High-throughput sequencing: extensive variation in human-specific L1 content in individual genomes

The aim of this study is to discover LINE-1 (L1) insertion sites present in humans that are absent from the reference genome sequence. We use the distinguishing nucleotide characteristics of human-specific L1 elements to resequence the L1 3' flanking regions thus locating the L1 insertion sites whether or not they are present in the reference genome assembly. In doing so, we have uncovered a higher than expected number of non-reference L1 insertions, an average of 152 insertions per individual. In addition, we find that any two individuals differ at an average of 285 sites with respect to presence or absence of L1 insertions in those sites, and use this to estimate the rate of retrotransposition in humans at 1 event per 140 live births (95% c.i. 1/95 to 1/270). We show that individual 'L1 profiles' recapitulate genetic ancestry as expected, and find that dimorphic insertions are more frequent in introns relative to fixed insertions.