Page 19 - Demo

Introduction and outline of this thesis171Figure 2. The regulation of tumor suppressor genes and oncogenes by miRNAs. A Regulation of tumor suppressor genes by miRNAs. miRNA levels can be increased due to a mutation (in red). When these overexpressed miRNAs are processed into a miRNA/RISC complex as described in Fig. 1, and target a specific tumor suppressor gene, these cells can become cancer cells. B Regulation of oncogenes by miRNAs. miRNA levels can be decreased due to for example a deletion in a miRNA gene. This can lead to overexpression of oncogenes, normally inhibited by miRNAs, resulting in increased proliferation and tumor formation (image with permission of dr D. Poel).miRNAs remain stable under different conditions, such as changes in temperature and pH. This stability allows miRNAs to remain detectable and quantifiable even in tissue that has been stored over extended periods (52). Innovative technologies now allow for the quantification of expression levels of numerous miRNAs from small tissue samples, which has stimulated further miRNA research (53, 54). Additionally, in 2008, circulating miRNAs (ci-miRNAs) released from tumor tissue were identified in blood using real-time polymerase chain reaction (RT-PCR). These ci-miRNAs were surprisingly stable in the blood circulation and therefore proposed as a new possible liquid biomarker for cancer (55, 56). In 2012, droplet digital PCR (ddPCR) revolutionized miRNA measurement. By partitioning samples into thousands of droplets for PCR reactions, ddPCR offers absolute miRNA copy counts, ensuring greater sensitivity compared to RT-qPCR (57). The definite role of miRNAs in EC and GC is unclear due to conflicting study outcomes (58-60) and limited data are available about the function of miRNAs in plasma of patients treated with palliative chemotherapy for EC and GC. We describe in chapter 5a study that focusses on the prognostic and predictive value of ci-miRNAs using droplet digital PCR (ddPCR) in plasma samples of patients with advanced EC and GC that were treated in the trial described in chapter 4.