Transforming Pluripotency: An Exon-Level Study of Malignancy-Specific Transcripts in Human Embryonal Carcinoma and Embryonic Stem Cells

To circumvent difficulties of isolating pure populations of cancer stem cells (CSCs) for the purpose of identifying malignancy-specific gene expression, we have compared exon-resolution transcriptomic profiles of 5 embryonal carcinoma (EC) cell lines, a histological subtype of germ cell tumor (GCT), to their nonmalignant caricature, specifically 6 human embryonic stem (ES) cell lines. Both cell types are readily accessible, and were purified for undifferentiated cells only. We identified a set of 28 differentially expressed genes, many of which had cancer and stemness roles. Overexpression of the recently discovered pluripotency gene NR5A2 in malignant EC cells revealed an intriguing indication of how WNT-mediated dysregulation of pluripotency is involved with malignancy. Expression of these 28 genes was further explored within 2 publically available data sets of primary EC tumors and normal testis. At the exon-level, alternative splicing events were detected in ZNF195, DNMT3B, and PMF1, and alternative promoters were detected for ASH2L and ETV5. These events were validated by reverse transcriptase-polymerase chain reaction-based methods in EC and ES lines, where the alternative splicing event in the de novo DNA methyltransferase DNMT3B may have functional consequences. In conclusion, we have identified malignancy-specific gene expression differences within a rigorous pluripotent stem cell context. These findings are of particular interest for both GCT and ES cell biology, and, in general, to the concept of CSCs.