dna methylation as a predisposition factor in the pathogenesis of congenital hypothyroidism

Congenital hypothyroidism (CH) is the most frequent endocrinopathy in newborn. If not promptly treated lead to a severe impairment of psychomotor development. The etiophatogenesis of CH is still poorly understood; although several causative genes are identified, they can explain only a small portion of the pathological phenotypes. This scenario is further complicated if we focused on the incidence of CH in different context. In fact, epidemiological data indicate that children born prematurely have a 3-5 fold higher risk of CH. In addition premature infants born small for gestational age (SGA) have a risk of 12% higher to develop IC compared to prematures with appropriate development (AGA). The mechanisms that justify the increased risk of IC are still unknown. Some studies report that aberrant methylation patterns are associated with prematurity, intrauterine fetal development and the onset of some diseases. This project is focused on the study of DNA methylation, as predisposing factor to permanent thyroid dysfunction with neonatal onset. Using the Illumina Infinium-HumanMethylation27 technology we analyzed the global DNA methylation patterns (AVG) and selected the differentially methylated genes (DMGs) between 31 CH-cases born premature, AGA or SGA, and 28 term or preterm controls. To better understand the relationship between the DNA methylation and the premature birth, the intrauterine growth and the thyroid defect, the following groups were selected according to the gestational age at birth: 12 CH-with very preterm birth (CH-VPB<32wks) and 19 CH-with preterm birth (CH-PB 32-37wks); Controls: 9-CVPB, 6-CPB, 12-term birth (CTB>37wks). The same subjects were then analyzed according to intrauterine growth (20 CH-SGA, 11 CH-AGA than 6 C-SGA and 20 C-AGA) or the degree of CH: 19 with overt CH (OH, TSH>10 U/L) and 12 with mild CH (MH, TSH<10 U/L) than 16 CPB and 12 CTB. The global methylation analysis showed that infants born prematurely and SGA have a significant hypomethylation than term-controls. These data were confirmed by the gene-specific methylation analysis, through which we selected a large group of differentially methylated genes (DMGs) in CH-cases than term controls. Interestingly, the 95% of the DMGs are hypomethylated and the 70% of them are represented by CpG sites located in DNA non-coding regions. The gene ontology analysis revealed that genes involved in fetal growth and thyroid hormone metabolism were included among DMGs. The analysis of nine maternal genomic DNA for polymorphisms at the MTHFR revealed the possible association with folate deficiency during pregnancy and the global hypomethylation status of affected newborns. This is the first work exploring the role of epigenetic influences in the predisposition to congenital hypothyroidism. Our results suggest that genomic instability caused by global hypomethylation of non-coding regions may be related to premature birth and fetal growth delay. Under these conditions, thyroid defects are more frequent than expected and could result from the increased expression of predisposing genes, rather than from the reduced expression of protective genes. The role of maternal conditions during pregnancy seems to be a key factor to determine a proper DNA methylation pattern on fetus. Based on the Developmental Origin of Health and Disease Theory, we can assume that adverse condition during pregnancy, such as folate deficiency, may produce a fetal epigenetic reprogramming and the adaptation of preterm neonate to the extrauterine life includes among other dysfunctions a thyroid functional impairment. If this data will be confirmed by further experiments, this could represent a new predisposing factor to take into account during pregnancy to prevent and improve the prenatal screening of CH.