The equine IGF genes : structural and transcriptional features

Insulin-like growth factors 1 and 2 (IGF1 and IGF2) are small mitogenic peptides. IGF! is mainly expressed in adult life, whereas IGF2 is one of the most ubiquitous growth factors in the mammalian embryo, where it plays an important role in the regulation of fetal growth. The encoding genes have been characterised in a number ofspecies and show a complex structural organisation and regulation of expression. Furthermore, IGF2 it is subject to epigenetic modification and genomic imprinting.The present study aimed at the basic structural and functional characterisation ofthe equine IGF1 and IGF2 genes.cDNAs for both equine IGF1 and IGF2 were cloned and showed high sequence homology when compared to other species. Even more conserved is the deduced amino add sequence of both peptides. The equine IGF2 gene was cloned and characterised. Sequence comparison revealed three non-coding leader exons and three coding exons. Downstream of the polyadenylation site a dinucleotide repeat sequence was identified. A novel structural element, an inverted repeat, is predicted. It is conserved between species and located in a region which is differentially methylated in the human and mouse genes and possibly involved in the imprinting mechanism. The inverted repeat acquires a stem-loop structure with a hybrid A/B-DNA conformation in the stem area and binds a specific, methylation sensitive protein which is developmentally regulated.A number oftranscripts were detected for both equine IGF1 and IGF2 genes. In the IGF2 gene, two promoters (P2 and P3) were shown to be active in fetal tissues and one promoter (P3) was found to be active in adult tissues. This represents a transcriptional pattern different from that in humans or rodents. Furthermore, IGF2 transcripts were located at tissue level using in situ hybridisation.The equine IGF2 gene was physically mapped to ECA 12ql3 and homoeology of this chromosome to HSA11 was revealed.