L1 recombination-associated deletions generate human genomic variation

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Abstract
Mobile elements have created structural variation in the human genome through their de novo insertions and post-insertional genomic rearrangements. L1 elements are a type of long interspersed element (LINE) that is dispersed at high copy numbers within most mammalian genomes. To determine the magnitude of L1 recombination-associated deletions (L1RADs), we computationally extracted L1RAD candidates by comparing the human and chimpanzee genomes and verified each of the L1RAD events by using wet-bench analyses. Through these analyses, we identified 73 human-specific L1RAD events that occurred subsequent to the divergence of the human and chimpanzee lineages. Despite their low frequency, the L1RAD events deleted approximately 450 kb of the human genome. One L1RAD event generated a large deletion of approximately 64 kb. Multiple alignments of prerecombination and postrecombination L1 elements suggested that two different deletion mechanisms generated the L1RADs: nonallelic homologous recombination (55 events) and nonhomologous end joining between two L1s (18 events). In addition, the position of L1RADs throughout the genome does not correlate with local chromosomal recombination rates. This process may be implicated in the partial regulation of L1 copy numbers by the finding that approximately 60% of the DNA sequences deleted by the L1RADs consist of L1 sequences that were either directly involved in the recombination events or located in the intervening sequence between recombining L1s. Overall, there is increasing evidence that L1RADs have played an important role in creating structural variation.