Birth weight and congenital absence of teeth in twins

Sex and ancestry patterning of residual correlations in human dental development: Cooperative genetic interaction and phenotypic plasticity

OBJECTIVES

Most research in human dental age estimation has focused on point estimates of age, and most research on dental development theories has focused on morphology or eruption. Correlations between developing teeth using ordinal staging have received less attention. The effect of demographic variables on these correlations is unknown. I tested the effect of reference sample demographic variables on the residual correlation matrix using the lens of cooperative genetic interaction (CGI).

MATERIALS AND METHODS

The sample consisted of Moorrees et al., Journal of Dental Research, 1963, 42, 1490-1502, scores of left mandibular permanent teeth from panoramic radiographs of 880 London children 3-22.99 years of age stratified by year of age, sex, and Bangladeshi or European ancestry. A multivariate cumulative probit model was fit to each sex/ancestry group (n = 220), each sex or ancestry (n = 440), and all individuals (n = 880). Residual correlation matrices from nine reference sample configurations were compared using Bartlett's tests of between-sample difference matrices against the identity matrix, hierarchical cluster analysis, and dendrogram cophenetic correlations.

RESULTS

Bartlett's test results were inconclusive. Cluster analysis showed clustering by tooth class, position within class, and developmental timing. Clustering patterns and dendrogram correlations showed similarity by sex but not ancestry.

DISCUSSION

Expectations of CGI were supported for developmental staging. This supports using CGI as a model for explaining patterns of variation within the dentition. Sex was found to produce consistent patterns of dental correlations, whereas ancestry did not. Clustering by timing of development supports phenotypic plasticity in the dentition and suggests shared environment over genetic ancestry to explain population differences.

Dental anomalies in first-degree relatives of transposed canine probands

The aim of this study was to investigate and compare the inheritance pattern and prevalence of inheritable dental anomalies in a sample of patients with maxillary canine—first premolar transposition and their first-degree relatives with a sample of palatally displaced canine families. Thirty-five consecutive maxillary canine—first premolar transposition probands and 111 first-degree relatives were matched to 35 consecutive palatally displaced canine probands and 115 first-degree relatives. These were assessed for palatally displaced canines and incisor-premolar hypodontia. Parental age at birth of the proband was also noted. The results revealed that (i) there is no difference in the overall prevalence of palatally displaced canine or incisor-premolar hypodontia between the groups of relatives; (ii) first-degree relatives of bilateral palatally displaced canine probands have a higher prevalence of palatally displaced canine and incisor-premolar hypodontia than those with unilateral palatally displaced canine; and (iii) maternal age at birth of the maxillary canine—first premolar transposition probands was significantly higher than that of the palatally displaced canine probands. The results suggest that maxillary canine—first premolar transposition and palatally displaced canine are unlikely to be different genetic entities and also indicate environmental or epigenetic influences on dental development.

Epigenetic influences may explain dental differences in monozygotic twin pairs

BACKGROUND

Comparisons between monozygotic (MZ) co-twins have tended to focus on the similarities between their dentitions rather than differences. The aim of this study was to determine the prevalence of discordant expression for simple hypodontia and supernumerary teeth in MZ twin pairs and to explain how phenotypic differences might occur despite their similar genotypes.

METHODS

Records of 278 pairs of MZ twins, including dental casts and radiographs, were examined and the prevalences of discordant expression for missing upper lateral incisors (ULI) or second premolars (PM2), and of mesiodentes, were determined. Zygosities were confirmed by comparisons of blood markers and DNA.

RESULTS

There was evidence of at least one missing ULI or PM2 in 24 of the 278 MZ pairs (8.6 per cent), with 21 of these 24 pairs (87.5 per cent) showing discordant expression. Nine of the 278 MZ pairs (3.2 per cent) displayed evidence of mesiodentes, with eight of these nine pairs (88.9 per cent) being discordant.

CONCLUSION

Our findings show that differences in the expression of missing or extra teeth occur often between MZ co-twins whose genetic make-up predisposes them to simple hypodontia or mesiodentes. We postulate that minor variations in epigenetic events during odontogenesis may account for these distinct differences.

Agenesis of permanent maxillary lateral incisors in South Australian twins

The frequency of agenesis of maxillary lateral incisors (I2) in a sample of South Australian twins was determined, and associations with gender, zygosity, birthweight and dental crown size were investigated. Ten of the 446 twins examined (2.24 per cent) displayed agenesis of one or both I2, a similar frequency to that reported for Australian singletons. Seven of the ten affected individuals were monozygous female twins, including two pairs, while the three affected dizygous twins were each from different male twin pairs. Five pairs of monozygous twins were identified who displayed varying expressions of normal, small, peg-shaped or missing I2. These twin pairs displayed disparate birthweights suggesting they may have been mono-chorionic. These findings are consistent with a multifactorial threshold model linking size and number of I2 with developmental influences modifying phenotypic expression in those monozygous twins whose genetic predisposition places them near to the threshold for agenesis.

Hypodontia in twins discordant or concordant for cleft lip and/or palate

The aims of this study were to determine the frequency of hypodontia of permanent teeth in twins discordant or concordant for clefts in each type of cleft, to determine the concordance of hypodontia in mono- and dizygotic twins, and to compare the findings with some earlier data. Thirty-nine pairs of twins between 7 and 23 yr of age were investigated. Six of 13 monozygotic and 24/26 dizygotic pairs were discordant for clefts. Orthopantomograms of the dentition, treatment records, and anamnestic data were studied. Twenty-three pairs of twins (59%) had at least one twin with hypodontia: 9/13 (69%) monozygotic and 14/26 (54%) dizygotic. The prevalence of hypodontia was 37% for monozygotic and 32.7% for dizygotic twins, 16.7% in the noncleft and cleft lip groups, 41.1% in the cleft palate group, and 64.3% in the cleft lip and palate group. All these were above the values observed earlier in the noncleft twins and in the Finnish normal and cleft group populations. Four of 13 pairs of monozygotic twins had no hypodontia, 8/9 pairs were discordant, and only 1/9 was concordant for hypodontia. The corresponding figures for the 26 dizygotic pairs were: 12/26, 11/14, and 3/14 pairs. Maxillary second premolars were the most frequently absent teeth, followed in order of frequency by the maxillary lateral incisors and the mandibular second premolars. Hence, for this sample of twins, the genetic component seems to be weak.

Twins and twinning, dentists and dentistry

Comparisons of physical features within identical (monozygous) and non-identical (dizygous) twin pairs have provided valuable insights into the relative contributions of genetic and environmental influences to observed variability. The special nature of the twinning process itself also provides an opportunity to learn more about early human development, including how body symmetry is determined. The mechanisms of twinning, mortality and morbidity in twins, determination of body symmetry including the phenomenon of mirror-imaging, postnatal growth and development of twins, and zygosity determination are discussed. Twin studies with direct relevance to clinical dentistry are reviewed and illustrated by examples from an ongoing investigation of dentofacial morphology in South Australian twins.

Orofacial pain associated with an infected complex odontome. Case report

A case of complex odontome of the mandible associated with severe pain was presented. The pain regressed after removal of the lesion.

Symmetry and combinations of hypodontia in non-cleft and cleft palate children

Symmetry and combinations of hypodontia was studied and compared in non-cleft (NONC) and cleft palate (CP) groups with different prevalences of hypodontia. The NONC group included 300 children with one or more congenitally absent permanent teeth, excluding the third molars, and 79 children with isolated cleft palate without accessory anomalies. There were no significant differences between the groups in distribution of children with agenesis of 1 to 11 teeth. Bilateral agenesis increased with increasing number of absent teeth per child, and it was more frequent in the maxillary teeth of the NONC group and in the mandibular teeth of the CP group. Concomitant agenesis was observed in one to four different dental fields per child, and the distribution of the subjects was similar in both groups. We conclude that the symmetry of agenesis may be dependent on the population frequency of hypodontia and on the number of absent teeth per child. The agenesis in the CP group may have a common genetic basis, but the substantially higher incidence of hypodontia in the maxilla is likely caused by factors similar to those causing the cleft itself.

Twin studies in craniofacial genetics: a review

A review of the literature on twin studies in craniofacial genetics is presented. The following areas are particularly dealt with: tooth form and size, tooth formation and eruption, dental arch and occlusion, facial form and size, and cleft lip and palate.

Prevalence of hypodontia and malformations of permanent teeth in Iceland

This study was performed to elucidate the prevalence of hypodontia and congenital malformations of permanent teeth in Iceland. Computer analysis was done on the basis of results from examining 1,116 children (521 boys and 595 girls), or about 9.5% of all schoolchildren in Reykjavík in stages DS 3 (canines and premolars erupting) and DS 4 (canines and premolars fully erupted) of dental development at the time of the study. The age range was mainly from 8-16 years. The prevalence of congenitally missing teeth (second and third molars excluded) was 6.7% in boys and somewhat higher, 8.9%, in girls, 7.9% for both sexes combined. On the average, each child was missing 1.9 teeth. In the girls, the most commonly absent teeth were the mandibular second premolar (55%), the maxillary second premolar (19%), and the maxillary lateral incisors (18%). In the boys the figures were 51%, 18%, and 10%, respectively. Malformation of permanent teeth, mainly in the form of pegshaped maxillary laterals, was also less common in the boys, 1.3% against 3.0% in the girls, and 2.2% for both sexes combined. Among Icelandic children hypodontia and/or congenital malformation of permanent teeth were found in 9.1%.