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Validation of Amelogenesis Imperfecta Inferred from Amelogenin Evolution

¹ UMR 7138, Equipe "Evolution & Développement du Squelette", Université Paris 6, Case 05, 7 quai St-Bernard, 75005 Paris, France; and
² Department of Histology, The Nippon Dental University, School of Dentistry, Niigata, Japan

^* corresponding author, sire{at}ccr.jussieu.fr

	ABSTRACT

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We used the evolutionary analysis of amelogenin (AMEL) in 80 amniotes (52 mammalian and 28 reptilian sequences) to aid in the genetic diagnosis of X-linked amelogenesis imperfecta (AIH1). Out of 191 residues, 77 were found to be unchanged in mammals, and only 34 in amniotes. The latter are considered crucial residues for enamel formation, while the 43 residues conserved only in mammals could indicate that they play new, important roles for enamel formation in this lineage. The 5 substitutions leading to AIH1 were validated when the mammalian dataset was used, and 4 of them with the amniote dataset. These 2 sequence datasets will facilitate the validation of any human AMEL mutation suspected of involvement in AIH1. This evolutionary analysis also revealed numerous residues that appeared to be important for correct AMEL function, but their role remains to be elucidated.

KEY WORDS: amelogenin • amelogenesis imperfecta • molecular evolution • enamel • teeth • mammals • reptiles

	INTRODUCTION

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Amelogenin (AMEL) is the major protein of forming enamel. In humans, the amelogenin genes (AMEL) are located on the X and Y chromosomes, but in males, 90% of the transcripts are expressed from AMELX (Salido et al. 1992). AMEL plays a crucial role in enamel formation, but its exact functions are not totally understood (Paine et al., 2003). Its importance is well-illustrated, however, by the occurrence of a genetic disease, X-linked amelogenesis imperfecta (AIH1), resulting from AMEL mutations leading to various hypoplastic and hypomature enamel phenotypes. To date, 14 mutations leading to AIH1 are known (Hart et al., 2002a; Kim et al., 2004). The characterization of these mutations helps in identifying particular regions, or specific residues, that play a crucial role in AMEL function (Collier et al., 1997; Ravindranath et al., 1999). However, the few AMEL mutations known so far are insufficient to target all important residues.

Mutational analyses are time-consuming and expensive: analysis of the individual’s pedigree, mapping the mutation on a chromosome to identify a candidate gene, sequencing, and sequence analysis to validate the mutation. Moreover, AMEL polymorphism could lead to diagnostic errors in the clinical context, and this possibility is largely underestimated. Indeed, if a person has an enamel defect, and there is a pedigree consistent with an X-linked mutation, then a polymorphism in AMELX is unlikely to be the cause of the defect.

Evolutionary analysis is an alternative for validating the AMEL mutations responsible for AIH1, and for highlighting all the residues that are important for the protein to function correctly (Delgado et al., 2005; Sire et al., 2005, 2006). Such an analysis is based on the following postulates: (i) Important residues must remain unchanged, because their change or loss could lead to severe enamel defaults; (ii) conversely, less important residues can be substituted without damage to enamel structure and organization, and must therefore be considered polymorphisms; and (iii) given the slow rate of mutations in most lineages, the studied sample must cover a large evolutionary period and must be representative of the various lineages in which the protein has similar functions. This is the case in mammals and reptiles (amniotes), in which enamel structure is roughly similar (Sander, 2000), although both lineages separated approximately 310 million years (my) ago (Hedges, 2002). Nevertheless, in reptiles, teeth are continuously replaced during life (polyphyodonty), and the constraints acting on enamel structure could be less important than in mammals, which are diphyodont or monophyodont.

In the present study, we compiled 52 mammalian and 28 reptilian AMEL sequences, with the aim of obtaining datasets that could be useful for a rapid and accurate validation of the mutations responsible for AIH1.

	MATERIALS & METHODS

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In humans, AMELX is composed of 7 exons. Exon 1 is not translated; exon 4 is subjected to alternative splicing (Hu et al., 1996; Yuan et al., 1996, 2001) and is absent in some mammals and in all reptiles (Ishiyama et al., 1998; Delgado et al., 2006); and exon 7 codes for a single amino acid. Nine exons have been identified in rat and mouse AMEL (Li et al., 1998), but exons 8 and 9 are absent in all other species studied so far. Therefore, only AMEL exons 2, 3, 5, and 6 were included in the present study. Because the sequences of the small exons 2, 3, and 5 (fewer than 60 bp each) are well-conserved, we have concentrated our efforts primarily on exon 6 (> 400 bp), which is more variable. AMELY has evolved separately in various mammalian lineages (Girondot and Sire, 1998), in relation to the particular pattern of Y chromosome evolution (Iwase et al., 2001, 2003). Therefore, AMELY was not included in our study.

Materials
Several AMEL sequences were found in GenBank, and one sequence was obtained from the literature (Yamamoto et al., 2002). We completed this dataset by blasting sequenced genomes and by sequencing AMEL in representative species of most amniote lineages (Fig. 1). A dataset of 80 sequences (52 mammals and 28 reptiles) was obtained. References to species and sequences are found in APPENDIX 1. Taxa which have either no teeth [e.g., baleen whales (Mysticeti), anteaters (Xenarthra), pangolins (Pholidota)] or no enamel [e.g., armadillos (Xenarthra), aardvarks (Tubulidentata)] were not included in this study.

View larger version (18K): [in this window] [in a new window]   Figure 1. Relationships of the amniote lineages (in bold) for which amelogenin was used in this study (adapted from Madsen et al., 2001; Murphy et al., 2001; Janke et al., 2005; Vidal and Hedges, 2005). The number of species in each clade is indicated between the brackets. See APPENDIX 1 for information on the species and sequences.

    Primers
Primers were defined from the alignment of known AMEL sequences (see APPENDIX 2).

    PCR Amplification
Genomic DNA or cDNA (1 µL) was amplified in a mixture composed of 5 µL Taq buffer (10x) (pH 8.8), 3L MgCl₂ 2 mM, and 1 µL dNTP 10 mM, in the presence of sense and antisense primers, and 0.3 µL Red Hot polymerase (Advanced Biotechnologies Ltd., Foster City, CA, USA). Amplification was performed in a thermocycler (Genius Techne) for 38 cycles, each cycle consisting of 1 min of denaturation at 94°C, 1 min of annealing at 59°C, and 1 min of extension at 72°C. The final extension was for 20 min at 72°C.

    Cloning
One microgram of PCR product was isolated, ligated to pCR 2.1-TOPO plasmid vector (Invitrogen SA, Carlsbad, CA, USA) by the TA-cloning method, then used to transform competent E. coli TOP10F bacteria. The transformed bacteria were grown overnight at 37°C in Luria-ampicillin broth, and subjected to lysis in 200 µL of NaOH 0.2 M-SDS 1%, at 0°C for 5 min. Subsequently, a 150- µL quantity of AcK 3 M was added at 0°C for 5 min to precipitate the proteins. The plasmids were purified in a phenol/chloroform mixture. Sequencing was done by Genome Express S.A (Meylan, France).

    Molecular Analyses
AMEL sequences were aligned via Clustal X 1.81 (Thompson et al., 1997), and checked by hand with Se-Al v2.0 (available at http://evolve.zoo.ox.ac.uk/).

	RESULTS

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The Mammalian and Reptilian AMEL Datasets
Of the 250 amino acids (aa) in the alignment of the 52 mammalian AMEL sequences (including residue insertions), 77 were unchanged, and 30 were substituted by a residue from the same group (APPENDIX 3A). Most of the conserved amino acids were located in the N- and C-terminal regions [coded by exons 2, 3, 5, and the begining of exon 6, up to the TRAP (tyrosine-rich amelogenin peptide) proteolytic sites (aa 1–64) and the end of exon 6 (aa 218–250), respectively]. In contrast, the central region of exon 6 (aa 65–217) showed numerous variations, with a particular region characterized by large sequence deletions or insertions (aa 130–208). Twelve AMEL sequences possessed triplet (PXQ or PXX) insertions (up to 10 in the water opossum), while 4 other sequences showed deletions (up to 17 in the dolphin). All positions currently considered important were unchanged, including the TRAP proteolytic loci (aa 59 and 61) and the LRAP (leucine-rich amelogenin peptide) intra-exonic splicing site (aa 223).

In crocodiles, the 6 AMEL sequences were highly similar (APPENDIX 3B). Of 199 aa in the alignment, only 11 were substituted, and most of these were by residues from the same group. In squamates, the 22 AMEL sequences showed a high degree of variation (APPENDIX 3C). Of 217 amino acids in the alignment, 53 were unchanged, and 18 were substituted by a residue from the same group. Most unchanged residues were located in the N (aa 1–64) and C (aa 192–217) terminal regions; nearly all positions in the variable region of exon 6 (aa 65–191) were substituted.

When we considered the complete alignment of amniote AMEL, we could not align most parts of exon 6 (from aa 68 onward in our alignment), due to the high number of variations (substitutions, deletions, and insertions) (APPENDIX 3D). Only the N- and C- terminal regions could be aligned. We found 34 unchanged residues in these regions and 15 residues that were substituted by a residue from the same group. The proteolytic loci leading to TRAP were conserved, while the intra-exonic splicing site for LRAP could not be identified in most squamates.

Validation of AIH1 Using Two Sequence Datasets
The results obtained from the analysis of mammalian (52 AMEL) and amniote (80 AMEL) sequences were transposed onto the human AMEL sequence, with indication of residues that were unchanged, substituted by an amino acid from the same group, or variable (Figs. 2A, 2B). Of the 5 residues known to lead to AIH1 when substituted (M1, W4, T37, P56, and H63 in our sequence; p.M1T, p.W4S, p.T51I, p.P70T, p.H77L, respectively, in the AIH1 nomenclature), 4 were validated (i.e., unchanged) in mammalian and amniote sequence datasets, and all when only AMEL sequences were used. Indeed, the p.H77L mutation was not validated by the amniote dataset: Histidine (H: basic group) was substituted by glutamine (Q: polar) in crocodiles and in a snake. In humans, this AIH1 resulted from substitution by a leucine (L: non-polar). Most residues known to be important for a correct function of AMEL were conserved in amniotes. In addition, the datasets revealed a high number of unchanged amino acids.

View larger version (34K): [in this window] [in a new window]   Figure 2. Amino-acid sequence of human amelogenin with indication of important residues inferred from the alignment of 52 mammalian sequences (A) and of 80 amniote sequences (52 mammals, 6 crocodiles, and 22 squamates) (B). (The alignments are presented in APPENDIX 2.) Exon 4 (14 residues) was not included, because it was absent in most species studied. Signal peptide is on the grey background. The protein sequence (191 amino acids) is numbered from methionine (1). Large characters = residues unchanged; italics = residues that can be substituted for by an amino acid from the same group only. Small characters = residues for which substitution can be made. Boldface characters = the 5 residues known to lead to amelogenesis imperfecta after substitution.

	DISCUSSION

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We have chosen to present 2 datasets, one based on AMEL sequences of 52 mammals, and the other on a compilation of 80 amniote sequences. Indeed, although enamel structure is roughly similar in mammals and reptiles, some enamel specificities could have been selected for during the long evolutionary period (310 my) that separates these lineages. In contrast to reptiles, in which some ancestral characters, such as polyphyodonty, have been conserved, mammals no longer replace their teeth continuously throughout life. Furthermore, from a structural viewpoint, Tomes’ processes, a feature of mammalian ameloblasts related to the prismatic structure of enamel, do not exist in reptiles, in which enamel is non-prismatic (Sander, 2000). These two mammalian novelties could have led to new constraints in the AMEL sequence. We hypothesized that the 34 AMEL residues which are unchanged at the amniote level are essential for the correct formation and mineralization of enamel, i.e., they are important for AMEL interactions with the cell membrane and/or the mineral crystals. This hypothesis was well-supported: All these conserved positions were found at the N- and C-terminal regions, which are known to exert such functions (Paine et al., 2003; Snead, 2003). We hypothesized also that the 43 residues that are conserved only in mammals are related to the peculiar features of enamel that were selected for during mammalian evolution (180 my). Half of the unchanged positions were found in the N-and C-terminal regions, reflecting a possible stronger constraint on the AMEL sequence in these regions in mammals than in reptiles. The other conserved positions were found in the region known to be variable (Delgado et al., 2005; Sire et al., 2005, 2006), either close to the N- and C-terminal regions or in the central region of exon 6. This could also reflect new constraints in this region, but we can also envisage that these positions are not really important for AMEL function. Perhaps 180 my are insufficient for random substitution of amino acids that are not really important.

The 5 amino acid substitutions known to lead to AIH1 were validated by our method with the mammalian dataset, and 4 of them with the amniote dataset. In reptiles, the substitution of H63 in our alignment (p.H77L: Hart et al., 2002b) by a glutamine (Q) could indicate that this locus has probably been constrained during mammalian evolution only. The presence of this basic residue probably plays a role in TRAP proteolysis by enamelysin (MMP20). Does this mean that there is no TRAP in crocodiles, or that a polar residue (Q) could replace a basic one (H)? Amino acids that were replaced by residues from the same group were also indicated in the human sequence. Indeed, if one considers that only the biochemical characteristics of a position are important, there would be no problem if the residue were substituted by an amino acid from the same group.

Our evolutionary analysis of AMEL at the amniote level confirmed our previous findings, inferred from the comparative study of mammalian AMEL, i.e., highly conserved residues in the N- and C-terminal regions, and a variable region in exon 6 (Delgado et al., 2005, 2006; Sire et al., 2006). In exon 6, the intra-exonic splicing site, which releases LRAP (a short peptide involved in cell signaling: Veis et al., 2000), was well-conserved in mammals, but not in reptiles. The ’hot spot’ of mutation (i.e., large insertions and/or deletions located in the central region of exon 6) in mammals (Delgado et al., 2005) was found in the present study in a few newly sequenced AMEL of mammalian species, but was absent in reptiles. These features were acquired recently in mammalian evolution.

In addition to proposed sequence dataset, which will help in the diagnosis of AIH1, this analysis has revealed 30 unchanged residues with unknown, but certainly important, function. These amino acids could be good candidates for AIH1 if they were substituted, and their role in AMEL function should be evaluated.

Our study showed how evolutionary analysis, when conducted within a phylogenetic framework, could help both in validating mutations in humans and in revealing amino acids that could play important roles in enamel structure and organization. In dental research, this method could be applied to the study of other genes—for instance, enamelin, which is known to be responsible for autosomal-dominant AI, and dentin sialophosphoprotein, responsible for dentinogenesis imperfecta. The large number of genomes currently being sequenced in mammals could be taken as an opportunity to build datasets that could be used to validate mutations responsible for a genetic disease.

	ACKNOWLEDGMENTS

 
We are grateful to Prof. Ann Huysseune (Ghent University, Belgium) for helpful criticism of the manuscript. We are grateful to the following colleagues for sending either DNA or tissue samples: F. Catzeflis (UMR 5554, Université de Montpellier 2, France); L. Fougeirol and S. Martin (La Ferme des Crocodiles, Pierrelatte, France); A. Lécu and F. Ollivet (Zoo de Vincennes, MNHN, France); G. Véron, V. de Buffrénil and N. Vidal (Muséum national d’Histoire naturelle, France); W. Dabin (Muséum de la Rochelle, France); T. Robinson (Stellenboch University, Afrique du Sud); and D.J. Harris (Centro de Estudos de Ciência Animal, Vila do Conde, Portugal). This work was financially supported by IFRO (Institut Français de Recherche Odontologiques).

Since our article was in press, "A Novel Missense Mutation (p.P52R) in Amelogenin Gene Causing X-linked Amelogenesis Imperfectca" was published in JDR, 86:69–72, 2007, by M. Kida et al. This substitution is validated by our evolutionary analysis (exon5, position 38 in our alignment).

	FOOTNOTES

 
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org.

Received July 11, 2006; Last revision November 23, 2006; Accepted November 29, 2006

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