Altered enamelin phosphorylation site causes amelogenesis imperfecta

Abstract

Defects in the enamelin gene (ENAM) cause amelogenesis imperfecta (AI). Our objective was to identify the genetic etiology of enamel hypoplasia in a Caucasian proband. Our hypothesis was that ENAM was defective. The proband and his father have an AG insertion (g.13185_13186insAG; p.422FsX448) in ENAM previously identified in AI kindreds from Slovenia and Turkey. The proband, his brother, and his mother have a novel missense mutation (g.12573C>T) that substitutes leucine for a phosphorylated serine (p.S216L) in the 32-kDa enamelin cleavage product. In this family, a defect in one ENAM allele caused minor pitting or localized enamel hypoplasia, whereas defects in both alleles caused severe enamel malformations, with little or no mineral covering dentin. Ser(216) is one of two serines on the 32-kDa enamelin that is phosphorylated by Golgi casein kinase and is thought to mediate calcium binding. We propose that phosphorylation of enamelin is critical for its function.

Figure 1.

Genotypes and phenotypes of the kindred. (A) Pedigree of the family with autosomal-dominant amelogenesis imperfecta. Two mutant ENAM alleles were identified in this family: mutation 1 (g.12573C>T; c.647C>T; p.S216L) and mutation 2 (g.13185_13186insAG; c.1259_1260insAG). The proband (III:I; arrow) is represented by a completely shaded square because he is a compound heterozygote for both mutations. The proband’s mother (II:3) and younger brother (III:2) are heterozygous for mutation 1, while the father (II:2) is heterozygous for mutation 2. (B) Oral photograph of the proband showing generalized severe enamel hypoplasia. (C) Oral photographs of the mother (p.S216L), showing highly polished enamel with localized pitting (arrowheads). (D) Oral photograph of the father (p.P422fsX448), showing polished enamel with localized pitting defects (arrowheads). (E) Oral photographs of the younger sibling (p.S216L), showing chalky-white enamel resembling fluorosis with localized pitting defects (arrowheads).

Figure 2.

SEM evaluation of primary incisors from the proband and his brother. (Top row) Scanning electron micrographs of maxillary right primary central incisor (tooth E) from the proband (III:I; p.S216L and p.P422fsX448). A thin layer of mineral measuring 22.5 µm thick covers dentin on the facial surface (arrowheads), but is absent from the cusp tip. (Bottom row) SEM of maxillary right primary central incisor from the proband’s younger brother (III:2; p.S216L). Bars = 100 µm.