AMELX Mutations and Genotype-Phenotype Correlation in X-Linked Amelogenesis Imperfecta

Area of Science:

• Genetics and Molecular Biology
• Dental and Oral Health
• Biochemistry

Background:

• X-linked amelogenesis imperfecta (AI) is a group of inherited disorders affecting tooth enamel formation.
• Mutations in the AMELX gene are a known cause of AI, but the precise genotype-phenotype correlations require further elucidation.
• Understanding the molecular mechanisms underlying different AI subtypes is crucial for diagnosis and potential therapeutic strategies.

Purpose of the Study:

• To identify disease-causing mutations in the AMELX gene in six families affected by X-linked AI.
• To investigate the molecular and cellular consequences of identified AMELX variants.
• To establish a genotype-phenotype relationship for AMELX-associated AI subtypes.

Main Methods:

• Whole exome sequencing was performed to identify genetic variants in affected families.
• Splicing assays, immunoblotting, and quantitative RT-PCR were utilized to assess the functional impact of mutations.
• In vitro overexpression studies were conducted to analyze mutant protein behavior and cellular stress responses.

Main Results:

• Four pathogenic AMELX variants (c.2T>C, c.29T>C, c.77del, c.145-1G>A) and a whole gene deletion were identified.
• Affected individuals presented with a spectrum of enamel defects, from hypomaturation ('snow-capped') to severe hypoplasia.
• Mutant amelogenin proteins showed impaired secretion, leading to endoplasmic reticulum stress and potential apoptosis, correlating with specific AI types.

Conclusions:

• Amorphic AMELX mutations (deletions, 5' truncations) result in hypoplastic-hypomaturation AI (types IIB, IIC) due to loss of function.
• Neomorphic AMELX variants (signal peptide defects, 3' truncations) cause severe hypoplastic/aplastic AI (type IE), likely via toxic cellular effects.
• This study clarifies the genotype-phenotype spectrum of AMELX-associated AI, aiding in diagnosis and genetic counseling.