Digenic Inheritance Accounts for Phenotypic Variability in Amelogenesis Imperfecta

Objectives: Amelogenesis imperfecta (AI) encompasses a range of clinically and genetically diverse disorders affecting enamel formation and mineralization. This study aimed to investigate the genetic etiology of hypoplastic and hypomineralized AI.
Methods: Two unrelated Chinese families with varying degrees of AI were recruited, and DNA was extracted from peripheral blood. Whole exome sequencing (WES), Sanger sequencing, and DNA quantitative PCR were employed to identify disease-causing mutations. RT-PCR and minigene splicing assays were conducted to assess the impact of splicing variants.
Results: Digenic mutations in unlinked AI-related genes were discovered in both probands. In family 1, the proband inherited a paternal frameshift mutation in LAMA3 (NM_198129.4:c.3712dup) and a maternal deletion spanning the entire AMELX gene. Consequently, the proband exhibited a combined hypoplastic and hypomineralized AI phenotype, distinct from the phenotypes observed in the parents. In family 2, WES analysis identified a maternal heterozygous splicing mutation in COL17A1 (NC_000010.11 (NM_000494.3):c.4156+2dup) and compound heterozygous mutations in RELT (paternal: NM_032871.4:c.260A>T; maternal: NM_032871.4:c.521T>G) in the proband. These genetic changes caused the abundant irregular enamel defects observed in the proband, whereas other affected family members carrying heterozygous mutations in both COL17A1 and RELT displayed only horizontal grooves as their phenotype. The pathogenicity of the novel COL17A1 splice site mutation was confirmed through RT-PCR and minigene assay.
Conclusions: This study reveals the potential association between co-occurring mutations in two genes and variable phenotypes in AI patients, indicating a complex genetic basis beyond a monogenic mode, especially in hypoplastic and hypomineralized cases. This knowledge is valuable for the clinical characterization and genetic counseling of individuals affected by this complex disorder.