Evaluation of seven kindreds with ADHCAI revealed 6 different FAM83H
mutations, including 4 novel nonsense mutations and 2 deletion mutations, resulting in a frameshift and premature truncation, bringing the total number of mutations reported in FAM83H
to 14. The current findings support and extend previous findings (Kim et al., 2008
) indicating that normal ameloblast function and enamel mineralization are critically dependent on the presence of sufficient amounts of the FAM83H protein. It has been suggested (Witkop and Sauk, 1977
) that ADHCAI was due to a defect in the initial mineralization of the enamel crystallites, resulting in sustained defective mineralization. If FAM83H
indeed functions as a transcription factor, then it must regulate the expression of genes critical for mineralization and/or protein processing during amelogenesis. Interestingly, the enamel proteins present in fully developed ADHCAI enamel have a different amino acid profile compared with those present in hypomaturation AI types, where a proline-rich amelogenin-like profile is observed (Wright et al., 2006
). This suggests that FAM83H
likely has a role in amelogenesis other than, or at least in addition to, regulating the enamel proteinases.
Prior to this report, all 10 previously described FAM83H
mutations were nonsense mutations (Kim et al., 2008
; Lee et al., 2008
). The occurrence of only truncating mutations suggests either haploinsufficiency or a dominant-negative effect as the underlying molecular mechanism in ADHCAI. Although the FAM83H
mutations all occur in the terminal exon, some terminal exon mutations can trigger nonsense-mediated decay (Tan et al., 2008
). Even if the mRNA transcript is not degraded prior to translation, it is possible that the improperly folded protein would be targeted for degradation. If haploinsufficiency is the underlying mechanism, then persons with terminal or interstitial 8q24.3 deletions would be expected to manifest hypocalcified AI. A search of the literature and the A Band Apart database (http://tomcat.esat.kuleuven.be/abandapart/
) failed to identify persons with terminal or interstitial deletions of 8q24.3. Given that mutations in terminal exons may escape nonsense-mediated decay, the more likely explanation for the molecular etiology appears to be a dominant-negative effect. The unique localized AI phenotype resulting from 2 novel mutations, p.E694X and p.L625fsX703, suggests that the putative truncated proteins function differently than do shorter proteins, allowing the coronal enamel to form more normally. The mutations p.L625fsX703 and p.E694X would putatively produce proteins slightly less than half the length of wild-type FAM83H. These truncated proteins, of approximately 700 amino acids, appear to attenuate the more severe phenotype that results from the mutations that truncate the protein at 677 or fewer amino acids. If indeed the protein products are translated and not degraded, there would appear to be a functional difference in FAM83H proteins that are 677 vs
. 694 amino acids or longer. Identification of additional mutations and phenotype characterization will help resolve our understanding of FAM83H function and variable AI phenotypes.
Several studies recognize the association of Class III malocclusions and/or open bites with AI, regardless of the molecular basis (Cartwright et al., 1999
; Ravassipour et al., 2005
). As has been reported in other families with FAM83H
mutations, there is a predilection for dental open bites (Kim et al., 2008
) and, as shown in the present study, for Class III malocclusion. Although the FAM83H protein appears to be expressed in a wide variety of tissues (Kim et al., 2008
), it remains to be determined whether abnormal expression of the gene or secondary factors associated with the primary dental changes—such as attrition, dental sensitivity, or bite force—contribute to the craniofacial changes observed in this population. Class III malocclusion is known to have a strong genetic predisposition and to be more prevalent in certain families (e.g
., Hapsburg) (El-Gheriani et al., 2003
; Cruzet al., 2008
). The present study shows individuals having the AI trait to have a greater frequency of Class III malocclusion compared with the non-AI-trait family members.
This investigation broadens our understanding of the AI conditions by identifying multiple allelic FAM83H mutations that cause ADHCAI, a relatively common form of AI in North America. Two of these mutations are associated with a unique localized cervical ADHCAI not previously described, suggesting that there are unique phenotypes associated with some FAM83H mutations. The roles of the FAM83H gene and its protein in enamel formation remain unclear; however, this study shows that amelogenesis occurring with inadequate FAM83H functional protein results in a severe diminution of mineralization, coupled with marked retention of protein that lacks an amelogenin-like composition. The enamel crystallites form in a relatively normal habit and are directionally ordered, producing a prismatic architecture that is poorly mineralized. When a truncated FAM83H protein of approximately 700 amino acids is potentially produced, there is an attenuated phenotype, where primarily the cervical enamel is affected and the coronal enamel forms normally. This suggests that the truncated protein has some activity and is able to sustain normal amelogenesis in the coronal areas, but not throughout the duration of crown formation. Taken together, these findings suggest a unique role for the FAM83H gene in, and clearly illustrate the complex nature of, enamel formation and its molecular regulation.