Methylation and expression profiles of MGMT gene in thymic epithelial tumors
Introduction
Thymic epithelial tumors (TET) represent a wide range of clinical and histological entities. This heterogeneity has long hindered the development of standardized protocol for diagnostic and prognostic evaluation of TET [1], [2]. In recent years, attempts have been made to verify the molecular characteristics of these tumors in hope that molecular profiling can be used to predict the prognosis or lead to the development of new treatment strategies [2], [3], [4], [5].
Aberrant DNA methylation is the most widespread epigenetic lesion in tumourigenesis. Neither gene mutations (nucleotide changes, recombination, and deletions) nor cytogenetic disorders are as common in human tumors as DNA methylation alterations [6]. Hypermethylation of promoter CpG islands results in silencing of tumor suppressor genes (TSG) in neoplasm [7], [8]. The reversibility of epigenetic changes, unlike genetic modifications, makes them an attractive therapeutic target since demethylating drugs can re-express and reverse the silencing of genes resulting from methylation [9].
MGMT gene is among a growing list of cancer related genes that develop significant DNA methylation at the promoter CpG islands [10]. MGMT hypermethylation hinders its function as a DNA repair gene that removes the mutagenic alkyl adducts from the O6 position of guanine produced by alkylating agents [11], [12]. Notably worth that aberrant methylation of MGMT was reported in TET [13]. However, the relation between MGMT methylation and its expression in TET and the effect of these epigenetic changes in aggressiveness of TET has not yet been fully explored.
Therefore, in this study we aimed at profiling the frequency of MGMT methylation and its expression in TET using MSP and immunohistochemistry to clarify the association between these epigenetic changes and clinicopathological features, response to chemotherapy and overall survival.
Section snippets
Patients
Tumor tissue samples were obtained from 66 patients with TET who underwent surgery (n = 57) or biopsy (n = 9) between 1985 and 2006 at Tokushima University Hospital and the related hospitals. Patients’ characteristics are shown in Table 1. All TET were classified according to the World Health Organization (WHO) histological classification system [14]. The clinical stage of each TET was determined according to the criteria of Masaoka's staging [15]. This study protocol as well as ethical issues were
Nested MSP of MGMT in TET
The results of nested-MSP for the MGMT in the representative cases of the TET are shown in Fig. 1. The primer sets used for the second PCR are designated as unmethylated (U) or methylated (M). The second PCR products were run on 3% agarose gel, stained with ethidium bromide, and visualized under UV illumination. Samples 53 and 54 which only had an unmethylated band, was classified as “unmethylated cases”. Samples 55, 57, and 58 showed both bands. These were classified as “methylated cases”. We
Discussion
Although thymoma and thymic carcinoma originate from the same origin, they are two distinct tumors regarding not only clinicopathological and morphological features but also molecular entities [3]. Thymomas show benign or low grade malignant behavior, while thymic carcinoma has more aggressive behavior [21], [22]. Some studies have addressed the genetic alterations involved in the tumourigenesis of TET [23], [24], [25], [26]. Unfortunately, there are relatively few reports about epigenetic
Conclusion
Our study uncovered that aberrant MGMT methylation is frequent in TET especially thymic carcinoma. Furthermore, there is a significant correlation between MGMT methylation and aggressiveness of TET. Therefore, MGMT methylation could be used as a marker for thymic carcinoma. Moreover, translation of our results from basic molecular research to clinical practice may have important implication for considering MGMT methylation as a target for future therapies in TET.
Conflict of interest
The authors declare no conflict of interest.
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2020, GeneCitation Excerpt :A lot of studies have been performed over the last two decades searching for methylation biomarkers of the commonest human cancers (Costa-Pinheiro et al., 2015), leading to very encouraging results that include the development of commercially non-invasive diagnostic assays (Molnar et al., 2015), the validation of methylation biomarkers of response to treatment (Zhao et al., 2018), and the FDA approval of epigenetic drugs targeting enzymes involved in DNA methylation for the treatment of myelodysplastic syndromes (Li et al., 2017). However, only a few studies have investigated DNA methylation in thymic epithelial tumors (TETs), and very few data are so far available in thymoma-associated MG (TAMG) samples (Hirabayashi et al., 1997; Suzuki et al., 2005; Chen et al., 2009; Hirose et al., 2009; Mokhtar et al., 2014; Lopomo et al., 2016; Kajiura et al., 2017). A recent epigenome-wide study in thymomas, that included only a few TAMG samples, revealed that TAMG are epigenetically distinct from thymomas that develops in people without MG.
A computational model to predict the structural and functional consequences of missense mutations in O <sup>6</sup> -methylguanine DNA methyltransferase
2019, Advances in Protein Chemistry and Structural BiologyCitation Excerpt :During DNA replication, the DNA polymerases stall at this position and insert incorrect bases, leading to transversion mutations (Gerson, 2004). This may lead to cancer such as esophageal squamous cell carcinoma (Su et al., 2014), colorectal (Shen et al., 2005), thyroid (Mokhtar et al., 2014), larynx (Paluszczak, Misiak, Wierzbicka, Woźniak, & Baer-Dubowska, 2011) and stomach cancer (Jin, Xie, Xie, & Zhou, 2014; Zou et al., 2009). The O6-methylguanine DNA methyltransferase (MGMT) gene located on the human chromosome 10 at the 10q26 position is referred to as a conserved sequence throughout evolution (Gerson, 2004), and comprises of five exons and four introns and encodes a protein of 207 amino acids in length.
Imbalance in DNA repair machinery is associated with BRAF <sup>V600E</sup> mutation and tumor aggressiveness in papillary thyroid carcinoma
2018, Molecular and Cellular EndocrinologyCitation Excerpt :It is possible due to a non-conservative substitution or deletion of amino acid 188, which can affect cellular sensitivity to DNA damage (Liu et al., 1998; Rafii et al., 2002). Under expression and hypermethylation of MGMT has been associated with tumorigenesis in several human cancer tissues, such as gastric carcinoma, thymic epithelial tumor, ductal breast carcinoma and colorectal cancer, leading to cell malignancy, modulation of chemotherapy responses and numerous clinical outcomes (Bardhan and Liu, 2013; Mokhtar et al., 2014; Yousuf et al., 2014; Asiaf et al., 2015; Leguisamo et al., 2017). Previous unpaired immunohistochemical studies have demonstrated that negative/weak MGMT immunoreactivity was significantly more frequently observed in malignant compared to benign thyroid lesions.
The thymidylate synthase enhancer region (TSER) polymorphism increases the risk of thymic lymphoid hyperplasia in patients with Myasthenia Gravis
2018, GeneCitation Excerpt :Similarly, MTRR is involved in tumourigenesis by regulating DNA methylation through activation of MTR (Han et al., 2012). The lack of association that we observed between MTHFR, MTR or MTRR polymorphisms and thymoma risk might be due to the fact that aberrant DNA methylation has been more frequently reported in other cancers than in thymoma, such as for example in thymic carcinoma, suggesting that increased DNA methylation in thymic epithelial tumours might be a late event linked to their malignant behaviour, rather than an early triggering event (Hirose et al., 2009; Mokhtar et al., 2014). Moreover, little is still known about impaired DNA methylation in MG, but a recent methylome and transcriptome profiling of monozygotic twins discordant for the disease revealed that only a relatively few genes differed between the discordant healthy and MG co-twins, and both their expression and methylation profiles demonstrated very high similarity (Mamrut et al., 2017).
State of the art of genetic alterations in thymic epithelial tumors
2014, Journal of Thoracic OncologyCitation Excerpt :Other genes affected by aberrant methylation include DAP-K, p16, MGMT, and HPP1.17 MGMT methylation and loss of its protein expression has been reported to be significantly more frequent in thymic carcinoma than in thymoma (74% versus 29%) and in advanced TETs (stages III/IV) compared with early stage tumors (stages I/II).18 Methylation of the promoter region of CDKN2 has been observed in up to 14% of thymomas and 25% of thymic carcinomas.19