Long interspersed element-1 ribonucleoprotein particles protect telomeric ends in alternative lengthening of telomeres dependent cells

Malignant cells ensure telomere maintenance by the alternative lengthening of telomeres (ALT) in the absence of telomerase activity (TA). The retrotransposons "long interspersed nuclear element-1'' (LINE-1, L1) are expressed in malignant cells and are primarily known to contribute to complex karyotypes. Here we demonstrate that LINE-1 ribonucleoprotein particles (L1-RNPs) expression is significantly higher in ALT(+)- versus in TA(+)-human glioma. Analyzing a role of L1-RNP in ALT, we show that L1-RNPs bind to telomeric repeat containing RNA (TERRA), which is critical for telomere stabilization and which is overexpressed in ALT(+) cells. In turn, L1-RNP knockdown (KD) abrogated the nuclear retention of TERRA, resulted in increased telomeric DNA damage, decreased cell growth and reduced expression of ALT characteristics such as c-circles and PML-bodies. L1-RNP KD also decreased the expression of Shelterin- and the ALT-regulating protein Topoisomerase III alpha (TopoIII alpha) indicating a more general role of L1-RNPs in supporting telomeric integrity in ALT. Our findings suggest an impact of L1-RNP on telomere stability in ALT(+) dependent tumor cells. As L1-RNPs are rarely expressed in normal adult human tissue those elements might serve as a novel target for tumor ablative therapy.