A new variant of type-AB GM2-gangliosidosis

Biochemical and molecular characterization of mutant hexosaminidase A in a Turkish family

BACKGROUND

Tay-Sachs disease is a form of monosialoganglioside triaose (GM2) gangliosidosis that results from the mutations in the alpha-subunit gene of hexosaminidase A. In the B1 variant, the active site of the alpha-subunit of the enzyme is thought to be affected. In the present study, a patient who had previously been diagnosed as a B1 variant is further analyzed. The patient's parents and brother were also analyzed.

METHODS

Single-stranded conformational polymorphism (SSCP) and DNA sequencing analysis were conducted in all cases. In addition, hexosaminidase A (Hex A) was isolated from leukocyte homogenates of the patient's parents and brother using DE 52 ion-exchange chromatography, and thermostability analyses of the isolated enzymes were performed.

RESULTS

Hexosaminidase A of the parents was found to be more thermostable than normal Hex A. DNA sequencing analysis revealed a 12-bp deletion mutation in exon 10 of the Hex A gene. The patient was a homozygote and the parents were heterozygotes for the mutation, which could also be observed at the DNA double strands by SSCP analysis. These deleted bases are located within the catalytic domain of the alpha-subunit.

CONCLUSIONS

The 12-bp deletion mutation in exon 10 of Hex A is responsible for the increased thermostability of the enzyme. Considering this mutation has previously been found in a Turkish Tay-Sachs patient, the patient in the present study may have another mutation on the Hex B gene that causes decreased thermostability of the enzyme. Thermal inactivation assay may not be sufficient for a correct diagnosis in such unusual cases.

Role of beta Arg211 in the active site of human beta-hexosaminidase B

Tay-Sachs or Sandhoff disease results from a deficiency of either the alpha- or the beta-subunits of beta-hexosaminidase A, respectively. These evolutionarily related subunits have been grouped with the "Family 20" glycosidases. Molecular modeling of human hexosaminidase has been carried out on the basis of the three-dimensional structure of a bacterial member of Family 20, Serratia marcescens chitobiase. The primary sequence identity between the two enzymes is only 26% and restricted to their active site regions; therefore, the validity of this model must be determined experimentally. Because human hexosaminidase cannot be functionally expressed in bacteria, characterization of mutagenized hexosaminidase must be carried out using eukaryotic cell expression systems that all produce endogenous hexosaminidase activity. Even small amounts of endogenous enzyme can interfere with accurate K(m) or V(max) determinations. We report the expression, purification, and characterization of a C-terminal His(6)-tag precursor form of hexosaminidase B that is 99.99% free of endogenous enzyme from the host cells. Control experiments are reported confirming that the kinetic parameters of the His(6)-tag precursor are the same as the untagged precursor, which in turn are identical to the mature isoenzyme. Using highly purified wild-type and Arg(211)Lys-substituted hexosaminidase B, we reexamine the role of Arg(211) in the active site. As we previously reported, this very conservative substitution nevertheless reduces k(cat) by 500-fold. However, the removal of all endogenous activity has now allowed us to detect a 10-fold increase in K(m) that was not apparent in our previous study. That this increase in K(m) reflects a decrease in the strength of substrate binding was confirmed by the inability of the mutant isozyme to efficiently bind an immobilized substrate analogue, i.e., a hexosaminidase affinity column. Thus, Arg(211) is involved in substrate binding, as predicted by the chitobiase model, as well as catalysis.

Molecular basis of an adult form of beta-hexosaminidase B deficiency with motor neuron disease

A patient (KL) with progressive motor neuron disease associated with partial Hex A (alpha beta) and no Hex B (beta beta) activity, synthesized beta-chains which only associated with alpha-chains. To identify the molecular basis of this inability of beta-chains to self associate, RNA from cultured fibroblasts was reverse transcribed, the cDNA encoding the beta-chain amplified by polymerase chain reaction, subcloned, and sequenced to reveal two types of single missense mutation. The first mutation, (Type I) 619A----G, was paternally inherited and converted a 207IIe----Val in a highly conserved region believed to be associated with catalytic activity and activator protein binding. Biochemical evidence for impaired activator protein binding was obtained by purifying Hex A from KL urine and demonstrating a greater than 50% reduction of in vitro GM2 hydrolysis compared to normal urinary Hex A. In other cDNA species (Type II), a maternally inherited 1367A----C mutation converted 456Tyr----Ser in another highly conserved region of the beta-chain and we propose that this mutation leads to the inability of the beta-chains to self associate and thus reach maturity. These same cDNA species contained a second 362A----G mutation which converted 121Lys----Arg, but is apparently a polymorphism since it also occurs in some normal subjects. We propose that the patient is a compound heterozygote in which a combination of no self-association of the mutant beta-chains and impaired activator protein binding to alpha-beta (mutant) (Hex A) required for GM2 hydrolysis result in total beta-Hex B deficiency and slow accumulation of GM2 ganglioside, primarily in motor neurons.

A new point mutation within exon 5 of beta-hexosaminidase alpha gene in a Japanese infant with Tay-Sachs disease

A new point mutation within exon 5 of beta-hexosaminidase alpha subunit gene (guanine509----adenine; arginine170----glutamine) has been identified as being responsible for the typical clinical and enzymological phenotype of infantile Tay-Sachs disease in a Japanese infant. Expression of the mutant enzyme protein in the COS I cell system indicated that it is catalytically inactive and also is unstable. The patient is a compound heterozygote, and the exact abnormality in the other allele could not be identified except that it is not any of the other nine known mutations of the beta-hexosaminidase alpha. The data collectively suggest that the other allele is not producing stable messenger RNA (mRNA). The rapidly increasing number of mutations responsible for clinical and enzymological phenotypes and the very large number of statistically possible combinations among them for compound heterozygosity pose a serious pragmatic problem for classification and nomenclature of this group of rare genetic disorders.

GM2-gangliosidosis B1 variant: a wide geographic and ethnic distribution of the specific beta-hexosaminidase alpha chain mutation originally identified in a Puerto Rican patient

A point mutation within exon 5 of beta-hexosaminidase alpha chain gene was identified earlier in a Puerto Rican patient with GM2-gangliosidosis B1 variant (the DN-allele) [K. Ohno and K. Suzuki: J. Neurochem. 50:316-318, 1988]. Oligonucleotide probes designed to detect either the normal or the DN-allele showed that four additional patients carried the same mutation. These patients were of Italian, French, Spanish, and English/Italian/Hungarian origin. Three of them, as well as our original patient, were compound heterozygotes with positive signals for both the mutant and normal probes, while the Spanish patient was positive only for the DN-allele. A patient from Czechoslovakia was negative for the DN-allele. Thus, the specific mutation originally found in the Puerto Rican patient has a surprisingly wide geographic and ethnic distribution. This mutation can account for the B1 variant phenotype in five of the six B1 variant patients so far examined.

Mutation in GM2-gangliosidosis B1 variant

Fibroblasts from a patient with GM2-gangliosidosis B1 variant contained mRNA of normal size but in reduced quantity for the beta-hexosaminidase alpha subunit. The nucleotide sequence of a cDNA clone that included the entire protein coding sequence was completely normal except for a single base substitution from G to A at no. 533, resulting in a change from arginine to histidine at amino acid no. 178. The same mutation was found in two other cDNA clones. The position of the mutation is approximately 90 amino acids from the N-terminus of the mature, processed enzyme. Computer analysis predicted substantial alterations in the secondary structure of the enzyme protein. These results provide new insight into functional domains of this enzyme.

Assay for cerebroside sulfate (sulfatide) sulfatase in cultured skin fibroblasts with the natural activator protein

A simple procedure was developed to assay the ability of arylsulfatase A in extracts of cultured skin fibroblasts to degrade the natural substrate, sulfatide, in the presence of the physiological activator protein but without detergents. Inhibitory substances were removed by dialysis and by batch-wise ion-exchange chromatography. The enzyme recoveries during purification were monitored with a newly developed method that employs the chromogenic substrate 4-nitrocatecholsulfate at an incubation temperature of 4 degrees C. The residual sulfatidase activities determined with this procedure in fibroblasts from patients with various forms of MLD correlated well with the severity of the disease.

Biochemical basis of type AB GM2 gangliosidosis in a Japanese spaniel

The biochemical basis of a case of GM2 gangliosidosis in a Japanese Spaniel was studied. This dog had a massive accumulation of GM2 ganglioside in the brain. The beta-hexosaminidase activity in this affected dog brain was approximately 12 times higher than that of normal brain. However, the activity toward p-nitrophenyl-6-sulfo-2-acetamido-2-deoxyglucopyranoside was only four times higher in the affected brain than in normal brain. The GM2 activator preparation obtained from the normal dog brain could stimulate the hydrolysis of GM2 ganglioside by beta-hexosaminidase isolated from the affected dog. However, the corresponding activator fraction from the affected dog could not stimulate such a reaction. It was concluded that the biochemical basis of the GM2 gangliosidosis in this Japanese Spaniel was due to the attenuation in the stimulatory activity of GM2 activator. This case represents the first animal form similar to the activator deficiency (or defect) of Type AB GM2 gangliosidosis in humans.

Regulation of GM2 ganglioside metabolism in cultured cells

GM2-ganglioside (II3NeuAcGgOse3Cer) is a minor component of adult nervous tissue, but is probably an oncofetal antigen. Its biological role is unknown, but several lines of evidence indicate its potential role in cell adhesion both in the retina and in oligodendrocytes. The biosynthesis of GM2-ganglioside appears to be tightly regulated, since it is a key intermediate in complex ganglioside synthesis. The specific GM3: hexosaminyl-transferase is activated under conditions which activate cyclic AMP-dependent protein kinase, and cell transformation with retroviruses inactivates it. Catabolism of GM2 requires the concerted action of three gene products (alpha-chain, beta-chain and activator protein in a thermolabile alpha beta 2 AP complex referred to as HexA). Defects in either three components results in the neuronal storage of GM2 ganglioside and the manifestations of Tay-Sachs Disease in children or motor neuron disease in adults.

Preliminary evidence for a processing error in the biosynthesis of Gaucher activator in mucolipidosis disease types II and III

Activator protein (AP), which stimulated fibroblast sphingomyelinase activity, was isolated from the spleen of a patient with Gaucher's disease type I by the combined techniques of heat and alcohol denaturation, DEAE-cellulose column chromatography, gel filtration, preparative polyacrylamide-gel electrophoresis and decyl-agarose chromatography. Urea/sodium dodecyl sulphate (SDS)/polyacrylamide-gel electrophoresis showed two bands, one with an Mr of approx. 3,000 and the other with an Mr of 5,000-6,500. Similarly, SDS/polyacrylamide-gel electrophoresis performed in the absence of urea revealed the presence of two components, one of which adsorbed to a concanavalin A (Con A) column. Both components stimulated sphingomyelinase activity. On a non-denaturing polyacrylamide gel containing Triton X-100, four major components, two of which bound to Con A, were detected with the dye Stains-All. Cross-reacting material (CRM) to polyclonal Gaucher spleen AP antibodies was detected in normal fibroblasts and in fibroblasts from patients with sphingomyelinase and beta-glucocerebrosidase deficiency states (Niemann-Pick and Gaucher's diseases respectively). CRM in normal fibroblasts adsorbed to Con A columns and had the same mobility on SDS/polyacrylamide-gel electrophoresis as Con A-adsorbing Gaucher spleen AP. Normal AP was not observed in mucolipidosis type II (I-cell disease) fibroblasts; instead, extracts from these cells revealed the presence of two closely migrating bands with higher Mr values than normal fibroblast CRM. Furthermore, extracts of media from I-cell fibroblast cultures, but not from control or Gaucher fibroblast cultures, contained AP activity towards sphingomyelinase and beta-glucocerebrosidase. Fibroblasts from a patient with mucolipidosis type III (pseudo-Hurler polydystrophy) showed an intermediate pattern consisting of normal as well as the higher-Mr CRM. Our data provide evidence for the existence of AP in cultured skin fibroblasts and suggest that these proteins may be targetted to the lysosome by post-translational modification in a similar manner to that reported for lysosomal enzymes.

Genetic complementation in somatic cell hybrids of four variants of infantile GM2 gangliosidosis

Cell hybridizations between fibroblasts of four variants (B, O, AB, and B1) of infantile GM2 gangliosidosis were performed. Cocultivated as well as hybrid cells were analyzed for their capability to degrade exogenously added [3H]-GM2. Hybridization of variant AB fibroblasts with fibroblasts of variant O, variant B, or variant B1 resulted in an enhanced rate of GM2 hydrolysis, showing intergenic complementation. Similar restoration of GM2 catabolism was observed after hybridization of variant B1 cells with variant O, but not with variant B cells. These results indicate that B1 cells carry a mutation in the gene locus for the alpha-subunit of beta-hexosaminidase. Studies of the processing of immature enzyme in variant B1 cells showed the presence of alpha-precursors and mature alpha-chains, but at a lower level as compared to normal cells.

Glycolipid-binding proteins

Proteins which bind glycolipids with high specificity are tentatively divided into two groups. One group consists of activator proteins involved in the catabolism of glycolipids by acid lysosomal hydrolases. Two activator proteins, GM2-activator and sphingolipid activator protein-1, are critically appraised on their glycolipid-binding properties and on their activity to facilitate the transfer of glycolipids. These proteins are glycoproteins localized in the lysosomes. Their molecular weights are in a range of 21 000-27 000, and isoelectric points are 4-5. Glycolipid transfer protein (GLTP) is included in the other group. GLTP purified from pig brain has a molecular weight of about 20 000 and an isoelectric point of 8.3. GLTP facilitates the transfer of various glycosphingolipids and glyceroglycolipids between membranes. The protein does not facilitate the transfer of phospholipids or cholesterol. GLTP binds galactosylceramide. The galactosylceramide-GLTP complex participates in the transfer reaction as the intermediate. Each protein in both groups binds glycolipids with a characteristic specificity to the sugar moiety. A stoichiometry of 1 mol of lipid per mol of protein has been found in all three proteins. Proteins in both groups seem to have a hydrophobic region on their surface, since all three proteins have been efficiently purified by hydrophobic chromatography.

Late onset GM2 gangliosidosis: an alpha-locus genetic compound with near normal hexosaminidase activity

A non-Jewish child with late onset GM2 gangliosidosis is described. Tissues from the patient had near normal hexosaminidase A (hex A) activity using 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside (MU-glcNAc) as substrate, and deficient activity when assayed with the 6-sulphate derivative of MU-glcNAc (MU-glcNAcS) or GM2 in the presence of activator. We present evidence that this patient is a genetic compound for different alpha-subunit mutations. The father's tissues have hex A activity in the heterozygote range when assayed with MU-glcNAcS, but normal activity using MU-glcNAc; the mother's tissues have activities toward both substrates in the heterozygote range. These results emphasize the pitfalls of using only MU-glcNAc for the diagnosis of unusual variants of GM2 gangliosidosis.

Synthesis of 4-methylumbelliferyl-beta-D-N-acetylglucosamine-6-sulfate and its use in classification of GM2 gangliosidosis genotypes

Measurement of hexosaminidase A (Hex A) is an important clinical chemical procedure in the classification of GM2 gangliosidosis genotypes. We have synthesized a new substrate which may be useful in both the biochemical diagnosis of GM2 gangliosidosis and the detection of heterozygotes for the Tay-Sachs disease (TSD) allele. 4-Methylumbelliferyl-beta-D-N-acetylglucosamine-6-sulfate (4MUGS) was synthesized by sulfation of 4MU-beta-D-N-acetylglucosamine (4MUG) with chlorosulfonic acid and purified through gel filtration and ion-exchange chromatography. The structure of 4MUGS was verified by elemental analysis and NMR. Hex A is approximately 100 times more active toward 4MUGS than Hex B. The advantage of this increased specificity is that Hex A can be determined in a one-step procedure which allows separation of normal control serum values from those of obligate heterozygotes. Alternatively, assay values obtained using both substrates can be transformed by application of an empirical equation that allows the calculation of both Hex A and Hex B without the requirement of thermal fractionation. Lower values for % Hex A in serum have been obtained for Tay-Sachs homozygotes using the 4MUGS assay procedure. The results of Hex A assays on fibroblast cell strains obtained from Tay-Sachs homozygotes, variant forms of GM2 gangliosidosis and normal controls are also discussed.

Diagnosis of infantile and juvenile forms of GM2 gangliosidosis variant 0. Residual activities toward natural and different synthetic substrates

p-Nitrophenyl-6-sulfo-2-acetamido-2-deoxy-beta-D-glucopyranoside, which is known to be a specific substrate for human hexosaminidase A, has recently been used successfully for diagnosis of variants B and B1 of GM2-gangliosidosis (Fuchs et al. 1983; Kytzia et al. 1983; Li et al. 1983). However, it is hydrolyzed by hexosaminidase S as well and is therefore not suitable for detection of patients with variant 0, who reach the normal range of activity toward this substrate. Assay of ganglioside GM2 cleaving activity in fibroblast extracts in the presence of the natural GM2 activator protein reveals residual hexosaminidase A activities of less than 2% of normal controls in two infantile and up to 7.5% in two juvenile patients with variant 0.

Enzyme-linked immunosorbent assay for the ganglioside GM2-activator protein. Screening of normal human tissues and body fluids, of tissues of GM2 gangliosidosis, and for its subcellular localization

An enzyme-linked immunosorbent assay was developed for the quantitation of the human activator protein for ganglioside GM2 degradation by beta-N-acetyl-D-hexosaminidase A. With this assay, various tissues and body fluids of normal subjects and of patients with variant forms of GM2 gangliosidosis were screened for their GM2-activator content. The highest content of this protein was found in kidney. Tissues from patients with variant AB of infantile GM2 gangliosidosis contained only low levels (up to 5% of that of normal controls) of cross-reacting material. On subcellular fractionation of normal human skin fibroblasts, the activator was found to co-distribute with the lysosomal marker enzyme beta-hexosaminidase.

Concentrations of an activator protein for sphingolipid hydrolysis in liver and brain samples from patients with lysosomal storage diseases

The hydrolysis of sphingolipids by lysosomal enzymes requires the presence of additional proteins, which have been called activator proteins. The number of activator proteins, their specificity, exact mechanism of action, and response to a storage process all remain to be determined. In this study, antibodies to an activator protein known to bind sphingolipids and activate the enzymatic hydrolysis of GM1 ganglioside and sulfatide were used to estimate the concentration of this activator protein in small samples of liver and brain from patients with lysosomal storage diseases. By using rocket immunoelectrophoresis, the concentration of cross-reacting material (CRM) was determined. Control livers had an average of 0.95 +/- 0.18 (mean +/- 1 SD) microgram CRM/mg protein in the extracts, and control brains had an average of 0.25 +/- 0.14 microgram CRM/mg protein. Extremely high levels of CRM were found in extracts of livers from patients with type 1 GM1 gangliosidosis (15.1 and 16.9), and type A Niemann-Pick disease (10.7). Extracts of brain samples revealed a large amount of CRM in type 1 GM1 gangliosidosis (14.8), Tay-Sachs disease (5.3 and 8.7), and Sandhoff disease (13.5). Significantly elevated CRM was also measured in brain samples from patients with type 2 GM1 gangliosidosis, type A Niemann-Pick disease, metachromatic leukodystrophy, and Krabbe disease. The highest levels are found in those genetic diseases where the lipids stored, primarily or secondarily to the genetic defect, bind to this activator protein. This activator protein may have an important function in regulating intralysosomal lipid catabolism, and changes in its concentration in certain genetic diseases may be the cause of clinical, biochemical, and pathological heterogeneity found in the patients.

Immunological evidence for deficiency in an activator protein for sulfatide sulfatase in a variant form of metachromatic leukodystrophy

Cultured skin fibroblasts from the patient described by Shapiro and co-workers as having a variant form of metachromatic leukodystrophy (MLD) [Shapiro, L.J., Aleck, K. A., Kaback, M.M., Itabashi, H., Desnick, R.J., Brand, N., Stephens, R.L., Fluharty, A.L. & Kihara, H. (1979) Pediatr. Res. 13, 1179-1181] were confirmed to have a partial deficiency (25-40% of controls) of arylsulfatase A activity in vitro and a severe inability to metabolize [14C]stearic acid-labeled sulfatide presented in the medium. When 150 micrograms of purified activator protein for GM1 ganglioside beta-galactosidase and sulfatide sulfatase was added in 4 ml of medium with the 14C-labeled sulfatide, correction of the sulfatide metabolism to the normal range was found. Monospecific antibodies to this activator protein were prepared in rabbits, and they were used to examine cultured cells for the presence of crossreacting material by Ouchterlony double immunodiffusion and rocket immunoelectrophoresis. Cell extracts from controls and from patients with GM1 gangliosidosis and MLD were found to have a single line of identity. By comparison to known concentrations of purified activator protein, cell extracts from controls were found to have 0.76 +/- 0.32 micrograms of activator protein (mean +/- 1 SD, n = 10) per mg of solubilized protein, whereas those from patients with type 1 GM1 gangliosidosis and late infantile MLD had 1.53 and 1.41 micrograms/mg, respectively. Cell extracts from the patient with a variant form of MLD had no visible precipitin line by Ouchterlony double immunodiffusion and only a diffuse nonspecific region of staining by rocket immunoelectrophoresis. These immunologic studies provide evidence for a deficiency in the activator protein required for normal catabolism of sulfatide in the cells from this patient and possibly provide a method for diagnosis of similar patients.

The protein activator specific for the enzymic hydrolysis of GM2 ganglioside in normal human brain and brains of three types of GM2 gangliosidosis

In order to understand the etiology of Type AB GM2 gangliosidosis, we have purified and characterized the activator protein (GM2 activator) specific for the enzymic hydrolysis of GM ganglioside from normal human brain. The purified activator from human brain moved as one major protein band in various electrophoretic systems. We have also prepared the antiserum against this activator. The levels and the nature of GM2 activator and beta-hexosaminidase A were examined in the brains of five cases of GM2 gangliosidosis-one Type B, two O, and two Type AB. We found that the levels of GM2 activator in both Type B and Type O cases were markedly elevated, and that the two Type AB cases were the results of different causes. One case had a defective beta-hexosaminidase A and an elevated level of GM2 activator. Although this defective beta-hexosaminidase A could hydrolyze synthetic substrates, it was inactive in the cleavage of natural glycosphingolipids in the presence of the GM2 activator. It could, however, hydrolyze asialo-GM2 and GbOse4Cer in the presence of sodium taurodeoxycholate. The other case had normal beta-hexosaminidase A, but had a very low level of GM2 activator when analyzed by in vitro assay, suggesting the deficiency of this activator. By immunoelectrophoresis, this case was found to be completely devoid of the protein that cross-reacts with the antiserum against the GM2 activator.

A protein activator of galactosylceramide beta-galactosidase

A heat-stable protein was isolated from the spleen of a patient with Gaucher's disease. This protein will activate glucosylceramide beta-glucosidase activity (Ho, M.W. and O'Brien, J.S. (1971) Proc. Natl. Acad. Sci. U.S.A. 68, 2810-2813). When the specificity of this activator was tested using other enzymes and substrates, it was found to activate galactosylceramide beta-galactosidase activity and sphingomyelinase but not GM1 beta-galactosidase or sulfatide sulfatase. The ability to stimulate galactosylceramide beta-galactosidase was optimum at pH 4.6 in the presence of pure phosphatidylserine or other acidic lipids such as sulfatide and phosphatidylinositol. The partially purified activator protein could stimulate galactosylceramide beta-galactosidase activity in brain, liver, leukocytes and cultured fibroblasts. It was not able to stimulate the activity of this enzyme in tissue samples from patients with Krabbe's disease, demonstrating that it was acting on galactosylceramide beta-galactosidase and not GM1 beta-galactosidase. It was slowly denatured by treatment with Pronase, reaching 16% of starting levels after 24 h at 50 degrees C. Attempts to separate the abilities of this activator preparation to stimulate several lysosomal hydrolases by column chromatography were not successful.