NEURONAL TRANSDIFFERENTIATION: UNCOVERING THE ROLE OF MLL1 AND MLL2 DURING LINEAGE CONVERSION

Neuronal transdifferentiation entails the direct conversion between mouse embryonic fibroblasts (MEFs) and induced neuronal cells (iNs), through the expression of the neuronal-specific transcription factors Brn2, Ascl1 and Myt1l (i.e., BAM factors). To date it is still unclear how BAM factors guide such epigenetic remodelling necessary to drive transdifferentiation. Some hints suggest the involvement of MLL1 and MLL2, two H3K4 trimethylases belonging to the Trithorax protein family, in both in vivo and in vitro neuronal differentiation. Therefore, I studied their role during MEF-to-iNs transdifferentiation. I showed that the absence of MLL1 does not affect either transdifferentiation efficiency or iNs neuronal morphology, but only the survival rate. On the contrary, transdifferentiation efficiency and neurite elongation are compromised in Mll2-/- iNs. The co-deletion of Mll1 and Mll2 impinges on cell viability as the knock-out of Mll1 and further exacerbates the Mll2-/- transdifferentiation defect, causing iNs to have very short neurites. These results suggest a role for MLL2-mediated H3K4 methylation in the control of transdifferentiation. Therefore, I defined the direct and indirect MLL2 targets through the integrative analysis of: i) the RNA-seq on iNs, ii) the ChIP-seq for MENIN, the specific common subunit of MLL1 and MLL2, and iii) the H3K4me3 ChIP-seq. I showed that in absence of Mll2 a conspicuous fraction of the transcriptome is down-regulated and/or looses the H3K4me3 mark, therefore highlighting the absence of MLL1-mediated compensation. Moreover, many deregulated genes (either differentially expressed or differentially marked by H3K4me3) are linked to neuronal differentiation and maturation, as expected by the phenotype analysis.