation. There are only a few reports on intermediate phenotypes between GMI gangliosidosis and MorB. The presentation of two new patients with this rare intermediate phenotype motivated us to review the literature, to study differences and similarities between GMI gangliosidosis and MorB, and to speculate about the possible mechanisms that may contribute to the differences in clinical presentation. In conclusion, we hypothesize that GMI gangliosidosis and MorB are part of one phenotypic spectrum of the same disease and that the classification of LSDs might need to be revised.

Disentangling molecular and clinical stratification patterns in beta-galactosidase deficiency

INTRODUCTION

This study aims to define the phenotypic and molecular spectrum of the two clinical forms of β-galactosidase (β-GAL) deficiency, GM1-gangliosidosis and mucopolysaccharidosis IVB (Morquio disease type B, MPSIVB).

METHODS

Clinical and genetic data of 52 probands, 47 patients with GM1-gangliosidosis and 5 patients with MPSIVB were analysed.

RESULTS

The clinical presentations in patients with GM1-gangliosidosis are consistent with a phenotypic continuum ranging from a severe antenatal form with hydrops fetalis to an adult form with an extrapyramidal syndrome. Molecular studies evidenced 47 variants located throughout the sequence of the GLB1 gene, in all exons except 7, 11 and 12. Eighteen novel variants (15 substitutions and 3 deletions) were identified. Several variants were linked specifically to early-onset GM1-gangliosidosis, late-onset GM1-gangliosidosis or MPSIVB phenotypes. This integrative molecular and clinical stratification suggests a variant-driven patient assignment to a given clinical and severity group.

CONCLUSION

This study reports one of the largest series of b-GAL deficiency with an integrative patient stratification combining molecular and clinical features. This work contributes to expand the community knowledge regarding the molecular and clinical landscapes of b-GAL deficiency for a better patient management.

Comprehensive behavioral and biochemical outcomes of novel murine models of GM1-gangliosidosis and Morquio syndrome type B

Deficiencies in the lysosomal hydrolase β-galactosidase (β-gal) lead to two distinct diseases: the skeletal disease Morquio syndrome type B, and the neurodegenerative disease GM1-gangliosidosis. Utilizing CRISPR-Cas9 genome editing, the mouse β-gal encoding gene, Glb1, was targeted to generate both models of β-gal deficiency in a single experiment. For Morquio syndrome type B, the common human missense mutation W273L (position 274 in mice) was introduced into the Glb1 gene (Glb1^W274L), while for GM1-gangliosidosis, a 20 bp mutation was generated to remove the catalytic nucleophile of β-gal (β-gal^-/-). Glb1^W274L mice showed a significant reduction in β-gal enzyme activity (8.4-13.3% of wildtype), but displayed no marked phenotype after one year. In contrast, β-gal^-/- mice were devoid of β-gal enzyme activity (≤1% of wildtype), resulting in ganglioside accumulation and severe cellular vacuolation throughout the central nervous system (CNS). β-gal^-/- mice also displayed severe neuromotor and neurocognitive dysfunction, and as the disease progressed, the mice became emaciated and succumbed to the disease by 10 months of age. Overall, in addition to generating a novel murine model that phenotypically resembles GM1-gangliosidosis, the first model of β-galactosidase deficiency with residual enzyme activity has been developed.

Rapid Targeted Genomics in Critically Ill Newborns

BACKGROUND

Rapid diagnostic whole-genome sequencing has been explored in critically ill newborns, hoping to improve their clinical care and replace time-consuming and/or invasive diagnostic testing. A previous retrospective study in a research setting showed promising results with diagnoses in 57%, but patients were highly selected for known and likely Mendelian disorders. The aim of our prospective study was to assess the speed and yield of rapid targeted genomic diagnostics for clinical application.

METHODS

We included 23 critically ill children younger than 12 months in ICUs over a period of 2 years. A quick diagnosis could not be made after routine clinical evaluation and diagnostics. Targeted analysis of 3426 known disease genes was performed by using whole-genome sequencing data. We measured diagnostic yield, turnaround times, and clinical consequences.

RESULTS

A genetic diagnosis was obtained in 7 patients (30%), with a median turnaround time of 12 days (ranging from 5 to 23 days). We identified compound heterozygous mutations in the EPG5 gene (Vici syndrome), the RMND1 gene (combined oxidative phosphorylation deficiency-11), and the EIF2B5 gene (vanishing white matter), and homozygous mutations in the KLHL41 gene (nemaline myopathy), the GFER gene (progressive mitochondrial myopathy), and the GLB1 gene (GM1-gangliosidosis). In addition, a 1p36.33p36.32 microdeletion was detected in a child with cardiomyopathy.

CONCLUSIONS

Rapid targeted genomics combined with copy number variant detection adds important value in the neonatal and pediatric intensive care setting. It led to a fast diagnosis in 30% of critically ill children for whom the routine clinical workup was unsuccessful.

Clinical Findings and Natural History in Ten Unrelated Families with Juvenile and Adult GM1 Gangliosidosis

We describe 12 subjects of ten unrelated families from the region of Campinas and the southern state of Minas Gerais, Brazil, who presented with juvenile (n = 4) and adult (n = 8) GM1 gangliosidosis. Data includes clinical history, physical examination, and ancillary exam findings. Six subjects presented initially with skeletal deformities, while the remaining six had neurological manifestations at onset. Over time, all exhibited a combination of osteoarticular and neurologic degeneration with varying degrees of severity. Corneal clouding, angiokeratomas, and inguinal hernia were seen in one individual each. Other features commonly described in lysosomal storage disorders were not found in this series, such as coarse faces, gingival hypertrophy, visceromegaly, and cherry red spot. All subjects presented with short stature, dysostosis multiplex, dysarthria, and impairment of activities of daily living, 10/12 had extrapyramidal signs, 8/12 had pyramidal signs, 8/12 had oculomotor abnormalities, 4/12 had behavioral alterations, and 2/12 had ataxia. None had seizures or Parkinsonism. All female subjects developed severe hip dysplasia and underwent arthroplasty due to chronic pain. A vertebral bone bar and os odontoideum, not previously described in this condition, were found in one patient each. There was no clear genotype-phenotype correlation regarding enzyme residual activity and clinical findings, since all subjects were compound heterozygous, but the p.T500A was the most frequent allele in eight families and was associated to Morquio B phenotype. Two sets of siblings allowed intrafamilial comparison revealing consistent features among the families. Interfamilial correlation among unrelated families presenting the same mutations was less consistent.

Beta-galactosidase deficiencies and novel GLB1 mutations in three Chinese patients with Morquio B disease or GM1 gangliosidosis

BACKGROUND

This paper aims to report GLB1 activities and mutation analysis of three patients from the mainland of China, one with Morquio B disease and two with GM1 gangliosidosis.

METHODS

GLB1 activity and GLB1 gene mutation were analyzed in the three patients who were clinically suspected of having Morquio B disease or GM1 gangliosidosis. Novel mutations were analyzed by aligning GLB1 homologs, 100 control chromosomes, and the PolyPhen-2 tool.

RESULTS

The enzymatic activity of GLB1 was found to be 5.03, 4.20, and 4.50 nmol/h/mg in the three patients, respectively. Patient 1 was a compound heterozygote for p.[Arg148Cys] and p.[Tyr485Cys] mutations in the GLB1 gene. Patient 2 was a compound heterozygote for p.[Tyr270Phe] and p.[Leu337Pro] mutations. Patient 3 was a homozygote for p.[Asp448Val] mutation. Three mutations (p.[Tyr485Cys], p.[Tyr270Phe] and p.[Leu337Pro]) were novel variants and were predicted to damage GLB1 function.

CONCLUSIONS

The enzymatic activity and related gene analysis of β-galactosidase should be performed in clinically suspected individuals to confirm diagnosis. The three novel mutations, p.[Tyr485Cys], p.[Tyr270Phe], and p.[Leu337Pro], are thought to be disease-causing mutations.

β-Galactosidosis in Patient with Intermediate GM1 and MBD Phenotype

A 5-year-old girl with clinical and biochemical phenotypes encompassing both GM1-gangliosidosis (GM1) and Morquio B disease (MBD) is described. Mild generalized skeletal dysplasia and keratan sulfaturia were consistent with a diagnosis of MBD, while developmental delay and GM1-specific oligosacchariduria were consistent with GM1 gangliosidosis. No observable β-galactosidase activity was detected in leukocytes, and two mutations, p.R201H (c.602G>A) and p.G311R (c.931G>A), were identified by gene sequencing. The R201H substitution has been previously reported in patients with both GM1 and MBD, and G311R is a novel mutation. Our patient represents a further example of the clinical heterogeneity that can result from mutations at the β-galactosidase locus.

Differential profiling studies of N-linked glycoproteins in glioblastoma cancer stem cells upon treatment with γ-secretase inhibitor

We have recently demonstrated that Notch pathway blockade by γ-secretase inhibitor (GSI) depletes cancer stem cells (CSCs) in Glioblastoma Multiforme (GBM) through reduced proliferation and induced apoptosis. However, the detailed mechanism by which the manipulation of Notch signal induces alterations on post-translational modifications such as glycosylation has not been investigated. Herein, we present a differential profiling work to detect the change of glycosylation pattern upon drug treatment in GBM CSCs. Rapid screening of differential cell surface glycan structures has been performed by lectin microarray on live cells followed by the detection of N-linked glycoproteins from cell lysates using multi-lectin chromatography and label-free quantitative mass spectrometry analysis. A total of 51 and 52 glycoproteins were identified in the CSC- and GSI-treated groups, respectively, filtered by a combination of decoy database searching and Trans-Proteomic Pipeline (TPP) processing. Although no significant changes were detected from the lectin microarray experiment, 7 differentially expressed glycoproteins with high confidence were captured after the multi-lectin column including key enzymes involved in glycan processing. Functional annotations of the altered glycoproteins suggest a phenotype transformation of CSCs toward a less tumorigenic form upon GSI treatment.

GM1 gangliosidosis and Morquio B disease: an update on genetic alterations and clinical findings

GM1 gangliosidosis and Morquio B syndrome, both arising from beta-galactosidase (GLB1) deficiency, are very rare lysosomal storage diseases with an incidence of about 1:100,000-1:200,000 live births worldwide. Here we report the beta-galactosidase gene (GLB1) mutation analysis of 21 unrelated GM1 gangliosidosis patients, and of 4 Morquio B patients, of whom two are brothers. Clinical features of the patients were collected and compared with those in literature. In silico analyses were performed by standard alignments tools and by an improved version of GLB1 three-dimensional models. The analysed cohort includes remarkable cases. One patient with GM1 gangliosidosis had a triple X syndrome. One patient with juvenile GM1 gangliosidosis was homozygous for a mutation previously identified in Morquio type B. A patient with infantile GM1 gangliosidosis carried a complex GLB1 allele harbouring two genetic variants leading to p.R68W and p.R109W amino acid changes, in trans with the known p.R148C mutation. Molecular analysis showed 27 mutations, 9 of which are new: 5 missense, 3 microdeletions and a nonsense mutation. We also identified four new genetic variants with a predicted polymorphic nature that was further investigated by in silico analyses. Three-dimensional structural analysis of GLB1 homology models including the new missense mutations and the p.R68W and p.R109W amino acid changes showed that all the amino acid replacements affected the resulting protein structures in different ways, from changes in polarity to folding alterations. Genetic and clinical associations led us to undertake a critical review of the classifications of late-onset GM1 gangliosidosis and Morquio B disease.

Population analysis of the GLB1 gene in South Brazil

Infantile GM1 gangliosidosis is caused by the absence or reduction of lysosomal beta-galactosidase activity. Studies conducted in Brazil have indicated that it is one of the most frequent lysosomal storage disorders in the southern part of the country. To assess the incidence of this disorder, 390 blood donors were tested for the presence of two common mutations (1622–1627insG and R59H) in the GLB1 gene. Another group, consisting of 26 GM1 patients, and the blood donors were tested for the presence of two polymorphisms (R521C and S532G), in an attempt to elucidate whether there is a founder effect. The frequencies of the R59H and 1622–1627insG mutations among the GM1 patients studied were 19.2% and 38.5%, respectively. The frequency of polymorphism S532G was 16.7%, whereas R521C was not found in the patients. The overall frequency of either R59H or 1622–1627insG was 57.7% of the disease-causing alleles. This epidemiological study suggested a carrier frequency of 1:58. Seven different haplotypes were found. The 1622–1627insG mutation was not found to be linked to any polymorphism, whereas linkage disequilibrium was found for haplotype 2 (R59H, S532G) (p < 0.001). These data confirm the high incidence of GM1 gangliosidosis and the high frequency of two common mutations in southern Brazil.

Lysosomal multienzymatic complex-related diseases: a genetic study among Portuguese patients

The functional activity of lysosomal enzymes sialidase, β-galactosidase and N-acetylaminogalacto-6-sulfate-sulfatase in the cell depends on their association in a multienzyme complex with cathepsin A. Mutations in any of the components of this complex result in functional deficiency thereby causing severe lysosomal storage disorders. Here, we report the molecular defects underlying sialidosis (mutations in sialidase; gene NEU1), galactosialidosis (mutations in cathepsin A; gene PPGB) and GM1 gangliosidosis (mutations in β-galactosidase; gene GLB1) in Portuguese patients. We performed molecular studies of the PPGB, NEU1 and GLB1 genes in biochemically diagnosed Portuguese patients. Gene expression was determined and the effect of each mutation predicted at protein levels. In the NEU1 gene, we found three novel missense mutations (p.P200L, p.D234N and p.Q282H) and one nonsense mutation (p.R341X). In the PPGB gene, we identified two missense mutations, one novel (p.G86V) and one already described (p.V104M), as well as two new deletions (c.230delC and c.991-992delT) that give rise to non-functional proteins. We also present the first molecular evidence of a causal missense mutation localized to the cathepsin A active site. Finally, in the GLB1 gene, we found six different mutations, all of them previously described (p.R59H, p.R201H, p.H281Y, p.W527X, c.1572-1577InsG and c.845-846delC). Seven novel mutations are reported here, contributing to our knowledge of the mutational spectrum of these diseases and to a better understanding of the genetics of the lysosomal multienzymatic complex. The results of this study will allow carrier detection in affected families and prenatal molecular diagnosis, leading to the improvement of genetic counseling.

Phenotype determining alleles in GM1 gangliosidosis patients bearing novel GLB1 mutations

GM1 gangliosidosis manifests with progressive psychomotor deterioration and dysostosis of infantile, juvenile, or adult onset, caused by alterations in the structural gene coding for lysosomal acid beta-galactosidase (GLB1). In addition, allelic variants of this gene can result in Morquio B disease (MBD), a phenotype with dysostosis multiplex and entire lack of neurologic involvement. More than 100 sequence alterations in the GLB1 gene have been identified so far, but only few could be proven to be predictive for one of the GM1 gangliosidosis subtypes or MBD. We performed genotype analyses in 16 GM1 gangliosidosis patients of all phenotypes and detected 28 different genetic lesions. Among these, p.I55FfsX16, p.W65X, p.F107L, p.H112P, p.C127Y, p.W161X, p.I181K, p.C230R, p.W273X, p.R299VfsX5, p.A301V, p.F357L, p.K359KfsX23, p.L389P, p.D448V, p.D448GfsX8, and the intronic mutation IVS6-8A>G have not been published so far. Due to their occurrence in homozygous patients, four mutations could be correlated to a distinct GM1 gangliosidosis phenotype. Furthermore, the missense mutations from heteroallelic patients and three artificial nonsense mutations were characterized by overexpression in COS-1 cells, and the subcellular localization of the mutant proteins in fibroblasts was assessed. The phenotype specificity of 10 alleles can be proposed on the basis of our results and previous data.

Impact of beta-galactosidase mutations on the expression of the canine lysosomal multienzyme complex

beta-galactosidase (GLB1) forms a functional lysosomal multienzyme complex with lysosomal protective protein (PPCA) and neuraminidase 1 (NEU1) which is important for its intracellular processing and activity. Mutations in the beta-galactosidase gene cause the lysosomal storage disease G(M1)-gangliosidosis. In order to identify additional molecular changes associated with the presence of beta-galactosidase mutations, the expression of canine lysosomal multienzyme complex components in GLB1(+/+), GLB1(+/-) and GLB1(-/-) fibroblasts was investigated by quantitative RT-PCR, Western blot and enzymatic assays. Quantitative RT-PCR revealed differential regulation of total beta-galactosidase, beta-galactosidase variants and protective protein for beta-galactosidase gene (PPGB) in GLB1(+/-) and GLB1(-/-) compared to GLB1(+/+) fibroblasts. Furthermore, it was shown that PPGB levels gradually increased with the number of mutant beta-galactosidase alleles while no change in the NEU1 expression was observed. This is the first study that simultaneously examine the effect of GLB1(+/+), GLB1(+/-) and GLB1(-/-) genotypes on the expression of lysosomal multienzyme complex components. The findings reveal a possible adaptive process in GLB1 homozygous mutant and heterozygous individuals that could facilitate the design of efficient therapeutic strategies.

GM1 gangliosidosis: review of clinical, molecular, and therapeutic aspects

GM(1) gangliosidosis is a lysosomal storage disorder due to deficiency of the beta-galactosidase enzyme. This deficiency results in accumulation of GM(1) gangliosides and related glycoconjugates in the lysosomes leading to lysosomal swelling, cellular damage, and organ dysfunction. The disease is lethal in the infantile and juvenile forms. To date, up to 102 mutations distributed along the beta-galactosidase gene (GLB1) have been reported. This review gives an overview of the clinical and molecular findings in patients with GM(1) gangliosidosis. Furthermore, it describes therapeutic approaches which are currently under investigation in animal models of the disease.