Small RNAs in differentiating mouse ES cells. Mus musculus

RNA-interference (RNAi) refers to a growing class of gene silencing phenomena defined by a requirement for small RNAs of 20-32 nt and the action of the Argonaute (Ago) family of ribonucleases. We have previously identified developmentally regulated small RNAs, using Northern blot analysis, that are expressed during X-chromosome inactivation in differentiating female mouse ES cels. We sought to identify these small RNAs using deep sequencing. We identified small RNAs that align to retrotransposon sequences and are enriched on the X-chromosome. LINE elements have been proposed to act as way stations during X-inactivation for the spreading of silencing along the entire chromosome, and our findings suggest that LINE elements found on the X-chromosome may be enriched for small RNAs relative to the genome. These results suggest that RNAi pathways are involved in regulating LINE elements during X-inactivation and ES cell differentiation. Overall design: We size-fractionated total RNA from differentiating female mouse ES cells (day 4) into 18-24 nt and 25-45 nt populations and sequenced each fraction separately using the 454 platform. We used a karyotypically stable 40XX female ES cell line (EL16.7) with one X each of 129 and M. castaneus origins: 129 x (M.castaneus x 129).