Characterization of the mutant β-subunit of β-hexosaminidase for dimer formation responsible for the adult form of Sandhoff disease with the motor neuron disease phenotype

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.

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.

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.

Motor neuron disease in inherited neurometabolic disorders

Inherited neurometabolic disorders represent a growing group of inborn errors of metabolism that present with major neurological symptoms or a complex spectrum of symptoms dominated by central or peripheral nervous system dysfunction. Many neurological presentations may arise from the same metabolic defect, especially in autosomal-recessive inherited disorders. Motor neuron disease (MND), mainly represented by amyotrophic lateral sclerosis, may also result from various inborn errors of metabolism, some of which may represent potentially treatable conditions, thereby emphasizing the importance of recognizing such diseases. The present review discusses the most important neurometabolic disorders presenting with motor neuron (lower and/or upper) dysfunction as the key clinical and neuropathological feature.

Tay-Sachs disease mutations in HEXA target the α chain of hexosaminidase A to endoplasmic reticulum-associated degradation

In Tay–Sachs disease, mutations in HEXA can lead to aberrant α subunits of the HexA enzyme. Two such mutants have folding defects and are cleared by endoplasmic reticulum-associated degradation. Toward the pursuit of therapeutic treatments, it was found that manipulating endoplasmic reticulum quality control can impair mutant α degradation and improve cellular Hex activity.

Loss of function of the enzyme β-hexosaminidase A (HexA) causes the lysosomal storage disorder Tay–Sachs disease (TSD). It has been proposed that mutations in the α chain of HexA can impair folding, enzyme assembly, and/or trafficking, yet there is surprisingly little known about the mechanisms of these potential routes of pathogenesis. We therefore investigated the biosynthesis and trafficking of TSD-associated HexA α mutants, seeking to identify relevant cellular quality control mechanisms. The α mutants E482K and G269S are defective in enzymatic activity, unprocessed by lysosomal proteases, and exhibit altered folding pathways compared with wild-type α. E482K is more severely misfolded than G269S, as observed by its aggregation and inability to associate with the HexA β chain. Importantly, both mutants are retrotranslocated from the endoplasmic reticulum (ER) to the cytosol and are degraded by the proteasome, indicating that they are cleared via ER-associated degradation (ERAD). Leveraging these discoveries, we observed that manipulating the cellular folding environment or ERAD pathways can alter the kinetics of mutant α degradation. Additionally, growth of patient fibroblasts at a permissive temperature or with chemical chaperones increases cellular Hex activity by improving mutant α folding. Therefore modulation of the ER quality control systems may be a potential therapeutic route for improving some forms of TSD.

Expression of human β-N-acetylhexosaminidase B in yeast eases the search for selective inhibitors

Human lysosomal β-N-acetylhexosaminidases from the family 20 of glycoside hydrolases are dimeric enzymes catalysing the cleavage of terminal β-N-acetylglucosamine and β-N-acetylgalactosamine residues from a broad spectrum of glycoconjugates. Here, we present a facile, robust, and cost-effective extracellular expression of human β-N-acetylhexosaminidase B in Pichia pastoris KM71H strain. The prepared Hex B was purified in a single step with 33% yield obtaining 10mg of the pure enzyme per 1L of the culture media. The enzyme was used in the inhibition assays with the known mechanism-based inhibitor NAG-thiazoline and a wide variety of its derivatives in the search for specific inhibitors of the human GH20 β-N-acetylhexosaminidases over the human GH84 β-N-acetylglucosaminidase, which was expressed, purified and used in the inhibition experiments as well. Moreover, enzyme-inhibitor complexes were analysed employing computational tools in order to reveal the structural basis of the results of the inhibition assays, showing the importance of water-mediated interactions between the enzyme and respective ligands. The presented method for the heterologous expression of human Hex B is robust, it significantly reduces the costs and equipment demands in comparison to the expression in mammalian cell lines. This will enhance accessibility of this human enzyme to the broad scientific community and may speed up the research of specific inhibitors of this physiologically important glycosidase family.

The Big Bluff of Amyotrophic Lateral Sclerosis Diagnosis: The Role of Neurodegenerative Disease Mimics

Neurodegenerative diseases include a significant number of pathologies affecting the nervous system. Generally, the primary cause of each disease is specific; however, recently, it was shown that they may be correlated at molecular level. This aspect, together with the exhibition of similar symptoms, renders the diagnosis of these disorders difficult. Amyotrophic lateral sclerosis is one of these pathologies. Herein, we report several cases of amyotrophic lateral sclerosis misdiagnosed as a consequence of features that are common to several neurodegenerative diseases, such as Parkinson's, Huntington's and Alzheimer's disease, spinal muscular atrophy, progressive bulbar palsy, spastic paraplegia and frontotemporal dementia, and mostly with the lysosomal storage disorder GM2 gangliosidosis. Overall reports highlight that the differential diagnosis for amyotrophic lateral sclerosis should include correlated mechanisms.

Clinical and biochemical characterization of 3-hydroxyisobutyryl-CoA hydrolase (HIBCH) deficiency that causes Leigh-like disease and ketoacidosis

3-Hydroxyisobutyryl-CoA hydrolase (HIBCH) deficiency is an autosomal recessive disorder characterized by episodes of ketoacidosis and a Leigh-like basal ganglia disease, without high concentrations of pyruvate and lactate in the cerebrospinal fluid. Only 4 cases of HIBCH deficiency have been reported. However, clinical-biochemical correlation in HIBCH deficiency by determining the detailed residual enzyme activities has not yet been elucidated. Here, we report a case of two Japanese siblings with HIBCH deficiency carrying a new homozygous missense mutation (c.287C > A, [p.A96D]) at the substrate-binding site. A transfection study using HIBCH expression vectors harboring wild type or 4 reported mutations, including the newly identified mutation (p.A96D, p.Y122C, p.G317E, and p.K74Lfs*13), revealed a correlation between residual HIBCH activities and the severity of the disease. All HIBCH mutants, except p.K74Lfs*13, showed residual enzyme activity and only the patient with p.K74Lfs*13 had congenital anomalies. p.G317E showed only low enzyme activity (~ 3%) of that of wild-type HIBCH. Although p.A96D had approximately 7 times higher enzyme activity than p.G317E, patients with p.A96D died during childhood. These findings are essential for clinical management, genetic counseling, and specific meal and concomitant drug considerations as part of the treatment for patients with HIBCH deficiency.

Natural history of motor neuron disease in adult onset GM2-gangliosidosis: A case report with 25 years of follow-up

An adult with Sandhoff disease presented with pure lower motor neuron phenotype. Twenty years later, he showed signs of upper motor neuron involvement. 25 years from the onset, his muscle weakness slightly worsened but he was fully independent in activities of daily living.

GM2-gangliosidosis can manifest as a motor neuron disease with a slowly progressive course. The correct knowledge of the natural history can be really important to achieve the diagnosis, design new therapies and evaluate clinical trials.

The spectrum of ZEB2 mutations causing the Mowat-Wilson syndrome in Japanese populations

Mowat-Wilson syndrome (MWS) is a multiple congenital anomaly syndrome characterized by moderate or severe intellectual disability, a characteristic facial appearance, microcephaly, epilepsy, agenesis or hypoplasia of the corpus callosum, congenital heart defects, Hirschsprung disease, and urogenital/renal anomalies. It is caused by de novo heterozygous loss of function mutations including nonsense mutations, frameshift mutations, and deletions in ZEB2 at 2q22. ZEB2 encodes the zinc finger E-box binding homeobox 2 protein consisting of 1,214 amino acids. Herein, we report 13 nonsense and 27 frameshift mutations from 40 newly identified MWS patients in Japan. Although the clinical findings of all the Japanese MWS patients with nonsense and frameshift mutations were quite similar to the previous review reports of MWS caused by nonsense mutations, frameshift mutations and deletions of ZEB2, the frequencies of microcephaly, Hirschsprung disease, and urogenital/renal anomalies were small. Patients harbored mutations spanning the region between the amino acids 55 and 1,204 in wild-type ZEB2. There was no obvious genotype-phenotype correlation among the patients. A transfection study demonstrated that the cellular level of the longest form of the mutant ZEB2 protein harboring the p.D1204Rfs*29 mutation was remarkably low. The results showed that the 3'-end frameshift mutation of ZEB2 causes MWS due to ZEB2 instability.

Mutations in HADHB, which encodes the β-subunit of mitochondrial trifunctional protein, cause infantile onset hypoparathyroidism and peripheral polyneuropathy

Mitochondrial trifunctional protein (MTP) is a hetero-octamer composed of four α- and four β-subunits that catalyzes the final three steps of mitochondrial β-oxidation of long chain fatty acids. HADHA and HADHB encode the α-subunit and the β-subunit of MTP, respectively. To date, only two cases with MTP deficiency have been reported to be associated with hypoparathyroidism and peripheral polyneuropathy. Here, we report on two siblings with autosomal recessive infantile onset hypoparathyroidism, peripheral polyneuropathy, and rhabdomyolysis. Sequence analysis of HADHA and HADHB in both siblings shows that they were homozygous for a mutation in exon 14 of HADHB (c.1175C>T, [p.A392V]) and the parents were heterozygous for the mutation. Biochemical analysis revealed that the patients had MTP deficiency. Structural analysis indicated that the A392V mutation identified in this study and the N389D mutation previously reported to be associated with hypoparathyroidism are both located near the active site of MTP and affect the conformation of the β-subunit. Thus, the present patients are the second and third cases of MTP deficiency associated with missense HADHB mutation and infantile onset hypoparathyroidism. Since MTP deficiency is a treatable disease, MTP deficiency should be considered when patients have hypoparathyroidism as the initial presenting feature in infancy.