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1 Diploma in Dental Science, Aristotle University of Thessaloniki, Greece;
2 Diploma in Biology, Aristotle University of Thessaloniki, Greece; and
3 Department of Endodontology, Dental School, Aristotle University of Thessaloniki, 23, Papafi Str., 54638 Thessaloniki, Greece
* corresponding author, lyroudia{at}zeus.csd.auth.gr
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONAMELOGENESIS IMPERFECTAFROM GENES TO PROTEINS...CONCLUSIONSREFERENCES |
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KEY WORDS: amelogenesis imperfecta • kallikrein-4 • enamelin • amelogenin • DLX3
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONAMELOGENESIS IMPERFECTAFROM GENES TO PROTEINS...CONCLUSIONSREFERENCES |
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AMELOGENESIS IMPERFECTA |
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TOPABSTRACTINTRODUCTIONAMELOGENESIS IMPERFECTAFROM GENES TO PROTEINS...CONCLUSIONSREFERENCES |
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FROM GENES TO PROTEINS AND AMELOGENESIS IMPERFECTA |
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TOPABSTRACTINTRODUCTIONAMELOGENESIS IMPERFECTAFROM GENES TO PROTEINS...CONCLUSIONSREFERENCES |
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). Exon 1 contains the 5' untranslated region of the mature mRNA. The remaining part of the 5' untranslated region is included in exon 2, where translation initiation and the signal peptide sequences are also encoded. A unique exon 4, not homologous to or evolved from the exon 4 segment expressed in humans and rodents, has been recently identified in the pig amelogenin gene (Hu CC et al., 2002). It is noteworthy that exon 6 shows considerable variation among species, due to additional internal sequence lengths (Brookes et al., 1995). Exon 7 codes for the final amino acid, Asp. It also contains the stop codon and, in humans, has been shown to contain a putative polyadenylation site (Salido et al., 1992). In addition to these known exons, Li et al.(1998) identified 2 additional exons at the 3' end of the amelogenin gene in genomic DNA of the rat. Exons 8 and 9 are 45 and 110 bp long, respectively, followed by a stop codon and a polyadenylation site. It has been confirmed that these two exons code for proteins present in the matrix of developing enamel in the rat (Baba et al., 2002). The presence of these exons in the mouse and the human genome was indicated by Southern blot analysis (Li et al., 1998). Another interesting feature is that of alternative splicing of the pre-mRNAs, resulting in multiple transcript variants that encode different isoforms (Simmer, 1995).
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Amelogenin and X-linked Amelogenesis Imperfecta
Molecular studies and mutational analyses in patients with X-linked AI have established its correlation with the amelogenin gene. To date, the studies have identified two gene loci that are correlated with this disorder (Xp22.1-Xp22.3 and Xq24-Xq27.1) (Lau et al., 1989; Lagerström et al., 1990; Aldred et al., 1992a). Thus, genetic heterogeneity in X-linked AI may exist. To date, there are 14 AMELX-associated AI mutations (Lagerström et al., 1991; Aldred et al., 1992b; Lench et al., 1994; Lagerström-Fermer et al., 1995; Lench and Winter, 1995; Collier et al., 1997; Kindelan et al., 2000; Sekigushi et al., 2001a,b; Hart et al., 2002b; Greene et al., 2002; Kim et al., 2004). With respect to the nomenclature system devised for AMELX-associated AI mutations, exon 4 is included in the numbering scheme, although exon 4 is present in less than 1% of all transcripts (Hart et al., 2002a). The above studies showed that mutations that cause changes in domains of the protein with different functions result in diverse and distinct clinical manifestations (Wright et al., 2003). The observed phenotypes in X-linked AI vary considerably in severity, as well as in their primary features. Great variation also exists between male and female patients, because males express only one mutant allele, whereas females show a mosaic pattern of expression, due to X-chromosome inactivation (Lyonization) (Crawford and Aldred, 1992; Collier et al., 1997). With respect to the reported mutations, four are considered to include the signal peptide (Lagerström-Fermer et al., 1995; Sekigushi et al., 2001a; Kim et al., 2004) (Fig. 1
). A single base substitution (g.11G>A) that resulted in a premature stop codon, two missense mutations in exon 2 that affected the translation initiation codon and/or the secretion of amelogenin (P.W4S and P.M1T), and a deletion of 9 nucleotides that replaced amino acids 5 through 8 with threonine (p.I5_A8delinsT) resulted in a clinical manifestation of hypoplasia. A combined phenotype of hypomineralization with hypomaturation, as a result of a 5-Kb deletion (g.1148_54del), including all genomic DNA from exon 3 through part of exon 7, was reported in two Swedish families (Lagerström et al., 1991). There are five described mutations concerning the C-terminal region of amelogenin (Fig. 1). All these mutations introduce a premature stop codon (Lench and Winter, 1995; Kindelan et al., 2000; Greene et al., 2002; Hart et al., 2002b). Four of these are single deletions in exon 6. The difference lies in the changes between the point of deletion and the stop codon. The loss of the C-terminal region results in a hypoplastic type of AI. The fifth mutation introduces a premature stop codon late in exon 6 (p.E191X) and a single nucleotide change (c.571G>T). There are four mutations that involve the amino-terminal region of amelogenin (Fig. 1). Three of them are single-nucleotide substitutions causing single amino-acid changes. The one that occurred in exon 5 (g.3455C>T) resulted in a phenotype described as hypomineralization/hypomaturation, while the other two, involving exon 6, resulted in a phenotype of hypomaturation with discolored enamel (Lench and Winter, 1995; Ravassipour et al., 2000; Hart et al., 2002b). The fourth was a single-nucleotide substitution that introduced a premature stop codon in exon 5 (Lench et al., 1994). Individuals appeared with a predominant combined phenotype of hypomineralization with hypomaturation, accompanied by various degrees of hypoplasia.
Enamelin Gene and Protein
The enamelin (ENAM) gene is a tooth-specific gene expressed predominantly by the enamel organ and, at a low level, in odontoblasts (Hu et al., 1998; Hu et al., 2001a; Nagano et al., 2003). ENAM cDNAs have been isolated and characterized from the mouse, the pig, and, more recently, from humans (Hu et al., 1997c, 1998; Hu CC et al., 2000). The human ENAM gene is localized on chromosome 4 (4q13.3) (Hu CC et al., 2000) and consists of only 9 exons and 8 introns (Fig. 2A) (the mouse and pig enamelin genes consist of 10 exons). Intron and exon sizes vary from 61 to 4171 bp and from 42 to 4805 bp, respectively. In humans, the sequence corresponding to mouse exon 2 is absent (Hu CC et al., 2000; Hu et al., 2001a). However, the search for the intron separating the first and second exons in the human gene showed a sequence homologous to mouse exon 2, flanked by appropriate splice junctions, raising the possibility that alternative splicing may occur (Hu et al., 2001a; Hart et al., 2003a). The translation initiation codon is assigned at the third ATG from the 5' end of the cDNA (exon 3). The first ATG is in the appropriate position for translation, but is followed in 4 codons by a stop signal (TGA). The second is not appropriate, since it contains a pyrimidine in the –3 position. The 3' untranslated region for humans is particularly long and contains 4 putative polyadenylation signals (Hu CC et al., 2000). The cDNA differs from the human gene at two positions that affect the deduced amino acid of the protein. These differences appeared to be polymorphisms. In contrast to what is observed in ameloblastin and amelogenin, no alternatively spliced enamelin mRNAs have been reported (Hu et al., 1997c; Simmer and Hu, 2002; Hart et al., 2003a).
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Enamelin and Autosomal-inherited Amelogenesis Imperfecta
One autosomal-inherited form of AI, namely, autosomal-dominant amelogenesis imperfecta (ADAI), was linked to a 4Mb region on 4q21 (Kärrman et al., 1997). The ENAM gene has been mapped within this locus by radiation hybrid analysis (RHA) and fluorescent in situ hybridization (FISH), and was therefore considered a candidate gene for this type of AI (Dong et al., 2000; Hu CC et al., 2000). The reported mutational analyses of families with AI support the linkage between the ENAM gene and autosomal amelogenesis imperfecta (AAI) (Hart et al., 2003). With respect to the nomenclature system, the ENAM exhibited 10 exons and 9 introns (Hart et al., 2003a). Kida et al.(2002) first reported an autosomal-dominant hypoplastic form of AI caused by a single G-deletion within a series of 7 G residues at the exon 9-intron 9 boundary of the ENAM gene (g.8344delG). The affected individuals were heterozygous for the mutation. It was suggested that this mutation resulted in an alteration of the reading frame from exon 9 and the introduction of a premature stop codon in the 5' region of exon 10. Hart et al.(2003a) reported a G-deletion in intron 9, suggesting a potential shortening of exon 9 and the introduction of a premature stop codon. Mårdh et al.(2002) described a nonsense mutation in exon 5 (g.2382A>T). It was a single-base substitution in position 438 that resulted in a change of lysine to a stop codon, and subsequently to a truncated protein comprised only of 52 aa, compared with the wild type. The affected individuals exhibited local hypoplastic ADAI. Rajpar et al.(2001) reported a heterozygous mutation in the splice donor site of intron 8 (g.6395G>A). The most possible scenario was that the mutation caused the skipping of exon 8, resulting in an in-frame deletion of the amino acid sequence encoded by this exon. The phenotype of the affected individuals was described as thin and smooth hypoplastic ADAI. This type has been shown to map to the critical region of the ADAI local hypoplastic form, and therefore these two AI subtypes were considered allelic. Recently, Hart et al.(2003) described a 2-bp insertion mutation that resulted in a premature stop codon in exon 10 (g.13185_131186insAG). All affected individuals were homozygous for the mutation and presented with open bite and generalized hypoplastic AI. The heterozygous carriers had localized enamel-pitting defects, and none had AI or open bite. The importance of this report is that it is the first to describe the correlation between ENAM and autosomal-recessive AI (ARAI). In the same report, genetic linkage studies were consistent for localization of an ARAI locus to the AI candidate region of chromosome 4q13.3. Kim et al.(2005a) identified two ENAM mutations in kindreds with hypoplastic ADAI, one novel (g.4806A>C) and one previously identified (g.8344delG). The novel mutation alters the intron 6 splice acceptor site. Two defective splicing outcomes are probable for this mutation. The first is the inclusion of intron 6 (1615bp). This would insert multiple, in-frame stop codons preceding the most 3' exon. Translation of this transcript would add 8 novel aa to 70 aa of the wild-type protein, with the first 39 aa constituting the signal peptide. A second possible scenario would be the skipping of exon 7. This would maintain the reading frame, but would delete 87 aa (71–157) from the N-terminal side of the enamelin protein.
Ameloblastin Gene and Protein
The ameloblastin (AMBN) gene is expressed at high levels by ameloblasts (Cerny et al., 1996; Fong et al., 1996a; Lee et al., 1996) and at low levels by odontoblasts and pre-odontoblasts (Fong et al., 1998; Nagano et al., 2003), while moderate expression is also observed in Hertwig’s epithelial root sheath (Fong et al., 1996b, 1998), and in odontogenic tumors, such as in ameloblastomas (Toyosawa et al., 2000). The first reports of cloning and characterization of cDNAs encoding ameloblastin rat homologues appeared in 1996 (Krebsbach et al., 1996). Later, cDNAs from other species, such as the pig and the mouse, were also published (Hu et al., 1997b; Simmons et al., 1998). The human AMBN is localized on chromosome 4 at locus 4q21 (MacDougall et al., 1997), near other genes associated with mineralized tissues: osteopontin, bone sialoprotein, and bone morphogenetic protein 3. It consists of 13 exons and 12 introns varying in size from 39 to 1101 bp, and from 105 bp to approximately 2300 bp, respectively (Mårdh et al., 2001). The translation initiation codon is located at the 3' end of exon 1 (putative start site at 84 bp), and the translation stop codon is located in the middle of exon 13. Splice site sequences for each intron follow the 5'-gt...ag-3' rule. The human AMBN transcript is alternatively spliced, since cDNA clones of different sizes have been identified (MacDougall et al., 2000). The clones differ in a 45-bp fragment that is included or excluded. This stretch of amino acids is also alternatively spliced in the rat, mouse, and pig (Cerny et al., 1996; Hu et al., 1997b; Simmons et al., 1998). According to the human data, this fragment does not correspond to an independent exon. The 5' end of the 45-bp sequence is adjacent to intron 5, while the 3' end is within exon 6. This suggests that part of exon 6 can be excluded by the use of an alternative splice site in exon 6. The above observations differ from those of Toyosawa et al.(2000) with regard to: (a) the location of the start site, suggesting that the putative start codon is at 66 bp (exon 1); (b) the exon-intron sizes; and (c) the number of putative polyadenylation signals.
Ameloblastin, or amelin (Cerny et al., 1996), or sheathlin (Hu et al., 1997a) is present in the organic matrix and accounts for about 5% of the total protein. During investigations of pig enamel proteins, the N-terminal (Fukae and Tanabe, 1987b) and C-terminal ends (Fukae and Tanabe, 1987a; Yamakoshi et al., 2001) of ameloblastin were separately discovered due to their different biochemical properties. A comparison of the existing protein sequences for the human, pig, cattle, rat, and mouse showed high similarity (MacDougall et al., 2000), and the sequences share several features, such as the presence of potential phosphorylation sites, similar patterns of hydrophilicity, and a high proportion of Pro (15.2%), Leu (10.2%), and Gly (9%) residues. The human precursor protein is composed of 447 aa, the signal peptide is composed of 26 aa, and the mature protein of 421 aa. The fate of ameloblastin in the matrix shares similarities with that of the other proteins. Soon after secretion, the initial cleavages of the nascent ameloblastin (65 kDa) generate relatively small polypeptides containing the N-terminal and relatively large polypeptides containing the C-terminal. The N-terminal polypeptides appear to be rather stable and are gradually degraded, but not lost, during the matrix formation stage, while the C-terminal large polypeptides appear to be degraded rapidly and are soon lost from the matrix (Uchida et al., 1997; Brookes et al., 2001). Intact ameloblastin and its cleavage products containing the C-terminal are found only in the outer enamel, concentrated among the crystallites in the rod and interrod enamel (Murakami et al., 1997). Ameloblastin cleavage products containing the N-terminal side are found at all depths within the enamel layer, but they do not present a random distribution within the tissue; they concentrate in the sheath space (Uchida et al., 1995). The functional significance of this spatial distribution is not yet understood.
Enamelysin and Kallikrein-4 Genes and Proteins
The human enamelysin (MMP-20) gene, located in chromosome 11 (11q22.3-q23) (Llano et al., 1997), is comprised of 10 exons and 9 introns (Caterina et al., 2000), with sizes varying from 104 to 310 bp and 806 to 14,296 bp, respectively. The start codon (ATG) is located in exon 1, while the stop codon is located in exon 10 (Fig. 2B). To date, MMP-20 is considered a tooth-specific gene, since Northern blot analysis of RNAs that were obtained from multiple human tissues failed to show any positive hybridization signal with human enamelysin probes (Llano et al., 1997). Additionally, no other intact, physiologically normal, tissue has been demonstrated to express MMP-20, apart from ameloblasts, pre-ameloblasts, and odontoblasts (Bartlett, 2004), whereas the expression of MMP-20 in human odontogenic tumors and carcinoma cell lines originating from the tongue has recently been described (Bègue-Kirn et al., 1998; Caterina et al., 2000; Takata et al., 2000; Väänänen et al., 2001). The cloning and characterization of cDNAs from different species, including humans, pigs, mice, and cattle, has been reported (Llano et al., 1997; Bartlett et al., 1998; Den Besten et al., 1998; Caterina et al., 2000).
MMP-20 gene codes for a calcium-dependent (Bartlett et al., 1998) proteinase that is a member of the matrix metallopeptidases family (MMPs) (Rawlings et al., 2004). This protein family is characterized by a common domain structure, which also applies in the case of MMP-20. MMP-20 is divided into the following domains: signal peptide (1–22), propeptide (23–107), catalytic domain (108–271), linker (272–295), and hemopexin (296–483). The pre-proprotein has 483 aa, the proprotein has 461 aa, while the active protein has 376 aa (Simmer and Hu, 2002). However, MMP-20 possesses several unique structural features that define it as a novel MMP. First, the MMP-20 amino acid sequence contains no N-linked glycosylation sites (Bartlett et al., 1996). Second, it lacks two of the three residues important in defining the active site of a collagenase, and lacks all three of the residues important in defining the active site of a stromelysin (Bartlett et al., 1996; Llano et al., 1997). Third, MMP-20, in contrast to stromelysins and collagenases, has a unique hinge region of 24 residues that connects the catalytic domain to the hemopexin-like domain (Bartlett et al., 1996; Llano et al., 1997; Caterina et al., 2000). An amino acid sequence comparison between human MMP-20 (Llano et al., 1997) and that reported for porcine MMP-20 revealed a high percentage of identity (89%). The active protease migrates as a doublet at 46 kDa and 41 kDa on zymograms (Yamada et al., 2003). MMP-20 is the early protease, and it is expressed throughout the secretory stage and part of the maturation stage (Bartlett et al., 1996; Bègue-Kirn et al., 1998; Bartlett and Simmer, 1999). Immunohistochemical studies have showed the presence of enamelysin within the secretory enamel, with the greatest staining occurring adjacent to the secretory face of Tomes’ process (Fukae et al., 1998).
Kallikrein-4 (KLK4), or prostase, or EMPS1, or KLK-L1, or PRSS17 (serine proteinase 17) was first extracted in 1977 from developing porcine enamel (Fukae et al., 1977), and the first cDNA and protein sequence was deduced from a cDNA library (Simmer et al., 1998). Different groups have isolated the human cDNA from a variety of tissues, such as the central nervous system and the prostate (Nelson et al., 1999; Stephenson et al., 1999), and, more recently, from developing human tooth tissues (Simmer et al., 2000). The KLK4 gene is located near the telomere of chromosome 19 (19q13.3–19q13.4) downstream of the KLK2 gene, and is considered a member of the human tissue kallikrein gene family (Du Pont et al., 1999; Diamantis et al., 2000; Hu JC et al., 2000). A further characterization of the KLK4 gene, extending its 3' downstream sequence and identifying potentially important polymorphisms, has been reported (Hu JC et al., 2000). These investigators also identified an additional exon. The human DNA sequence is of 7115 bp and consists of 6 exons (5 of which are coding) and 5 introns. The exons vary in sizes from 72 to 251 bp, and intron sizes vary from 83 to 1357 bp. The translation initiation codon (ATG) is located in exon 2 (first coding exon) (660–662 bp), and the termination codon (TGA) is located in exon 6 (fifth coding exon) (5108–5110) (Fig. 2C). The sequences encoding the amino acids of the catalytic triad presented in the enzyme are located at coding exons 2, 3, and 5, respectively. KLK4 is expressed by both ameloblasts and odontoblasts (Hu JC et al., 2000; Nagano et al., 2003). A splice variant lacking exon 6 has been described in skin and endometrial tumor cultures (Myers and Clements, 2001; Komatsu et al., 2003), but its role has not yet been defined.
KLK4 protein is a calcium-independent serine protease. KLK4 is secreted as an inactive zymogen of 230 aa that becomes the active protein (224 aa) with the removal of a 6 aa propeptide by MMP-20 (Ryu et al., 2002). Human KLK4 has six disulfide bridges and one potential N-linked glycosylation site away from the active site of the enzyme. Of great importance for KLK4’s function is a triad of catalytic amino acids (H71, S207, and D116) (Komatsu et al., 2003). Despite its most recent official designation, KLK4 is no more closely related to the original kallikreins than it is to the trypsins. The kallikrein loop, a structural feature common to the kallikrein proteases, is absent in KLK4. A single amino acid insertion that is common to the pig, mouse, and human gene is absent from the kallikreins (Hu JC et al., 2000; Ryu et al., 2002). KLK4 is the late protease; its expression by ameloblasts begins in the transition stage and continues throughout enamel maturation (Hu JC et al., 2000, 2002). The KLK4 activity observed in the highly mineralized enamel at the dentino-enamel junction during the secretory stage is due to the gene’s expression from the odontoblasts (Fukae et al., 2002). KLK4 is responsible for the degradation of the TRAP amelogenin cleavage product to smaller fragments.
DLX3 Gene and Protein
The DLX3 gene is a member of the family of homeobox genes that are homologous to the distalless (Dll) gene of Drosophila, known to be expressed during development of the chondrocranium, dermatocranium, sensory organs, brain, limbs, and appendages, and in the processes of osteogenesis and hematopoiesis (Robinson and Mahon, 1994; Weiss et al., 1998). The mammalian genes take the form of bigene clusters, namely, Dlx2-1, Dlx5-6, and Dlx3-7. Scherer et al.(1995) mapped the human homologue of the mouse Dlx3 gene to 17q21.3-q22, which consists of 3 exons, with the homeodomain contained in exons 2 and 3. Exons 1 and 2 are separated by a 1.1-kb intron; exons 2 and 3 are separated by a 1.6-kb intron (Fig. 2D). The start codon is located in exon 1, while the stop codon is in exon 3 (Price et al., 1998a). The encoded DLX3 human protein (GenBank accession number NP_005211) is a 31738-Da protein composed of 287 aa with a 60 aa homeodomain (129–188 aa). As with all DLX proteins, it shares similar DNA-binding sites and is thought to act as a homeodomain transcription factor, having N-terminal and C-terminal regions that act as transcriptional activators (Bryan and Morasso, 2000). Therefore, the presence of these proteins is considered critical for craniofacial, tooth, brain, hair, and neural development.
Ameloblastin, Enamelysin, Kallikrein-4, and DLX3 in Amelogenesis Imperfecta
AMBN gene loci are on chromosome 5 in the mouse (Krebsbach et al., 1996) and chromosome 4q21 in humans (MacDougall et al., 1997). AMBN maps within the critical region for autosomal-dominant AI, and therefore is considered as a candidate gene. However, it was excluded from a causative role by mutational analyses within the families studied by Mårdh et al.(2001). As far as KLK4 is concerned, Hart et al.(2004) identified the mutation (g.2142G>A) (Fig. 2C) that resulted in a truncated protein containing 152 aa and lacking the S207 site, which, as we previously mentioned, is essential for the function of the enzyme. Due to the abnormal enzymic activity, the crystallites of the enamel grew to the normal length but incompletely in thickness (Hart et al., 2004). This is the first report of mutation in this gene found to cause ADAI. Kim et al.(2005b) identified a homozygous mutation in the MMP-20 gene in two affected members of a family with autosomal-recessive pigmented hypomaturation AI. The mutation destroyed the splice acceptor at the 3' end of intron 6 (AG
TG) and resulted in a hypomatured enamel product. Two defective splicing outcomes seem to be most probable, both of which would introduce an upstream translation termination codon in the mRNA transcript. In the first scenario, the retention of intron 6 in the mRNA would generate a large mRNA with translation terminating in the first complete codon of the retained intron (p.I1319X). In the second splicing scenario, exon 7 would not be recognized as an exon and would be skipped. Skipping exon 7 would shift the reading frame after R318 and would introduce 19 extraneous aa before terminating translation (p.I1319fs338X) (Kim et al., 2005b) (Fig. 2B). A comparison of the dental phenotypes of the KLK4 and MMP-20 probands shows that they share many similar features (Kim et al., 2005b).
Dong et al.(2005) demonstrated that a mutation within the human DLX3 gene homeodomain is associated with amelogenesis imperfecta (hypoplastic-hypomaturation type), with taurodontism (AIHHT). This 2-bp deletion (CT) in exon 3 (Fig. 2D) causes a frameshift within the last two amino acids of the homeodomain, with a premature stop codon truncating the protein by 88 aa. This is the first report of a mutation within the homeodomain of DLX3. Previous studies have shown a DLX3 mutation, outside the homeodomain, associated with trichodento-osseous syndrome (TDO) (Price et al., 1998a,b). Dong et al.(2005) suggested that TDO and some forms of AIHHT are allelic. However, this requires further investigation.
However, AMELX, AMBN, ENAM, KLK4, and MMP-20 were excluded from a causative role in two families with autosomal-dominant hypocalcified AI, suggesting that this type of AI is caused by mutation of a gene that is either not known or not considered to be a major contributor to enamel formation (Hart et al., 2003b).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONAMELOGENESIS IMPERFECTAFROM GENES TO PROTEINS...CONCLUSIONSREFERENCES |
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Received January 25, 2005; Accepted May 19, 2005
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in diagram), and the translation stop codon is at the beginning of exon 7 (view shape 
in diagram). Vertical arrows show the location of the mutations, while the branched boxes include the genomic DNA (upper row) and the mutated protein (lower row). The phenotypic effects of these proteins, as well as the references where each mutation is described, are depicted below the gene representation.