Expression levels of growth-regulating imprinted genes in cloned piglets

Different from cloned sheep, mice, and cattle, which are often born with increased body weights, pigs produced through nuclear transfer have not been found to be overly large compared to their age-matched controls. In our study, cloned pigs were significantly smaller than controls of conventional reproduction (p < 0.05) in both the deceased newborn group (1.2 +/- 0.8 vs. 2.1 +/- 0.2 kg) and at 1 month of age (6.1 +/- 1.3 vs. 8.0 +/- 0.8 kg, mean +/- SD). Because imprinted genes are important regulators of fetal growth and may be subjected to faulty reprogramming during nuclear transfer, we aimed to determine the expression levels of both growth-enhancing and growth-inhibiting imprinted genes in these cloned pigs by quantitative real-time reverse transcription polymerase chain reaction. These genes include IGF2 and PEG3 (growth-promoting), as well as IGF2R and GRB10 (growth-inhibiting). Tissues from six major organs including heart, lung, liver, kidney, brain, and spleen were collected from clones and controls of both age groups. With the exception of IGF2, significant differences in the expression levels of the other three imprinted genes were found in certain organs of either group of cloned pigs when compared to their age-matched controls. However, no strong correlation was found between the levels of gene expression and the low-body-weight phenotype of these cloned pigs. Interestingly, larger variances of gene expression were found in identical clones at 1 month old when compared to the control animals, indicating the variability of the nuclear reprogramming process.