LINE1 EXPRESSION DYNAMICS IN COLORECTAL CANCER SENSITIVITY TO DNA DAMAGE

Transposable Elements (TEs) are repetitive DNA sequences that make up approximately 45% of the human genome. Among these TEs, LINE1 (autonomous interspersed nuclear elements, constituting 18% of the genome) have been reported to exert crucial epigenetic regulatory functions, and their expression is specific to the type of cancer. In this context, it has been found that p53 represses LINE1 transcription. TP53 mutations are associated with a higher mutation burden and LINE1 retrotransposition in various tumors, with the exception of Colorectal cancer (CRC), which displays a low degree of genome instability despite having a microenvironment characterized by high concentrations of free radicals, well-known inducers of DNA oxidation and damage. Thus, we have investigated LINE1 expression and possible function in CRC genome stability. Our initial observations revealed that LINE1 is specifically expressed in p53 mutated CRCs. This was determined by conducting RNA in situ hybridization (ISH) on formalin-fixed paraffin-embedded samples from CRC patients and normal-like colon tissues (n=17 patients). LINE1 is actively transcribed and remains associated with chromatin in p53 mutated CRC cell lines. Notably, LINE1 expression exhibits an inverse correlation with the abundance of CD3+ T Lymphocytes, indicative of tumor immunogenicity. Subsequently, we formulated the hypothesis that LINE1 transcription could contribute to genome stability in CRCs. We have assessed that CRCs with high LINE1 expression are less sensitive to ROS-like DNA-damaging agents compared to CRCs not expressing LINE1, and we addressed that downregulation of LINE1 could reverse this phenotype. We hypothesize that LINE1 RNA could act as scavenger for free radicals to protect CRC genome integrity. Thus, we found that H2O2 treatment induce DNA oxidation only in CRC in which LINE1 expression is repressed. Finally, we performed RNA immunoprecipitation and sequencing analyses detecting oxidation of LINE1 RNA in p53 mutated CRC cell lines exposed to ROS. Currently our current research is focused on determining the mechanism mediated by LINE1 RNAs in safeguarding the genome against DNA damage and we sought to understand if LINE1 expression could be a marker to predict resistance to chemotherapy, which is known to induce oxidative stress.