Perspectives for tailored chemoprevention and treatment of colorectal cancer in Lynch syndrome

In vitro study of radiosensitivity in colorectal cancer cell lines associated with Lynch syndrome

Introduction

Lynch syndrome patients have an inherited predisposition to cancer due to a deficiency in DNA mismatch repair (MMR) genes which could lead to a higher risk of developing cancer if exposed to ionizing radiation. This pilot study aims to reveal the association between MMR deficiency and radiosensitivity at both a CT relevant low dose (20 mGy) and a therapeutic higher dose (2 Gy).

Methods

Human colorectal cancer cell lines with (dMMR) or without MMR deficiency (pMMR) were analyzed before and after exposure to radiation using cellular and cytogenetic analyses i.e., clonogenic assay to determine cell reproductive death; sister chromatid exchange (SCE) assay to detect the exchange of DNA between sister chromatids; γH2AX assay to analyze DNA damage repair; and apoptosis analysis to compare cell death response. The advantages and limitations of these assays were assessed in vitro, and their applicability and feasibility investigated for their potential to be used for further studies using clinical samples.

Results

Results from the clonogenic assay indicated that the pMMR cell line (HT29) was significantly more radio-resistant than the dMMR cell lines (HCT116, SW48, and LoVo) after 2 Gy X-irradiation. Both cell type and radiation dose had a significant effect on the yield of SCEs/chromosome. When the yield of SCEs/chromosome for the irradiated samples (2 Gy) was normalized against the controls, no significant difference was observed between the cell lines. For the γH2AX assay, 0, 20 mGy and 2 Gy were examined at post-exposure time points of 30 min (min), 4 and 24 h (h). Statistical analysis revealed that HT29 was only significantly more radio-resistant than the MLH1-deficient cells lines, but not the MSH2-deficient cell line. Apoptosis analysis (4 Gy) revealed that HT29 was significantly more radio-resistant than HCT116 albeit with very few apoptotic cells observed.

Discussion

Overall, this study showed radio-resistance of the MMR proficient cell line in some assays, but not in the others. All methods used within this study have been validated; however, due to the limitations associated with cancer cell lines, the next step will be to use these assays in clinical samples in an effort to understand the biological and mechanistic effects of radiation in Lynch patients as well as the health implications.

Lynch Syndrome Germline Mutations in Breast Cancer: Next Generation Sequencing Case-Control Study of 1,263 Participants

Genome instability—the increased tendency of acquiring mutations in the genome and ability of a cell to tolerate high mutation burden—is one of the drivers of cancer. Genome instability results from many causes including defects in DNA repair systems. Previously, it has been shown that germline pathogenic mutations in DNA Mismatch Repair (MMR) pathway cause cancer-predisposing Lynch Syndrome. We proposed that Lynch Syndrome-related germline mutations (LS-mutations) are associated with breast cancer (BC). In this study, we performed Targeted Next-Generation Sequencing of MMR pathway genes MLH1, MSH2, MSH6, EPCAM, and PMS2 in a cohort of 711 patients with hereditary BC, 60 patients with sporadic BC, and 492 healthy donors. Sixty-nine patients (9.7%) with hereditary BC harbored at least one germline mutation in the MMR pathway genes, of them 32 patients (4.5%) harbored mutations in MMR pathway genes which we define as pathogenic or likely pathogenic, and of them 26 patients (3.6%) did not have any pathogenic mutations in DDR pathway genes, compared to two mutations in MMR pathway genes (0.4%) detected in a group of 492 healthy donors [p = 0.00013, OR = 8.9 (CI 95% 2.2–78.4)]. Our study demonstrates that LS-mutations are present in patients with hereditary BC more frequently than in healthy donors, and that there is an association of hereditary BC and mutations c.1321G>A in MLH1, c.260C>G and c.2178G>C in MSH2, c.3217C>T in MSH6, c.1268C>G and c.86G>C in PMS2 genes. This finding provides a rationale for including pathogenic LS-mutations into genetic counseling tests for patients with hereditary BC.

The mutational profile and infiltration pattern of murine MLH1-/- tumors: concurrences, disparities and cell line establishment for functional analysis

Mice lines homozygous negative for one of the four DNA mismatch repair (MMR) genes (MLH1, MSH2, PMS2, MSH6) were generated as models for MMR deficient (MMR-D) diseases. Clinically, hereditary forms of MMR-D include Lynch syndrome (characterized by a germline MMR gene defect) and constitutional MMR-D, the biallelic form. MMR-D knockout mice may be representative for both diseases. Here, we aimed at characterizing the MLH1^-/- model focusing on tumor-immune microenvironment and identification of coding microsatellite mutations in lymphomas and gastrointestinal tumors (GIT).

All tumors showed microsatellite instability (MSI) in non-coding mononucleotide markers. Mutational profiling of 26 coding loci in MSI⁺ GIT and lymphomas revealed instability in half of the microsatellites, two of them (Rfc3 and Rasal2) shared between both entities. MLH1^-/- tumors of both entities displayed a similar phenotype (high CD71, FasL, PD-L1 and CTLA-4 expression). Additional immunofluorescence verified the tumors’ natural immunosuppressive character (marked CD11b/CD200R infiltration). Vice versa, CD3⁺ T cells as well as immune checkpoints molecules were detectable, indicative for an active immune microenvironment. For functional analysis, a permanent cell line from an MLH1^-/- GIT was established. The newly developed MLH1^-/- A7450 cells exhibit stable in vitro growth, strong invasive potential and heterogeneous drug response. Moreover, four additional MSI target genes (Nktr1, C8a, Taf1b, and Lig4) not recognized in the primary were identified in this cell line.

Summing up, molecular and immunological mechanisms of MLH1^-/- driven carcinogenesis correlate well with clinical features of MMR-D. MLH1^-/- knockout mice combine characteristics of Lynch syndrome and constitutional MMR-D, making them suitable models for preclinical research aiming at MMR-D related diseases.

Epigenetic mechanisms in the pathogenesis of Lynch syndrome

Inherited defects in the DNA mismatch repair (MMR) system, MLH1, MSH2, MSH6, and PMS2 genes, underlie Lynch syndrome, one of the most prevalent cancer syndromes in man. The syndrome offers a model for cancers arising through MMR defects and microsatellite instability, which applies to ~ 15% of all colorectal, endometrial, and other cancers. Lynch syndrome also illustrates the significance of the epigenetic component in cancer development. Inactivation of tumor suppressor genes by epigenetic mechanisms is an acquired property of many tumors developing in Lynch syndrome. Furthermore, constitutional epimutations of MMR genes may explain a proportion of mutation-negative families lacking MLH1 or MSH2 protein expression in tumor tissue. This review provides an update of the molecular basis of Lynch syndrome by focusing on the role of epigenetic mechanisms in the pathogenesis of the disease.

Endoscopic and surgical management of hereditary nonpolyposis colorectal cancer

Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome is a disease characterized by autosomal dominant clustering of colorectal cancer (CRC) as well as other cancers. It is critical for clinicians and surgeons caring for patients with HNPCC to be familiar with their management related to CRC. Based on retrospective studies, screening colonoscopy is recommended every 1 to 2 years beginning at age 20 to 25, or 10 years younger than the earliest CRC in the family (whichever is earlier). HNPCC patients with colon cancer should be considered for total abdominal colectomy rather than a more limited segmental colon resection due to the increased risk of metachronous neoplasia associated with the condition. Rectal cancer in HNPCC has not been well studied, but discussions with the patient regarding surgical management should weigh the risks of metachronous CRC with the morbidity and quality of life issues associated with proctocolectomy. Regardless of the procedure, a patient with HNPCC requires close postoperative endoscopic surveillance of any remaining at-risk mucosa. In terms of chemoprevention, aspirin has been shown to be effective in preventing colorectal neoplasia in prospective trials and should be considered in patients who do not have a contraindication to the drug. Trials for other chemopreventative agents in HNPCC are ongoing. As more is learned about particular genotype-phenotype correlations with Lynch syndrome, this will likely affect surgical decision making. Despite all of these efforts in the management of patients with HNPCC or Lynch syndrome, incident CRCs still occur, thus reinforcing the need for further studies to better understand the optimal management of these patients.