Pericentromeric satellite repeat expansions through RNA-derived DNA intermediates in cancer

Aberrant transcription of the HSATII pericentromeric satellite repeat is present in a wide variety of epithelial cancers. We observed that HSATII expression is induced in colon cancer cells cultured as xenografts or under non-adherent conditions in vitro, but it is rapidly lost in standard 2D cultures. Unexpectedly, physiological induction of endogenous HSATII RNA, as well as introduction of synthetic HSATII transcripts, generate complementary DNA intermediates in the form of DNA:RNA hybrids. Single molecule sequencing of tumor xenografts shows that HSATII RNA-derived DNA (rdDNA) molecules are stably reincorporated within pericentromeric loci, leading to progressive expansion of these regions We analyzed the dynamics of HSATII RNA and DNA level changes using single molecule sequencing (Helicos/SeqLL) in SW620 colon cancer cells transitioned from 2D in vitro culture conditions to growth as mouse xenografts and vice versa.