Specificity and sensitivity of hexosaminidase assays and DNA analysis for the detection of Tay-Sachs disease gene carriers among Ashkenazic Jews

Prenatal Diagnosis of Tay-Sachs Disease

Tay-Sachs disease (TSD) is an autosomal recessive lysosomal storage disorder caused by mutations of the HEXA gene resulting in the deficiency of hexosaminidase A (Hex A) and subsequent neuronal accumulation of G_M2 gangliosides. Infantile TSD is a devastating and fetal neurodegenerative disease with death before the age of 3-5 years. A small proportion of TSD patients carry milder mutations and may present juvenile or adult onset milder disease. TSD is more prevalent among Ashkenazi Jewish (AJ) individuals and some other genetically isolated populations with carrier frequencies of approximately ~1:27 which is much higher than that of 1:300 in the general population. Carrier screening and prenatal testing for TSD are effective in preventing the birth of affected fetuses greatly diminishing the incidence of TSD. Testing of targeted HEXA mutations by genotyping or sequencing can detect 98% of carriers in AJ individuals; however, the detection rate is much lower for most other ethnic groups. When combined with enzyme analysis, above 98% of carriers can be reliably identified regardless of ethnic background. Multiplex PCR followed by allele-specific primer extension is one method to test for known and common mutations. Sanger sequencing or other sequencing methods are useful to identify private mutations. For prenatal testing, only predefined parental mutations need to be tested. In the event of unknown mutational status or the presence of variants of unknown significance (VUS), enzyme analysis must be performed in conjunction with DNA-based assays to enhance the diagnostic accuracy. Enzymatic assays involve the use of synthetic substrates 4-methylumbelliferyl-N-acetyl-β-glucosamine (4-MUG) and 4-methylumbelliferyl-6-sulfo-2-acetamido-2-deoxy-β-D-glucopyranoside (4-MUGS) to measure the percentage Hex A activity (Hex A%) and specific Hex A activity respectively. These biochemical and molecular tests can be performed in both direct specimens and cultured cells from chorionic villi sampling or amniocentesis.

Platelet hexosaminidase a enzyme assay effectively detects carriers missed by targeted DNA mutation analysis

Biochemical testing of hexosaminidase A (HexA) enzyme activity has been available for decades and has the ability to detect almost all Tay-Sachs disease (TSD) carriers, irrespective of ethnic background. This is increasingly important, as the gene pool of those who identify as Ashkenazi Jewish is diversifying. Here we describe the analysis of a cohort of 4,325 individuals arising from large carrier screening programs and tested by the serum and/or platelet HexA enzyme assays and by targeted DNA mutation analysis. Our results continue to support the platelet assay as a highly effective method for TSD carrier screening, with a low inconclusive rate and the ability to detect possible disease-causing mutation carriers that would have been missed by targeted DNA mutation analysis. Sequence analysis performed on one such platelet assay carrier, who had one non-Ashkenazi Jewish parent, identified the amino acid change Thr259Ala (A775G). Based on crystallographic modeling, this change is predicted to be deleterious, as threonine 259 is positioned proximal to the HexA alpha subunit active site and helps to stabilize key residues therein. Accordingly, if individuals are screened for TSD in broad-based programs by targeted molecular testing alone, they must be made aware that there is a more sensitive and inexpensive test available that can identify additional carriers. Alternatively, the enzyme assays can be offered as a first tier test, especially when screening individuals of mixed or non-Jewish ancestry.

Community genetics and dignity in diversity in the Quebec Network of Genetic Medicine

The Quebec Network of Genetic Medicine (QNGM), implemented in 1971, has been an integrated program of community genetics serving the population (approximately 7.5 million) of Quebec province in Canada. QNGM reported to the Minister of Social Affairs and operated under an umbrella of universal health insurance in the province. The Network's programs have been run by members of the four university medical schools of the province under the direction of a central committee. A global annual budget was awarded to QNGM from its inception. Among its many programs, QNGM supported: (1) two newborn screening programs (using blood and urine samples) for early diagnosis, treatment and research in phenylketonuria, hereditary tyrosinemia, congenital hypothyroidism, and in a large number of other hereditary metabolic diseases; (2) follow-up of confirmatory diagnostic tests at regional centers, followed by supervision of ambulatory treatment modalities; (3) carrier screening and reproductive counseling for Tay-Sachs and beta-thalassemia diseases; (4) a spectrum of feasibility (research) studies (e.g., screening for biotinidase deficiency, neuroblastoma, hemoglobinopathies, and cystic fibrosis) to inform policy decisions. QNGM performed economic analyses of its major programs and followed prevailing ethical guidelines. Its global budget and integrated structure terminated in 1994, although some of its programs continue independently.

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.

Molecular screening for diseases frequent in Ashkenazi Jews: lessons learned from more than 100,000 tests performed in a commercial laboratory

PURPOSE

To determine the frequency of carriers of Ashkenazi Jewish (AJ) genetic diseases in the US population and compare these numbers with previously published frequencies reported in smaller more isolated cohorts.

METHODS

A database containing more than 100,000 genotyping assays was queried. Assays for 10 separate AJ genetic diseases where comparisons were made with published data.

RESULTS

As expected, we observed lower carrier frequencies in a general, US population than those reported in literature. In 2427 patients tested for a panel of 8 AJ diseases, 20 (1:121) were carriers of two diseases and 331 (1:7) were carriers of a single disease. Fifty-three of 7184 (1:306) individuals tested for Gaucher disease had 2 Gaucher Disease mutations indicating a potentially affected phenotype.

CONCLUSIONS

As the number of AJ diseases increases, progressively more individuals will be identified as carriers of at least one disease.

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.

Human genetics: lessons from Quebec populations

The population of Quebec, Canada (7.3 million) contains approximately 6 million French Canadians; they are the descendants of approximately 8500 permanent French settlers who colonized Nouvelle France between 1608 and 1759. Their well-documented settlements, internal migrations, and natural increase over four centuries in relative isolation (geographic, linguistic, etc.) contain important evidence of social transmission of demographic behavior that contributed to effective family size and population structure. This history is reflected in at least 22 Mendelian diseases, occurring at unusually high prevalence in its subpopulations. Immigration of non-French persons during the past 250 years has given the Quebec population further inhomogeneity, which is apparent in allelic diversity at various loci. The histories of Quebec's subpopulations are, to a great extent, the histories of their alleles. Rare pathogenic alleles with high penetrance and associated haplotypes at 10 loci (CFTR, FAH, HBB, HEXA, LDLR, LPL, PAH, PABP2, PDDR, and SACS) are expressed in probands with cystic fibrosis, tyrosinemia, beta-thalassemia, Tay-Sachs, familial hypercholesterolemia, hyperchylomicronemia, PKU, oculopharyngeal muscular dystrophy, pseudo vitamin D deficiency rickets, and spastic ataxia of Charlevoix-Saguenay, respectively) reveal the interpopulation and intrapopulation genetic diversity of Quebec. Inbreeding does not explain the clustering and prevalence of these genetic diseases; genealogical reconstructions buttressed by molecular evidence point to founder effects and genetic drift in multiple instances. Genealogical estimates of historical meioses and analysis of linkage disequilibrium show that sectors of this young population are suitable for linkage disequilibrium mapping of rare alleles. How the population benefits from what is being learned about its structure and how its uniqueness could facilitate construction of a genomic map of linkage disequilibrium are discussed.

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.

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.

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

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

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 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.

Inherited colorectal polyposis and cancer risk of the APC I1307K polymorphism

Germ-line and somatic truncating mutations of the APC gene are thought to initiate colorectal tumor formation in familial adenomatous polyposis syndrome and sporadic colorectal carcinogenesis, respectively. Recently, an isoleucine-->lysine polymorphism at codon 1307 (I1307K) of the APC gene has been identified in 6%-7% of the Ashkenazi Jewish population. To assess the risk of this common APC allelic variant in colorectal carcinogenesis, we have analyzed a large cohort of unselected Ashkenazi Jewish subjects with adenomatous polyps and.or colorectal cancer, for the APC I1307K polymorphism. The APC I1307K allele was identified in 48 (10.1%) of 476 patients. Compared with the frequency in two separate population control groups, the APC I1307K allele is associated with an estimated relative risk of 1.5-1.7 for colorectal neoplasia (both P=.01). Furthermore, compared with noncarriers, APC I1307K carriers had increased numbers of adenomas and colorectal cancers per patient (P=.03), as well as a younger age at diagnosis. We conclude that the APC I1307K variant leads to increased adenoma formation and directly contributes to 3%-4% of all Ashkenazi Jewish colorectal cancer. The estimated relative risk for carriers may justify specific clinical screening for the 360,000 Americans expected to harbor this allele, and genetic testing in the setting of long-term-outcome studies may impact significantly on colorectal cancer prevention in this population.

Prenatal diagnosis for inborn errors of metabolism and haemoglobinopathies: the Montreal Children's Hospital experience

We reviewed all referrals for prenatal diagnosis for inborn errors of metabolism and haemoglobinopathies performed at the Montreal Children's Hospital Prenatal Diagnosis Centre/McGill University during the period 1990-1995; 92 procedures were performed for these indications (less than 1 per cent of all referrals for prenatal diagnosis). All prenatal diagnoses for haemoglobinopathies (n = 55) were exclusively DNA-based. The three most frequent referrals were for beta-thalassaemia, sickle cell anaemia, and Tay-Sachs disease, accounting for 68 per cent of cases; the other indications were predominantly for untreatable inborn errors of metabolism. Our unit maintains population-based carrier screening programmes in high schools for beta-thalassaemia and Tay-Sachs diseases. Carriers detected in these programmes accounted for the majority of referrals for these two conditions. This study indicates that carrier testing and screening for sickle cell anaemia may be also welcomed by at-risk groups in Quebec.

Population-specific screening by mutation analysis for diseases frequent in Ashkenazi Jews

We describe a partially automated DNA mutation assay for detecting the most frequent mutations in the alpha-subunit of beta-hexosaminidase A, the acid beta-glucosidase and the cystic fibrosis transmembrane conductance regulator genes for the Ashkenazi Jewish population. The assay detects carriers for Tay-Sachs disease, Gaucher disease, and cystic fibrosis with sensitivities of at least 92%, 96%, and 97%, respectively. Among 1,364 young adults of Ashkenazic ancestry in the Dor Yeshurim community who were tested, 52 were Tay-Sachs carriers, 110 were Gaucher carriers, and 62 were cystic fibrosis carriers. Ten individuals were carriers for two diseases, and three unsuspected cases were diagnosed with Gaucher disease based on mutation test results. In addition to Tay-Sachs mutation data, results for hexosaminidase A activity were also available. All of 1,254 samples normal by enzyme quantitation were also negative for the three alpha-subunit mutations tested, and all of 43 samples with 'inconclusive' enzyme results were negative by DNA. Only 52 of 67 samples positive by enzyme assay were also positive for one of the three mutations tested for Tay-Sachs disease. The data suggest a high degree of false positivity inherent in enzyme identification of carriers. There are no correlative methods to assess the sensitivity of Gaucher and CF carrier testing. The results show that population screening can be carried out efficiently by DNA analysis, with the accrual of carrier information for three separate diseases conducted as a single test. Furthermore, the DNA method for Tay-Sachs screening appears to exceed the specificity of hexosaminidase A enzyme testing.

A robotics-assisted procedure for large scale cystic fibrosis mutation analysis

We describe a convenient, efficient, semiautomated protocol for assaying large numbers of DNA samples for over 20 mutations causing cystic fibrosis. The protocol uses the following: (1) a programmable robotic workstation to perform rapid pipetting and dot-blotting operations, (2) an allele-specific oligonucleotide hybridization in a single water bath without correcting for G+C content of oligonucleotides, and (3) a combinatorial system that allows direct determination of the genotype for more frequent mutations. We have used this system routinely for 16 months for carrier detection and for diagnosis of cystic fibrosis. The method can be readily applied to any combination of allele-specific oligonucleotide assays whether for multiple alleles at one locus or for a few alleles at multiple loci.

Tay-Sachs disease screening and diagnosis: evolving technologies

Tay-Sachs disease (TSD) is an autosomal recessive, progressive, and fatal neurodegenerative disorder. Within the last 25 years, the discovery of the enzymatic basis of the disease, the deficiency of the enzyme hexosaminidase A, has made possible both enzymatic diagnosis of TSD and heterozygote identification. TSD is the first genetic condition for which a community-based heterozygote screening program was attempted with the intention of reducing the incidence of a genetic disease. In this article we review the clinical, biochemical, and molecular features of TSD as well as the development of laboratory technology that has been deployed in community genetic screening programs. We describe the assay procedures used and some of the limitations in their accuracy. We consider the impact of DNA-based technology on the process of identification of individuals carrying mutant genes associated with TSD and we discuss the social context within which genetic screening occurs.

What young people think and do when the option for cystic fibrosis carrier testing is available

We report findings in phase II of a pilot study of cystic fibrosis (CF) carrier screening/testing by mutation analysis. Phase I has been reported elsewhere. Eligible participants in phase II (n = 815) were students (15 to 17 years of age) in public high schools. An educational component (exchange of information and discussion about common genetic disorders including CF) preceded, by one week or more, voluntary participation in the screening component which required a blood sample. The uptake rate for screening was 42%. Nine carriers (2pq = 0.0260) were identified, all with the delta F508 mutation; students were also tested for G551D, G542X, W1282X, and -549-mutations, but no carriers of these alleles were found. Carriers had positive views of the education and testing experiences. Persons identified as 'non-carriers' were also surveyed (n = 135, response rate 41%). As in phase I, the majority (83%) again understood that a negative DNA test had not excluded them from possible carrier status. Students who participated in the informational component but were not screened served here as controls in the follow up survey (n = 208, response rate 53%). Their views were similar to those of the screened non-carriers, and similar also to those held by students, adults, pregnant women, couples, and CF relatives in other communities.