Mineral Metabolism Markers in a Pig Model of Preterm Birth Mimic Clinical Findings

Abstract Body : Introduction: Preterm births (< 37 weeks gestation) make up about 10% of all live human births [1]. The skeletal system may be particularly affected because approximately 80% of bone mineral accrual occurs during the third trimester of gestation [2]. While evidence indicates that preterm birth is associated with metabolic bone disease of prematurity, lower than expected peak bone mass, and increased risk for osteoporosis later in life [3-5], a major barrier to studying skeletal phenotypes after preterm birth is the lack of a translational animal model. The purpose of this study was to evaluate metabolic markers of mineral and bone metabolism in blood and urine in a pig model of preterm birth.
Methods: Naturally-delivered, sow-reared full-term pigs (Landrace x Yorkshire x Duroc) were sacrificed for sample collections on postnatal days 5 and 19 (n=7/day) and compared with corresponding caesarean-delivered, artificially-reared pigs delivered preterm (106 of 117 days gestation) sacrificed on the same postnatal days (n=14-18/day). Following caesarean delivery, preterm pigs were fitted with orogastric tubes for enteral feeding and umbilical catheters for parenteral nutrition and housed in incubators with supplemental oxygen for the first 12 hours. At sacrifice, body and organ weights were recorded and whole blood and urine were collected (n=46 total, 27 males/19 females). Plasma was assayed for biomarkers of mineral metabolism (phosphate, calcium, alkaline phosphatase, and creatinine). Plasma will be assayed for additional markers of bone and mineral metabolism (parathyroid hormone, fibroblast growth factor 23, vitamin D, osteocalcin, procollagen type I N-terminal propeptide, and C-terminal telopeptide of type I collagen). Urine was assayed for phosphate, calcium, and creatinine to calculate tubular mineral reabsorption rates. Blood and urine biomarker data were analyzed by two-way analysis of variance with birth status (full term vs preterm) and postnatal sacrifice day (day 5 vs 19) as fixed independent factors, and sex (male vs female) as a covariate. A threshold of p< 0.05 was used to determine statistical significance.
Results: Birth status, postnatal day, and the interaction between birth status and postnatal day had significant overall effects on the markers (all p< 0.001), while sex did not affect the outcomes (p=0.266). Plasma phosphate and calcium were each reduced in preterm pigs on postnatal days 5 and 19 compared to full term pigs. By day 19, alkaline phosphatase was elevated in preterm pigs compared to full term pigs. Calcium reabsorption was reduced in preterm pigs on days 5 and 19. By day 19, phosphate reabsorption was higher in preterm pigs compared to full term pigs.
Discussion: Elevated alkaline phosphatase and depressed phosphate are the most consistently used serum or plasma markers to diagnose metabolic bone disease of prematurity and these findings were replicated in the pig model. The altered levels of phosphate, calcium, and alkaline phosphatase are consistent with altered mineral reabsorption, similar to what is observed clinically in preterm neonates. Analysis of additional markers of mineral metabolism and bone turnover are ongoing.
Significance: The pig appears to be a potentially useful animal model to study preterm birth-associated skeletal phenotypes and interventions.