Haploinsufficiency of PAX9 is associated with autosomal dominant hypodontia

Novel missense mutations in PAX9 causing oligodontia

OBJECTIVE

We investigated the disease-causing gene of oligodontia in Chinese families and analysed the pathogenesis of mutations of this gene that results in oligodontia.

METHODS

Two families with oligodontia, but of different descent and 100 unrelated healthy controls were enrolled in our study. Genomic DNA was isolated from blood samples. Mutation analysis was performed by amplifying MSX1 and PAX9 exons and sequencing the products. After identifying the mutations, we performed site-directed mutagenesis to generate mutated vectors. The wild-type and mutated PAX9 vectors were then transfected separately to NIH3T3 cells. Immunolocalization, electrophoretic mobility shift assay (EMSA) and luciferase reporter assay were performed to analyse the effects of mutations on protein function.

RESULTS

We identified two novel missense mutations, Leu27Pro (L27P) and Ile29Thr (I29T) in the paired-domain of PAX9. Analysis of homologous PAX proteins indicated that these two substitutions may affect the function of the PAX9 protein. Results of immunofluorescence and western blot showed that the mutations did not alter the nuclear localization of PAX9. EMSA and luciferase reporter assays indicated that both the mutated proteins could not bind DNA or transactivate the BMP4 promoter.

CONCLUSIONS

Two novel missense mutations in PAX9 have been indentified in Chinese families causing oligodontia.

Case report: identical twins revealing discordant hypodontia. The rationale of dental arch differences in monozygotic twins

BACKGROUND

While it is generally accepted that monozygotic (MZ) twins are identical with respect to inherited traits, because they share 100% of their genetic material, clinical findings and scientific evidence does not support this belief. In addition to environmental factors and stochastic developmental events, a number of genetic mechanisms, detectable by new techniques in molecular genetics, explain the differences frequently observed in MZ twins.

CASE REPORT

Nine-year-old twin girls requested treatment for a dental malocclusion. Their facial and occlusal features were very similar. Panoramic radiographs revealed hypodontia of two permanent teeth in one twin (35 and 45) and of only one tooth in the other (45). An incorrect diagnosis of dizygosity (DZ) had been made at birth based on the presence of two amniotic sacs. Despite discordance in the dental complement of both girls their orthodontist suspected that the twins might be identical. A genetic study performed by quantitative fluorescence-polymerase chain reaction (QFPCR) analysis of chromosomes 13, 15, 16, 18, 21, 22, and X confirmed that the twins were MZ.

CONCLUSION

Discordances in dental complement between MZ twins are not uncommon and do not exclude monozygosity.

A novel nonsense mutation in PAX9 is associated with sporadic hypodontia

The most important events during the regulation of tooth development were inductive interactions between the epithelial and mesenchymal tissues. The expression of Pax9 had been shown to specifically mark the mesenchymal regions at the prospective sites of all teeth prior to any morphological manifestations. Here, we investigated the PAX9 gene as a candidate gene for hypodontia in five unrelated Chinese patients with tooth agenesis. Direct sequencing and restriction enzyme analysis revealed a novel heterozygous mutation c.480C>G (p.160Tyr>X, Y160X) in a patient who was missing 20 permanent teeth (the third molars excluded) and 6 primary teeth. The mutation was a nonsense mutation, leading to a premature stop codon in exon 2 of PAX9 gene. PCR analysis of complementary DNA from cultured lymphocytes of the affected individual could not indicate the complete degradation of the mutated transcript. Promoter reporter assays revealed reduced transcriptional activity of the mutated PAX9 protein suggesting that the severe phenotype may result from haploinsufficiency of PAX9. In another patient with 15 missing permanent teeth (the third molars excluded), we found the c.219insG mutation previously reported by Stockton.

Sequence analysis of PAX9, MSX1 and AXIN2 genes in a Chinese oligodontia family

OBJECTIVES

The goal of our research was to look into the clinical traits and genetic mutations in nonsyndromic oligodontia in a Chinese family and to gain insight into the role of mutations of PAX9, MSX1 and AXIN2 in oligodontia phenotypes.

MATERIALS AND METHODS

6 subjects from a family underwent complete oral examination, including panoramic radiographs. Retrospective data were reviewed and blood samples were collected. PCR primers for PAX9, MSX1, and AXIN2 were designed through the Oligo Primer Analysis Software. PCR products were purified and sequenced using the BigDye Terminator Kit and analysed by the 3730 DNA Analyzer.

RESULTS

The proband missed 4 permanent canines, 2 permanent maxillary lateral incisors, 2 permanent mandibular lateral incisors, and 2 permanent mandibular central incisors, whilst his maternal grandfather lacked only 2 permanent mandibular central incisors. Moreover, the size of some permanent teeth appeared smaller than normal values of crown width of Chinese people. Oligodontia and abnormalities of teeth were not present in other family members. Radiographic examination showed that the proband and the rest of family members retained all germs of the third molars. There was one known mutation A240P (rs4904210) of PAX9 in the coding region in the proband and the maternal family members (II-2, II-3, and II-4), which possibly contributed to structural and functional changes of proteins. No mutations were identified in MSX1 and AXIN2.

CONCLUSIONS

Our findings may imply that the PAX9 A240P mutation is a risk factor for oligodontia in the Chinese population. A240P is likely to be a genetic cause of oligodontia though previous literature suggested it as a polymorphism only.

Transcriptional analysis of the human PAX9 promoter

Objectives

PAX9 belongs to the Pax family of transcriptional factor genes. This gene is expressed in embryonic tissues such as somites, pharyngeal pouch endoderm, distal limb buds and neural crest-derived mesenchyme. Polymorphisms in the upstream promoter region of the human PAX9 have been associated with human non-syndromic tooth agenesis. In the present study, we verified the in vitro mRNA expression of this gene and the luciferase activity of two constructs containing promoter sequences of the PAX9 gene.

Material and Methods

Embryonic tissues were obtained from digits, face, and midbrain/hindbrain regions. Fragments containing PAX9 promoter sequences were cloned into reporter plasmids and were transfected into the different cell cultures. mRNA were extracted from primary cell cultures.

Results

The semi-quantitative RT-PCR results showed that in vitro E13.5 limb bud and CNS cells express PAX9, but cells derived from the facial region do not. Moreover, the luciferase assay showed that protein activity of the constructed vector was weaker than pgl3 -basic alone.

Conclusion

The present results suggest that the promoter sequences analyzed are not sufficient to drive PAX9 gene transcription.

Non-syndromic Oligodontia with a Novel Mutation of PAX9

Agenesis of the permanent teeth is a congenital anomaly that is frequently seen in humans. Oligodontia is a severe type of tooth agenesis involving 6 or more congenitally missing teeth, excluding the third molars. Previous studies have indicated that mutations in the homeobox gene MSX1, paired domain transcription factor PAX9, and EDA are associated with non-syndromic oligodontia. This study reports a Japanese family (eight of 14 family members affected) with non-syndromic oligodontia who preferentially lacked molar teeth. In this family, a novel frameshift mutation (321_322insG) was identified in the paired domain of PAX9. The frameshift mutation caused altered amino acids in the paired domain and premature termination of translation by 26 amino acids. When transfected into COS-7 cells, the mRNA expression of 321_322insG PAX9 was comparable with that of wild-type PAX9. However, the mRNA of 321_322insG PAX9 was more unstable than that of wild-type PAX9. This mRNA instability caused a marked decrease in protein production, as evaluated by Western blot analysis and immunostaining. These findings suggest that the 321_322insG mutation causes insufficient function of PAX9 protein and haploinsufficiency as a genetic model of familial non-syndromic oligodontia with a PAX9 mutation.

A New Hypo/Oligodontia Syndrome

Dental agenesis is either syndromic or non-syndromic. Here, we describe a familial case with Carvajal/Naxos syndrome associating woolly hair, palmoplantar keratoderma, and biventricular dilated cardiomyopathy. In addition to these signs, all three affected family members had hypo/oligodontia ranging from absence of the lower left second molar to 15 missing teeth, the typical pattern of oligodontia being absent 2nd premolars and absent 2nd and 3rd molars. Mutation screening in the desmoplakin gene ( DSP) revealed a de novo missense mutation (c.1790 C>T, p.Ser597Leu) changing a serine residue conserved in all vertebrates. In addition, this variation was absent from 100 control DNA samples. There were no mutations in the plakoglobin gene. This familial case report and two other previous reports demonstrate that autosomal-dominant mutations in the DSP gene are associated with hypo/oligodontia in the setting of Carvajal/Naxos syndrome. This study suggests that dentists discovering oligo/hypodontia should screen for woolly hair and palmoplantar keratoderma because of the probable cardiac involvement with an inherent high risk of severe cardiomyopathy. In addition, this study reveals the role of desmosomes in the development of teeth and suggests that other genes encoding proteins of the desmosome could be involved in oligo/hypodontia.

A 223-kb de novo deletion of PAX9 in a patient with oligodontia

The PAX (paired box) genes are a family of transcription factors critical for fetal growth and organogenesis. Abnormalities of PAX2, PAX3, PAX6, and PAX9 are associated with various congenital craniofacial anomalies, including tooth abnormalities. We present here a boy with oligodontia and language delay. Dental x-rays showed that he lacked primary molars and was missing most of his permanent teeth. A genome-wide, single-nucleotide polymorphism-based microarray revealed a de novo 223-kb heterozygous deletion on 14q13.3 that included the PAX9 gene. In addition, the array showed 2 copies of the X chromosome and 1 copy of the Y chromosome, diagnostic for Klinefelter syndrome. The findings in this patient illustrate the role of the PAX9 gene in tooth development and provide the first example of a de novo deletion of 14q13.3 manifesting primarily with oligodontia. This report also supports the utility of genome-wide microarrays in determining the genetic cause of craniofacial abnormalities.

A novel g.-1258G>A mutation in a conserved putative regulatory element of PAX9 is associated with autosomal dominant molar hypodontia

Mutations in the transcription factor PAX9 which plays a critical role in the switching of odontogenic potential from the epithelium to the mesenchyme during tooth development cause autosomal dominant non-syndromic hypodontia primarily affecting molars. Linkage analysis on a family segregating autosomal dominant molar hypodontia with markers flanking and within PAX9 yielded a maximum multipoint LOD score of 3.6. No sequence variants were detected in the coding or 5'- and 3'-untranslated regions (UTRs) of PAX9. However, we identified a novel g.-1258G>A sequence variant in all affected individuals of the family but not in the unaffected family members or in 3088 control chromosomes. This mutation is within a putative 5'-regulatory sequence upstream of PAX9 highly conserved in primates, somewhat conserved in ungulates and carnivores but not conserved in rodents. Bioinformatics analysis of the sequence determined that there was no abolition or creation of a putative binding site for known transcription factors. Based on our previous findings that haploinsufficiency for PAX9 leads to hypodontia, we postulate that the g.-1258G>A variant reduces the expression of PAX9 which underlies the hypodontia phenotype in this family.

Genetic basis of tooth agenesis

Tooth agenesis or hypodontia, failure to develop all normally developing teeth, is one of the most common developmental anomalies in man. Common forms, including third molar agenesis and hypodontia of one or more of the incisors and premolars, constitute the great majority of cases. They typically affect those teeth that develop latest in each tooth class and these teeth are also most commonly affected in more severe and rare types of tooth agenesis. Specific vulnerability of the last developing teeth suggests that agenesis reflects quantitative defects during dental development. So far molecular genetics has revealed the genetic background of only rare forms of tooth agenesis. Mutations in MSX1, PAX9, AXIN2 and EDA have been identified in familial severe agenesis (oligodontia) and mutations in many other genes have been identified in syndromes in which tooth agenesis is a regular feature. Heterozygous loss of function mutations in many genes reduce the gene dose, whereas e.g. in hypohidrotic ectodermal dysplasia (EDA) the complete inactivation of the partially redundant signaling pathway reduces the signaling centers. Although these mechanisms involve quantitative disturbances, the phenotypes associated with mutations in different genes indicate that in addition to an overall reduction of odontogenic potential, tooth class-specific and more complex mechanisms are also involved. Although several of the genes so far identified in rare forms of tooth agenesis are being studied as candidate genes of common third molar agenesis and incisor and premolar hypodontia, it is plausible that novel genes that contribute to these phenotypes will also become identified.

Pathogenic mechanisms of tooth agenesis linked to paired domain mutations in human PAX9

Mutations in the paired-domain transcription factor PAX9 are associated with non-syndromic tooth agenesis that preferentially affects posterior dentition. Of the 18 mutations identified to date, eight are phenotypically well-characterized missense mutations within the DNA-binding paired domain. We determined the structural and functional consequences of these paired domain missense mutations and correlated our findings with the associated dental phenotype variations. In vitro testing included subcellular localization, protein-protein interactions between MSX1 and mutant PAX9 proteins, binding of PAX9 mutants to a DNA consensus site and transcriptional activation from the Pax9 effector promoters Bmp4 and Msx1 with and without MSX1 as co-activator. All mutant PAX9 proteins were localized in the nucleus of transfected cells and physically interacted with MSX1 protein. Three of the mutants retained the ability to bind the consensus paired domain recognition sequence; the others were unable or only partly able to interact with this DNA fragment and also showed a similarly impaired capability for activation of transcription from the Msx1 and Bmp4 promoters. For seven of the eight mutants, the degree of loss of DNA-binding and promoter activation correlated quite well with the severity of the tooth agenesis pattern seen in vivo. One of the mutants however showed neither reduction in DNA-binding nor decrease in transactivation; instead, a loss of responsiveness to synergism with MSX1 in target promoter activation and a dominant negative effect when expressed together with wild-type PAX9 could be observed. Our structure-based studies, which modeled DNA binding and subdomain stability, were able to predict functional consequences quite reliably.

Molecular characterization of a novel PAX9 missense mutation causing posterior tooth agenesis

Autosomal dominant mutations in the gene encoding the paired box containing transcription factor PAX9 are associated with nonsyndromic human tooth agenesis that primarily affect posterior dentition. The molecular mechanisms contributing to its pathogenesis are poorly understood. In this study, we describe a novel mutation in PAX9 in a family with molar oligodontia. This heterozygous mutation results in the substitution of a highly conserved isoleucine residue by phenylalanine within the carboxyl-terminal subdomain of the paired domain. Immunolocalization and cell fractionation studies to ascertain the subcellular localization of the Ile87Phe protein showed that both wild-type and mutant proteins are synthesized in mammalian cells and that the mutation does not alter the nuclear localization of the mutant protein. Gel-shift assays using two cognate paired-domain recognition sequences, e5 and CD19-2(A-ins), revealed that while wild-type Pax9 binds to both sequences, the mutant protein was unable to bind these sites. In addition, the latter did not alter the DNA-binding activities of wild-type Pax9. Furthermore, we evaluated the ability of the Ile87Phe mutant protein to form a complex with a partner protein, Msx1, and found that the mutation under study has no effect on this interaction. Based on our observed defects in DNA binding by the mutant protein, we propose a loss-of-function mechanism that contributes to haploinsufficiency of PAX9 in this family with posterior tooth agenesis.

Deletion of PAX9 and oligodontia: a third family and review of the literature

OBJECTIVE

This study was conducted to report a family affected by benign hereditary chorea in which a large deletion including TTF1, PAX9, and other genes was identified and results in oligodontia.

METHODS

Clinical and radiological studies of the two affected members (mother and daughter) were used to describe the oligodontia present in both of them.

RESULTS

The missing teeth in both patients are described in detail, and these data are compared with the dental anomalies observed in the only two other families with deletions of PAX9 and with the data available for 12 previously reported families carrying different types of PAX9 mutations.

CONCLUSIONS

There is a clinical relevance for recognizing such families, and offering available therapies since childhood is stressed. Some genotype-phenotype correlations between PAX9 mutations and dental anomalies can be drawn.

Dental agenesis: genetic and clinical perspectives

Dental agenesis is the most common developmental anomaly in humans and is frequently associated with several other oral abnormalities. Whereas the incidence of missing teeth may vary considerably depending on dentition, gender, and demographic or geographic profiles, distinct patterns of agenesis have been detected in the permanent dentition. These frequently involve the last teeth of a class to develop (I2, P2, M3) suggesting a possible link with evolutionary trends. Hypodontia can either occur as an isolated condition (non-syndromic hypodontia) involving one (80% of cases), a few (less than 10%) or many teeth (less than 1%), or can be associated with a systemic condition or syndrome (syndromic hypodontia), essentially reflecting the genetically and phenotypically heterogeneity of the condition. Based on our present knowledge of genes and transcription factors that are involved in tooth development, it is assumed that different phenotypic forms are caused by different genes involving different interacting molecular pathways, providing an explanation not only for the wide variety in agenesis patterns but also for associations of dental agenesis with other oral anomalies. At present, the list of genes involved in human non-syndromic hypodontia includes not only those encoding a signaling molecule (TGFA) and transcription factors (MSX1 and PAX9) that play critical roles during early craniofacial development, but also genes coding for a protein involved in canonical Wnt signaling (AXIN2), and a transmembrane receptor of fibroblast growth factors (FGFR1). Our objective was to review the current literature on the molecular mechanisms that are responsible for selective dental agenesis in humans and to present a detailed overview of syndromes with hypodontia and their causative genes. These new perspectives and future challenges in the field of identification of possible candidate genes involved in dental agenesis are discussed.

Identification and functional analysis of two novel PAX9 mutations

The paired-domain transcription factor PAX9 plays a critical role in tooth development, as heterozygous mutations in PAX9 have been shown to be associated with human tooth agenesis. In this study, we report 2 novel missense mutations, gly6arg (G6R) and ser43lys (S43K), in the paired domain of PAX9 in Chinese patients with varying degrees of nonsyndromic tooth agenesis. Excluding third molars, the individual with the G6R mutation was missing 2 mandibular incisors and a maxillary premolar, while the phenotype of individuals with the S43K mutation consisted of peg-shaped upper lateral incisors and missing molars, premolars and canines. As these 2 mutations occur at highly conserved amino acids in the PAX gene family and between different species, we further analyzed the effects of the mutations on the function of the resulting proteins. Immunofluorescence and immunoblotting studies showed that the mutations did not alter nuclear localization in mammalian cells. Gel shift and super shift assays indicate that both mutant proteins bound DNA at a lower level than the normal protein, with G6R having a greater affinity for DNA than S43K. Likewise, the G6R protein was able to transcriptionally activate a Bmp4 promoter construct to a greater extent than S43K. Our finding that the severity of tooth agenesis in the patients was correlated to the DNA-binding capacity of the mutated PAX9 9proteins supports the hypothesis that DNA binding is responsible for the genetic defect.

Non-syndromic tooth agenesis in two Chinese families associated with novel missense mutations in the TNF domain of EDA (ectodysplasin A)

Here we report two unrelated Chinese families with congenital missing teeth inherited in an X-linked manner. We mapped the affected locus to chromosome Xp11-Xq21 in one family. In the defined region, both families were found to have novel missense mutations in the ectodysplasin-A (EDA) gene. The mutation of c.947A>G caused the D316G substitution of the EDA protein. The mutation of c.1013C>T found in the other family resulted in the Thr to Met mutation at position 338 of EDA. The EDA gene has been reported responsible for X-linked hypohidrotic ectodermal dysplasia (XLHED) in humans characterized by impaired development of hair, eccrine sweat glands, and teeth. In contrast, all the affected individuals in the two families that we studied here had normal hair and skin. Structural analysis suggests that these two novel mutants may account for the milder phenotype by affecting the stability of EDA trimers. Our results indicate that these novel missense mutations in EDA are associated with the isolated tooth agenesis and provide preliminary explanation for the abnormal clinical phenotype at a molecular structural level.

Investigating the etiology of multiple tooth agenesis in three sisters with severe oligodontia

OBJECTIVES

To describe the dentofacial phenotypes of three sisters with severe non-syndromic oligodontia, to report on the mutation analysis in three genes, previously shown to cause various phenotypes of non-syndromic oligodontia and in two other suspected genes. Based on the phenotypes in the pedigree of this family, the different possible patterns of transmission are discussed.

METHODS

Anamnestic data and a panoramic radiograph were taken to study the phenotype of the three sisters and their first-degree relatives. Blood samples were also taken to obtain their karyotypes and DNA samples. Mutational screening was performed for the MSX1, PAX9, AXIN2, DLX1 and DLX2 genes.

RESULTS

The probands' pedigree showed evidence for a recessive or multifactorial inheritance pattern. Normal chromosomal karyotypes were found and - despite the severe oligodontia present in all three sisters - no mutation appeared to be present in the five genes studied so far in these patients.

CONCLUSIONS

In the three sisters reported, their common oligodontia phenotype is not caused by mutations in the coding regions of MSX1, PAX9, AXIN2, DLX1 or DLX2 genes, but genetic factors most probably play a role as all three sisters were affected. Environmental and epigenetic factors as well as genes regulating odontogenesis need further in vivo and in vitro investigation to explain the phenotypic heterogeneity and to increase our understanding of the odontogenic processes.

PAX9 polymorphisms and susceptibility to sporadic tooth agenesis: a case-control study in southeast China

Tooth agenesis is one of the most common developmental disorders in humans. The PAX9 gene, which plays an important role in odontogenesis, is associated with familial and sporadic tooth agenesis. A case-control study was performed in 102 subjects with tooth agenesis (cases) and 116 healthy controls. We genotyped four PAX9 gene polymorphisms using a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay. The allele and genotype frequencies of the four polymorphisms were not significantly different between the controls and the subjects with tooth agenesis. Similar results were observed in a subgroup analysis of test subjects only with mandibular incisor agenesis. Further analysis showed no significant difference in the haplotype distribution between the controls and the subjects with tooth agenesis or mandibular incisor agenesis. However, we found that the AGGC haplotype was associated with a decreased risk of tooth agenesis, compared with the most common haplotype, AGCC (odds ratio, 0.14; 95% confidence interval: 0.00-0.95). These results suggest that the four PAX9 polymorphisms alone have a non-significant main effect on the risk of tooth agenesis but that the AGGC haplotype may have a protective effect associated with a decreased risk of tooth agenesis.

A novel missense mutation in the paired domain of human PAX9 causes oligodontia

PAX9 and MSX1 are transcription factors that play essential roles in craniofacial and limb development. In humans, mutations in both genes are associated with nonsyndromic and syndromic oligodontia, respectively. We screened one family with nonsyndromic oligodontia for mutations in PAX9 and MSX1. Single stranded conformational polymorphism (SSCP) analysis and sequencing revealed a novel heterozygous C139T transition in PAX9 in the affected members of the family. There were no mutations detected in the entire coding sequence of MSX1. The C139T mutation, predicted to result in the substitution of an arginine by a tryptophan (R47W) in the N-terminal subdomain, affected conserved residues in the PAX9 paired domain. To elucidate the pathogenic mechanism producing oligodontia phenotype caused by this mutation, we analyzed the binding of wild-type and mutant PAX9 paired domain protein to double-stranded DNA targets. The R47W mutation dramatically reduced DNA binding suggesting that the mutant protein with consequent haploinsufficiency results in a clinical phenotype.

Genetische Störungen der Zahnentwicklung und Dentition

Die Zähne sind Organe, die aus ektodermalen epithelialen Aussackungen im Bereich des 1. Kiemenbogens entstehen, gesteuert von epitheliomesenchymalen Interaktionen. Dabei spielen zahlreiche Signalmoleküle speziell der 4 großen Familien TGF-β, FGF, Hedgehog und WNT sowie diverse Transkriptionsfaktoren eine Rolle. Eine Beteiligung der Retinoide an der Odontogenese ist durch umfangreiche Befunde belegt, auch wenn die Inaktivierung relevanter Gene in Mausmodellen meist keine Zahnanomalien verursacht. Die Zahnentwicklung wird klassischerweise in verschiedene Stadien eingeteilt: Entstehung der Zahnleiste, der Zahnknospe, der Schmelzkappe, der Schmelzglocke, die Wurzelbildung und der Zahndurchbruch. Anomalien der Zahnentwicklung können isoliert oder gemeinsam mit anderen Symptomen im Zusammenhang mit Syndromen auftreten. Sie können genetisch bedingt sein oder unter Einwirkung teratogener Stoffe während der Bildung und Mineralisierung der Zahnkeime zustande kommen. Dentibukkale Entwicklungsanomalien treten im Kontext seltener Erkrankungen auf und finden zunehmend Beachtung, da sie bei bestimmten Erkrankungen in der Diagnostik und als prädikative Faktoren wichtige Anhaltspunkte geben können. Allerdings ist hierfür eine interdisziplinäre und internationale Kooperation notwendig, die bislang erst in Ansätzen verwirklicht wurde.

A novel nonsense mutation in PAX9 is associated with marked variability in number of missing teeth

Tooth development is under strict genetic control. During the last decade, studies in molecular genetics have led to the identification of gene defects causing the congenital absence of permanent teeth. Analyses of PAX9 and MSX1 in nine families with hypodontia and oligodontia revealed one new PAX9 mutation. A LOD score of Z = 1.8 (theta = 0.0) was obtained for D14S75 close to PAX9 in one three-generation family, and sequencing of the gene identified the nonsense mutation c.433C>T. The mutation results in a truncated PAX9 protein containing the paired domain region as a result of the Q145X stop mutation. The family showed a marked phenotypic variability in the number of missing teeth, ranging from 2 to 15 missing teeth. The highest frequency of missing teeth was found for second molars followed by second premolars.

Familial human hypodontia--is it all in the genes?

The congenital absence of teeth is one of the commonest developmental abnormalities seen in human populations. Familial hypodontia or oligodontia represents an absence of varying numbers of primary and/or secondary teeth as an isolated trait. While much progress has been made in understanding the developmental basis of tooth formation, knowledge of the aetiological basis of inherited tooth loss remains poor. The study of mouse genetics has uncovered a large number of candidate genes for this condition, but mutations in only three have been identified in human pedigrees with familial hypodontia or oligodontia: MSX1, PAX9 and AXIN2. This suggests that these conditions may represent a more complex multifactorial trait, influenced by a combination of gene function, environmental interaction and developmental timing. Completion of the human genome project has made available the DNA sequence of the collected human chromosomes, allowing the localisation of all human genes and, ultimately, determination of their function. Therefore it is likely that our understanding of this complex developmental process will continue to improve, not only during normal development but also when things go wrong.

Genes affecting tooth morphogenesis

The development of dentition is a fascinating process that encompasses a complex series of epithelial-mesenchymal interactions involving growth factors, transcription factors, signal receptors and other soluble morphogens. It is not surprising that such a complex process is prone to disturbances and may result in tooth agenesis. Initial discoveries indicating that the homeo-domain protein MSX1 and the paired-domain transcription factor PAX9 are causative genes in tooth morphogenesis were made in mice. Both genes are co-expressed in dental mesenchyme and either one, when homozygously deleted, results in an arrest at an early developmental stage. Heterozygous Pax9 or Msx1 mice have normal teeth, however, double heterozygous Pax9/Msx1 mice show a phenotype of arrested tooth development which can be rescued by transgenic expression of Bmp4, a very influential signaling factor in many developmental processes. We have obtained mounting evidence for a partnership between PAX9 and MSX1 within the tooth-specific Bmp4 signaling pathway. In humans, unlike in mice, a heterozygous mutation in either PAX9 or MSX1 suffices to cause tooth agenesis of a predominantly molar or more premolar pattern, respectively. Our laboratory and others have identified several PAX9 and MSX1 mutations in families with non-syndromic forms of autosomal dominant posterior tooth agenesis. We have also identified families with tooth agenesis in whom PAX9 and MSX1 mutations have been excluded opening up the possibilities for the discovery of other genes that contribute to human tooth agenesis.

New syndromic form of benign hereditary chorea is associated with a deletion of TITF-1 and PAX-9 contiguous genes

Benign hereditary chorea is a rare autosomal dominant disorder presenting with a childhood-onset and slowly progressive chorea. The objective of this study was to describe the clinical and genetic features of 3 patients who developed childhood-onset chorea. Three affected patients from three generations of a family with benign hereditary chorea associated with a multisystemic disorder of the basal ganglia, thyroid, lungs, salivary glands, bowels, and teeth. The TITF-1 gene was screened by microsatellite analysis, gene sequencing, and fluorescence in situ hybridization. Genetic analysis revealed a novel 0.9-Mb deletion on chromosome 14, which includes the TITF-1 and PAX9 genes. We have identified a novel deletion responsible for a new syndrome of benign hereditary chorea, including symptoms of brain-thyroid-lung syndrome associated with bowels, salivary glands, and teeth disorders. Associated signs, sometimes of slight expression, remain of high interest for the clinical and genetic diagnosis of benign hereditary chorea.

Exclusion of coding region mutations in MSX1, PAX9 and AXIN2 in eight patients with severe oligodontia phenotype

PURPOSE

This paper describes the screening of eight patients with severe oligodontia for PAX9 and AXIN2 mutations.

SUBJECTS AND METHODS

Anamnestic data and a panoramic radiograph were collected to study the phenotype of eight patients with oligodontia and their first-degree relatives. A blood sample was taken for a mutational screening for PAX9 and AXIN2 mutations.

RESULTS

No mutations were discovered, but a unique nucleotide change in a conserved 5' flanking region of PAX9 was revealed. Earlier screening of the same patients for MSX1 mutations also had a negative outcome.

CONCLUSIONS

Considering the discrepancy between the high incidence rate of agenesis and the relatively small number of reported causative mutations in PAX9, MSX1 and AXIN2 genes, the genetic contribution to oligodontia probably is much more heterogeneous than expected so far. Therefore negative results, like the present exclusion data, should be published more often in order to get a better appreciation of the relative contribution of these specific mutations causing oligodontia. In this context the exact number of tested probands also should be mentioned at all cases. Recent evidence of PAX9-MSX1 protein interactions in odontogenesis as well as other genes and developmental factors should receive more attention.

Novel MSX1 frameshift causes autosomal-dominant oligodontia

Can kindreds with tooth agenesis caused by MSX1 or PAX9 mutations be distinguished by their phenotypes? We have identified an MSX1second bicuspids and mandibular central incisors. The dominant phenotype is apparently due to haploinsufficiency. We analyzed patterns of partial tooth agenesis in seven kindreds with defined MSX1 mutations and ten kindreds with defined PAX9 mutations. The probability of missing a particular type of tooth is always bilaterally symmetrical, but differences exist between the maxilla and mandible. MSX1-associated oligodontia typically includes missing maxillary and mandibular second bicuspids and maxillary first bicuspids. The most distinguishing feature of MSX1-associated oligodontia is the frequent (75%) absence of maxillary first bicuspids, while the most distinguishing feature of PAX9-associated oligodontia is the frequent (> 80%) absence of the maxillary and mandibular second molars.

A novel mutation in PAX9 causes familial form of molar oligodontia

PAX9 is a paired domain transcription factor that plays a critical role in odontogenesis. All mutations of PAX9 identified to date have been associated with nonsyndromic form of tooth agenesis. The present report describes an unusual novel mutation in PAX9 identified in a family with severe molar oligodontia. This heterozygous deletion combined with 24 bp insertion (including a 5' splice site) is localized in the second exon beyond the highly conserved paired box sequence, and might result either in a premature termination of translation at aa 210 or in an aberrant splicing, leading to a frameshift and premature termination of translation at aa 314. Real-time PCR analysis revealed no mutated transcript in cultured lymphocytes of one of the affected individuals indicating that the novel mutation might result in rapid degradation of the mutated transcript leading to haploinsufficiency of PAX9. Our results support the view that mutations in PAX9 constitute a causative factor in nonsyndromic oligodontia.

A novel c.581C>T transition localized in a highly conserved homeobox sequence of MSX1: is it responsible for oligodontia?

Even though selective tooth agenesis is the most common developmental anomaly of human dentition, its genetic background still remains poorly understood. To date, familial as well as sporadic forms of both hypodontia and oligodontia have been associated with mutations or polymorphisms of MSX1, PAX9, AXIN2 and TGFa, whose protein products play a crucial role in odontogenesis. In the present report we described a novel mutation of MSX1, which might be responsible for the lack of 14 permanent teeth in our proband. However, this c.581C>T transition, localized in a highly conserved homeobox sequence of MSX1, was identified also in 2 healthy individuals from the proband's family. Our finding suggests that this transition might be the first described mutation of MSX1 that might be responsible for oligodontia and showing incomplete penetrance. It may also support the view that this common anomaly of human dentition might be an oligogenic trait caused by simultaneous mutations of different genes.

Gene discovery for dental anomalies

BACKGROUND

Thousands of inherited human disorders have been catalogued to date, but the underlying genetic causes of less than 20 percent of those disorders have been discovered.

TYPE OF STUDIES REVIEWED

The completion of the Human Genome Project (HGP) has made available the DNA sequence of all 24 human chromosomes, thereby allowing the localization of all human genes and, ultimately, determination of their function. Disease gene discovery is being expedited greatly by the data from the HGP, thereby paving the way for determination of the genetic etiology of most of these disorders.

RESULTS

While most dental anomalies can severely affect patients' quality of life, they are not fatal, which makes multigenerational families with these disorders available for study. These families are invaluable for genetic studies. Despite this fact, the discovery of genes underlying non-syndromic dental anomalies has lagged behind that for anomalies affecting other organ systems. The authors present an overview of the methodologies of disease gene identification using hypodontia, which is one of the most common anomalies of the dentition, to illustrate the application of these principles.

CLINICAL IMPLICATIONS

An understanding of the advances in human genetics should inspire the practicing dental professional to ascertain whether a dental anomaly is inherited and, if so, work with a human geneticist to identify its underlying genetic mechanism.

Natural selection and molecular evolution in primate PAX9 gene, a major determinant of tooth development

Large differences in relation to dental size, number, and morphology among and within modern human populations and between modern humans and other primate species have been observed. Molecular studies have demonstrated that tooth development is under strict genetic control, but, the genetic basis of primate tooth variation remains unknown. The PAX9 gene, which codes for a paired domain-containing transcription factor that plays an essential role in the development of mammal dentition, has been associated with selective tooth agenesis in humans and mice, which mainly involves the posterior teeth. To determine whether this gene is polymorphic in humans, we sequenced approximately 2.1 kb of the entire four-exon region (exons 1, 2, 3 and 4; 1,026 bp) and exon-intron (1.1 kb) boundaries of 86 individuals sampled from Asian, European, and Native American populations. We provided evidence that human PAX9 polymorphisms are limited to exon 3 only and furnished details about the distribution of a mutation there in 350 Polish subjects. To investigate the pattern of selective pressure on exon 3, we sequenced ortholog regions of this exon in four species of New World monkeys and one gorilla. In addition, orthologous sequences of PAX9 available in public databases were also analyzed. Although several differences were identified between humans and other species, our findings support the view that strong purifying selection is acting on PAX9. New World and Old World primate lineages may, however, have different degrees of restriction for changes in this DNA region.

A PANorama of PAX genes in cancer and development

Populations of self-renewing cells that arise during normal embryonic development harbour the potential for rapid proliferation, migration or transdifferentiation and, therefore, tumour generation. So, control mechanisms are essential to prevent rapidly expanding populations from malignant growth. Transcription factors have crucial roles in ensuring establishment of such regulation, with the Pax gene family prominent amongst these. This review examines the role of Pax family members during embryogenesis, and their contribution to tumorigenesis when subverted.

Molecular evolution of the primate developmental genes MSX1 and PAX9

In primates, the craniofacial skeleton and the dentition are marked by high levels of interspecific variation. Despite this, there are few comparative species studies conducted at the molecular level to investigate this functional diversity. We have determined nucleotide sequences of MSX1 and PAX9, two developmental genes, in a sample of 27 diverse primate species in order to identify coding or regulatory variation that may be associated with phenotypic diversity. Our analyses have identified four highly conserved noncoding sequences, including one that is conserved across primates and with dogs but not with mice. Although we find that substitution rates vary significantly across MSX1 exons, comparisons of nonsynonymous and synonymous substitution rates (dN/dS) suggest that, as a whole, MSX1 and PAX9 amino acid sequences have been under functional constraint throughout primate evolution. Compared to all other primates in our sample, our analysis of exon 1 in MSX1 finds an unusual pattern of amino acid substitution for Tarsius syrichta, a member of a lineage (tarsiers) that has many unique features among primates. For example, tarsiers are the only extant primates without deciduous incisors, and MSX1 is expressed exclusively in the incisor regions during the earliest stages of dental development. Our overall results provide insight into the utility of comparative species analyses of highly conserved developmental genes and their roles in the evolution of complex phenotypes.

PAX9 and TGFB3 are linked to susceptibility to nonsyndromic cleft lip with or without cleft palate in the Japanese: population-based and family-based candidate gene analyses

The prevalence of nonsyndromic cleft lip with or without cleft palate (CL/P) and cleft palate only (CPO) are believed to be higher in the Japanese than in Americans, Europeans or Africans. The purpose of this study was to investigate, in a Japanese population, relationships between CL/P or CPO and seven candidate genes (TGFB3, DLX3, PAX9, CLPTM1, TBX10, PVRL1, TBX22) that showed positive associations in other populations and are expressed in the oral/lip region in developing mice. We first searched for mutations in these genes among 112 CL/P and 16 CPO patients, and found a heterozygous missense mutation (640A > G, S214G) in exon 3 of PAX9 in two sibs with CL/P and their phenotypically normal mother from a Japanese family. A population-based case-control analysis and a family-based transmission disequilibrium test (TDT), using single nucleotide polymorphisms (SNPs), and two-SNP haplotypes of the genes, between the 112 CL/P cases with their parents and 192 controls indicated a significant association at one SNP site, IVS1 + 5321, in TGFB3 with a P-value of 0.0016. Population-based haplotyping revealed that the association was most significant for haplotype "A/A" consisting of IVS1 + 5321 and IVS1 - 1572; TDT also gave a P-value of 0.0252 in this haplotype.

Reduction of Pax9 gene dosage in an allelic series of mouse mutants causes hypodontia and oligodontia

Missing teeth (hypodontia and oligodontia) are a common developmental abnormality in humans and heterozygous mutations of PAX9 have recently been shown to underlie a number of familial, non-syndromic cases. Whereas PAX9 haploinsufficiency has been suggested as the underlying genetic mechanism, it is not known how this affects tooth development. Here we describe a novel, hypomorphic Pax9 mutant allele (Pax9neo) producing decreased levels of Pax9 wild-type mRNA and show that this causes oligodontia in mice. Homozygous Pax9neo mutants (Pax9neo/neo) exhibit hypoplastic or missing lower incisors and third molars, and when combined with the null allele Pax9lacZ, the compound mutants (Pax9neo/lacZ) develop severe forms of oligodontia. The missing molars are arrested at different developmental stages and posterior molars are consistently arrested at an earlier stage, suggesting that a reduction of Pax9 gene dosage affects the dental field as a whole. In addition, hypomorphic Pax9 mutants show defects in enamel formation of the continuously growing incisors, whereas molars exhibit increased attrition and reparative dentin formation. Together, we conclude that changes of Pax9 expression levels have a direct consequence for mammalian dental patterning and that a minimal Pax9 gene dosage is required for normal morphogenesis and differentiation throughout tooth development.

Association between PAX-9 promoter polymorphisms and hypodontia in humans

Hypodontia, the congenital absence of one or a few teeth, is one of the most common alterations of the human dentition. The most common permanent missing teeth are the third molars, second premolars, and maxillary lateral incisors. Hypodontia does not represent a serious public health problem, but it may cause masticatory and speech dysfunctions, and esthetic problems. PAX 9 is believed to play an important role in tooth development. It is expressed at initiation, bud, cap, and bell stages of odontogenesis. Mutations in PAX 9 coding sequences have been implicated in autosomal dominant oligodontia affecting predominantly permanent molars and second premolars. Here, we report two polymorphisms in the promoter region of PAX 9 gene that are associated with hypodontia. DNA was extracted from buccal epithelial cells of 106 healthy Control individuals and of 102 unrelated individuals with hypodontia. PCR-RFLP was employed in the investigation of G-1031 A and T-912 C polymorphisms. Significant differences were obtained comparing Control and Test groups. Alleles G and T were found at a significant higher frequency in individuals with hypodontia, whereas alleles A and C were more frequent in Control subjects, p=0.0094 and 0.0086, respectively. The GT haplotype was significantly more prevalent in the hypodontia group, while the AC haplotype was more frequent in the Control group. These results indicate that polymorphisms in the promoter region of PAX 9 gene may have an influence on the transcriptional activity of this gene and are associated with hypodontia in humans.

Defining a holoprosencephaly locus on human chromosome 14q13 and characterization of potential candidate genes

Holoprosencephaly (HPE) is the most common developmental field defect in patterning of the human prosencephalon and associated craniofacial structures. The genetics is complex, with 12 loci defined on 11 chromosomes. We defined a locus for HPE (HPE8) on human chromosome 14q13 between markers D14S49 and AFM205XG5, by mapping deletion intervals of affected subjects with proximal chromosome 14q interstitial cytogenetic deletions. A 35-BAC contig was built by chromosome walking. By annotation of the 2.82-Mb minimal critical region, we identified 28 possible genes. Seven genes were expressed in human fetal brain: NPAS3, SNX6, C14ORF11, C14ORF10, PAX9, NKX2.1, and C14ORF19, the last an apparent gene fragment. Molecular embryology, animal modeling, and human mutation studies were reported elsewhere for PAX9 and NKX2.1. We focused on three genes, SNX6, NPAS3, and C14ORF11, as potential candidates for HPE. Genomic structure, human expression patterns, protein cellular localization, and embryonic expression patterns of orthologous murine genes were determined, showing that the three genes have properties similar to those of known HPE genes.

Human blood genomics: distinct profiles for gender, age and neurofibromatosis type 1

Application of gene expression profiling to human diseases will be limited by availability of tissue samples. It was postulated that germline genetic defects affect blood cells to produce unique expression patterns. This hypothesis was addressed by using a test neurological disease-neurofibromatosis type 1 (NF1), an autosomal dominant genetic disease caused by mutations of the NF1 gene at chromosome 17q11.2. Oligonucleotide arrays were used to survey the blood gene expression pattern of 12 NF1 patients compared to 96 controls. A group of genes related to tissue remodeling, bone development and tumor suppression were down-regulated in NF1 blood samples. In addition, there were blood genomic patterns for gender and age: Y chromosome genes showing higher expression in males, indicating a gene-dosage effect; and genes related to lymphocyte functions showing higher expression in children. The results suggest that genetic mutations can be manifested at the transcriptional level in peripheral blood cells and blood gene expression profiling may be useful for studying phenotypic differences of human genetic diseases and possibly providing diagnostic and prognostic markers.

Studies on Pax9-Msx1 protein interactions

Pax9 belongs to the Pax family of transcriptional regulators that are defined by a highly conserved DNA-binding region, the paired domain. Drosophila, mouse and human genetics have shown that Pax proteins play multiple roles in tissue patterning and organogenesis by mediating their functions in a highly tissue-specific manner. Members of the Pax family, Pax9 and Pax1, act synergistically during vertebral formation. However, only Pax9 is essential for tooth formation. Furthermore, mutations of PAX9 are associated with human tooth agenesis. The highly tooth-specific molecular functions of Pax9 suggest that its activity is tightly regulated. Most likely, this occurs through interactions with other protein factors. Among the regulatory molecules that are expressed in dental mesenchyme, the Msx1 homeoprotein is of particular interest. The closely overlapping expression patterns of Pax9 and Msx1 are consistent with a role in epithelial-mesenchymal interactions. To demonstrate that Pax9 interacts with Msx1 physiologically in vivo and in vitro, we performed co-immunoprecipitation and GST interaction assays. Our results indicate that there is a physical association between the two proteins. Our biochemical data, coupled with human genetic studies and expression analysis in a mouse model, indicate a functional relationship between Pax9 and Msx1 during tooth development.

MSX1, PAX9, and TGFA contribute to tooth agenesis in humans

In this study, we sought to determine the association between tooth agenesis and DNA sequence variation in the genes MSX1 and PAX9 in an ethnically diverse human population. Since cleft lip/palate is also associated with both tooth agenesis and the gene TGFA, we included TGFA in the analysis as well. Cheek swab samples were obtained for DNA analysis from 116 case/parent trios. Probands had at least one developmentally missing tooth, excluding third molars. Genotyping was performed by single-strand conformational polymorphism or kinetic polymerase chain-reaction assays. Transmission distortion of the marker alleles and DNA sequence analysis was performed. Results showed that tooth agenesis is associated with markers of the genes MSX1 and TGFA. No mutations were found in MSX1 or PAX9 coding regions. There were statistically significant data suggesting that MSX1 interacts with PAX9. These findings suggest that MSX1, PAX9, and TGFA play a role in isolated dental agenesis.

MSX1 gene is deleted in Wolf-Hirschhorn syndrome patients with oligodontia

Abnormalities of the short arm of chromosome 4 cause multiple congenital malformations, including craniofacial, oral, and dental manifestations. A candidate gene for oral defects in this region is MSX1, which is mandatory for normal oral and tooth development. We examined the dentition and the presence of MSX1 in eight Finnish patients with abnormalities of 4p, including seven cases of Wolf-Hirschhorn syndrome. Five of the Wolf-Hirschhorn syndrome patients presented with agenesis of several teeth, suggesting that oligodontia may be a common (even though previously not well-documented) feature in Wolf-Hirschhorn syndrome. In fluorescence in situ hybridization (FISH) analysis, the five patients with oligodontia lacked one copy of MSX1, while the other three had two hybridization signals. One of these presented with the only case of cleft palate among the patients. Our result confirms that haploinsufficiency for MSX1 serves as a mechanism that causes selective tooth agenesis but, alone, is not enough to cause oral clefts.

A missense mutation in PAX9 in a family with distinct phenotype of oligodontia

Mutations in PAX9 have been described for families in which inherited oligodontia characteristically involves permanent molars. Our study analysed one large family with dominantly inherited oligodontia clinically and genetically. In addition to permanent molars, some teeth were congenitally missing in the premolar, canine, and incisor regions. Measurements of tooth size revealed the reduced size of the proband's and his father's deciduous and permanent teeth. This phenotype is distinct from oligodontia phenotypes associated with mutations in PAX9. Sequencing of the PAX9 gene revealed a missense mutation in the beginning of the paired domain of the molecule, an arginine-to-tryptophan amino-acid change occurring in a position absolutely conserved in all sequenced paired box genes. A mutation of the homologous arginine of PAX6 has been shown to affect the target DNA specificity of PAX6. We suggest that a similar mechanism explains these distinct oligodontia phenotypes.

Functional Analysis of a Mutation in PAX9 Associated with Familial Tooth Agenesis in Humans

Pax9 is a paired domain-containing transcription factor that plays an essential role in the patterning of murine dentition. In humans, mutations in PAX9 are associated with unique phenotypes of familial tooth agenesis that mainly involve posterior teeth. Among these, a frameshift mutation (219InsG) within the paired domain of PAX9 produces a protein product associated with a severe form of molar agenesis in a single family. The objectives of this study were to gain new insights into the molecular pathogenesis of the 219InsG mutation and its role in tooth agenesis. Here we describe functional defects in DNA binding and transactivation of mutant 219InsGPax9. Although wild type Pax9 binds to the high affinity paired domain recognition sequences, e5 and CD19-2(A-ins), the 219InsGPax9 mutant protein was unable to bind to these cognate DNA-binding sites. In co-transfection assays, wild type Pax9 activated reporter gene transcription although the mutant was transcriptionally inactive. Immunolocalization data show that Pax9 and 219InsGPax9 proteins are synthesized in mammalian cells but that the nuclear localization of the mutant Pax9 protein is altered. Furthermore, transactivation by the full-length Pax9 protein from paired domain binding sites was not impaired by the 219InsGPax9 mutant. The latter did not alter the DNA binding activities of wild type Pax9 in gel mobility shift assays. The combined defects in DNA binding activities and transactivation function of mutant 219InsGPAX9 likely alter the selective activation and/or repression of PAX9 effector genes during odontogenesis. This loss-of-function of PAX9 most likely results in its haploinsufficiency during the patterning of dentition and the subsequent loss of posterior teeth.

A novel missense mutation in the paired domain of PAX9 causes non-syndromic oligodontia

PAX9, a paired domain transcription factor, has important functions in craniofacial and limb development. Heterozygous mutations of PAX9, including deletion, nonsense, or frameshift mutations that lead to a premature stop codon, and missense mutations, were previously shown to be associated with autosomal dominant oligodontia. Here, we report a novel missense mutation that lies in the highly conserved paired domain of PAX9 and that is associated with non-syndromic oligodontia in one family. The mutation, 83G-->C, is predicted to result in the substitution of arginine by proline (R28P) in the N-terminal subdomain of PAX9 paired domain. To rule out the possibility that this substitution is a rare polymorphism and to test whether the predicted amino acid substitution disrupts protein-DNA binding, we analyzed the binding of wild-type and mutant PAX9 paired domain to double-stranded DNA targets. The R28P mutation dramatically reduces DNA binding of the PAX9 paired domain and supports the hypothesis that loss of DNA binding is the pathogenic mechanism by which the mutation causes oligodontia.

Growth and specification of the eye are controlled independently by Eyegone and Eyeless in Drosophila melanogaster

Control of growth determines the size and shape of organs. Localized signals known as 'organizers' and members of the Pax family of proto-oncogenes are both elements in this control. Pax proteins have a conserved DNA-binding paired domain, which is presumed to be essential for their oncogenic activity. We present evidence that the organizing signal Notch does not promote growth in eyes of D. melanogaster through either Eyeless (Ey) or Twin of eyeless (Toy), the two Pax6 transcription factors. Instead, it acts through Eyegone (Eyg), which has a truncated paired domain, consisting of only the C-terminal subregion. In humans and mice, the sole PAX6 gene produces the isoform PAX6(5a) by alternative splicing; like Eyegone, this isoform binds DNA though the C terminus of the paired domain. Overexpression of human PAX6(5a) induces strong overgrowth in vivo, whereas the canonical PAX6 variant hardly effects growth. These results show that growth and eye specification are subject to independent control and explain hyperplasia in a new way.