Project : Understanding transgenerational epigenetic inheritance
Epigenetic inheritance entails transmission of phenotypic traits not encoded in the DNA sequence and, in the most extreme case, transgenerational Epigenetic Inheritance (TEI) involves transmission of memory through multiple generations. Very little is known on the principles and the molecular mechanisms governing TEI. Chromatin has been linked to TEI but little is known on how chromatin modifications might be transmitted across generations. Polycomb components are involved at each level of chromatin folding, from post-translational histone modification all the way up to regulation of global chromosome architecture.
In our previous work, we established that perturbation of 3D chromatin architecture affected Polycomb function in a heritable way. In Drosophila lines carrying a transgene with a PRE from the Fab-7 region of the Hox cluster called bithorax complex (BX-C), we showed that the transgene establishes 3D contacts with the endogenous BX-C locus. The transgene is flanked by a reporter gene called mini-white, which is responsible for eye pigmentation. In the Fab2L line, the PRE in the transgene partially represses the reporter gene. Therefore flies have low levels of eye pigmentation that are variable between individuals, a phenomenon that reflects metastable silencing in somatic cells. These somatic epigenetic differences are not transgenerationally heritable, as self-crossing of flies with the most repressed or the most derepressed eye phenotypes does not cause any phenotypic shift in the progenies. However, we found that a transient perturbation of nuclear organization can induce TEI. We transiently removed one copy of the endogenous Fab-7 element by crossing Fab2L flies with a derivative carrying the transgene in the absence of the endogenous Fab-7 copy. This induces an increase in the contact frequency between the transgenic locus and the single remaining copy of the endogenous Fab-7 in the F1. We then restored the missing endogenous Fab-7 and obtained an F2, which is genotypically and phenotypically the same as the P0 parental generation. F2 flies with the most repressed and the most derepressed eye color were then segregated in two distinct groups, whose progenies were subjected to selection based on the eye color. Strikingly, starting at the F3, we observed the appearance of more pigmented flies in the selection of the active state and of more reduced pigment levels in the selection of the repressed state. This trend further amplified across generations, finally reaching the establishment of two “epilines”, which could stably maintain their different phenotypic trait.
We showed that this form of inheritance applies to multiple transgenic lines, to endogenous genes, that it involves components of the PRC2 complex and that it can be modulated by environmental conditions. Our future research aims at understanding the mechanisms of TEI and to uncover its role in natural processes.
