Similarities between long interspersed element-1 (LINE-1) reverse transcriptase and telomerase

Abstract

Long interspersed element-1 (LINE-1 or L1) retrotransposons encode two proteins (ORF1p and ORF2p) that contain activities required for conventional retrotransposition by a mechanism termed target-site primed reverse transcription. Previous experiments in XRCC4 or DNA protein kinase catalytic subunit-deficient CHO cell lines, which are defective for the nonhomologous endjoining DNA repair pathway, revealed an alternative endonuclease-independent (ENi) pathway for L1 retrotransposition. Interestingly, some ENi retrotransposition events in DNA protein kinase catalytic subunit-deficient cells are targeted to dysfunctional telomeres. Here we used an in vitro assay to detect L1 reverse transcriptase activity to demonstrate that wild-type or endonuclease-defective L1 ribonucleoprotein particles can use oligonucleotide adapters that mimic telomeric ends as primers to initiate the reverse transcription of L1 mRNA. Importantly, these ribonucleoprotein particles also contain a nuclease activity that can process the oligonucleotide adapters before the initiation of reverse transcription. Finally, we demonstrate that ORF1p is not strictly required for ENi retrotransposition at dysfunctional telomeres. Thus, these data further highlight similarities between the mechanism of ENi L1 retrotransposition and telomerase.L ong interspersed element-1 sequences (LINE-1s or L1s) are abundant non-long terminal repeat (non-LTR) retrotransposons that constitute approximately one-sixth (i.e., ≈17%) of human nuclear DNA (1). An estimated 80-100 full-length, retrotransposition competent L1s (RC-L1s) are present in a typical diploid human genome, and a small number, termed "hot L1s" exhibit high retrotransposition efficiencies in cultured human cells (2). Human RC-L1s are ≈6 kb (3, 4) and encode two proteins (ORF1p and ORF2p) required for retrotransposition (5). ORF1p is a 40-kDa protein (6) with RNA binding (7-12) and nucleic acid chaperone activities (13-15). ORF2p is a 150-kDa protein (16, 17) with endonuclease (EN) (18,19) and reverse transcriptase (RT) (20, 21) activities.The mobilization of RC-L1s can be mutagenic (22); germ line and, less often, somatic L1 insertions have led to ≈65 sporadic cases of disease in humans (reviewed in ref. 23). ORF1p and/or ORF2p also can act in trans to mobilize short interspersed elements [e.g., Alu (24) and SINE-VNTR-Alu (25) elements], certain noncoding RNAs [e.g., U6 snRNA and small nucleolar RNAs (24, 26-29)], and some cellular mRNAs, which leads to the formation of processed pseudogenes (30-32). In total, these trans retrotransposition events account for at least an additional 10% of human DNA (1). Moreover, several recent studies have further demonstrated that L1-mediated retrotransposition events are responsible for a significant proportion of interindividual genetic variation in the human population (33-39) and may cause intraindividual variation in the mammalian nervous system (40, 41). L1 retrotransposition likely occurs by a mechanism termed target-site primed reverse transcription (TPRT) (42)...

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“…Moreover, there is a stunning functional resemblance between telomerase and the RT enzyme encoded by retrotransposons (Belfort et al, 2011). John V Moran's laboratory discovered this similarity between L1 ORF2 protein and telomerase function (Kopera et al, 2011). Moreover, both retrotransposon and telomere complexes use a ribonucleoprotein particle system for their amplifications Zhang et al, 2011).…”

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“…Consistent with a common origin, LINE RT can prime reverse transcription from oligonucleotides that mimic telomere ends [34]. Moreover, primitive LINEs without the endonuclease function may have been able Main groups of retrotransposons.…”