Frequency of microsatellite instability (MSI) in upper tract urothelial carcinoma: comparison of the Bethesda panel and the Idylla MSI assay in a consecutively collected, multi-institutional cohort

Abstract

Aims: Upper tract urothelial carcinoma (UTUC) is a rare malignancy with a poor prognosis which occurs sporadically or in few cases results from a genetic disorder called Lynch syndrome. Recently, examination of microsatellite instability (MSI) has gained importance as a biomarker: MSI tumours are associated with a better response to immunomodulative therapies. Limited data are known about the prevalence of MSI in UTUC. New detection methods using the fully automated Idylla MSI Assay facilitate analysis of increased patient numbers.

Methods: We investigated the frequency of MSI in a multi-institutional cohort of 243 consecutively collected UTUC samples using standard methodology (Bethesda panel), along with immunohistochemistry of mismatch repair (MMR) proteins. The same tumour cohort was retested using the Idylla MSI Assay by Biocartis.

Results: Using standard methodology, 230/243 tumours were detected as microsatellite stable (MSS), 4/243 tumours as MSI and 9/243 samples as invalid. In comparison, the Idylla MSI Assay identified four additional tumours as MSS, equalling 234/243 tumours; 4/243 were classified as MSI and only 5/243 cases as invalid. At the immunohistochemical level, MSI results were supported in all available cases with a loss in MMR proteins. The overall concordance between the standard and the Idylla MSI Assay was 98.35%. Time to result differed between 3 hours for Idylla MSI Assay and 2 days with the standard methodology.

Conclusion: Our data indicate a low incidence rate of MSI tumours in patients with UTUC. Furthermore, our findings highlight that Idylla MSI Assay can be applied as an alternative method of MSI analysis for UTUC.

Keywords: methods; molecular; pathology; urological neoplasms.

Figure 1

(A) Representative capillary electrophoresis results of MSI tumour of (a) normal tissue and (b) tumour tissue. X-axis is the number of bases; y-axis is the number of amplicons reflected in fluorescence intensity, scaled to the variable amount of analysed DNA. Blue, green and black peaks are products of the amplifications of the different MSI markers: BAT25, D2S123, BAT26, D17S250 and D5S346. Comparing (a) and (b), we saw a shift and extension of base amplification product in each marker of tumour tissue compared with normal tissue. (B) Representative capillary electrophoresis results of MSS tumour of (a) normal tissue and (b) tumour tissue. X-axis is the number of bases; y-axis is the number of amplicons reflected in fluorescence intensity, scaled to the variable amount of analysed DNA. Blue, green and black peaks are products of the amplifications of the different MSI markers: BAT25, D2S123, BAT26, D17S250 and D5S346. No shift and no expansion of the base amplification product are seen in the markers of tumour tissue (b) compared with normal tissue (a). MSI, microsatellite instability; MSS, microsatellite stable.

Figure 2

Whole slides of immunohistochemical staining of MMR-proteins MLH1, MSH2, MSH6 and PMS2. Note the strong staining of inflammatory and stromal cells and additionally the strong nuclear expression of tumour cells for MLH1 and PMS2. In contrast, immune and stromal cells are strongly stained (internal positive control), but the tumour cells are showing a nuclear loss of expression for MSH2 and MSH6. MLH1, mutL homologue 1; MMR, mismatch repair; MSH2, mutS homologue2; MSH6, mutS homologue 6; PMS2; PMS1 homologue 2.

Figure 3

Comparison of workflow of (A) standard method and (B) Idylla MSI assay. FFPE, formalin-fixed paraffin-embedded; MSI, microsatellite instability.