Noncoding Mutations in a Thyroid Hormone Receptor Gene That Impair Cone Photoreceptor Function

Area of Science:

• Endocrinology
• Molecular Biology
• Genetics

Background:

• Hormone receptor function depends on precise gene expression control.
• Thyroid hormone receptor β2 (TRβ2) is selectively expressed in the pituitary and cone photoreceptors.
• TRβ2 regulates opsin gene expression essential for color vision.

Purpose of the Study:

• To investigate the physiological significance of a candidate enhancer for TRβ2 induction.
• To understand the role of specific DNA sequences within an intron of the Thrb gene.
• To examine enhancer function in the natural chromosomal context.

Main Methods:

• Site-directed mutagenesis of a conserved intron region in the endogenous Thrb gene in mice.
• Assessing the impact of mutations on TRβ2 expression in the pituitary and cone photoreceptors.
• Analyzing chromatin acetylation and opsin expression in mutated cones.

Main Results:

• Mutations in e-box sites preferentially impaired TRβ2 expression in cones.
• Mutations in nearby sequences preferentially impaired TRβ2 expression in the pituitary.
• A combined deletion impaired expression in both tissues, demonstrating bifunctional enhancer activity.
• Cone-specific e-box mutations disrupted chromatin acetylation, blunted TRβ2 induction, and impaired opsin expression and light sensitivity.

Conclusions:

• Enhancers must be studied in their natural chromosomal context to define biological relevance.
• Enhancer function exhibits critical nuances in expression level and timing.
• Noncoding DNA sequences significantly influence thyroid hormone functions.