Uncovering the genetic basis of the hereditary tooth enamel disorder Amelogenesis Imperfecta

Enamel is the wear-resistant outer layer of the dental crown and is the hardest tissue in the human body. Enamel is formed from the secretion of an extracellular matrix and subsequent mineralization by secretory ameloblasts, in a process termed amelogenesis. AI refers to a group of rare, Mendelian disorders caused by abnormal amelogenesis. AI can be isolated or part of syndromic conditions; to date, 20 genes are implicated in non-syndromic (NS) AI. To uncover the genetic basis of AI, a DNA sequencing method was developed to target AI-associated genes, using single molecule molecular inversion probes (smMIPs) technology. Using smMIPs, 181 unsolved AI cases were screened, with 36% of these solved using the assay. smMIP-unsolved samples were further analysed by whole exome sequencing, identifying several new candidate AI genes. Comprehensive genetic analysis of this AI cohort identified variants in COL17A1, previously overlooked, as the most frequent cause of AI. Homozygous COL17A1 variants cause the recessive skin disorder epidermolysis bullosa, highlighting a need for a multidisciplinary approach to the clinical management of these patients. This study also further added to our understanding of the molecular mechanisms underlying contrasting phenotypes in patients with AI due to dominant or recessive mutations in AMBN. Another intriguing finding was the detection of a 587 bp homozygous deletion in PLXNB2 in a syndromic patient, strengthening evidence for PLXNB2 as a novel gene involved in recessive syndromic AI. The smMIPs method proved to be an effective first-line screen, offering patients a rapid, low-cost, diagnosis. The technique could now be expanded to investigate whether AI genes contribute to the risk of other dental conditions such as fluorosis or molar incisor hypomineralisation. Cell modelling and functional experiments are planned to demonstrate the biological relevance of the candidate AI-associated genes that were identified. These experiments will advance our understanding of disease mechanisms with the aim of ultimately improving clinical care.