Intrauterine Growth Restriction Disrupts Nucleosome Positioning at a Growth Hormone Response Element in the Insulin‐like Growth Factor 1 Gene

Abstract

Intrauterine growth restriction (IUGR) increases the risk of adult onset diseases such as obesity and type II diabetes. Persistent downregulation of insulin‐like growth factor 1 (IGF1) has been implicated in these diseases. IUGR downregulates liver IGF1 expression in newborn and in day 21 (d21) mice. IGF1 expression at d21 is dependent on growth hormone (GH)‐mediated upregulation at multiple GH response elements (GHRE) including one containing tandem STAT5b binding sites within intron 2 (i2). We hypothesized that IUGR would disrupt prenatal epigenetic programming of the i2 GHRE and prevent postnatal i2 GHRE activation. IUGR mice were generated by thromboxane A2 analog infusion from gestation day 12.5 to term to elicit maternal hypertension. The GHRE chromatin state at the i2 GHRE was assessed by Na‐bisulfite sequencing for DNA methylation and GpC methyltransferase mapping for nucleosome positioning. In normal mice, the GHRE remains hypermethylated throughout gestation and begins to demethylate at d17. By d21 a single nucleosome is displaced. IUGR animals undergo similar DNA demethylation. However, we found two distinct nucleosome patterns at the d21 hepatic i2 GHRE. Some IUGR animals showed no nucleosome displacement, while in others, the nucleosome free region (NFR) was expanded on the 3′ side suggesting the NFR was misaligned. Evidence suggests different GHRE in IGF1 have unique functions and when tandem STAT5b sites are used, both must be functional for the enhancer to work. We speculate the altered NFR spacing could disrupt proper enhanceosome assembly and result in loss/altered GHRE activity.