Abstract
About 10% – 15% of cancer cell types do not express telomerase, but they use the Alternative Lengthening of Telomeres (ALT) pathway, to counteract telomere shortening. The molecular details of the ALT reaction are poorly characterized and the DNA polymerase (pol) involved in the synthesis step is not known. Here, we show that human DNA pol lambda can actively and efficiently promote annealing telomeric repeats to the complementary C‐rich strand and use it to prime DNA synthesis. DNA pol lambda is associated to telomeres of cells in vivo. Replication protein A negatively regulates this activity, but its inhibition can be counteracted by the Rad9/Hus1/Rad1 complex and the Protection of Telomeres protein 1, which physically interact with and stimulate DNA pol lambda. We show that the telomeric non‐coding RNAs, TERRA can compete with telomeric DNA for annealing and elongation by DNA pol lambda and that Pot1 can counteract this effect. We propose that R‐loops formed by TERRA RNAs at telomeres can function as entry point for the strand invasion during ALT, in a recombination‐independent mechanism. In addition, DNA pol lambda may help to bypass TERRA R‐loops during telomere DNA replication. Supported by an Italian Cancer Research Association AIRC grant.
Published Version
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