Kinetics of strain-dependent differential gene expression in oxygen-induced retinopathy in the rat.

Recent evidence suggests that retinopathy of prematurity, a potentially blinding condition of premature human neonates, has a genetically-determined component. Different inbred strains of rat exhibit differential susceptibility to oxygen-induced retinopathy (OIR), a well-established experimental model of retinopathy of prematurity. To explore the basis for this differential susceptibility, we quantified the retinal expression of 8 angiogenesis-related genes during early post-natal retinal development in rats with OIR. Inbred Fischer 344 (F344), Dark Agouti (DA) and Sprague Dawley (SPD) rat neonates were exposed to alternating cycles of 80% oxygen in air and normoxia for up to 14 days. After 14 days of cyclic hyperoxic exposure, some rats were exposed to normoxia for a further 4 days. Retinal mRNA for vascular endothelial growth factor (VEGF), VEGF receptor 2 (VEGFR2), pigment epithelium-derived factor (PEDF), angiopoietin-2 (Ang2), Tie2, cyclooxygenase-2 (COX2), insulin-like growth factor-1 (IGF1) and erythropoietin (EPO) were quantified by real-time reverse-transcriptase polymerase chain reaction at different time-points. Time-course analysis showed that expression of mRNA for VEGF, VEGFR2 and Ang2 was significantly greater in OIR-resistant (F344) retinae than in OIR-susceptible (DA) retinae during the first 9 days of cyclic hyperoxia. However, at post-natal days 14 and 18, retinal mRNAs for VEGF, EPO, VEGFR2, Ang2, IGF1, COX2 and PEDF were expressed to a significantly greater extent in OIR-susceptible (DA, SPD) than OIR-resistant (F344) retinae. The VEGF/PEDF ratio was greater in the F344 compared with the DA strain up to day 9, but was higher in the DA than the F344 strain at days 14 and 18. Thus, we found that retinal expression of angiogenesis-related genes was significantly higher in OIR-resistant rats than in OIR-susceptible rats during early retinal development, but the pattern reversed during the proliferative phase of OIR. We conclude that susceptibility to OIR correlates with differential gene expression very early in retinal microvascular development, during periods of cyclic hyperoxic exposure rather than during subsequent sustained hypoxia.