An artificial thyroid hormone receptor mutant without DNA binding can have dominant negative effect

Area of Science:

• Endocrinology
• Molecular Biology
• Genetics

Background:

• Resistance to thyroid hormone (RTH) is a syndrome caused by mutations in the thyroid hormone receptor beta 1 (TR beta 1).
• Mutant TRs often inhibit normal receptor activity through a dominant negative effect (DNE), consistent with RTH's dominant inheritance.
• Previous research suggested intact DNA-binding activity is crucial for this DNE.

Purpose of the Study:

• To investigate whether a mutation in the DNA-binding domain (DBD) of a TR can exert a dominant negative effect (DNE).
• To explore the mechanism of DNE for DBD mutants and compare it to known mechanisms.

Main Methods:

• Transient co-transfection studies were performed using wild-type and mutant TRs with reporter genes containing thyroid hormone response elements (TREs).
• The effects of co-expressing TR alpha 1, TRv alpha 2, and retinoid X receptor alpha were examined.
• Reporter gene assays were used to measure transcriptional activity.

Main Results:

• A Cys73Ser substitution in the TR's DBD resulted in a mutant TR that inhibited transcriptional activation by TR alpha 1, with or without T3.
• This DNE was observed on TRE-containing reporter genes but not on a negatively-regulated TSH alpha Luc reporter gene at equimolar ratios.
• The DNE of the DBD mutant was not reversed by excess retinoid X receptor alpha and could inhibit transcription independently of TREs.

Conclusions:

• A TR mutant with a substitution in the DNA-binding domain can exert a dominant negative effect.
• This DNE may occur through a mechanism involving interference with basal transcription factors, similar to TRv alpha 2.
• The clinical relevance of such DBD mutants in RTH remains to be determined but is plausible.