Recurrent and novel GLB1 mutations in India

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.

Extensive and Persistent Dermal Melanocytosis in a Male Carrier of Mucopolysaccharidosis Type IIIC (Sanfilippo Syndrome): A Case Report

Congenital dermal melanocytosis (DM) represents a common birthmark mainly found in children of Asian and darker skin phototype descent, clinically characterized by an oval blue-grey macule or macules, commonly located on the lumbosacral area. In rare DM cases, when presenting with diffuse macules persisting during the first years of life, it could represent a cutaneous feature of mucopolysaccharidoses (MPS). Extensive congenital DM is actually associated with Hurler syndrome (MPS type I) and Hunter syndrome (MPS type II), although several reports also described this association with MPS type VI and other lysosomal storage disorders (LySD), including GM1 gangliosidosis, mucolipidosis, Sandhoff disease, and Niemann-Pick disease. Here, we present the case of a two-year-old boy presenting with extensive dermal melanocytosis, generalized hypertrichosis, and chronic itch, harboring a heterozygous variant of uncertain significance, NM_152419.3: c.493C>T (p.Pro165Ser), in the exon 4 of HGSNAT gene, whose mutations are classically associated with MPS IIIC, also known as Sanfilippo syndrome. This is the first report that highlights the association between extensive congenital DM and MPS type IIIC, as well as a pathogenetic link between heterozygous LySD carrier status and congenital DM. We speculate that some cases of extensive congenital DM could be related to heterozygous LySD carriers, as a manifestation of a mild clinical phenotype.

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.

Lysosomal storage disorders: from biology to the clinic with reference to India

Lysosomal storage disorders (LSDs) are a group of seventy different metabolic storage diseases due to accumulation of substrate mainly in the form of carbohydrate, lipids, proteins, and cellular debris. They occur due to variant in different genes that regulate lysosomal enzymes synthesis, transport, and secretion. In recent years, due to an increased availability of various therapies to treat these disorders, and increased diagnostic tools, there has been an escalated awareness of LSDs. Due to heterogeneous population and various social reasons, India is likely to have a high frequency of LSDs. Therefore, to understand the burden of various LSDs, its molecular spectrum, and understanding the phenotype-genotype correlation, Indian Council of Medical Research (ICMR) and Department of Health Research (DHR), Government of India had set up a task force in the year 2015. It has resulted in identifying common LSDs, and founder variant for some of the storage disorders and molecular spectrum of various LSDs across the country. This review describes in detail the spectrum of LSDs, its molecular epidemiology and prevention in context to Indian population.

Emerging cellular themes in leukodystrophies

Leukodystrophies are a broad spectrum of neurological disorders that are characterized primarily by deficiencies in myelin formation. Clinical manifestations of leukodystrophies usually appear during childhood and common symptoms include lack of motor coordination, difficulty with or loss of ambulation, issues with vision and/or hearing, cognitive decline, regression in speech skills, and even seizures. Many cases of leukodystrophy can be attributed to genetic mutations, but they have diverse inheritance patterns (e.g., autosomal recessive, autosomal dominant, or X-linked) and some arise from de novo mutations. In this review, we provide an updated overview of 35 types of leukodystrophies and focus on cellular mechanisms that may underlie these disorders. We find common themes in specialized functions in oligodendrocytes, which are specialized producers of membranes and myelin lipids. These mechanisms include myelin protein defects, lipid processing and peroxisome dysfunction, transcriptional and translational dysregulation, disruptions in cytoskeletal organization, and cell junction defects. In addition, non-cell-autonomous factors in astrocytes and microglia, such as autoimmune reactivity, and intercellular communication, may also play a role in leukodystrophy onset. We hope that highlighting these themes in cellular dysfunction in leukodystrophies may yield conceptual insights on future therapeutic approaches.

Mongolian spots in GM1 gangliosidosis: a pictorial report

GM1 gangliosidosis is a lysosomal storage disorder, characterized by psychomotor deterioration, visceromegaly, facial coarseness, retinal cherry-red spots, and skeletal abnormalities. We report six unrelated patients with GM1 gangliosidosis with extensive Mongolian spots on the trunk and extremities that provided clue to clinical diagnosis. All patients exhibited psychomotor delay, coarse facies, hepatosplenomegaly, hypotonia, and dysostosis multiplex. Four patients had retinal cherry-red spots. The condition was confirmed by identification of very low activities of beta-galactosidase enzyme in peripheral leukocytes and biallelic pathogenic variants in the GLB1 gene. We identified one novel (c.1479G>T) and two known (c.75 + 2dup and c.1369C>T) pathogenic variants in homozygous state in them. Our work ascertains extensive Mongolian spots as a diagnostic handle for early recognition of GM1 gangliosidosis. Though a known feature of GM1 gangliosidosis, considerable variation in the prevalence and ethnic differences are observed. This report illustrates the Mongolian spots pictorially in Indian patients.

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.

Clinical and Laboratory Profile of Gangliosidosis from Southern Part of India

Gangliosidoses are progressive neurodegenerative disorders caused by the deficiency of enzymes involved in the breakdown of glycosphingolipids. There are not much data about gangliosidosis in India; hence, this study was planned. The aim is to study the clinical, biochemical, and molecular profile of gangliosidosis. A retrospective chart review, in the pediatric neurology department from January 2015 to March 2020, was performed. Children diagnosed with Gangliosidosis were included. The disorder was confirmed by reduced activity of enzymes and/or pathogenic or likely pathogenic variants in associated genes. We assessed age at presentation, gender, parental consanguinity, clinical manifestations, neuroimaging findings, enzyme level, and pathogenic or likely pathogenic variants. Clinical data for 32 children with gangliosidosis were analyzed, which included 12 (37.5%) with GM1 gangliosidosis, 8 (25%) with Tay-Sachs disease (TSD), 11 (34.37%) with Sandhoff disease (SD), and 1 AB variant of GM2 gangliosidosis that occurs due to GM2 ganglioside activator protein deficiency. Twenty-four (75%) children were the offspring of consanguineous parents. Thirty-one (97%) had developmental delay. The median age at presentation was 15.5 months. Nine (28.12%) had seizures. Five children (41.6%) with GM1 gangliosidosis and two with SD had extensive Mongolian spots. Ten children with GM1 gangliosidosis (83.3%) had coarse facial features. Cherry red spot was found in 24 out of 32 children (75%). All children with GM1 gangliosidosis and none with TSD had hepato-splenomegaly. Two children (2/8; 25%) with TSD and seven (7/11; 63%) with SD had microcephaly. One child with SD had coarse facies and three did not have hepato-splenomegaly. Neuroimaging findings revealed bilateral thalamic involvement in 20 (62.5%) patients and periventricular hypomyelination in all cases. One child had a rare AB variant of GM2 gangliosidosis. GM2 Gangliosidoses are more common compared with GM1 variety. All of them had infantile onset except one child with TSD. Microcephaly can be present while usually megalencephaly is reported in the literature. The absence of hepato-splenomegaly does not rule out SD. Extensive Mongolian spots can be seen in GM2 gangliosidosis. AB variant of GM2 gangliosidosis should be considered when the enzyme is normal in the presence of strong clinical suspicion.

Axonopathy and Reduction of Membrane Resistance: Key Features in a New Murine Model of Human GM1-Gangliosidosis

G_M1-gangliosidosis is caused by a reduced activity of β-galactosidase (Glb1), resulting in intralysosomal accumulations of G_M1. The aim of this study was to reveal the pathogenic mechanisms of G_M1-gangliosidosis in a new Glb1 knockout mouse model. Glb1^−/− mice were analyzed clinically, histologically, immunohistochemically, electrophysiologically and biochemically. Morphological lesions in the central nervous system were already observed in two-month-old mice, whereas functional deficits, including ataxia and tremor, did not start before 3.5-months of age. This was most likely due to a reduced membrane resistance as a compensatory mechanism. Swollen neurons exhibited intralysosomal storage of lipids extending into axons and amyloid precursor protein positive spheroids. Additionally, axons showed a higher kinesin and lower dynein immunoreactivity compared to wildtype controls. Glb1^−/− mice also demonstrated loss of phosphorylated neurofilament positive axons and a mild increase in non-phosphorylated neurofilament positive axons. Moreover, marked astrogliosis and microgliosis were found, but no demyelination. In addition to the main storage material G_M1, G_A1, sphingomyelin, phosphatidylcholine and phosphatidylserine were elevated in the brain. In summary, the current Glb1^−/− mice exhibit a so far undescribed axonopathy and a reduced membrane resistance to compensate the functional effects of structural changes. They can be used for detailed examinations of axon–glial interactions and therapy trials of lysosomal storage diseases.

The natural history of Type 1 infantile GM1 gangliosidosis: A literature-based meta-analysis

INTRODUCTION

Type 1 GM1 gangliosidosis is an ultra-rare, rapidly fatal lysosomal storage disorder, with life expectancy of <3 years of age. To date, only one prospective natural history study of limited size has been reported. Thus, there is a need for additional research to provide a better understanding of the progression of this disease. We have leveraged the past two decades of medical literature to conduct the first comprehensive retrospective study characterizing the natural history of Type 1 GM1 gangliosidosis.

OBJECTIVES

The objectives of this study were to establish a large sample of patients from the literature in order to identify: 1) clinically distinguishing factors between Type 1 and Type 2 GM1 gangliosidosis, 2) age at first symptom onset, first hospital admission, diagnosis, and death, 3) time to onset of common clinical findings, and 4) timing of developmental milestone loss.

METHODS

PubMed was searched with the keyword "GM1 Gangliosidosis" and for articles from the year 2000 onwards. A preliminary review of these results was conducted to establish subtype classification criteria for inclusion of only Type 1 patients, resulting in 44 articles being selected to generate the literature dataset of 154 Type 1 GM1 gangliosidosis patients. Key clinical events of these patient cases were recorded from the articles.

RESULTS

Comprehensive subtyping criteria for Type 1 GM1 gangliosidosis were created, and clinical events, including onset, diagnosis, death, and symptomology, were mapped over time. In this dataset, average age of diagnosis was 8.7 months, and average age of death was 18.9 months.

DISCUSSION

This analysis demonstrates the predictable clinical course of this disease, as almost all patients experienced significant multi-organ system dysfunction and neurodevelopmental regression, particularly in the 6- to 18-month age range. Patients were diagnosed at a late age relative to disease progression, indicating the need for improved public awareness and screening.

CONCLUSION

This study highlights the significant burden of illness in this disease and provides critical natural history data to drive earlier diagnosis, inform clinical trial design, and facilitate family counseling.

A review of skeletal dysplasia research in India

We aimed to review the contributions by Indian researchers to the subspecialty of skeletal dysplasias (SDs). Literature search using specific keywords in PubMed was performed to retrieve all the published literature on SDs as on July 6, 2017. All published literature on SDs wherein at least one author was from an Indian institute was included. Publications were grouped into different categories based on the major emphasis of the research paper. Five hundred and forty publications in English language were retrieved and categorized into five different groups. The publications were categorized as reports based on: (i) phenotypes (n = 437), (ii) mutations (n = 51), (iii) novel genes (n = 9), (iv) therapeutic interventions (n = 31), and (v) reviews (n = 12). Most of the publications were single-patient case reports describing the clinical and radiological features of the patients affected with SDs (n = 352). We enlisted all the significant Indian contributions. We have also highlighted the reports in which Indians have contributed to discovery of new genes and phenotypes. This review highlights the substantial Indian contributions to SD research, which is poised to reach even greater heights given the size and structure of our population, technological advances, and expanding national and international collaborations.

Multi-gene testing in neurological disorders showed an improved diagnostic yield: data from over 1000 Indian patients

BACKGROUND

Neurological disorders are clinically heterogeneous group of disorders and are major causes of disability and death. Several of these disorders are caused due to genetic aberration. A precise and confirmatory diagnosis in the patients in a timely manner is essential for appropriate therapeutic and management strategies. Due to the complexity of the clinical presentations across various neurological disorders, arriving at an accurate diagnosis remains a challenge.

METHODS

We sequenced 1012 unrelated patients from India with suspected neurological disorders, using TruSight One panel. Genetic variations were identified using the Strand NGS software and interpreted using the StrandOmics platform.

RESULTS

We were able to detect mutations in 197 genes in 405 (40%) cases and 178 mutations were novel. The highest diagnostic rate was observed among patients with muscular dystrophy (64%) followed by leukodystrophy and ataxia (43%, each). In our cohort, 26% of the patients who received definitive diagnosis were primarily referred with complex neurological phenotypes with no suggestive diagnosis. In terms of mutations types, 62.8% were truncating and in addition, 13.4% were structural variants, which are also likely to cause loss of function.

CONCLUSION

In our study, we observed an improved performance of multi-gene panel testing, with an overall diagnostic yield of 40%. Furthermore, we show that NGS (next-generation sequencing)-based testing is comprehensive and can detect all types of variants including structural variants. It can be considered as a single-platform genetic test for neurological disorders that can provide a swift and definitive diagnosis in a cost-effective manner.

Phenotyping and genotyping of skeletal dysplasias: Evolution of a center and a decade of experience in India

Genetic heterogeneity, high burden and the paucity of genetic testing for rare diseases challenge genomic healthcare for these disorders in India. Here we report our experience over the past decade, of establishing the genomic evaluation of skeletal dysplasia at a tertiary university hospital in India. Research or clinical genomic testing was carried out by Sanger sequencing and next-generation sequencing. Close national and international collaborations aided phenotyping and genotyping. We report 508 families (557 affected individuals) with the definitive molecular diagnosis of skeletal dysplasia. Dysostoses multiplex (n = 196), genetic inflammatory/rheumatoid-like osteoarthropathies (n = 114) and osteogenesis imperfecta and decreased bone density (n = 58) were the most common diagnoses. We enumerate the processes, clinical diagnoses and causal variants in the cohort with 48 novel variants in 21 genes. We summarize scientific contributions of the center to the description of clinical and mutation profiles and discovery of new phenotypes and genetic etiology. Our study illustrates the establishment and application of genomic testing tools for genetic disorders of skeleton in a large cohort. We believe this could be a model to emulate for other developing genetic centers.

Genetic Analysis of Undiagnosed Juvenile GM1-Gangliosidosis by Microarray and Exome Sequencing

GM1 gangliosidosis is an autosomal recessive lysosomal storage disorder due to mutations in the lysosomal acid 3-galactosidase gene, GLB1. It is usually classified into three forms, infantile, juvenile, or adult, based on age at onset and severity of central nervous system involvement. Because of their broad clinical spectrum and their similarity to many other aetiologies, including inherited neurodegenerative and metabolic diseases, it is often difficult to diagnose such diseases. Recently, whole exome sequencing (WES) has become increasingly used when a strong hypothesis cannot be formulated based on the clinical phenotype. Here, we present three patients belonging to a consanguineous Moroccan family with a GM1-gangliosidosis with unusual clinical onset and atypical radiological presentation that had eluded diagnosis for over a decade. To identify the disease-causing mutation, we performed a whole exome sequencing and a chromosomal microarray genotyping in order to reduce the number of genetic variants to be interpreted, by focusing the data analysis only on the linked loci. The already known pathogenic missense mutation c.601G>A in GLB1 (p.R201C) was found at homozygous state in the proband V.1 and at heterozygous state in his father IV.1. The mutation was validated by Sanger sequencing and segregated in all the family members according to a recessive mode of inheritance. Outside of the linked loci, we found the EXOSC8 p.Ser272Thr mutation at heterozygous state in all the patients and their mother IV.2. This mutation was reported to cause pontocerebellar hypoplasia type 1C and could act as a modifying factor that exacerbates the brain atrophy of patients. Our study identified the first GLB1 mutation in North Africa in patients with unexpected brain-MRI outcomes extending the clinical spectrum of the GM1-gangliosidosis.

Hypomyelinating disorders in China: The clinical and genetic heterogeneity in 119 patients

OBJECTIVE

Hypomyelinating disorders are a group of clinically and genetically heterogeneous diseases characterized by neurological deterioration with hypomyelination visible on brain MRI scans. This study was aimed to clarify the clinical and genetic features of HMDs in Chinese population.

METHODS

119 patients with hypomyelinating disorders in Chinese population were enrolled and evaluated based on their history, clinical manifestation, laboratory examinations, series of brain MRI with follow-up, genetic etiological tests including chromosomal analysis, multiplex ligation probe amplification, Sanger sequencing, targeted enrichment-based next-generation sequencing and whole exome sequencing.

RESULTS

Clinical and genetic features of hypomyelinating disorders were revealed. Nine different hypomyelinating disorders were identified in 119 patients: Pelizaeus-Merzbacher disease (94, 79%), Pelizaeus-Merzbacher-like disease (10, 8%), hypomyelination with atrophy of the basal ganglia and cerebellum (3, 3%), GM1 gangliosidosis (5, 4%), GM2 gangliosidosis (3, 3%), trichothiodystrophy (1, 1%), Pol III-related leukodystrophy (1, 1%), hypomyelinating leukodystrophy type 9 (1, 1%), and chromosome 18q deletion syndrome (1, 1%). Of the sample, 94% (112/119) of the patients were genetically diagnosed, including 111 with mutations distributing across 9 genes including PLP1, GJC2, TUBB4A, GLB1, HEXA, HEXB, ERCC2, POLR3A, and RARS and 1 with mosaic chromosomal change of 46, XX,del(18)(q21.3)/46,XX,r(18)(p11.32q21.3)/45,XX,-18. Eighteen novel mutations were discovered. Mutations in POLR3A and RARS were first identified in Chinese patients with Pol III-related leukodystrophy and hypomyelinating leukodystrophy, respectively.

SIGNIFICANCE

This is the first report on clinical and genetic features of hypomyelinating disorders with a large sample of patients in Chinese population, identifying 18 novel mutations especially mutations in POLR3A and RARS in Chinese patients, expanding clinical and genetic spectrums of hypomyelinating disorders.

Defining the genetic basis of early onset hereditary spastic paraplegia using whole genome sequencing

We performed whole genome sequencing (WGS) in nine families from India with early-onset hereditary spastic paraplegia (HSP). We obtained a genetic diagnosis in 4/9 (44 %) families within known HSP genes (DDHD2 and CYP2U1), as well as perixosomal biogenesis disorders (PEX16) and GM1 gangliosidosis (GLB1). In the remaining patients, no candidate structural variants, copy number variants or predicted splice variants affecting an extended candidate gene list were identified. Our findings demonstrate the efficacy of using WGS for diagnosing early-onset HSP, particularly in consanguineous families (4/6 diagnosed), highlighting that two of the diagnoses would not have been made using a targeted approach.