Novel Tay-Sachs disease mutations from China

We describe three HEXA mutations associated with infantile Tay-Sachs disease (TSD) in three unrelated nonconsanguineous Chinese families. Novel mutations were found in two of these families. The third is a previously reported mutation (G-->A transition at nt 1444) (Nakano et al., 1988). Direct sequencing of PCR products identified a novel insertion of an A after nt 547 in family 1. This change generates an early termination codon 6 bp downstream from the insertion site. Allele-specific oligonucleotide hybridization confirmed homozygosity in the proband. Single strand conformational polymorphism analysis and direct sequencing of amplified exon 13 revealed a T-->C transition at nt 1453 with the corresponding amino acid substitution W485R in the second family. This mutation creates an Fnu4HI restriction site. The proband is homozygous for this allele. When the site-specific mutagenized alpha cDNA carrying the T-->C transition at nt 1453 was expressed in COS 1 cells hexosaminidase S activity was not detectable above background. A G-->A transition at nt 1444 (exon 13) corresponding to the E482K substitution was found in the third family. This mutation occurs at a CpG dinucleotide. It has been reported in an Italian TSD proband and causes defective intracellular transport of the alpha-subunit from the rough endoplasmic reticulum to the Golgi apparatus.

A mutation common in non-Jewish Tay-Sachs disease: frequency and RNA studies

Tay-Sachs disease (TSD) is an autosomal recessive genetic disorder resulting from mutation of the HEXA gene encoding the alpha-subunit of the lysosomal enzyme, beta-N-acetylhexosaminidase A (Hex A). We have discovered that a Tay-Sachs mutation, IVS-9 + 1 G-->A, first detected by Akli et al. (Genomics 11:124-134, 1991), is a common disease allele in non-Jewish Caucasians (10/58 alleles examined). A PCR-based diagnostic test, which detects an NlaIII site generated by the mutation, revealed a frequency among enzyme-defined carriers of 9/64 (14%). Most of those carrying the allele trace their origins to the United Kingdom, Ireland, or Western Europe. It was not identified among 12 Black American TSD alleles or in any of 18 Ashkenazi Jewish, enzyme-defined carriers who did not carry any of the mutations common to this population. No normally spliced RNA was detected in PCR products generated from reverse transcription of RNA carrying the IVS-9 mutation. Instead, the low levels of mRNA from this allele were comprised of aberrant species resulting from the use of either of two cryptic donor sites, one truncating exon 9 and the other within IVS-9, spliced to exon 10. Numerous additional splice products were detected, most involving skipping of one or more surrounding exons. Together with a recently identified allele responsible for Hex A pseudodeficiency (Triggs-Raine et al. Am J Hum Genet, 1992), these two alleles accounted for almost 50% (29/64) of TSD or carrier alleles ascertained by enzyme screening tests in non-Jewish Caucasians.

A glycine250--> aspartate substitution in the alpha-subunit of hexosaminidase A causes juvenile-onset Tay-Sachs disease in a Lebanese-Canadian family

The mutation causing juvenile Tay-Sachs disease (TSD) in two sibs of Lebanese-Maronite origin is described. An mRNA-containing extract of cultured fibroblasts obtained from one of the probands was used as a template to amplify the coding sequence of the hexosaminidase A (Hex A) alpha-subunit. Sequencing of amplified cDNA fragments revealed a single alteration, guanine to adenine at nt 749 creating a G250D mutation. The mutation introduces a new recognition site for the restriction enzyme Eco RV, permitting identification of heterozygotes for this allele following PCR amplification and Eco RV digestion of exon 7 sequences from genomic DNA templates. In order to test the effect of this substitution, an in vitro mutagenized cDNA construct was introduced into a mammalian expression vector and transfected into monkey Cos-1 cells separately or along with a beta-cDNA expression vector. When the mutant alpha-cDNA was the only gene introduced into COS cells no enzymatic activity above endogenous COS cell activity was detected. Cotransfection of normal alpha-cDNA and beta-cDNA followed by immunoprecipitation of human Hex A resulted in 20-fold increase in the ratio between positive and negative (mock transfection) control values. This allowed the detection of some residual activity (12% of the positive control) when the mutant alpha-cDNA replaced its wild-type counterpart. The predicted protein environment in which the mutation occurs is compared to that of the adult-onset Tay-Sachs disease mutation caused by a Gly269-->Ser substitution in exon 7.(ABSTRACT TRUNCATED AT 250 WORDS)

beta-hexosaminidase in cultured normal and mutant human fibroblasts: an immunohistochemical and biochemical investigation

Fibroblasts from a genetically normal individual and mutant fibroblasts from patients with Tay-Sachs and Sandhoff's diseases were grown in vitro. The lysosomal enzyme beta-hexosaminidase (Hex) was determined biochemically and localized with monoclonal antibodies recognizing Hex A, and Hex A and Hex B, respectively. The biochemical results showed similar amounts of Hex A and Hex B in the normal fibroblasts, whereas only Hex B activity was detectable in the fibroblasts from the patient with Tay-Sachs disease. The fibroblasts from the patient with Sandhoff's disease showed small amounts of Hex A- and no Hex B activity. Immunohistochemically, Hex was detectable with both antibodies in the normal fibroblasts and in those from the patient with Tay-Sachs disease. The fibroblasts from the patient with Sandhoff's disease were reactive only with Hex A-specific antibody.

Marked variation in blood beta-hexosaminidase in Gaucher disease

Gaucher disease is due to a primary deficiency of acid beta-glucosidase activity and is associated with secondary elevations of several plasma/serum lysosomal enzyme activities, including beta-hexosaminidase. We analyzed plasma and serum beta-hexosaminidase A & B activities in 55 patients with enzyme-documented Gaucher disease. The mean beta-hexosaminidase activity was increased and the percent of the A isozyme decreased, consistent with earlier studies. Gaucher disease patients had 2,067 +/- 1,491 nmol ml-1 h-1 units of beta-hexosaminidase activity with 51.9 +/- 15.5% beta-hexosaminidase A compared to 1,086 +/- 260 nmol ml-1 h-1 and 67.8 +/- 4.0% beta-hexosaminidase A in normal controls and 965 +/- 261 nmol ml-1 h-1 and 43.6 +/- 5.5% beta-hexosaminidase A in Tay-Sachs disease heterozygotes. Contrary to previous reports, marked heterogeneity of both total plasma/serum enzyme activity and isozyme pattern was noted, as some patients had normal enzyme levels and others had severe reductions in the percent of hexosaminidase A. These data argue against the suggestions of recent studies that routine serum beta-hexosaminidase testing done in Tay-Sachs disease heterozygote detection programs can be effectively used to screen for patients with Gaucher disease.

GM2-gangliosidosis B1 variant: analysis of beta-hexosaminidase alpha gene mutations in 11 patients from a defined region in Portugal

The GM2-gangliosidosis B1 variant occurs at an exceptionally high frequency in the northern part of Portugal. In most patients, the disease manifests itself as a juvenile form, as opposed to the late-infantile form described for many patients from other parts of the world. We have analyzed the beta-hexosaminidase alpha gene in 11 patients, as well as in some relatives, in order to characterize the underlying abnormalities. They were screened for the two previously identified mutations responsible for the B1 variant phenotype (G533----A, also designated as the "DN allele," and C532---T) by PCR amplification of an 800-bp DNA fragment and subsequent dot-blot hybridization with allele-specific oligonucleotides. The fragment amplified from one patient was also subcloned and sequenced. Ten patients, constituting a clinically and biochemically homogeneous group, were found to be homozygous for the DN allele. The other, whose clinical profile more resembled the late-infantile phenotype often described in the literature, was a compound heterozygote carrying the DN allele and another, as yet unidentified, abnormal allele. Our results, corroborated by previously published data, suggest that homozygotes and compound heterozygotes for the DN allele may be distinguishable at the phenotypic level, depending on the nature of the abnormality in the other allele. A common ancestral origin for the DN allele can also be postulated.

Biochemistry and genetics of Tay-Sachs disease

Tay-Sachs disease is one of the few neurodegenerative diseases of known causes. It results from mutations of the HEXA gene encoding the alpha subunit of beta-hexosaminidase, producing a destructive ganglioside accumulation in lysosomes, principally in neurons. With the determination of the protein sequence of the alpha and beta subunits, deduced from cDNA sequences, the complex pathway of subcellular and lysosomal processing of the enzyme has been determined. More recently, detailed knowledge of the gene structure has allowed the determination of specific mutations causing Tay-Sachs disease. The high incidence of the disease in Ashkenazi Jews is attributed predominantly to three mutations present in high frequency, while in non-Jews some two dozen mutations have been identified thus far. The cataloguing of mutations has important implications for carrier screening and prenatal diagnosis for Tay-Sachs disease.

Tay-Sachs disease heterozygote detection: use of a centrifugal analyser for automation of hexosaminidase assays with two different artificial substrates

An assay for measuring hexosaminidase A in serum and leucocytes is described in which a centrifugal analyser is used for automation of the enzyme assays after manual heat inactivation. The assay was used in a screening programme to identify heterozygotes for Tay-Sachs disease in Ashkenazi Jewish subjects in the UK. The first results from this programme indicate a carrier frequency of 1 in 27. Automation of an assay for direct measurement of hexosaminidase A in serum using 4-methyl-umbelliferyl-beta-N-acetylglucosamine-6-sulphate as substrate is also described. Comparison of data obtained from 66 control and 30 obligate carrier sera tested by this method and by heat inactivation showed improved discrimination using the sulphated substrate. Results obtained using the sulphated substrate for screening serum during pregnancy are also presented.

Biochemical and molecular aspects of late-onset GM2-gangliosidosis: B1 variant as a prototype

Clinical phenotypes of GM2-gangliosidosis are complex. In the past 5 years it has become possible to dissect out the phenotypic complexity on the basis of abnormalities on the DNA level. Available data on the 18 disease-causing mutations so far identified in the beta-hexosaminidase alpha-gene allow an oversimplified generalization; mutations that produce no or highly unstable mRNA cause the most severe infantile forms of the disease, while all late-onset forms are due to point mutations within the protein-coding region, which generate stable mRNA and stable mutant protein. The mutation underlying the distinct phenotype of Jewish adult Tay-Sachs disease will be discussed separately by Navon. The prototype of juvenile Tay-Sachs disease is the B1 variant. The disease was first recognized by an apparent discrepancy in the beta-hexosaminidase activities toward the conventional artificial substrates and the natural lipid substrate, GM2-ganglioside. When assayed with the conventional artificial substrates, patients appear reasonably normal while they are severely deficient in hydrolysis of the natural substrate (and more recently the 'sulfated' artificial substrate). The majority of B1 patients fall in the clinical category of juvenile GM2-gangliosidosis. Some of the earlier juvenile patients reported to have partial hexosaminidase A deficiency are likely to be B1 variant. Two point mutations, occurring at a mutation hot spot, CpG, and both affecting the same codon, have been described as the causes of the B1 variant phenotype; G533----A, Arg178----His; and C532----T, Arg178----Cys. The latter mutation has been found so far only in one Czechoslovakian family. In contrast, the former mutation has a wide geographic and ethnic distribution.(ABSTRACT TRUNCATED AT 250 WORDS)

The clinical aspects of adult hexosaminidase deficiencies

The authors describe the clinical phenotypes of hexosaminidase deficiencies (GM2 gangliosidosis). The symptoms, differently combined, include cerebellar ataxia, motor neuron disease, dystonia, psychosis, neurovegetative troubles with different severity. Morphological changes are evident in rectal, muscle or nerve biopsies. Minor clinical changes are described in carriers from a family. A chronic GM2 gangliosidosis has to be suspected in any atypical case with the above-mentioned symptoms with autosomal-recessive inheritance.

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.

Juvenile GM2 gangliosidosis variant B1: clinical and biochemical study in seven patients

We describe seven patients from five different families with GM2 gangliosidosis, variant B1. To our knowledge these are the first juvenile cases reported in the literature.

GM2-gangliosidosis. Clinical and biochemical aspects of four cases

We discuss four cases of GM2-gangliosidosis. In one of them the biochemical diagnostic confirmation was difficult. This case revealed striking discrepancies between the results of different methods of enzyme assay. The hexosaminidase A determination based on pH inactivation is not always reliable; assay with natural substrate may be necessary. However, the results with the newly developed substrate 4-MU-GlcNac-6-SO4 are promising and it seems to be a good alternative to the traditional (pH or heat) inactivation procedures. The deficiency can be shown in leukocytes, plasma and fibroblasts with the 6-sulfated substrate. The carrier state seems better reflected in plasma hexosaminidase A than in leukocyte hexaminidase A levels.

[Isolation and characteristics of hexosaminidase A and activator protein from human kidney]

Hexosaminidase A (HA) was isolated from human kidney and purified to an electrophoretically homogeneous state. The purification procedure included ion-exchange chromatography on DEAE-cellulose, gel filtration on Toyopearl HW-55 and chromatofocusing on PBE 94 (enzyme yield 26.6%, 1133.6-fold purification). The physico-chemical and kinetic properties of HA are as follows: Mr of the purified enzyme is approximately 100,000; Km for 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside is 0.6 mM; pH optimum is at pH 4.4-4.6; pI is 5.0. The amino acid composition of the purified enzyme was determined. A specific anti-HA antiserum was raised, which did not immunoprecipitate with fibroblast extracts characterized by a mutational blockade of HA synthesis. GM2 was isolated and purified from murine liver as well as from the brain of a female patient who died of Tay-Sachs disease. The label was introduced by way of treatment of GM2 with tritiated acetic anhydride. The specific radioactivity of [3H]GM2 was 521 and 2065 Ci/M, respectively. The label was introduced into the N-acetylneuraminic acid and GalNAc residues of these GM2 preparations. An activator protein capable of solubilizing the natural substrate of HA was isolated from human kidney and partially purified (with a 19.9% yield and 480-fold purification). The Mr of the purified activator protein was approximately 21,000. Purified HA hydrolyzed [3H]GM2 only in the presence of the activator protein. An addition of the activator to the incubation medium containing normal fibroblast culture extracts and [3H]GM2 caused an increase in the rate of substrate hydrolysis, tenfold, on the average.

The juvenile and chronic forms of GM2 gangliosidosis: clinical and enzymatic heterogeneity

We report the cases of 5 patients from 2 sibships with the "adult" or chronic form of GM2 gangliosidosis and 2 patients from another sibship with the juvenile form. We demonstrated hexosaminidase A deficiency in all cases but in 1 sibship the enzymatic profile was identical to that in Tay-Sachs disease, whereas in the remaining 2 families it was that of the B1 variant. There was no correlation between the clinical features and the enzymatic profile. Hexosaminidase A deficiency should be considered in unexplained progressive neurologic disorders of childhood and adolescence, including isolated dementia. EMG evidence of anterior horn cell involvement in association with neurologic or cognitive deterioration may be a diagnostic clue in the juvenile forms.

Introduction of the alpha subunit mutation associated with the B1 variant of Tay-Sachs disease into the beta subunit produces a beta-hexosaminidase B without catalytic activity

Lysosomal beta-hexosaminidase (beta-N-acetylhexosaminidase, EC 3.2.1.52) occurs in two major isozyme forms, hexosaminidase A (alpha beta) and hexosaminidase B (beta beta). Although dimer formation is required for enzymatic activity, both subunits contain active sites which share many common substrates. However, the alpha subunit alone confers on hexosaminidase A the specificity for negatively charged substrates, e.g. GM2 ganglioside. Recently, a point mutation, producing a single amino acid substitution in the alpha subunit (Arg178-His), has been found to be associated with the B1 variant phenotype of Tay-Sachs disease (Ohno, K., and Suzuki, K. (1988) J. Neurochem. 50, 316-318). This variant is characterized by normal levels of hexosaminidase A as measured by a common artificial substrate, but an absence of activity toward alpha subunit-specific substrates. However, because of the presence of an active beta subunit in the mutant hexosaminidase A, it has not been possible to determine whether the affected alpha subunit has undergone a change in substrate specificity or become totally inactive. In order to define the full effect of the B1 mutation we have taken advantage of the common evolutionary origin of the genes coding for the alpha and beta subunits. Since the B1 mutation occurs in a region of extended identity between the two subunits, we have duplicated the Arg178-His mutation in a cDNA coding for the human beta subunit (Arg211-His). By expression of the mutant construct in monkey COS cells we have been able to examine the effect of this mutation on beta subunits which are capable of forming stable, active homodimers, an experiment that could not readily be accomplished with heterodimeric hexosaminidase A. Our data show that beta homodimers containing the Arg211-His substitution are formed and are transported into the lysosome in a manner identical to that of normal pro-hexosaminidase B. However, the mutant homodimers are processed at a slower rate and are less stable in the lysozyme. Their most striking feature was a total lack of normal hexosaminidase B activity. We conclude that while the effect of the Arg178-His substitution is not strictly limited to the active site, the severe B1 phenotype results from a totally inactive alpha-subunit in hexosaminidase A.

The mutation mechanism causing juvenile-onset Tay-Sachs disease among Lebanese

Expression of the hexosaminidase isozymes was evaluated in fibroblast cell lines obtained from two sibs of Lebanese-Christian origin who presented with juvenile-onset Tay-Sachs disease. In the normal control fibroblasts the alpha subunit of hexosaminidase A (hex A) is synthesized as a 67 KD precursor which is cleaved in lysosomes to a mature 54 KD peptide. The patients' fibroblasts were capable of synthesizing the 67 KD precursor but failed to convert it to the mature subunit. The alpha subunit precursor synthesized by patients' cells could not be phosphorylated, nor was the patients' alpha subunit precursor secreted into the medium in response to NH4Cl, which caused accumulation of both alpha and beta subunit precursor in the medium of the normal control fibroblasts. The measurement of residual enzyme activity in the fibroblasts of patients which best correlated with the onset of the illness was the ion exchange chromatographic separation of Hex A-associated hydrolysis of the synthetic substrate 4-methylumbelliferyl N-acetyl-beta-D-glucosamine-6-sulfate (4MUGS). The patients had 0.32% and 0.36% of Hex A-associated 4MUGS cleaving activity compared to normal control fibroblasts as compared to less than 0.016% for infantile Tay-Sachs disease fibroblasts. The residual Hex A activity in patients' cells had a pH optimum identical with normal enzyme (pH 3.9-4.0), a reduced specific activity for 4MUGS (relative to hydrolysis of unsulfated synthetic substrate), and a greatly enhanced thermal stability. The occurrence of this form of Tay-Sachs disease in Lebanon, the fact that the condition has been described in three unrelated Lebanese immigrant families in Canada, together with the fact that the grandparents of the unrelated probands come from villages in both the northern and southern regions of Lebanon, leads us to speculate that a gene causing juvenile-onset Tay-Sachs disease may not be infrequent in Lebanon.

B1 variant of GM2 gangliosidosis in a 12-year-old patient

A girl aged 12 y, 9 mo, suffered from a progressive neurodegenerative disorder marked by ataxia, extrapyramidal symptoms, and convulsions. A skin biopsy showed axonal pathology that emphasized axonal segments enlarged by mitochondria, dense bodies, and lysosomal residual bodies of the membranous cytoplasmic body type. This ultrastructural pathology suggested GM2 gangliosidosis which was shown to be a B1 variant by specific biochemical studies, although conventional techniques had failed to detect GM2 gangliosidosis. The B1 variant is marked by a deficient activity of beta-hexosaminidase A towards one substrate, and by an almost normal activity towards another. Both parents showed a diminished activity towards the sulfated substrate, suggesting a heterozygous state, and almost normal activity with the second substrate type.

A splicing defect due to an exon-intron junctional mutation results in abnormal beta-hexosaminidase alpha chain mRNAs in Ashkenazi Jewish patients with Tay-Sachs disease

Abnormal beta-hexosaminidase alpha chain mRNAs from an Ashkenazi Jewish patient with the classical infantile Tay-Sachs disease contained intact or truncated intron 12 sequences. Sequence analysis showed a single nucleotide transversion at the 5' donor site of intron 12 from the normal G to C. This provides the first evidence that this junctional mutation, also found independently in two other laboratories by analysis of genomic clones, results in functional abnormality. Analysis with normal and mutant oligonucleotides as probes indicated that our patient was a compound heterozygote with only one allele having the transversion. The patient studied in the other two laboratories was also a compound heterozygote. Another Ashkenazi Jewish patient was normal in this region in both alleles. Thus, the splicing defect is the underlying genetic cause in some but not all Ashkenazi Jewish patients with Tay-Sachs disease.

Normal transcription of the beta-hexosaminidase alpha-chain gene in the Ashkenazi Tay-Sachs mutation

Tay-Sachs disease is a biochemically heterogeneous lysosomal storage disorder caused by lack of the A isoenzyme of beta-hexosaminidase; the underlying defect is a mutation in the gene encoding the alpha-chain. It has been shown that fibroblasts isolated from Tay-Sachs patients of Ashkenazi Jewish origin contain no alpha-chain mRNA detectable on Northern blots. We now have compared run-on transcription in nuclei isolated from three strains of Ashkenazi Tay-Sachs fibroblasts and from a strain of normal (IMR90) cells. Using alpha-chain and beta-chain cDNAs as probes, we found no difference in the relative amount of [32P]ribonucleotide added to nascent transcripts; the average ratio of alpha/beta hybridizable radioactivity was 1.3 and 1.4 for mutant and normal cells, respectively. The identity of the Tay-Sachs alpha-chain transcript was confirmed by competition hybridization with excess alpha-chain mRNA. The results indicate that the Ashkenazi Tay-Sachs mutation permits a normal level of transcription of the alpha-chain gene and points to a posttranscriptional defect, such as RNA processing, transport, or stability.

A deletion involving Alu sequences in the beta-hexosaminidase alpha-chain gene of French Canadians with Tay-Sachs disease

French Canadians living in eastern Quebec are carriers of a severe type of Tay-Sachs disease, known as the classic form, 10 times more often than the general population. The alpha-chain of beta-hexosaminidase A, a lysosomal enzyme composed of two chains (alpha, beta), bears the mutation in this inherited disorder. We previously reported that the 5' end of the alpha-chain gene was deleted in two such patients (Myerowitz, R., and Hogikyan, N.D. (1986) Science, 232, 1646-1648). The present study reports the size, precise location, and environment of the deletion. A clone encompassing the deletion was isolated from a genomic library constructed in lambda EMBL3 with DNA from a patient's fibroblasts. Comparison of the restriction maps of the clone with that of the normal gene (Proia, R.L., and Soravia, E. (1987) J. Biol. Chem. 262, 5677-5681) showed that the deletion was 7.6 kilobases long and included part of intron 1, all of exon 1 and extended 2000 base pairs upstream past the putative promotor region of the alpha-chain gene. These data are consistent with the inability to detect mRNA and immunoprecipitable alpha-chain protein in this mutant. Sequence analysis of the deletion junction in the mutant and corresponding regions of the normal gene demonstrated the presence of similarly oriented Alu sequences at the 5' and 3' deletion boundaries. The data are in accord with the possibility that the deletion may have arisen during homologous recombination from unequal crossing over between Alu sequences.

Tay-Sachs disease with hexosaminidase A: characterization of the defective enzyme in two patients

Cases of infantile Tay-Sachs disease (TSD) with high residual hexosaminidase A (Hex A) activity have recently been described. The clinical presentation of the disease in these patients is identical to that found among Ashkenazi-Jewish patients. Fibroblasts from two such TSD patients had Hex A activity comprising 16% of total Hex when measured by thermal fractionation and quantitation with 4-methylumbelliferyl-beta-D-N-acetylglucosamine (4MUG). Hydrolysis of 4-methylumbelliferyl-beta-D-N-acetylglucosamine-6-SO4 (4MUGS) by patient fibroblast extracts is catalyzed by an enzyme activity that comprises less than 1% of total Hex. Kinetic analysis of patient Hex A by using 4MUGS revealed Km's similar to that of control Hex A but Vmax's significantly different from that of the control enzyme. The inhibitors N-acetylglucosamine and N-acetylglucosamine-6-PO4 were used to distinguish between active sites associated with the two different subunits of Hex A. A beta-subunit site with little activity toward 4MUGS is sensitive to N-acetylglucosamine but resistant to N-acetylglucosamine-6-PO4. This site accounts for most of the hydrolysis of 4MUG. By contrast, an alpha-subunit site that is sensitive to N-acetylglucosamine-6-PO4 but resistant to N-acetylglucosamine accounts for almost all of the hydrolysis of 4MUGS. In mutant cells, this site retains the ability to bind substrate but is deficient in catalytic activity toward 4MUGS. The pH optima of patients' Hex A is shifted to a more acidic range, and the enzymes are significantly more thermostable than control Hex A. By using the thermal fractionation procedure for serum isozyme discrimination, one parent of each patient is unambiguously classified as heterozygous for the TSD gene whereas the other parent has test values in the grey zone. When parents are tested by use of 4MUGS, however, all four parents are classified as heterozygotes. Comparison of the results of both assay procedures allows the carrier of the atypical TSD allele to be recognized and identifies the probands as compound heterozygotes.