Reproduction in Domestic Animals

Epigenetics of Foetal Development in Clones 303donor cell selection, cell cycle stage, culture conditions.For unknown reasons, approximately one-third of thedonor cell lines lead to a success rate of live calvesobtained from initiated pregnancy as high as 40%whereas approximately 25% of donor cell lines totallyfailed (Panarace et al. 2007). Those differences in therate of live calves even occur when donor cell linecultures are run simultaneously within the same experimentalprogram. The different cell lines with noevidence of abnormal chromosomal constitution gavethe same high number of blastocysts in vitro afterSCNT, irrespectively of the following success rate ofdevelopment (Renard et al. 2007).Higher success rates of SCNT in cattle are largely dueto the extensive knowledge on the female (and male)reproductive physiology in this economically importantspecies. In most mammalian species studied thus far, thesurvival rate to birth for cloned blastocysts is onlyapproximately 1–5%, compared with a 30–60% birthrate for IVF blastocysts. Many pathologies have beendescribed in conceptuses and neonates derived fromSCNT (Cibelli et al. 2002). Large offspring syndrome(LOS), which also occurs in cattle and sheep derivedfrom IVF, is common in cloned cattle, sheep and mice.LOS refers to a heterogeneous group of symptoms:notably, large size at birth and severe placental deficiency(Young et al.1998). Other symptoms are morevariable but include prolonged gestation, dystocia(strenuous labour), foetal and placental oedema, abnormalsize of organs, hydrallantois and hydramnios,respiratory problems and perinatal death (Young et al.1998; Farin et al. 2006). Most of the observed defectsinvolve abnormal placentation (Cibelli et al. 2002).Placentas of cloned calves have fewer but much largerplacentomes, presumably to compensate for the reducednumber of sites of foeto-maternal exchange (Chavatte-Palmer et al. 2002). In SCNT mice placentas are alwaysenlarged (Suemizu et al. 2003). It is possible that manyof the observed abnormalities are consequences ofprimary defects in placental function (Constant et al.2006). SCNT embryos from mice and cattle develop tothe blastocyst stage with a high success rate, and NTembryonicstem cells (NT-ESCs) can be derived fromsuch embryos with high efficiency (Wang et al. 2005;Wakayama et al. 2006) . Indeed, we have producednumerous NT-ESC lines in mouse from donor cellsincluding cumulus, fibroblast and ESCs. Gene expressionanalyses of such cell lines have shown almost nodifferences compared to the ESCs with the same geneticbackground or from each others, regardless of the celltype of donor cells (Dinnyes et al., unpublished results).The molecular, epigenetic mechanisms behind the placentalabnormalities are particularly interesting as aunique model for epigenetic regulation of the pregnancy,as discussed in details below.Molecular Level Changes in SCNT EmbryosGlobal gene expressionMost SCNT embryos have abnormalities at the molecularlevel. Early reports on reprogramming of variouscandidate genes in SCNT embryos are inconsistent(Daniels et al. 2000; Wrenzycki et al. 2001; Inoue et al.2006). For example, expression of the pluripotencymarker gene Pou5f1 (formerly known as Oct4) afterSCNT has been reported to be both normal (Danielset al. 2000; Jouneau et al. 2006), or abnormal (Bortvinet al. 2003). The global gene expression pattern inSCNT bovine blastocysts reflects well the extent ofnuclear reprogramming. In our studies the gene expressionprofiles of SCNT embryos were very different fromthose of somatic donor cells, and they resembled thoseof naturally fertilized embryos with