Differences in telomere length of bovine gametes, embryos, fetal tissues, and clones.

摘要

In 1997 it was announced that the first animal successfully cloned from an adult cell was born. This animal was the sheep Dolly. Dolly was created using nuclear transfer methods to reprogram an adult mammary cell. It was announced in 1999 that Dolly may be aging faster than normal. It is thought that she is prematurely aging because her telomeres are shorter than age-matched control sheep. Telomeres are a repeated DNA sequence located at the ends of linear chromosomes that allow for base pair loss during lagging strand synthesis of DNA replication. The loss of DNA during each replication is referred to as a "mitotic clock." Once the telomere has reached a critical shortened length, the cell will enter senescence and eventually die. In contrast, some cloned bovines have longer than normal telomeres. By using whole cell lysate and slot-blot analysis, we determined the telomere-to-centromere ratio (T/C) for bovine gametes, embryos, fetal tissues (brain, heart, lung, kidney, uterus, ovary, and skin), adult donor cells, and cloned embryos. Our data indicate that there is a consistency in T/C among the various fetal tissues. The T/C of sperm is significantly lower than in oocytes. The T/C decreases from the oocyte to the 2- to 8-cell stage embryo, increases dramatically at the morula stage, and again decreases at the blastocyst stage. According to our data there is no significant difference in the T/C between cloned embryos and in vitro fertilization (IVF) embryos, but there is a significant difference between cloned embryos and adult donor cells. In conclusion, the enucleated bovine oocyte has the ability to reestablish the telomere length of adult somatic cell donor nuclei. Reestablishment of telomere length may be an important component of the nuclear reprogramming process.