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MALDI mass spectrometric imaging based identification of clinically relevant signals in prostate cancer using large‐scale tissue microarrays

To identify molecular features associated with clinico‐pathological parameters and TMPRSS2‐ERG fusion status in prostate cancer, we employed MALDI mass spectrometric imaging (MSI) to a prostate cancer tissue microarray (TMA) containing formalin‐fixed, paraffin‐embedded tissues samples from 1,044 patients for which clinical follow‐up data were available. MSI analysis revealed 15 distinct mass per charge (m/z)‐signals associated to epithelial structures. A comparison of these signals with clinico‐pathological features revealed statistical association with favorable tumor phenotype such as low Gleason grade, early pT stage or low Ki67 labeling Index (LI) for four signals (m/z 700, m/z 1,502, m/z 1,199 and m/z 3,577), a link between high Ki67LI for one signal (m/z 1,013) and a relationship with prolonged time to PSA recurrence for one signal (m/z 1,502; p = 0.0145). Multiple signals were associated with the ERG‐fusion status of our cancers. Two of 15 epithelium‐associated signals including m/z 1,013 and m/z 1,502 were associated with detectable ERG expression and five signals (m/z 644, 678, 1,044, 3,086 and 3,577) were associated with ERG negativity. These observations are in line with substantial molecular differences between fusion‐type and non‐fusion type prostate cancer. The signals observed in this study may characterize molecules that play a role in the development of TMPRSS2‐ERG fusions, or alternatively reflect pathways that are activated as a consequence of ERG‐activation. The combination of MSI and large‐scale TMAs reflects a powerful approach enabling immediate prioritization of MSI signals based on associations with clinico‐pathological and molecular data.