Vertical growth in mono‐and dizygotic twins: A longitudinal cephalometric cohort study

Abstract

Objective The aims of this longitudinal analysis of untreated monozygotic and dizygotic twins were to investigate vertical changes of the craniofacial structures during growth, to determine the concordance between genetically twins and to assess the genetic component for the various aspects of vertical growth. Settings and sample population The sample consisted of 34 pairs of untreated monozygotic twins (23 male, 11 female) and 30 untreated dizygotic siblings of multiple birth (8 male, 8 female and 14 mixed) from the Forsyth Moorrees Twin Study (1959‐1975); lateral cephalograms taken from 6 to 18 years of age were analysed at 3‐year intervals. Materials and Methods Cephalograms were traced, and longitudinal changes between twins in six angular and proportional vertical cephalometric variables (SN‐NL, ML‐NL, SN‐ML, y‐axis, PFH/AFH and LAFH/AFH) were analysed with intraclass correlation coefficients and linear regression modelling. Results The concordance between monozygotic/dizygotic twins at 18 years of age was moderate to high with intraclass correlation coefficient values between 0.51 and 0.66. Additionally, sex differences in concordance at 18 years of age were found for three variables. High heritability (66%‐79%) was observed for 5 of the 6 variables (LAFH/AFH, ML‐NL, y‐axis, SN‐ML, PFH/AFH), while SN‐NL showed limited heritability (34%). Conclusions Although monozygotic/dizygotic twins share at least part of their genetic material, differences in the vertical dimension were found. This supports the complex developmental mechanism of the human face and the varying influence of genetic and environmental factors.

Genetic and environmental components of vertical growth in mono- and dizygotic twins up to 15–18 years of age

“…The analysis of the repeated measurements showed high reliability and small limits of agreement in all instances, which supported the robustness of the method. 17 In the main analysis, at 15 years of age, 125 participants were studied, including 64 MZ twins (n ¼ 20; 31% female) and 61 DZ twins (n ¼ 29; 48% female; Table 1). Small differences were found between MZ-DZ twins for all variables, and only the lower anterior face height/face height (LAFH/FH) showed a statistically significant difference of 1.4% (as the 95% CI excluded zero).…”

“…Other than LAFH/FH, all variables showed high between-twins concordance, which agreed with previous data. 17 This is understandable because twins brought up together experience a similar environment and have an identical or similar genome. 21 Nevertheless, classical heritability estimates revealed that not all variables presented similar heritability, while maxillary inclination showed the least heritability (0.22 at 15 years and 0.28 at 18 years).…”

“…This might be because the models were not adjusted for sex. Because heritability of craniofacial components can be influenced by sex, 17 models adjusted for sex were chosen a The most parsimonious model that best describes each cephalometric variable is shown in italics. A indicates additive genetic variance; AIC, Akaike information criterion; b, regression coefficient; C, shared environment variance; D, dominant genetic variance; E, unique environment variance; h 2 , classic heritability; rDZ, correlation of dizygotic twins; rMZ, correlation of monozygotic twins; SE, standard error.…”

“…14 To analyze the degree of genetic contribution to facial growth, several twin studies have been carried out. [15][16][17] Classical twin study methods compared the differences within monozygotic (MZ; identical) and dizygotic (DZ; fraternal) pairs, whereas the amount of the difference was interpreted as the relative genetic impact. 18 However, evidence of the specific contribution of genes and the environment on the development of each craniofacial characteristic remains limited.…”

Single Nucleotide Polymorphisms in RUNX2 and BMP2 contributes to different vertical facial profile

“…SNPs are characterized by the wild allele substitution, which may change the amino acid sequence of a protein or directly affect the quantity, quality, and stability of gene expression [ 11 ]. Although twin studies clearly indicate that the vertical growth pattern is strongly influenced by genetic factors [ 12 ], studies investigating the aetiology of skeletal facial growth patterns have been mainly focused on variations in anteroposterior dimensions of the face [ 8 , 9 ]. The role of SNPs, however, in the development of vertical facial proportions is still poorly understood [ 7 , 8 ].…”