Role of the syncytin genes –of retroviral origin- in the formation of the placental syncytiotrophoblast and in materno-fetal tolerance – RETRO-PLACENTA

We have generated genetically modified mice in which “syncytin” genes can be selectively switched off. The impact of these targeted disruptions on the ability of mice to produce offspring and the associated placental defects are investigated at both the cellular and tissular levels. The ubiquity of syncytins is otherwise scrutinized by searching for and studying these genes in the major mammalian lineages, as well as in some other vertebrates that were unexpectedly found to have acquired the capacity to form a placenta (e.g. the Mabuya lizard).

The results obtained up to now have shown that “syncytins” could be identified in all the placental mammalian species that we had investigated. This further supports the data obtained with genetically modified mice in which these genes were disrupted, allowing us to demonstrate the “required” role of syncytins in the mouse. Moreover, mice in which the syncytin genes were not switched off but only deprived by targeted mutagenesis of their immunosuppressive function have now been established, in a genetic background allowing us to investigate their role in maternal-fetal tolerance. The first results should be obtained before the end of the current year.
Finally, a series of experiments are being conducted to assess the possible “collateral” contribution of syncytins in the formation –through cell fusion- of the muscular fiber, using once more the genetically modified mice generated in our laboratory.

The present research program aims at understanding the role of “syncytins”, these “novel” genes which are still ill-characterized, in the physiology and pathophysiology of placentation. These genes are found of course in humans, but also in placental mammals, including some –such as the mouse- which can be considered as model systems for these investigations and for the design of possible therapeutic treatments targeting these genes.

Two publications during the first year (2013) of the project, in high impact journals (PNAS, PLoS Genet), and several manuscripts in preparation. Participation of two members of the research team to 3 international meetings as invited speakers on these subjects in 2013.

Articles
- Cornelis G, Heidmann O, Degrelle SA, Vernochet C, Lavialle C, Letzelter C, Bernard-Stoecklin S, Hassanin H, Mulot B, Guillomot M, Hue I, Heidmann T* and Dupressoir A*. (2013) Captured retroviral envelope syncytin gene associated with the unique placental structure of higher ruminants. Proc. Natl. Acad. Sci. USA (*co- authors) Feb 26;110(9):E828-37

- Esnault C, Cornelis G, Heidmann O and Heidmann T. (2013). Differential Evolutionary Fate of an Ancestral Primate Endogenous Retrovirus Envelope Gene, the EnvV “Syncytin”, Captured for a Function in Placentation. PloS Genetics, 9:e1003400.

Reviews
- Dewannieux M, Heidmann T.(2013) Endogenous retroviruses: acquisition, amplification and taming of genome invaders. Curr Opin Virol. S1879-6257(13)00141-7.

- Lavialle C, Cornelis G, Dupressoir A, Esnault C, Heidmann O, Vernochet C, Heidmann T. (2013). Paleovirology of 'syncytins', retroviral env genes exapted for a role in placentation. Phil.Trans R Soc 368 : 20120507.

The placenta is a structure of embryonic origin that allows metabolic exchanges between mother and fetus and mediates tolerance of the fetus by the maternal immune system. The placenta is an invasive organ that comes into contact and anchors itself in the maternal uterine wall during implantation. In humans and mice, for instance, a particular population of placental cells, the cytotrophoblasts, invades the uterine epithelium. A subset of these cells migrates toward the maternal arteries and disrupts the wall of these vessels, leading to the formation of blood lacunae in direct contact with the placenta. Other cytotrophoblasts differentiate and fuse together to form giant multinucleated cells called “syncytiotrophoblasts”. Syncytiotrophoblasts are in direct contact with maternal blood and constitute the main exchange area between mother and fetus. These cells of embryonic origin express at their surface paternal antigens and are recognized as foreign by the maternal immune system. However, despite this recognition, both the placenta and the fetus are tolerated throughout pregnancy. This mechanism of immune tolerance is still poorly understood.
In humans, syncytin-1 and -2 encode envelope glycoproteins involved in syncytiotrophoblast formation. These genes of retroviral origin have been captured during evolution and are today an integral part of our genome. The envelope proteins of retroviruses exhibit two types of activity. First, a so-called fusogenic activity allows fusion between the viral and cellular membranes and entry of the viral particle inside the infected cells. The second activity is denoted “immunosuppressive” and allows the retrovirus to inhibit the host immune system thus favoring viral propagation. The capture of syncytin-1 and -2 during evolution allowed subversion of the fusogenic property of retroviral envelopes to fuse cytotrophoblast membranes, thus leading to the formation of the syncytiotrophoblast. In our laboratory, we have also characterized 4 additional syncytin genes, syncytin-A and –B in the mouse, syncytin-Ory1 in the rabbit, and syncytin-Car1 in carnivores, providing evidence that the cooptation of retroviral genes for their fusogenic function has been a frequent occurrence in most mammals. Syncytin genes are required for syncytiotrophoblast formation, since mutant mice lacking these genes display a syncytialization defect resulting in growth retardation and embryonic death in utero. One hypothesis is that the immunosuppressive property of syncytins, inherited from the initial retroviral envelopes, also contributes to materno-fetal tolerance.
The objectives of the present program are both to elucidate the properties and mechanisms of syncytins in placentation and to assess the impact of mutations/malfunctions of these genes on the physiopathology of pregnancy (pre-eclampsia, intrauterine growth restriction) and on some gynecological tumors (hydatiform mole, choriocarcinoma). These studies will be carried out both on humans and on animal models, including a large panel of placental mammals, and notably on the mouse with which it is possible to engineer knock-out and knock-in mutants for both syncytin genes. Our working hypothesis is that the transition during evolution from oviparous to viviparous animals is directly linked to the capture of retroviral envelope genes, which have provided their host with both their immunosuppressive and fusogenic property. This hypothesis will be tested by a systematic search for syncytin genes in placental animals (eutherian mammals, marsupials, some lizards), and by analyzing mice genetically engineered as to be specifically deprived of the immunosuppressive function of their syncytins. Analysis of the impact of syncytin mutations will be carried out both on this animal model and in humans by systematic DNA sequencing. Ultimately, novel therapeutic approaches could be developed taking into account the alterations identified in these genes.