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Low birth rate after cloning: the consequences of highly disruptive interactions between uterus and embryo
When comparing cloned animal embryos and embryos that come about as a result of artificial insemination, there is a difference in gene expression at the moment implantation occurs in the uterus for more than 5 000 genes. Researchers at INRA and the University of California are shining the spotlight on this critical step for the survival of cloned embryos in cattle. Published in the 8 December 2016 issue of the journal Proceedings of the National Academy of Sciences, these findings are furthering knowledge of hitherto poorly-understood mechanisms that govern interactions between cloned embryos and the uterus. They may hold the key to improving cloning in mammals.
In cattle, cloning by transferring the nucleus of an adult cell into an unfertilised egg (or somatic cell nuclear transfer) leads to the birth of a healthy calf in only 5-15% of cases (compared with 30-60% for in vitro fertilisation). How can such a low rate be explained? What mechanisms come into play? What is the role of the interactions between the embryo and the uterus in the early stages of gestation? In 2009, researchers from INRA (Joint Research Unit for Developmental Biology and Reproduction in Jouy en Josas) and the University of California unveiled the “biosensory” role of the uterus when it comes to the quality of embryos. They demonstrated how the uterus is made up of dynamic and active tissue, capable of recognising, by way of a very fine-tuned regulation mechanism, the type of embryo it is coming into contact with at the moment of implantation. This varies depending on whether the embryo is a result of cloning, in vitro fertilisation, or artificial insemination.
More recently, the same researchers set out to compare the level of gene expression both in the uterus and extra-embryonic tissue (whether from cloning or artificial insemination) during implantation (18th and 34th day of gestation). They carried out high-throughput RNA sequencing in extra-embryonic tissue (future placenta) and the endometrium (uterine tissue where an embryo implants itself). The results showed major effects associated with cloned embryos from the 18th day of gestation: differential expression for more than 5 000 genes compared with tissue from control embryos (from artificial insemination). Of those, more than 250 genes are associated with lethal phenotypes (causing defects in embryonic development or extra-embryonic tissue) in mice. These findings suggest highly disruptive defects in the signals that govern the interactions between cloned embryos and the uterus, upsetting the natural process of gestation.
This is the first study to analyse gene expression in both cloned embryos and the uterus where they are implanted. The data gathered are advancing knowledge on as yet poorly-understood mechanisms that lead to gestational failures in cloning. The studies are opening up new avenues to finding improved cloning techniques for animals.
The first cloned animal, Dolly the sheep, was born some 20 years ago, in 1996. Two years later, INRA’s Jouy-en-Josas laboratory gave the world its first cloned calves. The technique used in both cases was “somatic cell nuclear transfer”, in which a differentiated cell is taken from an individual (somatic tissue from skin, muscle, etc.) and transferred to an egg whose nucleus has been removed. This de-differentiates, or re-programs, the cell. The transplanted nucleus gets a new start in the embryonic state and the embryo thus formed is then implanted into a surrogate mother. In this model, there is no trace of the surrogate mother’s genome. The cloned animal only inherits the genes of the cell that was transplanted.
Massive dysregulation of genes involved in cell signaling and placental development in cloned cattle conceptus and maternal endometrium. Fernando H. Biase, Chanaka Rabel, Michel Guillomot, Isabelle Hue, Kalista Andropolis, Colleen A. Olmstead, Rosane Oliveira, Richard Wallace, Daniel Le Bourhis, Christophe Richard, Evelyne Campion, Aurélie Chaulot-Talmon, Corinne Giraud-Delville, Géraldine Taghouti, Hélène Jammes, Jean-Paul Renard, Olivier Sandra and Harris A. Lewin, PNAS, 8 December 2016, DOI: 10.1073/pnas.1520945114