Apoptosis induced by overexpression of hMSH2 or hMLH1.

Mutations of the mismatch repair genes hMSH2 and hMLH1 have been found in a high proportion of individuals with hereditary nonpolyposis colon cancer (HNPCC), establishing the link between mismatch repair and cancer. Tumor cell lines that are deficient in mismatch repair develop a mutator phenotype that appears to drive the accumulation of mutations required for tumor development. However, mutations of other mismatch repair genes such as hPMS2 can lead to a mutator phenotype, although inherited mutations of these genes are rare in HNPCC families. Here, we show that overexpression of hMSH2 or hMLH1 but not of hMSH3, hMSH6, or hPMS2 induces apoptosis in either repair-proficient or -deficient cells. Furthermore, primary mouse embryo fibroblasts derived from Msh2-deficient mice lose their ability to undergo apoptosis after treatment with N-methyl-N'-nitro-N-nitrosoguanidine. These results suggest that the mismatch repair proteins hMSH2 and hMLH1 may be components of a pathway that influences apoptosis. We consider the possibility that loss of apoptosis as a result of hMSH2 or hMLH1 deficiency may be an additional factor in cancer predisposition in HNPCC.     

The role of hPMS1 and hPMS2 in predisposing to colorectal cancer.

Hereditary nonpolyposis colorectal cancer (HNPCC) is attributable to a deficiency of mismatch repair. Inactivation of DNA mismatch repair underlies the genesis of microsatellite instability in colorectal cancer. Germline mutations in three DNA mismatch repair genes, hMSH2, hMLH1, and hMSH6, have been found to segregate in HNPCC and HNPCC-like families. The two DNA mismatch repair genes hPMS1 and hPMS2 have also been suggested to predispose to HNPCC. In this study, 84 HNPCC and HNPCC-like kindreds without known mutations in the other three known DNA mismatch repair genes were screened for germline mutations in the hPMS1 or hPMS2 gene. No clear-cut pathogenic mutations were identified. Conversion technology was used to detect a large hMSH2 deletion in two affected members of the kindred in which the hPMS1 mutation was originally reported, whereas the hPMS1 mutation was only present in one of these two individuals. Since the hPMS1 and hPMS2 genes were first reported, germline mutations in hPMS2 have been demonstrated primarily in patients with Turcot's syndrome. However, no mutation in any of the two genes has been found to segregate in HNPCC families. Until there is better evidence for an increased colorectal cancer risk associated with germline mutations in these genes, a conservative interpretation of the role of mutations in these genes is advised.     

食管癌和贲门癌组织中错配修复基因表达的研究

目的　研究错配修复基因 (MMR)表达与食管癌和贲门癌癌变的关系和甲基苄基亚硝胺 (NMBzA)对其影响的研究。方法　采用多重逆转录聚合酶链反应 (RT -PCR)。结果　 31例食管癌hMSH2基因表达明显降低占 2 2 .6 % ;hMLH1基因表达明显降低占 6 .45 % ;未发现hPMS2 ,GTBP和hPMS1基因表达降低。 1 1例食管癌旁组织分析 :hMSH2 ,hPMS2 ,GTBP ,hMLH1基因表达明显降低占 9.0 9% ;未发现hPMS1基因表达明显降低。 1 2例贲门癌组织分析 :hMSH2基因表达明显降低占 41 .7% ;GTBP ,hMLH1 ,hPMS1基因表达明显降低占 8.3 % ;未发现hPMS2基因表达明显降低。 8例贲门癌旁组织分析 :3例hMSH2基因表达明显降低 ;1例hPMS2基因表达明显降低 ;3例hMLH1基因表达明显降低 ;未发现GTBP ,hPMS1基因表达明显降低。对NMBzA致癌物诱发的人胎儿食管上皮癌细胞株分析发现 :hMLH1和hPMS1基因低表达 ;hMSH2 ,hPMS2 ,GTBP基因不表达。结论　食管癌和贲门癌癌变可能与错配修复基因失活有关 ;甲基苄基亚硝胺能引起错配修复基因表达异常。     

Deletion Mutations in an Australian Series of HNPCC Patients

Hereditary non polyposis colorectal cancer (HNPCC) is characterized by the presence of early onset colorectal cancer and other epithelial malignancies. The genetic basis of HNPCC is a deficiency in DNA mismatch repair, which manifests itself as DNA microsatellite instability in tumours. There are four genes involved in DNA mismatch repair that have been linked to HNPCC; these include hMSH2, hMLH1, hMSH6 and hPMS2. Of these four genes hMLH1 and hMSH2 account for the majority of families diagnosed with the disease. Notwithstanding, up to 40 percent of families do not appear to harbour a change in either hMSH2 or hMLH1 that can be detected using standard screening procedures such as direct DNA sequencing or a variety of methods all based on a heteroduplex analysis.In this report we have screened a series of 118 probands that all have the clinical diagnosis of HNPCC for medium to large deletions by the Multiplex Ligation-Dependent Probe Amplification assay (MLPA) to determine the frequency of this type of mutation. The results indicate that a significant proportion of Australian HNPCC patients harbour deletion or duplication mutations primarily in hMSH2 but also in hMLH1.     

Deletion Mutations in an Australian Series of HNPCC Patients

Hereditary non polyposis colorectal cancer (HNPCC) is characterized by the presence of early onset colorectal cancer and other epithelial malignancies. The genetic basis of HNPCC is a deficiency in DNA mismatch repair, which manifests itself as DNA microsatellite instability in tumours. There are four genes involved in DNA mismatch repair that have been linked to HNPCC; these include hMSH2, hMLH1, hMSH6 and hPMS2. Of these four genes hMLH1 and hMSH2 account for the majority of families diagnosed with the disease. Notwithstanding, up to 40 percent of families do not appear to harbour a change in either hMSH2 or hMLH1 that can be detected using standard screening procedures such as direct DNA sequencing or a variety of methods all based on a heteroduplex analysis.In this report we have screened a series of 118 probands that all have the clinical diagnosis of HNPCC for medium to large deletions by the Multiplex Ligation-Dependent Probe Amplification assay (MLPA) to determine the frequency of this type of mutation. The results indicate that a significant proportion of Australian HNPCC patients harbour deletion or duplication mutations primarily in hMSH2 but also in hMLH1.     

Allelic losses and DNA methylation at DNA mismatch repair loci in sporadic colorectal cancer

Normal and tumor DNA samples of 35 patients with sporadic colorectal carcinoma were analyzed for microsatellite alterations at 12 markers linked to mismatch repair loci: hMLH1, hMSH2, hMSH3, hMSH6, hPMS1 and hPMS2. Remarkably, no correlation was observed between the replication error phenotype (RER+) and allelic losses at these loci. Hemizygous deletions, seen in 6/35 (17%) informative cases at hMLH1, 4/27 (15%) at hMSH2/hMSH6 and 6/34 (18%) at hMSH3, were rarely found in RER+ tumors. Since mismatch repair protein components act in molecular complexes of defined stoichiometry we propose that hemizygous deletion of the corresponding loci may be involved in colorectal tumorigenesis through defects in cellular functions other than replication error correction. The analysis of the methylation status of the promoter region of hMLH1 revealed that methylation might be an important mechanism of this locus inactivation in RER+ sporadic colorectal cancer.      

Conditional nuclear localization of hMLH3 suggests a minor activity in mismatch repair and supports its role as a low-risk gene in HNPCC

DNA mismatch repair (MMR) mechanism contributes to the maintenance of genomic stability. Loss of MMR function predisposes to a mutator cell phenotype, microsatellite instability (MSI) and cancer, especially hereditary non-polyposis colorectal cancer (HNPCC). To date, five MMR genes, hMSH2, hMSH6, hMLH1, hPMS2, and hMLH3 are associated with HNPCC. Although, hMLH3 is suggested to be causative in HNPCC, its relevance to MMR needs to be confirmed to reliably assess significance of the inherited sequence variations in it. Recently, a human heterodimer hMLH1/hMLH3 (hMutLgamma) was shown to be able to assist hMLH1/hPMS2 (hMutLalpha) in the repair of mismatches in vitro. To repair mismatches in vivo, hMLH3 ought to localize in the nucleus. Our immunofluorescence analyses indicated that when all the three MutL homologues are natively expressed in human cells, endogenous hMLH1 and hPMS2 localize in the nucleus, whereas hMLH3 stays in the cytoplasm. Absence of hPMS2 and co-expression of hMLH3 with hMLH1 results in its partial nuclear localization. Our results are clinically relevant since they show that in the nuclear localization hMLH3 is dependent on hMLH1 and competitive with hPMS2. The continuous nuclear localization of hMLH1 and hPMS2 suggests that in vivo, hPMS2 (hMutLalpha) has a major activity in MMR. In absence of hPMS2, hMLH3 (hMutLgamma) is located in the nucleus, suggesting a conditional activity in MMR and supporting its role as a low-risk gene in HNPCC.     

Involvement of hMSH6 in the development of hereditary and sporadic colorectal cancer revealed by immunostaining is based on germline mutations, but rarely on somatic inactivation.

Germline mutations in human mismatch repair (MMR) genes yield a predisposition for the hereditary nonpolyposis colon cancer (HNPCC) syndrome. In contrast to hMLH1 and hMSH2, little is known about the overall involvement of hMSH6 in colorectal cancer. We investigated 82 tumors from patients who fulfilled the Bethesda guidelines for HNPCC as well as 146 sporadic tumors, analyzing microsatellite instability and expression of the 4 MMR proteins hMSH6, hMSH2, hMLH1 and hPMS2. Four tumors with lost expression and 1 tumor with cytoplasmic expression of hMSH6 were identified. Sequence analysis revealed germline mutations in 4 of the 5 patients, including 1 patient with sporadic disease. The lost or reduced expression of hMSH2 and hMLH1 was always identical to its heterodimerization partners, hMSH6 and hPMS2, respectively. Furthermore, hMSH2 expression was reduced upon hMSH6 deficiency. Abnormal expression of 1 or more of the 4 proteins was always associated with a high level of microsatellite instability (MSI-H). Conversely, all but 1 of the 44 MSI-H tumors had abnormal expression of 1 or more of the proteins, basically excluding additional genes associated with the MSI-H phenotype. We conclude that the involvement of somatic or epigenetic hMSH6 inactivation in colorectal cancer is rare.     

Mismatch repair and microsatellite instability in esophageal cancer cells

Using in vitro mismatch repair (MMR) assay, we have identified 3 of 22 esophageal cancer cell lines exhibiting reduced MMR activity. By means of gel-shift assay, decreased binding ability to GT mismatch and CA loop was observed in these 3 cell lines. However, we could not find any mutations in the hMSH2, hMSH3 and hMSH6 genes, the protein products of which exhibit mismatch binding activity in human cells. In addition, when using antibodies against 5 MMR-related proteins (hMSH2, hMSH3, hMSH6, hPMS2 and hMLH1), no aberrant expression was detected in any of them. When we examined 9 microsatellite loci in endogenous genomic DNA, these 3 esophageal cancer cell lines, deficient in MMR, did not exhibit microsatellite instability. However, when we examined the repetitious sequence on exogenous plasmid DNA which was introduced into these 3 esophageal cancer cells, the results suggested that MMR deficiency in esophageal cancer cells could result in moderate instability of the exogenous sequence.     

Loss or Somatic Mutations of hMSH2 Occur in Hereditary Nonpolyposis Colorectal Cancers with hMSH2 Germline Mutations

Hereditary nonpolyposis colorectal cancer (HNPCC) is a major cancer susceptibility syndrome known to be caused by the inheritance of mutations in DNA mismatch repair genes, such as hMSH2, hMLH1, hPMS1 and hPMS2 . To investigate the role of genetic alterations of hMSH2 in HNPCC tumorigenesis, we analyzed 36 Japanese HNPCC kindreds as to hMSH2 germline mutations. Moreover, we also examined somatic mutations of hMSH2 or loss of heterozygosity at or near the hMSH2 locus in the tumors from the hMSH2 -related kindreds. Germline mutations were detected in five HNPCC kindreds (5/36, 14%). Among them, three were nonsense mutations, one was a frameshift mutation and the other was a mutation in an intron where the mutation affected splicing. Loss of heterozygosity in four and somatic mutations in one were detected among the eight tumors with hMSH2 germline mutations. All these alterations were only detected in genomic instability(+) tumors, i.e., not in genomic instability(-) ones, indicating that mutations of hMSH2 were responsible for at least some of the tumors with genomic instability. These data establish a basis for the presymptomatic diagnosis of HNPCC patients, and constitute further evidence that both DNA mismatch repair genes and tumor suppressor genes may share the same requirement, i.e., two hits are necessary to inactivate the gene function.     

Expression of DNA mismatch repair proteins hMSH2 and hMLH1 and the cyclin G1 inhibitor, p21(waf1/cip1) in pediatric tumors: correlation with response to therapy

The expression of the DNA mismatch repair proteins hMSH2 and hMLH1 and p21(waf1) the cyclin G1 inhibitor, may determine response of adult cancers to anti-cancer drugs, that include alkylating agents and platinum-based drugs. The role of DNA mismatch repair proteins (hMSH2 and hMLH1) and p21(waf1) in pediatric tumor responses to chemotherapy and irradiation is described in the present study of 23 pediatric solid cancers (4 wilms' tumors, 9 neuroblastomas, 3 hepatoblastomas, 3 lymphomas and 4 sarcomas) using immunohistochemical methods. Immunostaining was scored for intensity (0-3) and extent (0-3). Most tumors stained strongly for hMSH2 and weakly or negative for hMLH1. All the hMLH1 negative tumors (1 wilms', 1 hepatoblastoma, 1 sarcoma, 2 lymphomas and 2 neuroblastomas) achieved complete response. p21(waf1) negative and positive tumors achieved relatively similar treatment response. The results suggest that the expression of DNA mismatch repair proteins hMLH1 and hMSH2, and p21(waf1) do not influence individual cancer responses to treatment and the results may reflect the use of multiple drugs and irradiation that cause many different types of DNA damage.     

Microsatellite Instability Assessment by Immunohistochemistry in Acute Myeloid Leukemia: A Reappraisal and Review of the Literature

BackgroundMicrosatellite instability (MSI) is caused by defects in DNA mismatch repair (MMR) components. Inactivation of any MMR gene(s), including hMLH1, hMSH2, hMSH6, and hPMS2, can result in MSI. Immunohistochemistry (IHC) is a sensitive and specific screening tool for MSI that can detect loss of expression of one or more MMR components. Of the four MMR markers, hMLH1 and hMSH2 are considered most informative of MSI status. There has been renewed interest in MSI status in view of its favorable association with response to immune checkpoint inhibitors in some cancers. MMR expression patterns in acute myeloid leukemia (AML) have not been evaluated systematically.MethodsWe used clinically-validated IHC assays to assess the expression of hMLH1, hMSH2, hMSH6, and/or hPMS2 in formalin-fixed paraffin-embedded tissue sections of bone marrow core biopsies from patients diagnosed with AML. Mutation profiling was performed using next-generation sequencing to assess for mutations in MMR genes.ResultsThe study group included 236 patients with AML, including a cohort treated on a clinical trial of azacitidine and nivolumab (NCT02397720). In addition, hMSH6, and/or hPMS2 expression was assessed in 99 AML patients with diploid karyotype. All patients, except two, had retained expression of all MMR markers assessed: One patient from the azacytidine+nivolumab group had zonal patchy loss of staining of hMLH1 and, to a lesser extent, a similar staining pattern of hMSH2; and one patient from the AML with diploid karyotype group had loss of hMSH2 but retained expression of hMLH1, hMSH6 and hPMS2. In addition, a retrospective analysis on a separate cohort of 139 patients with primary AML, on which next generation sequencing profiling was performed, identified 14 cases with alterations in MMR genes.Conclusion and remarksMMR loss is a rare event in AML, thus does not appear to underlie response patterns to anti-PD1 therapy.     

散发性大肠癌的错配修复缺陷研究

目的探讨错配修复缺陷的散发性大肠癌的临床病理特征及错配修复缺陷检测手段的应用。方法对71例散发性大肠癌行hMLH1启动子甲基化检测、微卫星不稳定检测以及hMLH1和hMSH2的免疫组化检测,分析错配修复缺陷的散发性大肠癌的临床病理特征,探讨三种检测方法的应用价值。结果hMLH1基因启动子甲基化、微卫星不稳定和错配修复蛋白表达的阳性率分别为9.9%,9.9%和71.0%,三者密切相关。hMLH1启动子甲基化和微卫星不稳定的散发性大肠癌均具有结肠癌多发和低分化腺癌相对多见的特征。错配修复蛋白表达阴性的散发性大肠癌仅具有低分化腺癌相对多见的特征。结论错配修复缺陷的散发性大肠癌具有结肠癌和低分化腺癌多发的倾向,hMLH1启动子甲基化和微卫星不稳定以及错配修复蛋白的失表达三者密切相关。     

Frequent Microsatellite Instabilities and Analyses of the Related Genes in Familial Gastric Cancers

Microsatellite instability or replication error seems to be related to defective DNA mismatch repair genes, such as hMSH2 , hMLH1 , hPMS1 and hPMS2 , which have been identified as causative genes of hereditary nonpolyposis colorectal cancers (HNPCC). Recently, it was reported that mutations at the simple repeated sequences in the transforming growth factor-β type II receptor ( TGF-β RII ) gene occurred in replication error-positive colorectal cancers. To determine genetic alterations in familial gastric cancers (FGC), we examined replication error using eight microsatellite DNA markers, and screened mutations in the hMSH2 , hMLH1 and TGF-β RII genes in six cases from four FGC kindreds. Moreover, hMTH1 , a human homolog of the bacterial mutT gene, was also screened. Four of six (67%) cancers showed the replication error-positive phenotype, indicating that microsatellite instability is highly associated with not only HNPCC, but also FGC. No germline mutation was found in the whole coding sequences of hMSH2 and hMTH1 , or in the conservative regions of hMLH1 in any patient, while one cancer DNA showed a somatic mutation at codon 682 (threonine to alanine) in hMSH2 . No alteration was found at the small repeated sequences in TGF-β RII in FGC tumor DNA. These results indicate that the carcinogenetic process of FGC may be different from that of HNPCC.     

High resolution deletion mapping reveals frequent allelic losses at the DNA mismatch repair loci hMLH1 and hMSH3 in non-small cell lung cancer

To study the involvement of DNA mismatch repair genes in non-small cell lung cancer, matched normal and tumoral DNA samples from 31 patients were analyzed for both LOH and microsatellite instability with 34 markers at or linked to hMLH1(3p21),hMSH2(2p16), hMSH3(5q11-q13),hMSH6(2p16), hPMS1(2q32), and hPMS2 (7p22) loci. Chromosomal regions 3p21 and 5q11-q13 were found to be hemizygously deleted in 55% and 42% of the patients, respectively. Sixty five percent of the patients deleted at hMLH1 were also deleted at hMSH3. The shortest regions of overlap for 3p21 and 5q11-q13 deletions delimited by D3S1561/D3S1612 and D5S2107/D5S624, respectively, were restricted to genetic distances of only 1 cM. Currently, the hMLH1 (3p21) and hMSH3 (5q11-q13) genes are the only known candidates located within these regions. The mutational analysis of hMLH1 and hMSH3 in hemizygously deleted patients led to the detection of 2 new polymorphisms in hMSH3. The consequence of these allelic losses remains unclear, but the lack of inactivating mutation might explain that replication error, the hallmark of mismatch repair genes inactivation in cancer cells, was quasi-absent in tumors. We suggest that hMLH1 and hMSH3 genes could be involved in lung tumorigenesis through dosage effect in cellular functions other than replication error correction. Int. J. Cancer 77:173–180, 1998. © 1998 Wiley-Liss, Inc.     

Mismatch repair deficiency in hematological malignancies with microsatellite instability

Mutations in human mismatch repair (MMR) genes are the genetic basis for certain types of solid tumors displaying microsatellite instability (MSI). MSI has also been observed in hematological malignancies, but whether these hematological malignancies are associated with MMR deficiency is still unclear. Using both biochemical and genetic approaches, this study analysed MMR proficiency in 11 cell lines derived from patients with hematological malignancies and demonstrated that six out of seven hematological cancer cell lines with MSI were defective in strand-specific MMR. In vitro complementation experiments, using characterized MMR mutant extracts or purified proteins, showed that these hematological cancer cells were defective in either hMutS(alpha) (a heterodimer of hMSH2 and hMSH6) or hMutL(alpha) (a heterodimer of hMLH1 and hPMS2). Furthermore, cell lines deficient in hMutS(alpha) showed large deletions or point mutations in hMSH2, while those deficient in hMutL(alpha) exhibited point mutations in hMLH1 or a lack of expression of hPMS2. From these results, we conclude that, as in solid tumors, hematological malignancies with MSI are also associated with MMR deficiency, and that the cause of MMR deficiency in these cell lines is due to a defective MutS(alpha) or MutL(alpha). We also report here, for the first time, that an MSI-positive cell line derived from Burkitt's lymphoma is proficient in MMR.     

A novel mechanism for aspirin-mediated growth inhibition of human colon cancer cells.

PURPOSE: The molecular mechanisms by which aspirin and other nonsteroidal anti-inflammatory drugs exert chemopreventative effects in colon cancer are unclear and complex. Current investigations focus on the chemopreventive properties of nonsteroidal anti-inflammatory drugs, independent of their ability to inhibit cyclooxygenase (COX) activity, and presumably, identification of non-COX pathways will suggest new targets for clinical use. It was demonstrated recently that aspirin results in reduced microsatellite instability in colorectal cancer cells. We hypothesized that aspirin treatment might alter expression of DNA mismatch repair (MMR) proteins, representing another potential non-COX mechanism for its action. EXPERIMENTAL DESIGN: In this study, we have examined the effects of aspirin on the cellular growth rates, MMR protein levels, cell cycle analysis and apoptosis in MMR-deficient (HCT116) and MMR-proficient (HCT116+chr3 and SW480) human colon cancer cell lines. RESULTS: We found that treatment with aspirin inhibited the growth of these three cancer cell lines. In HCT116+chr3 cells, treatment with 1 mM of aspirin increased expression of the hMLH1 and hPMS2 proteins by 2.5-fold and 2-fold, respectively, and increased expression of the hMSH2 and hMSH6 proteins by 2-3-fold. For SW480 cells, treatment with 1 and 5 mM of aspirin increased expression of the hMLH1 and hPMS2 proteins by 2-4-fold and 3-5-fold, respectively, and increased expression of the hMSH2 and hMSH6 proteins by 3-7-fold. For all three of the cell lines, treatment with 1 and 2.5 mM of aspirin induced apoptosis at 48 and 72 h. Aspirin induced G(0)/G(1) cell cycle arrest in HCT116 cells. CONCLUSIONS: We conclude that aspirin acts through COX-independent mechanisms by resulting in an increase in MMR protein expression and subsequent apoptosis, which might serve as an additional means of growth inhibition in aspirin-treated human colon cancer cells.     

The reliability of immunohistochemistry as a prescreening method for the diagnosis of hereditary nonpolyposis colorectal cancer (HNPCC)--results of an international collaborative study

Hereditary nonpolyposis colorectal cancer syndrome (HNPCC) is an autosomal dominant condition accounting for 2-5% of all colorectal carcinomas as well as a small subset of endometrial, upper urinary tract and other gastrointestinal cancers. An assay to detect the underlying defect in HNPCC, inactivation of a DNA mismatch repair enzyme, would be useful in identifying HNPCC probands. Monoclonal antibodies against hMLH1 and hMSH2, two DNA mismatch repair proteins which account for most HNPCC cancers, are commercially available. This study sought to investigate the potential utility of these antibodies in determining the expression status of these proteins in paraffin-embedded formalin-fixed tissue and to identify key technical protocol components associated with successful staining. A set of 20 colorectal carcinoma cases of known hMLH1 and hMSH2 mutation and expression status underwent immunoperoxidase staining at multiple institutions, each of which used their own technical protocol. Staining for hMSH2 was successful in most laboratories while staining for hMLH1 proved problematic in multiple labs. However, a significant minority of laboratories demonstrated excellent results including high discriminatory power with both monoclonal antibodies. These laboratories appropriately identified hMLH1 or hMSH2 inactivation with high sensitivity and specificity. The key protocol point associated with successful staining was an antigen retrieval step involving heat treatment and either EDTA or citrate buffer. This study demonstrates the potential utility of immunohistochemistry in detecting HNPCC probands and identifies key technical components for successful staining.     

DNA mismatch repair genes hMLH1, hMSH2, and hMSH6 are not inactivated in bronchioloalveolar carcinomas of the lung.

BACKGROUND: Defective DNA mismatch repair (MMR) appears to be rare in nonsmall cell carcinomas of the lung. Defective DNA MMR results from genetic or epigenetic alterations that inactivate the DNA MMR genes hMLH1 or hMSH2, and rarely hMSH6. The loss of normal DNA MMR is thought to promote tumorigenesis by accelerating the accumulation of mutations in oncogenes and tumor suppressor genes. Inactivation of hMLH1, hMSH2, and hMSH6 is observed as a loss of expression of these proteins by immunohistochemistry. Bronchioloalveolar carcinoma is a subtype of adenocarcinoma with distinctive clinical and pathologic features. MATERIALS AND METHODS. An immunohistochemical study was performed on paraffin embedded sections of 33 bronchioloalveolar carcinomas (20 nonmucinous and 13 mucinous) for hmlh1, hmsh2, and hmsh6 proteins. RESULTS All the tumors showed normal expression of hmlh1, hmsh2, and hmsh6. CONCLUSIONS: These findings suggest that defective DNA MMR due to inactivation of hMLH1, hMSH2, or hMSH6 does not play a significant role in the pathogenesis of bronchioloalveolar carcinomas.