Biblioteca Virtual en Salud

Background:

Since the first success in sheep, the production of viable cloned offspring by somatic cell nuclear transfer (SCNT) in various mammals has increased significantly. The incidence of pregnancy failure and fetal death, however, is still very high, whatever the species, and impairs the commercial development of this technology, even in the bovine species where the success rates are highest compared to other species. Review In cattle, most gestation losses are initially due to abnormal implantation and poor placental development leading to fetal demise during the early post-implantation period (30 to 70 days of pregnancy). Thereafter, in continuing pregnancies, losses usually occur in the last third of gestation and affect about 25% of the on-going pregnancies, with very large differences according to phenotype. These are currently referred to as the Large Offspring Syndrome (LOS), Large Placenta Syndrome or Abnormal Offspring Syndrome. In all cases, the placenta appears to be central to the onset of the pathology, with placentomegaly and hydrallantois being the most common features. Clinically, transabdominal ultrasound monitoring of fetal and placental development as well as the measurement of maternal plasma concentrations of pregnancy associated glycoproteins (PAG) are recommended in order to monitor the pregnancies. Humane termination of the pregnancies by Caesarian section or slaughtering of the affected animals is recommended when the pathology onset is diagnosed more than 2 weeks prior to term. Underlying mechanisms include abnormal placental vascularization, which is present early in SCNT placental development. Enzymatic response to oxidative stress is also modified. In the first trimester, several genes expressed in the trophoblast have been found to be differentially expressed between SCNT and control conceptuses, including placental lactogen (PL), the PAG, prolactin related protein-1 (PRP-1) and Dickkopf-1(DKK-1), to name a few. All these proteins are expressed in the Binucleate cells (BNC) of the trophoblast and thus, indicate that BNC function may be affected in SCNT from very early in gestation, thereby compromising placental development. Later in pregnancy, it has been shown that transplacental exchanges are disturbed, in particular those related to glucose metabolism. Moreover, endocrine function is altered compared to controls, with decreased estrogen secretion and modifications in PAG secretion, resulting in largely elevated maternal plasma concentrations. Gene expression patterns are affected, with most prominent functional effects involving cell cycle, cell signaling pathways, molecular transport, DNA replication, recombination and repair. Most of the affected genes are downregulated. Finally, many of the pathologies reported with SCNT pregnancies resemble abnormalities reported with either mutations or deletions of imprinted genes or dysregulation of imprinted gene expression, and the expression of several imprinted genes have been shown to be abnormal in SCNT placenta.

Conclusions:

In conclusion, pregnancy failure after SCNT is due to multiple factors affecting, implantation, placental development, morphology, vascularization, responses to oxidative stress and the epigenetic control of gene expression. If abnormal nuclear reprogramming may induce long term effects in bovine SCNT, these effects may also be due to fetal programming due to abnormal placental function and perturbed fetal development.