Functional genomics of imprinted genes
par- 4 février
We originally got interested in genomic imprinting during the functional characterization of a zinc finger transcription factor that we named Zac1. We showed that this transcription factor is able to induce apoptosis and cell cycle arrest in different cell types (Spengler et al. 1997 ; Varrault et al. 1998 ; Bilanges et al. 2001), suggesting a possible role as a tumor suppressor gene (Bilanges et al. 1999 ; Basyuk et al. 2005).
Figure 1 : Epithelial LLC-PK1 cells were transfected by the Tet-off system that allows regulated Zac1 expression. In the presence of tetracycline (left panel), Zac1 is not expressed whereas in the absence of the antibiotic (right panel) Zac1 is induced (green label). Cortical actin was labeled by phalloidine (red label). Somme of the cells expressing Zac1 display a fragmented nucleus indicative of apoptosis.
More recently, ZAC was shown to be maternally imprinted in humans (Kamiya et al. 2000) and mice (Piras et al. 2000). ZAC is located in the 450 kb minimal interval for Transient Neonatal Diabetes Mellitus (TNDM) (Gardner et al. 2000), a rare genetic disease due to the overexpression of a maternally imprinted gene that affect the maturation and/or differentiation of pancreatic beta-cells. Because of its functional properties, its chromosomal localization, its maternal imprinting, and the regulation of its promoter (Varrault et al. 2001), ZAC is the TNDM candidate gene. We presently look for Zac1 target genes, and want to understand the mechanims that underly Zac1 extinction in some tumors, its mechanisms of action, and its role during embryonic development.
Figure 2 : Zac1 is widely expressed during mouse embryonic development.
We generated a Zac1 nullizygote mouse mutant, which, as is the case for a number of other imprinted genes, displays alterations of embryonic growth (Varrault et al. 2006). Using the results of a meta-analysis of microarray data, we showed that Zac1 belongs to a gene network that comprises other imprinted genes and controls embryonic growth (Varrault et al. 2006).
Figure 3 : The Imprinted Gene Network (IGN). A network of co-regulated genes was discovered using the results of a meta-analysis of 116 microarray data sets. Imprinted genes are in bold. The edge thickness is proportional to the number of experiments in which these 2 genes are co-regulated. Details regarding the construction of this network are found in Varrault et al. (2006) Dev. Cell 11:711-22.
We presently better characterize this network, and seek for the bioological process its controls and the mechanisms that ensure the cooridinated regulation of this set of genes. For that purpose, we use an in vitro model of corticogenesis that allows to generate every cortical neuron subtype from embryonic stem cells (Gaspard et al. 2008, 2009).
Figure 4 : Embryonic stem cells were induced to differentiate in cortical neurons (green) and glial cells (red) for 21 days. Cell nuclei were counterstained with DAPI (blue).