Human Type 1 Iodothyronine Deiodinase (DIO1) Mutations
Cause Abnormal Thyroid Hormone Metabolism

Background: Iodothyronine deiodinase-1 (D1) selenoenzyme regulates the systemic supply of active thyroid
hormone (TH). Transient decrease in D1 enzymatic activity is clinically relevant and adaptive in nonthyroidal
illness such as fasting or acute illness. However, DIO1 gene defects have not been reported in humans.
Methods: Genetic analysis was performed using whole-exome sequencing in members of two unrelated families presenting with abnormal serum thyroid function tests. Plasmid constructs containing the two pathogenic
DIO1 variants were used for in vitro studies assessing the kinetics of their enzymatic activity. Thyroid function
tests were measured in Dio1 heterozygous-null mice.
Results: We report the novel identification and characterization of two missense DIO1 pathogenic variants
(resulting in p.Asn94Lys and p.Met201Ile) in two unrelated families presenting with abnormal TH metabolism
with elevated serum reverse triiodothyronine (rT3) levels and rT3/T3 ratios. These characteristic in vivo parameters are also present in Dio1 heterozygous-null mice. Kinetic studies of the resulting mutant D1 proteins
demonstrate two- to threefold higher Km indicating lower substrate affinity and slower enzyme velocity.
Conclusions: We report the identification and characterization of two missense DIO1 pathogenic variants identified
in families with abnormal TH metabolism. This is the first demonstration of inherited D1 deficiency in humans.