Prematurity and early life programming
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Preterm infants are at increased risk of cardiometabolic and neurodevelopmental disorders in later life. The typical postnatal growth pattern of failure to achieve the equivalent of a normal fetal growth rate, followed up by catch-up growth, altered adiposity and altered hypothalamic-pituitary-adrenal axis (HPA) activity may be predisposing factors. Potential mechanisms that may mediate such programmed effects include altered DNA methylation and faster telomere attrition. A prospective cohort of 46 very preterm (25+2 to 31+5 weeks’ gestation, mean 28.6) and 40 full term (38+3 to 42+2 weeks’ gestation, mean 40.2) infants was established to investigate potential mechanisms. Infants were studied at birth, term equivalent age, 3 months and 1 year corrected for prematurity. At all time points, linear growth and body composition (by densitometry) were measured and buccal (epithelial) cells was collected for measurement of DNA methylation (5mC) and relative telomere length. Compared with full term infants, preterm infants were lighter (p < 0.001) and had a smaller head circumference (p < 0.05) at all time-points and were shorter at term equivalent (p < 0.001) and 3 months corrected age (p = 0.002). Preterm infants also had greater percentage body fat at term equivalent age (mean difference = 5.5%, p < 0.001), which normalised by 3 months corrected (mean difference = 0.9%, p = 0.4). Preterm infants had a blunted salivary cortisol response (mean difference 0.4 μg/dL, p = 0.02) to a stressor (physical examination) at 3 months compared to term infants at this age, suggesting altered activity of the HPA axis. 5mC is fundamental in the control of expression of imprinted genes involved in fetal growth. Notably, a number of studies in humans exposed to an adverse environment in early life have demonstrated altered 5mC at the differentially methylated regions (DMRs) controlling the expression of the key fetal growth factor insulin like growth factor 2 (IGF2) and at the linked H19 imprinting control region (H19 ICR). At birth, preterm infants had a significant decrease in 5mC at DMR2 compared with term infants at birth (β = –11.5, p < 0.001) and compared with preterm infants at term equivalent age (mean difference = -7.4, p = 0.01). By term equivalent age, preterm infants had decreased 5mC at both DMR2 (β = –2.8, p = 0.01) and the H19 ICR (β = –2.3, p = 0.048) compared with term infants at birth, although this difference disappeared at 1 year corrected. Although research has suggested that catch up growth may confer an unfavourable metabolic phenotype, poor initial weight gain can associate with worse cognitive outcome. A pathway was established for obtaining advanced magnetic resonance images of the preterm brain. 5mC at H19 ICR and DMR2 in buccal DNA showed no association with measures of white matter microstructure or whole brain volumes. Term infants demonstrated telomere lengthening over the first year of life (mean difference = -0.3, p = 0.02). There was no significant change in telomere length over the first year of life in preterm infants (mean difference = 0.2, p = 0.34). However, as preterm infants at term equivalent age had longer telomeres compared to term infants at birth (β = 0.6, p < 0.001), ultimately there were no differences between the term and the preterm groups at 1 year corrected age (β = 0.3, p = 0.07). The DNA modification 5-hydroxymethylcytosine (5hmC) is a stable modification in its own right and is also thought to be an intermediate step in DNA demethylation. 5hmC is abundant in the placenta but has not been studied in the context of fetal programming. Additionally, previous research using methods such as bisulphite conversion would not have discriminated between 5mC and 5hmC and therefore the role of 5mC may not have been accurately measured. To study the relationship between 5mC, 5hmC and fetal growth, gene expression of candidate imprinted and non-imprinted genes in full term placental samples from the Edinburgh Reproductive Tissue BioBank was analysed. 5mC and 5hmC within the IGF2/H19 and KvDMR (controlling CDKN1C) loci was estimated using chemical capture and immunoprecipitation techniques that discriminate between modifications. Relationships between the expression of IGF2 (r = 0.3, p = 0.02) and CDKN1C (r = -0.3, p = 0.01) and birth weight across the normal range were found and in keeping with the known action of these genes. 5mC at IGF2 DMR0 (β = 0.3, p = 0.02) and KvDMR (β = 0.3, p = 0.02) and 5hmC at H19 gene body (β = 0.2, p = 0.04) associated with birth weight. Thus, DNA modifications at imprinted DMRs may modulate environmental influences on fetal growth across the normal range. DNA methylation at IGF2/H19 can be influenced by early life events. It remains to be seen whether any changes are present later in childhood and whether they associate with risk factors for the metabolic syndrome.