An estimated frequency of endogenous insertional mutations in humans (original) (raw)

Nature Genetics volume 22, page 130 (1999)Cite this article

A potential undesirable side effect of somatic gene therapy is that, during gene delivery, foreign DNA reaches the gonadal tissue and inserts into the germ cell genome, with the risk of transmission to subsequent generations. If exogenous insertions indeed occur, what level of insertion is tolerable? This has been the subject of public debate. The Federal Drug Administration in the United States has proposed that gene delivery strategies should have a limit of haploid genome insertion of less than 50 per μg of DNA, or 1 event in roughly 6,000 sperm. In this context, it is important to consider the extent to which endogenous insertions occur in the human genome. Although the frequency of endogenous insertions has not been determined empirically, it can be estimated based on available data.

The major causative agents of endogenous genomic insertions are LINE-1 (L1) retrotransposons. These elements, of which there are more than 100,000 forms, comprise approximately 15% of the human genome1. Most L1s are truncated or rearranged, and only about 3,000 are full-length. Of these full-length L1s, approximately 40–50 are active retrotransposons2. Retrotransposition occurs in a series of steps: transcription, endonucleolytic nicking of genomic DNA, reverse transcription of L1 RNA and integration of L1 DNA at the endonuclease cleavage site3. The L1 endonuclease has limited sequence specificity3, thus retrotransposition events occur at many sites in the genome, including within genes. A HeLa cell culture assay was used to show that human L1 elements autonomously retrotranspose and to estimate the frequency at which L1s insert into genes4. As there is little, if any, bias against genes as sites of L1 retrotransposition in cultured cells5, L1-mediated insertions in the human genome are likely to occur at essentially random sites. Alu elements and processed pseudogenes insert at genomic sequences that closely resemble the sequences at L1 insertion sites6, suggesting that L1 endonuclease is also responsible for the retrotransposition of these elements.

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Acknowledgements

I thank R. Spielman, W. Ewens, E. Luning Prak, J. Moran, P. Green and K. High for helpful comments. This work was supported by a grant from the NIH.

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  1. Department of Genetics, University of Pennsylvania School of Medicine, Philadephia, 19104, Pennsylvania, USA
    Haig H. Kazazian Jr

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Kazazian, H. An estimated frequency of endogenous insertional mutations in humans.Nat Genet 22, 130 (1999). https://doi.org/10.1038/9638

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