Biochemical characterization of the Cys138Arg substitution associated with the AB variant form of GM2 gangliosidosis: evidence that Cys138 is required for the recognition of the GM2 activator/GM2 ganglioside complex by beta-hexosaminidase A

Atypical juvenile presentation of GM2 gangliosidosis AB in a patient compound-heterozygote for c.259G > T and c.164C > T mutations in the GM2A gene

G_M2-gangliosidosis, AB variant is an extremely rare autosomal recessive inherited disorder caused by mutations in the GM2A gene that encodes G_M2 ganglioside activator protein (GM2AP). GM2AP is necessary for solubilisation of G_M2 ganglioside in endolysosomes and its presentation to β-hexosaminidase A. Conversely GM2AP deficiency impairs lysosomal catabolism of G_M2 ganglioside, leading to its storage in cells and tissues. We describe a 9-year-old child with an unusual juvenile clinical onset of G_M2-gangliosidosis AB. At the age of 3 years he presented with global developmental delay, progressive epilepsy, intellectual disability, axial hypertonia, spasticity, seizures and ataxia, but without the macular cherry-red spots typical for G_M2 gangliosidosis. Brain MRI detected a rapid onset of diffuse atrophy, whereas whole exome sequencing showed that the patient is a compound heterozygote for two mutations in GM2A: a novel nonsense mutation, c.259G > T (p.E87X) and a missense mutation c.164C > T (p.P55L) that was recently identified in homozygosity in patients of a Saudi family with a progressive chorea-dementia syndrome. Western blot analysis showed an absence of GM2AP in cultured fibroblasts from the patient, suggesting that both mutations interfere with the synthesis and/or folding of the protein. Finally, impaired catabolism of G_M2 ganglioside in the patient's fibroblasts was demonstrated by metabolic labeling with fluorescently labeled G_M1 ganglioside and by immunohistochemistry with anti-G_M2 and anti-G_M3 antibodies. Our observation expands the molecular and clinical spectrum of molecular defects linked to G_M2-gangliosidosis and suggests novel diagnostic approach by whole exome sequencing and perhaps ganglioside analysis in cultured patient's cells.

GM2 gangliosidosis AB variant: novel mutation from India - a case report with a review

Background

GM2 gangliosidosis-AB variants a rare autosomal recessive neurodegenerative disorder occurring due to deficiency of GM2 activator protein resulting from the mutation in GM2A gene. Only seven mutations in nine cases have been reported from different population except India.

Case presentation

Present case is a one year old male born to 3rd degree consanguineous Indian parents from Maharashtra. He was presented with global developmental delay, hypotonia and sensitive to hyperacusis. Horizontal nystagmus and cherry red spot was detected during ophthalmic examination. MRI of brain revealed putaminal hyperintensity and thalamic hypointensity with some unmyelinated white matter in T2/T1 weighted images. Initially he was suspected having Tay-Sachs disease and finally diagnosed as GM2 gangliosidosis, AB variant due to truncated protein caused by nonsense mutation c.472 G > T (p.E158X) in GM2Agene.

Conclusion

Children with phenotypic presentation as GM2 gangliosidosis (Tay-Sachs or Sandhoff disease) and normal enzyme activity of β-hexosaminidase-A and -B in leucocytes need to be investigated for GM2 activator protein deficiency.

Hexosaminidase assays

beta-Hexosaminidases (EC 3.2.1.52) are lysosomal enzymes that remove terminal beta-glycosidically bound N-acetylglucosamine and N-acetylgalactosamine residues from a number of glycoconjugates. Reliable assay systems are particularly important for the diagnosis of a family of lysosomal storage disorders, the GM2 gangliosidoses that result from inherited beta-hexosaminidase deficiency. More recently, aberrant hexosaminidase levels have also been found to be associated with a variety of inflammatory diseases. Apart from patient testing and carrier screening, practical in vitro assays are indispensable for the characterization of knock-out mice with potentially altered hexosaminidase activities, for detailed structure-function studies aimed at elucidating the enzymatic mechanism, and to characterize newly described enzyme variants from other organisms. The purpose of this article is to discuss convenient hexosaminidase assay procedures for these and other applications, using fluorogenic or chromogenic artificial substrates as well as the physiological glycolipid substrate GM2. Attempts are also made to provide an overview of less commonly used alternative techniques and to introduce recent developments enabling high-throughput screening for enzyme inhibitors.

Ligand extraction properties of the GM2 activator protein and its interactions with lipid vesicles

The GM2 activator protein (GM2AP) is an accessory protein required for the enzymatic conversion of GM2 to GM3 by hydrolases in the lysosomal compartments of cells. Here, GM2AP interactions with lipid vesicles are investigated by sucrose-loaded vesicle sedimentation and gel filtration assays, and the effects of pH and lipid composition on membrane binding and lipid extraction are characterized. The sedimentation experiments allow for facile quantification of the percentage of protein in solution and on the bilayer surface, with detailed analysis of the protein:lipid complex that remains in solution. Optimum binding and ligand extraction is found for pH 4.8 where <15% of the protein remains surface associated regardless of the lipid composition. In addition to extracting GM2, we find that GM2AP readily extracts dansyl-headgroup-labeled lipids as well as other phospholipids from vesicles. The ability of GM2AP to extract dansyl-DHPE from vesicles is altered by pH and the specific ligand GM2. Although the unique endosomal lipid, bis(monoacylglycero)phosphate, is not required for ligand extraction, it does enhance the extraction efficiency of GM2 when cholesterol is present in the vesicles.

Identification and tissue-specific expression of amphioxus GM2 activator protein gene from amphioxus Branchiostoma belcheri

An amphioxus cDNA, AmphiGM2AP, encoding GM2 activator protein was isolated from the gut cDNA library of Branchiostoma belcheri. It is 907 bp long, and its longest open reading frame codes for a precursor protein consisting of 242 amino acid residues with a signal peptide of 14 amino acids. The deduced amino acid sequence includes a conserved domain typical of GM2APs between residues 53 and 224, a single N-linked glycosylation site at position 65 and 8 conserved cysteines. Phylogenetic analysis showed that amphiGM2AP forms a club together with invertebrate GM2APs, indicating that AmphiGM2AP is evolutionarily closely related to invertebrate GM2APs rather than vertebrate ones. Both Northern blotting and in situ hybridization histochemistry analyses revealed a tissue-specific expression pattern of AmphiGM2AP in adult amphioxus with the strongest expression in the digestive system, which is in contrast to the widespread expression pattern of human, mouse and sheep GM2AP genes. It is suggested that AmphiGM2AP is possibly involved in the take-in of digested food components like lipid molecules.

The enzyme-binding region of human GM2-activator protein

The GM2-activator protein (GM2AP) is an essential cofactor for the lysosomal degradation of ganglioside GM2 by beta-hexosaminidase A (HexA). It mediates the interaction between the water-soluble exohydrolase and its membrane-embedded glycolipid substrate at the lipid-water interface. Functional deficiencies in this protein result in a fatal neurological storage disorder, the AB variant of GM2 gangliosidosis. In order to elucidate this cofactor's mode of action and identify the surface region of GM2AP responsible for binding to HexA, we designed several variant forms of this protein and evaluated the consequences of these mutations for lipid- and enzyme-binding properties using a variety of biophysical and functional studies. The point mutants D113K, M117V and E123K showed a drastically decreased capacity to stimulate HexA-catalysed GM2 degradation. However, surface plasmon resonance (SPR) spectroscopy showed that the binding of these variants to immobilized lipid bilayers and their ability to solubilize lipids from anionic vesicles were the same as for the wild-type protein. In addition, a fluorescence resonance energy transfer (FRET)-based assay system showed that these variants had the same capacity as wild-type GM2AP for intervesicular lipid transfer from donor to acceptor liposomes. The concentration-dependent effect of these variants on hydrolysis of the synthetic substrate 4-methylumbelliferyl-2-acetamido-2-deoxy-6-sulfo-beta-D-glucopyranoside (MUGS) indicated a weakened association with the enzyme's alpha subunit. This identifies the protein region affected by these mutations, the single short alpha helix of GM2AP, as the major determinant for the interaction with the enzyme. These results further confirm that the function of GM2AP is not restricted to a biological detergent that simply disrupts the membrane structure or lifts the substrate out of the lipid plane. In contrast, our data argue in favour of the critical importance of distinct activator-hexosaminidase interactions for GM2 degradation, and corroborate the view that the activator/lipid complex represents the true substrate for the degrading enzyme.

Mutation of the GM2 activator protein in a feline model of GM2 gangliosidosis

The G(M2) activator protein is required for successful degradation of G(M2) ganglioside by the A isozyme of lysosomal beta-N-acetylhexosaminidase (EC 3.2.1.52). Deficiency of the G(M2) activator protein leads to a relentlessly progressive accumulation of G(M2) ganglioside in neuronal lysosomes and subsequent fatal deterioration of central nervous system function. G(M2) activator deficiency has been described in humans, dogs and mice. This manuscript reports the discovery and characterization of a feline model of G(M2) activator deficiency that exhibits many disease traits typical of the disorder in other species. Cats deficient in the G(M2) activator protein develop clinical signs at approximately 14 months of age, including motor incoordination and exaggerated startle response to sharp sounds. Affected cats exhibit central nervous system abnormalities such as swollen neurons, membranous cytoplasmic bodies, increased sialic acid content and elevated levels of G(M2) ganglioside. As is typical of G(M2) activator deficiency, hexosaminidase A activity in tissue homogenates appears normal when assayed with a commonly used synthetic substrate. When the G(M2) activator cDNA was sequenced from normal and affected cats, a deletion of 4 base pairs was identified as the causative mutation, resulting in alteration of 21 amino acids at the C terminus of the G(M2) activator protein.

Structural analysis of lipid complexes of GM2-activator protein

The GM2-activator protein (GM2-AP) is a small lysosomal lipid transfer protein essential for the hydrolytic conversion of ganglioside GM2 to GM3 by beta-hexosaminidase A. The crystal structure of human apo-GM2-AP is known to consist of a novel beta-cup fold with a spacious hydrophobic interior. Here, we present two new structures of GM2-AP with bound lipids, showing two different lipid-binding modes within the apolar pocket. The 1.9A structure with GM2 bound shows the position of the ceramide tail and significant conformational differences among the three molecular copies in the asymmetric unit. The tetrasaccharide head group is not visible and is presumed to be disordered. However, its general position could be established through modeling. The structure of a low-pH crystal, determined at 2.5A resolution, has a significantly enlarged hydrophobic channel that merges with the apolar pocket. Electron density inside the pocket and channel suggests the presence of a trapped phospholipid molecule. Structure alignments among the four crystallographically unique monomers provide information on the potential role for lipid binding of flexible chain segments at the rim of the cavity opening. Two discrete orientations of the S130-T133 loop define an open and a closed configuration of the hydrophobic channel that merges with the apolar pocket. We propose: (i) that the low-pH structure represents an active membrane-binding conformation; (ii) that the mobile S130-T133 loop serves as a gate for passage of ligand into the apolar pocket; and (iii) that this loop and the adjacent apolar V59-W63 loop form a surface patch with two exposed tryptophan residues that could interface with lipid bilayers.

Crystal structure of human beta-hexosaminidase B: understanding the molecular basis of Sandhoff and Tay-Sachs disease

In humans, two major beta-hexosaminidase isoenzymes exist: Hex A and Hex B. Hex A is a heterodimer of subunits alpha and beta (60% identity), whereas Hex B is a homodimer of beta-subunits. Interest in human beta-hexosaminidase stems from its association with Tay-Sachs and Sandhoff disease; these are prototypical lysosomal storage disorders resulting from the abnormal accumulation of G(M2)-ganglioside (G(M2)). Hex A degrades G(M2) by removing a terminal N-acetyl-D-galactosamine (beta-GalNAc) residue, and this activity requires the G(M2)-activator, a protein which solubilizes the ganglioside for presentation to Hex A. We present here the crystal structure of human Hex B, alone (2.4A) and in complex with the mechanistic inhibitors GalNAc-isofagomine (2.2A) or NAG-thiazoline (2.5A). From these, and the known X-ray structure of the G(M2)-activator, we have modeled Hex A in complex with the activator and ganglioside. Together, our crystallographic and modeling data demonstrate how alpha and beta-subunits dimerize to form either Hex A or Hex B, how these isoenzymes hydrolyze diverse substrates, and how many documented point mutations cause Sandhoff disease (beta-subunit mutations) and Tay-Sachs disease (alpha-subunit mutations).

Eight novel mutations in the HEXA gene

PURPOSE

To characterize novel mutations in the HEXA gene (alpha-subunit beta-hexosaminidase A).

METHODS

Subjects included participants in the California Tay-Sachs disease prevention program. DNA samples from 49 subjects (47 enzymatically defined carriers and 2 disease afflicted) who were negative for the four common disease-associated and the two pseudodeficient mutations, were subjected to single-strand conformation polymorphism (SSCP) analysis over 14 exons.

RESULTS

Targeted sequencing of the 39 electrophoretic variants from SSCP analysis revealed eight novel and deleterious mutations and 31 with previously described mutations. Six novel mutations were found in non-Jewish carriers, and two were found in two patients with infantile Tay-Sachs disease.

CONCLUSION

Identification of these eight novel mutations provides additional insight to the mutational spectrum for the HEXA gene. Furthermore, this knowledge should enhance diagnosis and prognosis for Tay-Sachs disease, carrier identification, and fundamental studies in structure/function relationships between this gene and its enzymatic product.

Crystal structure of human GM2-activator protein with a novel beta-cup topology

GM2 activator protein (GM2-AP) belongs to a small group of non- enzymatic lysosomal proteins that act as cofactors in the sequential degradation of gangliosides. It has been postulated that GM2-AP extracts single GM2 molecules from membranes and presents them in soluble form to beta-hexosaminidase A for cleavage of N-acetyl-d-galactosamine and conversion to GM3. The high affinity of GM2-AP for GM2 is based on specfic recognition of the oligosaccharide moiety as well as the ceramide lipid tail. Genetic defects in GM2-AP result in an atypical form of Tay-Sachs disease known as variant AB GM2 gangliosidosis. The 2.0 A resolution crystal structure of GM2-AP reported here reveals a previously unobserved fold whose main feature is an eight-stranded cup-shaped anti-parallel beta-pleated sheet. The striking feature of the GM2-AP structure is that it possesses an accessible central hydrophobic cavity rather than a buried hydrophobic core. The dimensions of this cavity (12 Ax14 Ax22 A) are suitable for binding 18-carbon lipid acyl chains. Flexible surface loops and a short alpha-helix decorate the mouth of the beta-cup and may control lipid entry to the cavity.

The GM2 gangliosidoses databases: allelic variation at the HEXA, HEXB, and GM2A gene loci

The GM2 gangliosidoses are a group of recessive disorders characterized by accumulation of GM2 ganglioside in neuronal cells. The genes responsible for these disorders are HEXA (Tay-Sachs disease and variants), HEXB (Sandhoff disease and variants), and GM2A (AB variant of GM2 gangliosidosis). We report the establishment of three relational locus-specific databases recording allelic variation at the HEXA, HEXB, and GM2A genes and accessed at the GM2 gangliosidoses home page (http://data.mch.mcgill.ca/gm2-gangliosidoses). Submission forms are available for the addition of new mutations to the databases. The databases are available online for users to search and retrieve information about specific alleles by a number of fields describing mutations, phenotypes, or author(s).

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.

Catabolism of asialo-GM2 in man and mouse. Specificity of human/mouse chimeric GM2 activator proteins

Tay-Sachs disease is an inborn lysosomal disease characterized by excessive cerebral accumulation of GM2. The catabolism of GM2 to GM3 in man requires beta-hexosaminidase A (HexA) and a protein cofactor, the GM2 activator. Thus, Tay-Sachs disease can be caused by the deficiency of either HexA or the GM2 activator. The same cofactor found in mouse shares 74.1% amino acid identity (67% nucleotide identity) with the human counterpart. Between the two activators, the mouse GM2 activator can effectively stimulate the hydrolysis of both GM2 and asialo-GM2 (GA2) by HexA and, to a lesser extent, also stimulate HexB to hydrolyze GA2, whereas the human activator is ineffective in stimulating the hydrolysis of GA2 (Yuziuk, J. A., Bertoni, C., Beccari, T., Orlacchio, A., Wu, Y.-Y., Li, S.-C., and Li, Y.-T. (1998) J. Biol. Chem. 273, 66-72). To understand the role of these two activators in stimulating the hydrolyses of GM2 and GA2, we have constructed human/mouse chimeric GM2 activators and studied their specificities. We have identified a narrow region (Asn(106)-Tyr(114)) in the mouse cDNA sequence that might be responsible for stimulating the hydrolysis of GA2. Replacement of the corresponding site in the human sequence with the specific mouse sequence converted the ineffective human activator into an effective chimeric protein for stimulating the hydrolysis of GA2. This chimeric activator protein, like the mouse protein, is also able to stimulate the hydrolysis of GA2 by HexB. The mouse model of human type B Tay-Sachs disease recently engineered by the targeted disruption of the Hexa gene showed less severe clinical manifestation than found in human patients. This has been considered to be the result of the catabolism of GM2 via converting it to GA2 and further hydrolysis of GA2 to lactosylceramide by HexB with the assistance of mouse GM2 activator protein. The chimeric activator protein that bears the characteristics of the mouse GM2 activator may therefore be able to induce an alternative catabolic pathway for GM2 in human type B Tay-Sachs patients.

The GM2 activator protein, a novel inhibitor of platelet-activating factor

The GM2 activator protein is required as a substrate-specific cofactor for beta-hexosaminidase A to hydrolyze GM2 ganglioside. The GM2 activator protein reversibly binds and solubilizes individual GM2 ganglioside molecules, making them available as substrate. Although GM2 ganglioside is the strongest binding ligand for the activator protein, it can also bind and transport between membranes a series of other glycolipids, even at neutral pH. Biosynthetic studies have shown that a large portion of newly synthesized GM2 activator molecules are not targeted to the lysosome, but are secreted and can then be recaptured by other cells through a carbohydrate independent mechanism. Thus, the GM2 activator protein may have other in vivo functions. We found that the GM2 activator protein can inhibit, through specific binding, the ability of platelet activating factor (PAF) to stimulate the release of intracellular Ca2+ pools by human neutrophils. PAF is a biologically potent phosphoacylglycerol. Inhibitors for PAF's role in the pathogenesis of inflammatory bowel disease and asthma have been sought as potential therapeutic agents. The inherent stability and protease resistance of the small, monomeric GM2 activator protein, coupled with the ability to produce large quantities of the functional protein in transformed bacteria, suggest it may serve as such an agent.

The GM2 activator protein, its roles as a co-factor in GM2 hydrolysis and as a general glycolipid transport protein

Although there is only one documented function carried out by the GM2 activator protein in the lysosome, new information suggests that other less obvious roles may also be played by this protein in vivo. This information includes data demonstrating that the GM2 activator is a secretory, as well as a lysosomal protein, and that cells possess a carbohydrate-independent mechanism to re-capture the activator, with or without bound lipid, from the extracellular fluid. Additionally the GM2 activator has been shown to bind, solubilize and transport a broad spectrum of lipid molecules, such as glycolipids, gangliosides and at least one phosphoacylglycerol, between liposomes. At pH 7 the GM2 activator's rate of lipid transport is reduced by only 50% from its maximum rate which is achieved at approx. pH 5, suggesting that the GM2 activator may serve as a general intra- and/or inter-cellular lipid transport protein in vivo. Since the late 1970s the lysosomal form of the GM2 activator has been known to act as a substrate-specific co-factor for the hydrolysis of GM2 ganglioside by beta-hexosaminidase A. Gangliosides are a class of negatively charged glycolipids particularly abundant in neuronal cells which have been linked to numerous in vivo functions, such as memory formation and signal transduction events. Deficiency of the GM2 activator protein results in the storage of GM2 ganglioside and severe neurological disease, the AB-variant form of GM2 gangliosidosis, usually culminating in death before the age of 4 years. The exact mode-of-action of the GM2 activator in its role as a co-factor, and its specificity for various glycolipids are currently matters of debate in the literature.