ESPE Abstracts

Down syndrome (DS) is characterized by a higher incidence of congenital hypothyroidism (CH) and a high prevalence of subclinical hypothyroidism (SH) early in life. Children and adults with DS have an increased risk of developing autoimmune thyroid disease, however CH and early SH cannot be explained by thyroid autoimmunity. The etiology of CH and early SH in DS remains to be elucidated. Considering the recently discovered genome-wide transcriptional and epigenetic alterations in DS, we hypothesized that CH and early SH might be caused by promoter region hypermethylation of genes involved in thyroid gland function or development, which in general is associated with transcriptional downregulation. To investigate this, we performed linear regression on blood DNA methylation (DNAm) profiles of DS and non-DS subjects, focusing on thyroid-related genes. DNAm profiles were extracted from four independent Illumina array-based datasets, including 252 DS subjects and 519 non-DS subjects, spanning three age groups: newborns (n=650), toddlers (n=34) and adolescents/adults (n=87). We replicated the findings of the largest dataset in an aggregate validation dataset (composed of the remaining three datasets) and performed a meta-analysis. In the largest (discovery) dataset we found significant associations for DS in 18 thyroid-related genes. 21 of 30 differentially methylated positions (DMPs) were replicated in the validation set. The meta-analysis identified 31 DMPs with concordant methylation patterns across the datasets, and two differentially methylated regions (DMRs). Of these DMPs, 27 were hypomethylated and promoter-associated. The difference in mean methylation between cases and controls of these hypomethylated thyroid-related DMPs decreased with age (ANOVA P value = < 0.0001). Contrary to our hypothesis, we mainly found promoter region hypomethylation, suggesting transcriptional upregulation of thyroid-related genes. According to these data, we can infer that CH and early SH in DS are probably not caused by differential methylation/expression of thyroid function and development genes. Since epigenetic regulation is dynamic by definition, we may speculate that the identified changes to DNAm of thyroid-related genes could be a rescue phenomenon to ameliorate the phenotype. This hypothesis aligns with the decreasing methylation difference of thyroid-related genes with age. The prevalence of early (non-autoimmune) SH declines with age, so hypothetically, the need for epigenetic upregulation of thyroid-related genes also diminishes. However, the etiology of CH and early SH in DS remains unknown. Focus of further research should be centered around obtaining and analyzing DS thyroid tissues and thyroid hormone target tissues, and age-matched controls, for (multi-)omics analyses.