Differences in microRNA expression during tumor development in the transition and peripheral zones of the prostate

• Background: The prostate is divided into three glandular zones, the peripheral zone (PZ), the transition zone (TZ) and the central zone. Most prostate tumors arise in the peripheral zone (70-75%) and in the transition zone (20-25%) while only 10% arise in the central zone. The aim of this study was to investigate if differences in miRNA expression could be a possible explanation to the difference in propensity of tumors in the zones of the prostate.Methods:The expression of 667 unique miRNAs was investigated in normal and malignant transition zone and peripheral zone prostate tissues using TaqMan low density arrays for miRNAs. Student’s t-test was used in order to identify differentially expressed miRNAs and hierarchical clustering and principal component analyses were used to note the separation of the tissues. A cross-validation test using the ADtree algorithm was used to test the generality of the identified miRNA signatures.Results:The Student’s t-tests revealed that the major differences in miRNA expression are found between normal and malignant tissues. Hierarchical clustering and PCA based on differentiallyexpressed miRNAs between normal and malignant tissues showed a perfect separation between samples while analyses based on differentially expressed miRNAs between the two zones showed several misplaced samples. A classification and cross-validation procedure confirmedt hese results and several potential miRNA markers were identified.Conclusions:The results of this study indicate that the major differences in the transcription programme are those arising during tumor development, rather than during normal tissue development. In conclusion, tumors arising in the TZ have more unique differentially expressed miRNAs compared to the PZ, indicating that the PZ are more prone to tumor development than the TZ. The results also indicate that separate miRNA expression signatures for diagnosis might be needed for tumors arising in the transition- or peripheral zone.

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