Permanent congenital hypothyroidism with GBP1 p.Leu187Pro: clinical follow-up data and epigenetic insights

Abstract Objectives GBP1 variants have recently been identified as a novel cause of congenital hypothyroidism (CH), but the clinical spectrum remains incompletely defined. Longitudinal data on methylation dynamics at cg12054698, an open-sea CpG site within the GBP1 5′ untranslated region (5′ UTR), and normative methylation levels in euthyroid controls are lacking. Methods We identified three unrelated CH probands carrying the heterozygous GBP1 p.Leu187Pro variant by trio whole-exome sequencing and systematically screened 61 known CH-related genes. Clinical phenotypes and longitudinal outcomes data were collected. For epigenetic analysis, cg12054698 methylation was quantified by pyrosequencing in the three probands, their carrier parents, 30 age-stratified euthyroid controls (newborns, toddlers, and adults), 10 CH patients without GBP1 variants, and paired longitudinal samples from two probands. Results All three probands presented with moderate-to-severe CH at neonatal diagnosis. Thyroid ultrasonography revealed heterogeneous phenotypes, including a normal in situ gland, thyroid agenesis, and a normal-sized gland with diffuse hypoechogenicity. All three cases were confirmed as permanent CH. In euthyroid controls, cg12054698 methylation was significantly higher in newborns than in toddlers and adults (p<0.001). Neonatal methylation levels did not differ significantly among probands with GBP1 variants, CH patients without GBP1 variants, and euthyroid controls (p=0.260). Conclusions In this cohort, GBP1 -associated CH was permanent and showed a marked phenotypic heterogeneity. cg12054698 was hypermethylated during the neonatal period across study groups, supporting a developmentally regulated, stage-specific epigenetic pattern rather than a disease-specific pathogenic “second hit.”