Heterozygosity for Tay-Sachs disease in non-Jewish Americans with ancestry from Ireland or Great Britain

The incidence and carrier frequency of Tay-Sachs disease in the French-Canadian population of Quebec based on retrospective data from 24 years, 1992-2015

Tay-Sachs disease (TSD) is a hereditary neurodegenerative condition inherited through an autosomal recessive pattern. The incidence and carrier frequency of infantile TSD were found to be increased among French Canadians in specific areas of the province of Quebec or calculated from New England populations with French-Canadian heritage. No accurate infantile TSD carrier frequency for the whole French-Canadian population in Quebec has been published. In this study, we estimate the incidence and carrier frequency of infantile TSD in the Quebec French-Canadian population. The number of TSD cases was ascertained during the 1992-2015 period, as well as the number of births to mothers whose language of use is French. Seven cases of TSD have been diagnosed in Quebec during the period of ascertainment. This corresponds to an incidence of 1/218,144, which in turn corresponds to a carrier frequency of 1/234. In the same 24-year period, there are two French-Canadian couples who had a fetus prenatally diagnosed with TSD. If these cases are included, the incidence of TSD in the French-Canadian population of Quebec is 1/169,668 and the carrier frequency 1/206. These findings can be used for genetic counseling and policy decisions regarding carrier screening for TSD in populations of French-Canadian descent.

Patients' reactions and follow-up testing decisions related to Tay-Sachs (HEXA) variants of uncertain significance results

JScreen is a national public health initiative based out of Emory University that provides reproductive carrier screening through an online portal and follow-up genetic counseling services. In 2014, JScreen began reporting to patients variants of uncertain significance (VUSs) in the gene that causes Tay-Sachs disease (HEXA). Genetic counseling was provided to discuss the VUS and patients were offered hexosaminidase A (HEXA) blood enzyme testing to assist with VUS reclassification. To identify patient reactions and factors influencing their follow-up testing decisions after receiving these results, we conducted a retrospective quantitative study by administering online surveys to 62 patients with HEXA VUSs. Participants who pursued enzyme testing and those who did not both experienced low levels of distress when receiving the VUS results. Perceptions of HEXA carrier status after genetic counseling, decisional conflict levels, plans to have children in the near future, time available to pursue enzyme testing, and eligibility for research were significant factors influencing decision-making to pursue or not pursue enzyme testing. Genetic counseling played an important role in helping patients understand the VUS and follow-up testing options. When discussing VUSs with patients, it would be beneficial for genetic counselors to focus on the patient's perception of the VUS, anxiety related to the uncertainty of their results, and follow-up options, when available.

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.

Committee Opinion No. 691: Carrier Screening for Genetic Conditions

Carrier screening is a term used to describe genetic testing that is performed on an individual who does not have any overt phenotype for a genetic disorder but may have one variant allele within a gene(s) associated with a diagnosis. Information about carrier screening should be provided to every pregnant woman. Carrier screening and counseling ideally should be performed before pregnancy because this enables couples to learn about their reproductive risk and consider the most complete range of reproductive options. A patient may decline any or all screening. When an individual is found to be a carrier for a genetic condition, his or her relatives are at risk of carrying the same mutation. The patient should be encouraged to inform his or her relatives of the risk and the availability of carrier screening. If an individual is found to be a carrier for a specific condition, the patient's reproductive partner should be offered testing in order to receive informed genetic counseling about potential reproductive outcomes. If both partners are found to be carriers of a genetic condition, genetic counseling should be offered. What follows is a detailed discussion of some of the more common genetic conditions for which carrier screening is recommended in at least some segments of the population.

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.

GM2 gangliosidosis in a UK study of children with progressive neurodegeneration: 73 cases reviewed

AIM

To report the demographic, phenotypic, and time-to-diagnosis characteristics of children with GM2 gangliosidosis referred to the UK study of Progressive Intellectual and Neurological Deterioration.

METHOD

Case notification is made via monthly surveillance card, administered by the British Paediatric Surveillance Unit to all UK-based paediatricians; children with GM2 gangliosidosis were identified from cases satisfying inclusion in the UK study of Progressive Intellectual and Neurological Deterioration and analysed according to phenotypic and biochemical categories.

RESULTS

Between May 1997 and January 2010, 73 individuals with GM2 gangliosidoses were reported: 40 with Tay-Sachs disease, 31 with Sandhoff disease, and two with GM2 activator protein deficiency. Together they account for 6% (73/1164) of all diagnosed cases of progressive intellectual and neurological deterioration. The majority (62/73) were sporadic index cases with no family history. Children of Pakistani ancestry were overrepresented in all subtypes, particularly juvenile Sandhoff disease, accounting for 10 of 11 notified cases. Infantile-onset variants predominated (55/73); the mean age at onset of symptoms was 6.2 and 4.7 months for infantile-onset Tay-Sachs and Sandhoff disease respectively, and 26.2 and 34.7 months for the corresponding juvenile-onset variants. Time to diagnosis averaged 7.4 months and 28.0 months in infantile- and juvenile-onset disease respectively.

INTERPRETATION

GM2 gangliosidosis is a significant cause of childhood neurodegenerative disease; timely diagnosis relies upon improved clinical recognition, which may be increasingly important as specific therapies become available. There is a potential benefit from the introduction of screening programmes for high-risk ethnic groups.

From new screens to discovered genes: the successful past and promising present of single gene disorders

Prenatal screening for single gene disorders, which include over 10,000 diverse diseases, presents a great challenge. The major approach to identifying high-risk groups for diseases, from Tay Sachs Disease to sickle cell disease, has historically centered on ethnic-based screening. A major concern in an ethnic-based approach is that carriers belonging to less-traditionally considered populations will be missed. In the United States, the paradigm for a more modern pan-ethnic approach has become exemplified by cystic fibrosis (CF), although considerable debate about future directions remains. CF screening brings several additional issues to the forefront, including that the largest molecular prenatal genetic screening program is based on a single gene disorder that is not necessarily severely disabling. On the other hand, several devastating disorders where screening is indeed available remain relatively inaccessible to prenatal patients in the general population. Future candidates to consider for broad-based screening programs include spinal muscular atrophy (SMA), fragile X, and inborn errors of metabolism. As prenatal screening for single gene disorders expands, issues to consider include inclusion criteria and risk versus benefit.

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.

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.

Carrier screening for Gaucher disease in couples of mixed ethnicity

With the advent of mutational analysis for Gaucher disease, carrier screening has been incorporated into many Jewish genetic disease screening programs. Frequencies and mutations for Gaucher disease in non-Jewish populations are less well established and the detection rate of carriers are lower. Testing is problematic for resolving residual risk in a couple of mixed ethnicity. We report the testing choices made by 20 consecutive couples of mixed ethnicity where the Ashkenazi Jewish partner was identified to be a Gaucher disease gene carrier. Carrier studies of the non-Jewish partner were elected as follows: DNA studies alone, 5 (25%); enzymatic assay, 2 (10%); both, 6 (30%); no carrier studies, 7 (35%). Of the 7 couples not electing carrier studies, one was not in a pregnancy and 6 elected prenatal diagnosis in lieu of parental testing by enzymatic analysis of amniocytes. One couple elected parental carrier studies as well as prenatal diagnosis. All couples electing prenatal Gaucher determination had amniocentesis for other indications as well (4, advanced maternal age; 4, parental anxiety). We conclude that Gaucher screening is feasible for couples of mixed ethnicity if appropriate counseling and testing are offered.

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.

Heterozygosity for Tay-Sachs and Sandhoff diseases among Massachusetts residents with French Canadian background

OBJECTIVES

The frequency of Tay-Sachs disease (TSD) heterozygosity is increased among French Canadians in eastern Quebec. A large proportion of the New England population has French Canadian heritage; thus, it is important to determine if they too are at increased risk for TSD heterozygosity. This prospective study was designed to assess the TSD heterozygote frequency among people with French Canadian background living in Massachusetts. A simultaneous screen for heterozygosity for Sandhoff disease, a related genetic disorder, was also undertaken.

METHODS

1260 non-pregnant subjects of French Canadian background were included in the study. beta hexosaminidase activity was measured in blood samples, and results were evaluated for TSD and Sandhoff disease heterozygosity. Samples from the TSD heterozygotes were also subjected to mutation analysis.

RESULTS

Of the 1260 samples studied, 22 (1 in 57; CI 1 in 41, 1 in 98) were identified as TSD heterozygotes by enzymatic analyses and 11 subjects (1 in 114; CI 1 in 72, 1 in 280) were identified as Sandhoff disease heterozygotes. Three of the 22 TSD heterozygotes were found to have benign pseudodeficiency mutations, resulting in a maximum TSD heterozygote frequency of 19 in 1260 (1 in 66; CI 1 in 46, 1 in 120). Together, these data provide a maximum frequency of heterozygosity for TSD or Sandhoff disease of 30 in 1260 (1 in 42; CI 1 in 31, 1 in 64) in this population.

CONCLUSIONS

Simultaneous screening for TSD and Sandhoff disease heterozygosity by assay of beta hexosaminidases A and B activities provides a possible method for use with subjects of French Canadian background. The relevance of some of the novel mutations identified in this group needs further study. However, the comparatively high combined frequency of TSD and Sandhoff disease heterozygosity indicates a need for discussion regarding the appropriateness of carrier testing for these disorders for persons of French Canadian background in Massachusetts.

Novel mutations and DNA-based screening in non-Jewish carriers of Tay-Sachs disease

We have evaluated the feasibility of using PCR-based mutation screening for non-Jewish enzyme-defined carriers identified through Tay-Sachs disease-prevention programs. Although Tay-Sachs mutations are rare in the general population, non-Jewish individuals may be screened as spouses of Jewish carriers or as relatives of probands. In order to define a panel of alleles that might account for the majority of mutations in non-Jewish carriers, we investigated 26 independent alleles from 20 obligate carriers and 3 affected individuals. Eighteen alleles were represented by 12 previously identified mutations, 7 that were newly identified, and 1 that remains unidentified. We then investigated 46 enzyme-defined carrier alleles: 19 were pseudodeficiency alleles, and five mutations accounted for 15 other alleles. An eighth new mutation was detected among enzyme-defined carriers. Eleven alleles remain unidentified, despite the testing for 23 alleles. Some may represent false positives for the enzyme test. Our results indicate that predominant mutations, other than the two pseudodeficiency alleles (739C-->T and 745C-->T) and one disease allele (IVS9+1G-->A), do not occur in the general population. This suggests that it is not possible to define a collection of mutations that could identify an overwhelming majority of the alleles in non-Jews who may require Tay-Sachs carrier screening. We conclude that determination of carrier status by DNA analysis alone is inefficient because of the large proportion of rare alleles. Notwithstanding the possibility of false positives inherent to enzyme screening, this method remains an essential component of carrier screening in non-Jews. DNA screening can be best used as an adjunct to enzyme testing to exclude known HEXA pseudodeficiency alleles, the IVS9+1G-->A disease allele, and other mutations relevant to the subject's genetic heritage.