Final Report Summary - SOMATIC_MOSAICISM (LINE-1 retrotransposition in human somatic cells) In the project SOMATIC MOSAICISM, we are developing a mouse model to get insight into a major recent discovery in human biology: due to the activity of Transposable Elements (TE), the genome of our brain (or: of all somatic cells of our body) is a mosaic. The host lab and others recently demonstrated that some types of TEs are active in the human brain, generating variation in neuronal genomes. LINE elements are expressed in human Neuronal Stem Cells (NSCs) and their activity impact the functioning of NSCs genomes1. However, it is not known if the somatic activity of LINEs is restricted to NSCs or if it also occurs in other somatic cells. Recent results indicate that most LINE activity occurs in pluripotent stem cells during early embryogenesis. Thus, to deeply understand the impact of the somatic activity of LINE elements, we are developing and we will use a model organism to answer the following questions: 1) Are LINEs active in all three germ layers?; 2) Are LINEs differentially regulated, depending on the germ layer?; 3) What’s the impact of LINE activity in the somatic tissues?. In order to answer the first aim, we successfully carried out retrotransposition assays in different (tissue) stem cells, like hESCs, NPCs, MSCs, HSCs, etc. We found high levels of retrotransposition only in NPCs and NPCs in vitro differentiated to mature neurons. This is a remarkable result, as it shows true somatic retrotransposition in non-dividing neurons. In a second approach, we will use a construct allowing inducible LINE-1 retrotransposition. We are currently generating hESC cell lines with one stable insertion of this vector in characterized loci in the genome.Once we have obtained and characterized these cell lines, we will differentiate them in vitro and in vivo (in mice) into the three germ layers. Switching on retrotransposition will allow us to determine its relative rate depending on the germ layer. We have furthermore characterized the endogenous expression of LINEs on the RNA and protein level in the different germ layers, with the result that NPCs moderately express LINE-1 RNA and protein, while the other germ layers show much lower levels. This could explain the potential of NPCs to sustain high levels of somatic retrotransposition.Finally, we have been able to successfully map several new LINE-1 insertions in hESCs with a new mapping protocol. This will allow us to detect hot spots of insertion depending on each germ layer.In parallel, using other funds of the host lab and with the help of a talented PhD student, we have been developing a similar approach using an inducible LINE element in zebrafish (see attached report on the zebrafish project). The comparison of both model organisms will allow us to draw conclusions on whether somatic LINE activity is evolutionary conserved and what is its impact in vertebrates. Related documents final1-annex-i-final-report.pdf
