Characterization of seven novel mutations on the HEXB gene in French Sandhoff patients

P. Ala278Val mutation might cause a pathogenic defect in HEXB folding leading to the Sandhoff disease

Sandhoff disease is a rare neurodegenerative and autosomal recessive disorder, which is characterized by a defect in ganglioside metabolism. Also, it is caused by mutations in the HEXB gene for the β-subunit isoform 1 of β-N-acetyl hexosaminidase. In the present study, an Iranian 14- month -old girl with 8- month history of unsteady walking and involuntary movements was described. In this regard, biochemical testing showed some defects in the normal activity of beta-hexosaminidase protein. Following sequencing of HEXB gene, a homozygous c.833C > T mutation was identified in the patient's genome. After recognition of p.A278V, several different in silico methods were used to assess the mutant protein stability, ranging from mutation prediction methods to ligand docking. The p.A278V mutation might be disruptive because of changing the three-dimensional folding at the end of the 5th alpha helix. According to the medical prognosis, in silico and structural analyses, it was predicted to be disease cause.

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.

Analysis of the HEXA, HEXB, ARSA, and SMPD1 Genes in 68 Iranian Patients

Lysosomal storage diseases (LSDs) are known as genetic disorders with an overall prevalence of 1 per 7700 live births. Sphingolipidosis, which is a subgroup of LSDs, is resulted from mutations in the coding genes of specific enzymes of sphingolipid hydrolases. The current study aimed to provide additional knowledge on the genotype of sphingolipidoses disease among Iranian patients affected by the disease. In this research, we studied 68 unrelated Iranian patients diagnosed with one kind of sphingolipidoses from 2014 to 2019. Thereafter, genomic DNA was isolated from their peripheral blood leukocytes samples in EDTA in terms of the manufacturer's protocol. All the coding exons and exon-intron boundaries of the related genes were sequenced and then analyzed using the NCBI database. Finally, they were reviewed using some databases such as the Human Gene Mutation Database (HGMD) and ClinVar ( https://www.ncbi.nlm.nih.gov/clinva ). By studying 22 MLD patients, 18 different variations of the ARSA gene were found, one of which was new including, named as c.472 T > G p. (Cys158Gly). Out of 15 Sandhoff disease (SD) patients, 11 different variations of the HEXB gene were found. Correspondingly, the c.1083-2delA was not reported earlier. By investigating 21 Iranian patients with Tay-Sachs disease (TSD), one new variant was found as c.622delG. The study of 10 Niemann-Pick disease A/B (NPDA/B (patients has led to the identification of 9 different SMPD1 gene variations, among which 3 variations were novel mutations. The results of the present study can be expanded to the genotypic spectrum of Iranian patients with MLD, SD, TSD, and NPD diseases and also used to innovate more effective methods for the detection of genetic carriers as well as diagnosing and counseling of Iranian patients affected with these disorders.

Decrease in Myelin-Associated Lipids Precedes Neuronal Loss and Glial Activation in the CNS of the Sandhoff Mouse as Determined by Metabolomics

Sandhoff disease (SD) is a lysosomal disease caused by mutations in the gene coding for the β subunit of β-hexosaminidase, leading to deficiency in the enzymes β-hexosaminidase (HEX) A and B. SD is characterised by an accumulation of gangliosides and related glycolipids, mainly in the central nervous system, and progressive neurodegeneration. The underlying cellular mechanisms leading to neurodegeneration and the contribution of inflammation in SD remain undefined. The aim of the present study was to measure global changes in metabolism over time that might reveal novel molecular pathways of disease. We used liquid chromatography-mass spectrometry and ¹H Nuclear Magnetic Resonance spectroscopy to profile intact lipids and aqueous metabolites, respectively. We examined spinal cord and cerebrum from healthy and Hexb ^-/- mice, a mouse model of SD, at ages one, two, three and four months. We report decreased concentrations in lipids typical of the myelin sheath, galactosylceramides and plasmalogen-phosphatidylethanolamines, suggesting that reduced synthesis of myelin lipids is an early event in the development of disease pathology. Reduction in neuronal density is progressive, as demonstrated by decreased concentrations of N-acetylaspartate and amino acid neurotransmitters. Finally, microglial activation, indicated by increased amounts of myo-inositol correlates closely with the late symptomatic phases of the disease.

Natural History of Adult Patients with GM2 Gangliosidosis

OBJECTIVE

GM2 gangliosidoses are lysosomal diseases due to biallelic mutations in the HEXA (Tay-Sachs disease [TS]) or HEXB (Sandhoff disease [SD]) genes, with subsequent low hexosaminidase(s) activity. Most patients have childhood onset, but some experience the first symptoms during adolescence/adulthood. This study aims to clarify the natural history of adult patients with GM2 gangliosidosis.

METHODS

We retrospectively described 12 patients from a French cohort and 45 patients from the literature.

RESULTS

We observed 4 typical presentations: (1) lower motoneuron disorder responsible for proximal lower limb weakness that subsequently expanded to the upper limbs, (2) cerebellar ataxia, (3) psychosis and/or severe mood disorder (only in the TS patients), and (4) a complex phenotype mixing the above 3 manifestations. The psoas was the first and most affected muscle in the lower limbs, whereas the triceps and interosseous were predominantly involved in the upper limbs. A longitudinal study of compound motor action potentials showed a progressive decrease in all nerves, with different kinetics. Sensory potentials were sometimes abnormally low, mainly in the SD patients. The main brain magnetic resonance imaging feature was cerebellar atrophy, even in patients without cerebellar symptoms. The prognosis was mainly related to gait disorder, as we showed that beyond 20 years of disease evolution, half of the patients were wheelchair users.

INTERPRETATION

Improved knowledge of GM2 gangliosidosis in adults will help clinicians achieve correct diagnoses and better inform patients on the evolution and prognosis. It may also contribute to defining proper outcome measures when testing emerging therapies. ANN NEUROL 2020;87:609-617.

Clinical and Molecular Characteristics of Two Chinese Children with Infantile Sandhoff Disease and Review of the Literature

Infantile Sandhoff disease is an autosomal recessive inherited disease primarily characterized by cherry red spots in the retina, muscle weakness, seizure, truncal hypotonia, hyperacusis, developmental delay and regression. The pathogenic genetic defects of the HEXB gene, which encodes the β subunit of the hexosaminidase A (ɑβ) and hexosaminidase B (ββ) enzymes, cause deficiency of both the Hex A and Hex B enzymes, resulting in the deposition of GM2 ganglion glycerides in the lysosomes of the central nervous system and somatic cells. The aim of this study was to discover disease-causing variants of the HEXB gene in two Chinese families through the use of exome sequencing. By characterizing three novel variants by molecular genetics, bioinformatics analysis, and three-dimensional structure modeling, we showed that all these novel variants influenced the protein structure. The results broaden the variant spectrum of HEXB in different ethnic groups. Furthermore, not all patients diagnosed with infantile Sandhoff disease had characteristic cranial imaging findings, which can only be used as supplementary information for diagnosis. The results of this study may contribute to clinical management, genetic counseling, and gene-targeted treatments for Sandhoff disease.

Homozygous variants in the HEXB and MBOAT7 genes underlie neurological diseases in consanguineous families

BACKGROUND

Neurological disorders are a common cause of morbidity and mortality within Pakistani populations. It is one of the most important challenges in healthcare, with significant life-long socio-economic burden.

METHODS

We investigated the cause of disease in three Pakistani families in individuals with unexplained autosomal recessive neurological conditions, using both genome-wide SNP mapping and whole exome sequencing (WES) of affected individuals.

RESULTS

We identified a homozygous splice site variant (NM_000521:c.445 + 1G > T) in the hexosaminidase B (HEXB) gene confirming a diagnosis of Sandhoff disease (SD; type II GM2-gangliosidosis), an autosomal recessive lysosomal storage disorder caused by deficiency of hexosaminidases in a single family. In two further unrelated families, we identified a homozygous frameshift variant (NM_024298.3:c.758_778del; p.Glu253_Ala259del) in membrane-bound O-acyltransferase family member 7 (MBOAT7) as the likely cause of disease. MBOAT7 gene variants have recently been identified as a cause of intellectual disability (ID), seizures and autistic features.

CONCLUSIONS

We identified two metabolic disorders of lipid biosynthesis within three Pakistani families presenting with undiagnosed neurodevelopmental conditions. These findings enabled an accurate neurological disease diagnosis to be provided for these families, facilitating disease management and genetic counselling within this population. This study consolidates variation within MBOAT7 as a cause of neurodevelopmental disorder, broadens knowledge of the clinical outcomes associated with MBOAT7-related disorder, and confirms the likely presence of a regionally prevalent founder variant (c.758_778del; p.Glu253_Ala259del) in Pakistan.

Genotype-phenotype correlation of gangliosidosis mutations using in silico tools and homology modeling

Gangliosidoses, including GM1-gangliosidosis and GM2-gangliosidosis (Tay-Sachs disease and Sandhoff disease), are lysosomal disorders resulting from enzyme deficiencies and accumulation of gangliosides. Phenotypes of gangliosidoses range from infantile, late-infantile, juvenile, and to the adult form. The genotype-phenotype correlation is essential for prognosis and clinical care planning for patients with a gangliosidosis condition. Previously, we have developed a method to establish the genotype-phenotype correlation of another lysosomal disease, mucopolysaccharidosis type I, with in silico tools. This same method was applied to analyze the genotype and phenotype of 38 patients diagnosed with a gangliosidosis disease in the United States. Out of 40 mutations identified, 3 were novel, including p.Tyr192His and p.Phe556Ser of the GLB1 gene and p.Gly461Val of the HEXA gene. Furthermore, the mutant protein structure of all missense mutations was constructed by homology modeling. A systemic structural analysis of these models revealed the specific mechanisms of how each mutation may lead to the disease. In summary, the method developed in this study holds promise as a tool that can be broadly applicable to other lysosomal diseases and monogenic diseases.

Clinical presentation and outcome in infantile Sandhoff disease: a case series of 25 patients from Iranian neurometabolic bioregistry with five novel mutations

BACKGROUND

Infantile Sandhoff disease (ISD) is a GM2 gangliosidosis that is classified as a lysosomal storage disorder. The most common symptoms of affected individuals at presentation are neurologic involvement. Here we report clinical course and demographic features in a case series of infantile Sandhoff disease. Enzymatically and some genetically proven cases of ISD were extracted from the Iranian Neurometabolic Registry (INMR) in Children's Medical Center, Iran, Tehran from December 2010 to December 2016.

RESULT

Twenty five cases of infantile SD (13 female, 12 male) were included in this study. The age range of patients was 9-24 months with a mean of 15.8 months. The consanguinity rate of parents affected families was about 80%. The mean age of patients at disease onset was 6.4 months and the mean age at diagnosis was 14 months. Patients were diagnosed with a mean delay of 7.8 months. Eleven of patients died due to aspiration pneumonia and intractable seizure. The most common features at presentation (92%) were developmental delay or regression in speech and cognitive domains. Cherry red spots were detected in 17 patients (68%). Organomegaly was detected only in two patients. Enzyme studies showed marked reductions of both Hexosaminidase A and B in all patients. HEXB gene mutation studies performed in eight patients identified 6 different mutations, which five of them were novel.

CONCLUSION

Infantile SD should be considered for each child presented with neurologic symptoms such as developmental delay and regression and cherry red spots in ophthalmic examination. Organomegaly is not a frequent clinical finding in infantile SD. Additionally; there are a genetic heterogenisity among Iranian patients.

Genotype, phenotype and in silico pathogenicity analysis of HEXB mutations: Panel based sequencing for differential diagnosis of gangliosidosis

OBJECTIVES

Gangliosidosis is an inherited metabolic disorder causing neurodegeneration and motor regression. Preventive diagnosis is the first choice for the affected families due to lack of straightforward therapy. Genetic studies could confirm the diagnosis and help families for carrier screening and prenatal diagnosis. An update of HEXB gene variants concerning genotype, phenotype and in silico analysis are presented.

PATIENTS AND METHODS

Panel based next generation sequencing and direct sequencing of four cases were performed to confirm the clinical diagnosis and for reproductive planning. Bioinformatic analyses of the HEXB mutation database were also performed.

RESULTS

Direct sequencing of HEXA and HEXB genes showed recurrent homozygous variants at c.509G>A (p.Arg170Gln) and c.850C>T (p.Arg284Ter), respectively. A novel variant at c.416T>A (p.Leu139Gln) was identified in the GLB1 gene. Panel based next generation sequencing was performed for an undiagnosed patient which showed a novel mutation at c.1602C>A (p.Cys534Ter) of HEXB gene. Bioinformatic analysis of the HEXB mutation database showed 97% consistency of in silico genotype analysis with the phenotype. Bioinformatic analysis of the novel variants predicted to be disease causing. In silico structural and functional analysis of the novel variants showed structural effect of HEXB and functional effect of GLB1 variants which would provide fast analysis of novel variants.

CONCLUSIONS

Panel based studies could be performed for overlapping symptomatic patients. Consequently, genetic testing would help affected families for patients' management, carrier detection, and family planning's.

Identification of microRNAs with Dysregulated Expression in Status Epilepticus Induced Epileptogenesis

The involvement of miRNA in mesial temporal lobe epilepsy (MTLE) pathogenesis has increasingly become a focus of epigenetic studies. Despite advances, the number of known miRNAs with a consistent expression response during epileptogenesis is still small. Addressing this situation requires additional miRNA profiling studies coupled to detailed individual expression analyses. Here, we perform a miRNA microarray analysis of the hippocampus of Wistar rats 24 hours after intra-hippocampal pilocarpine-induced Status Epilepticus (H-PILO SE). We identified 73 miRNAs that undergo significant changes, of which 36 were up-regulated and 37 were down-regulated. To validate, we selected 5 of these (10a-5p, 128a-3p, 196b-5p, 352 and 324-3p) for RT-qPCR analysis. Our results confirmed that miR-352 and 196b-5p levels were significantly higher and miR-128a-3p levels were significantly lower in the hippocampus of H-PILO SE rats. We also evaluated whether the 3 miRNAs show a dysregulated hippocampal expression at three time periods (0h, 24h and chronic phase) after systemic pilocarpine-induced status epilepticus (S-PILO SE). We demonstrate that miR-128a-3p transcripts are significantly reduced at all time points compared to the naïve group. Moreover, miR-196b-5p was significantly higher only at 24h post-SE, while miR-352 transcripts were significantly up-regulated after 24h and in chronic phase (epileptic) rats. Finally, when we compared hippocampi of epileptic and non-epileptic humans, we observed that transcript levels of miRNAs show similar trends to the animal models. In summary, we successfully identified two novel dysregulated miRNAs (196b-5p and 352) and confirmed miR-128a-3p downregulation in SE-induced epileptogenesis. Further functional assays are required to understand the role of these miRNAs in MTLE pathogenesis.

Homozygous p.R284* mutation in HEXB gene causing Sandhoff disease with nystagmus

Sandhoff disease is a rare, genetic, lipid storage disorder characterized by progressive degeneration of the nerve cells (neurons) in the brain and spinal cord. This disease is caused by mutations in the beta-hexosaminidase beta-subunit (HEXB) gene. Here, we investigated the clinical characteristics and molecular basis of Sandhoff disease in an infant female patient from Jordan. The initial sign was nystagmus, which was noted at birth. To our knowledge, this is the first report of Sandhoff disease from Jordan. Introducing lysosomal enzyme assays to the testing of children with global developmental delay with unknown etiology in countries with high rates of consanguinity will not only increase the percentage of diagnosed cases, but will also help orient genetic counseling and prenatal diagnosis and eventually will reduce the overall burden of disabilities in these countries.

Incidence and carrier frequency of Sandhoff disease in Saskatchewan determined using a novel substrate with detection by tandem mass spectrometry and molecular genetic analysis

Sandhoff disease is a rare progressive neurodegenerative genetic disorder with a high incidence among certain isolated communities and ethnic groups around the world. Previous reports have shown a high occurrence of Sandhoff disease in northern Saskatchewan. Newborn screening cards from northern Saskatchewan were retrospectively screened in order to investigate the incidence and determine the carrier frequency of Sandhoff disease in these communities. PCR-based screening was conducted for the c.115delG (p.(Val39fs)) variant in the HEXB gene that was previously found in 4 Sandhoff disease patients from this area. The carrier frequency for this allele was estimated to be ~1:27. MS/MS-based screening of hexosaminidase activity along with genetic sequencing allowed for the identification of additional variants based on low total hexosaminidase activity and high % hexosaminidase A activity relative to c.115delG carriers. In total 4 pathogenic variants were discovered in the population (c.115delG, c.619A>G, c.1601G>T, and c.1652G>A) of which two are previously unreported (c.1601G>T and c.1652G>A). The combined carrier frequency of these alleles in the study area was estimated at ~1:15. Based on the number of cases of Sandhoff disease from this area we estimate the incidence to be ~1:390 corresponding to a child being born with the disease every 1-2 years on average. The results from our study were then compared with variants in the HEXB gene from the genomes available from the 1000 Genomes project. A total of 19 HEXB variants were found in the 1092 genomes of which 5 are suspected of having a deleterious effect on hexosaminidase activity. The estimated carrier frequency of Sandhoff disease in Saskatchewan at 1:15 is more than 3 times higher than the carrier frequency in the global sample provided by the 1000 Genomes project at 1:57.