Mutation Report for: E497V_human-BCHE

A recent study has linked the butyrylcholinesterase (BChE) K-variant and the apolipoprotein epsilon4 isoform to late-onset Alzheimer's disease. These findings have been controversial and have led us to examine the differences between wild-type and K-variant BChE in enzyme activity, protein stability, and quaternary structure. J-variant BChE (E497V/A539T) was also studied because it is associated with the K-variant mutation. The K-variant mutation (A539T) is located in the C-terminal tetramerization domain. Wild-type, K-variant, and J-variant BChE were expressed in Chinese hamster ovary cells and purified. The purified enzymes had similar binding affinity (Km) values and catalytic rates for butyrylthiocholine and benzoylcholine. In pulse-chase studies the K-variant, J-variant, and wildtype BChE were degraded rapidly within the cell, with a half-time of approximately 1.5 h. Less than 5% of the intracellular BChE was exported. The C-terminal peptide containing the K-variant mutation interacted with itself as strongly as did the wild-type peptide in the yeast two-hybrid system. Both K-variant and wild-type BChE assembled into tetramers in the presence of poly-L-proline or the proline-rich attachment domain of the collagen tail. The native K-variant BChE in serum showed the same proportion of tetramers as the native serum wild-type BChE. We conclude that the K-variant BChE is similar to wild-type BChE in enzyme activity, protein turnover, and tetramer formation.

The time course of inhibition of butyrylcholinesterase (EC 3.1.1.8) by the dimethylcarbamate Ro 02-0683 in sera taken from patients heterozygous for the usual (U), atypical (A), K or J variants was followed using propionylthiocholine as substrate. Data obtained were used to determine rate constants of inhibition together with the contribution made by each variant to total enzyme activity. The findings substantiate earlier reports that J and K mutations lead to quantitative changes in the concentration of usual enzyme in contrast to the qualitative changes of the atypical variant. The contribution of the atypical enzyme to the total activity in serum from UA, AK and AJ heterozygotes was respectively 17-20, 24-31 and 34-53%. The altered ratios of atypical to usual, K or J enzyme in UA, AK and AJ together with the constants on the usual enzyme alone, explain the differences in observed inhibitor numbers which enable these heterozygotes to be identified.

Catalysed hydrolysis of butyrylthiocholine (BTCh) by the usual (UU), fluoride-resistant (FS), AK, AJ and atypical (AA) human serum butyrylcholinesterase (EC 3.1.1.8) variants was measured in phosphate buffer pH 7.4 at 25 degrees C. pS-curves for all phenotypes were S-shaped; the activities rose to a plateau with increasing substrate concentration except at 100 mM where there was a small decrease. To obtain the catalytic constants, three equations were applied: Michaelis-Menten equation (Eq. 1), Hill equation (Eq. 2) and an equation which assumes simultaneous binding of the substrate to the catalytic site and to a peripheral site on the enzyme (Eq. 3). Over a range from 0.01 to 50 mM BTCh, the activity versus substrate concentration relationship deviated from Michaelis-Menten kinetics (Eq. 1) while data fitted well with Eqs. 2 and 3. The Michaelis-Menten equation was applied separately to two BTCh concentration ranges: the corresponding Km constants for the UU, FS, AK, AJ and AA phenotypes ranged from 0.1 to 0.2 mM (at 0.01-1.0 mM BTCh) and from 0.3 to 2.0 mM (at 1.0-50 mM BTCh). Hill coefficients (nH) calculated from Eq. 2 were similar for all phenotypes (nH approximately 0.5). The dissociation constants K1 and K2 calculated from Eq. 3 for two sites on the enzyme fell between 0.02 and 0.12 mM (K1) and 0.89 and 4.9 mM (K2) for the five phenotypes. Experimental data support the assumption that the phenotypes studied have two substrate binding sites.

We have identified 12 kinds of genetic mutations of butyrylcholine esterase (BCHE) from phenotypic abnormalities, showing that BCHE activities were deficient or diminished in sera. These genetic mutations, detected by PCR-single-strand conformation polymorphism analysis and direct sequencing, consisted of one deletion (BCHE*FS4), nine missense (BCHE*24 M, *1005, *250P, *267R, *330I, *365R, *418S, *515C, *539T), and two nonsense mutations (BCHE*119STOP, *465STOP). All of the individuals deficient in serum BCHE activity were homozygous for silent genes (6 of 6). Fifty-eight percent of the individuals (31 of 53) with slightly reduced serum BCHE activity were heterozygous for silent genes. They also showed a higher frequency (47% as allele frequency) of the K-variant than the general population (17.5%). Finally, we confirmed low serum BCHE activity in 10 of 23 individuals heterozygous for silent genes.

Variant alleles of the butyrylcholinesterase gene, BCHE, have often been used to trace the genetic histories of populations. The D70G substitution in BCHE causes prolonged postanesthesia apnea ("atypical" phenotype); H322N substitution in the closely related acetylcholinesterase gene, ACHE, is the basis of the mutually incompatible Yt blood groups. In both genes, additional point mutations were reported to be linked to these phenotypically evident ones. To examine whether the intragenic linkage reported for the ACHE and BCHE mutations in Americans is universal, we studied frequencies of these mutations in trans-Caucasian Georgian Jews, a population that has remained relatively isolated for 1500 years. To this end we employed PCR amplification followed by DNA sequencing and enzymatic restriction and compared the frequencies we found to corresponding reported phenotype data. Georgian Jews' N322 ACHE was a rather low 7.0% and was totally linked to a P446 mutation, in agreement with a recent report. In BCHE, however, G70 was a relatively high 5.8%, and the V497 and T539 mutations were not found, either in Georgian or in Ashkenazi Jews, in contrast to reported findings in Americans. Our findings reveal distinct displays of ACHE and BCHE haplotypes in Georgian Jews and suggest different founder effects, genetic drifts, and/or selection pressures in the evolution of each of these genes.

The J-variant of human serum butyrylcholinesterase (BChE) causes both an approximately two-thirds reduction of circulating enzyme molecules and a corresponding decrease in the level of BChE activity present in serum. Since the level of serum BChE activity and the duration of succinylcholine apnea are inversely correlated, this marked decrease in activity makes individuals with the J-variant more susceptible than usual subjects to prolonged apnea from succinylcholine. We reinvestigated the same family in which Garry et al. identified the J-variant phenotype. The atypical, fluoride, and K-variant mutations were also identified in members of the 47-person pedigree. DNA amplification by PCR, followed by direct sequencing of the amplified DNA, led to the finding that the J-variant phenotype of human serum BChE was associated with two DNA point mutations in the coding region. One of these was the mutation previously identified with the K-variant phenotype (GCA----ACA; Ala539----Thr). The other was an adenine-to-thymine transversion at nucleotide 1490, which changed amino acid 497 from glutamic acid to valine (GAA----GTA; Glu497----Val). This latter point mutation was named the J-variant mutation (formal name BCHE*497V). The J-variant mutation has not been identified without the K-variant mutation. The J-variant mutation created an RsaI-enzyme RFLP. Two additional point mutations, located in the noncoding regions of the gene, were also found to be linked with the J-variant and K-variant point mutations on the same allele. These noncoding polymorphic mutations had previously been found linked to the atypical and K-variant point mutations. A summary table shows dibucaine, fluoride, and Hoffmann-La Roche compound Ro 2-0683 inhibition numbers for 119 samples whose DNA has been sequenced. Eighteen BChE genotypes are represented.

An analysis of investigations performed between December 1978 and September 1982 into the cholinesterase status of 795 Caucasian patients has revealed an E1aE1j genotype in three (0.4%) and an E1aE1k genotype in 22 (2.8%). Both groups of patients are at increased risk of sensitivity to suxamethonium. Inhibitor numbers characteristic of these genotypes are reported which it is hoped will assist other workers to identify them more easily. While the J allele is probably rare among the general population it is suggested that as many as one person in 76 could be a KK homozygote. Our findings provide a possible explanation of the low cholinesterase activities seen in some patients for which there is no other obvious cause.

A family (H-J pedigree) segregating for the A and F alleles at cholinesterase locus 1 is described. Apparent anomalous results led to the recognition of a new allele (E1j) also segregating in the family. The data are consistent with the hypothesis that the the E1j causes reduction of 'usual' (E1u) molecules by about 66%. Whether this is because of retarded synthesis or accelerated degradation of serum cholinesterase remains to be determined.

Sera of various phenotypes at serum cholinesterase locus 1, including the newly recognized phenotypes E1 aE1j, E1 uE1j, and E1 fE1J, were studied by immunodiffusion and rocket immunoelectrophoresis. The sera containing the E1j allele show reduced numbers of immunologically active cholinesterase molecules. This finding is consistent with the previously advanced hypothesis that E1j results in reduced numbers of circulating 'usual' (E1u) molecules. Whether this reduction is the result of the low rate of synthesis or of an increased rate of degradation of the cholinesterase remains to be determined.