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of dental features possible using 3-D laser technology. The value of twin studies in dental phenomics Our studies of Australian twins com- menced in the School of Dentistry at the University of Adelaide in the early 1980s, and currently there are records of more than 1,200 pairs of twins across three main cohorts of partici- pants. The first cohort consists of around 300 pairs of teenage twins, for whom various records have been collected, including dental casts, facial photo- graphs, finger and palm prints and information on laterality, including handedness. The second cohort comprises around 300 pairs of twins who have been ex- amined at three stages of dental de- velopment from approximately 4 to 14 years: at primary, mixed and perma- nent dentition stages. The most recent study of tooth emer- gence and oral health provides a third cohort of around 600 twin pairs, aged from birth to around 5 to 6 years. Our broad aim in these studies has been to improve our understanding of how genetic, environmental and epigenetic factors contribute to varia- tion in dental and facial features, and to oral health.7 Findings from our studies showing dental discordance between MZ co- twins in relation to tooth size, missing teeth, extra teeth and asymmetrical expression, not only highlight the im- portance of epigenetic influences on human dental development but are consistent with the concept that there is a group of genes that not only influ- ence the size and shape of teeth but also the expression of missing or extra teeth, i.e., there are pleiotropic genetic effects operating on the human denti- tion, as well as spatial and/or temporal variations in local epigenetic events during odontogenesis, that lead to distinct phenotypic differences in the dentition, even in genetically identical twin pairs.1 While the twin pair described in this article illustrates the value of identify- ing differences in MZ co-twins as well as their similarities (i.e., the MZ co- twin model), there are other twin mod- els available to researchers. These in- clude the traditional or classical twin model (comparisons between MZ twin pairs and dizygotic (DZ) twin pairs, which enable heritability estimates to be calculated); the twins-reared-apart model; the investigation of twins and other family members; the MZ half- sibling model; and the DZ opposite sex model.8 Case study: MZ co-twins showing similarities, dissimilarities and asymmetry in the expression of various dental features We have found many examples of MZ twin pairs who exhibit varying de- grees of similarity and dissimilarity in the expression of various dental fea- tures. This particular case study describes a pair of female MZ co-twins selected from the second cohort for whom fa- cial photographs, and study models of the primary and mixed dentition are available. Zygosity was determined by analysis of up to six highly variable genetic loci (FES, vWA31, F13A1, THO1, D21S11, FGA) on six different chromo- somes, using DNA obtained from buc- cal cells. This particular MZ co-twin pair has been selected to highlight how similar- ities, dissimilarities and asymmetry of dental features may all occur in a pair of genetically identical twins. Similarities in the expression of Carabelli trait Carabelli trait is evident on the lingual aspect of the mesio-lingual cusp for all primary maxillary second molars and permanent maxillary first molars (Figure 2) for both Twin A and Twin B (arrowed). Qualitative comparison of the size and position of the feature suggests a high degree of similarity in the expression of the trait for both dentitions (primary and permanent), whether making intra-twin and/or in- ter-twin comparisons. Dissimilarities in the expression of fused primary teeth Study models of the lower primary dentition for Twins A and B are shown in Figure 3. Twin B exhibits fused later- al incisor and canine teeth, bilaterally (arrowed), while Twin A does not exhib- it any fused teeth. The assumption that the teeth are fused (in contrast to be- ing geminated) is based on the fact that there is one less tooth than expected in each quadrant. Asymmetry (and mirror-imaging) in the expression of retained primary maxillary lateral incisor teeth One particularly interesting expres- sion of asymmetry that can be ob- served in MZ twin pairs is the phe- nomenon of mirror imaging, where one twin mirrors the other for one or more features. An example of mirror imaging is shown in Figure 4, which presents a frontal view of the upper primary dentition of Twins A and B. The upper left lateral incisor of Twin B is worn and retained (with the upper right lateral incisor of Twin B missing), while Twin A exhibits a mirror image of this, i.e., the upper right lateral in- cisor is worn and retained (with the upper left lateral incisor missing) (ar- rowed). There is evidence to suggest that many of these dental features dis- cussed may be inter-related, for ex- ample, Carabelli trait and the size of teeth,9 and anomalies of number, size and shape of teeth.10,11 This case study highlights how MZ co-twins provide an extremely valu- able research model, for example, just one pair of MZ co-twins displaying similarities, dissimilarities and asym- metry in their dentitions offers great opportunity to understand more deep- ly the underlying biological processes of tooth formation. Use of high-precision 2-D and 3-D imaging equipment will not only en- able us to quantify and describe den- tal variations, such as Carabelli trait, in more detail than has been possible previously, but to also define new phe- notypes that we have not been able to measure previously, including small grooves and tubercles, as well as crown contours, areas and volumes. How can dental phenomics enhance future understanding of biological processes related to dental development? We plan to maximize the use of the longitudinal data and DNA we have collected, and continue to collect, by performing genome-wide scans for pu- tative genetic linkage peaks for a range of dental features, and then to test for association between a series of likely candidate genes and our phenotypes. Identifying the key genes for dental development in humans would not only provide clinicians with a sounder scientific basis for monitoring indi- viduals predisposed to developmental problems (e.g., missing teeth, maloc- clusions) but assist when counselling patients, especially where there is a fa- milial history. By developing the field of dental phe- nomics we hope to better understand how genetic, environmental and epi- genetic factors interact to produce the extensive range of variation observed in the human dentition. Authors’ contact information Suzanna Mihailidis, BDS, BEc, PhD Craniofacial Biology Research Group Lecturer, School of Dentistry The University of Adelaide, South Australia 5005, Australia suzanna.mihailidis@adelaide.edu.au ˙ References 1. Townsend G, Brook A. Genetic, epigenetic and environmental influences on dental development. Ortho Tribune 2008;3:3–6. 2. Townsend G, Bockmann M, Hughes T, Mi- hailidis S, Seow WK, Brook A. New ap- proaches to dental anthropology based on the study of twins. In: Townsend G, Kanazawa E, Takyama H (Eds.) New Direc- tions in Dental Anthropology: paradigms, methodologies and outcomes. University of Adelaide Press, Adelaide, 2012. 3. Houle D, Govindaraju DR, Omholt S. Phe- nomics: the next challenge. Nature Rev Genet 2010;11:855–866. 4. Pillas D, Hoggart CJ, Evans DM, O’Reilly PF, Sipila K, Lahdesmaki R, et al. Genome- wide association study reveals multiple loci associated with primary tooth devel- opment during infancy. PLoS Genet 2010;6:1–7. 5. Ashar A, Hughes T, James H, Kaidonis J, Khamis F, Townsend G. Dental crown and arch size in Europeans, Australian Aborigi- nals and Malaysians: 2-D and 3-D ap- proaches. In: Townsend G, Kanazawa E, Takyama H (Eds.) New Directions in Den- tal Anthropology: paradigms, methodolo- gies and outcomes. University of Adelaide Press, Adelaide, 2011. 6. Smith R, Zaitoun H, Coxon T, Karmo M, Gurpeet K, Townsend G, Harris EF, Brook A. Defining new dental phenotypes using 3-D image analysis to enhance discrimina- tion and insights into biological process- es. Arch Oral Biol 2009;54S:S118–S125. 7. Townsend G, Richards L, Brearley Messer L, Hughes T, Pinkerton S, Seow K, Gotja- manos T, Gully N, Bockmann M. Genetic and environmental influences on dento- facial structures and oral health: studies of Australian twins and their families. Twin Res Hum Genet 2006;9:727–732. 8. Townsend G, Hughes T, Luciano M, Bock- mann M, Brook A. Genetic and environ- mental influences on human dental varia- tion: A critical evaluation of studies involving twins. Arch Oral Biol 2009;54S:S45–S51. 9. Kondo S, Townsend GC. Associations be- tween Carabelli trait and cusp areas in hu- man permanent maxillary first molars. Am J Phys Anthropol 2006;129:196-203. 10. Townsend GC, Richards L, Hughes T, Pinkerton S, Schwerdt W. Epigenetic influ- ences may explain dental differences in monozygotic twin pairs. Aust Dent J 2005;50:95–100. 11. Brook AH. Multilevel complex interac- tions between genetic, epigenetic and environmental factors in the aetiology of anomalies of dental development. Arch Oral Biol 2009;54S:S3–S17. Figs. 2a, 2b: Photo image of upper primary study models for MZ co-twins A [Fig. 2a] and B [Fig. 2b] showing Carabelli trait (occlusal view). Figs. 3a, 3b: Photo image of lower study models for MZ co-twins A [Fig 3a] and B [Fig 3b] showing frontal view of fused primary lateral incisor and canine teeth in co-twin B. Figs. 4a, 4b: Photo image of upper study models for MZ co-twins A [Fig. 4a] and B [Fig 4b] showing frontal view of asymmetrically retained primary lateral incisor teeth. Fig. 2a Fig. 2b Fig. 3a Fig. 3b Fig. 4a Fig. 4b Dental Tribune U.S. Edition | April 2013 A5CLINICAL