ENAM Mutations Can Cause Hypomaturation Amelogenesis Imperfecta.

Area of Science:

• Genetics and Molecular Biology
• Dental and Craniofacial Development
• Cellular Pathology

Background:

• Amelogenesis imperfecta (AI) encompasses inherited enamel malformations due to defects in enamel formation stages.
• Mutations in the ENAM gene, encoding enamelin, are known to cause hypoplastic AI.
• The specific mechanisms linking ENAM mutations to hypomaturation AI remain incompletely understood.

Purpose of the Study:

• To investigate the genetic basis and pathogenic mechanisms of AI in two families with distinct enamel defects.
• To characterize the functional consequences of novel ENAM mutations identified in these families.
• To elucidate the role of ENAM mutations in ameloblast pathology and the development of hypomaturation AI.

Main Methods:

• Genetic analysis to identify mutations in the ENAM gene in affected families.
• Minigene splicing assays to assess the impact of mutations on ENAM protein processing.
• In situ hybridization, protein secretion assays, endoplasmic reticulum stress analysis, and apoptosis assays (TUNEL, MTT) to evaluate cellular effects.

Main Results:

• Two distinct indel mutations (c588+1del and c.588+1dup) in ENAM were identified, causing frameshifts and truncated ENAM proteins (p.Asn197Ilefs*81 and p.Asn197Glufs*25).
• Truncated ENAM proteins were improperly secreted, impeded wild-type ENAM secretion, and potentially formed amyloid aggregates.
• Overexpression of mutant ENAM induced endoplasmic reticulum stress, unfolded protein response (UPR), and significant ameloblast apoptosis, leading to cell death.

Conclusions:

• ENAM mutations can cause generalized hypomaturation AI through ameloblast pathology and cell death, not just loss of function.
• Proteinopathy, characterized by misfolded and aggregated proteins, is a potential pathogenic mechanism for ENAM-associated AI.
• These findings expand the understanding of AI pathogenesis and highlight the critical role of proper enamelin protein processing and secretion.