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Background Analysis of high microsatellite instability (MSI-H) phenotype in colorectal carcinoma (CRC) is important for evaluating prognosis and choosing a proper adjuvant therapy. Although the conventional MSI analysis methods such as polymerase chain reaction (PCR) fragment analysis and immunohistochemistry (IHC) show high specificity and sensitivity, there are substantial barriers to their use. Methods In this study, we analyzed the MSI detection performance of three molecular tests and IHC. For the molecular tests, we included a recently developed peptide nucleic acid probe (PNA)-mediated real-time PCR-based method using five quasi-monomorphic mononucleotide repeat markers (PNA method) and two conventional PCR fragment analysis methods using NCI markers (NCI method) or five quasi-monomorphic mononucleotide repeat markers (MNR method). IHC analysis was performed with four mismatch repair proteins. The performance of each method was validated in 166 CRC patient samples, which consisted of 76 MSI-H and 90 microsatellite stable (MSS) CRCs previously diagnosed by NCI method. Results Of the 166 CRCs, 76 MSI-H and 90 MSS CRCs were determined by PNA method. On the other hand, 75 MSI-H and 91 MSS CRCs were commonly determined by IHC and MNR methods. Based on the originally diagnosed MSI status, PNA showed 100% sensitivity and 100% specificity while IHC and MNR showed 98.68% sensitivity and 100% specificity. When we analyzed the maximum sensitivity of MNR and PNA method, which used the same five markers, PNA method could detect alterations in all five mononucleotide repeat markers in samples containing down to 5% MSI-H DNAs, whereas MNR required at least 20% MSI-H DNAs to achieve the same performance. Conclusions Based on these findings, we suggest that PNA method can be used as a practical laboratory test for the diagnosis of MSI. Electronic supplementary material The online version of this article (10.1186/s12885-018-5127-6) contains supplementary material, which is available to authorized users.

Background SnoN is an important regulator of the transforming growth factor beta (TGFβ) signalling pathway and has been shown to exhibit both tumour promotion and suppression activity. Methods To further explore the role of this complex molecule in colorectal tumorigenesis, we examined 52 paired normal and tumour colorectal specimens stratified by level of microsatellite instability; 18 with high-level microsatellite instability (MSI-H) and 34 microsatellite stable (MSS). SnoN transcript expression was quantitated by real-time PCR and analysed with respect to clinical indicators of prognosis. Results Within the MSI-H subgroup, SnoN was commonly either up-regulated (6/18, 33%) or down-regulated (7/18, 39%). A significantly different distribution of SnoN expression was observed in MSS cancers compared with MSI-H (P ≤ 0.001). Whilst 17/34 (50%) of MSS tumours demonstrated up-regulation, none showed down-regulated expression. Within the MSI-H subgroup, up-regulation was significantly correlated with lack of repeat tract mutation in the TGFβRII gene (P ≤ 0.025), suggesting that SnoN is more frequently up-regulated in the presence of functional TGFβ signalling. Conclusion Together these data support the notion that SnoN has both oncogenic and tumour suppressive properties depending on other genetic changes within the tumour, and that the MSI-H pathway of colorectal tumorigenesis presents an excellent model for the study of these opposing functions.