The intron 7 donor splice site transition: a second Tay-Sachs disease mutation in French Canada

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.

Identification of novel mutations in HEXA gene in children affected with Tay Sachs disease from India

Tay Sachs disease (TSD) is a neurodegenerative disorder due to β-hexosaminidase A deficiency caused by mutations in the HEXA gene. The mutations leading to Tay Sachs disease in India are yet unknown. We aimed to determine mutations leading to TSD in India by complete sequencing of the HEXA gene. The clinical inclusion criteria included neuroregression, seizures, exaggerated startle reflex, macrocephaly, cherry red spot on fundus examination and spasticity. Neuroimaging criteria included thalamic hyperdensities on CT scan/T1W images of MRI of the brain. Biochemical criteria included deficiency of hexosaminidase A (less than 2% of total hexosaminidase activity for infantile patients). Total leukocyte hexosaminidase activity was assayed by 4-methylumbelliferyl-N-acetyl-β-D-glucosamine lysis and hexosaminidase A activity was assayed by heat inactivation method and 4-methylumbelliferyl-N-acetyl-β-D-glucosamine-6-sulphate lysis method. The exons and exon-intron boundaries of the HEXA gene were bidirectionally sequenced using an automated sequencer. Mutations were confirmed in parents and looked up in public databases. In silico analysis for mutations was carried out using SIFT, Polyphen2, MutationT@ster and Accelrys Discovery Studio softwares. Fifteen families were included in the study. We identified six novel missense mutations, c.340 G>A (p.E114K), c.964 G>A (p.D322N), c.964 G>T (p.D322Y), c.1178C>G (p.R393P) and c.1385A>T (p.E462V), c.1432 G>A (p.G478R) and two previously reported mutations. c.1277_1278insTATC and c.508C>T (p.R170W). The mutation p.E462V was found in six unrelated families from Gujarat indicating a founder effect. A previously known splice site mutation c.805+1 G>C and another intronic mutation c.672+30 T>G of unknown significance were also identified. Mutations could not be identified in one family. We conclude that TSD patients from Gujarat should be screened for the common mutation p.E462V.

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.

Population history and its impact on medical genetics in Quebec

Knowledge of the genetic demography of Quebec is useful for gene mapping, diagnosis, treatment, community genetics and public health. The French-Canadian population of Quebec, currently about 6 million people, descends from about 8500 French settlers who arrived in Nouvelle-France between 1608 and 1759. The migrations of those settlers and their descendants led to a series of regional founder effects, reflected in the geographical distribution of genetic diseases in Quebec. This review describes elements of population history and clinical genetics pertinent to the treatment of French Canadians and other population groups from Quebec and summarizes the cardinal features of over 30 conditions reported in French Canadians. Some were discovered in French Canadians, such as autosomal recessive ataxia of the Charlevoix-Saguenay (MIM 270550), agenesis of corpus callosum and peripheral neuropathy (MIM 218000) and French-Canadian-type Leigh syndrome (MIM 220111). Other conditions are particularly frequent or have special genetic characteristics in French Canadians, including oculopharyngeal muscular dystrophy, hepatorenal tyrosinaemia, cystic fibrosis, Leber hereditary optic neuropathy and familial hypercholesterolaemia. Three genetic diseases of Quebec First Nations children are also discussed: Cree encephalitis (MIM 608505), Cree leukoencephalopathy (MIM 603896) and North American Indian childhood cirrhosis (MIM 604901).

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.

Tay-Sachs disease screening and counseling families at risk for metabolic disease

Carrier testing for Tay-Sachs disease should be offered to couples when at least one individual is of Ashkenazi Jewish (carrier frequency 1/30), Pennsylvania Dutch, Southern Louisiana Cajun, or Eastern Quebec French Canadian descent. Ideally, testing is done prior to conception. For Ashkenazi Jews, in whom DNA testing identifies 99.9% of carriers, DNA testing is the preferred method to ascertain carriers [14]. For non-Jewish individuals seeking carrier testing, enzyme assay should be done initially and positive or indeterminate results should be confirmed by DNA mutation analysis. If only one partner is descended from a high-risk group, that person should be tested first; only if he/ she is a carrier should the other partner be tested. If the couple is pregnant at the time carrier testing is requested, both partners should have enzyme testing (leukocyte assay for the pregnant woman and serum assay for the father) and DNA testing sent concomitantly to expedite counseling and action. Carriers are individuals with a disease causing DNA mutation or carrier range enzyme analysis results on both serum and leukocytes with no detectable mutation and no pseudodeficiency alleles. Noncarriers are individuals with normal enzyme results or carrier range enzyme results and a pseudodeficiency allele on DNA mutation analysis. If both partners are found to be carriers they should be counseled of a 25% risk of having an affected child with each pregnancy. Options to modify this risk include prenatal diagnosis by amniocentesis or chorionic villus sampling, egg or sperm donation, preimplantation diagnosis or adoption.

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.

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.

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.

Multiple mutations in a specific gene in a small population

Hereditary diseases have been reported with relatively high frequency in some small populations. Founder effect and genetic drift, associated or not with selective advantage of heterozygotes in case of recessive diseases, are the main explanations. Therefore, if we consider one population and one particular disease, only one deleterious allele should be observed. Determination of mutations has shown that in most cases the situation is more complex; more than one mutation is found among the patients. This finding can be explained by a multiple founder effect, with genetic drift and new mutations.

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

We performed a genetic epidemiological analysis of American non-Jewish people with ancestry from Ireland or Great Britain with regard to heterozgosity for Tay-Sachs disease (TSD). This study was prompted by a recent report that the frequency of heterozygosity for TSD among Irish Americans was 1 in 8, a frequency much higher than that recognised for any other population group. We identified 19 of 576 (3.3%) people of Irish background as TSD heterozygotes by the standard thermolability assay for beta-hexosaminidase A (Hex A) activity. Three of 289 people of non-Irish British Isles background (1%) were also identified as heterozygotes by biochemical testing. Specimens from the biochemically identified Irish heterozygotes were analysed for seven different Hex A alpha subunit gene mutations; three (15.8%) had a lethal +1 IVS-9 G to A mutation, previously noted to be a common mutation among TSD heterozygotes of Irish ancestry. Eight of 19 (42.1%) had one of two benign or pseudodeficiency mutations, and no mutation was found in 42.1% of the heterozygotes analysed. These data indicate that non-Jewish Americans with Irish background have a significantly increased frequency of heterozygosity at the Hex A alpha subunit gene locus, but that approximately 42% of the biochemically ascertained heterozygotes have clinically benign mutations. A pseudodeficiency mutation was identified in one of the three TSD heterozygotes of non-Irish British Isles background; no mutations were found in the other two. The data allow for a frequency estimate of deleterious alleles for TSD among Irish Americans of 1 in 192 to 1 in 52. Non-Jewish Americans with ancestry from Great Britain have a minimal, if any, increase in rate of heterozygosity at the TSD gene locus relative to the general population.

Tay-Sachs disease in persons of French-Canadian heritage in northern New England

This study sought to determine whether persons of French-Canadian heritage in northern New England are at high risk for the lethal infantile form of Tay-Sachs disease. In order to accomplish this, death records and laboratory diagnostic records were surveyed to ascertain Tay-Sachs deaths in a cohort of 372,000 live births between 1977-1986. The proportion of the total population with French-Canadian or Jewish heritage was determined from census and birth records, and the ethnic background of Tay-Sachs cases was determined from the corresponding birth records. In 1,860 births, both parents were of Ashkenazi Jewish heritage. One of those children was diagnosed with Tay-Sachs disease. In 41,000 births, both parents were of French-Canadian heritage, and in an additional 93,000 births, one parent was of French-Canadian heritage. No cases of Tay-Sachs disease were identified in the offspring of those individuals. Approximately 14 cases (95% confidence interval 8-20) would be expected, if the gene frequency approximated that reported for individuals of Ashkenazi Jewish heritage. Based on the results of this study, routine testing for Tay-Sachs disease heterozygosity is not indicated for persons of French-Canadian heritage in northern New England. This conclusion may not necessarily be valid for persons of French-Canadian heritage residing in other states. Further studies of Tay-Sachs disease mutations and prevalence among persons of French-Canadian heritage will be important to determine possible regional variations in gene frequencies.

Geographic distribution of 18 autosomal recessive disorders in the French Canadian population of Saguenay-Lac-Saint-Jean, Quebec

We analysed the geographic distribution of 770 patients and 1084 obligate carriers of 18 autosomal recessive disorders that have a high incidence in Saguenay-Lac-Saint-Jean (SLSJ) (Quebec). The places of birth of the patients and the obligate carriers were found to be unevenly distributed in SLSJ. A statistically significant higher number of places of birth than expected (p < or = 0.05) was observed in Saguenay and Lac-Saint-Jean (LSJ) East, where the immigration was mainly the fact of individuals coming from Charlevoix. A significantly lower number of places of birth than expected was found in LSJ West, and in some municipalities in which the immigration and the contribution of the ancestors from Charlevoix was low. The geographic distribution of inbreeding and kinship only partially explained the geographic distribution of the disorders. The individuals who had both parents and all four grandparents born in SLSJ were at a higher risk of having a recessive disorder. While the genes of these disorders are still being introduced in SLSJ, they are also being dispersed in other regions and outside Quebec by emigrants from SLSJ.

Tay-Sachs disease: intron 7 splice junction mutation in two Portuguese patients

A single nucleotide transversion (G-->C) in the 5' donor site of intron 7 of the beta-hexosaminidase alpha-chain gene was identified in two Portuguese patients with infantile Tay-Sachs disease. One patient was found to be homozygous and the other a compound heterozygote with the four-base insertion in exon 11 on the other allele. In fibroblasts from the homozygous patient the beta-hexosaminidase alpha mRNA was observed as a nearly undetectable fast migrating band. Through cDNA-PCR amplification and hybridization with full length alpha cDNA several fragments of smaller size than the normal transcript were detected, most of them lacking exon 7. We propose that this point mutation in the 5' donor site of intron 7 of the beta-hexosaminidase alpha-chain gene is responsible for an inefficient and abnormal processing of the mutant transcript, resulting in functional abnormality.

High frequencies of human genetic diseases: founder effect with genetic drift or selection?

Rare genetic diseases have been reported with high frequency in some populations. The mechanisms which were proposed to explain most of these observations include founder effect, genetic drift or selective advantage. In recent years, many genes have been sequenced and mutations causing some of these disorders were characterized. According to the analysis of haplotypes and/or mutations, it may be possible to distinguish 3 groups of disorders frequent in isolated populations. In the first group, all the affected patients have only one frequent mutation, suggesting a founder effect with genetic drift. In the second group, more than one mutation is found among the patients; however, most of the patients are homozygotes for one frequent mutation which most probably originated from a common founder; the other patients are compound heterozygotes for the common mutation and a rare mutation. In the third group, more than one frequent mutation is found responsible for each disease. This may be due to a selective advantage which allows the expansion of each new mutation in the particular population or to multiple founder effect with genetic drift in smaller communities which thereafter mixed to form the larger population.

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.