sign in sign up
<<
Home , Anodontia

Braz J Oral Sci. April/June 2005 - Vol. 4 - Number 13

Hypodontia: genetics and future perspectives

Trevor J Pemberton1 Parimal Das3 Abstract Pragna I Patel1,2 development is a complex process of reciprocal interactions 1Institute for Genetic Medicine and the 2Center that we have only recently begun to understand. With the large number for Craniofacial Molecular Biology, Keck School of genes involved in the odontogenic process, the opportunity for of Medicine, University of Southern California, mutations to disrupt this process is high. Tooth agenesis () 3 CA-USA and Department of Surgical Oncology, is the most common craniofacial malformation with patients missing M.D. Anderson Cancer Center, Houston, TX-USA anywhere from one tooth to their entire dentition. Hypodontia can occur in association with other developmental anomalies (syndromic) or as an isolated condition (non-syndromic). Recent advances in genetic techniques have allowed us to begin understanding the genetic processes that underlie the odontogenic process and to identify the mechanisms responsible for tooth agenesis. Thus far two genes have been identified Received for publication: February 14, 2005 by mutational analysis as the major causes of non-syndromic Accepted: May 10, 2005 hypodontia; PAX9 and MSX1. Haploinsufficiency of either has been observed to cause the more severe forms of hypodontia whilst point mutations cause hypodontia to varying degrees of severity. With the prevalence of hypodontia having been observed to have increased during the 20th century, the future identification and analysis of its genetic basis is essential to allow us to better treat the condition. The clinician can facilitate this process by collaborating with the human geneticist and referring patients/families with familial hypodontia for investigative research.

Key Words: hypodontia, tooth agenesis, PAX9, MSX1, prevalence, mutations

Correspondence to: Pragna I Patel Institute for Genetic Medicine, Keck School of Medicine, University Of Southern California, 2250 Alcazar Street, CSC-240, Los Angeles, CA 90033, USA. Tel: +1 (323) 442 2751 Fax: +1 (323) 442 2764 E-mail: [email protected]

695 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

Introduction The online catalog of inherited human diseases (Online The smile is a unique facial expression distinct to primates. The Mendelian Inheritance in Man; OMIM18) currently lists main physical component of the smile is a complete dentition almost 16000 inherited human disease genes. However, the set comprising four different types of teeth. Vertebrate molecular etiology for only about 2000 of these has been comparative histology has indicated that the continued determined successfully. The cloning of disease genes is evolution of teeth throughout the emergence of modern man is the first step in understanding the detailed molecular basis due to the increased fitness they have offered us. Our modern of the disease that ultimately facilitates the development of lifestyle also has a special attachment to a complete set of suitable diagnostic and therapeutic agents. There are mainly dentition aside from their use as merely mastigatory appendages two different strategies that have been employed for for food. For this reason, naturalists, biologists and dentists identifying human disease genes: functional cloning and have for a long time been trying to unravel the cause for the positional cloning (Figure I). As the name implies, functional congenital loss of teeth leading to several clinical phenotypes cloning identifies genes based on the known biochemical such as hypodontia, oligodontia and anodontia. function of their encoded proteins. For example, the While a number of clinical studies have been carried out on identification of the gene responsible for phenylketonuria disorders that involve the congenital lack of teeth1-11, until was achieved by purifying the mRNA for phenylalanine recently very little effort has been made to understand the hydroxylase, the enzyme normally responsible for the genetic component responsible for mammalian tooth oxidation of phenylalanine to tyrosine but absent in development. Advancements in molecular biology sufferers, and using this to screen a cDNA library for the approaches coupled with the now complete human genome corresponding DNA sequence19 which facilitated the sequence12 has allowed a number of putative disease genes/ identification of its chromosomal location20-21. However, in loci associated with the hypodontia/oligodontia phenotypes reality for the vast majority of inherited disorders, knowledge to be identified6,13-16. Functional studies of these disease about their basic biochemical defect is unknown making the genes have started to reveal their precise role in tooth use of functional cloning impossible which has led to a development allowing us to better understand their role in second strategy, positional cloning. disease pathology and the molecular morphogenetic fields The positional cloning approach employs one or both of the within which they function17. following strategies; (a) analysis of genetic linkage in families The congenital lack of one or more is a with a disease and/or (b) identification of a specific common anomaly in man. By definition, congenitally missing chromosomal aberration(s) in the diseased individual. teeth are those that fail to erupt in the oral cavity and remain Successful genetic analysis depends on the following invisible in a radiograph, which implies that this is caused requirements; (a) identification of large families segregating by disturbances during the early stages of tooth the disease phenotype, (b) a distinctive diagnostic criterion development. These phenotypes most frequently involve to distinguish the affected individuals, (c) accurate the second premolars and upper lateral and assessment of the individuals of the family for a known commonly lead to mild phenotypes that have no associated Mendelian or complex pattern of segregation and (d) highly systemic disorder. The term hypodontia has most frequently polymorphic DNA markers. been used for describing the phenomenon of congenitally In principle, the basis of linkage analysis is to observe the missing teeth, although a large number of missing teeth is closeness of two alleles for two different genes on a defined as oligodontia and the complete absence of teeth is chromosome. The closer the two alleles are to each other, defined as anodontia. Hypodontia and oligodontia are also the more likely they are to segregate together during meiosis classified as either nonsyndromic (isolated) or syndromic due to the reduced chance of a recombinatorial event (associated with their syndromes). A literature survey shows occurring between them. Logically, this principle is applied various other terminologies describing a reduction in teeth to determine the genetic distance of known genetic markers, number; teeth aplasia, congenitally missing teeth, absence or “site locators”, from a disease locus by tracing its of teeth, agenesis of teeth, and lack of teeth. Although no segregation pattern in affected families. In figure II, the distinct definition and classification exists in literature, the disease locus D is closer to the marker locus represented by following definitions have been widely used in scientific allele X/x but distant from the locus represented by Y/y. Due literature: to this, the disease locus co-segregates with the X locus 1. Hypodontia: one of six missing teeth excluding 3rd whereas it shows random segregation pattern with respect 2. Oligodontia: More than six missing teeth excluding 3rd to the Y locus. Essentially, good genetic markers are those molar which are polymorphic and can be easily assayed in genomic 3. Anodontia: Complete absence of teeth DNA isolated either from lymphocytes or buccal cells. The traditional definition of polymorphism is a natural Identifying disease genes variation in nucleotide sequence which is observed in at 696 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

least 1% of a population and is capable of distinguishing the parental chromosomes which otherwise seem virtually identical in every respect. The degree of polymorphism of the genetic marker has direct bearings in scoring the distinction. The information on segregation of polymorphic markers with respect to the disease phenotype is then analyzed using statistical programs in order to derive a LOD score. LOD score, or the logarithm to the base 10 of the odds, is defined as the relative probability of the observed data being the result of true linkage versus the probability due to chance. Conventionally, linkage is considered established if the LOD score at any given recombination fraction (θ) is equal to or greater than 3, while linkage is excluded if the LOD score is equal to or less than –2. Statistically, a LOD score of 3 means that it is 103, or 1000 times more likely that the observed pattern of segregation of one marker with respect to another marker or disease Fig. II: Methodology of positional and functional cloning of a single gene defect. locus occurred because of linkage rather than chance. Multipoint linkage analysis for determining the disease gene position with respect to a number of genetic markers in the candidate region is then used to localize the position of the disease gene between two flanking genetic markers. The success Tooth Development of positional cloning efforts requires very tightly linked markers. In mammals, tooth development is a complex process with An alternative approach to associate gene(s) with a genetic reciprocal interactions between the dental epithelium and disorder is to analyze mutations in candidate genes, those mesenchyme involving the shifting of the odontogenic that have either a proven or speculated association either potential between these tissues (figure III). The first sign of directly or indirectly leading to the disease state. With the tooth development is the appearance of the primary epithelial advancement of our present day knowledge about disease band within which the odontogenic process initiates with pathology and its consequences at both the biochemical the formation of an epithelial bud. Mesechymal cells then and physiological levels, mapping human disease genes by differentiate around the bud to form the dental papilla, the the candidate gene approach plays an important role precursor of the tooth pulp and dentin-secreting especially in excluding the involvement of a gene for a given odontoblasts that appear after a few additional soft-tissue phenotype. Using this approach, the association of multiple phases that include the cap stage leading to the bell stage growth factor and growth factor receptor gene(s) with when the enamel-depositing ameloblasts are formed. The congenital teeth agenesis has been excluded22. dentinal matrix then forms at the periphery of the dental papilla during dentinogenesis and subsequently enamel deposition, or amelogenesis, occurs at the dentino-enamel junction after a few micrometers of dentin has been deposited. Finally, apposition of dentin and enamel gives way to tooth eruption and function. Transcription factors and signaling molecules, which operate both intra- and extra-cellularly, are expressed in a spatially- and temporally-restricted pattern in the epithelium and mesenchyme tissues throughout the odontogenic process and guide tooth development (figure III). Tissue- recombination experiments have helped greatly in developing our understanding of the hierarchy and roles of the various factors with the odontogenic process23-24. BMP4, a member of the transforming growth factor-β (TGF-β) Fig. I: Segregation of loci within a two generation pedigree. The family, and the transcription factors PAX9 and MSX1, disease locus is labelled “N” for normal or “D” for disease with the members of the paired-box domain and homeobox domain two marker loci marked as X/x or Y/y where the change in case indicates different alleles at that position. Allele x appears to gene families respectively, are examples of controlling factors segregate with the disease (D) given their close proximity. during the odontogenic process. The odontogenic potential 697 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

shifts from the epithelium to the dental mesenchyme missing tooth is reported to be the maxillary lateral concomitantly with BMP4 expression25. Both PAX9 & MSX1 by some investigators30,35-36 or the mandibular second are expressed in the dental mesenchyme and their expression premolar by others37-38. The lowest incidence of tooth is key to maintaining the odontogenic potential following agenesis occurs in the lower central and lateral permanent this shift25. PAX9 has been identified as a key controlling incisors with agenesis of maxillary permanent central incisors, factor during the odontogenic process with its expression maxillary permanent cuspids and maxillary permanent first found specifically at the prospective sites of all teeth prior molars also rare39. to there being any morphological signs of odontogenesis25. Hypodontia affecting the primary dentition is rare (prevalence A general role for MSX1 in the development of ectodermal rate of <0.5%) and has been observed to afflict both sexes derivatives has been suggested14 with it strongly expressed equally39-40. It is often followed by hypodontia in the same in the dental mesenchyme but notably absent from the dental region of the permanent dentition, which itself has a epithelia during the bud, cap and bell stages of tooth prevalence, excluding third molars (wisdom teeth), ranging development26. Tooth development in both PAX9- and MSX1- between 2.3% and 10%41 and a third of sufferers typically mutant mice is arrested at the bud stage27-28, suggesting they have at least one first-degree relative also afflicted40,42. Severe have similar, non-redundant roles in signal progression to hypodontia, also referred to as oligodontia, involves the the cap stage of tooth development. Interestingly, PAX9 agenesis of six or more teeth and like hypodontia of the and MSX1 have been reported to have an important primary dentition it is rare afflicting approximately 0.5% of regulatory role in the maintenance of BMP4 expression and the population43. signaling25 implying they may also have a role in odontogenic Hypodontia has been identified as both non-syndromic, where it is an independent congenital oral trait, or syndromic, where it is acquired as part of a specific disease. It is an associated finding in at least 49 syndromes listed in the Online Mendelian Inheritance in Man database18 implying some factors involved in tooth development have a wider role within the human body. Other anomalies associated with hypodontia include small tooth size (), large tooth size () and anomalies in tooth shape, most commonly tapering or “peg-shaped” teeth40,44. The non-syndromic form of hypodontia can be sporadic or familial and it has been most frequently reported as inherited in an autosomal dominant45-52 (AD) fashion where it displays phenotypic heterogeneity as measured by the nature of the missing teeth and other alterations in the teeth. However, autosomal recessive53 (AR), X-linked54-56 and polygenic57-60 Fig. III: Known protein factors involved in tooth development. Names in bold indicate that they have only been identified as involved at inheritance has also been reported. one stage and names in italics indicate those factors that have been found to be involved in signalling from both the epithelium and Geographical, population and gender prevalence mesenchyme. Hypodontia potential shifts. Variation is seen in the number of teeth found in both the primary and permanent dentition, although it is less common Clinical features of Hypodontia in the primary dentition39, 41. Several population studies have A congenital anomaly affecting the formation of the dentition been carried out in the past to establish the epidemiological, that results in a reduction in the usual number of the human clinical and genetic characteristics and prevalence of permanent dentition (a total of 32 teeth in both jaws) and/or hypodontia both in primary and permanent dentition through the deciduous dentition (20 total teeth in both jaws) is the collection of the dental history of individuals belonging commonly referred to as hypodontia. Recent studies have to several families representing various countries/ shown that the occurrence of hypodontia has increased geographical locations and races. Table I shows the during the 20th century29. 80-85% of hypodontia cases summarized data representing the prevalence of hypodontia studied involved the agenesis of just one or two teeth30, in primary and permanent dentition in various geographical indicating that most people afflicted suffer from a mild form locations. It is apparent from these studies that the population of the disease. The tooth most commonly missing is the prevalence of hypodontia varies with geographical location. third molar (or ) which is absent in as much as In the primary dentition, Japan shows the greatest prevalence 20% of the population2,31-34. The second most commonly of hypodontia which is almost three times greater than the 698 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

next highest, Finland. Great Britain shows the lowest this63. The data in (table II) shows no clear pattern of gender prevalence which is an eighth of that of Japan with the preference with fluctuations between male and female bias remaining countries in table I also showing low prevalences where most studies show ratios that are close to parity. between 0.4-0.6%. In the permanent dentition, Saudi Arabia shows the lowest prevalence whilst Iceland exhibits the Oligodontia highest. The greatest differences are found in the The congenital lack of more than six permanent teeth populations of both Iceland and Sweden which exhibit a (oligodontia; a severe hypodontia) is also quite prevalent in prevalence of hypodontia in their permanent dentition that the population. Studies on different populations have shown is 16 or 19 (respectively) times greater than in their primary variation in the prevalence of oligodontia with the difference dentition. Europe has a prevalence of hypodontia in the in the frequency of oligodontia between males and females permanent dentition that is at least ten times greater than found to be not statistically significant64-65, nor is the any of the European component countries have exhibited in difference in distribution of missing teeth over mandible/ the primary dentition. This pattern is also seen between the maxilla and right/left sides64-65. Collective data from six primary dentition of New Zealand and the permanent dentition different studies does however indicate that the frequency of its neighbor country Australia, whereas Japan has a of oligodontia is lower in males than females and that the prevalence in its primary dentition that is only a third of that frequency of missing second premolars or upper lateral in the permanent dentition of its neighbor country China. incisors is higher in congenital oligodontia66. Interestingly, Saudi Arabia exhibits an equal prevalence of hypodontia in both its primary and permanent dentition. It Etiology therefore appears that people of Scandinavian decent are Both genetic and environmental factors have been found to the most susceptible to hypodontia in the permanent contribute to the etiology of tooth agenesis with many dentition whilst those of Asian or Arabic descent are the theories having been suggested to explain their affects, most susceptible in the primary dentition. There are particularly prior to the intensive genetic studies performed apparently no published reports of large scale studies of in recent years38,49,67-68. populations in Latin American countries. A number of investigations have attempted to take into Environmental Factors account any possible gender preference in tooth agenesis Environmental factors can cause tooth agenesis by a variety (table II). Several reports mentioned a lower prevalence of of means69 that can be broadly placed into two groups: tooth agenesis in males30,38,61 with one study reporting a male invasive and non-invasive. These can act either additively to female ratio62 of 2:3 whilst others have failed to confirm or independently to affect the positioning and physical

Table I: Prevalence of hypodontia in the primary and permanent dentition in different countries/continents.

699 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

Table II: Studies comparing gender bias of tooth agenesis (excluding third molar) in the permanent dentition in various countries.

nr = not reported development of the tooth. development of the main innervation paths75-76 where it was Jaw fractures, surgical procedures, extraction of the noted that the regions most commonly affected by preceding primary tooth and changes in muscle pressure hypodontia were the last to undergo innervation. Brainstem from the facial and lingual sides are all examples of invasive anomalies have been shown not to affect tooth factors that can affect tooth development and positioning development77 indicating that it is local rather than global leading to tooth agenesis and impaction38,49,65. nerve development that affects tooth agenesis. It has also been shown that developing teeth are irreversibly affected by chemotherapy and irradiation in an age- and dose- Genetic Factors dependent manner, with the latter having been shown to In the majority of cases, hypodontia has a genetic basis. cause the more severe effects4-5. Thalidomide (N- Tooth agenesis is found more commonly among individuals phthaloylglutamimide) has been reported to cause related to hypodontia patients than in the population in congenitally missing teeth in children whose mothers took it general56 identifying it as a genetic disease. An exhaustive during their pregnancy1,70-71. Nutrient deprivation and serious study on a Swedish family with 685 family members, including illness have also been linked to tooth developmental 171 probands affected with hypodontia, showed that problems, although no definite etiological relationship has hypodontia involving permanent teeth is primarily been found between hypodontia and systemic diseases38,49, determined by genetic factor(s)38. The frequency of endocrine disturbances72 or ectodermal dysplasia73. hypodontia among races varies30,78 and greater concordance A developmental relationship has been proposed between of hypodontia is apparent in identical twins than non- nerve and hard tissues74 with tooth agenesis linked to the identical79-80 with no environmental etiology apparent in 700 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

afflicted individuals. of positional candidate genes present in the candidate Familial hypodontia is reported to exhibit mainly autosomal interval. Using this gene mapping strategy, autosomal dominant inheritance with incomplete penetrance and dominant hypodontia has been localized to at least three variable expressivity49-52. However, an autosomal recessive chromosomal loci to date; MSX183, PAX913 and an unknown mode of inheritance for hypodontia has been reported in a locus on chromosome 106. Five mutations have thus far Pakistani family which mapped to chromosome 16q12.153 and been identified within MSX1 and ten within PAX9 (table III) in another report on Finnish patients that are afflicted with a with both genes also having been found to be deleted in specific type of hypodontia (Recessive Incisor Hypodontia; separate studies of familial hypodontia45,84. RIH), where patients notably lacked both deciduous and permanent incisors81. It has also been suggested that it can MSX1 follow sex-linked42,59,82 (Patel et al., unpublished results) or Although one report has excluded the MSX1 gene as the polygenic inheritance patterns54-55,57-58. gene responsible for tooth agenesis85, recent research has Recently, direct evidence was gathered for the genetic basis identified MSX1 as the causative gene for some forms of of tooth agenesis thanks to the mapping of human disease congenital teeth agenesis with five mutations having been genes using linkage analysis followed by mutation analysis identified within MSX1 thus far (table III). All of these

Table III: Identified mutations in MSX1 and PAX9 that have been found in people afflicted with hypodontia. Tooth is considered absent if not present in >50% of patients. Mutation nomenclature used as previously described 124 and ∆ is used to denote gene deletion or protein absence.

= tooth present; = tooth absent in both; = tooth absent in either mandibular/maxillary; AD = autosomal dominant.

701 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

mutations have been point mutations with two leading to a substitution mutations disrupt MSX1 activity they do not substitution mutation within the protein and the remaining abolish it whereas the truncation mutations appear to abolish three form a stop codon that prematurely truncates the MSX1 activity leading to a more severe phenotype. This is protein. Two mutations fall within the N-terminal region prior supported by another study that identified to the central homeodomain (M61K & S105X) with the haploinsufficiency of MSX1 as the cause of severe remaining three (Q187X, R196P & S202X) all falling within oligodontia within unrelated Finnish patients who also had the homeodomain itself (figure 4(A)). Wolf-Hirschhorn syndrome84. However, there is no clear Of the two substitution mutations, the M61K mutation15 falls correlation between the severity of the hypodontia and the outside of the homeodomain of MSX1 and how it affects its severity of the effect on the MSX1 protein caused by the function remains unknown but it has been proposed that it identified missense mutations. may be through the disruption of protein interactions15. The R196P mutation83 falls within helix-I of the MSX1 PAX9 homeodomain disrupting its stability and functional In contrast to MSX1, both missense and frame-shift mutations activity86. in PAX9 have been associated with hypodontia (table III). Of the three premature termination mutations, S105X is the Of the seven missense mutations identified to date, one is a only mutation to occur prior to the homeodomain of MSX1 premature termination mutation (K114X) and the remaining (figure 4(A)). It was identified in a Dutch family suffering six are all residue substitution mutations. Of these latter from cleft lip-palate and hypodontia that were found to be mutations, only five generate a substitution in the protein heterozygous for the 314C>A nucleotide substitution, which (L21P, R26W, R28P, G51S & K91E) with one believed to creates a stop codon in MSX1 exon 1 truncating the protein prevent PAX9 expression (1A>G). Three frame-shift prior to the homeodomain87. The remaining two termination mutations have been identified, two of which are caused by mutations fall within the central region of the MSX1 the insertion of a single nucleotide (G73fsX316 & homeodomain. A 559C>T nucleotide substitution was V265fsX316) and the other by the deletion of 8 nucleotides identified in a Flemish family suffering from cleft lip-palate with the insertion of 288 foreign nucleotides (R59fsX177). and hypodontia where it forms a stop codon that truncates All but one of these mutations (V265fsX316) falls within the the protein within helix-I of the MSX1 homeodomain (N187X; N-terminal paired-domain (Figure 4(B)) with the exception figure 4(A))88. The S202X mutation, caused by a 605C>A falling in the approximate middle of the C-terminal region. nucleotide substitution, was identified in a patient with Witkop This single nucleotide insertion falls within exon 4 of the syndrome who was suffering from hypodontia89 where it PAX9 gene creating a frame-shift at amino acid 264 (Figure generates a stop codon in helix-I of the homeodomain region 4(B)) which leads to premature truncation of the protein91. (figure 4(A)) truncating the protein and disrupting its The other single nucleotide insertion was a guanine nucleotide that extends a series of five guanines to six causing a frame-shift between the N- and C-terminal DNA binding domains of the PAX9 paired-domain (Figure 4(B)) abolishing the C-terminal DNA binding domain13. By far the most severe frame-shift mutation is caused by a 288bp insertion in the paired domain of PAX9 (Figure 4(B)) in place of eight deleted nucleotides which leads to a frame-shift that disrupts the C-terminal DNA binding region of the paired- domain47. Two nucleotide substitutions within the PAX9 paired domain were also identified during this study (L21P and K91E), both of which fall within the DNA binding regions Fig. 4: Location of the mutations stated in table III in the MSX1 and of the PAX9 paired domain (Figure 4(B)), N- and C-terminal PAX9 proteins. respectively, which may affect PAX9’s ability to associate with DNA and thus its transcriptional activity. functional activity86,90. The only substitution mutation to cause premature Interestingly, two of the premature termination mutations termination was an A340T switch that creates a stop codon at were identified in patients afflicted with both oligodontia lysine 114 producing a truncated PAX9 protein that terminates and cleft lip-palate (S105X & Q187X) with the third mutation at the end of the N-terminal DNA binding region of the PAX9 having been identified in an individual with Witkop tooth- Paired-box domain92 (figure 4(B)). The remaining three nail syndrome (S202X). They all suffer from additional missense mutations that lead to a residue substitution in the pathologies other than hypodontia which also afflicts those PAX9 protein were all identified recently. An R26W mutation with substitution mutations indicating that that whilst the was identified in the N-terminal DNA binding region of the 702 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

PAX9 paired domain which has been hypothesized to affect effect on dentition. Dtsch Zahnarztl Z 1965; 20: 1278-83. its target DNA specificity93. A mutation two residues further 2. Shapiro SD, Farrington FH. A potpourri of syndromes with into the N-terminal DNA binding region was identified (R28P) anomalies of dentition. Birth Defects Orig Artic Ser 1983; 19: 129-40. and shown to dramatically reduce the DNA binding ability 3. Bella G, Caltabiano M, Russo S, Messina G. Statistical study of PAX994. The final missense mutation (G51S) lies within of the incidence of agenesis in a sample of 447 cases of the boundary region between the N- and C-terminal DNA dysgnathia. Minerva Stomatol 1984; 33: 609-14. binding regions of the PAX9 paired domain95. 4. Maguire A, Craft AW, Evans RG, Amineddine H, Kernahan J, Macleod RI. The long-term effects of treatment on the Haploinsufficiency of PAX9 has been reported in two studies dental condition of children surviving malignant disease. although its cause has been by two drastically different Cancer 1987; 60: 2570-5. mechanisms. One study identified a nucleotide substitution 5. Näsman M, Forsberg C-M, Dahllöf G. Long-term dental in the first position of the ATG start codon that has been development in children after treatment for malignant disease. Eur J Orthod 1997; 19: 151-9. hypothesized to abolish its expression48 whilst another 6. Liu W, Wang H, Zhao S, Zhao W, Bai S, Zhao Y, et al. The 45 identified a deletion of the entire PAX9 gene . novel gene locus for agenesis of permanent teeth (He- All but one of the PAX9 mutations appear to disrupt the Zhao deficiency) maps to chromosome 10q11.2. J Dent DNA binding ability of its paired domain, thus reducing its Res 2001; 80: 1716-20. transcriptional activity, which appears to be the likely cause 7. Ranta R. A Review of tooth formation in children with cleft lip/palate. Am J Orthod Dentofacial Orhop 1986; 90: 11-8. of the hypodontia phenotype associated with them. It is 8. Pinheiro M, Freire-Maia N. Ectodermal dysplasias: a clinical interesting to note that most of the PAX9 frame-shift, deletion classification and a causal review. Am J Med Genet 1994; and missense termination mutations cause hypodontia in 53: 153-62. both the permanent and the primary dentition, whereas 9. Shapira J, Chaushu S, Becker A. Prevalence of tooth transposition, third molar agenesis, and maxillary canine missense substitution mutations affect the permanent impaction in individuals with Down syndrome. Angle Orthod dentition only. 2000; 70: 290-6. 10. Mhanni A, Cross H, Chudley AE. Kabuki syndrome: Locus 10q11.2 description of dental findings in 8 patients. Clin Genet A study on He-Zhao deficiency, a distinct form of permanent 1990; 56: 154-7. 11. Karlstedt E, Kaitila I, Pirinen S. Phenotypic features of teeth agenesis which is different from other previously dentition in diastrophic dysplasia. J Craniofac Genet Dev described disorders, in members of a large Chinese kindred Biol 1996; 16: 164-73. has identified an unknown locus on chromosome 10q11.2 12. Venter JC. The Sequence of the Human Genome. Science using multipoint linkage analysis6. 2001; 291: 1304-51. 13. Stockton DW, Das P, Goldenberg M, D’Souza RN, Patel With the increase in prevalence of hypodontia observed over PI. Mutation of PAX9 is associated with oligodontia. Nat th the 20 century, the identification of its causative factors is Genet 2000; 24: 18-9. essential for providing treatment to those afflicted in the 14. Vieira AR, Meira R, Modesto A, Murray JC. MSX1, PAX9, future. Modern molecular genetic techniques have allowed and TGFα Contribute to Tooth Agenesis in Humans. J Dent us to start to identify the genetic factors responsible for Res 2004; 83: 723-7. 15. Lidral AC, Reising BC. The Role of MSX1 in tooth agenesis but more work is required to discover how Agenesis. J Dent Res 2002; 81: 274-8. malfunctions in these factors disrupt tooth development. 16. Line SR. Variation of tooth number in mammalian dentition: The identification of more families afflicted with hypodontia connecting genetics, development, and evolution. Evol Dev is key to identifying the molecular processes that underlie 2003; 5: 295-304. 17. Line SRP. Molecular morphogenetic fields in the this pathology and our knowledge of tooth development. development of human dentition. J Theor Biol 2001; 211: More patients/families presenting familial hypodontia need 67-75. to be referred to the geneticist for investigative research to 18. Online Mendelian Inheritance in Man, OMIM (TM); 2000. increase the known population of mutations affecting the Available from: URL: http://www.ncbi.nlm.nih.gov/omim. dentition. To achieve this, dental professionals need to 19. Robson KJ, Chandra T, MacGillivray RT, Woo SL. Polysome immunoprecipitation of phenylalanine hydroxylase mRNA collaborate with human geneticists like ourselves after the from rat liver and cloning of its cDNA. Proc. Nat. Acad. identification of probands presenting familial hypodontia and Sci. USA 1982;79(15):4701-05. join in our gene discovery efforts. 20. Lidsky AS, Robson KJ, Thirumalachary C, Barker PE, Ruddle FH, Woo SL. The PKU locus in man is on chromosome 12. Am. J. Hum. Genet. 1984;36(3):527-33. Acknowledgements 21. Lidsky ASL, M.L., Morse HG, Kao FT, Rabin M, Ruddle Work in the authors’ laboratory was supported by NIH grant FH, Woo SL. Regional mapping of the phenylalanine DE014102 (to PIP). hydroxylase gene and the phenylketonuria locus in the human genome. Proc Nat Acad Sci USA 1985; 82: 6221-5. 22. Arte S, Nieminen P, Pirinen S, Thesleff I, Peltonen L. References Gene defect in hypodontia: exclusion of EGF, EGFR, and 1. Schubel F, Partsch CJ. Thalidomide-embryopathies and their FGF-3 as candidate genes. J Dent Res 1996; 73: 1346-52. 703 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

23. Mina M, Kollar EJ. The induction of odontogenesis in severe hypodontia. J Oral Rehabil 1980; 7: 289-98. non-dental mesenchyme combined with early murine 44. McKeown HF, Robinson DL, Elcock C, Al-Sharood M, mandibular arch epithelium. Arch Oral Biol 1987; 32: 123-7. Brook AH. Tooth dimensions in hypodontia patients, their 24. Lumsden AG. Spatial organization of the epithelium and unaffected relatives and a control group measured by a new the role of neural crest cells in the initiation of the image analysis system. Eur J Orthod 2002; 24: 131-41. mammalian tooth germ. Development 1988; 103: 155-69. 45. Das P, Stockton DW, Bauer C, Shaffer L, D’Souza RN, 25. Neubuser A, Peters H, Balling R, Martin GR. Antagonistic Wright JT, et al. Haploinsufficiency of PAX9 is associated interactions between FGF and BMP signaling pathways: a with autosomal dominant hypodontia. Hum Genet 2002; mechanism for positioning the sites of tooth formation. 110: 371-6. Cell 1997; 90: 247-55. 46. Goldenberg M, Das P, Messersmith M, Stockton DW, Patel 26. MacKenzie A, Leeming GL, Jowett AK, Ferguson MWJ, PI, D’Souza RN. Clinical, Radiographic, and Genetic Sharpe PT. The homeobox gene 7.1 has specific regional Evaluation of a Novel Form of Autosomal-dominant and temporal expression patterens during early murine Oligodontia. J Dent Res 2000; 79: 1469-75. craniofacial embryogenesis, especially tooth development 47. Das P, Hai M, Elcock C, Leal SM, Brown DT, Brook AH. in vivo and in vitro. Development 1991; 11: 269-85. Novel missense mutations and a 288-bp exonic insertion 27. Peters H, Neubuser A, Kratochwil K, Balling R. Pax9- in PAX9 in families with autosomal dominant hypodontia. deficient mice lack pharyngeal pouch derivatives and teeth Am J Med Genet 2003; 118: 35–42. and exhibit craniofacial and limb abnormalities. Genes Dev 48. Klein ML, Nieminen P, Lammi L, Niebuhr E, Kreiborg S. 1998; 12: 2735-47. Novel Mutation of the Initiation Codon of PAX9 Causes 28. Satokata I, Maas R. Msx1 deficient mice exhibit cleft palate Oligodontia. J Dent Res 2005; 84: 43-7. and abnormalities of craniofacial and tooth development. 49. Arte S. Phenotypic and Genotypic features of Familial Nat Genet 1994; 6: 348-56. Hypodontia. Doctoral Thesis. Helsinki, Finland: University 29. Mattheeuws N, Dermaut L, Martens G. Has hypodontia of Helsinki; 2001. increased in Caucasians during the 20th century? A meta- 50. Arte S, Nieminen P, Apajalahti S, Haavikko K, Thesleff I, analysis. Eur J Orthod 2004; 26: 99-103. Pirinen S. Characteristics of incisor-premolar hypodontia 30. Muller TP, Hill IN, Petersen AC, Blayney JR. A survey of in families. J Dent Res 2001; 80: 1445-50. congenitally missing permanent teeth. J Am Dent Assoc 51. Phillip MJ, Caurdy JC. Inheritance of Hypodontia in 1970; 81: 101-7. Consanguineous Families of Arabic Decent. Ann Dent 1985; 31. Lavelle CL, Ashton EH, Flinn RM. Cusp pattern, tooth 44: 39-41. size and third molar agenesis in the human mandibular 52. Tal H. Familial hypodontia in the permanent dentition: a dentition. Arch Oral Biol. 1970; 15: 227-37. case report. J Dent 1981; 9: 260-4. 32. Schalk-van der Weide Y. Oligodontia. A clinical, 53. Ahmad W, Brancolini V, ul Faiyaz MF, Lam H, ul Haque S, radiographic and genetic evaluation. Doctoral Thesis. Haider M, et al. A locus for autosomal recessive hypodontia Utrecht, Finland: University of Utrecht; 1992. with associated dental anomalies maps to chromosome 33. Graber LW. Congenital absence of teeth: a review with 16q12.1. Am J Hum Genet 1998; 62: 987-91. emphasis on inheritance patterns. J Am Dent Assoc 1978; 54. Alvesalo L. The influence of sex-chromosome genes on 96: 266-75. tooth size in man: A genetic and quantitative study. Am J 34. Cohen MMJ. Craniofacial disorders caused by mutations in Orthod 1971; 60: 420. homeobox genes MSX1 and MSX2. J Craniofac Genet Dev 55. Huskins CI. On the inheritance of an anomaly of human Biol 2000; 20: 19-25. dentition. J Hered 1930; 21: 279-82. 35. Brekhus P, Oliver C, Montelinus G. A study of the pattern 56. Burzynski NJ, Escobar VH. Classification and genetics of and combination of congenitally missing teeth in man. J numeric anomalies of dentition. Birth Defects Orig Artic Dent Res 1944; 23: 117-31. Ser 1984; 19: 95-106. 36. Symons AL, Stritzel F, Stamation J. Anomalies associated 57. Suarez BK, Spence MA. The genetics of hypodontia. J with hypodontia of the permanent later incisor and second Dent Res 1974; 53: 781-5. premolar. J Clin Pediatr Dent. 1993; 17: 109-11. 58. Chosack A, Eidelman E, Cohen T. Hypodontia: A polygenic 37. Dolder E. Zahn-Unterzahl. Diagnostik, Statistik, trait, a family study among Israeli Jews. J Dent Res 1975; Artikulation. Schweiz Monatsschr Zahnmed 1936; 46: 663-71. 54: 16-9. 38. Grahnen H. Hypodontia in the permanent dentition. 59. Peck L, Peck S, Attia Y. Maxillary canine-first premolar Aclinical and genetical investigation. Odontol Revy 1956; transposition, associated dental anomalies and genetic bases. 7(Suppl.3): 1-100. Angle Orthod 1993; 63: 99-109. 39. Tavajohi-Kermani H, Kapur R, Sciote JJ. Tooth agenesis 60. Peck S, Peck L, Kataja M. The palatally displaced canine and craniofacial morphology in an orthodontic population. as a dental anomaly of genetic origin. Angle Orthod 1994; Am J Orthod Dentofacial Orthop 2002; 122: 39-47. 64: 249-56. 40. Brook AH, Elcock C, Al-Sharood MH, McKeown HF, Khalaf 61. Glenn FB. Incidence of congenitally missing permanent K, Smith RN. Further Studies of a Model for the Etiology teeth in a private pedodontic practice. ASDC J Dent Child of Anomalies of Tooth Number and Size in Humans. Connect 1961; 28: 317-20. Tissue Res 2002; 43: 289-95. 62. Egermark-Eriksson I, Lind V. Congenital numerical 41. Rose JS. A survey of congenitally missing teeth, excluding variation in the permanent dentition. D. Sex distribution third molars, in 6000 orthodontic patients. Dent Pract of hypodontia and hyperodontia. Odontol Revy 1971; 22: Dent Rec 1966; 17: 107-14. 309-15. 42. Brook AH. A unifying aetiological explanation for 63. Eidelman E, Chosack A, Rosenzweig KA. Hypodontia: anomalies of human tooth number and size. Arch Oral Biol prevalence amongst Jewish populations of different origin. 1984; 29: 373-8. Am J Phys Anthropol 1973; 39: 129-33. 43. Hobkirk JA, Brook AH. The management of patients with 64. Rasmussen P. Severe hypodontia: diversities in 704 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

manifestations. J Clin Pediatr Dent 1999; 23: 179-88. selective tooth agenesis. Mol Cell Biol 1998; 18: 6044-51. 65. Schalk-van der Weide Y, Steen WHA, Bosman F. 87. van den Boogaard M-JH, Dorland M, Beemer FA, van Distribution of missing teeth and tooth morphology in Amstel HK. MSX1 mutation is associated with orofacial patients with oligodontia. J Dent Child 1992; 59: 133-40. clefting and tooth agenesis in humans. Nat Genet 2000; 66. Rolling S, Poulsen S. Oligodontia in Danish schoolchildren. 24: 342-3. Acta Odontol Scand 2001; 59: 111-2. 88. De Muynck S, Schollen E, Matthijs G, Verdonck A, 67. Jorgenson RJ. Clinician´s view of hypodontia. J Am Dent Devriendt K, Carels C. A novel MSX1 mutation in Assoc 1980; 101: 283-6. hypodontia. Am J Med Genet 2004; 128: 401-3. 68. Vastardis H. The genetics of human tooth agenesis: new 89. Jumlongras D, Bei M, Stimson JM, Wang W-F, DePalma discoveries for understanding dental anomalies. Am J Orthod SR, Seidman CE. A nonsense mutation in MSX1 causes Dentofacial Orthop 2000; 117: 650-6. Witkop syndrome. Am J Hum Genet 2001; 69: 67-74. 69. Riesenfeld A. The effect of environmental factors on tooth 90. Isaac VE, Sciavolino P, Abate C. Multiple amino acids development: an experimental investigation. Acta Anat determine DNA binding specificity of the Msx-1 1970; 77: 188-215. homeodomain. Biochemistry 1995; 34: 7127-34. 70. Speirs AL. Thalidomide. Lancet 1965; 2: 1074. 91. Frazier-Bowers SA, Guo DC, Cavender A, Xue L, Evans B, 71. Axrup K, D´ Avignon M, Hellgren K, Herikson CO, Juhlin King T, et al. A Novel Mutation in Human PAX9 Causes IM, Larsson KS. Children with thalidomide embryopathy: Molar Oligodontia. J Dent Res 2002; 81: 129-33. odontological observations and aspects. Acta Odontol Scand 92. Nieminen P, Arte S, Tanner D, Paulin L, Alaluusua S, 1966; 24: 3-21. Thesleff I, et al. Identification of a nonsense mutation in 72. Cohen MM. Congenital, genetic, and endocrinologic the PAX9 gene in molar oligodontia. Eur J Hum Genet influences on dental occlusion. Dent Clin North Am 1975; 2001; 9: 743-6. 19: 499-514. 93. Lammi L, Halonen K, Pirinen S, Thesleff I, Arte S, 73. Schalk-van der Weide Y, Beemer FA, Faber JAJ, Bosman F. Nieminen P. A missense mutation in PAX9 in a family with Symptomatology of patients with Oligodontia. J Oral distinct phenotype of oligodontia. Eur J Hum Genet 2003; Rehabil 1994; 21: 247-61. 11: 866-71. 74. Kjaer I. Neuro-osteology. Crit Rev Oral Biol Med 1998; 9: 94. Jumlongras D, Lin JY, Chapra A, Seidman CE, Seidman JG, 224-44. Maas RL, et al. A novel missense mutation in the paired 75. Kjaer I, Kocsis G, Nodal M, Christensen LR. Aetiological domain of PAX9 causes non-syndromic oligodontia. Hum aspects of mandibular tooth agenesis—focusing on the role Genet 2004; 114: 242-9. of nerve, , and supporting tissues. Eur J Orthod 95. Mostowska A, Kobielak A, Biedziak B, Trzeciak WH. Novel 1994; 16: 371-5. mutation in the paired box sequence of PAX9 gene in a 76. Kjaer I. Can the location of tooth agenesis and the location sporadic form of oligodontia. Eur J Oral Sci 2003; 111: 272-6. of initial bone loss seen in juvenile periodontitis be explained 96. Brook AH. Dental anomalies of number, form and size: by neural developmental fields in the jaws? Acta Odontol their prevalence in British schoolchildren. J Int Assoc Dent Scand 1997; 55: 70-2. Child 1974; 5: 37-53. 77. Linderstrom A, Samuelsson L, Huggare J. Is tooth agenesis 97. Grahnen H, Granath LE. Numerical variations in primary related to brainstem anomalies in myelomeningocele dentition and their correlation with the permanent patients with Chiari II malformations? Acta Odontol Scand dentition. Odontol Revy 1961; 12: 348-57. 2002; 60: 337-40. 98. Nik-Hussein NN. Hypodontia in the permanent dentition: 78. Polder BJ, Van’t Hof MA, Van der Linden FPGM, Kuijpers- a study of its prevalence in Malaysian children. Aust Orthod Jagtman AM. A meta-analysis of the prevalence of dental J 1989; 11: 93-5. agenesis of permanent teeth. Community Dent Oral 99. Carvalho JC, Vinker F, Declerck D. Malocclusion, dental Epidemiol 2004; 32: 217-26. injuries and dental anomalies in the primary dentition of 79. Lapter M, Slaj M, Skrinjaric I, Muretic Z. Inheritance of Belgian children. Int J Paediatr Dent 1998; 8: 137-41. hypodontia in twins. Coll Antropol 1998; 22: 291-8. 100. Whittington BR, Durward CS. Survey of anomalies in 80. Markovic M. Hypodontia in twins. Swed Dent J Suppl 1982; primary teeth and their correlation with the permanent 15: 153-62. dentition. N Z Dent J 1996; 92: 4-8. 81. Pirinen S, Kentala A, Nieminen P, Varilo T, Thesleff I, 101. Nordgarden H, Jensen JL, Storhaug K. Reported prevalence Arte S. Recessively inherited lower incisor hypodontia. J of congenitally missing teeth in two Norwegian counties. Med Genet 2001; 38: 551-8. Community Dent Health 2002; 19: 258-62. 82. Dahlberg AA. Inherited congenital absense of six incisors, 102. Magnusson TE. Hypodontia, hyperodontia, and douple deciduous and permanent. J Dent Res 1937; 16: 59-62. formation of primary teeth in Iceland. An epidemiological 83. Vastardis H, Karimbux N, Guthua SW, Seidman JG, Seidman study. Acta Odontol Scand 1984; 42: 137-9. CE. A human MSX1 homeodomain missense mutation 103. Ravn JJ. Aplasia, supernumerary teeth and fused teeth in causes selective tooth agenesis. Nat Genet 1996; 13: 417-21. the primary dentition. An epidemiologic study. Scand J 84. Nieminen P, Kotilainen J, Aalto Y, Knuutila S, Pirinen S, Dent Res 1971; 79: 1-6. Thesleff I. MSX1 Gene is Deleted in Wolf-Hirschhorn 104. Järvinen S, Lehtinen L. Supernumerary and congenitally Syndrome Patients with Oligodontia. J Dent Res 2003; 82: missing primary teeth in Finnish children. An epidemiologic 1013-7. study. Acta Odontol Scand 1981; 39: 83-6. 85. Scarel RM, Trevilatto PC, Di Hipolito OJ, Camargo LEA, 105. Ng’ang’a RN, Ng’ang’a PM. Hypodontia of permanent Line SRP. Absence of mutations in the homeodomain of teeth in a Kenyan population. East Afr Med J 2001; 78: the MSX1 gene in patients with hypodontia. Am J Med 200-3. Genet 2000; 92: 346-9. 106. Yonezu T, Hayashi YS, J., Machida Y. Prevalence of 86. Hu G, Vastardis H, Bendall AJ, Wang Z, Logan M, Zhang H, congenital dental anomalies of the deciduous dentition in et al. Haploinsufficiency of Msx1: a mechanism for Japanese children. Bull Tokyo Dent Coll 1997; 38: 27-32. 705 Braz J Oral Sci. 4(13):695-706 Hypodontia: genetics and future perspectives

107. Salama FS, Abdel-Megid FY. Hypodontia of primary and permanent teeth in a sample of Saudi children. Egypt Dent J 1994; 40: 525-632. 108. Backman B, Wahlin YB. Variations in number and morphology of permanent teeth in 7-year-old Swedish children. Int J Paediatr Dent 2001; 11: 11-7. 109. Magnusson TE. Prevalence of hypodontia and malformations of permanent teeth in Iceland. Community Dent Oral Epiodemiol 1977; 5: 173-8. 110. O’Dowling IB. Hypo- in an Irish population. J Ir Dent Assoc 1989; 35: 114-7. 111. Rolling S. Hypodontia of permanent teeth in Danish schoolchildren. Scand J Dent Res 1980; 88: 365-9. 112. Ravn J, Nielsen L. Supernumerary teeth and aplasia among Copenhagen schoolchildren. Tandlaegebladet 1973; 77: 12-21. 113. Haavikko K. Hypodontia of permanent teeth. An orthopantomographic study. Suom Hammaslääk Toim 1971; 67: 219-25. 114. Hunstadbraten K. Hypodonti i det permanente tannsett. Nor Tannlaegeforen Tid 1964; 74: 64-9. 115. Hundstadbraten K. Hypodontia in the permanent dentition. J Dent Child 1973; 73: 115-7. 116. Wisth PJ, Thunold K, Böe OE. Frequency of hypodontia in relation to tooth size and dental arch width. Acta Odontol Scand 1974; 32: 201-6. 117. Aasheim B, Ögaard B. Hypodontia in 9-year-old Norwegians related to need of orthodontic treatment. Scand J Dent Res 1993; 101: 257-60. 118. Bergström K. An orthopantomographic study of hypodontia, supernumeraries and other anomalies in school children between the ages of 8-9 years. An epidemiological study. Swed Dent J 1977; 1: 145-57. 119. Thompson GW, Popovich F. Probability of congenitally missing teeth: results in 1,191 children in Burlington Growth Centre in Toronto. Community Dent Oral Epiodemiol 1974; 2: 26-32. 120. Davis PJ. Hypodontia and hyperdontia of permanent teeth in Hong Kong schoolchildren. Community Dent Oral Epiodemiol 1987; 15: 218-20. 121. Niswander JD, Sujaku C. Congenital anomalies of teeth in Japanese Children. Am J Phys Anthropol 1963; 21 :569-74. 122. Lai PY, Seow WK. A controlled study of the association of various dental anomalies with hypodontia of permanent teeth. Pediatr Dent 1989; 11: 291-6. 123. Salem G. Prevalence of selected dental anomalies in Saudi children from Gizan region. Community Dent Oral Epiodemiol 1989; 17: 162-3. 124. den Dunnen JT, Antonarakis SE. Mutation Nomenclature Extensions and Suggestions to Describe Complex Mutations: A Dissussion. Hum Mut 2000; 15: 7-12.

706