The mutations in Ashkenazi Jews with adult GM2 gangliosidosis, the adult form of Tay-Sachs disease

Changes of gut microbiota reflect the severity of major depressive disorder: a cross sectional study

Disturbed gut microbiota is a potential factor in the pathogenesis of major depressive disorder (MDD), yet whether gut microbiota dysbiosis is associated with the severity of MDD remains unclear. Here, we performed shotgun metagenomic profiling of cross-sectional stool samples from MDD (n = 138) and healthy controls (n = 155). The patients with MDD were divided into three groups according to Hamilton Depression Rating Scale 17 (HAMD-17), including mild (n = 24), moderate (n = 72) and severe (n = 42) individuals, respectively. We found that microbial diversity was closely related to the severity of MDD. Compared to HCs, the abundance of Bacteroides was significantly increased in both moderate and severe MDD, while Ruminococcus and Eubacterium depleted mainly in severe group. In addition, we identified 99 bacteria species specific to severity of depression. Furthermore, a panel of microbiota marker comprising of 37 bacteria species enabled to effectively distinguish MDD patients with different severity. Together, we identified different perturbation patterns of gut microbiota in mild-to-severe depression, and identified potential diagnostic and therapeutic targets.

Diagnostic Tips from a Video Series and Literature Review of Patients with Late-Onset Tay-Sachs Disease

Background

Late-Onset Tay-Sachs (LOTS) disease is a rare, progressive neurological condition that can dramatically affect the life of these patients. The diagnosis of LOTS is easily missed because of the multifaced presentation of these patients, who can initially be assessed by neuromuscular or movement disorder specialists, or psychiatrists. Clinical trials are now becoming available for LOTS. Therefore, early diagnosis can be detrimental for these patients and for insuring informative research outcomes.

Methods

We characterized a cohort of nine patients with LOTS through a detailed clinical and video description. We then reviewed the available literature regarding the clinical description of patients with LOTS. Our findings were summarized based on the predominant phenotype of presentation to highlight diagnostic clues to guide the diagnosis of LOTS for different neurology specialists (neuromuscular, movement disorders) and psychiatrist.

Results

We described a cohort of 9 new patients with LOTS seen at our clinic. Our literature review identified 76 patients mainly presenting with a neuromuscular, cerebellar, psychiatric, stuttering, or movement disorder phenotype. Diagnostic tips, such as the triceps sign, distinct speech patterns, early psychiatric presentation and impulsivity, as well as neurological symptoms (cerebellar or neuromuscular) in patients with a prominent psychiatric presentation, are described.

Discussion

Specific diagnostics clues can help neurologists and psychiatrists in the early diagnosis of LOTS disease. Our work also represent the first video presentation of a cohort of patients with LOTS that can help different specialists to familiarize with these features and improve diagnostic outcomes.

Highlights

Late-Onset Tay-Sachs (LOTS) disease, a severe progressive neurological condition, has multifaced presentations causing diagnostic delays that can significantly affect research outcomes now that clinical trials are available. We highlight useful diagnostic clues from our cohort (including the first video representation of a LOTS cohort) and comprehensive literature review.

Whole exome sequencing reveals a homozygous nonsense mutation in HEXA gene leading to Tay-Sachs disease in Saudi Family

Objective:

To study the causative variants in affected member of a Saudi family with Tay-Sachs disorder. This disorder includes paralysis, decreasing in attentiveness, seizures, blindness, motor deterioration progresses rapidly leading to a completely unresponsive state and a cherry-red spot visible on the eye.

Methods:

Whole exome sequencing (WES) and Sanger sequencing was performed to study the variant leading to the disease.

Results:

WES data analysis and Sanger sequencing validation, identifies a homozygous nonsense mutation c.1177C>T, p.Arg393Ter as a result in protein change. This mutation was also studied in 100 unrelated healthy controls.

Conclusions:

We detected homozygous mutation in HEXA gene that may lead to cause Tay-Sachs disorder. Moreover, explain the possibility that HEXA gene may play important role for multiple aspects of normal human neurodevelopment.

Genotype-phenotype correlation of gangliosidosis mutations using in silico tools and homology modeling

Gangliosidoses, including GM1-gangliosidosis and GM2-gangliosidosis (Tay-Sachs disease and Sandhoff disease), are lysosomal disorders resulting from enzyme deficiencies and accumulation of gangliosides. Phenotypes of gangliosidoses range from infantile, late-infantile, juvenile, and to the adult form. The genotype-phenotype correlation is essential for prognosis and clinical care planning for patients with a gangliosidosis condition. Previously, we have developed a method to establish the genotype-phenotype correlation of another lysosomal disease, mucopolysaccharidosis type I, with in silico tools. This same method was applied to analyze the genotype and phenotype of 38 patients diagnosed with a gangliosidosis disease in the United States. Out of 40 mutations identified, 3 were novel, including p.Tyr192His and p.Phe556Ser of the GLB1 gene and p.Gly461Val of the HEXA gene. Furthermore, the mutant protein structure of all missense mutations was constructed by homology modeling. A systemic structural analysis of these models revealed the specific mechanisms of how each mutation may lead to the disease. In summary, the method developed in this study holds promise as a tool that can be broadly applicable to other lysosomal diseases and monogenic diseases.

Amyotrophy, cerebellar impairment and psychiatric disease are the main symptoms in a cohort of 14 Czech patients with the late-onset form of Tay-Sachs disease

BACKGROUND

Tay-Sachs disease (TSD) is an inherited neurodegenerative disorder caused by a lysosomal β-hexosaminidase A deficiency due to mutations in the HEXA gene. The late-onset form of disease (LOTS) is considered rare, and only a limited number of cases have been reported. The clinical course of LOTS differs substantially from classic infantile TSD.

METHODS

Comprehensive data from 14 Czech patients with LOTS were collated, including results of enzyme assays and genetic analyses.

RESULTS

14 patients (9 females, 5 males) with LOTS were diagnosed between 2002 and 2018 in the Czech Republic (a calculated birth prevalence of 1 per 325,175 live births). The median age of first symptoms was 21 years (range 10-33 years), and the median diagnostic delay was 10.5 years (range 0-29 years). The main clinical symptoms at the time of manifestation were stammering or slurred speech, proximal weakness of the lower extremities due to anterior horn cell neuronopathy, signs of neo- and paleocerebellar dysfunction and/or psychiatric disorders. Cerebellar atrophy detected through brain MRI was a common finding. Residual enzyme activity was 1.8-4.1% of controls. All patients carried the typical LOTS-associated c.805G>A (p.Gly269Ser) mutation on at least one allele, while a novel point mutation, c.754C>T (p.Arg252Cys) was found in two siblings.

CONCLUSION

LOTS seems to be an underdiagnosed cause of progressive distal motor neuron disease, with variably expressed cerebellar impairment and psychiatric symptomatology in our group of adolescent and adult patients. The enzyme assay of β-hexosaminidase A in serum/plasma is a rapid and reliable tool to verify clinical suspicions.

Tay-Sachs disease mutations in HEXA target the α chain of hexosaminidase A to endoplasmic reticulum-associated degradation

In Tay–Sachs disease, mutations in HEXA can lead to aberrant α subunits of the HexA enzyme. Two such mutants have folding defects and are cleared by endoplasmic reticulum-associated degradation. Toward the pursuit of therapeutic treatments, it was found that manipulating endoplasmic reticulum quality control can impair mutant α degradation and improve cellular Hex activity.

Loss of function of the enzyme β-hexosaminidase A (HexA) causes the lysosomal storage disorder Tay–Sachs disease (TSD). It has been proposed that mutations in the α chain of HexA can impair folding, enzyme assembly, and/or trafficking, yet there is surprisingly little known about the mechanisms of these potential routes of pathogenesis. We therefore investigated the biosynthesis and trafficking of TSD-associated HexA α mutants, seeking to identify relevant cellular quality control mechanisms. The α mutants E482K and G269S are defective in enzymatic activity, unprocessed by lysosomal proteases, and exhibit altered folding pathways compared with wild-type α. E482K is more severely misfolded than G269S, as observed by its aggregation and inability to associate with the HexA β chain. Importantly, both mutants are retrotranslocated from the endoplasmic reticulum (ER) to the cytosol and are degraded by the proteasome, indicating that they are cleared via ER-associated degradation (ERAD). Leveraging these discoveries, we observed that manipulating the cellular folding environment or ERAD pathways can alter the kinetics of mutant α degradation. Additionally, growth of patient fibroblasts at a permissive temperature or with chemical chaperones increases cellular Hex activity by improving mutant α folding. Therefore modulation of the ER quality control systems may be a potential therapeutic route for improving some forms of TSD.

Tay-Sachs disease: current perspectives from Australia

Tay-Sachs disease (TSD) is a fatal, recessively inherited neurodegenerative condition of infancy and early childhood. Although rare in most other populations, the carrier frequency is one in 25 in Ashkenazi Jews. Australian high-school-based TSD preconception genetic screening programs aim to screen, educate, and optimize reproductive choice for participants. These programs have demonstrated high uptake, low psychological morbidity, and have been shown to result in fewer than expected Jewish TSD-affected births over 18 years of operation. The majority of Jewish individuals of reproductive age outside of the high school screening program setting in Australia have not accessed screening. Recent recommendations advocate supplementing the community high school screening programs with general practitioner- and obstetrician-led genetic screening of Ashkenazi Jewish individuals for TSD and other severe recessive diseases for which this group is at risk. Massively parallel DNA sequencing is expected to become the testing modality of choice over the coming years.

Next-generation DNA sequencing of HEXA: a step in the right direction for carrier screening

Tay-Sachs disease (TSD) is the prototype for ethnic-based carrier screening, with a carrier rate of ∼1/27 in Ashkenazi Jews and French Canadians. HexA enzyme analysis is the current gold standard for TSD carrier screening (detection rate ∼98%), but has technical limitations. We compared DNA analysis by next-generation DNA sequencing (NGS) plus an assay for the 7.6 kb deletion to enzyme analysis for TSD carrier screening using 74 samples collected from participants at a TSD family conference. Fifty-one of 74 participants had positive enzyme results (46 carriers, five late-onset Tay-Sachs [LOTS]), 16 had negative, and seven had inconclusive results. NGS + 7.6 kb del screening of HEXA found a pathogenic mutation, pseudoallele, or variant of unknown significance (VUS) in 100% of the enzyme-positive or obligate carrier/enzyme-inconclusive samples. NGS detected the B1 allele in two enzyme-negative obligate carriers. Our data indicate that NGS can be used as a TSD clinical carrier screening tool. We demonstrate that NGS can be superior in detecting TSD carriers compared to traditional enzyme and genotyping methodologies, which are limited by false-positive and false-negative results and ethnically focused, limited mutation panels, respectively, but is not ready for sole use due to lack of information regarding some VUS.

Late onset GM2 gangliosidosis mimicking spinal muscular atrophy

A case of late onset GM2 gangliosidodis with spinal muscular atrophy phenotype followed by cerebellar and extrapyramidal symptoms is presented. Genetic analysis revealed compound heterozygous mutation in exon 10 of the HEXA gene. Patient has normal intelligence and emotional reactivity. Neuroimaging tests of the brain showed only cerebellar atrophy consistent with MR spectroscopy (MRS) abnormalities. (18)F-fluorodeoxyglucose positron emission tomography (18)F-FDG PET/CT of the brain revealed glucose hypometabolism in cerebellum and in temporal and occipital lobes bilaterally.

GM2 gangliosidosis in an adult pet rabbit

A 1.5-year-old neutered male rabbit was presented with chronic nasal discharge and ataxia. Rapid progression of neurological signs was noted subsequent to general anaesthesia and the rabbit was humanely destroyed due to the poor prognosis. At necropsy examination there were no gross changes affecting the brain or spinal cord. Microscopical examination revealed that the perikarya of numerous neurons in the brain and spinal cord were distended by the intracytoplasmic accumulation of pale, finely granular to vacuolar material. Transmission electron microscopy showed this to be composed of concentric membranous cytoplasmic bodies. Thin layer chromatography revealed elevation of GM2 ganglioside in the brain of this rabbit compared with that of an unaffected control rabbit. Enzymatically, there was markedly reduced activity of tissue β-hexosaminidase A in brain and liver tissue from the rabbit. This was a result of an almost complete absence of the enzymatic activity of the α-subunit of that enzyme. These findings are consistent with sphingolipidosis comparable with human GM2 gangliosidosis variant B1.

Tay-Sachs disease in Jacob sheep

Autopsy studies of four Jacob sheep dying within their first 6-8 months of a progressive neurodegenerative disorder suggested the presence of a neuronal storage disease. Lysosomal enzyme studies of brain and liver from an affected animal revealed diminished activity of hexosaminidase A (Hex A) measured with an artificial substrate specific for this component of β-hexosaminidase. Absence of Hex A activity was confirmed by cellulose acetate electrophoresis. Brain lipid analyses demonstrated the presence of increased concentrations of G(M2)-ganglioside and asialo-G(M2)-ganglioside. The hexa cDNA of Jacob sheep was cloned and sequenced revealing an identical number of nucleotides and exons as in human HexA and 86% homology in nucleotide sequence. A missense mutation was found in the hexa cDNA of the affected sheep caused by a single nucleotide change at the end of exon 11 resulting in skipping of exon 11. Transfection of normal sheep hexa cDNA into COS1 cells and human Hex A-deficient cells led to expression of Hex S but no increase in Hex A indicating absence of cross-species dimerization of sheep Hex α-subunit with human Hex β-subunits. Using restriction site analysis, the heterozygote frequency of this mutation in Jacob sheep was determined in three geographically separate flocks to average 14%. This large naturally occurring animal model of Tay-Sachs disease is the first to offer promise as a means for trials of gene therapy applicable to human infants.

Two novel exonic point mutations in HEXA identified in a juvenile Tay-Sachs patient: role of alternative splicing and nonsense-mediated mRNA decay

We have identified three mutations in the beta-hexoseaminidase A (HEXA) gene in a juvenile Tay-Sachs disease (TSD) patient, which exhibited a reduced level of HEXA mRNA. Two mutations are novel, c.814G>A (p.Gly272Arg) and c.1305C>T (p.=), located in exon 8 and in exon 11, respectively. The third mutation, c.1195A>G (p.Asn399Asp) in exon 11, has been previously characterized as a common polymorphism in African-Americans. Hex A activity measured in TSD Glial cells, transfected with HEXA cDNA constructs bearing these mutations, was unaltered from the activity level measured in normal HEXA cDNA. Analysis of RT-PCR products revealed three aberrant transcripts in the patient, one where exon 8 was absent, one where exon 11 was absent and a third lacking both exons 10 and 11. All three novel transcripts contain frameshifts resulting in premature termination codons (PTCs). Transfection of mini-gene constructs carrying the c.814G>A and c.1305C>T mutations proved that the two mutations result in exon skipping. mRNAs that harbor a PTC are detected and degraded by the nonsense-mediated mRNA decay (NMD) pathway to prevent synthesis of abnormal proteins. However, although NMD is functional in the patient's fibroblasts, aberrant transcripts are still present. We suggest that the level of correctly spliced transcripts as well as the efficiency in which NMD degrade the PTC-containing transcripts, apparently plays an important role in the phenotype severity of the unique patient and thus should be considered as a potential target for drug therapy.

Chapter 9 GM(2) gangliosidoses

LOGM(2)G results from the defective activity of the lyosomal enzyme beta-hexosaminidase A. Continued accumulation of undegraded substrate results in pathology in the central nervous system. The disease is progressive and disease dynamics may vary throughout life. Clinically, the disease variants present a remarkable spectrum of phenotypes ranging from the lethal form to a slowly progressive disease type. Genotype/phenotype correlations are imperfect. Homozygosity for the L444P genotype is almost always associated with the infantile form. The pathological mechanism of the central nervous system damage is still not fully understood. Neuronal loss and neurodegeneration have been reported, as well as gray matter and white matter involvement, leading to multisystem expression. Recently, the possibilities of using stem cells to replace damaged neurons or enzyme replacement therapy have been suggested for several neurodegenerative diseases. The anterior horn cells can theoretically be a target for this procedure. In fact, using stem cells engineered to carry a normal HEXA gene is potentially more likely to benefit LOGM(2)G cases than patients with other forms of motor neuron disease, since in ALS the degenerative process will continue and is likely to affect the transplanted cells whereas in LOGM(2)G a real correction of the metabolic abnormality could replace damaged motor neurons by normal ones.

Spontaneous appearance of Tay-Sachs disease in an animal model

Tay-Sachs disease (TSD) is a progressive neurodegenerative disorder due to an autosomal recessively inherited deficiency of beta-hexosaminidase A (Hex A). Deficiency of Hex A in TSD is caused by a defect of the alpha-subunit resulting from mutations of the HEXA gene. To date, there is no effective treatment for TSD. Animal models of genetic diseases, similar to those known to exist in humans, are valuable and essential research tools for the study of potentially effective therapies. However, there is no ideal animal model of TSD available for use in therapeutic trials. In the present study, we report an animal model (American flamingo; Phoenicopterus ruber) of TSD with Hex A deficiency occurring spontaneously in nature, with accumulation of G(M2)-ganglioside, deficiency of Hex A enzymatic activity, and a homozygous P469L mutation in exon 12 of the hexa gene. In addition, we have isolated the full-length cDNA sequence of the flamingo, which consists of 1581 nucleotides encoding a protein of 527 amino acids. Its coding sequence indicates approximately 71% identity at the nucleotide level and about 72.5% identity at the amino acid level with the encoding region of the human HEXA gene. This animal model, with many of the same features as TSD in humans, could represent a valuable resource for investigating therapy of TSD.

Rapid detection of fetal Mendelian disorders: Tay-Sachs disease

Tay-Sachs disease is an autosomal recessive storage disease caused by the impaired activity of the lysosomal enzyme hexosaminidase A. In this fatal disease, the sphingolipid GM2 ganglioside accumulates in the neurons. Due to high carrier rates and the severity of the disease, population screening and prenatal diagnosis of Tay-Sachs disease are routinely carried out in Israel. Laboratory diagnosis of Tay-Sachs is carried out with biochemical and DNA-based methods in peripheral and umbilical cord blood, amniotic fluid, and chorionic villi samples. The assay of hexosaminidase A (Hex A) activity is carried out with synthetic substrates, 4-methylumbelliferyl-6-sulfo-N-acetyl-beta-glucosaminide (4-MUGS) and 4-methylumbelliferil-N-acetyl-beta-glucosamine (4-MUG), and the DNA-based analysis involves testing for the presence of specific known mutations in the alpha-subunit gene of Hex A. Prenatal diagnosis of Tay-Sachs disease is accomplished within 24-48 h from sampling. The preferred strategy is to simultaneously carry out enzymatic analysis in the amniotic fluid supernatant or in chorionic villi and molecular DNA-based testing in an amniotic fluid cell-pellet or in chorionic villi.

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

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

Evaluation of the Risk for Tay-Sachs Disease in Individuals of French Canadian Ancestry Living in New England

Background: The assessment of risk for Tay-Sachs disease (TSD) in individuals of French Canadian background living in New England is an important health issue. In preliminary studies of the enzyme-defined carrier frequency for TSD among Franco-Americans in New England, we found frequencies (1:53) higher than predicted from the incidence of infantile TSD in this region. We have now further evaluated the risk for TSD in the Franco-American population of New England.

Methods: Using a fluorescence-based assay for β-hexosaminidase activity, we determined the carrier frequencies for TSD in 2783 Franco-Americans. DNA analysis was used to identify mutations causing enzyme deficiency in TSD carriers.

Results: We determined the enzyme-defined carrier frequency for TSD as 1:65 (95% confidence interval 1:49 to 1:90). DNA-based analysis of 24 of the enzyme-defined carriers revealed 21 with sequence changes: 9 disease-causing, 4 benign, and 8 of unknown significance. Six of the unknowns were identified as c.748G>A p.G250S, a mutation we show by expression analysis to behave similarly to the previously described c.805G>A p.G269S adult-onset TSD mutation. This putative adult-onset TSD c.748G>A p.G250S mutation has a population frequency similar to the common 7.6 kb deletion mutation that occurs in persons of French Canadian ancestry.

Conclusions: We estimate the frequency of deleterious TSD alleles in Franco-Americans to be 1:73 (95% confidence interval 1:55 to 1:107). These data provide a more complete data base from which to formulate policy recommendations regarding TSD heterozygosity screening in individuals of French Canadian background.

Tay Sachs disease carrier screening in schools: educational alternatives and cheekbrush sampling

PURPOSE

Tay Sachs disease carrier screening programs have been offered successfully worldwide since 1970. The programs typically offer education, testing, and counseling to provide reproductive choices. One such program has been offered to Jewish school students in Melbourne since 1998. In a time of increasing public awareness of genetics, programs require continuous evaluation and updating.

METHODS

Over 2 successive years, a longitudinal evaluation involved students attending Jewish schools in Melbourne. Both qualitative and quantitative techniques were used to analyze alternative methods for education and sampling procedures. Comparisons involved (1) a computer-based resource versus an oral educational presentation and (2) blood sampling for enzyme and genetic testing versus cheekbrush testing for genetic sampling alone.

RESULTS

The education session was effective in significantly increasing students' knowledge (10.5% +/- 1.2%, P < .0001) and decreasing their anxiety about being a carrier (-12.2% +/- 1.6%, P < .0001). For the students, no significant differences were found between the computer-based resource and oral presentation. There were significantly more students accepting a carrier test and anxiety was lower when a cheekbrush test was offered compared with when a blood test was offered.

CONCLUSIONS

Computer-based instruction is equally effective, in addition to offering advantages of self-paced learning and minimization of human resources as an oral presentation within a genetic carrier screening program. Cheekbrush sampling is preferred to blood sampling and should be implemented into current practices for offering genetic screening programs. These results present alternatives to practices for genetic screening reflecting the current developing technology.

Diagnosis and molecular characterization of non-classic forms of Tay-Sachs disease in Brazil

Molecular analysis of five Brazilian families, including eight patients presenting with nonclassic Tay-Sachs disease, was performed to identify frequent causative mutations and their correlation with clinical course. Three patients were affected by the B1 subacute variant and were shown to carry the R178H mutation (the DN allele), which is also common among Portuguese patients. Two of them were compound heterozygotes, whereas the third presented with the mutation in both alleles. Since Brazil was a Portuguese colony for over two centuries, common ancestry might be the probable explanation. The fourth patient presented with a juvenile phenotype and carries the R499H mutation, which has been reported only once worldwide and is associated with residual enzyme activity, responsible for a slower clinical course. The fifth family, of an Ashkenazi Jewish background, showed an extensive intrafamilial clinical variability among three affected sibs presenting with muscle atrophy, ataxia, and psychiatric symptoms. They were first diagnosed as having atypical spinal muscular atrophy and, subsequently, spinocerebellar ataxia, but, recently, the diagnosis of late-onset Tay-Sachs disease was confirmed based on reduced plasma hexosaminidase A activity and the G269S/InsTATC1278 genotype. It is therefore highly recommended to test patients with a similar clinical history for Tay-Sachs disease. In the same family, one first cousin committed suicide at the age of 24 years, presenting with a clinical phenotype that suggested an undiagnosed case and highlighting the effect of the intrafamilial clinical variability in delaying a prompt diagnosis. It is now recognized that his parents are, in fact, a carrier couple. Additionally, another relative had been previously identified as a heterozygote in a Tay-Sachs disease screening program, but the information was not shared among the family. Since this information might anticipate diagnosis and genetic counseling, it is advisable that heterozygote screening programs encourage families to share genetic information.

Crystallographic structure of human beta-hexosaminidase A: interpretation of Tay-Sachs mutations and loss of GM2 ganglioside hydrolysis

Lysosomal beta-hexosaminidase A (Hex A) is essential for the degradation of GM2 gangliosides in the central and peripheral nervous system. Accumulation of GM2 leads to severely debilitating neurodegeneration associated with Tay-Sachs disease (TSD), Sandoff disease (SD) and AB variant. Here, we present the X-ray crystallographic structure of Hex A to 2.8 A resolution and the structure of Hex A in complex with NAG-thiazoline, (NGT) to 3.25 A resolution. NGT, a mechanism-based inhibitor, has been shown to act as a chemical chaperone that, to some extent, prevents misfolding of a Hex A mutant associated with adult onset Tay Sachs disease and, as a result, increases the residual activity of Hex A to a level above the critical threshold for disease. The crystal structure of Hex A reveals an alphabeta heterodimer, with each subunit having a functional active site. Only the alpha-subunit active site can hydrolyze GM2 gangliosides due to a flexible loop structure that is removed post-translationally from beta, and to the presence of alphaAsn423 and alphaArg424. The loop structure is involved in binding the GM2 activator protein, while alphaArg424 is critical for binding the carboxylate group of the N-acetyl-neuraminic acid residue of GM2. The beta-subunit lacks these key residues and has betaAsp452 and betaLeu453 in their place; the beta-subunit therefore cleaves only neutral substrates efficiently. Mutations in the alpha-subunit, associated with TSD, and those in the beta-subunit, associated with SD are discussed. The effect of NGT binding in the active site of a mutant Hex A and its effect on protein function is discussed.

Severe subacute GM2 gangliosidosis caused by an apparently silent HEXA mutation (V324V) that results in aberrant splicing and reduced HEXA mRNA

We have characterized the molecular basis of beta-hexosaminidase A (HEX A) deficiency in a patient ascertained through an ophthalmologic examination that revealed cherry red spots on his retina. The absence of neurological deficit in this child until 3 3/4 years of age indicated residual HEX A must be present. Three HEXA mutations, 10T > C (S4P) and 972T > A (V324V) on the maternal allele, and 1A > T (M1L) on the paternal allele were identified. The effects of the amino acid substitutions on HEX A expressed in COS-7 cells were analyzed; as expected, no HEX A activity was associated with the M1L mutation but surprisingly, the S4P mutation resulted in 59% of the HEX A activity expressed by the wild type cDNA. The effect of the S4P change was much less than that of another HEXA mutation, G269S, associated with an adult onset form of G(M2) gangliosidosis. This indicated that the S4P change was not the cause of disease and suggested that one of the mutations on the maternal allele, 10T > C or 972T > A, had its effect at the mRNA level. This was confirmed by Northern blot analysis that showed only 7% of the normal level of HEXA mRNA in proband fibroblasts. Analysis of the residual mRNA by RT/PCR and sequencing revealed normal transcripts from both the maternal and paternal allele, as well as a low abundance aberrant transcript from the maternal allele. Sequencing of this aberrant transcript revealed a new exon 8 donor site created by the 972T > A mutation that resulted in a 17 bp deletion and destabilization of the resulting abnormal transcript. The remaining normal mRNA produced from the 972T > A allele must account for the delayed onset of clinical symptoms in this child.

DNA-based carrier screening in the Ashkenazi Jewish population

Several relatively rare genetic diseases are found at greater frequencies in Ashkenazi Jewish populations. Most of these conditions are untreatable and shorten life expectancy. Genetic screening using molecular detection of a few common mutations for each of these diseases facilitates their prevention by identification of carrier couples. Conversely, couples with negative results are reassured by reduced carrier risks. Using a standardized format, a brief overview for each of the nine genetic diseases is presented. Known mutations, a short clinical summary, clinical and laboratory diagnostic methods and information on supportive treatments is provided for each. Finally, a brief discussion of available DNA testing technologies and a review of platforms for expanded testing options for Ashkenazi Jewish diseases under development are presented.

Heterozygosity for Tay-Sachs and Sandhoff Diseases in Non-Jewish Americans with Ancestry from Ireland, Great Britain, or Italy

Previous reports have found that non-Jewish Americans with ancestry from Ireland have an increased frequency of heterozygosity for Tay-Sachs disease (TSD), although frequency estimates are substantially different. Our goal in this study was to determine the frequency of heterozygosity for TSD and Sandhoff diseases (SD) among Irish Americans, as well as in persons of English, Scottish, and/or Welsh ancestry and in individuals with Italian heritage, who were referred for determination of their heterozygosity status and who had no known family history of TSD or SD or of heterozygosity for these conditions. Of 610 nonpregnant subjects with Irish background, 24 TSD heterozygotes were identified by biochemical testing, corresponding to a heterozygote frequency of 1 in 25 (4%; 95% CI, 1/39-1/17). In comparison, of 322 nonpregnant individuals with ancestry from England, Scotland, or Wales, two TSD heterozygotes were identified (1 in 161 or 0.62%; 95% CI, 1/328-1/45), and three TSD heterozygotes were ascertained from 436 nonpregnant individuals with Italian heritage (1 in 145 or 0.69%; 95% CI, 1/714-1/50). Samples from 21 Irish heterozygotes were analyzed for HEXA gene mutations. Two (9.5%) Irish heterozygotes had the lethal + 1 IVS-9 G --> A mutation, whereas 9 (42.8%) had a benign pseudodeficiency mutation. No mutation was found in 10 (47.6%) heterozygotes. These data allow for a frequency estimate of deleterious alleles for TSD among Irish Americans of 1 in 305 (95% CI, 1/2517-1/85) to 1 in 41 (95% CI, 1/72-1/35), depending on whether one, respectively, excludes or includes enzyme-defined heterozygotes lacking a defined deleterious mutation. Pseudodeficiency mutations were identified in both of the heterozygotes with ancestry from other countries in the British Isles, suggesting that individuals with ancestry from these countries do not have an increased rate of TSD heterozygosity. Four SD heterozygotes were found among individuals of Italian descent, a frequency of 1 in 109 (0.92%; 95% CI, 1/400-1/43). This frequency was higher than those for other populations, including those with Irish (1 in 305 or 0.33%; 95% CI, 1/252-1/85), English, Scottish, or Welsh (1 in 161 or 0.62%; 95% CI, 1/1328-1/45), or Ashkenazi Jewish (1 in 281 or 0.36%; 95% CI, 1/1361-1/96) ancestry. Individuals of Irish or Italian heritage might benefit from genetic counseling for TSD and SD, respectively.

Hexosaminidase A deficiency is an uncommon cause of a syndrome mimicking amyotrophic lateral sclerosis

Patients with adult hexosaminidase A (Hex A) deficiency may have clinical manifestations similar to amyotrophic lateral sclerosis (ALS). Mutations in the hexosaminidase A (HEXA) gene are common in the Jewish Ashkenazi population in Israel. Serum samples of 115 Israeli patients with sporadic ALS were screened for enzymatic activity to detect "enzyme-based carriers." Fifteen samples with low (< 50%) enzymatic activity were subjected to mutation analysis, which included the two common mutations in the HEXA gene among Ashkenazi Jews (+1278TATC and IVS12+1G-->C). Three "enzymatic carrier" patients of Moroccan origin were checked for two additional mutations (DeltaF304/305 and Arg170-->Gln), specific to this ethnic group. Two "enzymatic carrier" patients of Iraqi origin were analyzed for the mutation Gly250-->Val, specific to this population. The mutation Gly 269-->Ser was screened in carriers of Ashkenazi origin only (n = 10). The only abnormalities found were heterozygous +1278TATC mutations in two Ashkenazi patients. Their clinical presentation was not different from that usually encountered in ALS. The frequency of mutations in the HEXA gene among Israeli ALS patients was not higher than in the healthy Israeli population. Therefore, Hex A deficiency seems to be a very unlikely cause of an ALS-mimic syndrome.

Evaluation of a Tay-Sachs disease screening program

Tay-Sachs Disease (TSD) is an autosomal recessive neurodegenerative disorder. TSD is prevalent in the Ashkenazi Jewish population, and carrier screening programs have been implemented worldwide in these communities. A screening program initiated in 1997 involving the Melbourne Jewish community (Australia) incorporated education, counselling and carrier testing of high-school students aged 15 to 18 years. This study aimed to assess the participation rates, level of knowledge obtained and predicted feelings and attitudes of the students involved. Seven hundred and ten students participated, there was a 67% uptake for testing with a carrier rate of 1 in 28 determined. The level of knowledge of the students following education was high and of relative importance in regard to decision making, as were their feelings and attitudes about genetic testing for carrier status. A significant impediment to test uptake was the need for blood sampling, resulting in a recommendation for the introduction of DNA analysis on cheek brush samples. The evaluation of this program has given a wider scope for further development as well as providing valuable information for the implementation of community screening programs.

Experiences in molecular-based prenatal screening for Ashkenazi Jewish genetic diseases

Multiple-option prenatal carrier testing in the Ashkenazi Jewish community for three and now eight disorders has been readily accepted in this prenatal, health-oriented and knowledgeable population. Counseling of screenees concerning the nature (severity, treatability, etc.), inheritance, and frequencies of each disorder was essential for informed test choices and future reproductive decision making. The value of couple testing for a group of disorders when 1 in about 6 would be found to be a carrier of at least one disease was emphasized. These studies identified issues of education, confidentiality, posttest anxiety, and self-esteem that must be continuously addressed in the Ashkenazi population. However, an important value of these studies is that they provide a framework for the development of mass carrier screening programs in the general population or in specific segments of the population with similar demographic characteristics and in more diverse prenatal populations.

The frequency of Tay-Sachs disease causing mutations in the Brazilian Jewish population justifies a carrier screening program

CONTEXT

Tay-Sachs disease is an autosomal recessive disease characterized by progressive neurologic degeneration, fatal in early childhood. In the Ashkenazi Jewish population the disease incidence is about 1 in every 3,500 newborns and the carrier frequency is 1 in every 29 individuals. Carrier screening programs for Tay-Sachs disease have reduced disease incidence by 90% in high-risk populations in several countries. The Brazilian Jewish population is estimated at 90,000 individuals. Currently, there is no screening program for Tay-Sachs disease in this population.

OBJECTIVE

To evaluate the importance of a Tay-Sachs disease carrier screening program in the Brazilian Jewish population by determining the frequency of heterozygotes and the acceptance of the program by the community.

SETTING

Laboratory of Molecular Genetics--Institute of Biosciences--Universidade de São Paulo.

PARTICIPANTS

581 senior students from selected Jewish high schools.

PROCEDURE

Molecular analysis of Tay-Sachs disease causing mutations by PCR amplification of genomic DNA, followed by restriction enzyme digestion.

RESULTS

Among 581 students that attended educational classes, 404 (70%) elected to be tested for Tay-Sachs disease mutations. Of these, approximately 65% were of Ashkenazi Jewish origin. Eight carriers were detected corresponding to a carrier frequency of 1 in every 33 individuals in the Ashkenazi Jewish fraction of the sample.

CONCLUSION

The frequency of Tay-Sachs disease carriers among the Ashkenazi Jewish population of Brazil is similar to that of other countries where carrier screening programs have led to a significant decrease in disease incidence. Therefore, it is justifiable to implement a Tay-Sachs disease carrier screening program for the Brazilian Jewish population.

Prenatal genetic screening in the Ashkenazi Jewish population

The Ashkenazi Jewish community is a unique and ideal population in which to provide multiple disease screening because detection rates are high (> 95%) by testing a limited number of mutations. The residual risk that remains is very low. In addition, the lessons learned from carrier screening in this community indicate that only through genetic counseling and education can screening in the general population gain wide acceptance and provide maximum benefit.

31Phosphorus magnetic resonance spectroscopy in late-onset Tay-Sachs disease

The late-onset form of GM2 gangliosidosis (Tay-Sachs disease) is an autosomal-recessive disorder with progressive neurologic disease, mainly characterized by motor neuron and spinocerebellar dysfunction. The majority of patients are of Ashkenazi Jewish origin. 31Phosphorus magnetic resonance spectroscopy of the brain was performed to study the metabolic changes of a 16-year-old patient with late-onset Tay-Sachs disease who had a heterozygous Gly269-->Ser mutation in the hexosaminidase A encoding gene in compound heterozygosity with another, yet unidentified mutation. Severe changes in phosphorus metabolism with a decreased amount of phosphodiesters and membrane-bound phosphates were demonstrated, suggesting an activation of phosphodiesterases by accumulating gangliosides. The clinical findings were well related to the changes in spectroscopically determined metabolites.

Biallelic discrimination assays for the three common Ashkenazi Jewish mutations and a common non-Jewish mutation, in Tay-Sachs disease, using fluorogenic TaqMan probes

We have developed rapid semiautomated fluorogenic TaqMan assays for the three common Jewish mutations that occur in Tay-Sachs disease, the TATC 4-bp insertion in exon 11 (1,278insTATC), the IVS 12 + 1G --> C, splice site mutation in intron 12 (1421 + 1 G --> C), and the G --> A change at the 3' end of exon 7 (G269S), as well as for a non-Jewish mutation, IVS9 + I G --> A, believed to be prevalent in patients of Celtic descent. The TaqMan assays are designed to run on the ABI SDS 7700 sequence detection system, using allele-specific probes that carry a reporter dye at the 5' end and a quencher dye at the 3' end. Using a 96-well format, all four assays can be performed simultaneously on the same plate, with real-time fluorescence detection or just an end-point plate read. DNA samples from 78 patients identified as carriers by biochemical screening and genotyped by conventional techniques were used to assess the accuracy and efficiency of the probes in allelic discrimination assays. There were no discrepancies noted between previously assigned genotypes and the results obtained by application of this methodology.

Tay-Sachs screening in the Jewish Ashkenazi population: DNA testing is the preferred procedure

A unique screening program for the identification of Tay-Sachs Disease (TSD) heterozygotes has been performed in the tradi- tional Orthodox Ashkenazi Jewish (AJ) community since 1983. In recent years the program has utilized the biochemical assay for the determination of hexosaminidase A levels by the heat inactivation technique as well as by direct DNA analysis. The three mutations which were analyzed were those that have been shown to be prevalent among AJ TSD patients and carriers, namely the four nucleotide insertion mutation in exon 11 (1278+TATC), the splice mutation at the 5' end of intron 12 (1421+1g-->c), and the adult mutation, a Gly(269)-->Ser substitution in exon 5 (G269S). A total of 103,133 individuals were tested by biochemical analysis, and 38,197 of them were also assayed by DNA testing. Furthermore, 151 chromosomes from TSD patients or obligate heterozygotes were subjected to DNA analysis for one of the three mutations. DNA testing of the latter identified one of the three AJ mutations in every case, predicting a very high detection rate of heterozygotes in this community by this method. By contrast, the sensitivity of the enzyme assay ranged from 93.1% to 99.1% depending on the exclusion (inclusion) of inconclusive results as positive, while the specificity ranged from 88.1% to 98.8% depending on the inclusion (exclusion) of inconclusive results as positive. Our results strongly support the use of DNA testing alone as the most cost-effective and efficient approach to carrier screening for TSD in individuals of confirmed Ashkenazi Jewish ancestry.

Two mutated HEXA alleles in a Druze patient with late-infantile Tay-Sachs disease

Two affected HEXA alleles were found in an Israeli Druze Tay-Sachs child born to first-cousin parents. His paternal allele contained two adjacent changes in exon 5: delta496C, which resulted in a frameshift and premature termination codon 96 nucleotides downstream, and 498C-->G, a silent mutation. The maternal allele had a 835T-->C transition in exon 8 (S279P). Phosphoimaging quantitation of the parents' RNAs showed that the steady-state levels of mRNAs of the mutant exons 5 and 8 were 5% and 50%, respectively, of normal levels. The exon 5 mutated allele with the premature translation termination resulted in severe deficiency of Hex A. Transient expression of the exon 8 mutated alpha-chain cDNA in COS-1 cells resulted in deficiency of enzymatic activity. The child exhibited a late-infantile-type disease.

W474C amino acid substitution affects early processing of the alpha-subunit of beta-hexosaminidase A and is associated with subacute G(M2) gangliosidosis

Mutations in the HEXA gene, encoding the alpha-subunit of beta-hexosaminidase A (Hex A), that abolish Hex A enzyme activity cause Tay-Sachs disease (TSD), the fatal infantile form of G(M2) gangliosidosis, Type 1. Less severe, subacute (juvenile-onset) and chronic (adult-onset) variants are characterized by a broad spectrum of clinical manifestations and are associated with residual levels of Hex A enzyme activity. We identified a 1422 G-->C (amino acid W474C) substitution in the first position of exon 13 of HEXA of a non-Jewish proband who manifested a subacute variant of G(M2) gangliosidosis. On the second maternally inherited allele, we identified the common infantile disease-causing 4-bp insertion, +TATC 1278, in exon 11. Pulse-chase analysis using proband fibroblasts revealed that the W474C-containing alpha-subunit precursor was normally synthesized, but not phosphorylated or secreted, and the mature lysosomal alpha-subunit was not detected. When the W474C-containing alpha-subunit was transiently co-expressed with the beta-subunit to produce Hex A (alphabeta) in COS-7 cells, the mature alpha-subunit was present, but its level was much lower than that from normal alpha-subunit transfections, although higher than in those cells transfected with an alpha-subunit associated with infantile TSD. Furthermore, the precursor level of the W474C alpha-subunit was found to accumulate in comparison to the normal alpha-subunit precursor levels. We conclude that the 1422 G-->C mutation is the cause of Hex A enzyme deficiency in the proband. The resulting W474C substitution clearly interferes with alpha-subunit processing, but because the base substitution falls at the first position of exon 13, aberrant splicing may also contribute to Hex A deficiency in this proband.

Tay-Sachs disease and HEXA mutations among Moroccan Jews

Moroccan Jewry (N>750,000) is the only non-Ashkenazi Jewish community in which Tay-Sachs disease (TSD) is not extremely rare. Previous studies among Moroccan Jewish TSD families identified three HEXA mutations. In this study, extended to enzyme-defined and new obilgate TSD carriers, we found four additional mutations. One of them is a novel, IVS5-2(A-->G) substitution, resulting in exon skipping, and it was found only among enzyme-defined carriers. The seven HEXA identified mutations among Moroccan Jews are: deltaF(304/305), R170Q, IVS-2(A-->G), Y180X, E482K, 1278+TATC, and IVS12+1(G-->C). Their respective distribution among 51 unrelated enzyme-defined and obligate carriers is 22:19:6:1:1:1:1. The mutation(s) remain unknown in only three enzyme-defined carriers. Five of the seven Moroccan mutations, including the three most common ones, were not found among Ashkenazi Jews. Compared with the much larger and relatively homogeneous Ashkenazi population, the finding among Moroccan Jews probably reflects their much longer history.

Tay-Sachs disease-causing mutations and neutral polymorphisms in the Hex A gene

Tay-Sachs disease is an autosomal recessive disorder affecting the central nervous system. The disorder results from mutations in the gene encoding the alpha-subunit of beta-hexosaminidase A, a lysosomal enzyme composed of alpha and beta polypeptides. Seventy-eight mutations in the Hex A gene have been described and include 65 single base substitutions, one large and 10 small deletions, and two small insertions. Because these mutations cripple the catalytic activity of beta-hexosaminidase to varying degrees, Tay-Sachs disease displays clinical heterogeneity. Forty-five of the single base substitutions cause missense mutations; 39 of these are disease causing, three are benign but cause a change in phenotype, and three are neutral polymorphisms. Six nonsense mutations and 14 splice site lesions result from single base substitutions, and all but one of the splice site lesions cause a severe form of Tay-Sachs disease. Eight frameshift mutations arise from six deletion- and two insertion-type lesions. One of these insertions, consisting of four bases within exon 11, is found in 80% of the carriers of Tay-Sachs disease from the Ashkenazi Jewish population, an ethnic group that has a 10-fold higher gene frequency for a severe form of the disorder than the general population. A very large deletion, 7.5 kilobases, including all of exon 1 and portions of DNA upstream and downstream from that exon, is the major mutation found in Tay-Sachs disease carriers from the French Canadian population, a geographic isolate displaying an elevated carrier frequency. Most of the other mutations are confined to single pedigrees. Identification of these mutations has permitted more accurate carrier information, prenatal diagnosis, and disease prognosis. In conjunction with a precise tertiary structure of the enzyme, these mutations could be used to gain insight into the structure-function relationships of the lysosomal enzyme.

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.

Tay-Sachs disease carrier screening: a model for prevention of genetic disease

Tay-Sachs disease (TSD) is an autosomal-recessive, progressive, and ultimately fatal neurodegenerative disorder. Within the last 30 years, the discovery of the enzymatic basis of the disease, namely deficiency of the enzyme hexosaminidase A, made possible both enzymatic diagnosis of TSD and heterozygote identification. In the last decade, the cloning of the HEXA gene and the identification of more than 80 associated TSD-causing mutations has permitted molecular diagnosis in many instances. TSD was the first genetic condition for which community-based screening for carrier detection was implemented. As such, the TSD experience can be viewed as a prototypic effort for public education, carrier testing, and reproductive counseling for avoiding fatal childhood disease. More importantly, the outcome of TSD screening over the last 28 years offers convincing evidence that such an effort can dramatically reduce incidence of the disease.

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

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

The beta-hexosaminidase deficiency disorders: development of a clinical paradigm in the mouse

Tay-Sachs disease and Sandhoff disease are severe neurodegenerative disorders caused by a deficiency of beta-hexosaminidase A and resultant accumulation of its substrate, GM2 ganglioside, in neuronal lysosomes. The three clinical forms of the disorders (infantile, juvenile and adult) are of varying severity and onset, and have been correlated with the amount of residual GM2 ganglioside-degrading activity present in patients' cells. Through targeted disruption of the murine beta-hexosaminidase genes in embryonic stem cells, we have developed a set of mice that vary in their GM2 ganglioside-degrading capacity and exhibit many of the clinical features of the human diseases. These mice are valuable for the study of pathogenic mechanisms and for devising novel therapeutic strategies in these disorders.