Endocrine Abstracts

There is evidence that the 'large offspring syndrome' of in-vitro produced ruminant fetuses results from altered expression of imprinted genes which regulate fetal growth. DNA was isolated from tissue samples which were obtained at a public slaughterhouse, and a new length polymorphism was identified in the 3`UTR of the bovine IGF2-gene. This polymorphism was used to identify heterozygous animals and to characterize IGF2-imprinting in normal fetal and adult bovine tissues. Total RNA was isolated from heart, liver, brain and placenta of heterozygous fetuses (n=4) and from heart (n=2) and kidney (n=2) of adult heterozygous animals. After DNAse treatment IGF2-mRNA was amplified by RT-PCR and PCR-products were analysed with a sequencer. Predominant expression of one IGF2-allel was observed in heart, placenta and liver of all fetuses, the brain samples of the two younger fetuses (4th month of gestation), and in the adult kidneys. IGF2 expression was biallelic in the brain samples of the elder fetuses (8th and 9th month of gestation) and in the adult livers. Two fetuses had homozygous mothers, which allowed the identification of the imprinted (inactive) allele as maternal.

These results show that the bovine IGF2-gene is imprinted in a tissue specific pattern, which changes dependent on the developmental stage. The peri- or postnatal switch from monoallelic towards biallelic expression in the liver is phylogenetically conserved between ruminants and humans, but does not occur in rodents. This is presumably due to the activation of the non-imprinted IGF2-promotor P1, which is common to ruminants and humans, but has no equivalent in rodents. The change from monoallelic cerebral IGF2 expression in the two youger fetuses towards biallelic expression in the elder fetuses suggest an imprinting switch during the second half of gestation, which has not been described in human or rodents.