Insulin-like growth factor-II (IGF-II) is a fetal growth factor that is expressed at high levels in many tissues during fetal and early postnatal development but only in the central nervous system thereafter.1 The Igf2 gene occurs within a cluster of genes that are regulated by genomic imprinting, an epigenetic process by which the two parental alleles of a gene are differentially expressed.2 Though the mechanism of imprinting has not been defined completely, it appears to involve cis-acting sequences within the genomic DNA, differential methylation and a non-translated RNA that is encoded by a nearby gene.2,3
Expression of Igf2 comes from the paternal allele, which is differentially methylated in a specific region within the gene.3 The non-methylated maternal allele is normally silent.
|In the figure: Fetal and postnatal overgrowth in transgenic chimeras. Chimeras were made by microinjection of transgenic ES cells (B11 and P5) into F2 host blastocysts and analyzed on day 13 and the day of birth. a) Day-13 B11 chimeras with theri wit weight and percentage chimerism. Note the elongated anterior-posterior axis of the hight-level chimera (left). b) B11 chimera (left) on the day of birth together with a non-chimeric littermate (right). Note the kink in the terminal third of the tail in the overgrown chimeric pup. c) Overgrown chimera (right) of the line P5 together with the non-chimeric littermate (left) on the day of birth.
Reprinted with permission of F-L. Sun, (Sun, F-L. et al. (1997) Nature 389:809).
When the imprinting mechanism fails to function properly, expression levels are altered for the genes within the imprinted cluster leading to a developmental abnormality called Beckwith-Wiedemann Syndrome (BWS).3 The symptoms include pre- and postnatal overgrowth, overgrowth of several organs, abdominal wall defects, skeletal abnormalities, excess amniotic fluid, hypoglycemia and an increased risk of developing certain childhood tumors.
Igf2 overexpression has long been considered a likely cause of several of the BWS symptoms. It is expressed at highest levels in those tissues that are most affected by BWS, and Igf2 imprinting is lost in 80% of BWS cases resulting in expression from both alleles.3,4
Direct confirmation of the role of Igf2 in the symptoms of BWS has been elusive, because the surrounding cluster of genes, which shares the imprinting regulatory mechanism, could also contribute to the symptoms, and transgenic mice expressing additional copies of Igf2 have not been produced.
Sun et al.3 used a chimeric mouse model to confirm that overproduction of IGF-II causes several, but not all, of the BWS symptoms. Embryonic stem cell lines transfected with an additional copy of Igf2 were injected into blastocysts to produce chimeric embryos and neonates. An interesting twist on the imprinting regulatory mechanism was observed in these chimeric animals. Elevated levels of IGF-II were observed, but they were found to arise completely from hyperactivation of the endogenous gene. The transgene was repressed during development, presumably reflecting the absence of appropriate cis-linked enhancers. Upregulation of the endogenous gene might imply repressor competition that is sensitive to gene dosage.
Igf2 overexpression resulted in pre- and postnatal overgrowth up to 160% at birth, disproportionate organ overgrowth, skeletal abnormalities, and excess amniotic fluid.3 Moreover, the magnitude of the abnormality was proportional to the percent chimerism of the animal. Thus, overexpression of Igf2 causes many of the symptoms of BWS, though some symptoms result from improper regulation of other genes within the imprinted cluster.