Published OnlineFirst July 18, 2016; DOI: 10.1158/0008-5472.CAN-15-3421 Cancer Meeting Report Research Meeting Report: The Role of the Mobilome in Cancer Daniel Ardeljan1,2, Martin S. Taylor3, Kathleen H. Burns1,4, Jef D. Boeke5, Michael Graham Espey6, Elisa C. Woodhouse6, and Thomas Kevin Howcroft6 Abstract Approximately half of the human genome consists of repetitive in developmental and pathologic contexts, including many sequence attributed to the activities of mobile DNAs, including types of cancers. However, we have limited knowledge of the DNA transposons, RNA transposons, and endogenous retro- extent and consequences of L1 expression in premalignancies viruses. Of these, only long interspersed elements (LINE-1 or and cancer. Participants in this NIH strategic workshop consid- L1) and sequences copied by LINE-1 remain mobile in our species ered key questions to enhance our understanding of mechan- today. Although cells restrict L1 activity by both transcriptional isms and roles the mobilome may play in cancer biology. and posttranscriptional mechanisms, L1 derepression occurs Cancer Res; 76(15); 1–4. Ó2016 AACR. Introduction demonstration of retrotransposition in cultured HeLa cells (2). A somatically acquired L1 insertion was shown to disrupt the Mobile DNAs, including DNA transposons, RNA transposons, adenomatous polyposis coli (APC) tumor suppressor gene in a and endogenous retroviruses, are highly abundant sequences that case of colorectal cancer (3). Nearly a decade of advances in DNA make up a major portion of eukaryotic genomes. Although sequencing have both underscored the importance of L1 activity human genomes no longer possess active DNA transposons, in causing heritable variation through retrotransposition in the which integrate via a "cut-and-paste" mechanism, they are instead germline and also demonstrated that widespread somatic retro- enriched in active retrotransposons that integrate via RNA inter- transposition occurs in many cancers. On September 25, 2015, the mediates. Human retrotransposons include long and short inter- NCI (Rockville, MD) sponsored a strategic workshop to assess the spersed elements (LINE and SINE), composite SINE-R, VNTR, and potential impact of somatic retrotransposition on cancer initia- long terminal repeat (LTR) elements. LINEs and LTRs are auton- tion and progression. A panel of experts in mammalian mobile omous or protein-coding elements, although only LINE-1 (L1) is DNAs, chaired by K.H. Burns and J.D. Boeke, considered recent considered competent to mobilize in humans. L1 sequences advances and challenges in the field. The goals were to define key represent approximately 17% of the human genome; however, research priorities and discuss ways to accelerate our understand- the vast majority of the estimated 500,000 copies is incapable of ing of mobile DNAs in cancer. The following is a summary of the mediating retrotransposition due to substantial truncations in the key topics discussed. 50 region. In contrast, recently inserted, full-length L1s encode proteins that can "copy-and paste" L1 sequences to new locations Retroelement Biology and Its Regulation in the genome. In addition to mobilizing L1 RNA (in cis), L1 0 proteins can also transpose other repetitive elements and endog- L1 is an approximately 6-kb DNA sequence comprised of a 5 enous RNAs in trans. untranslated region (UTR); two sense stranded open reading Interest in L1 relationships to cancer biology dates back to the frames (ORF), which encode two proteins, ORF1p and ORF2p; 0 0 1980s. Early L1 work included sequence characterization of a3 UTR; and a poly(A) homopolymer. The 5 UTR is rich in CpG expressed full-length L1 in teratocarcinoma cells (1) and the dinucleotides and contains an antisense promoter activity and a recently discovered antisense ORF protein dubbed ORF0 (4, 5). Transcribed L1 mRNA is exported to the cytoplasm as a bicistronic transcript that encodes both ORF1p, an RNA-binding protein, and 1McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins Uni- versity School of Medicine, Baltimore, Maryland. 2Medical Scientist ORF2p, which possesses endonuclease and reverse transcriptase Training Program, Johns Hopkins University School of Medicine, Bal- domains required for L1-mediated retrotransposition. ORF1p timore, Maryland. 3Department of Pathology, Massachusetts General and ORF2p form a ribonucleoprotein (RNP) complex with target 4 Hospital, Boston, Massachusetts. Department of Pathology, Johns RNA (i.e., L1, Alu, SVA) or endogenous mRNAs in the cytoplasm Hopkins University School of Medicine, Baltimore, Maryland. 5Institute for Systems Genetics, New York University Langone Medical Center, and gains access to the nucleus through a poorly understood New York, New York. 6Division of Cancer Biology, NCI, NIH, Rockville, process. Transposition of L1 sequences to new positions in the Maryland. genome takes place through target-primed reverse transcription Corresponding Author: Elisa C. Woodhouse, NCI, 9609 Medical Center Dr., (TPRT). In this reaction, the ORF2p endonuclease first nicks the Tumor Biology and Metastasis Branch, Room 6W416, Bethesda, MD 20892- DNA double helix at a TTTT/A consensus motif to allow for an 9748. Phone: 240-276-6220; Fax: 240-276-7861; E-mail: interaction between the DNA and the poly(A) tail of the mRNA [email protected] and then uses this DNA to prime a reverse transcription reaction. doi: 10.1158/0008-5472.CAN-15-3421 Canonical L1-mediated integrations can be recognized by target Ó2016 American Association for Cancer Research. site duplications flanking the insertion site, the presence of poly-A www.aacrjournals.org OF1 Downloaded from cancerres.aacrjournals.org on September 29, 2021. © 2016 American Association for Cancer Research. Published OnlineFirst July 18, 2016; DOI: 10.1158/0008-5472.CAN-15-3421 Ardeljan et al. tracts at the 30 end of the insertion, and in the case of processed cellular and organismal aging and, by extension, how this relates pseudogenes their lack of intronic sequences. to cancer. It is important to consider that although derepression of J.D. Boeke discussed the identification of host proteins that retrotransposons in advanced age may have negligible conse- interact with L1 ORF1 and ORF2 proteins with the hypothesis that quences for the fecundity of a species, its occurrence may create this will elucidate mechanisms of TPRT and promote a better susceptibility to age-related diseases. Retroelements, including L1, understanding of how cells contend with LINE-1 activity (6). He have been shown to increase expression as a consequence of aging, also described a transgenic mouse model harboring an inducible a change that can be attenuated by caloric restriction in mice (14). L1 gene-trap cassette useful for forward genetic screens in cancer. Vera Gorbunova (University of Rochester, Rochester, NY) dis- These mice surprisingly demonstrated L1 activity–dependent, cussed the role of SIRT6, a chromatin regulator that mono-ADP- nonheritable coat color phenotypes, suggesting cell populations ribosylates KAP1 and PARP1 and deacetylates histones, in regu- highly susceptible to somatic L1 activity (7). Tim Bestor (Colum- lating L1 expression. SIRT6 binds L1 50UTR sequences in the bia University, New York, NY) provided evidence for an L1- genome and directs heterochromatin formation to suppress tran- derived gene involved in control of L1 activity, designated scription. Exposure to ionizing radiation or oxidative stress results ECAT11 or L1TD1. Evidence suggests that ECAT11/L1TD1 is a in a displacement of SIRT6 away from L1 50UTRs to sites of DNA partially rearranged L1 ORF1, under strong selection in species strand breaks, thereby allowing reexpression of L1 genes. These with active L1, and neutral selection in species without active L1. findings suggest a relationship between accumulated DNA dam- John Moran (University of Michigan, Ann Arbor, MI) emphasized age and increased L1 expression with age (15). the need to elucidate L1 genomic insertion patterns, highlighting a L1 activity also seems to be influenced by environmental key technical challenge in the field. Somatically acquired L1 factors, such as inversion of day–night cycles that disrupt circadian insertions, he explained, may be targeted to specific genomic pathways. Victoria Belancio (Tulane University, New Orleans, LA) intervals owing to biases of either the retroelement or the chro- highlighted the role of the circadian-responsive melatonin recep- matin state of a particular cell, that is, certain intervals may be tor 1 (MT1) in regulating L1 activity. MT1 is capable of reducing L1 more accessible than others. Alternatively, de novo insertions may transcript and protein levels and also reduces L1-mediated retro- occur randomly and subsequently be subject to natural selection transposition when overexpressed in cell culture systems. Fur- pressures during the expansion of a cell lineage. A need for more thermore, perfusion of human prostate cancer xenografted in in vivo experimental model systems to query the L1 life cycle, its nude rats with serum collected from human subjects at night, interactions with host factors, and characteristics of insertion site which contains high levels of melatonin, suppressed L1 expres- preferences was stressed. sion in tumor cells (16). In contrast, L1 RNA is expressed when tumors are perfused with serum collected from the same subjects Roles of Retroelements in Neurobiology during the daytime or at night after bright light exposure. Over the past several years,