DNA mutations associated with the human butyrylcholinesterase J-variant.

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.     

The butyrylcholinesterase K-variant shows similar cellular protein turnover and quaternary interaction to the wild-type enzyme

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.     

Biochemical and genetic analysis of butyrylcholinesterase (BChE) in a family, due to prolonged neuromuscular blockade after the use of succinylcholine

Butyrylcholinesterase (BChE) is a plasma enzyme that catalyzes the hydrolysis of choline esters, including the muscle-relaxant succinylcholine and mivacurium. Patients who present sustained neuromuscular blockade after using succinylcholine usually carry BChE variants with reduced enzyme activity or an acquired BChE deficiency. We report here the molecular basis of the BCHE gene underlying the slow catabolism of succinylcholine in a patient who underwent endoscopic nasal surgery. We measured the enzyme activity of BChE and extracted genomic DNA in order to study the promoter region and all exons of the BCHE gene of the patient, her parents and siblings. PCR products were sequenced and compared with reference sequences from GenBank. We detected that the patient and one of her brothers have two homozygous mutations: nt1615 GCA > ACA (Ala539Thr), responsible for the K variant, and nt209 GAT > GGT (Asp70Gly), which produces the atypical variant A. Her parents and two of her brothers were found to be heterozygous for the AK allele, and another brother is homozygous for the normal allele. Sequence analysis of exon 1 including 5'UTR showed that the proband and her brother are homozygous for -116GG. The AK/AK genotype is considered the most frequent in hereditary hypocholinesterasemia (44%). This work demonstrates the importance of defining the phenotype and genotype of the BCHE gene in patients who are subjected to neuromuscular block by succinylcholine, because of the risk of prolonged neuromuscular paralysis.     

Characterization of a Novel BCHE “Silent” Allele: Point Mutation (p.Val204Asp) Causes Loss of Activity and Prolonged Apnea with Suxamethonium

Butyrylcholinesterase deficiency is characterized by prolonged apnea after the use of muscle relaxants (suxamethonium or mivacurium) in patients who have mutations in the BCHE gene. Here, we report a case of prolonged neuromuscular block after administration of suxamethonium leading to the discovery of a novel BCHE variant (c.695T>A, p.Val204Asp). Inhibition studies, kinetic analysis and molecular dynamics were undertaken to understand how this mutation disrupts the catalytic triad and determines a “silent” phenotype. Low activity of patient plasma butyrylcholinesterase with butyrylthiocholine (BTC) and benzoylcholine, and values of dibucaine and fluoride numbers fit with heterozygous atypical silent genotype. Electrophoretic analysis of plasma BChE of the proband and his mother showed that patient has a reduced amount of tetrameric enzyme in plasma and that minor fast-moving BChE components: monomer, dimer, and monomer-albumin conjugate are missing. Kinetic analysis showed that the p.Val204Asp/p.Asp70Gly-p.Ala539Thr BChE displays a pure Michaelian behavior with BTC as the substrate. Both catalytic parameters K_m = 265 µM for BTC, two times higher than that of the atypical enzyme, and a low V_max are consistent with the absence of activity against suxamethonium. Molecular dynamic (MD) simulations showed that the overall effect of the mutation p.Val204Asp is disruption of hydrogen bonding between Gln223 and Glu441, leading Ser198 and His438 to move away from each other with subsequent disruption of the catalytic triad functionality regardless of the type of substrate. MD also showed that the enzyme volume is increased, suggesting a pre-denaturation state. This fits with the reduced concentration of p.Ala204Asp/p.Asp70Gly-p.Ala539Thr tetrameric enzyme in the plasma and non-detectable fast moving-bands on electrophoresis gels.     

Identification of two different point mutations associated with the fluoride-resistant phenotype for human butyrylcholinesterase.

The fluoride variant of human butyrylcholinesterase owes its name to the observation that it is resistant to inhibition by 0.050 mM sodium fluoride in the in vitro assay. Individuals who are heterozygous for the fluoride and atypical alleles experience about 30 min of apnea, rather than the usual 3-5 min, after receiving succinyldicholine. Earlier we reported that the atypical variant has a nucleotide substitution which changes Asp 70 to Gly. In the present work we have identified two different point mutations associated with the fluoride-resistant phenotype. Fluoride-1 has a nucleotide substitution which changes Thr 243 to Met (ACG to ATG). Fluoride-2 has a substitution which changes Gly 390 to Val (GGT to GTT). These results were obtained by DNA sequence analysis of the butyrylcholinesterase gene after amplification by PCR. The subjects for these analyses were 4 patients and 21 family members.     

Ethnic differences in the frequency of distribution of serum cholinesterase activity in the Iranian population

One thousand Iranians belonging to 5 different Iranian ethnic groups were tested for butyrylcholinesterase (BChE) activity and phenotype. The phenotype was measured as percent inhibition in the presence of dibucaine. It was found that the Iranian population had an extraordinarily high frequency of the atypical variant of butyrylcholinesterase. 70% to 80% of Iranians carried the atypical mutation (Asp70Gly) on one allele. This contrasts with European and American populations where only 4% carry the atypical allele. The atypical variant of butyrylcholinesterase is known to be associated with prolonged apnea after administration of the muscle relaxants succinylcholine and mivacurium, and is also thought to be associated with abnormal sensitivity to cocaine toxicity. This study demonstrates that the ethnic background of a person has an important role in a person's response to drugs.     

Identification of a frameshift mutation responsible for the silent phenotype of human serum cholinesterase, Gly 117 (GGT----GGAG).

A frameshift mutation that causes a silent phenotype for human serum cholinesterase was identified in the DNA of seven individuals of two unrelated families. The mutation, identified using the polymerase chain reaction, causes a shift in the reading frame from Gly 117, where GGT (Gly)----GGAG (Gly+ 1 base) to a new stop codon created at position 129. This alteration is upstream of the active site (Ser 198), and, if any protein were made, it would represent only 22% of the mature enzyme found in normal serum. Results of analysis of the enzymatic activities in serum agreed with the genotypes inferred from the nucleotide sequence. Rocket immunoelectrophoresis using alpha-naphthyl acetate to detect enzymatic activity showed an absence of cross-reactive material, as expected. One additional individual with a silent phenotype did not show the same frameshift mutation. This was not unexpected, since there must be considerable molecular heterogeneity involved in causes for the silent cholinesterase phenotype. This is the first report of a molecular mechanism underlying the silent phenotype for serum cholinesterase. The analytical approach used was similar to the one we recently employed to identify the mutation that causes the atypical cholinesterase variant.     

Normal and Atypical Butyrylcholinesterases in Placental Development, Function, and Malfunction

1. In utero exposure to poisons and drugs (e.g., anticholinesterases, cocaine) is frequently associated with spontaneous abortion and placental malfunction. The major protein interacting with these compounds is butyrylcholinesterase (BuChE), which attenuates the effects of such xenobiotics by their hydrolysis or sequestration. Therefore, we studied BuChE expression during placental development.2. RT-PCR revealed both BuChEmRNA and acetylcholinesterase (AChE) mRNA throughout gestation. However, cytochemical staining detected primarily BuChE activity in first-trimester placenta but AChE activity in term placenta.3. As the atypical variant of BuChE has a narrower specificity for substrates and inhibitors than the normal enzyme, we investigated its interactions with -solanine and cocaine, and sought a correlation between the occurrence of this variant and placental malfunction.4. Atypical BuChE of serum or recombinant origin presented >10-fold weaker affinities than normal BuChE for cocaine and -solanine. However, BuChE in the serum of a heterozygote and a homozygous normal were similar in their drug affinities. Therefore, heterozygous serum or placenta can protect the fetus from drug or poison exposure, unlike homozygous atypical serum or placenta.5. Genotype analyses revealed that heterozygous carriers of atypical BuChE were threefold less frequent among 49 patients with placental malfunction than among 76 controls or the entire Israeli population. These observations exclude heterozygote carriers of atypical BuChE from being at high risk for placental malfunction under exposure to anticholinesterases.     

Molecular genetics of GM2-gangliosidosis AB variant: a novel mutation and expression in BHK cells

The GM2 activator is a hexosaminidase A-specific glycolipid-binding protein required for the lysosomal degradation of ganglioside GM2. Genetic deficiency of GM2 activator leads to a neurological disorder, an atypical form of Tay-Sachs disease (GM2 gangliosidosis variant AB). Here, we describe a G⁵⁰⁶ to C transversion (Arg¹⁶⁹ to Pro) in the mRNA of an infantile patient suffering from GM2-gangliosidosis variant AB. Using the polymerase chain reaction amplification and direct-sequencing technique, we found the patient to be homozygous for the mutation, whereas the parents were, as expected, heterozygous. BHK cells transfected with a construct of mutant cDNA gave no GM2 activator protein detectable by the Western blotting technique, whereas those transfected by a wild-type cDNA construct showed a significant level of human GM2 activator protein. The substitution of proline for the normal Arg¹⁶⁹ therefore appears to result in premature degradation of the mutant GM2 activator, either during the post-translational processing steps or after reaching the lysosome. The basis for the phenotype of GM2 gangliosidosis variant AB may therefore be either inactivation of the physiological activator function by the point mutation or instability of the mutant protein.     

Estimates of the rate and distribution of fitness effects of spontaneous mutation in Saccharomyces cerevisiae

The per-genome, per-generation rate of spontaneous mutation affecting fitness (U) and the mean fitness cost per mutation (s) are important parameters in evolutionary genetics, but have been estimated for few species. We estimated U and sh (the heterozygous effect of mutations) for two diploid yeast strains differing only in the DNA mismatch-repair deficiency used to elevate the mutation rate in one (mutator) strain. Mutations were allowed to accumulate in 50 replicate lines of each strain, during 36 transfers of randomly chosen single colonies (approximately 600 generations). Among wild-type lines, fitnesses were bimodal, with one mode showing no change in mean fitness. The other mode showed a mean 29.6% fitness decline and the petite phenotype, usually caused by partial deletion of the mitochondrial genome. Excluding petites, maximum-likelihood estimates adjusted for the effect of selection were U = 9.5 x 10(-5) and sh = 0.217 for the wild type. Among the mutator lines, the best fit was obtained with 0.005 < or = U < or = 0.94 and 0.049 > or = sh > or = 0.0003. Like other recently tested model organisms, wild-type yeast have low mutation rates, with high mean fitness costs per mutation. Inactivation of mismatch repair increases the frequency of slightly deleterious mutations by approximately two orders of magnitude.     

The molecular defect of albumin Tagliacozzo: 313 Lys----Asn

Albumin Tagliacozzo is a fast-moving genetic variant of human serum albumin found in 19 unrelated families. The protein was isolated from the serum of a heterozygous healthy subject. Analysis of CNBr fragments by isoelectric focusing allowed us to localize the mutation to CNBr fragment IV (residues 299-329). This fragment was isolated on a preparative scale and subjected to tryptic digestion. Sequential analysis of the abnormal tryptic peptide, purified by RP-HPLC, revealed the variant was caused by 313 Lys----Asn substitution, probably due to a point mutation in the structural gene. The lack of a lysine residue accounts for the electrophoretic behavior of albumin Tagliacozzo.     

An adenosine kinase mutation in baby hamster kidney cells causing increased sensitivity to adenosine

A class of aravinosyladenine (araA)-resistant mutants of baby hamster kidney (BHK 21/C13) cells exhibits multiple phenotypes: resistance to araA and deoxyadenosine, extreme sensitivity to adenosine (Ado) and varying degrees of deficiency in adenosine kinase (AK) activity. One of these Ado^s/araA^r strains, ara-S10d, was isolated without mutagenesis and was shown to possess about 59% level of the wild-type AK activity. The AK from ara-S10d had an altered K_m and pH optimum and was stimulated by K⁺ cations. A number of Ado^s to Ado^r revertants were isolated from araS10d, and in all of the 7 examined, the AK activity was reduced to a nondetectable level. The altered kinetic parameters of the AK enzyme in ara-S10d cells suggest a mutation of the AK gene that leads to the synthesis of an altered enzyme. The loss of AK activity in the Ado^r revertants suggests an association of the enhanced Ado sensitivity to the AK mutation.     

Molecular genetics of phenylketonuria in Mediterranean countries: a mutation associated with partial phenylalanine hydroxylase deficiency.

We report the characterization of a mutation in the phenylalanine hydroxylase (PAH) gene associated with partial residual activity of the enzyme. This point mutation (280glu----lys) was found by sequencing a mutant cDNA clone derived from a needle biopsy of the liver in a child with variant form of phenylketonuria. There is a strict concordance between homozygosity for the mutation and this particular phenotype. The (280glu----lys) mutation is linked to an original and rare RFLP haplotype at the PAH locus found in south Europe and North Africa. So far, this genotype-haplotype association is both inclusive and exclusive. Thirty-three PAH-deficient patients were screened for the mutation by using polymerase chain-reaction amplification of their genomic DNA extracted from Guthrie cards. Since a large number of patients can be screened for a particular mutation by using Guthrie cards, the possibility arises of using these samples collected by national newborn screening centers for prospective and retrospective detection of other mutations in the human genome.     

Population genetic studies of the Philippine Negritos. I. A pilot survey of red cell enzyme and serum protein groups

Electrophoretic surveys of red cell enzyme and serum protein systems representing 21 genetic loci were carried out on 129 blood samples of the Negritos of Pampanga, Central Luzon, the Philippines. Nine (out of 16) red cell enzyme loci and four (out of five) serum protein loci showed polymorphic variation. Low frequencies of ACP 1A, GPTs1, ESD2, and Hp1, and a markedly high frequency of PGM12 were contrasted to those in non-Negrito Filipinos. Variant ESD phenotypes with a slowly migrating isozyme occurred in high frequency. The new allele designated as ESD3Negrito (ESD3N) had a frequency of .10 +/- .019. In AK, a variant phenotype indistinguishable from AK 2-1 was observed in 14% of the sample. In the Gc system, a fast migrating variant was discovered in high frequency which was distinct from Gc Ab and Gc J. The variant allele, denoted GcNegrito (GcN), had a frequency of .21 +/- .025. A relatively high degree of allelic diversity in the Negrito sample was also suggested by the average heterozygosity for 21 loci screened (.165), which is compared to that of the Japanese population (.140).     

Single base mutation in alpha 5(IV) collagen chain gene converting a conserved cysteine to serine in Alport syndrome.

We have identified a point mutation in the type IV collagen alpha 5 chain gene (COL4A5) in Alport syndrome. Variant PstI (Barker et al., 1990, Science 248, 1224-1227), and BglII restriction sites with complete linkage with the Alport phenotype have been found in the 3' end of the COL4A5 gene in the large Utah Kindred P. The approximate location of the variant sites was determined by restriction enzyme mapping, after which this region of the gene (1028 bp) was amplified with the polymerase chain reaction (PCR) from DNA of normal and affected individuals for sequencing analysis. The PCR products showed the absence or presence of the variant PstI and BglII sites in DNA from normal and affected individuals, respectively. DNA sequencing revealed a single base change in exon 3 (from the 3' end) in DNA from affected individuals, changing the TGT codon of cysteine to the TCT codon for serine. This single base mutation also generated new restriction sites for PstI and BglII. The mutation involves a cysteine residue that has remained conserved in the carboxyl-end noncollagenous domain (NC domain) of all known type IV collagen alpha chains from Drosophila to man. It is presumably crucial for maintaining the right conformation of the NC domain, which is important for both triple-helix formation and the formation of intermolecular cross-links of type IV collagen molecules.     

Clinical and biochemical manifestations and molecular characterization of the mutation HPRT Jerusalem

A novel point mutation (I137T) was identified in the hypoxanthine-guanine phosphoribosyltransferase (HPRT) encoding gene, in a patient with partial deficiency of the enzyme. The mutation, ATT to ACT (substitution of isoleucine to threonine), occurred at codon 137, which is within the region encoding the binding site for 5-phosphoribosyl-1-pyrophosphate (PRPP). The mutation caused decreased affinity for PRPP, manifested clinically as a Lesch-Nyhan variant (excessive purine production and delayed acquisition of language skills). The partial HPRT deficiency could be detected only by measuring HPRT activity in intact fibroblasts (uptake of hypoxanthine into nucleotides).     

Assessment of carriers' frequency of a novel MRE11 mutation responsible for the rare ataxia telangiectasia-like disorder

Ataxia telangiectasia-like disorder (ATLD) is a very rare variant of ataxia telangiectasia. ATLD is caused by mutations in MRE11 gene. Recently, a new missense mutation, a G-to-C change at nucleotide 630 of the MRE11 gene, was described in 10 ATLD Saudi Arabian patients from three unrelated families. This is the biggest ATLD group of patients that may suggest noticeable heterozygous carriers of G630C mutation in the general population. The aim of the present study was to assess the allelic frequency of this mutation. A cohort of 428 Saudi nationals was studied. The 630G > C mutation was genotyped by direct sequencing. Two individuals with heterozygous G630C mutation were found giving a G/C genotype frequency of 0.5% and a mutant C allele frequency of 0.2%. This indicates the presence of this rare mutation in our population with heterozygous carriers' frequency of 0.5%. The frequency could be higher in geographically isolated families with high consanguinity. Premarital, preimplementation, and prenatal screening for MRE11 G630C mutation could be useful to limit the risk of genetic diseases.     

Immunological comparison of the usual and atypical human serum cholinesterase phenotypes

Antiserum prepared against highly purified usual human serum cholinesterase (the most common phenotype) cross-reacted identically with the atypical serum cholinesterase. The level of circulating atypical enzyme protein, determined immunologically, was about 30% lower when the enzyme came from an atypical rather than a usual phenotype, and the level of enzyme activity measured enzymatically atV _max with eithero-nitrophenylbutyrate or benzoylcholine as substrate showed approximately the same degree of reduction. The average specific activity (activity atV _max per microgram of enzyme protein) in sera from 28 usual and 20 atypical individuals did not differ significantly. These findings suggest that the atypical enzyme not only has altered catalytic properties (K)_mbut also might be synthesized more slowly, or clearedin vivo more rapidly, than the usual enzyme. This work was supported by U.S. Public Health Service Grants NS 15871 and GM 27028 and by a grant from the Hoffmann-La Roche Foundation.     

Functional mutation of DNA polymerase beta found in human gastric cancer--inability of the base excision repair in vitro.

DNA polymerase beta (polbeta) is one of mammalian DNA polymerases and is known to be involved in a G:T/G:U mismatch repair. In order to investigate an involvement of this enzyme in a base excision repair, we searched a mutation of human polbeta in human gastric cancer and studied a function of the mutation. We observed cancer-specific missense mutations in 6 of 20 samples. All of these mutations were, however, heterozygous. We further analyzed the base excision repair activity of these mutants to know whether these mutants cause an error of mismatch repair. One of these mutants, which resulted in an amino acid substitution of Glu for Lys at codon 295, showed an inhibitory effect by in vitro base excision repair assay, suggesting that this mutation might play some role in carcinogenesis of the gastric mucosa.