Genetic analysis of thiopurine methyltransferase polymorphism in a Japanese population

Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathiopurine. Several mutations in the TPMT gene have been identified which correlate with a low activity phenotype. The molecular basis for the genetic polymorphism of TPMT has been established for European Caucasians, African-Americans, Southwest Asians and Chinese, but it remains to be elucidated in Japanese populations. The frequency of the four allelic variants of the TPMT gene, TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3C (A719G) were determined in Japanese samples (n=192) using polymerase chain reaction (PCR)-RFLP and allele-specific PCR-based assays. TPMT*3C was found in 0.8% of the samples (three heterozygotes). The TPMT*2, TPMT*3A and TPMT*3B alleles were not detected in any of the samples analyzed. This study provides the first analysis of TPMT mutant allele frequency in a sample of Japanese population and indicates that TPMT*3C is the most common allele in Japanese subjects.     

The frequency and distribution of thiopurine S-methyltransferase alleles in south Iranian population

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathiopurine. Variability in TPMT activity is mainly due to genetic polymorphism. The frequency of the four allelic variants of the TPMT gene, TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3C (A719G) were determined in an Iranian population from south of Iran (n = 500), using polymerase chain reaction (PCR)-RFLP and allele-specific PCR-based assays. Four hundred seventy four persons (94.8%) were homozygous for the wild type allele (TPMT*1/*1) and twenty five people were TPMT*1/*3C (5%). One patient was found to be heterozygous in terms TPMT*1 and *2 alleles with genotype of TPMT*1/*2 (0.2%). None of the participants had both defective alleles. The TPMT*3C and *2 were the only variant alleles observed in this population. The total frequency of variant alleles was 2.6% and the wild type allele frequency was 97.4%. The TPMT*3B and *3A alleles were not detected. Distributions of TPMT genotype and allele frequency in Iranian populations are different from the genetic profile found among Caucasian or Asian populations. Our findings also revealed inter-ethnic differences in TPMT frequencies between different parts of Iran. This view may help clinicians to choose an appropriate strategy for thiopurine drugs and reduce adverse drug reactions such as bone marrow suppression.     

Thiopurine S-methyltransferase deficiency: two nucleotide transitions define the most prevalent mutant allele associated with loss of catalytic activity in Caucasians.

The autosomal recessive trait of thiopurine S-methytransferase (TPMT) deficiency is associated with severe hematopoietic toxicity when patients are treated with standard doses of mercaptopurine, azathioprine, or thioguanine. To define the molecular mechanism of this genetic polymorphism, we cloned and characterized the cDNA of a TPMT-deficient patient, which revealed a novel mutant allele (TPMT*3) containing two nucleotide transitions (G460-->A and A719-->G) producing amino acid changes at codons 154 (Ala-->Thr) and 240 (Tyr--> Cys), differing from the rare mutant TPMT allele we previously identified (i.e., TPMT*2 with only G238-->C). Site-directed mutagenesis and heterologous expression established that either TPMT*3 mutation alone leads to a reduction in catalytic activity (G460-->A, ninefold reduction; A719-->G, 1.4-fold reduction), while the presence of both mutations leads to complete loss of activity. Using mutation specific PCR-RFLP analysis, the TPMT*3 allele was detected in genomic DNA from approximately 75 percent of unrelated white subjects with heterozygous phenotypes, indicating that TPMT*3 is the most prevalent mutant allele associated with TPMT-deficiency in Caucasians.     

The Clinical Impact of Thiopurine Methyltransferase Polymorphisms on Thiopurine Treatment

Acute lymphoblastic leukaemia (ALL) is the most common malignancy of childhood. Although current treatment results in long term survival in over 70% of cases there is evidence that as many as 50% could have been cured using a less complex regimen with a lower incidence of long term side effects. In previous studies it has been found that thiopurines given as part of continuing therapy are key agents in preventing relapse. However, optimal administration during continuing therapy is often not achieved. Variation in the level of thiopurine methyltransferase (TPMT) activity appears to be a major molecular determinant of the extent of thiopurine metabolism. TPMT activity shows a trimodal distribution pattern. A lack of activity is found in approximately one in 300 Caucasians; approximately 11% have intermediate activity and the remaining 89% high activity. Congenital loss of activity is associated with grossly elevated levels of active drug and profound myelosuppression on exposure to thiopurines. This loss of activity has been attributed to single nucleotide polymorphisms (SNPs) within the TPMT gene. The frequency of SNPs is related to ethnicity, with the most common in Caucasians being TPMT*3A which is characterized by a G to A transition at position 460 with a substitution of alanine for tyrosine at amino acid 154 (A154Y) and a transition of A to G at nucleotide 719 resulting in a change of tyrosine to cysteine at position 240 (Y240C). Polymorphisms have also been identified within the 5′ flanking promoter region of the TPMT gene due to a variable number of tandem repeats (VNTR*3–*8). An overview of the polymorphisms identified to date, their implication on the metabolism of the thiopurine drugs and therapeutic importance will be discussed.     

The clinical impact of thiopurine methyltransferase polymorphisms on thiopurine treatment

Acute lymphoblastic leukaemia (ALL) is the most common malignancy of childhood. Although current treatment results in long term survival in over 70% of cases there is evidence that as many as 50% could have been cured using a less complex regimen with a lower incidence of long term side effects. In previous studies it has been found that thiopurines given as part of continuing therapy are key agents in preventing relapse. However, optimal administration during continuing therapy is often not achieved. Variation in the level of thiopurine methyltransferase (TPMT) activity appears to be a major molecular determinant of the extent of thiopurine metabolism. TPMT activity shows a trimodal distribution pattern. A lack of activity is found in approximately one in 300 Caucasians; approximately 11% have intermediate activity and the remaining 89% high activity. Congenital loss of activity is associated with grossly elevated levels of active drug and profound myelosuppression on exposure to thiopurines. This loss of activity has been attributed to single nucleotide polymorphisms (SNPs) within the TPMT gene. The frequency of SNPs is related to ethnicity, with the most common in Caucasians being TPMT*3A which is characterized by a G to A transition at position 460 with a substitution of alanine for tyrosine at amino acid 154 (A154Y) and a transition of A to G at nucleotide 719 resulting in a change of tyrosine to cysteine at position 240 (Y240C). Polymorphisms have also been identified within the 5' flanking promoter region of the TPMT gene due to a variable number of tandem repeats (VNTR*3-*8). An overview of the polymorphisms identified to date, their implication on the metabolism of the thiopurine drugs and therapeutic importance will be discussed.     

In vitro characterization of four novel non-functional variants of the thiopurine S-methyltransferase

Human thiopurine S-methyltransferase (TPMT) is an enzyme responsible for the detoxification of widely used thiopurine drugs such as azathioprine (Aza). Its activity is inversely related to the risk of developing severe hematopoietic toxicity in certain patients treated with standard doses of thiopurines. DNA samples from four leucopenic patients treated with Aza were screened by PCR-SSCP analysis for mutations in the 10 exons of the TPMT gene. Four missense mutations comprising two novel mutations, A83T (TPMT*13, Glu(28)Val) and C374T (TPMT*12, Ser(125)Leu), and two previously described mutations, G430C (TPMT*10, Gly(144)Arg) and T681G (TPMT*7, His(227)Gln) were identified. Using a recombinant yeast expression system, kinetic parameters (K(m) and V(max)) of 6-thioguanine S-methylation of the four TPMT variants were determined and compared to those obtained with wild-type TPMT. This functional analysis suggests that these rare allelic variants are defective TPMT alleles. The His(227)Gln variant retained only 10% of the intrinsic clearance value (V(max)/K(m) ratio) of the wild-type enzyme. The Ser(125)Leu and Gly(144)Arg variants were associated with a significant decrease in intrinsic clearance values, retaining about 30% of the wild-type enzyme, whereas the Glu(28)Val variant produced a more modest decrease (57% of the wild-type enzyme). The data suggest that the sporadic contribution of the rare Glu(28)Val, Ser(125)Leu, Gly(144)Arg, and His(227)Gln variants may account for the occurrence of altered metabolism of TPMT substrates. These findings improve our knowledge of the genetic basis of interindividual variability in TPMT activity and would enhance the efficiency of genotyping methods to predict patients at risk of inadequate responses to thiopurine therapy.     

Mercaptopurine pharmacogenetics: Monogenic inheritance of erythrocyte thiopurine methyltransferase activity

Thiopurine methyltransferase (TPMT) catalyzes thiopurine S-methylation, an important metabolic pathway for drugs such as 6-mercaptopurine. Erythrocyte (RBC) TPMT activity was measured in blood samples from 298 randomly selected subjects. Of the subjects, 88.6% were included in a subgroup with high enzyme activity (13.50 ± 1.86 U, mean ± SD), 11.1% were included in a subgroup with intermediate activity (7.20 ± 1.08 U), and 0.3% had undetectable activity. This distribution conforms to Hardy-Weinberg predictions for the autosomal codominant inheritance of a pair of alleles for low and high TPMT activity, TPMT^L and TPMT^H, with gene frequencies of .059 and .941, respectively. If RBC TPMT activity is inherited in an autosomal codominant fashion, then subjects homozygous for TPMT^H would have high enzyme activity, subjects heterozygous for the two alleles would have intermediate activity, and subjects homozygous for TPMT^L would have undetectable activity. The segregation of RBC TPMT activity among 215 first-degree relatives in 50 randomly selected families and among 35 members of two kindreds and one family selected because they included probands with undetectable RBC enzyme activity were also compatible with the autosomal codominant inheritance of RBC TPMT. For example, in eight matings between subjects with intermediate activity (presumed genotype TPMT^L TPMT^H) and subjects with high activity (presumed genotype TPMT^H TPMT^H), 47% (8/17) of the offspring had intermediate activity. This value is very similar to the 50% figure expected on the basis of autosomal codominant inheritance (χ²_[1] = .059). Further experiments are required to determine whether this genetic polymorphism for an important drug metabolizing enzyme may represent one factor in individual variations in sensitivity to thiopurines.     

Structural Characteristics Determine the Cause of the Low Enzyme Activity of Two Thiopurine S-Methyltransferase Allelic Variants: A Biophysical Characterization of TPMT*2 and TPMT*5

The enzyme thiopurine S-methyltransferase (TPMT) is involved in the metabolism of thiopurine drugs used to treat acute lymphoblastic leukemia and inflammatory bowel disease. Thus far, at least 29 variants of the TPMT gene have been described, many of which encode proteins that have low enzyme activity and in some cases become more prone to aggregation and degradation. Here, the two naturally occurring variants, TPMT*2 (Ala80 → Pro) and TPMT*5 (Leu49 → Ser), were cloned and expressed in Escherichia coli. Far-UV circular dichroism spectroscopy showed that TPMT*2 was substantially destabilized whereas TPMT*5 showed much greater stability comparable to that of wild-type TPMT (TPMTwt). The extrinsic fluorescent molecule anilinonaphthalene sulfonate (ANS) was used to probe the tertiary structure during thermal denaturation. In contrast to TPMTwt, neither of the variants bound ANS to a large extent. To explore the morphology of the TPMT aggregates, we performed luminescent conjugated oligothiophene staining and showed fibril formation for TPMT*2 and TPMT*5. The differences in the flexibility of TPMTwt, TPMT*2, and TPMT*5 were evaluated in a limited proteolysis experiment to pinpoint stable regions. Even though there is only one amino acid difference between the analyzed TPMT variants, a clear disparity in the cleavage patterns was observed. TPMT*2 displays a protected region in the C-terminus, which differs from TPMTwt, whereas the protected regions in TPMT*5 are located mainly in the N-terminus close to the active site. In conclusion, this in vitro study, conducted to probe structural changes during unfolding of TPMT*2 and TPMT*5, demonstrates that the various causes of the low enzyme activity in vivo could be explained on a molecular level.     

Genotype Frequencies of Drug-Metabolizing Enzymes Responsible for Purine and Pyrimidine Antagonists in a Healthy Asian-Indian Population

Purine and pyrimidine antimetabolites are used to treat leukemias, autoimmune diseases, and solid tumors. Detection of slow metabolizers before administration of the drugs is necessary to prevent any subsequent drug toxicity. With this aim, we determined the frequencies of normal and slow alleles in our population. Polymorphisms in genes encoding cytidine deaminase (CDA), dihydropyrimidine dehydrogenase (DPYD), and thiopurine-S-methyltransferase (TPMT) were documented in 225 healthy volunteers. The polymorphisms typed included CDA*3, DPYD*2A, TPMT*2A, TPMT*3B, and TPMT*3C. Methods used for genotyping included standard PCR-RFLP and allele-specific PCR reactions. The frequencies were 0.44?% for DPYD*2A, 0.67?% for TPMT*3B, and 0.89?% for TPMT*3C. The CDA*3 and TPMT*2A alleles were not detected. Although these polymorphisms have been demonstrated to be associated with drug toxicity in other populations, they appear to be very rare in the adult Indian population.     

Four human thiopurine s-methyltransferase alleles severely affect protein structure and dynamics

Thiopurine S-methyltransferase (TPMT) metabolizes cytotoxic thiopurine drugs used in the treatment of leukemia and inflammatory bowel disease. TPMT is a major pharmacogenomic target with 23 alleles identified to date. Several of these alleles cause rapid protein degradation and/or aggregation, making it extremely difficult to study the structural impact of the TPMT polymorphisms experimentally. We, therefore, have performed multiple molecular dynamics simulations of the four most common alleles [TPMT*2 (A80P), *3A (A154T/Y240C), *3B (A154T) and *3C (Y240C)] to investigate the molecular mechanism of TPMT inactivation at an atomic level. The A80P polymorphism in TPMT*2 disrupts helix α3 bordering the active site, which breaks several salt-bridge interactions and opens up a large cleft in the protein. The A154T polymorphism is located within the co-substrate binding site. The larger threonine alters the packing of substrate-binding residues (P68, L69, Y166), increasing the solvent exposure of the polymorphic site. This packing rearrangement may account for the complete lack of activity in the A154T mutant. The Y240C polymorphism is located in β-strand 9, distant from the active site. Side-chain contacts between residue 240 and helix α8 are lost in TPMT*3C. Residues 154 and 240 in TPMT*3A are connected through a hydrogen-bonding network. The dual polymorphisms result in a flattened, slightly distorted protein structure and an increase in the thiopurine-binding site solvent accessibility. The two variants that undergo the most rapid degradation in vivo, TPMT*2 and *3A, are also the most deformed in the simulations.     

Screening strategies for a highly polymorphic gene: DHPLC analysis of the Fanconi anemia group A gene

Introduction: Patients with Fanconi anemia (Fanc) are at risk of developing leukemia. Mutations of the group A gene (FancA) are most common. A multitude of polymorphisms and mutations within the 43 exons of the gene are described. To examine the role of heterozygosity as a risk factor for malignancies, a partially automatized screening method to identify aberrations was needed. We report on our experience with DHPLC (WAVE (Transgenomic)). Methods: PCR amplification of all 43 exons from one individual was performed on one microtiter plate on a gradient thermocycler. DHPLC analysis conditions were established via melting curves, prediction software, and test runs with aberrant samples. PCR products were analyzed twice: native, and after adding a WT-PCR product. Retention patterns were compared with previously identified polymorphic PCR products or mutants. Results and discussion: We have defined the mutation screening conditions for all 43 exons of FancA using DHPLC. So far, 40 different sequence variations have been detected in more than 100 individuals. The native analysis identifies heterozygous individuals, and the second run detects homozygous aberrations. Retention patterns are specific for the underlying sequence aberration, thus reducing sequencing demand and costs. DHPLC is a valuable tool for reproducible recognition of known sequence aberrations and screening for unknown mutations in the highly polymorphic FancA gene.     

Thiopurine S-Methyltransferase Gene Polymorphisms in a Healthy Slovak Population and Pediatric Patients with Inflammatory Bowel Disease

Thiopurine methyltransferase (TPMT) is a key component in thiopurine metabolism. There is an insufficient evidence about the distribution of the genotype frequencies of TPMT variants and frequencies of TPMT alleles associated with intermediate and deficient activity in a healthy Slovak population and pediatric patients with inflammatory bowel disease (IBD). TPMT variant alleles (*1,*2, *3A, *3B, and *3C) were determined in 114 children treated for IBD and in 281 healthy volunteers. Mutant alleles were present in 9/114 (7.89%) in the IBD patients and in 23/281 (8.19%) of probands. The distribution of the most frequent variants of TPMT gene was similar in a healthy population and patients with IBD.     

Structural basis of allele variation of human thiopurine-S-methyltransferase

Human thiopurine S-methyltransferase (TPMT) exhibits considerable person-to-person variation in activity to thiopurine drugs. We have produced an N-terminal truncation of human TPMT protein, crystallized the protein in complex with the methyl donor product S-adenosyl-L-homocysteine, and determined the atomic structure to the resolution of 1.58 and 1.89 A, respectively, for the seleno-methionine incorporated and wild type proteins. The structure of TPMT indicates that the naturally occurring amino acid polymorphisms scatter throughout the structure, and that the amino acids whose alteration have the most influence on function are those that form intra-molecular stabilizing interactions (mainly van der Waals contacts). Furthermore, we have produced four TPMT mutant proteins containing variant alleles of TPMT*2, *3A, *3B, and *3C and examined the structure-function relationship of the mutant proteins based on their expression and solubility in bacteria and their thermostability profile.     

High-throughput detection of multiple genetic polymorphisms influencing drug metabolism with mismatch primers in allele-specific polymerase chain reaction

Because of genetic polymorphisms of drug-metabolizing enzyme genes, the activities of the enzymes in humans vary widely and alter the metabolism of commonly used clinical agents. Severe adverse effects or resistance to therapy may result. We have developed a rapid and high-throughput genotyping method for detecting polymorphisms of the drug-metabolizing enzyme genes CYP2C9*3, CYP2C19*2, *3, CYP2D6*2, *4, *10, *14, *21, NAT2*5, *6, *7, and TPMT*3 using allele-specific polymerase chain reaction (PCR) with mismatch primers (ASPCR-MP) and CYP2D6*5, *36, and CYP2D6xN using stepdown PCR with detection by SYBR Green I. We analyzed genomic DNA from 139 Japanese volunteers. Identical genotyping results were obtained by using ASPCR-MP, stepdown PCR, and conventional PCR. We found that the methods clearly differentiate three specific profiles with no overlap in the signals. Moreover, both ASPCR-MP and stepdown PCR for genotyping took less than 3-4h. To our knowledge, this is the first report of successful simultaneous detection of multiple genetic polymorphisms with point mutations using ASPCR-MP or multiple genetic polymorphisms with large structural alterations using stepdown PCR. In conclusion, ASPCR-MP and stepdown PCR appear to be suitable for large clinical and epidemiological studies as methods that enable highly sensitive genotyping and yield a high-throughput.     

不明原因男性不育人群中睾丸特异性乳酸脱氢酶基因突变的发现及其意义

为了研究睾丸特异性乳酸脱氢酶,即乳酸脱氢酶C4(LDH-C4)基因突变在男性不育发病中的作用,利用LDH-C4特异性底物对100名不明原因男性不育症患者的精子LDH-C4进行活性显色,用变性高效液相色谱(DHPLC)技术对LDH-C4活性低下的患者进行LDHC基因PCR产物的突变筛查,对DHPLC峰形异常的PCR产物进行序列测定.筛选到一组精子LDH-C4活性明显下降的患者,其中1名患者的LDHC基因PCR产物在DHPLC中呈异常洗脱峰.对这一PCR产物进行序列测定,发现患者LDHC基因第5外显子的115位碱基发生了T→A的杂合改变(GenBank登录号GU479375),该突变使LDHC基因的178位密码子由原来的TTG(编码亮氨酸)变为TAG(终止密码子),形成截短的C亚基.T克隆-测序进一步证实了该无义突变的杂合状态.这是在人类LDHC基因上发现的第一个突变,提示LDHC基因突变可能是男性不育发病的原因之一.     

Polymorphisms of the TPMT gene in the Czech healthy population and patients with inflammatory bowel disease

Genetic variation in thiopurine S-methyltransferase (TPMT) is a major factors for wide variation in the metabolism and safety of thiopurine drugs. We investigated the frequency of functional gene polymorphisms in 396 patients with inflammatory bowel disease and 300 healthy subjects. Frequencies of functionally deficient alleles TPMT*2, TPMT*3A, TPMT*3B, and TPMT*3B in the patient group were 0.1%, 4.3%, 0.1%, and 0.4%, respectively, and were similar to those of healthy subjects in the Czech population. Our results provide necessary information for pharmacoeconomic studies in the Czech Republic.     

Denaturing high-performance liquid chromatography is a suitable method for PMM2 mutation screening in carbohydrate-deficient glycoprotein syndrome type IA patients

The phosphomannomutase 2 gene (PMM2; MIM 601785) has been identified as the carbohydrate-deficient glycoprotein syndrome type 1A gene (CDGS type 1A; MIM 212065). The gene spans 8 exons and 741 bp of coding DNA. Previously, we have identified 20 different mutations in the PMM2 gene using mutation screening with single-stranded conformation polymorphism (SSCP) and sequencing of DNA from 61 CDGS type 1A patients. Because eight of these could not be detected by SSCP, we were not satisfied with the sensitivity of the mutation detection technique used. Thus, we wanted to investigate if denaturing high-performance liquid chromatography (DHPLC) was a more suitable mutation screening method for PMM2. DHPLC was set up for PMM2 by optimizing eight different PCR fragments, one for each exon. The mutation detection was optimized empirically with PCR fragments from controls. First, control samples were run at a universal gradient and after modification and shortening of the gradient, also run at 10 different temperatures, 50-70 degrees C with 2-degree intervals, to enable setting of the temperature with the highest resolution. Then, PCR products with known mutations from the previous study were analyzed, and the results were compared to the control chromatograms for aberrations. We detected 19/20 mutations with DHPLC, and several mutations not detected by earlier screening techniques were readily detected by DHPLC. We conclude that DHPLC is a suitable detection technique for a rapid and reliable first scan of CDGS type 1A patients.     

Denaturing high performance liquid chromatography (DHPLC) used in the detection of germline and somatic mutations.

Denaturing high performance liquid chromatography (DHPLC) has been described recently as a method for screening DNA samples for single nucleotide polymorphisms and inherited mutations. Thirty-eight DNAs, 22 of which were heterozygous for previously characterized rearranged transforming gene (RET) or cystic fibrosis transmembrane conductance regulator gene (CFTR) mutations or polymorphisms, were examined using DHPLC analysis to assess the accuracy of this scanning method. Ninety-one per cent (20/22) of the PCR amplicons from specimens with heterozygous RET or CFTR sequence showed elution profiles distinct from corresponding homozygous normal patterns; whether the profiles for two amplicons containing heterozygous RET sequence were distinct from homozygous cases was equivocal. To investigate the usefulness of this method for detecting mutations in tumor DNAs, each of the phosphatase and tensin homologue deleted on chromosome ten gene (PTEN) exons were examined for mutations in 63 malignant gliomas. Seventeen PTEN PCR products from this series of brain tumors showed elution profiles indicating sample heterozygosity and in each instance conventional sequencing confirmed the presence of a mutation. PTEN amplicons containing exons 1, 3 and 5 were sequenced for each of the 63 tumor DNAs to determine whether any mutations may have escaped DHPLC detection, and this analysis identified one such alteration in addition to the eight mutations that DHPLC had revealed. In total, DHPLC identified 37 of 40 (92.5%) PCR products containing defined sequence variation and no alterations were indicated among 196 amplicons containing homozygous normal sequence.     

DHPLC mutation analysis of the hereditary nonpolyposis colon cancer (HNPCC) genes hMLH1 and hMSH2

Denaturing high-performance liquid chromatography (DHPLC) is an efficient method for detection of mutations involving a single or few numbers of nucleotides, and it has been successfully used for mutation detection in disease-related genes. Colorectal cancer is one of the most common cancers, and mutations in the genes for hereditary nonpolyposis colon cancer (HNPCC), hMLH1 and hMSH2, also involve mainly point mutations. Sequence analysis is supposed to be a screening method with high sensitivity; however, it is time-consuming and expensive. We therefore decided to test sensitivity and reproducibility of DHPLC for 71 sequence variants in hMLH1 and hMSH2 initially found by sequence analysis in DNA samples of German HNPCC patients. DHPLC conditions of the PCR products were based on the melting pattern of the wild-type sequence of the corresponding PCR fragments. All but one of the 71 mutations was detected using DHPLC (sensitivity of 97%). Running time per sample averaged only 7 min, and the system is highly automated. Thus DHPLC is a rapid and sensitive method for the detection of hMLH1 and hMSH2 sequence variants.