Molecular basis of an adult form of Sandhoff disease: substitution of glutamine for arginine at position 505 of the beta-chain of beta-hexosaminidase results in a labile enzyme

Combination of single-nucleus and bulk RNA-seq reveals the molecular mechanism of thalamus haemorrhage-induced central poststroke pain

Central poststroke pain (CPSP) induced by thalamic haemorrhage (TH) can be continuous or intermittent and is accompanied by paresthesia, which seriously affects patient quality of life. Advanced insights into CPSP mechanisms and therapeutic strategies require a deeper understanding of the molecular processes of the thalamus. Here, using single-nucleus RNA sequencing (snRNA-seq), we sequenced the transcriptomes of 32332 brain cells, which revealed a total of four major cell types within the four thalamic samples from mice. Compared with the control group, the experimental group possessed the higher sensitivity to mechanical, thermal, and cold stimuli, and increased microglia numbers and decreased neuron numbers. We analysed a collection of differentially expressed genes and neuronal marker genes obtained from bulk RNA sequencing (bulk RNA-seq) data and found that Apoe, Abca1, and Hexb were key genes verified by immunofluorescence (IF). Immune infiltration analysis found that these key genes were closely related to macrophages, T cells, related chemokines, immune stimulators and receptors. Gene Ontology (GO) enrichment analysis also showed that the key genes were enriched in biological processes such as protein export from nucleus and protein sumoylation. In summary, using large-scale snRNA-seq, we have defined the transcriptional and cellular diversity in the brain after TH. Our identification of discrete cell types and differentially expressed genes within the thalamus can facilitate the development of new CPSP therapeutics.

Molecular Characterization of Portuguese Patients with Hereditary Cerebellar Ataxia

Hereditary cerebellar ataxia (HCA) comprises a clinical and genetic heterogeneous group of neurodegenerative disorders characterized by incoordination of movement, speech, and unsteady gait. In this study, we performed whole-exome sequencing (WES) in 19 families with HCA and presumed autosomal recessive (AR) inheritance, to identify the causal genes. A phenotypic classification was performed, considering the main clinical syndromes: spastic ataxia, ataxia and neuropathy, ataxia and oculomotor apraxia (AOA), ataxia and dystonia, and ataxia with cognitive impairment. The most frequent causal genes were associated with spastic ataxia (SACS and KIF1C) and with ataxia and neuropathy or AOA (PNKP). We also identified three families with autosomal dominant (AD) forms arising from de novo variants in KIF1A, CACNA1A, or ATP1A3, reinforcing the importance of differential diagnosis (AR vs. AD forms) in families with only one affected member. Moreover, 10 novel causal-variants were identified, and the detrimental effect of two splice-site variants confirmed through functional assays. Finally, by reviewing the molecular mechanisms, we speculated that regulation of cytoskeleton function might be impaired in spastic ataxia, whereas DNA repair is clearly associated with AOA. In conclusion, our study provided a genetic diagnosis for HCA families and proposed common molecular pathways underlying cerebellar neurodegeneration.

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.

Short-term effects of Dechlorane Plus on the earthworm Eisenia fetida determined by a systems biology approach

Dechlorane Plus (DP), a chlorinated flame retardant, has been widely detected in environmental matrices, especially in sediment and soil. DP has characteristics similar to persistent organic pollutants. However, no toxicity data of DP on terrestrial invertebrate are available. In this study, earthworms Eisenia fetida were exposed to 0.1, 1, 10, and 50mg/kg DP for 14 days. Lethality, oxidative stress and damage, neurotoxicity, and transcriptomic profiles of E. fetida were assessed on day 7 and day 14 of exposure. Results showed that the acute toxicity of DP was very low. However, DP exposure induced an increase in the oxidative stress markers malonaldehyde (MDA) and 8-Hydroxy-2'-deoxyguanosine (8-OHdG), and altered acetylcholinesterase (AChE) activities. High throughput sequencing-based transcriptomic analysis showed that DP exposure significantly altered gene expression and pathways related to antioxidant enzymes, stress responses, neurological dysfunctions, calcium binding, and signal transduction. The results from different toxicological endpoints indicate that DP toxicity on the earthworm is primarily through oxidative damage and neurotoxicity. Based on these results, we deduce that changes in oxidative stress and neurotoxicity might be the primary mechanisms of DP toxicity. This study provides insight into the toxicological effects of DP on earthworm model, and may be useful for risk assessment of DP on soil ecosystems.

Chaperone therapy for GM2 gangliosidosis: effects of pyrimethamine on β-hexosaminidase activity in Sandhoff fibroblasts

Sphingolipidoses are inherited genetic diseases due to mutations in genes encoding proteins involved in the lysosomal catabolism of sphingolipids. Despite a low incidence of each individual disease, altogether, the number of patients involved is relatively high and resolutive approaches for treatment are still lacking. The chaperone therapy is one of the latest pharmacological approaches to these storage diseases. This therapy allows the mutated protein to escape its natural removal and to increase its quantity in lysosomes, thus partially restoring the metabolic functions. Sandhoff disease is an autosomal recessive inherited disorder resulting from β-hexosaminidase deficiency and characterized by large accumulation of GM2 ganglioside in brain. No enzymatic replacement therapy is currently available, and the use of inhibitors of glycosphingolipid biosynthesis for substrate reduction therapy, although very promising, is associated with serious side effects. The chaperone pyrimethamine has been proposed as a very promising drug in those cases characterized by a residual enzyme activity. In this review, we report the effect of pyrimethamine on the recovery of β-hexosaminidase activity in cultured fibroblasts from Sandhoff patients.

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

The adult form of Sandhoff disease with the motor neuron disease phenotype is a rare neurodegenerative disorder caused by mutations in HEXB encoding the β-subunit of β-hexosaminidase, yet the properties of mutant β-subunits of the disease have not been fully determined. We identified a novel mutation (H235Y) in the β-sheet of the (β/α)₈-barrel domain, in addition to the previously reported P417L mutation that causes aberrant splicing, in a Japanese patient with the motor neuron disease phenotype. Enzyme assays, gel filtration studies and immunoprecipitation studies with HEK293 cells transiently expressing mutant β-subunits demonstrated that the H235Y mutation abolished both α-β and β-β dimer formation without increasing β-hexosaminidase activity, whereas other reported mutant β-subunits (Y456S, P504S or R533H) associated with the motor neuron disease phenotype formed dimers. Structural analysis suggested that the H235Y mutation in the β-sheet of the (β/α)₈-barrel domain changed the conformation of the β-subunit by causing a clash with the E288 side chain. In summary, H235Y is the first mutation in the β-sheet of the (β/α)₈-barrel domain of the β-subunit that abolishes α-β and β-β dimer formation; the presented patient is the second patient to exhibit the motor neuron disease phenotype with P417L and a non-functional allele of HEXB.

Integrated multiplex ligation dependent probe amplification (MLPA) assays for the detection of alterations in the HEXB, GM2A and SMARCAL1 genes to support the diagnosis of Morbus Sandhoff, M. Tay-Sachs variant AB and Schimke immuno-osseous dysplasia in humans

Multiplex ligation dependent probe amplification (MLPA) assays were designed for the genes HEXB (OMIM: 606873), GM2A (OMIM: 613109) and SMARCAL1 (OMIM: 606622) of humans. Two sets of synthetic MLPA probes for these coding exons were tested. Changes in copy numbers were detected as well as single nucleotide polymorphisms (SNPs) by complementary DNA sequence analyses. The MLPA method was shown to be reliable for mutation detection and identified five published and 12 new mutations. In all cases from a Morbus Sandhoff cohort of patients, exclusively one variation in copy number was observed and linked to a nucleotide alteration called c.1614-14C>A. This deletion comprised exons 1-5. One of these cases is described in detail. Deletions were neither detected in the GM2A nor the SMARCAL1 genes. The MLPA assays complement routine diagnostics for M. Sandhoff (OMIM: 268800), M. Tay-Sachs variant AB (OMIM: 272750) and Schimke immuno-osseous dysplasia (OMIM: 242900).

A novel HEXB mutation and its structural effects in juvenile Sandhoff disease

Mutations in HEXB, encoding the beta-subunit common to hexosaminidases A and B, cause the neurodegenerative condition, Sandhoff disease. A homozygous missense HEXB mutation (p. D459A) was discovered in six patients with a rare juvenile variant: we show that this disrupts a salt bridge between aspartate D459 and arginine 505 at the subunit interface; R505 mutations are reported in late-onset Sandhoff disease. Identification of D459A contributes to diagnosis and molecular understanding of attenuated Sandhoff disease variants.

Toxicogenomic analysis provides new insights into molecular mechanisms of the sublethal toxicity of 2,4,6-trinitrotoluene in Eisenia fetida

Xenobiotics such as explosives and pesticides released into the environment can have lethal and sublethal impacts on soil organisms such as earthworms with potential subsequent impacts at highertrophic levels. To better understand the molecular toxicological mechanisms of 2,4,6-trinitrotoluene (TNT), a commonly used explosive, in Eisenia fetida, earthworms were exposed to a gradient of TNT-spiked soils for 28 days and impacts on gene expression were examined using a 4032 cDNA microarray. Reproduction was increased at low doses of TNT, whereas high doses of TNT reduced juvenile production. On the basis of reproduction responses to TNT, four treatments, that is, control, 2, 10.6, and 38.7 mg/kg, were selected for gene expression studies in a balanced interwoven loop design microarray experiment in which the expression of 311 transcripts was significantly affected. Reverse-transcription quantitative polymerase chain reaction (RT-QPCR) data on 68 selected differentially and nondifferentially expressed transcripts showed a significant correlation with microarray results. The expression of genes involved in multiple biological processes was altered, including muscle contraction, neuronal signaling and growth, ubiquitinylation, fibrinolysis and coagulation, iron and calcium homeostasis, oxygen transport, and immunity. Chitinase activity assays confirmed down-regulation of chitinase genes as indicated by array and RT-QPCR data. An acute toxicity test provided evidence that dermal contact with TNT can cause bleeding, inflammation, and constriction, which may be explained by gene expression results. Sublethal doses of TNT affected the nervous system, caused blood disorders similar to methemoglobinemia, and weakened immunity in E. fetida.

Chronic GM2 gangliosidosis type Sandhoff associated with a novel missense HEXB gene mutation causing a double pathogenic effect

We identified a novel c.1556A>G transition in exon 12 of the HEXB gene associated with chronic Sandhoff's disease, changing a conserved aspartic acid to glycine at position 494 of the Hex beta-subunit; moreover, RT-PCR showed aberrant exon 12 skipping, causing a frame-shift and premature stop codon, consequent to the disruption of an exonic splicing enhancer motif by the mutation. These data suggest that the c.1556 A>G transition would affect both HEXB mRNA processing and biochemical properties of the beta-subunit.

Pyrimethamine as a potential pharmacological chaperone for late-onset forms of GM2 gangliosidosis

Late-onset GM2 gangliosidosis is composed of two related, autosomal recessive, neurodegenerative diseases, both resulting from deficiency of lysosomal, heterodimeric beta-hexosaminidase A (Hex A, alphabeta). Pharmacological chaperones (PC) are small molecules that can stabilize the conformation of a mutant protein, allowing it to pass the quality control system of the endoplasmic reticulum. To date all successful PCs have also been competitive inhibitors. Screening for Hex A inhibitors in a library of 1040 Food Drug Administration-approved compounds identified pyrimethamine (PYR (2,4-diamino 5-(4-chlorophenyl)-6-ethylpyrimidine)) as the most potent inhibitor. Cell lines from 10 late-onset Tay-Sachs (11 alpha-mutations, 2 novel) and 7 Sandhoff (9 beta-mutations, 4 novel) disease patients, were cultured with PYR at concentrations corresponding to therapeutic doses. Cells carrying the most common late-onset mutation, alphaG269S, showed significant increases in residual Hex A activity, as did all 7 of the beta-mutants tested. Cells responding to PC treatment included those carrying mutants resulting in reduced Hex heat stability and partial splice junction mutations of the inherently less stable alpha-subunit. PYR, which binds to the active site in domain II, was able to function as PC even to domain I beta-mutants. We concluded that PYR functions as a mutation-specific PC, variably enhancing residual lysosomal Hex A levels in late-onset GM2 gangliosidosis patient cells.

Pharmacological enhancement of beta-hexosaminidase activity in fibroblasts from adult Tay-Sachs and Sandhoff Patients

Tay-Sachs and Sandhoff diseases are lysosomal storage disorders that result from an inherited deficiency of beta-hexosaminidase A (alphabeta). Whereas the acute forms are associated with a total absence of hexosaminidase A and early death, the chronic adult forms exist with activity and protein levels of approximately 5%, and unaffected individuals have been found with only 10% of normal levels. Surprisingly, almost all disease-associated missense mutations do not affect the active site of the enzyme but, rather, inhibit its ability to obtain and/or retain its native fold in the endoplasmic reticulum, resulting in its retention and accelerated degradation. By growing adult Tay-Sachs fibroblasts in culture medium containing known inhibitors of hexosaminidase we have raised the residual protein and activity levels of intralysosomal hexosaminidase A well above the critical 10% of normal levels. A similar effect was observed in fibroblasts from an adult Sandhoff patient. We propose that these hexosaminidase inhibitors function as pharmacological chaperones, enhancing the stability of the native conformation of the enzyme, increasing the amount of hexosaminidase A capable of exiting the endoplasmic reticulum for transport to the lysosome. Therefore, pharmacological chaperones could provide a novel approach to the treatment of adult Tay-Sachs and possibly Sandhoff diseases.

The X-ray crystal structure of human beta-hexosaminidase B provides new insights into Sandhoff disease

Human lysosomal beta-hexosaminidases are dimeric enzymes composed of alpha and beta-chains, encoded by the genes HEXA and HEXB. They occur in three isoforms, the homodimeric hexosaminidases B (betabeta) and S (alphaalpha), and the heterodimeric hexosaminidase A (alphabeta), where dimerization is required for catalytic activity. Allelic variations in the HEXA and HEXB genes cause the fatal inborn errors of metabolism Tay-Sachs disease and Sandhoff disease, respectively. Here, we present the crystal structure of a complex of human beta-hexosaminidase B with a transition state analogue inhibitor at 2.3A resolution (pdb 1o7a). On the basis of this structure and previous studies on related enzymes, a retaining double-displacement mechanism for glycosyl hydrolysis by beta-hexosaminidase B is proposed. In the dimer structure, which is derived from an analysis of crystal packing, most of the mutations causing late-onset Sandhoff disease reside near the dimer interface and are proposed to interfere with correct dimer formation. The structure reported here is a valid template also for the dimeric structures of beta-hexosaminidase A and S.

Western blotting analysis of the beta-hexosaminidase alpha- and beta-subunits in cultured fibroblasts from cases of various forms of GM2 gangliosidosis

OBJECTIVES

The GM2 gangliosidoses are a group of genetic disorders caused by the accumulation of ganglioside GM2 in neuronal cells. We examined the alpha- and beta-subunits of beta-hexosaminidases by a non-radioisotopes detecting system to evaluate whether it was a useful method for understanding of the pathophysiologies of GM2 gangliosidoses.

MATERIALS AND METHODS

We investigated the alpha- and beta-subunits of beta-hexosaminidases in cultured fibroblasts from cases of various forms of GM2 gangliosidosis by means of Western blotting and a chemiluminescence detection system.

RESULTS

In a patient with infantile Tay-Sachs disease [HEXA genotype, Int5-SA(g-1-->t)/Int5-SA(g-1-->t)], the mature alpha-subunit was undetectable. In a patient with infantile Sandhoff disease (HEXB genotype, C534Y/C534Y), the mature beta-subunit was deficient. However, a small amount of the mature beta-subunit was detected in a patient with adult Sandhoff disease (HEXB genotype, R505Q(+I207V)/R505Q(+I207V)), which may have resulted in the residual enzyme activity and mild clinical course. Normal amounts of alpha- and beta-subunits were detected in a patient with GM2 activator deficiency.

CONCLUSION

This method is easy and sensitive for detecting target proteins, and is useful for clarification of the pathophysiologies of GM2 gangliosidoses.

Molecular basis of heat labile hexosaminidase B among Jews and Arabs

Genotyping individuals for Tay-Sachs disease (TSD) is mainly based on the heat lability of beta-hexosaminidase (Hex) A (alphabeta) and the heat stability of Hex B (betabeta). Mutations in the HEXB gene encoding the beta-subunits of Hex that result in heat-labile Hex B thus may lead to erroneous enzymatic genotyping regarding TSD. Utilizing single strand conformation polymorphism (SSCP) analysis for all 14 exons of HEXB followed by direct sequencing of aberrant fragments, we screened individuals whose Hex B was heat labile. A novel heat labile mutation, previously concluded to exist in the HEXB gene, was identified among Jews and Arabs as 1627 G-->A. One individual with heat labile Hex B (HLB) was negative for this novel mutation and for the known 1514 G-->A HLB mutation, proving that there exists at least one other unidentified HLB mutation. Based on these results, it is advisable to perform DNA tests for 1627 G-->A mutation in suspected HLB individuals.

Biochemical consequences of mutations causing the GM2 gangliosidoses

The hydrolysis of GM2-ganglioside is unusual in its requirements for the correct synthesis, processing, and ultimate combination of three gene products. Whereas two of these proteins are the alpha- (HEXA gene) and beta- (HEXB) subunits of beta-hexosaminidase A, the third is a small glycolipid transport protein, the GM2 activator protein (GM2A), which acts as a substrate specific co-factor for the enzyme. A deficiency of any one of these proteins leads to storage of the ganglioside, primarily in the lysosomes of neuronal cells, and one of the three forms of GM2-gangliosidosis, Tay-Sachs disease, Sandhoff disease or the AB-variant form. Studies of the biochemical impact of naturally occurring mutations associated with the GM2 gangliosidoses on mRNA splicing and stability, and on the intracellular transport and stability of the affected protein have provided some general insights into these complex cellular mechanisms. However, such studies have revealed little in the way of structure-function information on the proteins. It appears that the detrimental effect of most mutations is not specifically on functional elements of the protein, but rather on the proteins' overall folding and/or intracellular transport. The few exceptions to this generalization are missense mutations at two codons in HEXA, causing the unique biochemical phenotype known as the B1-variant, and one codon in both the HEXB and GM2A genes. Biochemical characterization of these mutations has led to the localization of functional residues and/or domains within each of the encoded proteins.

A Pro504 --> Ser substitution in the beta-subunit of beta-hexosaminidase A inhibits alpha-subunit hydrolysis of GM2 ganglioside, resulting in chronic Sandhoff disease

The GM2 gangliosidoses are caused by mutations in the genes encoding the alpha- (Tay-Sachs) or beta- (Sandhoff) subunits of heterodimeric beta-hexosaminidase A (Hex A), or the GM2 activator protein (AB variant), a substrate-specific co-factor for Hex A. Although the active site associated with the hydrolysis of GM2 ganglioside, as well as part of the binding site for the ganglioside-activator complex, is associated with the alpha-subunit, elements of the beta-subunit are also involved. Missense mutations in these genes normally result in the mutant protein being retained in the endoplasmic reticulum and degraded. The mutations associated with the B1-variant of Tay-Sachs are rare exceptions that directly affect residues in the alpha-active site. We have previously reported two sisters with chronic Sandhoff disease who were heterozygous for the common HEXB deletion allele. Cells from these patients had higher than expected levels of mature beta-protein and residual Hex A activity, approximately 20%. We now identify these patients' second mutant allele as a C1510T transition encoding a beta-Pro504 --> Ser substitution. Biochemical characterization of Hex A from both patient cells and cotransfected CHO cells demonstrated that this substitution (a) decreases the level of heterodimer transport out of the endoplasmic reticulum by approximately 45%, (b) lowers its heat stability, (c) does not affect its Km for neutral or charged artificial substrates, and (d) lowers the ratio of units of ganglioside/units of artificial substrate hydrolyzed by a factor of 3. We concluded that the beta-Pro504 --> Ser mutation directly affects the ability of Hex A to hydrolyze its natural substrate but not its artificial substrates. The effect of the mutation on ganglioside hydrolysis, combined with its effect on intracellular transport, produces chronic Sandhoff disease.

Adult Sandhoff's disease: R505Q and I207V substitutions in the HEXB gene of the first Japanese case

We describe a 31-year-old Japanese man with adult Sandhoff s disease presenting as spinocerebellar degeneration. There was a marked cerebellar atrophy on MRI, and proliferation of abundant PAS-positive foamy macrophages in the rectal mucosa. The activities of total beta-Hex, beta-Hex A, and beta-Hex B in leucocytes of the patient were 14%, 15%, and 6% of control values, respectively. However, oligosacchariduria or ultrastructural storage materials in liver tissue were nil. Direct sequencing of cDNA and genomic DNA, and restriction digestion revealed two different homozygous base substitutions in the HEXB gene: the G1514-->A substitution (R505Q) and the A619-->G substitution (1207V). The parents were consanguineous. His healthy mother, an enzymatic heterozygous carrier, was homozygous for 1207V, but heterozygous for R505Q mutation. Thus, the patient is probably homozygous for both base substitutions and a R505Q mutation may be linked to the phenotype of adult Sandhoff's disease.

Mutation of the gene encoding cellular retinaldehyde-binding protein in autosomal recessive retinitis pigmentosa

Inadequate levels of all-trans-retinol in the blood cause retinal dysfunction; hence, genes implicated in retinal vitamin-A metabolism represent candidates for inherited retinal degenerations. In the current study, molecular genetic analysis of a consanguineous pedigree segregating for non-syndromic autosomal recessive retinitis pigmentosa (arRP) indicated that the affected siblings were homozygous by descent for a G4763A nucleotide substitution in RLBP1, the gene encoding cellular retinaldehyde-binding protein (CRALBP). This substitution is predicted to replace an arginine with glutamine at residue 150. CRALBP is not expressed in photoreceptors but is abundant in the retinal pigment epithelium (RPE) and Müller cells of the neuroretina, where it carries 11-cis-retinol and 11-cis-retinaldehyde. When expressed in bacteria, recombinant CRALBP (rCRALBP) containing the R150Q substitution was less soluble than wild-type rCRALBP. Mutant rCRALBP was purified from the soluble cell lysate and the protein structure was verified by mass spectrometry. The mutant protein lacked the ability to bind 11-cis-retinaldehyde. These findings suggest that arRP in the current pedigree results from a lack of functional CRALBP, presumably leading to disruption of retinal vitamin-A metabolism.

Significance of two point mutations present in each HEXB allele of patients with adult GM2 gangliosidosis (Sandhoff disease) homozygosity for the Ile207-->Val substitution is not associated with a clinical or biochemical phenotype

The molecular defects in the HEXB gene encoding the common beta-subunit of lysosomal beta-hexosaminidase A (beta-Hex A, alpha beta) and beta-Hex B (beta beta) were investigated in a Portuguese family affected with late onset Sandhoff disease (GM2-gangliosidosis variant 0). This family comprised two unaffected daughters and three affected sibs who developed at about age 17 cerebellar ataxia and mental deficiency. Their parents were consanguineous and clinically asymptomatic. There was no detectable beta-Hex B activity and a profound reduction in the activity of beta-Hex A in the leukocytes and transformed lymphoid cell lines from the affected sibs. The expected intermediate values were observed in the parents as well as in one daughter and her children. Western analysis revealed the presence of reduced, but detectable amounts of mature beta-chain protein in cell lysates from the probands and intermediate levels in the parents. Nucleotide sequencing of amplified, reverse-transcribed beta-chain mRNA demonstrated the presence of two single point mutations: an A619 to G transition in exon 5 (Ile207-->Val), and a G1514 to A transition in exon 13 (Arg505-->Gln). Both of these two mutations have been previously linked to the adult form of Sandhoff disease in compound heterozygote patients. All three affected sibs were found to be homoallelic for both mutations. Interestingly, while the mother was heterozygous for each mutation, the father was homozygote for the A619-->G substitution and heterozygote for the G1514-->A transition. Since the father is homozygote for the A619-->G mutation but expresses a biochemical phenotype consistent with a carrier of Sandhoff disease and is clinically asymptomatic, this substitution is likely a neutral mutation. We confirmed this hypothesis by finding this transition present in 4 of 30 alleles from normal individuals. We conclude that homozygosity for the G1514-->A mutation is exclusively responsible for the adult form of Sandhoff disease in this family, and that the A619-->G substitution is not a deleterious mutation but rather a common HEXB polymorphism.

Bacterial chitobiase structure provides insight into catalytic mechanism and the basis of Tay-Sachs disease

Chitin, the second most abundant polysaccharide on earth, is degraded by chitinases and chitobiases. The structure of Serratia marcescens chitobiase has been refined at 1.9 A resolution. The mature protein is folded into four domains and its active site is situated at the C-terminal end of the central (beta alpha)8-barrel. Based on the structure of the complex with the substrate disaccharide chitobiose, we propose an acid-base reaction mechanism, in which only one protein carboxylate acts as catalytic acid, while the nucleophile is the polar acetamido group of the sugar in a substrate-assisted reaction. The structural data lead to the hypothesis that the reaction proceeds with retention of anomeric configuration. The structure allows us to model the catalytic domain of the homologous hexosaminidases to give a structural rationale to pathogenic mutations that underlie Tay-Sachs and Sandhoff disease.