RNA editing deficient mutants of APOBEC1: from genome editing to cancer.

APOBEC-1 is a cytosine-to-uracil deaminase that exerts its primary physiological role in the editing of the Apolipoprotein B mRNA at C6666. Recent studies outlined the ability of APOBEC1 to edit also DNA, thus linking its mutational property to the its overexpression in cancer. Several reports suggest that APOBEC1 can alter the cellular state by targeting RNA molecules. Our hypothesis portrays APOBEC1 as the trigger for a dual path towards cancer through its DNA- and RNA- targeting abilities, and its activity as central to the onset of tumorigenic features. Aim of my PhD project is to understand whether APOBEC1 oncogenic potential is mainly related to its ability to edit RNA or DNA. To this aim, I assessed the tumorigenic potential of a set of APOBEC1 mutants, selected for a dissociation in their RNA and DNA editing ability: they are DNA editing proficient but unable to edit RNA. I characterized these mutants and I tested them in mice to assess their ability to trigger liver tumors. Even if unable to edit RNA, the APOBEC1 mutants are still able to induce tumor formation. This means that RNA editing does not play a central role in the oncogenic potential of APOBEC1. Moreover, considering the recent developments in genome editing, I have exploited these mutants as base editors -fusions of the DNA targeting Cas9 with a DNA deaminase- that allow the correction of single bases: the most common mutator moiety in cytosine-targeting base editors is based on APOBEC1, but overexpression of the APOBEC1-Cas9 chimera results in a substantial amount of RNA and DNA off-target alterations. Once fused with Cas9, our RNA-editing deficient mutants were able to mutate the target DNA. On the other hand, I demonstrated that their off-target activity on RNA is orders of magnitude lower than that of wild type APOBEC1, at the same levels of the negative controls. Exploitation of these mutants could thus provide tools for safer base editing both in vitro and in vivo.