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Amelogenesis imperfecta  (AI) is a group of inherited conditions featuring isolated enamel malformations. About 5% of AI cases show an X-linked pattern of inheritance, which are caused by mutations in  AMELX . In humans there are two, non-allelic amelogenin genes:  AMELX  (Xp22.3) and  AMELY  (Yp11.2). About 90% of amelogenin expression is from  AMELX , which is nested within intron 1 of the gene encoding Rho GTPase activating protein 6 ( ARHGAP6 ). We recruited two AI families and determined that their disease-causing mutations were partial deletions in  ARHGAP6  that completely deleted  AMELX . Affected males in both families had a distinctive enamel phenotype resembling “snow-capped” teeth. The 96,240 bp deletion in family 1 was confined to intron 1 of  ARHGAP6  (g.302534_398773del96240), but removed alternative  ARHGAP6  promoters 1c and 1d. Analyses of developing teeth in mice showed that  ARHGAP6  is not expressed from these promoters in ameloblasts. The 52,654 bp deletion in family 2 (g.363924_416577del52654insA) removed  ARHGAP6  promoter 1d and exon 2, precluding normal expression of ARHGAP6. The male proband of family 2 had slightly thinner enamel with greater surface roughness, but exhibited the same pattern of enamel malformations characteristic of males in family 1, which themselves showed minor variations in their enamel phenotypes. We conclude that the enamel defects in both families were caused by amelogenin insufficiency, that deletion of  AMELX  results in males with a characteristic snow-capped enamel phenotype, and failed  ARHGAP6  expression did not appreciably alter the severity of enamel defects when  AMELX  was absent.

Amelogenesis Imperfecta in Two Families with Defined AMELX Deletions in ARHGAP6