Frameshift mutations in peripheral blood as a biomarker for surveillance of Lynch syndrome.

BACKGROUND: Lynch syndrome is a hereditary cancer predisposition syndrome caused by germline mutations in DNA mismatch repair genes, which lead to high microsatellite instability and frameshift mutations at coding mononucleotide repeats in the genome. Recurrent frameshift mutations in these regions are thought to play a central role in the increased risk of various cancers, but no biomarkers are currently available for the surveillance of high microsatellite instability-associated cancers. METHODS: A frameshift mutation-based biomarker panel was developed and validated by targeted next-generation sequencing of supernatant DNA from cultured high microsatellite instability colorectal cancer cells. This panel supported selection of 122 frameshift mutation targets as potential biomarkers. This biomarker panel was then tested using matched tumor, adjacent normal tissue, and buffy coat samples (53 samples) and blood-derived cell-free DNA (cfDNA) (38 samples) obtained from 45 high microsatellite instability and mismatch repair-deficient patients. We also sequenced cfDNA from 84 healthy participants to assess background noise. RESULTS: Recurrent frameshift mutations at coding mononucleotide repeats were detectable not only in tumors but also in cfDNA from high microsatellite instability and mismatch repair-deficient patients, including a Lynch syndrome carrier, with a varying range of target detection (up to 85.2%), whereas they were virtually undetectable in healthy participants. Receiver operating characteristic curve analysis showed high sensitivity and specificity (area under the curve = 0.94) of the investigated panel. CONCLUSIONS: We demonstrated that frameshift mutations can be detected in cfDNA from high microsatellite instability and mismatch repair-deficient patients and asymptomatic carriers. The 122-target frameshift mutation panel described here has promise as a tool for improved surveillance of high microsatellite instability and mismatch repair-deficient patients, with the potential to reduce the frequency of invasive screening methods for this high-cancer-risk cohort.

Circulating miRNA Signature Predicts Cancer Incidence in Lynch Syndrome-A Pilot Study.

Lynch syndrome (LS) is the most common autosomal dominant cancer syndrome and is characterized by high genetic cancer risk modified by lifestyle factors. This study explored whether a circulating miRNA (c-miR) signature predicts LS cancer incidence within a 4-year prospective surveillance period. To gain insight how lifestyle behavior could affect LS cancer risk, we investigated whether the cancer-predicting c-miR signature correlates with known risk-reducing factors such as physical activity, body mass index (BMI), dietary fiber, or NSAID usage. The study included 110 c-miR samples from LS carriers, 18 of whom were diagnosed with cancer during a 4-year prospective surveillance period. Lasso regression was utilized to find c-miRs associated with cancer risk. Individual risk sum derived from the chosen c-miRs was used to develop a model to predict LS cancer incidence. This model was validated using 5-fold cross-validation. Correlation and pathway analyses were applied to inspect biological functions of c-miRs. Pearson correlation was used to examine the associations of c-miR risk sum and lifestyle factors. hsa-miR-10b-5p, hsa-miR-125b-5p, hsa-miR-200a-3p, hsa-miR-3613-5p, and hsa-miR-3615 were identified as cancer predictors by Lasso, and their risk sum score associated with higher likelihood of cancer incidence (HR 2.72, 95% confidence interval: 1.64-4.52, C-index = 0.72). In cross-validation, the model indicated good concordance with the average C-index of 0.75 (0.6-1.0). Coregulated hsa-miR-10b-5p, hsa-miR-125b-5p, and hsa-miR-200a-3p targeted genes involved in cancer-associated biological pathways. The c-miR risk sum score correlated with BMI (r = 0.23, P < 0.01). In summary, BMI-associated c-miRs predict LS cancer incidence within 4 years, although further validation is required. PREVENTION RELEVANCE: The development of cancer risk prediction models is key to improving the survival of patients with LS. This pilot study describes a serum miRNA signature-based risk prediction model that predicts LS cancer incidence within 4 years, although further validation is required.

Combined Gemcitabine and Immune-Checkpoint Inhibition Conquers Anti-PD-L1 Resistance in Low-Immunogenic Mismatch Repair-Deficient Tumors.

Tumors arising in the context of Lynch Syndrome or constitutional mismatch repair deficiency are hypermutated and have a good response towards immune-checkpoint inhibitors (ICIs), including α-PD-L1 antibodies. However, in most cases, resistance mechanisms evolve. To improve outcomes and prevent resistance development, combination approaches are warranted. Herein, we applied a combined regimen with an α-PD-L1 antibody and gemcitabine in a preclinical tumor model to activate endogenous antitumor immune responses. Mlh1-/- mice with established gastrointestinal tumors received the α-PD-L1 antibody (clone 6E11; 2.5 mg/kg bw, i.v., q2wx3) and gemcitabine (100 mg/kg bw, i.p., q4wx3) in mono- or combination therapy. Survival and tumor growth were recorded. Immunological changes in the blood were routinely examined via multi-color flow cytometry and complemented by ex vivo frameshift mutation analysis to identify alterations in Mlh1-/--tumor-associated target genes. The combined therapy of α-PD-L1 and gemcitabine prolonged median overall survival of Mlh1-/- mice from four weeks in the untreated control group to 12 weeks, accompanied by therapy-induced tumor growth inhibition, as measured by [18F]-FDG PET/CT. Plasma cytokine levels of IL13, TNFα, and MIP1ß were increased and also higher than in mice receiving either monotherapy. Circulating splenic and intratumoral myeloid-derived suppressor cells (MDSCs), as well as M2 macrophages, were markedly reduced. Besides, residual tumor specimens from combi-treated mice had increased numbers of infiltrating cytotoxic T-cells. Frameshift mutations in APC, Tmem60, and Casc3 were no longer detectable upon treatment, likely because of the successful eradication of single mutated cell clones. By contrast, novel mutations appeared. Collectively, we herein confirm the safe application of combined chemo-immunotherapy by long-term tumor growth control to prevent the development of resistance mechanisms.

Advances in epithelial ovarian cancer.

BACKGROUND: Most epithelial ovarian cancer occurs in older women, with a mean age at diagnosis of 62 years and an overall five­year survival rate in Australia of 43%. Most women are diagnosed with advanced disease of high-grade serous type with 20-30% five-year survival; 70% relapse within three years of initial treatment. There is no available screening test for ovarian cancer. OBJECTIVE: The aim of this article is to highlight current management and future directions for women diagnosed with epithelial ovarian cancer, particularly the high incidence of underlying genetic mutations and new options for treatment. DISCUSSION: Risk-reducing surgery with bilateral salpingo-oophorectomy is recommended for women at high risk of developing ovarian cancer. Ovarian cancer treatment still centres on surgery and chemotherapy, with aggressive cytoreductive techniques and intraperitoneal treatments being evaluated in advanced disease. Molecular targeting agents are revolutionising treatment options, particularly the poly adenosine diphosphate-ribose polymerase inhibitors, and especially for patients with an underlying BRCA mutation. Other molecular targeting agents, such as vascular endothelial growth factor (VEGF) receptor inhibitors and newer approaches using immunotherapy and molecular targeting, aim to individualise treatment and improve survival in the future.

Microsatellite instability driven metastatic parathyroid carcinoma managed with the anti-PD1 immunotherapy, pembrolizumab.

The present case report describes a 65-year-old man with Lynch syndrome and hypercalcaemia associated with hyperparathyroidism. Parathyroid surgery confirmed the diagnosis of parathyroid carcinoma. Serum calcium and parathyroid hormone (PTH) concentrations serially increased after initial surgery. Imaging study and subsequent biopsy confirmed lung metastases with mismatch repair deficiency. Pembrolizumab was initiated achieving 60% reduction in tumour burden.

High-sensitivity microsatellite instability assessment for the detection of mismatch repair defects in normal tissue of biallelic germline mismatch repair mutation carriers.

INTRODUCTION: Lynch syndrome (LS) and constitutional mismatch repair deficiency (CMMRD) are hereditary cancer syndromes associated with mismatch repair (MMR) deficiency. Tumours show microsatellite instability (MSI), also reported at low levels in non-neoplastic tissues. Our aim was to evaluate the performance of high-sensitivity MSI (hs-MSI) assessment for the identification of LS and CMMRD in non-neoplastic tissues. MATERIALS AND METHODS: Blood DNA samples from 131 individuals were grouped into three cohorts: baseline (22 controls), training (11 CMMRD, 48 LS and 15 controls) and validation (18 CMMRD and 18 controls). Custom next generation sequencing panel and bioinformatics pipeline were used to detect insertions and deletions in microsatellite markers. An hs-MSI score was calculated representing the percentage of unstable markers. RESULTS: The hs-MSI score was significantly higher in CMMRD blood samples when compared with controls in the training cohort (p<0.001). This finding was confirmed in the validation set, reaching 100% specificity and sensitivity. Higher hs-MSI scores were detected in biallelic MSH2 carriers (n=5) compared with MSH6 carriers (n=15). The hs-MSI analysis did not detect a difference between LS and control blood samples (p=0.564). CONCLUSIONS: The hs-MSI approach is a valuable tool for CMMRD diagnosis, especially in suspected patients harbouring MMR variants of unknown significance or non-detected biallelic germline mutations.

Type 2 diabetes mellitus, blood cholesterol, triglyceride and colorectal cancer risk in Lynch syndrome.

BACKGROUND: Type 2 diabetes mellitus and high total cholesterol and triglycerides are known to be associated with increased colorectal cancer risk for the general population. These associations are unknown for people with a germline DNA mismatch repair gene mutation (Lynch syndrome), who are at high risk of colorectal cancer. METHODS: This study included 2023 (56.4% female) carriers with a mismatch repair gene mutation (737 in MLH1, 928 in MSH2, 230 in MSH6, 106 in PMS2, 22 in EPCAM) recruited by the Colon Cancer Family Registry between 1998 and 2012. Weighted Cox regression was used to estimate the hazard ratios (HR) and 95% confidence intervals (CI) for the associations between self-reported type 2 diabetes, high cholesterol, triglyceride and colorectal cancer risk. RESULTS: Overall, 802 carriers were diagnosed with colorectal cancer at a median age of 42 years. A higher risk of colorectal cancer was observed in those with self-reported type-2 diabetes (HR 1.92; 95% CI, 1.03-3.58) and high cholesterol (HR 1.76; CI 1.23-2.52) compared with those without these conditions. There was no evidence of high triglyceride being associated with colorectal cancer risk. CONCLUSION: For people with Lynch syndrome, self-reported type-2 diabetes mellitus and high cholesterol were associated with increased colorectal cancer risk.

Medical Devices; Hematology and Pathology Devices; Classification of Lynch Syndrome Test Systems. Final order.

The Food and Drug Administration (FDA or we) is classifying Lynch syndrome test systems into class II (special controls). The special controls that apply to the device type are identified in this order and will be part of the codified language for the Lynch syndrome test systems' classification. We are taking this action because we have determined that classifying the device into class II (special controls) will provide a reasonable assurance of safety and effectiveness of the device. We believe this action will also enhance patients' access to beneficial innovative devices, in part by reducing regulatory burdens.

A cryptic paracentric inversion of MSH2 exons 2-6 causes Lynch syndrome.

Lynch syndrome is an autosomal dominant disorder that predisposes carriers of DNA mismatch repair (MMR) gene mutations to early-onset cancer. Germline testing screens exons and splice sites for mutations, but does not examine introns or RNA transcripts for alterations. Pathogenic mutations have not been detected in ~30% of suspected Lynch syndrome cases with standard screening practices. We present a 38-year-old male with a clinicopathological and family history consistent with Lynch syndrome, including loss of MSH2 expression in his tumor. Germline testing revealed normal MSH2 coding sequence, splice sites and exon copy number, however, cDNA sequencing identified an aberrant MSH2 transcript lacking exons 2-6. An inversion PCR on germline DNA identified an ~18kb unbalanced, paracentric inversion within MSH2, with breakpoints in a long terminal repeat in intron 1 and an Alu repeat in intron 6. The 3' end of the inversion had a 1.2 kb deletion and an 8 bp insertion at the junction with intron 6. Screening of 55 additional Australian patients presenting with MSH2-deficient tumors who were negative in germline genetic tests for MSH2 mutations identified another inversion-positive patient. We propose an Alu-mediated recombination model to explain the origin of the inversion. Our study illustrates the potential value of cDNA screening to identify patients with cryptic MMR gene rearrangements, clarifies why standard testing may not detect some pathogenic alterations, and provides a genetic test for screening individuals with suspected Lynch syndrome that present with unexplained MSH2-deficient tumors.

Comparative polymorphism of BAT-26 between healthy individuals and cancer patients and its cancer risk implication for local Chinese.

PURPOSE: BAT-26 is one of the representative markers for microsatellite instability evaluation and presents different polymorphisms in different ethnic populations. The current knowledge of its comparative polymorphism between healthy individuals and cancer patients in the Chinese population is insufficient. This study aims to analyze germline polymorphic variations of BAT-26 between healthy individuals and cancer patients in Chinese from Jiangsu province and the associated cancer risk implications. METHODS: The various BAT-26 alleles and their percentages in cervical cells from 500 healthy women were assessed by direct sequencing. Twenty of these samples were also analyzed by fragment analysis. BAT-26 of blood DNA from 24 healthy individuals and 247 cancer patients was analyzed by fragment analysis. RESULTS: Compared with the sequencing results, 122.6-122.9 bp, 123.4-123.8 bp and 124.1-124.8 bp corresponded to the A25, A26 and A27 alleles, respectively. The 524 healthy individuals showed 4.58%, 92.18% and 3.24% of A25, A26 and A27, respectively. The variant alleles A18, A24, A28, A29 and A32 were only found in cancer patients, accounting for 0.81%, 0.40%, 0.40%, 0.40% and 0.40%, respectively; the A25, A26 and A27 alleles in cancer patients accounted for 6.48%, 77.33% and 13.77%. CONCLUSIONS: Healthy individuals had a stable BAT-26 profile within the quasimonomorphic variation range (QMVR), but cancer patients harbored variant alleles outside QMVR and showed a trend from quasimonomorph to polymonomorph, suggesting that variant alleles of BAT-26 in germline cells may be regarded as a potential marker of higher cancer risk in the Chinese population from Jiangsu province.

Dietary B vitamin and methionine intake and MTHFR C677T genotype on risk of colorectal tumors in Lynch syndrome: the GEOLynch cohort study.

PURPOSE: Dietary intake of B vitamins and methionine, essential components of DNA synthesis and methylation pathways, may influence colorectal tumor (CRT) development. The impact of B vitamins on colorectal carcinogenesis in individuals with Lynch syndrome (LS) is unknown but is important given their high lifetime risk of developing neoplasms. The role of MTHFR C677T genotype in modifying these relationships in LS individuals is also unclear. We investigated associations between dietary intakes of folate, vitamins B2, B6, B12, and methionine and CRT development in a prospective cohort study of 470 mismatch repair gene mutation carriers. METHODS: Dietary intakes were assessed by food frequency questionnaire. Cox regression models with robust sandwich covariance estimation, adjusted for age, sex, physical activity, number of colonoscopies during person-time, NSAID use, and mutual vitamins were used to calculate hazard ratios (HRs) and 95 % confidence intervals (95 % CIs). Analyses were also stratified by MTHFR C677T genotype. RESULTS: During a median person-time of 28.0 months, 131 persons developed a CRT. Fifty-one of these persons developed an incident colorectal adenoma, while there were four persons who developed an incident colorectal carcinoma. Compared to the lowest tertile of intake, adjusted HRs (95 % CIs) for CRT development in the highest tertile were 1.06 (0.59-1.91) for folate, 0.77 (0.39-1.51) for vitamin B2, 0.98 (0.59-1.62) for vitamin B6, 1.24 (0.77-2.00) for vitamin B12, and 1.36 (0.83-2.20) for methionine. Low vitamin B2 and low methionine intake were statistically significantly associated with an increased risk of CRT in MTHFR 677TT individuals compared to a combined reference of persons with low intake and CC genotype. CONCLUSIONS: There was no suggestion that intake of any dietary B vitamin or methionine was associated with CRT development among those with LS.

Longer telomeres are associated with cancer risk in MMR-proficient hereditary non-polyposis colorectal cancer.

Aberrant telomere length measured in blood has been associated with increased risk of several cancer types. In the field of hereditary non-polyposis colorectal cancer (CRC), and more particularly in Lynch syndrome, caused by germline mutations in the mismatch repair (MMR) genes, we recently found that cancer-affected MMR gene mutation carriers had shorter telomeres and more pronounced shortening of telomere length with age than controls and unaffected MMR gene mutation carriers. Here we evaluate blood telomere length in MMR-proficient hereditary non-polyposis CRC, i.e. familial CRC type X (fCRC-X). A total of 57 cancer-affected and 57 cancer-free individuals from 34 Amsterdam-positive fCRC-X families were analyzed and compared to the data previously published on 144 cancer-affected and 100 cancer-free MMR gene mutation carriers, and 234 controls. Relative telomere length was measured using a monochrome multiplex quantitative PCR method, following strict measures to avoid sources of bias and adjusting by age. Despite the retrospective nature of our study, the results show that longer telomeres associate with cancer risk in fCRC-X, thus identifying different patterns of telomere length according to the status of the MMR system.

Analysis of telomere dynamics in peripheral blood cells from patients with Lynch syndrome.

BACKGROUND: In patients with Lynch syndrome, germline mutations in DNA mismatch repair (MMR) genes cause a high risk of developing a broad spectrum of cancers. To date, the management of patients with Lynch syndrome has represented a major challenge because of large variations in age at cancer onset. Several factors, including genetic anticipation, have been proposed to explain this phenotypic heterogeneity, but the molecular mechanisms remain unknown. Telomere shortening is a common event in tumorigenesis and also has been observed in different familial cancers. In this study, the authors investigated the possibility of a relation between telomere length and cancer onset in patients with Lynch syndrome. METHODS: The mean telomere length was measured using quantitative polymerase chain reaction in peripheral blood samples from a control group of 50 individuals, from 31 unaffected mutation carriers, and from 43 affected patients, and the results were correlated with both gene mutation and cancer occurrence. In affected patients, telomere attrition was correlated with age at cancer onset. In all patients, a t test was used to assess the linearity of the regression. RESULTS: A significant correlation between telomere length and age was observed in both affected and unaffected mutation carriers (P = .0016 and P = .004, respectively) and in mutS homolog 2 (MSH2) mutation carriers (P = .0002) but not in mutL homolog 1 (MLH1) mutation carriers. Telomere attrition was correlated significantly with age at onset in MSH2 carriers (P = .004), whereas an opposite trend toward longer telomeres in patients with delayed onset was observed in MLH1 carriers. CONCLUSIONS: The current data suggested that telomere dynamics differ between MLH1 and MSH2 mutation carriers. It is possible that subtle, gene-specific mechanisms can be linked to cancer onset and anticipation in patients with Lynch syndrome.

Analysis of microsatellite instability and BRCA1 mutations in patients from hereditary nonpolyposis colorectal cancer (HNPCC) family.

Susceptibility to colorectal cancer appears to be linked to germ-line mutations in genes causing various familial cancer syndromes. The objectives of this study were to determine microsatellite instability and the frequency of BRCA1 germ-line mutations in patients with family history of cancer. The study population consisted of 30 patients from HNPCC family. Patients completed a family history questionnaire and provided blood for mutation analysis. Seven out of 30 investigated samples (23%) were found to be MSI-positive, 6 MSI-high and 1 MSI-low. Ex20insC and ExII17delA mutations of BRCA1 gene were identified in MSI-positive samples from HNPCC families. In the present study one Ex20insC mutation and two ExII17delA mutations were detected only in MSI-high samples. Genetic alterations seem to be a risk factor of colorectal cancer in subjects belonging to HNPCC families with high incidence of this cancer. The lack of detectable germ-line mutations in most cases suggests that there are probably additional, as yet unidentified genes predisposing to this disease.

Detection of germline mutations of hMLH1 and hMSH2 based on cDNA sequencing in China.

AIM: To detect the germline mutations of hMLH1 and hMSH2 based on mRNA sequencing to identify hereditary non-polyposis colorectal cancer (HNPCC) families. METHODS: Total RNA was extracted from peripheral blood of 14 members from 12 different families fulfilling Amsterdam criteria II. mRNA of hMLH1 and hMSH2 was reversed with special primers and heat-resistant reverse transcriptase. cDNA was amplified with expand long template PCR and cDNA sequencing analysis was followed. RESULT: Seven germline mutations were found in 6 families (6/12, 50%), in 4 hMLH1 and 3 hMSH2 mutations (4/12, 33.3%); (3/12, 25%). The mutation types involved 4 missense, 1 silent and 1 frame shift mutations as well as 1 mutation in the non-coding area. Four out of the seven mutations have not been reported previously. The 4 hMLH1 mutations were distributed in exons 8, 12, 16, and 19. The 3 hMSH2 mutations were distributed in exons 1 and 2. Six out of the 7 mutations were pathological, which were distributed in 5 HNPCC families. CONCLUSION: Germline mutations of hMLH1 and hMSH2 can be found based on cDNA sequencing so as to identify HNPCC family, which is highly sensitive and has the advantages of cost and time saving.

Genetic analysis of microsatellite markers in patients from hereditary nonpolyposis colorectal cancer (HNPCC) families.

AIM: Microsatellite instability (MSI) is due to defective DNA mismatch repair (MMR) and is characteristic of hereditary nonpolyposis colorectal cancer (HNPCC) tumors. The role of MSI in familial predisposition to colorectal cancer was investigated in this study by both microsatellite analysis and mutation screening of the two major MMR genes MLH1 and MSH2 among familial cases. METHODS: PCR-based microsatellite analysis was performed in blood obtained from 30 members from HNPCC families. Blood samples age matched healthy individuals (n = 28) served as control. MSI was studied at five loci containing single- or dinucleotide repeat sequences and mapping to different chromosomal locations: BAT-25 (at locus 4q12), BAT-26 (2p16), D2S123 (2p16-p21), D5S346 (5q21-q22) and D17S250 (17q11.2-q12). RESULTS: MSI frequency was higher in member of HNPCC families [7/30 (23%)] than in control [3/28 (10.7%)] cases. Two MLH1 and one MSH2 mutations were identified in 7 MSI positive samples from HNPCC families. MLH1/MSH2 mutations were only in MSI high samples detected. CONCLUSION: Genetic alterations seem to be a risk factor of colorectal cancer in subjects belonged to HNPCC families with high incidence of this cancer.

A hereditary nonpolyposis colorectal carcinoma case associated with hypermethylation of the MLH1 gene in normal tissue and loss of heterozygosity of the unmethylated allele in the resulting microsatellite instability-high tumor.

Fourteen suspected hereditary nonpolyposis colorectal carcinoma cases with microsatellite unstable(microsatellite instability-high; MSI-H) tumors but no germ-line MSH2, MSH6, or MLH1 mutations were examined for hypermethylation of CpG sites in the critical promoter region of MLH1. The methylation patterns were determined using methylation-specific PCR and by sequence analysis of sodium bisulfite-treated genomic DNA. In one case, DNA hypermethylation of one allele was detected in DNA isolated from blood. In the MSI-H tumor from this case, the unmethylated MLH1 allele was eliminated by loss of heterozygosity, and the methylated allele was retained. This biallelic inactivation resulted in loss of expression of MLH1 in the tumor as confirmed by immunohistochemistry. These results suggest a novel mode of germ-line inactivation of a cancer susceptibility gene.

[Colorectal cancer mortality can be reduced. Fecal blood screening is an efficient test]. / Dödligheten i kolorektal cancer kan reduceras. Screening för blod i avföringen effektivt test.

Findings in large randomised studies in Europe and America show that colorectal cancer mortality can be reduced by faecal occult blood screening. Moreover, in the American study it was shown for the first time that the risk of colorectal cancer is reduced by removal of benign polyps larger than 9 mm. Partly as a result of uncontrolled studies showing repeat examination with the rigid rectoscope and removal of benign adenomas to reduce the risk of cancer, flexible sigmoidoscopy has been adopted in two controlled studies, one British the other American, though the problem remains of diagnosing the 30-40 per cent of tumours that are inaccessible to the sigmoidoscope. Together, the two types of hereditary colorectal cancer, familial adenomatous polyposis and hereditary non-polyposis colon cancer, account for only 5-10 per cent of all colorectal cancer, the overwhelming majority of cases being sporadic.

DNA mismatch repair in lymphoblastoid cells from hereditary non-polyposis colorectal cancer (HNPCC) patients is normal under conditions of rapid cell division and increased mutational load.

Patients with hereditary non-polyposis colorectal cancer (HNPCC) have germ-line mutations in one of four DNA mismatch repair genes (hMSH2, hMLH1, hPMS1 and hPMS2). It is supposed that a single functional copy of these genes is sufficient for normal mismatch repair, but it is not certain that this is the case under abnormal conditions such as rapid cell division or an increased tendency to DNA replication errors (RERs). We have analysed mismatch repair by examining replication errors in immortalised lymphoblastoid cells derived from two HNPCC patients heterozygous for mismatch repair defects (one hMSH2 mutant and one hMLH1 mutant), and from control individuals. Three conditions of cell culture have been used: (i) relatively slow cell growth and division; (ii) relatively fast growth and division; and (iii) chronic perturbation of the intracellular dNTP pool to promote a increased frequency of replication errors. No significant differences in microsatellite instability were found between HNPCC patients and controls in any of these environments. Lymphoblastoid cells from hMSH2 and hMLH1 mutant/wild-type heterozygotes appear, therefore, to have normal levels of mismatch repair, even under conditions that increase the requirement for repair. The pool bias cultures from the HNPCC patients and controls did, however, show similar, increased frequencies of RERs, suggesting that the mismatch repair capacity of the cells had been overloaded, but that the number of normal HNPCC alleles was not the limiting factor.