GM2 gangliosidoses in Spain: Analysis of the HEXA and HEXB genes in 34 Tay–Sachs and 14 Sandhoff patients

Clinical and genetic features of a case with juvenile onset sandhoff disease

BACKGROUND

Sandhoff disease (SD) is a rare neurological disease with high clinical heterogeneity. SD in juvenile form is much rarer and it is often misdiagnosed in clinics. Therein, it is necessary to provide more cases and review the literature on juvenile onset SD.

CASE PRESENTATION

A 14 years-old boy with eight years of walking difficulties, and was ever misdiagnosed as spinocerebellar ataxia. We found this patient after genetic testing carried rs201580118 and a novel gross deletion in HEXB (g.74012742_74052694del). Through review the literature, we found that was the first gross deletion identified at the 3'end of HEXB, associated with juvenile onset SD from China.

CONCLUSION

This case expanded our knowledge about the genotype and phenotype correlations in SD. Comprehensive genetic testing is important for the diagnosis of unexplained ataxia.

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.

Aberrant Ganglioside Functions to Underpin Dysregulated Myelination, Insulin Signalling, and Cytokine Expression: Is There a Link and a Room for Therapy?

Gangliosides are molecules widely present in the plasma membranes of mammalian cells, participating in a variety of processes, including protein organization, transmembrane signalling and cell adhesion. Gangliosides are abundant in the grey matter of the brain, where they are critically involved in postnatal neural development and function. The common precursor of the majority of brain gangliosides, GM3, is formed by the sialylation of lactosylceramide, and four derivatives of its a- and b-series, GM1, GD1a, GD1b and GT1b, constitute 95% of all the brain gangliosides. Impairments in ganglioside metabolism due to genetic abnormalities of GM-synthases are associated with severe neurological disorders. Apart from that, the latest genome-wide association and translational studies suggest a role of genes involved in brain ganglioside synthesis in less pervasive psychiatric disorders. Remarkably, the most recent animal studies showed that abnormal ganglioside functions result in dysregulated neuroinflammation, aberrant myelination and altered insulin receptor signalling. At the same time, these molecular features are well established as accompanying developmental psychiatric disorders such as attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASD). This led us to hypothesize a role of deficient ganglioside function in developmental neuropsychiatric disorders and warrants further gene association clinical studies addressing this question. Here, we critically review the literature to discuss this hypothesis and focus on the recent studies on ST3GAL5-deficient mice. In addition, we elaborate on the therapeutic potential of various anti-inflammatory remedies for treatment of developmental neuropsychiatric conditions related to aberrant ganglioside functions.

A 23-year follow-up report of juvenile-onset Sandhoff disease presenting with a motor neuron disease phenotype and a novel variant

BACKGROUND

The clinical severity of Sandhoff disease is known to vary widely. Furthermore, long-term follow-up report is very limited in the literature.

CASE PRESENTATION

We present a long-term follow-up report of a patient with juvenile-onset Sandhoff disease with a motor neuron disease phenotype. The patient had compound heterozygous variants of HEXB (p.Trp460Arg, p. Arg533His); the Trp460Arg was a novel variant. Long-term follow-up revealed no intellectual deterioration, swallowing dysfunction, or respiratory muscle dysfunction despite progressive weakness of the extremities and sensory disturbances.

CONCLUSION

We need to be aware of Sandhoff disease in patients with juvenile-onset motor neuron disease.

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.

Utility of Gene Panels for the Diagnosis of Inborn Errors of Metabolism in a Metabolic Reference Center

Next-generation sequencing (NGS) technologies have been proposed as a first-line test for the diagnosis of inborn errors of metabolism (IEM), a group of genetically heterogeneous disorders with overlapping or nonspecific phenotypes. Over a 3-year period, we prospectively analyzed 311 pediatric patients with a suspected IEM using four targeted gene panels. The rate of positive diagnosis was 61.86% for intermediary metabolism defects, 32.84% for complex molecular defects, 19% for hypoglycemic/hyperglycemic events, and 17% for mitochondrial diseases, and a conclusive molecular diagnosis was established in 2-4 weeks. Forty-one patients for whom negative results were obtained with the mitochondrial diseases panel underwent subsequent analyses using the NeuroSeq panel, which groups all genes from the individual panels together with genes associated with neurological disorders (1870 genes in total). This achieved a diagnostic rate of 32%. We next evaluated the utility of a tool, Phenomizer, for differential diagnosis, and established a correlation between phenotype and molecular findings in 39.3% of patients. Finally, we evaluated the mutational architecture of the genes analyzed by determining z-scores, loss-of-function observed/expected upper bound fraction (LOEUF), and haploinsufficiency (HI) scores. In summary, targeted gene panels for specific groups of IEMs enabled rapid and effective diagnosis, which is critical for the therapeutic management of IEM patients.

Late onset Sandhoff disease presenting with lower motor neuron disease and stuttering

Defects in the HEXB gene which encodes the β-subunit of β-hexosaminidase A and B enzymes, cause a GM2 gangliosidosis, also known as Sandhoff disease, which is a rare lysosomal storage disorder. The most common form of the disease lead to quickly progressing mental and motor decline in infancy; however there are other less severe forms with later onset that can also involve lower motor neurons. The diagnosis of this disease is based on low serum β-hexosaminidases A and B levels and confirmed using genetic test. We report two siblings with compound heterozygous HEXB mutations whose phenotype was extremely mild consisting in stuttering in both cases associated to mild proximal weakness in one of the cases, broadening the clinical spectrum of late onset Sandhoff disease.

Sandhoff disease in the elderly: a case study

Sandhoff disease is an infrequent, genetically caused disorder with a recessive autosomal inheritance pattern. It belongs to the gangliosidosis GM2 group and is produced by mutations in gen HEXB leading to reduction in enzymatic activity of enzymes β-hexosaminidase A and B. Adult-onset GM2 gangliosidosis is rare. Here we report a white male who presented at age 69 with a fast-progression, motor neuron disease, mimicking amyotrophic lateral sclerosis (ALS), combined with autonomic dysfunction, sensory ataxia, and exaggerated startle to noise. Enzymatic assays demonstrated deficiency of both Hexosaminidases A and B leading to the diagnosis of Sandhoff disease.

Sequence analysis of the Hex A gene in Jacob sheep from Bulgaria

Background and Aim:

Jacob sheep are a rare ancient breed of sheep believed to have originated from the Mediterranean area but which are now kept throughout the world. These sheep have recently attracted medical interest due to the observation of a genetic disorder in the breed that can be used as an animal model of Tay–Sachs disease (TSD). This study aims to detect mutations in the Hexosaminidase A gene in Jacob sheep based on sequence analysis of the 284-bp fragment situated between exon 11 and intron 11 of the gene, a target sequence for site-specific mutation. This is the first study that has investigated Jacob sheep in Bulgaria for gene-specific mutations.

Materials and Methods:

A total of 20 blood samples were collected from Jacob sheep from the Rhodope Mountains. DNA was isolated from these samples, and a specific 284-bp fragment was amplified. The amplified products were purified using a polymerase chain reaction purification kit and sequenced in both directions.

Results:

Target sequences were successfully amplified from all 20 investigated sheep. Sequence analysis did not show the homozygous, recessive, missense (G-to-C transition) mutation at nucleotide position 1330 (G1330→C) in exon 11, demonstrating that all of these sheep were a normal genotype (wild-type).

Conclusion:

Jacob sheep are considered a potentially useful animal model in advancing the understanding of pathogenesis and developing potential therapies for orphan diseases, such as those characterized by mutant GM2 gangliosides. The clinical and biochemical features of the Jacob sheep model of TSD represent well the human classical late-infantile form of this disorder, indicating that the model can serve as a possible new research tool for further study of the pathogenesis and treatment of TSD.

Infantile onset Sandhoff disease: clinical manifestation and a novel common mutation in Thai patients

Background

Sandhoff disease (SD) is an autosomal recessive lysosomal storage disorder, resulting in accumulation of GM2 ganglioside, particular in neuronal cells. The disorder is caused by deficiency of β-hexosaminidase B (HEX-B), due to pathogenic variant of human HEXB gene.

Method

This study describes clinical features, biochemical, and genetic defects among Thai patients with infantile SD during 2008–2019.

Results

Five unrelated Thai patients presenting with developmental regression, axial hypotonia, seizures, exaggerated startle response to noise, and macular cherry red spot were confirmed to have infantile SD based on deficient HEX enzyme activities and biallelic variants of the HEXB gene. In addition, an uncommon presenting feature, cardiac defect, was observed in one patient. All the patients died in their early childhood. Plasma total HEX and HEX-B activities were severely deficient. Sequencing analysis of HEXB gene identified two variants including c.1652G>A (p.Cys551Tyr) and a novel variant of c.761T>C (p.Leu254Ser), in 90 and 10% of the mutant alleles found, respectively. The results from in silico analysis using multiple bioinformatics tools were in agreement that the p.Cys551Tyr and the p.Leu254Ser are likely pathogenic variants. Molecular modelling suggested that the Cys551Tyr disrupt disulfide bond, leading to protein destabilization while the Leu254Ser resulted in change of secondary structure from helix to coil and disturbing conformation of the active site of the enzyme. Genome-wide SNP array analysis showed no significant relatedness between the five affected individuals. These two variants were not present in control individuals. The prevalence of infantile SD in Thai population is estimated 1 in 1,458,521 and carrier frequency at 1 in 604.

Conclusion

The study suggests that SD likely represents the most common subtype of rare infantile GM2 gangliosidosis identified among Thai patients. We firstly described a potential common variant in HEXB in Thai patients with infantile onset SD. The data can aid a rapid molecular confirmation of infantile SD starting with the hotspot variant and the use of expanded carrier testing.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12887-020-02481-3.

GM2 Gangliosidoses: Clinical Features, Pathophysiological Aspects, and Current Therapies

GM2 gangliosidoses are a group of pathologies characterized by GM2 ganglioside accumulation into the lysosome due to mutations on the genes encoding for the β-hexosaminidases subunits or the GM2 activator protein. Three GM2 gangliosidoses have been described: Tay-Sachs disease, Sandhoff disease, and the AB variant. Central nervous system dysfunction is the main characteristic of GM2 gangliosidoses patients that include neurodevelopment alterations, neuroinflammation, and neuronal apoptosis. Currently, there is not approved therapy for GM2 gangliosidoses, but different therapeutic strategies have been studied including hematopoietic stem cell transplantation, enzyme replacement therapy, substrate reduction therapy, pharmacological chaperones, and gene therapy. The blood-brain barrier represents a challenge for the development of therapeutic agents for these disorders. In this sense, alternative routes of administration (e.g., intrathecal or intracerebroventricular) have been evaluated, as well as the design of fusion peptides that allow the protein transport from the brain capillaries to the central nervous system. In this review, we outline the current knowledge about clinical and physiopathological findings of GM2 gangliosidoses, as well as the ongoing proposals to overcome some limitations of the traditional alternatives by using novel strategies such as molecular Trojan horses or advanced tools of genome editing.

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.

The Classification of Autosomal Recessive Cerebellar Ataxias: a Consensus Statement from the Society for Research on the Cerebellum and Ataxias Task Force

There is currently no accepted classification of autosomal recessive cerebellar ataxias, a group of disorders characterized by important genetic heterogeneity and complex phenotypes. The objective of this task force was to build a consensus on the classification of autosomal recessive ataxias in order to develop a general approach to a patient presenting with ataxia, organize disorders according to clinical presentation, and define this field of research by identifying common pathogenic molecular mechanisms in these disorders. The work of this task force was based on a previously published systematic scoping review of the literature that identified autosomal recessive disorders characterized primarily by cerebellar motor dysfunction and cerebellar degeneration. The task force regrouped 12 international ataxia experts who decided on general orientation and specific issues. We identified 59 disorders that are classified as primary autosomal recessive cerebellar ataxias. For each of these disorders, we present geographical and ethnical specificities along with distinctive clinical and imagery features. These primary recessive ataxias were organized in a clinical and a pathophysiological classification, and we present a general clinical approach to the patient presenting with ataxia. We also identified a list of 48 complex multisystem disorders that are associated with ataxia and should be included in the differential diagnosis of autosomal recessive ataxias. This classification is the result of a consensus among a panel of international experts, and it promotes a unified understanding of autosomal recessive cerebellar disorders for clinicians and researchers.

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.

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.

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.

Clinical,biochemical and molecular analysis of five Chinese patients with Sandhoff disease

Sandhoff disease (SD) is a rare autosomal recessive lysosomal storage disorder of sphingolipid metabolism resulting from the deficiency of β-hexosaminidase (HEX). Mutations of the HEXB gene cause Sandhoff disease. In order to improve the diagnosis and expand the knowledge of the disease, we collected and analyzed relevant data of clinical diagnosis, biochemical investigation, and molecular mutational analysis in five Chinese patients with SD. The patients presented with heterogenous symptoms of neurologic deterioration. HEX activity in leukocytes was severely deficient. We identified seven different mutations, including three known mutations: IVS12-26G > A, p.T209I, p.I207V, and four novel mutations: p.P468PfsX62, p.L223P, p.Y463X, p.G549R. We also detected two different heterozygous mutations c.-122delC and c.-126C > T in the promoter which were suspected to be deleterious mutations. We attempted to correlate these mutations with the clinical presentation of the patients. Our study indicates that the mutation p.T209I and p.P468PfsX62 may link to the infantile form of SD. Our study expands the spectrum of genotype of SD in China, provides new insights into the molecular mechanism of SD and helps to the diagnosis and treatment of this disease.

Tay-Sachs Carrier Screening by Enzyme and Molecular Analyses in the New York City Minority Population

Background and Aims: Carrier screening for Tay-Sachs disease is performed by sequence analysis of the HEXA gene and/or hexosaminidase A enzymatic activity testing. Enzymatic analysis (EA) has been suggested as the optimal carrier screening method, especially in non-Ashkenazi Jewish (non-AJ) individuals, but its utilization and efficacy have not been fully evaluated in the general population. This study assesses the reliability of EA in comparison with HEXA sequence analysis in non-AJ populations. Methods: Five hundred eight Hispanic and African American patients (516 samples) had EA of their leukocytes performed and 12 of these patients who tested positive by EA (“carriers”) had subsequent HEXA gene sequencing performed. Results: Of the 508 patients, 25 (4.9%) were EA positive and 40 (7.9%) were inconclusive. Of the 12 patients who were sequenced, 11 did not carry a pathogenic variant and one carried a likely deleterious mutation (NM_000520.4(HEXA):c.1510C>T). Conclusions: High inconclusive rates and poor correlation between positive/inconclusive enzyme results and identification of pathogenic mutations suggest that ethnic-specific recalibration of reference ranges for EA may be necessary. Alternatively, HEXA gene sequencing could be performed.

Expanding the spectrum of HEXA mutations in Indian patients with Tay-Sachs disease

Tay-Sachs disease is an autosomal recessive neurodegenerative disorder occurring due to impaired activity of β-hexosaminidase-A (EC 3.2.1.52), resulting from the mutation in HEXA gene. Very little is known about the molecular pathology of TSD in Indian children except for a few mutations identified by us. The present study is aimed to determine additional mutations leading to Tay-Sachs disease in nine patients confirmed by the deficiency of β-hexosaminidase-A (< 2% of total hexosaminidase activity for infantile patients) in leucocytes. The enzyme activity was assessed by using substrates 4-methylumbelliferyl-N-acetyl-β-d-glucosamine and 4-methylumbelliferyl-N-acetyl-β-d-glucosamine-6-sulfate for total-hexosaminidase and hexosaminidase-A respectively, and heat inactivation method for carrier detection. The exons and exon-intron boundaries of the HEXA gene were bi-directionally sequenced on an automated sequencer. 'In silico' analyses for novel mutations were carried out using SIFT, Polyphen2 and MutationT@ster software programs. The structural study was carried out by UCSF Chimera software using the crystallographic structure of β-hexosaminidase-A (PDB-ID: 2GJX) as the template. Our study identified four novel mutations in three cases. These include a compound heterozygous missense mutation c.524A>C (D175A) and c.805G>C (p.G269R) in one case; and one small 1 bp deletion c.426delT (p.F142LfsX57) and one splice site mutation c.459+4A>C in the other two cases respectively. None of these mutations were detected in 100 chromosomes from healthy individuals of the same ethnic group. Three previously reported missense mutations, (i) c.532C>T (p.R178C), (ii) c.964G>T (p.D322Y), and (iii) c.1385A>T (p.E462V); two nonsense mutations (i) c.709C>T (p.Q237X) and (ii) c.1528C>T (p.R510X), one 4 bp insertion c.1277_1278insTATC (p.Y427IfsX5) and one splice site mutation c.459+5G>A were also identified in six cases. We observe from this study that novel mutations are more frequently observed in Indian patients with Tay-Sachs disease with clustering of ~ 73% of disease causing mutations in exons 5 to 12. This database can be used for a carrier rate screening in the larger population of the country.

Polycystic kidneys and GM2 gangliosidosis-like disease in neonatal springboks (Antidorcas marsupialis)

Clinical, gross, histopathologic, electron microscopic findings and enzymatic analysis of 4 captive, juvenile springboks (Antidorcas marsupialis) showing both polycystic kidneys and a storage disease are described. Springbok offspring (4 of 34; 12%) were affected by either one or both disorders in a German zoo within a period of 5 years (2008-2013). Macroscopic findings included bilaterally severely enlarged kidneys displaying numerous cysts in 4 animals and superior brachygnathism in 2 animals. Histopathologically, kidneys of 4 animals displayed cystic dilation of the renal tubules. In addition, abundant cytoplasmic vacuoles with a diameter ranging from 2 to 10 μm in neurons of the central and peripheral nervous system, hepatocytes, thyroid follicular epithelial cells, pancreatic islets of Langerhans and renal tubular cells were found in 2 springbok neonates indicative of an additional storage disease. Ultrastructurally, round electron-lucent vacuoles, up to 4 μm in diameter, were present in neurons. Enzymatic analysis of liver and kidney tissue of 1 affected springbok revealed a reduced activity of total hexosaminidase (Hex) with relatively increased HexA activity at the same level of total Hex, suggesting a hexosaminidase defect. Pedigree analysis suggested a monogenic autosomal recessive inheritance for both diseases. In summary, related springboks showed 2 different changes resembling both polycystic kidney and a GM2 gangliosidosis similar to the human Sandhoff disease. Whether the simultaneous occurrence of these 2 entities represents an incidental finding or has a genetic link needs to be investigated in future studies.

Next-generation DNA sequencing of HEXA: a step in the right direction for carrier screening

Tay-Sachs disease (TSD) is the prototype for ethnic-based carrier screening, with a carrier rate of ∼1/27 in Ashkenazi Jews and French Canadians. HexA enzyme analysis is the current gold standard for TSD carrier screening (detection rate ∼98%), but has technical limitations. We compared DNA analysis by next-generation DNA sequencing (NGS) plus an assay for the 7.6 kb deletion to enzyme analysis for TSD carrier screening using 74 samples collected from participants at a TSD family conference. Fifty-one of 74 participants had positive enzyme results (46 carriers, five late-onset Tay-Sachs [LOTS]), 16 had negative, and seven had inconclusive results. NGS + 7.6 kb del screening of HEXA found a pathogenic mutation, pseudoallele, or variant of unknown significance (VUS) in 100% of the enzyme-positive or obligate carrier/enzyme-inconclusive samples. NGS detected the B1 allele in two enzyme-negative obligate carriers. Our data indicate that NGS can be used as a TSD clinical carrier screening tool. We demonstrate that NGS can be superior in detecting TSD carriers compared to traditional enzyme and genotyping methodologies, which are limited by false-positive and false-negative results and ethnically focused, limited mutation panels, respectively, but is not ready for sole use due to lack of information regarding some VUS.

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

Sandhoff disease (SD) is an autosomal recessive lysosomal storage disease caused by mutations in the HEXB gene encoding the beta subunit of hexosaminidases A and B, two enzymes involved in GM2 ganglioside degradation. Eleven French Sandhoff patients with infantile or juvenile forms of the disease were completely characterized using sequencing of the HEXB gene. A specific procedure was developed to facilitate the detection of the common 5'-end 16kb deletion which was frequent (36% of the alleles) in our study. Eleven other disease-causing mutations were found, among which four have previously been reported (c.850C>T, c.793T>G, c.115del and c.800_817del). Seven mutations were completely novel and were analyzed using molecular modelling. Two deletions (c.176del and c.1058_1060del), a duplication (c.1485_1487dup) and a nonsense mutation (c.552T>G) were predicted to strongly alter the enzyme spatial organization. The splice mutation c.558+5G>A affecting the intron 4 consensus splice site led to a skipping of exon 4 and to a truncated protein (p.191X). Two missense mutations were found among the patients studied. The c.448A>C mutation was probably a severe mutation as it was present in association with the known c.793T>G in an infantile form of Sandhoff disease and as it significantly modified the N-terminal domain structure of the protein. The c.171G>C mutation resulting in a p.W57C amino acid substitution in the N-terminal region is probably less drastic than the other abnormalities as it was present in a juvenile patient in association with the c.176del. Finally, this study reports a rapid detection of the Sandhoff disease-causing alleles facilitating genetic counselling and prenatal diagnosis in at-risk families.