Antibody to the phosphomannosyl receptor inhibits recycling of receptor in fibroblasts

The 215-kd phosphomannosyl receptor is involved in the transport of newly synthesized acid hydrolases to lysosomes and also mediates the pinocytosis of lysosomal enzymes by fibroblasts in culture. Recycling of receptors to the sorting sites is an integral part of both these processes. In this report, we describe the inhibition in human fibroblasts of both functions of the phosphomannosyl receptor by a rabbit antiserum to the bovine liver receptor. This inhibition cannot be completely accounted for by inhibition of ligand-receptor interaction. Rather the antibody appears to cross-link receptors and cause a removal of receptors from the sorting sites (plasma membrane and Golgi apparatus) and their accumulation in a compartment from which they do not recycle. Removal of receptors from the recycling pool by antibody is irreversible, and return of receptors requires synthesis of new protein. Degradation of "trapped receptors" is enhanced (t1/2 = 7.5 hr), but much more gradual than their removal from the functional receptor pool (t1/2 = 30 min).

Intrafamilial variability in lysosomal storage diseases

In most lysosomal storage diseases clinical variability is found and affects age-of-onset, severity, and the degree of neurological involvement. Very often the variability is due to the existence of different mutations leading to the same enzyme deficiency. In most of the families with more than one person affected, the clinical picture is very similar. Intrafamilial variation has been reported in the lysosomal storage diseases related or not related to genetic heterogeneity. In families in which different affected persons have the same genotype, as in X-linked disorders, or autosomal recessive diseases in which both parents of the affected sibs are carriers and healthy, the variability must be due to factors not related to the genotype. On the other hand, when different mutations are present in the same family, the variability may be related to the differences in the genotype of the affected persons. Knowledge of the possibility of intrafamilial variability and its genetic basis is essential in clinical and prenatal diagnosis of the lysosomal storage diseases.

A shortened beta-hexosaminidase alpha-chain in an Italian patient with infantile Tay-Sachs disease

Fibroblasts derived from a beta-hexosaminidase A (HexA)-deficient infant with clinically classic Tay-Sachs disease synthesized a precursor alpha-chain that was smaller than its normal counterpart. Fibroblasts from the infant's parents, who were consanguinous, produced both normal and mutant alpha-chains. The size difference, estimated to be 2-3 kilodaltons on the basis of sodium dodecyl sulfate-polyacrylamide-gel electrophoresis, persisted after removal of oligosaccharides with endo-H and is therefore attributable to a shortened polypeptide. The mutant alpha-chain did not undergo the further posttranslational modifications characteristic of its normal counterpart--i.e., synthesis of the mannose phosphate recognition marker, association with the beta-chain to give HexA, and proteolytic conversion to the mature form. Nor was it secreted, even in the presence of NH4Cl. Instead, it disappeared in the course of a 20-h chase. These results suggest that the mutant alpha-chain was trapped in an early biosynthetic compartment, either the endoplasmic reticulum or the cis-Golgi. The mutation appears to be different from all those previously described in patients with clinically classic Tay-Sachs disease.

Isolation of cDNA clones coding for the alpha-subunit of human beta-hexosaminidase. Extensive homology between the alpha- and beta-subunits and studies on Tay-Sachs disease

The lysosomal beta-hexosaminidases (N-acetyl-beta-glucosaminidase, EC 3.2.1.30) occur as two major isozymes, hexosaminidase A (alpha beta a beta b) and hexosaminidase B (2(beta a beta b)). To facilitate the investigations of the biosynthesis and structure of the enzymes and the nature of mutation in Tay-Sachs disease, we have isolated cDNA clones coding for the alpha-subunit. The polypeptide chains of hexosaminidase A (30 mg) were digested with trypsin, and peptides were isolated by reverse phase high pressure liquid chromatography and their amino acid sequences determined. One of alpha-chain peptides contained a string of seven amino acids from which two sets of oligonucleotides were specified. They were used to screen the SV40-transformed human fibroblast cDNA library of Okayama and Berg. Three cDNA clones, designated pHexA, identified from among 5 X 10(5) clones screened, contained the deduced amino-acid sequences of five alpha-chain peptides. Genomic DNA homologous to pHexA cDNA mapped to human chromosome 15 in somatic cell hybrids, as expected for the pre-alpha-polypeptide. Two of the clones contained identical polyadenylation sites, while the third was polyadenylated about 450 base pairs downstream. The two types of clones were found to correspond to a major 2.0-kilobase pair and a minor 2.3-kilobase pair mRNA species. Blot hybridizations of mRNA and DNA from Tay-Sachs variant fibroblasts revealed absence or reduction of levels of both mRNA species among infantile and juvenile variants, but no observable DNA alterations. Alignment of the pre-alpha- and pre-beta-polypeptides revealed 55% nucleotide and 57% amino acid homology. These data suggest a common origin of the HEXA and HEXB genes and account for the similar substrate specificities of the alpha-dimer subunit, hexosaminidase S, and hexosaminidase B.

Prenatal diagnosis of GM2 gangliosidosis with high residual hexosaminidase A activity (variant B1; pseudo AB variant)

A case of infantile GM2 gangliosidosis with high residual beta-hexosaminidase A activity toward the synthetic substrate 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside was diagnosed prenatally. Extracts from cultured amniotic fluid cells of the fetus had a hexosaminidase A activity of 27% of total hexosaminidase but were almost completely unable to degrade [3H]ganglioside GM2 (less than 0.5% of control values) when assayed in the presence of the natural activator protein. These results were confirmed by analyses of fetal muscle fibroblasts, liver, and brain. All tissues examined showed a profound deficiency of ganglioside GM2 galactosaminidase despite hexosaminidase A levels in the heterozygote range. In brain tissue, ganglioside GM2 content was elevated more than 4-fold. Hydrolysis of p-nitrophenyl glucosaminide-6-sulfate, a substrate specific for hexosaminidases A and S, by tissue extracts was also markedly reduced but the residual activities found (5% in liver, 12% in fibroblasts, and 16% in brain) were much higher than those with the physiological lipid substrate, ganglioside GM2.

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.

Sandhoff disease heterozygote detection: a component of population screening for Tay-Sachs disease carriers. I. Statistical methods

Serum and leukocyte hexosaminidase profiles (total activity and percent heat-labile activity levels) in obligate Sandhoff disease (SHD) heterozygotes differ from those of obligate Tay-Sachs disease (TSD) heterozygotes and noncarrier individuals. We have developed a procedure to identify, with 95% sensitivity, carriers of the allele(s) for SHD among individuals screened in a TSD heterozygote identification program. Using multivariate statistical methods of cluster analysis and discriminant analysis on serum and leukocyte hexosaminidase profiles from 102 potential SHD carriers, a linear discriminant function to classify individuals as SHD carriers or SHD noncarriers was constructed. This function classifies the serum and leukocyte profiles from all 15 obligate SHD heterozygotes studied, as those of SHD carriers. A 95% isodensity ellipse derived from only the serum hexosaminidase profiles of the 15 SHD obligate carriers has been applied to a TSD screened sample of 37,843 Jewish and non-Jewish individuals. A potential recall rate of screened individuals for serum retests and leukocyte assays of 2.01% has been estimated. These statistical methods enhance the TSD heterozygote screening program by permitting one to detect SHD heterozygotes within the screened population.

Chromatofocusing coupled with automated assay for beta-hexosaminidase isoenzymes in GM2 gangliosidosis

Good separations of the two major beta-hexosaminidase forms from human leukocytes were achieved by chromatofocusing, a technique which separates proteins on the basis of their isoelectric points. The use of an automated and reliable method is described for the identification of homozygotes and carriers of the GM2 gangliosidosis.

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.

Epstein-Barr virus transformed lymphoid cell lines as a new model system in culture for the study of GM2-gangliosidoses: Tay-Sachs and Sandhoff diseases

Epstein-Barr Virus transformed cell lines (LCL) were established from blood B-lymphocytes of patients affected with GM2-gangliosidoses variant O (Sandhoff disease, SD) and variant B (Tay-Sachs disease, TSD). LCL from SD showed a severe deficiency of activity of the major lysosomal beta-N-acetylhexosaminidase isoenzymes, Hex A and B; the residual activity was due to Hex S and Hex C. In LCL from TSD, the whole Hex activity was not deficient but isoenzyme composition was completely abnormal. Ultrastructural investigations showed the presence of pleiomorphic enlarged lysosomes appearing as clear vacuoles containing a finely fibrillo-granular material characteristic of the visceral lysosomal storage of gangliosidoses.

Hereditary heat-labile hexosaminidase B: a variant whose homozygotes synthesize a functional HEX A

Homozygosity for a mutant allele at the beta-chain locus of hexosaminidase (HEX), resulting in a variant of heat-labile HEX B, is reported for the first time in two healthy children. HEX activity in their sera, leukocytes, and cultured skin fibroblasts is severely deficient when measured on the synthetic substrate 4-MU-GLcNAc. However, their cultured skin fibroblasts synthesize and process both alpha and beta chains of HEX, and their lymphoid cells hydrolyze normally the natural ganglioside GM2. This mutation is, therefore, different from at least one of the beta-chain mutations found in previously published families with heat-labile HEX B.

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.

Faulty association of alpha- and beta-subunits in some forms of beta-hexosaminidase A deficiency

We have previously described the kinetics of association of the alpha- and beta-subunits of beta-hexosaminidase A in intact cultured human fibroblasts, using biosynthetic labeling and immunoprecipitation with antisera that distinguish between monomeric and associated alpha-chains (Proia, R. L., d'Azzo, A., and Neufeld, E. F. (1984) J. Biol. Chem. 259, 3350-3354). We now show lack of alpha-beta association in fibroblasts of several individuals deficient in beta-hexosaminidase A (5 patients with nonclassic forms of Tay-Sachs disease and 2 asymptomatic siblings). Defective association was accompanied by markedly reduced (less than one-tenth of normal) conversion of the alpha-chain precursor of Mr = 67,000 to the mature lysosomal form of Mr = 54,000. Analysis by hybridization with fibroblasts lacking the alpha- or beta-chain showed that the association defect resided in the alpha-chain. Most of the cell strains studied also had decreased synthesis of the alpha-chain, suggesting compound heterozygosity with the Ashkenazi Tay-Sachs (no synthesis) allele. An unusual feature of the association defect is the variability in the resulting clinical manifestations, even within families, implying that other factors determine the adequacy of the residual associated beta-hexosaminidase A in vivo.

cDNA clone for the alpha-chain of human beta-hexosaminidase: deficiency of alpha-chain mRNA in Ashkenazi Tay-Sachs fibroblasts

We have isolated a cDNA clone containing sequences complementary to mRNA encoding the alpha-chain of the lysosomal enzyme beta-hexosaminidase. RNA from a human lung fibroblast strain, IMR90, was enriched for beta-hexosaminidase messenger by polysome immunoselection with antiserum against beta-hexosaminidase A. This preparation was used to construct cDNA recombinant plasmids by the Okayama-Berg vector primer procedure. After transformation of Escherichia coli, 385 ampicillin-resistant colonies were obtained, 44 of which contained inserts in the plasmid DNA. Differential hybridization, with cDNA probes prepared from polysomal RNA enriched or depleted for beta-hexosaminidase messenger, was used to screen the recombinant plasmids for sequences encoding beta-hexosaminidase. One clone, p beta H alpha-1, containing a cDNA insert of approximately equal to 240 base pairs, was identified in this manner. The plasmid hybrid-selected a messenger from placental RNA that programed a translation system to synthesize the alpha-chain of beta-hexosaminidase. p beta H alpha-1 hybridized to an mRNA of approximately equal to 1.9 kilobases in preparations enriched separately in messenger for the alpha-chain or for both alpha- and beta-chains (by polysome immunoselection with antiserum against isolated alpha-chain or against beta-hexosaminidase A, respectively). It did not hybridize to an RNA preparation enriched for messenger of beta-chain by immunoselection with antiserum against beta-hexosaminidase B. The 1.9-kilobase mRNA was observed in poly(A)+ RNA preparations from control fibroblasts and from fibroblasts of a Tay-Sachs patient that synthesize an altered alpha-chain; however, it was not seen in similar preparations from fibroblasts of four Ashkenazi Tay-Sachs patients.

[Use of fluorescence-labelled cerebroside as a substrate for galactosylceramidase of human skin fibroblasts]

Possible use of the fluorescently labelled cerebroside, 1-O-(beta-D-galactosyl)-2-N-[6-(2-antroyl)hexanoyl]-4-sphingeni n (Gal-A-sphingenin) as a substrate for galactosylceramidase (GC) from human skin fibroblasts was investigated. Studies involving TLC and fluorimetric methods revealed enzymatic splitting of Gal-A-sphingenin whose degree correlated with the amount of the enzyme and incubation time. Some kinetic parameters of GC were determined, using Gal-A-sphingenin as substrate. It was shown that the values of specific activity of GC (1.6 nmol/mg protein/hr) and Km (0.025 mM) for Gal-A-sphingenin agree well with the corresponding values obtained with the use of the galactocerebroside as a natural substrate. In experiments with mixtures of Gal-A-sphingening and galactocerebroside used as substrates in different molar ratios, it was shown that the enzyme splits each of the substrates with equal velocity. The results of experiments with the enzyme samples from skin fibroblasts of healthy individuals and of patients with GM1-gangliosidosis (GM1-beta-D-galactosidase deficiency) suggest that Gal-A-sphingenin is a specific substrate for GC.

Clinical and genetic variations in the syndrome of adult GM2 gangliosidosis resulting from hexosaminidase A deficiency

Six patients from three families developed adult GM2 gangliosidosis resulting from severe beta-hexosaminidase A deficiency. The clinical picture varied between and within families and included spinocerebellar, various motor neuron, and cerebellar connection syndromes. Psychosis appeared in each family. Involvement of three generations was recorded in one family. The phenotype of adult GM2 gangliosidosis is variable and cannot form a basis for genetic classification. Detailed hexosaminidase determinations in eight patients from five families revealed all patients to have minor quantities of hexosaminidase A (about 15% of normal), with marked increase in hexosaminidase I isozyme.

Hexosaminidase A deficient adults: presence of alpha chain precursor in cultured skin fibroblasts

Cultured skin fibroblasts from hexosaminidase A deficient adults synthesize the alpha and beta chain precursors of beta-hexosaminidase (EC 3.2.1.30) of the same molecular weight as that synthesized by normal fibroblasts. However, the amount of the alpha chain precursor is greatly reduced. The alpha chain precursor in secretions from these fibroblasts consists of 19% of the total beta-hexosaminidase secreted compared to about 50% in normal cells. Attempts to increase the amount of detectable cellular alpha chain precursor by addition of protease inhibitors or extensive extraction methods have failed. Mature alpha chains were not detected. The presence of alpha chain precursor in fibroblasts from hexosaminidase A deficient adults can be used to distinguish between them and true Tay-Sachs disease homozygotes.

Tay-Sachs disease heterozygote detection in Brazil: comparison between tears and leukocytes as beta-hexosaminidase A source

Hexosaminidase A activities were determined in tears and peripheral leukocytes of carriers and noncarriers for Tay-Sachs Disease (TSD) as a percentage of total hexosaminidase activity. Correlation between enzyme activities in tears and leukocytes was highly significant (r = 0.75, p less than 0.01). Compared to leukocytes, screening in tears revealed 100% sensitivity, 100% negative predictive value, 86% specificity and 80% positive predictive value. These results indicate that tears are a satisfactory material for mass-screening of TSD-carrier state, but positive results must be confirmed in peripheral leukocytes. Seven heterozygotes were detected among 298 young Ashkenazi Jewish volunteers screened, giving an adjusted frequency of the TSD gene of 0.024.

Prenatal diagnosis of Tay-Sachs disease. Reflectometry of hexosaminidase A, B, and C/S bands on zymograms

Hexosaminidase (Hex) A, B, and C/S were electrophoretically separated from cultured amniotic fluid cells, fetal brain, and white blood cells. Photographs of cellulose acetate zymograms were evaluated by reflectometric scanning. The usefulness and limitations of this rapid method were shown. Hex A was completely absent in the amniotic fluid cells of one out of three pregnancies at risk for Tay-Sachs disease, but Hex C/S was present in this case. The prenatal diagnosis of Tay-Sachs disease was made, and confirmed with the fetal material after abortion. Hex C/S was distinguishable from a residual or "heterozygous" Hex A activity. In the two other risk pregnancies, reflectometric Hex A activities were found to be 50 and 34% of control; the heterozygous stage was presumed for the fetuses.

A new form of residual hexosaminidase activity in infantile Tay Sachs disease fibroblasts

Fibroblast cell lines obtained from five patients with the early onset form of Tay Sachs disease (TSD) possess a species of beta-N-acetylhexosaminidase (Hex) which is more anionic than Hex B but which is stable to heating under conditions which completely inactivate Hex A. This species, which comprised between 3 and 20% of the total hexosaminidase activity in homozygous TSD fibroblasts, appeared to be unstable and upon isoelectric focussing produced a mixture of Hex B (pI = 7.2) and an isozyme with a pI of 6.2. This intermediate form of hexosaminidase was not seen in two normal fibroblast cell lines but was observed following anion exchange chromatography of extracts of fibroblast cell lines obtained from two obligate heterozygotes. A species of hexosaminidase with the same chromatographic properties, thermostability and isoelectric point as the intermediate form found in fibroblasts with the TSD genotypes can be recovered after anion exchange chromatography of a partially purified preparation of human liver Hex A that had been treated with merthiolate. We hypothesize that in TSD cells a form of the beta subunit which is usually incorporated into Hex A accumulates due to the absence of alpha subunits. This form of the beta subunit is more anionic than the beta subunit found in Hex B. In the absence of alpha subunits these anionic beta subunits form tetramers with a pI = 6.2. This form of the enzyme is unstable in the presence of cellular proteases and may be modified to Hexosaminidase B.