Tay-Sachs disease heterozygote detection: use of a centrifugal analyser for automation of hexosaminidase assays with two different artificial substrates

β-Hexosaminidase isoenzyme profiles in serum, plasma, platelets and mononuclear, polymorphonuclear and unfractionated total leukocytes

OBJECTIVES

The relative proportion in percentage of the isoenzyme A of beta-hexosaminidase (Hex) is the single discriminatory function most frequently used for the biochemical screening of heterozygote Tay-Sachs disease carriers. It has been suggested that the assay of the Hex isoenzymes in homogeneous cell preparations is preferable to that in mixed total leukocytes which present greater interindividual variation. The major aim of our study was the evaluation of this hypothesis.

DESIGN AND METHODS

Total Hex and its Hex A and Hex B isoenzymes were determined in different samples of serum and plasma (n = 81) as well as in lysates of platelets (n = 75), and mononuclear (n = 81), polymorphonuclear (n = 81) and mixed total leukocytes (n = 33).

RESULTS

The interindividual variations found for % Hex A in the different biological samples were: plasma (CV = 23.4%), platelets (CV = 10.2%), mononuclear (CV = 5.7%), polymorphonuclear (CV = 5.3%) and total leukocytes (CV = 7.1%). Although the relative proportion of Hex A was significantly greater in polymorphonuclear than in mononuclear leukocytes (P < 0.001), a statistical significance was not attained for the correlation between the relative proportions of blood polymorphonuclear cells and Hex A in mixed total leukocytes (r = 0.220).

CONCLUSIONS

The use of total leukocyte lysates does not appear to introduce a significant increase for the interindividual variation of the Hex A isoenzyme relative proportion in relation to the use of homogeneous cell preparations.

Thermodynamic determination of plasma and leukocyte beta-hexosaminidase isoenzymes in homozygote and heterozygote carriers for the GM2 gangliosidosis B1 variant

In the GM2 gangliosidosis B1 variant, the mutated isoenzyme A of beta-hexosaminidase (Hex) is incapable of hydrolyzing ganglioside GM2 and negatively charged substrates. Biochemical characterization of this lysosomal disease is carried out using synthetic alpha-subunit-specific sulfated substrates, as heat-inactivation assays are not applicable. The apparent enzyme activation energy of Hex using the chromogenic substrate 3,3'-dichlorophenolsulfonphthaleinyl N-acetyl-beta-D-glucosaminide is related directly to the relative proportions of Hex A and Hex B isoenzymes. This thermodynamic variable was used for the study of Hex enzyme heterogeneity in 3 patients with the GM2 gangliosidosis B1 variant and 6 heterozygote carriers. Hex activity was determined at 25 degrees C, 30 degrees C, 35 degrees C, and 37 degrees C in a Cobas Bio analyzer (Roche Diagnostics, Basel, Switzerland), and Arrhenius plot slopes and apparent activation energies were calculated in plasma samples and mononuclear and polymorphonuclear leukocyte lysates. The determination of the Hex isoenzymes in plasma presented a high discrimination power for B1 variant patients but not for heterozygote carriers, in whom false-negative results may be obtained. However, thermodynamic evaluation of the isoenzyme composition of Hex in leukocyte lysates permits the biochemical identification of patients with the GM2 gangliosidosis B1 variant and of heterozygote carriers.

Effect of collection, transport, processing and storage of blood specimens on the activity of lysosomal enzymes in plasma and leukocytes

This study was designed to evaluate the effect of different conditions of collection, transport and storage on the quality of blood samples from normal individuals in terms of the activity of the enzymes ss-glucuronidase, total hexosaminidase, hexosaminidase A, arylsulfatase A and ss-galactosidase. The enzyme activities were not affected by the different materials used for collection (plastic syringes or vacuum glass tubes). In the evaluation of different heparin concentrations (10% heparin, 5% heparin, and heparinized syringe) in the syringes, it was observed that higher doses resulted in an increase of at least 1-fold in the activities of ss-galactosidase, total hexosaminidase and hexosaminidase A in leukocytes, and ss-glucuronidase in plasma. When the effects of time and means of transportation were studied, samples that had been kept at room temperature showed higher deterioration with time (72 and 96 h) before processing, and in this case it was impossible to isolate leukocytes from most samples. Comparison of heparin and acid citrate-dextrose (ACD) as anticoagulants revealed that ss-glucuronidase and hexosaminidase activities in plasma reached levels near the lower normal limits when ACD was used. In conclusion, we observed that heparin should be used as the preferable anticoagulant when measuring these lysosomal enzyme activities, and we recommend that, when transport time is more than 24 h, samples should be shipped by air in a styrofoam box containing wet ice.

Novel splice site mutation at IVS8 nt 5 of HEXB responsible for a Greek-Cypriot case of Sandhoff disease

Sandhoff disease is caused by abnormalities in HEXB gene encoding the beta-subunit of beta-hexosaminidase. In this study, we analyzed the HEXB gene of a Sandhoff carrier in the Greek-Cypriot community. A G to C transversion was identified in one allele of her HEXB gene at position 5 of the 5'-splice site of intron 8 (IVS8 nt5). One of 13 cDNA clones derived from her lymphocyte HEXB mRNA lacked the last four nucleotides "GTTG" of exon 8, which created a premature termination codon at 11 codons downstream. In vivo transcription of the mutant HEXB gene fragment in CHO cells resulted in deletion of the "GTTG." The mutation has not been found in 40 DNA samples from anonymous donors, indicating that this is not a polymorphism in the Cypriot population. These results clearly indicate that the splice site mutation at IVS8 nt5 is responsible for this case of Sandhoff disease.

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.

Detection of Tay-Sachs disease carriers among individuals with thermolabile hexosaminidase B

The determination of hexosaminidases A and B in most programmes for Tay-Sachs disease carrier detection is based on their different heat sensitivity (hexosaminidase A is the heat labile isoenzyme). This routine cannot be employed for individuals who also possess a thermolabile hexosaminidase B. In Israel, 0.6% of the screened samples have a labile hexosaminidases B (about 110 each year) and the assessment of their hexosaminidase A activity has hitherto been based on isoenzyme separation by ion exchange chromatography. The latter requires relative large serum samples, and the individuals must usually be reappointed. In order to avoid the thermal treatment we have used the alternative technique, which employs two substrates with different specificities for the two isoenzymes: 1. The fluorogenic substance, 4-methylumbelliferyl-N-acetyl-glucopyranoside, which measures total hexosaminidase activity and 2. the derivative, 4-methylumbelliferyl-N-acetyl glucosamine-6-sulphate, which is considerably more specific toward hexosaminidase A. Hexosaminidase A activity was expressed as a ratio of total activities (the ratio of the assay with 4-methylumbelliferyl-N-acetyl glucosamine-6-sulphate to that with 4-methyllumbelliferyl-N-acetyl-glucopyranoside). Using the results from 65 obligate heterozygotes for Tay-Sachs disease, we established our reference ranges for assigning the genotypes with respect to the Tay-Sachs gene. Comparison of the results from 182 unrelated and randomly chosen sera screened by the ratio method and by heat inactivation, showed a very high correlation (r = 0.996). Sixty eight sera with thermolabile hexosaminidase B were tested by ion exchange chromatography and by the double substrate method, and they yielded identical diagnoses with regard to the Tay-Sachs locus.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-hexosaminidase splice site mutation has a high frequency among non-Jewish Tay-Sachs disease carriers from the British Isles

In the course of defining mutations causing Tay-Sachs disease (TSD) in non-Jewish patients and carriers from the British Isles, we identified a guanine to adenine change (also previously described) in the obligatory GT sequence of the donor splice site at the 5' end of intron 9 of the hexosaminidase alpha peptide gene. Of 24 unrelated mutant chromosomes from 20 non-Jewish subjects (15 TSD carriers, four TSD patients, and one TSD fetus), five had mutations common in the Ashkenazi Jewish community, and 10 had the intron 9 splice site mutation. This is an unexpected result considering the diverse origin of the population of the British Isles. This mutation was not found in 28 control UK subjects or 11 Jewish carriers of known TSD mutations. Before attempting detection of unknown mutations, non-Jewish TSD carriers from the British Isles should be screened for the intron 9 donor splice site mutation as well as those mutations which predominate in the Jewish community.

Frequency of the Tay-Sachs disease splice and insertion mutations in the UK Ashkenazi Jewish population

Tay-Sachs disease is a lethal neurodegenerative disorder caused by deficiency of the lysosomal enzyme beta-hexosaminidase A and inherited in an autosomal recessive fashion; carriers of the disease are 10 times more frequent in the Ashkenazi Jewish community than in the general population. Over 90% of North American Ashkenazi carriers tested have been shown to have either a splice site mutation at the boundary of exon 12 and intron 12 in the beta-hexosaminidase alpha subunit gene, or a 4 base pair insertion in exon 11. We describe simple assays involving amplification of DNA across these two mutation sites by polymerase chain reaction and the results of screening 75 subjects are given. The frequencies of the splice and insert mutations in 41 UK Ashkenazi carriers (20% to 80%, respectively) were similar to those found in the North American community. Twelve Ashkenazi subjects classified as non-carriers on enzyme assay were found to be negative for both mutations tested. All Ashkenazi carriers tested (both obligate carriers and those picked up by population screening) had either the splice or insert mutations; in contrast to this, only 21% of the non-Ashkenazi carriers had one or other of these mutations. It is concluded that within the carrier screening programmes for the Ashkenazi community, assays for the splice and insert mutations, together with an assay recently described for a mutation causing the rarer adult onset form of the disease, will prove useful as confirmatory tests for subjects who give positive or borderline results when screened on enzyme assay.