Functional Domain Mapping of TPO: Insights from Six Variants in Sudanese Kindreds with Congenital Hypothyroidism.

Context: Congenital hypothyroidism (CH) is a leading cause of preventable intellectual disability worldwide if left untreated. Thyroid peroxidase (TPO) is a key enzyme that uses hydrogen peroxide from the DUOX/DUOXA system to oxidize iodide for thyroid hormone synthesis.

Objective: Identification of the pathogenic TPO variants responsible for CH.

Methods: Variants identified by whole-exome sequencing were analyzed using in silico tools and structural modeling for pathogenicity. TPO function was assessed through in vitro studies on intracellular trafficking, enzymatic activity, and interaction with DUOX/DUOXA proteins.

Results: Six TPO variants were identified: p.G395D, p.V618M, p.M706V, and p.T725P in Family 1, and p.R648G and p.G771R in Families 2 and 3, respectively. Affected individuals in Family 1 exhibited compound heterozygous or homozygous variants for the four variants. In silico analyses showed incomplete concordance in predicting pathogenicity. In vitro studies confirmed p.G395D as the primary pathogenic variant in Family 1, and p.R648G and p.G771R in Families 2 and 3. Notably, p.V618M, p.M706V, and p.T725P did not impair TPO function, either individually or in combination, suggesting that these regions are not critical for enzymatic activity. Further functional analyses revealed that p.G771R is essential for proper membrane insertion, whereas p.R648G is necessary for enzymatic activity.

Conclusion: When multiple TPO variants occur within the same family, a combination of in silico and in vitro analyses can help identify the variant responsible for the phenotype. In silico methods, however, cannot predict the different mechanisms of impairment, such as enzyme activity versus cellular localization, where the protein's topology is essential for normal function.