LINE1

Line1, also referred to as L1, are "non-LTR retrotransposons" that represent the greatest reshaping force of the human genome during evolution. They are transposable class I elements (retrotransposons) in the DNA of organisms. They belong to the group of long interspersed nuclear elements (LINEs).

Of the human genome, about 45% consists of transposable elements (long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and transposons); 5-8% are from viral sequences with similarity to infectious retroviruses. When retrovirus integration occurs in the germline, the integrated viral sequences are heritable (Katsura Y et al. 2019).

L1s make up approximately 17% of the human genome (Cervantes-Ayalc A et al. 2020).

The majority of L1s in the human genome are inactive; however, approximately 80-100 have retained the ability to retrotranspose, with significant variation between individuals. These active L1s can alter the genome through insertions, deletions, rearrangements, and copy number variations (CNVs).

L1 activity has contributed to genome instability and evolution and is tightly regulated in the germline by DNA methylation, histone modifications, and piRNA. L1s, due to their repetitive DNA sequences, can further influence genome variation through mismatches and unequal crossing-over during meiosis. LINE-1 has been associated with tumorigenesis (Rodic N 2018).

1. Cervantes-Ayalc A et al. (2020) Long Interspersed Nuclear Elements 1 (LINE1): The chimeric transcript L1-MET and its involvement in cancer. Cancer Genet 241:1-11.
2. Katsura Y et al (2019) Evolutionary Medicine of Retroviruses in the Human Genome. Am J Med Sci 358:384-388.
3. Rodic N (2018) . LINE-1 activity and regulation in cancer. Front Biosci (Landmark Ed) 23:1680-1686.