Characterization of the Linked Ovine Insulin and Insulin-Like Growth Factor-II Genes

Abstract

The ovine insulin-like growth factor-II (oIGF-II) gene is comprised of 9 exons that span approximately 25 kb. Approximately 750 nucleotides upstream of oIGF-II exon 1 are the three exons of the ovine insulin gene that are transcribed in the same direction as oIGF-II. The genomic organization and expression of the oIGF-II gene is similar to that of the human IGF-II gene. Four putative promoters direct the transcription of six 5' noncoding exons (1, 3, 4, 5, 6, and 7), which are alternatively spliced to the shared exons 8, 9, and 10. An ovine exon comparable to human exon 2 has not been identified. Multiple transcription initiation sites were identified for exons 1 and 6 by primer extension analysis. Using a reverse transcriptase polymerase chain reaction (RT-PCR) assay, exon 1 and 3 transcripts were shown to be expressed in adult but not fetal liver. In addition, a novel transcript, which contained exon 1 spliced directly to exon 8, was detected in adult liver. Exon 4 transcripts were not detected in either fetal or adult liver, whereas exon 6 and 7 transcripts were detected in both fetal and adult liver. Exon 5 transcripts were also expressed in both fetal and adult liver, which is in contrast to the tumor cell-specific expression of human exon 5. Like the human and rodent genes, the regulation of expression of the oIGF-II gene is under complex control.

Transcriptional Regulation of Insulin-like Growth Factor-II Gene Expression by Cortisol in Fetal Sheep during Late Gestation

“…The contiguous sequence for exons 6 and 7 and their 5Ј regions shown in Fig. 1c extends the partial sequences of Ohlsen et al (8) for the ovine IGF-II gene and provides information for mapping transcriptional start sites. The initiation of transcription of exon 6 appears to be from a unique site 1125 bp upstream of the 3Ј splice site for this exon.…”

“…The IGF-II genes in human, rat, and mouse have been characterized (4 -7), and a recent report on the structure of the ovine IGF-II gene (8) showed it to be most similar to that of the human gene. In the human IGF-II gene nine exons including six 5Ј-untranslated leader exons, and four promoters, P1-P4, have been identified.…”

Molecular Biology of IGF-I and IGF-II

“…Extensive structural information is available for the IGF-II gene and the IGF-II chromosomal locus from several mammalian species (57)(58)(59)(60)(61)(62)(63)(64)(65)(66). In humans, the single-copy 10 exoniGF-II gene has been mapped to chromosome llp15.5,just 3' to the insulin gene and less than 200 kb 5' to the H19 gene (82,83).…”

“…This conserved linkage group has been mapped in mice to chromosome 7 ( 83). IGF-II and insulin genes also have been shown to be contiguous in sheep and rats (63,66).…”

“…The first publication ofiGF-II cON As appeared in 1984, with analysis of the IGF-II gene following soon thereafter (55)(56)(57). IGF-II cON As now have been cloned from several mammalian and nonmammalian vertebrates, and the gene has been characterized in detail from at least four different mammalian species (57)(58)(59)(60)(61)(62)(63)(64)(65)(66).…”