Ultrastructural analyses of deciduous teeth affected by hypocalcified amelogenesis imperfecta from a family with a novel Y458X FAM83H nonsense mutation

Abstract

Background: Nonsense mutations in FAM83H are a recently described underlying cause of autosomal dominant (AD) hypocalcified amelogenesis imperfecta (AI).

Objective: This study aims to report a novel c.1374C>A p.Y458X nonsense mutation and describe the associated ultrastructural phenotype of deciduous teeth.

Methods: A family of European origin from the Iberian Peninsula with AD-inherited AI was ascertained. Family members were assessed through clinical examination and supporting investigations. Naturally exfoliated deciduous teeth from 2 siblings were investigated by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and transverse microradiography (TMR).

Results: On clinical and radiographic investigation the appearances of the affected deciduous and permanent teeth were consistent with hypocalcified AI with small focal areas of more normal looking enamel. DNA sequencing identified a novel c.1374C>A p.Y458X FAM83H nonsense mutation in affected, but not in either unaffected family members or unrelated controls. Exfoliated teeth were characterised by substantial post-eruptive enamel loss on gross examination. Irregular, poor quality enamel prisms were observed on SEM. These were coated in amorphous material. TMR and EDX confirmed reduced mineral and increased organic content in enamel, respectively.

Conclusions: FAM83H nonsense mutations have recently been recognised as a cause of AD hypocalcified AI. We report a novel nonsense FAM83H mutation and describe the associated preliminary ultrastructural phenotype in deciduous teeth. This is characterised by poorly formed enamel rods with inappropriate retention of amorphous material, which is likely to represent retained organic matrix that contributes to the overall hypomineralised phenotype.

Figure 1. Clinical & radiographic phenotype, pedigree and FAM83H mutation

A) Mixed dentition of the 10-year old proband. i) The buccal surfaces of the erupting permanent premolar teeth (arrows) are paler and exhibit less post-eruptive enamel loss compared to teeth have been present in the mouth for longer. There is a malocclusion that in part reflects the transition from deciduous to permanent dentition. ii) Focal islands of pale, harder enamel are most obvious at the cusp tips of the first permanent molar teeth (arrows) and to a lesser extent involve the cusp tips of the permanent premolar teeth. The permanent upper left central incisor labial face has been restored. iii) An exfoliated deciduous incisor tooth exhibits typical loss of most of the enamel with relative sparing of the cervical enamel. A small island of harder enamel has persisted close to the DEJ (arrow). B. Detail from a panoramic radiograph of the proband. Unerupted teeth have a normal morphology, but the enamel is characterised by diminished radiodensity that contrasts from that observed in normal teeth (inset box). Erupted teeth exhibit significant premature post-eruptive loss of enamel. C. Family pedigree and confirmation of the c.1374C>A mutation.

Figure 2. Ultrastructural phenotyping of deciduous teeth

A) SEM of exfoliated deciduous tooth cervical enamel. The appearances of control enamel (i) contrast with the poorly formed enamel rods that are partially obscured by amorphous material (open arrow heads) in affected incisor (ii) and molar (iii) teeth. R is embedding resin and not dental tissue; bar 50μm. B) EDX spectra. In control enamel (i), the C:O ratio is low compared to that in affected enamel (ii). A small N peak was observed (between the C & O peaks) in affected, but not control teeth. Similar peaks are observed in affected and control teeth for Ca and P.