The value of twins in dental research

Facial morphology differences in monozygotic twins: a retrospective stereophotogrammetric study

OBJECTIVE

To assess soft tissue differences between monozygotic twins (MZ) for the total face and between facial regions using three-dimensional (3D) stereophotogrammetry and quantitative surface-based 3D deviation analyses.

MATERIALS AND METHODS

The study sample consisted of 14 untreated MZ twins (6 males, 8 females, mean age: 14.75 years) from the archive of Marmara University, Department of Orthodontics. The images were taken by the 3dMDface system, and 3dMDvultus software was used for removal of undesired areas and approximation of the images. Then, stereolithography (.stl) format images were superimposed using the best-fit algorithm using 3-matic software. The face was divided into facial thirds, and upper lip and lower lip + chin regions were created. For the comparison, 3D deviation analyses were performed, and a color map and histogram were created. The data were presented as mean deviation, root mean square (RMS), median, and interquartile range.

RESULTS

Between the facial thirds, there was no significant difference in soft tissue differences for mean deviation. A statistically significant difference was found between the upper and lower face for the RMS value. For the comparison of upper lip and lower lip + chin region, the only significant difference was for the RMS. When the data were presented as median and interquartile range, there were no statistically significant differences between any facial regions.

CONCLUSIONS

Lower facial third and lower lip + chin regions had the greatest differences within MZ twin pairs. The genetic and environmental influences might not be the same for different parts of the face.

The contribution of genetics to dental caries, oral habits and occlusal traits in Turkish twins: A comparative study

OBJECTIVE

The aims of this study were to investigate and compare the prevalence of dental caries and the frequency of oral habits, molar relationships and occlusal traits between children of multiple births, and singletons, and to determine the relative contributions of genetics and environmental factors to these parameters by using twin study design.

METHODS

The study group consisted of 345 multiple births (34 monozygotic and 122 dizygotic twin pairs, 11 sets of triplets) and 345 singletons between the ages of 2 and 17. The prevalence of dental caries, and the frequency of tooth brushing, the children's oral habits, molar relationships, and occlusal traits were recorded.

RESULTS

The percentage of children who brushed their teeth more than twice daily was statistically significantly higher in multiple births than in singletons. Higher correlation coefficients were found in dental caries index, except for decayed, filled (df) (2-5 age group) and filled (f) (6-11 age group), in the monozygotic twin pairs compared to those in the dizygotic twin pairs. In children between the ages of 6 and 11 years, mouth breathing, bruxism, lip biting, and pencil biting were higher in singletons than in children of multiple births. There were statistically significant differences between children of multiple births and singletons, with increased overjet in the 2-5 year age group being observed.

CONCLUSION

When analyzing these parameters, environmental factors must also be investigated. Due to the low incidence of twin births, longitudinal follow-up studies with more twin pairs are necessary to determine whether these results are generalizable.

Morphological Diversity of Permanent Maxillary Lateral Incisors and Their Impact on Aesthetics and Function in Orthodontically Treated Patients

The aim of this study was to determine the frequency of different crown shapes and associated dental anomalies of the permanent maxillary lateral incisor (PMLI) and their impact on aesthetics and occlusion (function) in orthodontically treated patients. Materials and Methods: The records of 372 subjects, which consisting of study casts, panoramic radiographs and anamnestic data, were investigated for crown shape and associated dental anomalies of PMLI and potential dental treatment to achieve satisfactory aesthetics and function. Descriptive statistics, including means, standard deviations and percentages for the observed variables, were calculated. Data were analyzed using the chi-square test. p-Values < 0.05 were considered as statistically significant. Results: The results showed that the most common crown shapes of PMLI were trapezoidal-shaped (59.8%), followed by central incisor-shaped (26.7%), canine-shaped (11.2%) and peg-shaped (2.3%), without statistically significant difference between genders. Developmental anomalies were found in 86 (11.6%) PMLI. All subjects with developmental anomalies were included in orthodontic treatment, and 91.2% of them need interdisciplinary treatment to achieve satisfactory aesthetics and function. Conclusions: The morphological diversity and developmental anomalies of the PMLI may affect aesthetics and function and should be considered in treatment planning.

Oral microbiota and dental caries data from monozygotic and dizygotic twin children

There are recent studies which aimed to detect the inheritance on the etiology of dental caries exploring oral composition. We present data on the oral microbiota and its relation with dental caries and other factors in monozygotic (MZ) and dizygotic (DZ) twin children. Following clinical investigation, DNA samples were collected and isolated from saliva of 198 patients (49 MZ and 50 DZ twins) with an average age of 9.7 ± 2.7 years. Salivary bacterial microbiota analysis was performed using high throughput amplicon sequencing method targeting V3-V4 region of the 16S rRNA gene. A total of 8,297,859 raw reads corresponding to 41,908 reads per sample were obtained on average. The QIIME2-deblur workflow was used for 16S rRNA amplicon analysis. Microbiome similarity analyses between twins (based on Bray-Curtis dissimilarity, weighted and unweighted Unifrac distances) showed that monozygotic twins share more bacterial microbial content compared to dizygotic twins. This is a large microbial community dataset of MZ and DZ twins with or without dental findings which can be further used for children oral microbiome profile explorations.

Measurement(s)	Oral Microbiome • DNA • dental health
Technology Type(s)	twin design • amplicon sequencing • Examination
Factor Type(s)	zygosity status • age • sex
Sample Characteristic - Organism	Microbiota • Homo sapiens
Sample Characteristic - Environment	saliva

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12987797

Heritability of Craniofacial Characteristics in Twins - Cephalometric Study

Introduction:

The growth and development of the craniofacial complex are under the influence of genetic and environmental factors, which determine its morphological and functional characteristics. Twin studies provide significant insight into how many genetic and environmental factors determine dental and craniofacial characteristics.

Aim:

The aim of the study was to determine the genetic influence on craniofacial complex using a twin study model.

Methods:

The study sample comprised 52 pairs of twins who were referred to the Orthodontic Department, School of Dental medicine, University of Sarajevo. Informed consent was obtained by the parents of the children included in the study. Twenty pairs of twins were diagnosed as monozygotic while 32 pairs were diagnosed as dizygotic. Zygosity was diagnosed by physical characteristics similarity. Nineteen variables were measured: 10 dental variables, 9 cephalometric.

Results:

Based on the findings of this study, t-test showed significant genetic effect on the length of cranial base (p = 0.03), corpus of maxilla (p = 0.02) and mandibular length (p = 0.03), and also for B-angle (p = 0.04). Environmental factors are more involved in determining dental traits (e.g., the inclination of the incisors).

Conclusion:

There is a significant genetic effect on the linear cephalometric variables: the length of the cranial base, maxillary length and mandibular length.

Facial soft tissue growth in identical twins

INTRODUCTION

The aims of this longitudinal analysis of untreated monozygotic twins were to investigate the change of the facial soft tissues during growth, to determine the concordance of soft tissue growth patterns between genetically identical twins, and to assess the genetic component of soft tissue development.

METHODS

The sample consisted of 33 pairs of untreated monozygotic twins (23 male, 10 female) from the Forsyth Moorrees Twin Study (1959-1975); lateral cephalograms taken from 6 to 18 years of age were analyzed at 3-year intervals. Cephalograms were traced, and longitudinal changes in the soft tissue profile between twins were analyzed with intraclass correlation coefficients and linear regression modelling.

RESULTS

The concordance between monozygotic twins at 18 years of age was moderate to high with intraclass correlation coefficients values between 0.37 and 0.87. Additionally, female twins showed higher concordance at 18 years of age than did male twins for all included variables. However, about 10% to 46% of the twin pairs had large differences in their soft tissue parameters, even after the growth period.

CONCLUSIONS

Although monozygotic twins possess the same genetic material, differences in the soft tissues were found. This supports the complex developmental mechanism of the human face and the varying influence of genetic and environmental factors.

Research on oral microbiota of monozygotic twins with discordant caries experience - in vitro and in vivo study

Oral microbiome is potentially correlated with many diseases, such as dental caries, periodontitis, oral cancer and some systemic diseases. Twin model, as an effective method for studying human microbiota, is widely used in research of relationship between oral microbiota and dental caries. However, there were few researches focusing on caries discordant twins. In this study, in vitro assays were conducted combined with 16S rRNA sequencing analysis on oral microbiota sampled from twins who presented discordant caries experience and mice model was developed as well. Results showed that oral microbiota from caries-active twin possessed higher metabolic activity and produced more lactic production. 16S rRNA sequencing analysis showed that more than 80% of family taxa could be transferred into gnotobiotic-mice. Key caries-associated genera were significantly different between twins and the same difference in genus level could be found in mice as well (p < 0.05). This study suggested that oral microbiota of twins could be distinguished from each other despite the similarities in genetic make-up, living environment, and lifestyle. The difference in microbiota was applied to develop a mice model which may facilitate the investigation of core microbiota of dental caries.

Exploring the association between genetic and environmental factors and molar incisor hypomineralization: evidence from a twin study

BACKGROUND

The etiology of molar-incisor hypomineralization (MIH) remains unknown. Studies indicate that it is multifactorial, and that genetic and environmental factors are involved. Research with twins provides important subsidy to investigate the Influence of genetics and environmental factors that act during pregnancy on the etiology of alterations.

AIM

This cross-sectional study evaluated the agreement of molar incisor hypomineralization (MIH) between monozygotic and dizygotic twin pairs and the association with environmental factors.

DESIGN

The sample consisted of 167 pairs of twins (8-15 years old), 94 monozygotic and 73 dizygotic. The parents answered a questionnaire on sociodemographic data and pre-, peri-, and postnatal health. A dental examination was performed by two calibrated examiners (Kappa ≥0.88) for MIH diagnosis, following the criteria proposed by the European Academy of Pediatric Dentistry in 2003.

RESULTS

The prevalence of MIH was 29.3%. There was greater concordance of MIH between monozygotic twins for affected first molars and permanent incisors (P = 0.0012) and pairs of twins assessed (P = 0.0211). The presence of MIH was associated with family income between one and two wages (P = 0.009, prevalence ratio [PR] = 3.82, confidence interval [CI 95%] 1.40-10.44), above two wages (P = 0.007, PR = 4.60, 95% CI: 1.51-14.05), and gestational hemorrhage (P = 0.032, PR = 5.70, 95% CI: 1.16-28.14).

CONCLUSIONS

The greater concordance in the diagnosis of MIH among monozygotic twins indicates a genetic influence, although environmental factors, such as family income and hemorrhage during pregnancy, are also associated with the occurrence of MIH.

Growth and Development of Dentofacial Complex influenced by Genetic and Environmental Factors using Monozygotic Twins

AIM

The purpose of this study was to determine the possible effects of genetic and environmental factors on dentofacial complex using monozygotic twins.

MATERIALS AND METHODS

The study sample was made of 21 pairs of monozygotic twins (14 female pairs and seven male pairs) between 10 and 25 years. Pretreatment lateral cephalo-grams were used which were traced and digitized, and various landmarks to determine the anteroposterior and vertical proportions were marked. Samples were divided into two groups. The correlation between groups was found by calculating Pearson's product moment correlation coefficients.

RESULTS

The range of the correlation coefficient was from 0.705 to 0.952. Gonial angle showed the highest correlation coefficient (0.952), while saddle angle showed the lowest correlation coefficient (0.705).

CONCLUSION

The growth and development of craniofacial complex is under mutifactorial control. However, genetic influences do tend to play a dominant role.

CLINICAL SIGNIFICANCE

By studying identical twins, we can study about the interaction of the environment with the genes and how it affects the growth and development of the body in general and dentofacial complex in particular. By utilizing twin studies, we can identify whether a particular trait, disease, or disorder is influenced more strongly by genetics or by the environment. Success of orthodontic treatment depends on a proper diagnosis of the problem including its etiological factors. Genetic studies let the orthodontists to understand the effects of genetic and environmental factors in the growth and development of dentofacial complex better and allows to prevent or treat malocclusions and skeletal anomalies in better ways.

Handedness and schizotypy: The potential effect of changing the writing-hand

Mixed-handedness, which has been associated with schizotypy in recent studies, might exist for at least two different reasons: it is innate or it has been forced. We examined whether the association between mixed-handedness and schizotypy differs depending on its source. We enrolled 3485 college students in Taiwan. We used both the Perceptual Aberration Scale and Schizotypal Personality Questionnaire to assess schizotypy, and the Annett Hand Preference Questionnaire to assess handedness. Two ways of classifying handedness were examined: the three-way classification based on Annett's categories and mixed- vs. non-mixed-handedness based on Degree of Handedness. Both mixed-handedness groups showed higher positive schizotypy scores. Among mixed handers, those who had been required to change their writing hand from left to right had higher positive schizotypy scores. Being forced to change writing hand seemed to be related to a higher level of schizotypy. The potential effect of the social pressure against using the left hand for writing is discussed.

Estimation of heritability for varied indexes of handedness

Inconsistent results of the molecular studies for handedness have been reported. One of the key issues involved could be ways of assessing handedness. The current study aimed to identify the index of handedness better reveal the genetic component, which showed higher heritability. We measured handedness using the Annett's handedness questionnaire. The college students participating in this study filled the questionnaire in the class while their first-degree relatives returned questionnaires one to two weeks later. A total of 1,968 subjects returned their questionnaires, including 640 college students and 1,328 first-degree relatives. Among the 449 college students returning at least one handedness questionnaire for their parents, a total of 449 fathers, 440 mothers, and 425 siblings participated in the study. The index of mixed-handedness (e.g., Degree of Handedness) showed highest heritability (0.67), followed by the Hand Preference Index (0.52), and then the Direction of Handedness (0.39). Using an index of mixed-handedness for future molecular studies was suggested.

Analysis of polymorphisms in the lactotransferrin gene promoter and dental caries

Regarding host aspects, there has been strong evidence for a genetic component in the etiology of caries. The salivary protein lactotransferrin (LTF) exhibits antibacterial activity, but there is no study investigating the association of polymorphisms in the promoter region of LTF gene with caries. The objective of this study was firstly to search the promoter region of the human LTF gene for variations and, if existent, to investigate the association of the identified polymorphisms with dental caries in 12-year-old students. From 687 unrelated, 12-year-old, both sex students, 50 individuals were selected and divided into two groups of extreme phenotypes according to caries experience: 25 students without (DMFT = 0) and 25 with caries experience (DMFT ≥ 4). The selection of individuals with extreme phenotypes augments the chances to find gene variations which could be associated with such phenotypes. LTF gene-putative promoter region (+39 to −1143) of the selected 50 individuals was analyzed by high-resolution melting technique. Fifteen students, 8 without (DMFT = 0) and 7 with caries experience (mean DMFT = 6.28), presented deviations of the pattern curve suggestive of gene variations and were sequenced. However, no polymorphisms were identified in the putative promoter region of the LTF gene.

Timing of colonization of caries-producing bacteria: an approach based on studying monozygotic twin pairs

Findings are presented from a prospective cohort study of timing of primary tooth emergence and timing of oral colonization of Streptococcus mutans (S. mutans) in Australian twins. The paper focuses on differences in colonization timing in genetically identical monozygotic (MZ) twins. Timing of tooth emergence was based on parental report. Colonization timing of S. mutans were established by plating samples of plaque and saliva on selective media at 3 monthly intervals and assessing colony morphology. In 25% of individuals colonization occurred prior to emergence of the first tooth. A significant proportion of MZ pairs (21%) was discordant for colonization occurring before or after first tooth emergence, suggesting a role of environmental or epigenetic factors in timing of tooth emergence, colonization by S. mutans, or both. These findings and further application of the MZ co-twin model should assist in development of strategies to prevent or delay infection with S. mutans in children.

Ankylosis of permanent first molars: genetics or environment? A case report of a discordant twin pair

The rare condition of secondary retention has been reported in the literature as being of genetic origin, with some authors suggesting an autosomal dominant pattern. We report the unusual case of two monozygotic biamniotic, bichorionic male twins, who were discordant for permanent first molar secondary retention, involving ankylosis. Twin A showed normal occlusion and eruption patterns, whereas Twin B displayed a left open bite, in relation with a totally submerged primary second molar leading to retention of the underlying premolar (35), and severe infraocclusion of the adjacent permanent molar (36). After orthodontic failure to close the open bite, ankylosis of 36 was confirmed, whereas 26 became severely infraoccluded.The mother had a history of bilateral molar ankylosis and presented reduced posterior alveolar height. Discordance in this twin pair demonstrates that environmental influences, in addition to epigenetic and local factors, may play a role in secondary retention, which is difficult to diagnose and challenging to treat.

Genetic influences on handedness: data from 25,732 Australian and Dutch twin families

Handedness refers to a consistent asymmetry in skill or preferential use between the hands and is related to lateralization within the brain of other functions such as language. Previous twin studies of handedness have yielded inconsistent results resulting from a general lack of statistical power to find significant effects. Here we present analyses from a large international collaborative study of handedness (assessed by writing/drawing or self report) in Australian and Dutch twins and their siblings (54,270 individuals from 25,732 families). Maximum likelihood analyses incorporating the effects of known covariates (sex, year of birth and birth weight) revealed no evidence of hormonal transfer, mirror imaging or twin specific effects. There were also no differences in prevalence between zygosity groups or between twins and their singleton siblings. Consistent with previous meta-analyses, additive genetic effects accounted for about a quarter (23.64%) of the variance (95%CI 20.17, 27.09%) with the remainder accounted for by non-shared environmental influences. The implications of these findings for handedness both as a primary phenotype and as a covariate in linkage and association analyses are discussed.

Heritability of oral microbial species in caries-active and caries-free twins

Oral microbes that colonize in the mouths of humans contribute to disease susceptibility, but it is unclear if host genetic factors mediate colonization. We therefore tested the hypothesis that the levels at which oral microbes colonize in the mouth are heritable. Dental plaque biofilms were sampled from intact tooth surfaces of 118 caries-free twins. An additional 86 caries-active twins were sampled for plaque from carious lesions and intact tooth surfaces. Using a reverse capture checkerboard assay the relative abundance of 82 bacterial species was determined. An integrative computational predictive model determined microbial abundance patterns of microbial species in caries-free twins as compared to caries-active twins. Heritability estimates were calculated for the relative microbial abundance levels of the microbial species in both groups. The levels of 10 species were significantly different in healthy individuals than in caries-active individuals, including, A. defectiva, S. parasanguinis, S. mitis/oralis, S. sanguinis, S. cristatus, S. salivarius, Streptococcus sp. clone CH016, G. morbillorum and G. haemolysans. Moderate to high heritability estimates were found for these species (h(2) = 56%-80%, p < .0001). Similarity of the overall oral microbial flora was also evident in caries-free twins from multivariate distance matrix regression analysis. It appears that genetic and/or familial factors significantly contribute to the colonization of oral beneficial species in twins.

Morphological study on quadruplets by cephalometric and model analyses

Clarifying the genetic factors involved in maxillofacial growth and development is very important in orthodontic treatment planning and prognosis. However, few dental studies have examined multiple births. The present orthodontic evaluation was conducted using orthodontic data from a set of quadruplets. Orthodontic evaluation was performed on a set of quadruplets (1 girl and 3 boys) aged 9 years and 7 months at the initial visit. Although all 4 children weighed only about 1,400 g each at birth, height and body weight subsequently normalized. Mean skeletal age of the quadruplets was 10 years and 2 months, about 6 months ahead of their calendar age. In all 4 children, facial profile was mostly symmetrical and convex. Intraoral findings showed a Hellman's dental age of IIIA, together with spacing of the upper anterior teeth. Both overbite and overjet were 5-7 mm, and mesial step of the terminal plane was noted. Model analysis showed that tooth materials were on the large side, while arch width was narrow. Cephalometric analysis revealed that the ANB of the first- and fourth-born children was 6 degrees , and skeletal maxillary protrusion due to mandibular retrusion was diagnosed. The second- and thirdborn children exhibited no marked skeletal abnormalities.

Mutans streptococci in preschool twins

OBJECTIVE

The purpose of this study was to determine whether genetic factors influence variation in salivary levels of the mutans streptococci (MS) in preschool twins.

DESIGN

The study population consisted of 48 pairs of monozygotic (MZ) twins and 54 pairs of dizygotic (DZ) twins. Genotyping eight highly polymorphic DNA markers determined zygosity. Demographic data and antibiotic exposure as reported by mothers were obtained by a face-to-face interview. Salivary levels of MS were determined by the Stripmutanstrade mark test. Test results are reported on a scale of 0-3. Regression analysis, two sample t-tests and structural equation modeling were applied to analyse the data.

RESULTS

Only 6% of the participants ever visited a dentist (F component of the DMFS was 0 for this population). Average salivary scores for the mutans streptococci were significantly different between MZ (1.1+/-0.1) and DZ (0.8+/-0.1) twins (p=0.021). High salivary levels of MS (scores 2 and 3) were observed in 42% of MZ twins and in 26% of DZ twins. Regression analysis on the MS salivary levels revealed no significant effects of age, gender and antibiotic intake for MZ or DZ twins. The heritability of mutans streptococci colonisation in 48 pairs MZ and in 26 pairs of DZ twins was estimated to be 52%. The non-shared environment contribution was estimated to be 48%.

CONCLUSIONS

These results suggest that variation in the salivary levels of the mutans streptococci is significantly contributed by genetic factors.