Reconsidering LINE-1's role in cancer: does LINE-1 function as a reporter detecting early cancer-associated epigenetic signatures?

Long interspersed nuclear element-1 (LINE-1 or L1) is the only autonomously active retrotransposon in humans. While L1 has been implicated in several pathologies and the aging process, I present a model which challenges an understanding of L1 as predominantly antagonistic to human health. I hypothesize that L1 serves as a reporter in an early cancer alert system: a tripwire strung throughout the genome poised to trigger p53 and a type I interferon (IFN-1) response when the epigenetic landscape portends cancer. Cell proliferation and a shift to aerobic glycolysis cause dramatic changes in the epigenome which are permissive to L1's escape from suppression. L1 has several properties which make it particularly apt to fulfill this hypothesized sentinel function. Being present in many copies spread throughout the genome allows it to monitor many regions for epigenetic instability and renders it robust to deactivation by mutation. This proposed cancer alert system would alter the cancer cell fitness landscape discouraging the use of growth-favoring aerobic glycolysis by threatening the activation of tumor-suppressive mechanisms. It also imposes costs on a strategy of non-specific global transcriptional derepression aimed at activating oncogenes. Erroneous activations of this system are predicted to increase the rate of aging, suggesting this represents a case of antagonistic pleiotropy trading prolonged youth for cancer prevention. More research is needed to assess this model. Lay summary: During carcinogenesis the epigenome is remodeled by the Warburg effect and cellular proliferation. These processes globally relax chromatin. This epigenetic environment is permissive to the retrotransposon long interspersed nuclear element-1's (LINE-1 or L1) escape from suppression. I hypothesize and present evidence for the notion that L1 has been co-opted to serve as a reporter in an early cancer alert system, poised to trigger tumor suppressive mechanisms when the epigenetic landscape portends cancer. This hypothesis describes a potentially major means by which transformation is thwarted early on.