Identification of <i>Rtl1</i>, a Retrotransposon-Derived Imprinted Gene, as a Novel Driver of Hepatocarcinogenesis

<div><p>We previously utilized a Sleeping Beauty (SB) transposon mutagenesis screen to discover novel drivers of HCC. This approach identified recurrent mutations within the <i>Dlk1-Dio3</i> imprinted domain, indicating that alteration of one or more elements within the domain provides a selective advantage to cells during the process of hepatocarcinogenesis. For the current study, we performed transcriptome and small RNA sequencing to profile gene expression in SB–induced HCCs in an attempt to clarify the genetic element(s) contributing to tumorigenesis. We identified strong induction of <i>Retrotransposon-like 1</i> (<i>Rtl1</i>) expression as the only consistent alteration detected in all SB–induced tumors with <i>Dlk1-Dio3</i> integrations, suggesting that <i>Rtl1</i> activation serves as a driver of HCC. While previous studies have identified correlations between disrupted expression of multiple <i>Dlk1-Dio3</i> domain members and HCC, we show here that direct modulation of a single domain member, <i>Rtl1</i>, can promote hepatocarcinogenesis <i>in vivo</i>. Overexpression of <i>Rtl1</i> in the livers of adult mice using a hydrodynamic gene delivery technique resulted in highly penetrant (86%) tumor formation. Additionally, we detected overexpression of <i>RTL1</i> in 30% of analyzed human HCC samples, indicating the potential relevance of this locus as a therapeutic target for patients. The <i>Rtl1</i> locus is evolutionarily derived from the domestication of a retrotransposon. In addition to identifying <i>Rtl1</i> as a novel driver of HCC, our study represents one of the first direct <i>in vivo</i> demonstrations of a role for such a co-opted genetic element in promoting carcinogenesis.</p>