Genetic modeling of primary tooth emergence: a study of Australian twins

Genetic Correlation, Pleiotropy, and Molar Morphology in a Longitudinal Sample of Australian Twins and Families

This study aims to expand our understanding of the genetic architecture of crown morphology in the human diphyodont dentition. Here, we present bivariate genetic correlation estimates for deciduous and permanent molar traits and evaluate the patterns of pleiotropy within (e.g., m1–m2) and between (e.g., m2–M1) dentitions. Morphology was observed and scored from dental models representing participants of an Australian twin and family study (deciduous n = 290, permanent n = 339). Data collection followed Arizona State University Dental Anthropology System standards. Genetic correlation estimates were generated using maximum likelihood variance components analysis in SOLAR v.8.1.1. Approximately 23% of deciduous variance components models and 30% of permanent variance components models yielded significant genetic correlation estimates. By comparison, over half (56%) of deciduous–permanent homologues (e.g., m2 hypocone–M1 hypocone) were significantly genetically correlated. It is generally assumed that the deciduous and permanent molars represent members of a meristic molar field emerging from the primary dental lamina. However, stronger genetic integration among m2–M1/M2 homologues than among paired deciduous traits suggests the m2 represents the anterior-most member of a “true” molar field. The results indicate genetic factors act at distinct points throughout development to generate homologous molar form, starting with the m2, which is later replaced by a permanent premolariform crown.

Emergence stages of permanent teeth in twins: A comparative study

BACKGROUND

The relative effects of genetic factors on the timing of tooth eruption can be demonstrated through twin studies; however, twin studies on the eruption time of permanent teeth were limited.

AIM

To compare the emergence stages of permanent teeth in twins and non-twin children.

DESIGN

A total of 480 patients aged from 5.0 to 13.11 years who attended to the Department of Pediatric Dentistry for any dental reason were included in the study. Twins were compared with the control group in terms of the emergence stages for all permanent teeth.

RESULTS

At the age of 5 years, girls in the control group showed statistically significant differences related to the central incisors, the first molars in the mandibular dentition and the first molars in the maxillary dentition, showing more advanced emergence stages. In the 6- to 8- and 12- to 13-year-old age groups, no gender-related differences in emergence stages were observed. Gestation age was found to have a statistically significant effect on the emergence stages of permanent teeth. There were no statistically significant differences between birthweights and tooth emergence stages in twins.

CONCLUSION

Longitudinal studies are needed to determine whether there is a strong genetic effect on emergence stages of permanent teeth in twins.

Dependence of Deciduous Tooth Eruption Terms and Tooth Growth Rate on the Weight-Height Index at Birth in Macrosomic Children over the First Year of Life

The aim of this research is to study the effect of body overweight at birth (fetal macrosomia) on the processes of tooth eruption and tooth growth during the first year of life in children in the Kharkiv City (Ukraine) population. One of the research tasks is to examine the features of deciduous teeth eruption in children who were born with macrosomia with different values of the weight-height index at birth. Materials and methods. The medical records of the children born between 1977 and 2013 have been analyzed. The database has been collected in one of the Kharkiv City clinic. The Main Group is comprised of the medical records of the children (separately for boys and girls) born with fetal macrosomia. All the medical records of the Main Group have been divided into subgroups taking into account the gender and the harmonious (well-balanced) development coefficient. The Comparison Group is comprised of the medical records of the children also born within the normal term range, but with weight and height that correspond to the gestation age (fetal normosomia). To determine the average time of the first tooth eruption, as well as the deciduous teeth growth rate for each of the groups under the study, we have used the hypothesis about a linear dependence between the number of erupted teeth and the age of the child. The statistical data processing and verification of the consistency of this hypothesis is performed using the multiple linear regression analysis with the STATISTICA 6.0 software package (Multiple Regression module). The number of delayed eruption and premature eruption cases observed is calculated along with the corresponding confidence intervals for the significance level, p, of less than 0.05, taking into account the binomial distribution of the random variable. The results of the study indicate a slowed growth rate of deciduous teeth in children born with macrosomia, as well as an increased number of cases (by a factor of 2 to 4 times) of deviations in the timing of teeth eruption compared to regional norms. The smallest growth rate of deciduous teeth and the smallest number of teeth at the age of one year are registered in macrosomic boys and macrosomic girls with a long body and a relatively reduced birth weight, as well as in macrosomic girls with intrauterine obesity. The macrosomic girls with intrauterine acceleration with obesity at the background have the largest average tooth growth rate and the largest percentage of premature eruption cases among all subgroups. Conclusions. The somatometric features of fetal macrosomia suggest the influence on the number of teeth that erupt by a certain age. The data on the deviation from the generally accepted terms of teeth eruption in children born with macrosomia, can be the basis for developing new and improving existing prevention programs aimed at preserving dental health.

RANKL is associated with persistent primary teeth and delayed permanent tooth emergence

BACKGROUND

Tooth eruption is a process that is not fully understood.

AIM

To evaluate whether genetic polymorphisms for RANK/RANKL/OPG are associated with delayed tooth emergence. To evaluate whether the relative expression of this genes is associated with persistent primary teeth.

DESIGN

To evaluate whether genetic polymorphisms for RANK/RANKL/OPG could be involved in delayed tooth emergence, saliva samples from 160 children, aged 6-13 years old, were analysed. To test if there is correlation between gene expression of RANK/RANKL/OPG in children with delayed tooth emergence and persistent primary teeth, periapical tissue from 15 children with persistent primary teeth and from 15 control subjects were collected for qPCR analysis.

RESULTS

Fifty-six children with delayed tooth emergence (35%) had at least one permanent tooth with delayed emergence. The T allele in RANKL (rs9594738) increased the risk of delayed tooth emergence (P = 0.02; OR = 1.71, 95%CI 1.09-2.75). The relative gene expression for RANKL and the ratio RANKL/OPG in children with delayed tooth emergence and persistent primary teeth were lower compared to controls (P = 0.02 and P = 0.005, respectively).

CONCLUSIONS

Data suggest that the polymorphism rs9594738 in RANKL is associated with delayed permanent tooth emergence. Moreover, reduced relative gene expression of RANKL in periapical tissue is associated with persistent primary teeth.

Coordination of bilateral tooth replacement in the juvenile gecko is continuous with in ovo patterning

We performed a test of how function impacts a genetically programmed process that continues into postnatal life. Using the dentition of the polyphyodont gecko as our model, tooth shedding was recorded longitudinally across the jaw. We compared two time periods: one in which teeth were patterned symmetrically in ovo and a later period when teeth were initiated post-hatching. By pairing shedding events on the right and left sides, we found the patterns of tooth loss are symmetrical and stable between periods, with only subtle deviations. Contralateral tooth positions shed within 3-4 days of each other in most animals (7/10). A minority of animals (3/10) had systematic tooth position shifts between right and left sides, likely due to changes in functional tooth number. Our results suggest that in addition to reproducible organogenesis of individual teeth, there is also a neotenic retention of jaw-wide dental patterning in reptiles. Finer analysis of regional asymmetries revealed changes to which contralateral position shed first, affecting up to one quarter of the jaw (10 tooth positions). Once established, these patterns were retained longitudinally. Taken together, the data support regional and global mechanisms of coordinating tooth cycling post-hatching.

The Influence of Chorion Type on Health Measures at Birth and Dental Development in Australian and Dutch Twins: A Comparative Study

Chorion type may significantly influence the prenatal environment of twins. This study explored the associations between chorion type and gestational age, birth weight, birth length, and the timing of emergence of the first primary tooth in two populations of twins, Australian and Dutch. Additionally, we investigated the relationship between chorion type and birth weight discordance (BWD) in order to determine whether a significant relationship existed between discordance in birth weight and discordance in the timing of emergence of the first primary tooth. The two study samples consisted of 409 Australian twin pairs and 301 Dutch twin pairs, all of European ancestry. Data were collected through a combination of questionnaires and recording charts administered to the parents and through linkage with biological databases. In the Australian sample, monozygotic monochorionic (MZMC) twins experienced the shortest mean gestation time (35 weeks), the lowest mean birth length (46 cm) and the lowest mean birth weight (2.3 kg) compared with other twin groups. For the same variables in the Dutch sample, these trends with MZMC twinning were not observed. Chorion type did not significantly affect the mean timing of emergence of the first primary tooth in either sample. Monochorionicity was found to be significantly associated with BWD in both samples, but there was a significant association between BWD in MZMC twin pairs and timing of emergence of the first primary tooth only in the Australian sample. Results from this study support previous findings that the timing of emergence of the first primary tooth is influenced strongly by genetic factors and is well protected from environmental disturbances.

Genetic determinants and dynamics of permanent teeth emergence in Northwest Indian twins: a chronogenetic study

The understanding of the role of genetic factors in phenotypic variation in the emergence of secondary teeth in humans remains is incomplete. Dental emergence data based on a mixed longitudinal study were collected on 111 twin pairs from an urban population of Chandigarh. The observations over time on a single individual varied from one to nine, thus giving a total of 595 entities. Female twins manifested emergence priority over males. The differences between zygosities in mean emergence ages were significant for only 6 of 16 (37%) instances. Magnitude of variations seen between twins and singletons in their mean emergence timings and duration of the hiatus between two dental phases of emergence were of the order observed among different samples from the same population/ethnic group. Heritability estimates for the specified number of the teeth emerged showed age variations. These estimates were highest in the first two age groups (from 5 to 7 years), when the first molars and incisors emerged. Maxilla-mandible differences were seen for tooth emergence timings and sequence patterns. Heritability for tooth emergence timings was higher in maxilla than in mandible. Multifactorial model of inheritance was the best fit model to explain variations observed in dental emergence timings and dental sequence pattern polymorphisms and there were significant genetic components of variation for both of these. There were sex differences in heritability; females had higher estimates than males. Genetic factors accounted for about 60% of the total phenotypic variation in the length of hiatus interval between two active stages of permanent teeth emergence.

Bacterial colonization, enamel defects and dental caries in 4-6-year-old mono- and dizygotic twins

BACKGROUND

Knowledge of the genetic and environmental influences in caries aetiology has relevance for preventive dentistry.

AIMS

This classical twin study compared concordance of mutans streptococci (MS) and lactobacilli (LB) colonization, enamel defects, and caries in a cohort of 4-6-year-old mono- (MZ) and dizygotic (DZ) twin pairs.

DESIGN

The twins were examined for prevalence and concordance of enamel opacities and hypoplasia, oral counts of MS and LB, and dental caries. Bacterial counts were assessed using a commercial microbiological kit.

RESULTS

Thirty-four MZ and 50 DZ twins (mean gestational age 35.0 ± 2.4 weeks, and birthweight 2.4 ± 0.6 kg) were examined. There were no statistically significant differences between MZ and DZ twins in the prevalence of MS, LB, and enamel hypoplasia. Concordance rates for MS and LB presence and prevalence of enamel defects within MZ and DZ twin pairs were not significantly different. There were more children with caries in DZ compared with MZ twins (18% vs 3%, P = 0.0029), most likely due to increased daily frequency of sugar consumption and less toothbrushing.

CONCLUSIONS

Concordance data from MZ and DZ twins did not demonstrate any statistically significant difference in susceptibility for enamel defects and colonization of MS and LB.

Genetic, Epigenetic, and Environmental Influences on Dentofacial Structures and Oral Health: Ongoing Studies of Australian Twins and Their Families

The Craniofacial Biology Research Group in the School of Dentistry at The University of Adelaide is entering an exciting new phase of its studies of dental development and oral health in twins and their families. Studies of the teeth and faces of Australian twins have been continuing for nearly 30 years, with three major cohorts of twins recruited over that time, and currently we are working with twins aged 2 years old to adults. Cross-sectional data and records relating to teeth and faces of twins are available for around 300 pairs of teenage twins, as well as longitudinal data for 300 pairs of twins examined at three different stages of development, once with primary teeth, once at the mixed dentition stage, and then again when the permanent teeth had emerged. The third cohort of twins comprises over 600 pairs of twins recruited at around birth, together with other family members. The emphasis in this third group of twins has been to record the timing of emergence of the primary teeth and also to sample saliva and dental plaque to establish the timing of colonization of decay-forming bacteria in the mouth. Analyses have confirmed that genetic factors strongly influence variation in timing of primary tooth emergence. The research team is now beginning to carry out clinical examinations of the twins to see whether those who become colonized earlier with decay-forming bacteria develop dental decay at an earlier age. By making comparisons within and between monozygotic (MZ) and dizygotic (DZ) twin pairs and applying modern molecular approaches, we are now teasing out how genetic, epigenetic, and environmental factors interact to influence dental development and also oral health.

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.