Impaired T3 transport and action underlies the pathophisiology of the Allan-Herdon-Dudley syndrome

Patients with mutations in the thyroid hormone (TH) cell transporter MCT8 gene develop severe neuropsychomotor retardation known as the Allan-Herndon-Dudley syndrome (AHDS). It is assumed that this is caused by a reduction in TH signaling in the developing brain during both intrauterine and postnatal developmental stages, and treatment remains understandably challenging. Given species differences in brain TH transporters and the limitations of studies in mice, we generated cerebral organoids (COs) using human iPSCs from MCT8-deficient patients. We found that MCT8-deficient COs exhibit (i) alterations in early neurodevelopment, resulting in smaller neural rosettes with thinner cortical units, (ii) impaired T3 transport in developing neural cells, as assessed through deiodinase-3-mediated T3 catabolism, (iii) reduced expression of genes involved in cerebral cortex development, and (iv) reduced T3-inducibility of TH-regulated genes. In contrast, the TH-analogs 3,5-diiodothyropropionic acid and 3,3’,5-triiodothyroacetic acid triggered normal responses (induction/repression of T3-responsive genes) in MCT8-deficient COs, constituting a proof-of-concept that lack of T3 transport underlies the pathophysiology of AHDS, and demonstrating the clinical potential for TH analogues to be used in treating AHDS patients. MCT8-deficient COs represent a species-specific relevant preclinical model that can be utilized to screen drugs with potential benefits as personalized therapeutics for AHDS patients.