Functional Analysis of Genetic Variation In The Secis Element Of Thyroid Hormone Activating Type 2 Deiodinase.

Context: Thyroid hormone (TH) is important for normal brain development. The type 2 deiodinase (D2) controls TH action in the brain by activating T4 to T3. The enzymatic activity of D2 depends on the incorporation of selenocysteine (Sec) for which the SECIS element located in the 3`UTR is indispensable. We hypothesized that mutations in the SECIS element could impact on D2 function, resulting in a neurocognitive phenotype.

Objective: To identify mutations in the SECIS element of DIO2 in patients with intellectual disability (ID) and to test their functional consequences.

Design, setting and patients: The SECIS element of DIO2 was sequenced in 387 patients with unexplained ID, based on a predefined pattern of thyroid function tests. SECIS element read-through in wild-type (WT) or mutant D2 was quantified by a luciferase reporter system in transfected cells. Functional consequences were assessed by quantifying D2 activity in cell lysate or intact cell metabolism studies.

Results: Sequence analysis revealed 2 heterozygous mutations: c.5703C>T and c.5730A>T, which were also present in unaffected family members. Functional evaluation showed that both mutations did not affect D2 enzyme activity in cell lysates or intact cells, although the 5730A>T mutation decreased SECIS element read-through by 75%. In the patient harboring the c.5730A>T variant, whole genome sequencing revealed a pathogenic deletion of the STXBP1 gene.

Conclusion: We report 2 families with mutations in the SECIS element of D2. Although functional analysis shows that nucleotide 5730 is important for normal SECIS element read-through, both variants do not segregate with a distinct phenotype.