Retrotransposons hijack alt-EJ DNA repair process for mobilization and eccDNA biogenesis

Parasitic retrotransposons populate in the genome to rewrite the hosts’ genetic code. How retrotransposons use the host factors to achieve their propagation remains largely unclear. Here we report retrotransposons hijack the alternative end-joining (alt-EJ) DNA repair process to accomplish mobilization cycle. We applied Nanopore sequencing to examine the fates of replicated retrotransposon DNA, and found that only 10% of them achieve integration, while 90% exist as extrachromosomal circular DNA (eccDNA). Using eccDNA production as a readout, further genetic screens identified factors from alt-EJ as essential for retrotransposon replication. alt-EJ drives the 2 nd -strand synthesis of the retrotransposon DNA via a circularization process, thus is necessitated for eccDNA production and integrations. Our study reveals a conserved function of alt-EJ in promoting selfish element propagation, which potentially causes disease or drives evolution.