Male dogs with prostate carcinoma are often diagnosed at a late stage and therapeutic approaches are limited. Therefore, new diagnostic strategies for early detection are needed. Whereas prostate-specific antigen (PSA) as a biomarker is controversially discussed for early detection in human medicine, potential biomarkers have not yet been established for dogs. Gene expression data sets, generated by next-generation sequencing, offer new possibilities in cancer research for biomarker discovery, understanding of carcinogenesis and development of therapeutic strategies. Since ultrasound-guided fine-needle aspiration biopsies (US-FNA) of the prostate are routinely used for cytology in dogs, aspirated cells can represent a source for gene expression studies. The aim of the present study was to evaluate US-FNA material for routine cytologic diagnosis and leftover cells for molecular biological analysis. US-FNAs were taken prospectively from 16 male dogs after clinical examination. Collected cells were used for cytological examination as well as molecular biological analysis. Prostate tissue samples were taken from 18 euthanized dogs. All samples were immediately frozen in liquid nitrogen. RNA was extracted from US-FNA samples using the miRNeasy® Micro Kit and RNA concentration was measured with Qubit. Prostate tissue samples were classified histopathologically. RNA isolation from prostate tissue was performed with AllPrep® DNA/RNA/miRNA Universal Kit, and quantity was measured with the Synergy system. A bioanalyzer was used to determine RNA integrity number (RIN). Whole transcriptome next-generation sequencing (NGS) was performed on an Illumina NextSeq500 system. Tissue samples were examined histologically: nine were diagnosed as malignant and nine were classified as non-neoplastic. Cytological examination of US-FNA was possible in 14 cases, 11 being diagnosed as normal to hyperplastic and three specimens being classified as prostate carcinoma. RNA concentration was detectable in all samples ranging from 9 ng/µl to 99 ng/µl. RNA quantity was sufficient as starting material for NGS. Transcriptome analyses from samples with RIN value ≥5.5 were successful. Data sets of differentially expressed genes (DEGs) were summarized in principal component analysis and showed major variances in DEGs between non-neoplastic and malignant samples and minor differences between US-FNA and tissue with comparable diagnosis. Globin genes were identified and significantly upregulated in US-FNA samples. Based on isolated total RNA concentration and integrity, residual cells from diagnostic US-FNA of the canine prostate can be considered as an adequate sample source for gene expression studies, biomarker research and a potential tool for advanced diagnostics of canine prostatic diseases.

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