Loss of PIWI-interacting RNAs regulation drives the expression of LINE1-derived oncogenic lncRNAs in testicular seminoma

Background The development of targeted therapies is an unmet clinical need for testicular cancer, especially for those patients that do not respond to platinum-based therapy. While testicular germ cell tumors (TGCTs) are generally curable, a significant number of patients develop refractory metastatic disease, underscoring an urgent need for the discovery of novel therapeutic targets. Epigenetic dysregulation and activation of LINE1 retrotransposons are frequent in seminoma due to genome hypomethylation. Beyond retrotransposition, LINE1 elements can regulate gene expression through bidirectional promoter activity and generate antisense transcripts. As LINE1 activity is tightly controlled by PIWI-interacting RNAs (piRNAs), we investigated whether disruption of this pathway may contribute to TGCTs tumorigenesis by impacting on the expression of LINE1-derived lncRNAs with oncogenic activity. Results Through integrated analyses, we identified a novel oncogenic axis involving transposable element (TE)-derived lncRNAs associated with dysregulated piRNA expression. A TGCT-specific piRNA signature was identified and mapping of piRNAs to LINE1 sequences identified piR-hsa-7221 as a transcriptional regulator of the lncRNA CASC9 in seminoma. The effect of piR-hsa-7221 was mediated by binding to a complementary LIPA5 sequence and regulating the expression of CASC9 driven by the LINE1 antisense promoter. Loss of piR-hsa-7221 resulted in CASC9 upregulation and oncogenic activity, which was reduced by CASC9 silencing, leading to decreased cancer cell proliferation and invasion, as well as increased sensitivity to cisplatin treatment. Transcriptomic analysis linked these effects to the regulation of the cell cycle, developmental pathways, extracellular matrix, hormone metabolism and immune responses, with WNT signalling emerging as a key downstream target. Conclusions This study describes a novel epigenetic mechanism in TGCTs involving piRNA-mediated regulation of LINE1-derived oncogenic lncRNAs. This mechanism provides new insights into seminoma pathogenesis and identifies the piR-hsa-7221 and CASC9 axis, together with associated gene networks as potential biomarkers and therapeutic targets.