Tay-Sachs Screening Research Articles

A Universal Carrier Test for the Long Tail of Mendelian Disease

Abstract Background: Mendelian disorders are individually rare but collectively common, forming a "long tail" of genetic disease. More than 20 million people worldwide suffer from a disease in this long tail before the age of 25, with minorities and developing countries at highest risk and with the number of carriers far in excess of this figure. Importantly, the Jewish community’s campaign for universal Tay-Sachs screening shows that these incurable diseases can nevertheless be prevented if carrier status is known before conception. A single highly-accurate assay for the long tail of Mendelian disease would allow us to scale this successful campaign up to the general population, thereby improving millions of lives, greatly benefiting minority health, and saving billions of dollars.*Methods and Findings:*We have addressed the need for such an assay by designing the Universal Genetic Test (UNIT), a non-invasive, saliva-based carrier test for more than 100 Mendelian diseases across all major population groups. We exhaustively validated the test with a median of 147 positive and 525 negative samples per variant. By combining probes for risk alleles with family history information, we show that we can achieve extremely high levels of accuracy (median 95% CI [0.99988, 0.999999]), precision (median 95% CI [0.99993, 0.99999]), sensitivity (median 95% CI [0.99988, 0.999999]), and specificity (median 95% CI [0.99643, 1]) at the level of individual mutations. In particular, through a combination of replicated probes and confirmatory testing, we are able to reliably detect rare alleles at q ≈ 1/1000 with positive predictive values above 0.995. To put this in context, this performance for a multiplex assay compares favorably with FDA-approved single-gene carrier tests. Conclusions: The UNIT represents a dramatic reduction in the cost and complexity of large scale population screening. With a single inexpensive assay for a substantial fraction of the global Mendelian disease burden, an end to many preventable genetic diseases is now in sight. Moreover, given that the assay requires only a saliva sample, it is for the first time feasible to contemplate an "at-home carrier test" as a successor to the at-home pregnancy test.*Authors' note:*_Nature Precedings is a preprint server used by scientists to communicate results in advance of the often lengthy publication process. This manuscript is a preprint and has not yet been accepted for publication by a peer-reviewed journal. It is currently undergoing expedited review at a peer-reviewed journal and is posted publicly to allow collegial feedback in advance of publication. Please address technical comments to balajis@stanford.edu_

The troubled dream of genetic medicine Ethnicity and innovation in Tay-Sachs, cystic fibrosis, and sickle cell disease

The major scientific advance of the late 20th and early 21st centuries was the determination of the sequence of the entire human genome and, in turn, the repertoire of human genes. As part of the Human Genome Project, a program was established to study the ethical, legal, and social issues (ELSI) surrounding the availability of genetic information. The troubled dream of genetic medicine grew out of an extended collaboration between the authors, Keith Wailoo and Stephen Pemberton, originally for an ELSI-funded project about gene therapy. The authors came to appreciate the troubled history of gene therapy, a proposed panacea of much hype and few successful therapeutic developments. Wailoo received funding to “expand the scope of the work, by fitting the stories of cystic fibrosis, Tay-Sachs, and sickle cell disease into the broader context of racial politics, pain, and the politics of biomedicine in twentieth-century America.” As a result, they dreamt up a hodgepodge of a book that unfairly brands a small, but important, medical specialty. Is genetic medicine a “troubled dream”? During the last 50 years, genetic medicine developed as a specialty. Indeed, The metabolic and molecular bases of inherited disease (1), first published in 1956, defined the practice of the field. The editors and contributing authors’ approaches were reductionistic, methodical, and novel, often requiring patients or their advocates to make decisions about experimental therapies or reproductive choices. This resulted in a less paternalistic, more participatory physician-patient relationship that has since permeated most aspects of medicine. The field was also fueled by the discovery of genes and genetic disorders (catalogued in the database Online Mendelian Inheritance in Man; http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM), which has occurred at an exponential pace due to advancements in scientific methodology. Consumers became a driving force behind genetic research, funding efforts to develop diagnostic methods, treatments, and cures. Medical genetics became not only a diagnostic and therapeutic specialty, but also a preventive strategy. The possibility of preventing mental retardation in children by early intervention was apparent in the mid-1960s and led to the establishment of newborn screening programs, initially for phenylketonuria, but subsequently for other conditions, including sickle cell disease and cystic fibrosis — although this is not discussed by Wailoo and Pemberton. Reproductive genetic screening followed soon thereafter, so that by the early 1970s, amniocentesis to test for chromosomal and biochemical disorders gained a foothold. The newly created profession of genetic counseling, providing education and guidance related to the conditions for which testing was offered, extended the range of medical genetics. In the same era, the first Tay-Sachs screening programs were implemented. The National Academy of Sciences appreciated the importance of this emerging field of genetic screening and commissioned an expert panel that provided a canon for performing genetic screening. As testing for other conditions became available, medical geneticists, pediatricians, and obstetricians grappled with whether these ought to become part of panels of routine screening tests. The “failures” that Wailoo and Pemberton discuss are new treatments for cystic fibrosis and sickle cell disease, including gene therapy, heart-lung transplantation, bone marrow transplantation, and drug (specifically urea and hydroxyurea) therapies. The documentation of the failure of gene therapy trials is the true strength of the book (2). Organ and bone marrow transplantation and hydroxyurea treatment have been shown to be effective — although transplants are expensive, risky, and limited by donor availability. In general, drug therapies are expensive to develop, and the failure rate is high; however, high cost is not the rule. Some treatments, such as nighttime feeding for medium-chain acyl-CoA dehydrogenase deficiency and penicillin for sickle cell disease, have been remarkably effective and cheap. The authors’ comments about racial politics are unsubstantiated and verge on being calumnious. Most notable are their comments about the Dor Yesharim program, established to prevent autosomal recessive genetic conditions in the ultra-Orthodox Jewish population. The program tests potential couples prior to matchmaking to determine whether both are carriers for an autosomal recessive condition, in which case they are urged not to marry. They do not learn of the condition for which they are a carrier and thus are not branded or stigmatized. This program has tested over 150,000 people and has been instrumental in introducing screening tests. It is most certainly not a “nightmare,” as Wailoo and Pemberton call it. So genetic medicine is not a “troubled dream” but rather an active medical specialty that provides care to millions across the globe. It has had its therapeutic triumphs and failures — much like any other specialty in medicine.

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.

Tay-Sachs disease carrier screening: a 21-year experience.

This paper presents the findings of a community-based carrier screening program for Tay-Sachs disease, initiated on the University of Wisconsin-Madison campus in 1978. The Madison Community Tay-Sachs Screening Program (MCTSSP) is a collaborative, interdisciplinary program that organizes and conducts periodic screening for Tay-Sachs disease (TSD) for the purpose of identifying Tay-Sachs carriers. We present and analyze data on carrier detection with regard to various demographics, including family history of TSD, ancestry, gender, medication exposure, and illness. Individuals participating in the MCTSSP between 1978 and 1999 were primarily of the target population, and the carrier rate was within the expected range (1/25). Despite aggressive publicity efforts and a well-established program, attendance at the screens has declined. A recent survey of Jewish undergraduate students at the University of Wisconsin-Madison showed poor recall of family screen history and carrier status and reinforced the perception that utilization of the Madison screening program has been low. Ways to increase awareness of and interest in carrier screening for TSD are explored.

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 α-subunit of β-hexosaminidase A, the acid β-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 α-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. © 1996 Wiley-Liss, Inc.

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.

The molecular basis of Tay-Sachs disease: Mutation identification and diagnosis

Tay-Sachs disease is the prototype of lysosomal storage disease. While it was first described over a century ago, the defective enzyme was not identified until 1969, making possible the development of enzyme-based diagnostic and carrier screening techniques. This led to the establishment of the successful international Tay-Sachs screening program, primarily for the high risk Ashkenazi Jewish population. In the past five years the development of recombinant DNA technology has allowed researchers to characterize 95-99% of the mutations causing Tay-Sachs disease in this high risk ethnic group. Knowledge of the exact mutations responsible for the disease coupled with the powerful polymerase chain reaction technique has now made DNA-based screening and diagnosis possible. While the enzyme-based test has proven to be reliable and economical, it cannot differentiate variant phenotypes and requires the presence of specialized testing centers. Although the DNA-based test is presently less economical, it can provide carrier couples with their exact genotype and thus, predict the general phenotype of an unborn child. Furthermore, as the catalogue of mutations leading to human disease increases, more economical DNA methodologies will be developed. In the future it would be expected that a laboratory using a single DNA-based technology could diagnose and screen for a myriad of human diseases including Tay-Sachs disease.

Evaluation of a Tay-Sachs screening program on a college campus.

A screening program for Tay-Sachs disease was done on a predominantly unmarried, university student population and later evaluated. Ten carriers were detected among a population of 390 screened. Voluntarily screened individuals were given a questionnaire at the time of screening and after their carrier status was revealed. In addition, individuals in a target population were queried as to why they did not attend the screening. Results of evaluation indicate that both the screened and unscreened students were able to answer approximately two thirds of a set of questions on Tay-Sachs correctly, but there was widespread lack of knowledge regarding the implications of carrier status and inheritance of the disease. In general, respondents to the questionnaires were not opposed to abortion. However, when asked about the family planning alternatives that they would consider if they were carriers, more subjects preferred adoption, artificial insemination, or avoidance of pregnancy to abortion.

Inborn errors of lysosomal catabolism--principles of heterozygote detection.

Carriers of an inborn error of lysosomal catabolism can be recognized, as they have enzyme levels approximately half those of normal individuals. Of the various tissues readily available for assay, plasma and leukocytes and, in some situations, tears are preferred. Although mixed leukocytes have proved satisfactory in Tay-Sachs screening programs, purified preparations of granulocytes or lymphocytes will allow better discrimination in most situations. Enzymes are assayed relative to some other reference parameter which must be a constant or highly correlated with test enzyme activity. In the two mass screening programs in operation, beta-hexosaminidase A and alpha-mannosidase have both been assayed relative to total beta-hexosaminidase activity. Carrier detection is particularly important in X-linked diseases. The techniques used mostly involve hair roots or fibroblasts and depend on random inactivation of the X chromosome. In the mucolipidoses II and III, in which there are a number of deficient enzymes in cells, carriers may be identified on the basis of the ratio of beta-hexosaminidase I1 and I2 to total hexosaminidase.

Psychosocial aspects of a Tay-Sachs screening clinic.

The authors administered questionnaires to participants in a Tay-Sachs screening clinic. They received 322 responses. Although the majority of the respondents expressed anxiety about the possibility that they might be carriers, most indicated that this anxiety brought them to the clinic. The authors conclude that a well-publicized and well-organized screening clinic can function successfully and deal with the issue of anticipated loss without producing mass fear. An important factor in their clinic was the fact that they were able to offer an alternative to a Tay-Sachs child through amniocentesis.

“Instrumental coping” and “Affective defense”: An additional perspective in health promoting behavior

Abstract A study intended to reveal factors that may further or impede engagement in preventive behavior when both engagement and non-engagement in the prescribed behavior will have the probability of imposing a high cost, was carried out among women who participated in a Tay-Sachs screening program. Data revealed that pregnant women, for whom both participation as well as nonparticipation may incur a higher cost than for others, tended more than others to participate, but they also tended more than others to employ defense mechanisms. Based on these results it has been suggested that in such situations an individual's engagement in preventive behavior will be a result of “Instrumental coping” as well as of “Affective defense”. The study may be considered as a further step in the understanding of the inclination to engage in preventive behavior in situations which are fairly typical to screening programs, where both participation and non-participation may involve the probability of a relatively high cost. The practical contribution of this study may be in its stressing the differential appeals to be employed in a campaign aimed at encouraging participation of various population groups in screening programs, by the level of the possible stresspotential involved in engaging in the prescribed behavior.

Ms. Clow and Dr. Scriver Reply

Dr. Kuhr can cite the “bad” aspects of our Tay-Sachs screening program among adolescents; others may choose to cite the “good.”Both facets are presented in our report. The open-minded reader will find Dr. Kuhr's selective quotations less than the whole story. For example, he will learn that later attitudes among Tay-Sachs screenees, after initial worry has been dispelled, are largely benign and very similar in screened carriers and noncarriers. Moreover, 88.4% of carriers say

Letter: Tay-Sachs screening.

Letter: Tay-Sachs screening.

Letter: Benefits of Tay-Sachs screening.

Letter: Benefits of Tay-Sachs screening.

Letter: Doubtful benefits of Tay-Sachs screening.

Letter: Doubtful benefits of Tay-Sachs screening.