Research Article

Extensive transduction of nonrepetitive DNA mediated by L1 retrotransposition in cancer genomes

Science  01 Aug 2014:
Vol. 345, Issue 6196,
DOI: 10.1126/science.1251343

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Structured Abstract

Introduction

The human genome is peppered with mobile repetitive elements called long interspersed nuclear element–1 (L1) retrotransposons. Propagating through RNA and cDNA intermediates, these molecular parasites copy and insert themselves throughout the genome, with potentially disruptive effects on neighboring genes or regulatory sequences. In the germ line, unique sequence downstream of L1 elements can also be retrotransposed if transcription continues beyond the repeat, a process known as 3′ transduction. There has been growing interest in retrotransposition and 3′ transduction as a possible source of somatic mutations during tumorigenesis.

Embedded Image

The activity of individual L1 elements fluctuates during tumor evolution. In a lung tumor, hundreds of 3′ transductions arose from a small number of active L1 source elements (colored circles on outer rim of circle). As the tumor evolved from the preinvasive common ancestor to invasive cancer, individual elements exhibited variable activity over time.

Rationale

To explore whether 3′ transductions are frequent in cancer, we developed a bioinformatic algorithm for identifying somatically acquired retrotranspositions in cancer genomes. We applied our algorithm to 290 cancer samples from 244 patients across 12 tumor types. The unique downstream sequence mobilized with 3′ transductions effectively fingerprints the L1 source element, providing insights into the activity of individual L1 loci across the genome.

Results

Across the 290 samples, we identified 2756 somatic L1 retrotranspositions. Tumors from 53% of patients had at least one such event, with colorectal and lung cancers being most frequently affected (93% and 75% of patients, respectively). Somatic 3′ transductions comprised 24% of events, half of which represented mobilizations of unique sequence alone, without any accompanying L1 sequence. Overall, 95% of 3′ transductions identified derived from only 72 germline L1 source elements, with as few as four loci accounting for 50% of events. In a given sample, the same source element could generate 50 or more somatic transductions, scattered extensively across the genome. About 5% of somatic transductions arose from L1 source elements that were themselves somatic retrotranspositions. In three of the cases in which we sequenced more than one sample from a patient’s tumor, we were able to place 3′ transductions on the phylogenetic tree. We found that the activity of individual source elements fluctuated during tumor evolution, with different subclones exhibiting much variability in which elements were “on” and which were “off.” The ability to identify the individual L1 source elements active in a given tumor enabled us to study the promoter methylation of those elements specifically. We found that 3′ transduction activity in a patient’s tumor was always associated with hypomethylation of that element. Overall, 2.3% of transductions distributed exons or entire genes to other sites in the genome, and many more mobilized deoxyribonuclease I (DNAse-I) hypersensitive sites or transcription factor binding sites identified by the ENCODE project. Occasionally, somatic L1 insertions inserted near coding sequence and redistributed these exons elsewhere in the genome. However, we found no general effects of retrotranspositions on transcription levels of genes at the insertion points and no evidence for aberrant RNA species resulting from somatically acquired transposable elements. Indeed, as with germline retrotranspositions, somatic insertions exhibited a strong enrichment in heterochromatic, gene-poor regions of the genome.

Conclusion

Somatic 3′ transduction occurs frequently in human tumors, and in some cases transduction events can scatter exons, genes, and regulatory elements widely across the genome. Dissemination of these sequences appears to be due to a small number of highly active L1 elements, whose activity can wax and wane during tumor evolution. The majority of the retrotransposition events are likely to be harmless “passenger” mutations.

Hitchhiking through the tumor genome

Retrotransposons are DNA repeat sequences that are constantly on the move. By poaching certain cellular enzymes, they copy and insert themselves at new sites in the genome. Sometimes they carry along adjacent DNA sequences, a process called 3′ transduction. Tubio et al. found that 3′ transduction is a common event in human tumors. Because this process can scatter genes and regulatory sequences across the genome, it may represent yet another mechanism by which tumor cells acquire new mutations that help them survive and grow.

Science, this issue p. 10.1126/science.1251343

Abstract

Long interspersed nuclear element–1 (L1) retrotransposons are mobile repetitive elements that are abundant in the human genome. L1 elements propagate through RNA intermediates. In the germ line, neighboring, nonrepetitive sequences are occasionally mobilized by the L1 machinery, a process called 3′ transduction. Because 3′ transductions are potentially mutagenic, we explored the extent to which they occur somatically during tumorigenesis. Studying cancer genomes from 244 patients, we found that tumors from 53% of the patients had somatic retrotranspositions, of which 24% were 3′ transductions. Fingerprinting of donor L1s revealed that a handful of source L1 elements in a tumor can spawn from tens to hundreds of 3′ transductions, which can themselves seed further retrotranspositions. The activity of individual L1 elements fluctuated during tumor evolution and correlated with L1 promoter hypomethylation. The 3′ transductions disseminated genes, exons, and regulatory elements to new locations, most often to heterochromatic regions of the genome.

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