“The International Mesothelioma Interest Group (IMIG) recommends that a diagnostic marker of MPM have sensitivity/specificity of >0.80, and these criteria are fulfilled by our miRNA classifier,” comments Dr. Santoni-Rugiu. The authors suggest that diagnostic accuracy can be further improved by adding immunohistochemical testing of miRNA targets in biopsy tissue to their miRNA assay. This combined assay could enable analysis of samples with low tumor cell count.
MPM, which is linked to long-term asbestos exposure, is an aggressive cancer originating from the mesothelial cells that line the membrane surrounding each lung, known as the pleura. Distinguishing MPM from noncancerous abnormalities, such as reactive mesothelial hyperplasia or fibrous pleurisy (organizing pleuritis), can be challenging as there are no generally accepted diagnostic biomarkers for differentiating these two conditions. As a result, patients often present with the disease when they are already at an advanced stage, and less than 20% of patients can be successfully treated surgically.
“Our goal was to identify microRNAs (miRNAs) that can aid in the differential diagnosis of MPM from RMPs,” says lead investigator Eric Santoni-Rugiu, MD, PhD, of the Laboratory of Molecular Pathology at the Department of Pathology of Rigshospitalet, Copenhagen University Hospital, Copenhagen, Denmark. miRNAs, which are small, non-coding RNA strands composed of approximately 22 nucleotides, have been shown to be potential diagnostic, prognostic, and predictive markers in other cancers.
The current study, however, suggests that miRNAs may provide new opportunities for improving the accuracy of the differential diagnosis between MPM and noncancerous pleural conditions. If further validated, the combination of ISH for miRNAs with immunohistocemical testing of miRNA targets may therefore have the potential to aid in the diagnosis, and thus outcome, of MPM.
Validation of the obtained miRNA-expression profile was performed on surgically removed tissue samples from 40 MPM patients and 14 patient-matched NNP samples as well as 12 preoperative diagnostic biopsies and five non-neoplastic reactive-mesothelial proliferation due to pneumothorax. By performing binary logistic regression on the RT-qPCR data for the four miRNAs, the classifier differentiated MPM from NNP with high sensitivity and specificity. The classifier’s optimal logit(P) value of 0.62 separated NNP and MPM samples with high sensitivity, specificity, and accuracy (all ≥0.93).
For immunohistochemistry, FFPE tissue sections underwent staining using antibodies to the known miR-126 targets LAT1 and Crk-II, were evaluated by light microscopy, and scored by a semiquantitative H score. Although no significant differences were found between MPM and NNP samples for Crk-II, the MPM samples had a median H score of 2 for LAT1 immunostaining, which was significantly higher than the 0.5 median score for the NNP samples (P < 0.01). Furthermore, the level of LAT1 in MPM inversely correlated with that of miR-126.