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Heritability of dental and skeletal cephalometric variables in monozygous and dizygous Iranian twins
orthodontic waves 68 (2009) 72–79
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Research paper
Heritability of dental and skeletal cephalometric variables in monozygous and dizygous Iranian twins Fariborz Amini a,*, Ali Borzabadi-Farahani b a b
Department of Orthodontics and Dentofacial Orthopaedics, School of Dentistry, Islamic Azad Medical University, Tehran, Iran Orthoworld, Leamington Spa, Warwickshire, UK
article info
abstract
Article history:
Background and purpose: To assess the influence of genetic and environmental factors on
Received 11 October 2008
craniofacial morphology in an Iranian study sample.
Received in revised form
Setting and sample population: The department of orthodontics at Islamic Azad Medical
23 December 2008
University and Iranian Twin Centre. Fifty pairs of twins were selected (25 monozygotic,
Accepted 5 January 2009
MZs and 25 dizygotic, DZs). The mean age of subjects in MZ and DZ groups were 16.2 (13.4–
Published on line 20 February 2009
19.8) and 16.6 (13.8–20.1) years, respectively. Methods and materials: A cross-sectional twin study carried out using lateral cephalograms.
Keywords:
The subjects were required to pass their pubertal growth spurts and received no previous
Iranian twin study
orthodontic treatment. Thirty-three linear and angular cephalometric variables were iden-
Cephlometric variables
tified and used. The heritability assessments were undertaken according to the Path
Heritability
Analysis model and also using the Holzinger’s equation. For each cephalometric variables Pearson’s intra-pair correlation coefficients were calculated for MZ and DZ twin pairs. The estimate of heritability (h2) and coefficient of cultural heritability (c2) were then calculated for cephalometric variables. Results: Overall vertical variables showed higher heritability than horizontal variables. The anterior cranial base (S-N), saddle angle (NSBa), total anterior facial height (N-Me), lower anterior facial height (ANS-Me), SNA, SNB, SNPog, Gonial angle, SN-GoGn angle and SNMaxillary plane angle showed high heritability. Heritability was low to moderate for the dento-alveolar variables. Conclusions: Vertical variables (in particular total anterior facial height, TAFH and lower anterior facial height, LAFH) showed more heritability than horizontal ones. Heritability seems to be expressed more anteriorly than posteriorly. The lower third of the face seems to be under strong genetic control. # 2009 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.
1.
Introduction
Craniofacial morphology has long captured the interest and imagination of orthodontists. The aetiology of malocclusion
thought to involve both major and minor genetic influences with variable interactions from environmental factors [1,2]. The separation of these factors in the contribution of severity of malocclusion is significant for clinical orthodontics. Facial
* Corresponding author. E-mail address: [email protected] (F. Amini). 1344-0241/$ – see front matter # 2009 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved. doi:10.1016/j.odw.2009.01.001
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bone sizes and their growth potential would be determined genetically. However according to the Moss’s functional matrix theory, environment also plays a part and these bones are subjected to related functional muscle attachments and oro-naso-pharyngeal function, which determines their eventual characteristics [3–7]. The prognosis of orthodontic treatment is determined largely by therapeutic environmental interventions. In this context, orthodontists have used different functional appliances, headgears and expanders to correct the anterior–posterior, vertical, and transverse discrepancies of the jaws and the facial skeleton [8–13]. The ‘‘heritability’’ means the strength of the genetic control and ‘‘high heritability’’ means small room for environmental intervention. Heritability is a ratio. It ranges from 0 to 1, indicating the relative importance of genetics compared with environment on a given trait’s variability. Orthodontists usually assume that genetic factors can significantly affect the development of malocclusion. There is now a considerable body of evidence that genes play a dominant role in the aetiology of malocclusion [14]. Studies of monozygotic (MZ) and dizygotic (DZ) twins offer a powerful method of partitioning genetic and environmental sources of covariance of quantitative traits. MZ twins are genetically identical, whereas DZ twins share their genes similar to siblings (i.e. fraternal). When only trait values, not genotypic information, are available, variance-component estimation can also be used to estimate heritability of a quantitative trait. The trait differences in MZ twins are considered to be due to environmental differences and those in DZ twins due to both genetic and environmental factors. Therefore if a trait has no genetic component, for example due to chance or trauma, concordance rates would be expected to be similar for MZ and DZ twins. For a single-gene trait or a chromosomal disorder the MZ concordance rate will be 100%, whereas the DZ rate will be less than this and equal to the rate in siblings. For discontinuous multi-factorial traits with both genetic and environmental contributions, the rate in MZ twins, although less than 100%, will exceed the rate in DZ twins. Previous studies investigated the heritability of malocclusion and craniofacial morphology among siblings and the twin study method is one of the most commonly used methods available for investigating genetically determined variables in orthodontics [16–24]. We know from previous studies that measurements of facial height are more genetically determined and more difficult to alter than measurements of facial depth [16–24]. An expansion of the ethnic variety of twin studies away from the predominantly Euro-American whites would be of particular interest. The objective of this study was to estimate the heritability of different cephalometric parameters, both skeletal and dento-alveolar, between MZ and DZ healthy twins in an Iranian study sample and compare it with previous studies.
2.
Materials and methods
2.1.
Study sample
The ethical approval for the present study was given by the Research Ethics Committee, School of Dentistry, Islamic
Table 1 – Age descriptive statistics (in years) for MZ and DZ twins. Groups studied
Mean age
Minimum
Maximum
MZ pairs Male (n = 13) Female (n = 12)
16.2 16.6 14.8
13.4 15.0 13.4
19.8 19.8 19.0
DZ pairs Male (n = 13) Female (n = 12)
16.6 15.9 15.1
13.8 14.9 13.8
20.1 20.1 19.8
Azad Medical University, Tehran, Iran. Subjects for this study were selected from the pool of patients who registered with the Iranian Twin Center based in Tehran, Iran. A total of 53 pairs of twins were identified and then allocated to MZ and DZ groups. Initially 28 pairs of DZ twins were selected, however 3 DZ pairs showed extreme discordance and were excluded from the study as an outlier. Finally, 25 pairs of MZ twins and 25 pairs of DZ twins were selected for this study (50 pairs). It was decided to select pairs who have passed their peak pubertal growth spurts therefore subjects who were younger than 13 years old were excluded from this study. The sample’s age characteristics are shown in Table 1. Twin zygosity was initially determined in Iranian Twin Center, by means of evaluating the concordance or discordance of various physical characteristics (standing height, finger print, tooth size, and hair and eye colour). Subsequently all subjects were genotyped and zygosity reconfirmed by means of hematological studies. Informed consent was obtained after the objective of the study was fully explained. All participants were offered free orthodontic consultation and as a part of initial examination and standardized lateral cephalographs were taken. They were healthy and had no previous orthodontic treatment or permanent dental extractions. None of the participants received any facial trauma that could have resulted in bony fracture.
2.2.
Cephalometric measurements
Cephalographs were obtained by the Brodbent cephalometric technique which positions the subject’s head in a cephalostat oriented to the Frankfort horizontal plane. An anode-to-object distance of 5 feet was used [25]. Following landmarks were identified on each lateral X-ray film: Sella turcica (S), sphenoethmoidale (Se), nasion (N), basion (Ba), orbitale (Or), anterior nasal spine (ANS), posterior nasal spine (PNS), porion (Po), point A (A), point B (B), pogonion (Pog), gnathion (Gn), gonion (Go), menton (Me) and articulare (Ar), incisor inferior (Ii), incisor superior (Is). Overall 33 linear and angular cephalometric measurements were identified and later digitized and processed using Dentofacial Planner software (Dentofacial Software, Toronto, Ontario, Canada) (Table 2, Figs. 1–3). Linear parameters were either actual lengths or projected lengths (projected lengths on the FH plane for horizontal variables and FH-vertical plane for vertical variables) (Fig. 3). The variables were divided into four groups: cranial base/skeletal variables, maxillary skeletal variables, mandibular skeletal variables and dentoalveolar variable.
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Table 2 – Cephalometric linear and angular variables used in the study. Cranial base/skeletal variables Linear (actual): S-N (Anterior cranial base), S-Ar, S-Go (TPFH) Linear (projected): N-Me (TAFH), ANS-Me (LAFH) Angular: NSBa (saddle angle), SN-GoMe, GoMe-Occl.Plane, GoMe-ANS.PNS, SN-ANS.PNS Maxillary skeletal variables Linear (actual): S-A, ANS-PNS Linear (projected): N-ANS (UAFH), Se-PNS (UPFH), A horizontal Angular: SNA Mandibular skeletal variables Linear (actual): Go-Gn (mandibular body length), Ar-Go (ramus height) Linear (projected): B Horizontal, Pog Horizontal Angular: SNB, SNPog, ArGoMe (Gonial angle), SArGo Dento-alveolar variables Lineara: UI ? ANS.PNS, UM ? ANS.PNS, LI ? GoMe, LM ? GoMe Linear (projected): UI horizontal, LI horizontal Angular: UI-SN, LI-GoMe, UI-LI (interincisal angle) a
Perpendicular distance from ANS.PNS and GoMe plane.
Fig. 2 – Cephalometric planes and angles used in the study.
own pair mean), d is the difference between the first and second recordings, and n is the number of radiographs replicated. In addition to the above method, Paired t-test analysis was also used to compare the original and repeated data.
Fig. 1 – Cephalometric landmarks used in the study.
2.3.
Measurement error
To avoid inter-examiner error, one investigator (FA) traced and digitized all radiographs. To determine the measurement error, all cephalometric measurements were repeated on 20 randomly selected radiographs 1 week apart. The cephalograms were retraced and re-evaluated 1 week apart. The method error values calculated with the following formula described by Dahlberg [26] Se = H(Sd2/2n). Where, Se is the method error (or standard deviation of the difference of each of the paired measurements from its
Fig. 3 – Linear measurements used in the study. Projected horizontal variables are parallel to Frankfurt horizontal plane. Projected vertical variables are perpendicular to Frankfort horizontal plane.
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2.4.
Statistical analysis
All analysis was carried out with the SPSS for Windows statistical package (Version 14.0, SPSS Inc., Chicago, IL). We used the estimate of heritability (h2) to assess the heritability. It expresses the proportion of total phenotypic variance that is attributable to the average effect of inherited genes. Theoretically, h2 value should range from zero for no heritability to 1.0 for a completely inherited trait. This is based on the assumption that MZ twins are genetically identical (share 100% of their genes) and differences between them can be due only to environmental factors, whereas DZ twins share on average half of their segregating genes and differences between them can be due to a combination of both environmental and genetic factors. To measure the resemblances between the twins, for each cephalometric variable Pearson’s intra-pair correlation coefficients (rMZ and rDZ) were first calculated for MZ and DZ twin pairs. The estimates of heritability (h2) or coefficient of cultural heritability (c2) was then calculated for cephalometric variables. To compare our findings with previous works we calculated heritability estimates according to the Dr Lundstro¨m’s approach [19], the Path Analysis model [27]. However, this approach does not always yields h2 values between 0 and 1.0. Therefore we also used the Holzinger equation [28]. Path Analysis calculates the proportion of the total variance explained by additive genes (heritability), and by the common and specific environment. The estimate of heritability (h2) and coefficient of cultural heritability (c2) are based on intra-pair correlation coefficients calculated from the MZ and DZ twins with the following formulas: h2 = 2(rMZ rDZ), c2 = 2rDZ rMZ. As mentioned earlier, to compare the results with previous studies we also calculated the Holzinger heritability estimate [28] using the following formula: Holzinger’s heritability estimate ¼
3.
Table 3 – Dahlberg’s error of method (Se) for repeatability of the angular and linear measurements and paired ttest results. Cephalometric variables S-N (mm), anterior cranial base S-Ar (mm) S-Go (mm), TPFH N-Me (mm), TAFH ANS-Me (mm), LAFH NSBa (8), saddle angle SN-GoMe(8) GoMe-Occl.Plane (8) GoMe-ANS.PNS (8) SN-ANS.PNS (8) S-A (mm) ANS-PNS (mm) N-ANS (mm), UAFH Se-PNS (mm), UPFH A horizontal (mm) SNA (8) Go-Gn (mm), mandibular body length Ar-Go (mm), ramus height B Horizontal (mm) Pog Horizontal (mm) SNB (8) SNPog (8) ArGoMe (8), Gonial angle SArGo (8) UI ? ANS-PNS (mm) UM ? ANS-PNS (mm) LI ? GoMe (mm) LM ? GoMe (mm) UI horizontal (mm) LI horizontal (mm) UI-SN (8) LI-GoMe (8) UI-LI (8), interincisal angle
Se 0.29 0.30 0.36 0.31 0.37 0.43 0.50 0.45 0.52 0.48 0.33 0.35 0.38 0.39 0.39 0.51 0.36 0.29 0.30 0.31 0.39 0.41 0.54 0.50 0.34 0.32 0.35 0.39 0.34 0.36 0.41 0.47 0.42
t-Test NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS
Se = error of method = H(Sd2/2n). NS; not significant.
rMZ rDZ 1 rDZ
Results
The measurement error values for angular and linear measurements on the lateral cephalograms did not exceed 0.548 and 0.39 mm (Table 3). None of the angular or linear measurements showed any statistical difference (P < 0.05) between the original and repeated values. Table 4 summarizes the Pearson’s intra-pair correlation coefficients, estimates of genetic heritability (h2), cultural inheritance (c2) and Holzinger’s heritability values between MZ and DZ twins. Overall in MZ twin pairs, intra-pair correlations were considerably higher than DZ twin pairs. Excluding the TPFH and UAFH, all angular and linear vertical variables showed considerably higher intra-pair correlation coefficients (r) in MZ pairs when compared with DZ pairs. Heritability estimate for linear measurement of the anterior cranial base (S-N) was high (h2 = 0.80). The flexure of cranial base (saddle angle) presented with one of the highest estimates of heritability (h2 = 1). Relative sagittal position of maxilla to the anterior cranial base (SNA) and its linear measurement (S-A) showed high h2 value compared to the variables representing the vertical
position of maxilla (UPFH). Upper anterior facial height (UAFH) showed one of the lowest h2 values (h2 = 0.18). Conversely lower anterior facial height (LAFH) showed a very high heritability. Reviewing different mandibular variables; Gonial angle presented with a considerably high h2 value (h2 = 1.62). Relative sagittal position of mandible to the anterior cranial base (SNB, SNPog) also showed a high coefficient of heritability. Mandibular body length showed a very low heritability. Overall heritability estimates for variables representing the vertical dimension were higher than those representing the horizontal variables. Dental variables in general showed low to moderate heritability. Facial plane angles (SN-ANS.PNS, GoMe-ANS.PNS and SN-GoMe) presented with high h2 values. Reviewing the Holzinger values, they ranged from 0.20 (linear measurements of UAFH and LM ? GoMe) to 0.93 (Gonial angle).
4.
Discussion
Heritability estimation is usually a first step in genetic studies, because it provides an estimate of how much phenotypic variation is attributable to genetic effects. Present study used Dr. Lundstro¨m’s approach, the Path Analysis model [27], and
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Table 4 – Intra-pair correlation coefficients (r) followed by estimates of genetic heritability (h2) and cultural inheritance (c2) according to the Path Analysis model [27] and Holzinger [28] values are listed in the last column. Cephalometric variables
rMZ
rDZ
h2
c2
Holzinger, h 2
Cranial base/skeletal variables S-N (mm), anterior cranial base S-Ar (mm) S-Go (mm), TPFH N-Me (mm), TAFH ANS-Me (mm), LAFH NSBa (8), saddle angle SN-GoMe (8) GoMe-Occl.Plane (8) GoMe-ANS.PNS (8) SN-ANS.PNS (8)
0.82 0.89 0.87 0.94 0.90 0.95 0.90 0.88 0.82 0.80
0.42 0.68 0.78 0.40 0.36 0.45 0.29 0.67 0.21 0.19
0.80 0.42 0.18 1.08 1.08 1.00 1.22 0.42 1.22 1.22
0.02 0.47 0.69 0.14 0.18 0.05 0.32 0.46 0.40 0.42
0.69 0.66 0.41 0.90 0.84 0.91 0.86 0.64 0.77 0.75
Maxillary skeletal variables S-A (mm) ANS-PNS (mm) N-ANS (mm), UAFH Se-PNS (mm), UPFH A horizontal (mm) SNA (8)
0.75 0.67 0.64 0.88 0.82 0.92
0.40 0.50 0.55 0.62 0.60 0.49
0.70 0.34 0.18 0.52 0.44 0.86
0.05 0.33 0.46 0.36 0.38 0.06
0.58 0.34 0.20 0.68 0.55 0.84
Mandibular skeletal variables Go-Gn (mm), mandibular body length Ar-Go (mm), ramus height B Horizontal (mm) Pog Horizontal (mm) SNB (8) SNPog (8) ArGoMe (8), Gonial angle SArGo (8)
0.80 0.83 0.87 0.81 0.83 0.81 0.94 0.59
0.67 0.58 0.665 0.58 0.34 0.32 0.13 0.46
0.26 0.50 0.41 0.46 0.98 0.98 1.62 0.26
0.54 0.33 0.46 0.35 0.15 0.17 0.68 0.33
0.39 0.60 0.61 0.55 0.74 0.72 0.93 0.24
Dento-alveolar variables UI ? ANS.PNS (mm) UM ? ANS.PNS (mm) LI ? GoMe (mm) LM ? GoMe (mm) UI horizontal (mm) LI horizontal (mm) UI-SN (8) LI-GoMe (8) UI-LI (8), interincisal angle
0.90 0.92 0.88 0.88 0.73 0.87 0.80 0.93 0.87
0.60 0.52 0.50 0.85 0.45 0.52 0.71 0.45 0.70
0.60 0.80 0.76 0.06 0.56 0.70 0.18 0.96 0.34
0.30 0.12 0.12 0.82 0.17 0.17 0.62 0.03 0.53
0.75 0.83 0.76 0.20 0.51 0.73 0.31 0.87 0.57
h2 = 2(rMZ rDZ), c2 = 2rDZ rMZ; Holzinger heritability estimate = (rMZ rDZ)/(1 rDZ).
Holzinger equation [28] in an attempt to investigate the heritability of craniofacial variables within pairs of MZ and DZ young, healthy twins. Craniofacial morphology is the result of complex interaction between inheritance and environmental factors and is therefore multi-factorial. The final product, the phenotype, represents the combination of genetic and environmental influences. Path Analysis model calculates the proportion of the total variance explained by additive genes (heritability), and by the common and specific environment. This is a common method previously used in twin studies [19,24]. The overall pattern of craniofacial development (short or long facial height) is established early and on average does not change with age [29]. It is also acknowledged that facial maturity develops in females between 10 and 13 years and 2 years later for males. Interestingly faces of pre-pubertal boys and girls may appear very similar [30]. Harris and Johnson [23] studied craniofacial dimensions using cephalometric radiographs obtained at regular intervals during the age period
from 4 to 20 years. They reported that heritability estimates of craniofacial variables increased with age. Therefore comparison of hereditary characteristics is more valid in the postadolescent period when growth is almost completed and in the present study, subjects who were younger than 13 years old were excluded from the study sample. Overall many skeletal cephalometric variables showed high heritability, including the total anterior facial height and facial plane angles (SN-ANS.PNS, GoMe-ANS.PNS and SNGoMe). As previously shown [24,31–33]; we also observed higher heritability estimates for many vertical linear measurements, compared with horizontal linear measurements. However this is in contrast to the findings of Lundstro¨m and McWilliam [19]. They did not find significant differences between the horizontal and vertical measurements. The present study is consistent with the findings of previous studies [15,24,34] and showed high genetic determination for total anterior facial height (TAFH) and for its lower component (LAFH).
orthodontic waves 68 (2009) 72–79
Using the present findings, we agree with Manfredi et al. [24] in that, heritability seemed to be more influential on the anterior vertical variables of the face than its posterior vertical variables. In the present study, the UAFH and posterior facial height (S-Go) showed low heritability. Ramus height (Ar-Go) also showed a moderate heritability (h2 = 0.50). In reviewing the sagittal maxillary variables, we found high heritability estimate for SNA angle and also its linear measurement (S-A). However heritability values for vertical variables (UAFH and UPFH) are suggestive of more environmental influence. This is consistent with the findings of Manfredi et al. [24] and again it may suggest that there is more likelihood of changing the vertical position of maxilla than its sagittal position. From a clinical perspective, it seems that the vertical position of the maxilla is much less under the control of genetics than its sagittal position. This result is also consistent with the earlier findings of Lundstro¨m and McWilliam [19]; they reported lower heritability for UAFH than other vertical dimensions. However, heritability is a function of one trait, in one sample, in one population, at one point in time. To be more precise, heritability is a population concept the concept of h2 is irrelevant to the individual. It would be misleading to suggest that features with low heritability might be more amenable to prevention or treatment in individuals .The ability to move the teeth or have an orthopedic effect on bone involves a complex mix of biomechanical properties of bone and other biologic tissues, the direction, strength, and duration of the mechanical force. The ability of the individual to respond to changes in the environment (including treatment) will define the amount of treatment effect. This is not measured or determined by estimating heritability. Therefore using the present findings it is difficult to conclude that orthopedic treatment is more successful in changing the vertical position of maxilla or in altering the posterior facial complex. Carels et al. [32] using the model fitting approach [35] calculated the heritability estimates. Although there are some inconsistencies with our results but the overall pattern is in agreement with our findings. The calculated heritability estimates were higher for the vertical variables compared to the horizontal variables. In contrast to our findings, Carels et al. [32] reported higher heritability value for the UAFH. However when they calculated the heritability estimate for UAFH, using the Path Analysis, their result was comparable to our findings. In the present study the linear measurement of anterior cranial base (S-N) showed high genetic determination. Horowitz [15], Dudas and Sassouni [34], Carels et al. [32] and Nakata et al. [36] also reported similar findings. The saddle angle revealed one of the highest heritability estimates. Johannsdottir et al. [37] investigated the heritability of different cephalometric parameters between parents and their offspring. They reported high heritability estimate for saddle angle that increases with age. This is in agreement with our results. However, Carels et al. [32], using the model fitting approach [35] reported a moderate heritability for the saddle angle. We found one of the highest heritability values for the Gonial angle. This is consistent with the findings of Manfredi et al. [24]. In contrast to our result Carels et al. [32] and Johannsdottir et al. [37] reported a moderate heritability estimate for the Gonial angle. Similar to the work of Nakata
77
et al. [36], we found low heritability for mandibular body length (h2 = 0.26) and moderate heritability estimate for ramus height (h2 = 0.50). Dudas and Sassouni [34] and Carels et al. [32] reported greater genetic determination for mandibular length. Carels et al. [32] suggested that there are no dominant genes involved for determining the anterior and posterior cranial bases, whereas mandibular length seems to be determined by dominant genes. The design of the study, unfortunately, did not allow investigating the influence of genetic dominance. As Neale and Cardon [35] stated, it is difficult to investigate the influence of genetic dominance in studies of twins raised together unless the sample size is very large. We found high heritability estimates for SNA and SNB angles. This is consistent with the findings of Harris and Johnson [23]. However this is in contrast with the work of Lobb [20] and Carels et al. [32]. According to Carels et al. [32], only an environmental influence explains the variations in SNA and SNB angles which is different to the concept of Path Analysis. This can partially explains the differences between the results. In addition, twin pairs, especially MZ ones, can give the same reply to similar environmental factors. This can lead to exaggerated h2 values. Previous studies showed that heritability estimates for inter-arch variables such as overbite and overjet were considerably lower than skeletal variables [18,31,38]. In reviewing the dento-alveolar variable there was some inconsistency. Some variables showed high heritability (linear measurement of upper molars and lower incisors, SNA and SNB angles). However other dento-alveolar variables showed low to moderate heritability (interincisal angle, linear measurement of lower molars, horizontal linear measurements of point A and B). Environmental factors like lips, tongue and cheeks, active muscles and certain functions (breathing and mastication) play an important part in the development of tooth position and the occlusion [3–7,39]. It is frequently assumed that environmental factors are exogenous (e.g., trauma), however, variety of endogenous events leading to alterations in cellular development and differentiation may also lead to differences in phenotypic expression between MZ twin pairs [40,41]. Examples include variations in expression of missing, peg-shaped, diminutive and normally formed upper lateral incisors within pairs of MZ twins [40,42]. This study was the first attempt to assess the heritability of different cephalometric variables in an Iranian study sample and further research using model fitting approaches are recommended, so that various models can be fitted to the data and goodness-of-fit assessed statistically. Although new findings are not many, it is still useful to validate the evidence of previous studies and to verify contradictory findings in other studies. This is important to understand that h2 values can only imply the extent to which genetic affects the trait variation and it would be wrong to say ‘‘how much’’ of a trait’s size is genetic. Instead, heritability estimates the extent to which genetics affects a trait’s variation in a specific population at a specific time. There are potential limitations that need to be considered in interpreting the current findings. MZ twins can give the same reply to the similar environmental factors and they tend to be more similar than DZ twins, not just because they are identical genetically, but also because society treats them more similarly and they spend more time
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together [43,44]. The fundamental precept of the twin model is that the environment affects MZ and DZ twins equally. When this assumption fails, h2 estimates can be exaggerated [45]. As Harris [46] stated there are numerous sources of environmental covariation, most of which cause family members’ phenotypes to converge, and consequently produce an inflated h2 values. Environmental covariation can lead to positive and sometimes substantial h2 estimates in the absence of any genetic contribution at all [45]. The inaccuracies introduced by the unequal variances in MZ and DZ groups can also result in distorted h2 value. The fact that some c2 estimates are negative could also imply some type of environmental divergence. Corruccini et al. [47] based on the earlier work of Christian et al. [48,49] showed that with appropriate corrections, the heritability for some dental variables like overjet is close to zero. Finally no formula for calculating h2 is without its legitimate critics because none accounts for all sources of bias inherent in the required simplifying assumptions [50–54]. Therefore full understanding of the interplay between environmental and genetic patterns involved in the development of craniofacial complex is required for better interpretation of our results and values presented here might be taken as conservative estimates for the heritable values.
5.
Conclusion
Our results suggest that many skeletal cephalometric variables investigated are under strong genetic control. Vertical variables (in particular TAFH and LAFH) showed more heritability compared to horizontal variables. Heritability seems to be expressed more anteriorly than posteriorly. The lower third of the face seems to be under strong genetic control.
Acknowledgements We sincerely thank the patients, their families and the Iranian Twin Center for making this study possible. We would like to thank the staff at the Islamic Azad Medical University Dental School for their assistance.
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journal homepage: www.elsevier.com/locate/odw
Research paper
Heritability of dental and skeletal cephalometric variables in monozygous and dizygous Iranian twins Fariborz Amini a,*, Ali Borzabadi-Farahani b a b
Department of Orthodontics and Dentofacial Orthopaedics, School of Dentistry, Islamic Azad Medical University, Tehran, Iran Orthoworld, Leamington Spa, Warwickshire, UK
article info
abstract
Article history:
Background and purpose: To assess the influence of genetic and environmental factors on
Received 11 October 2008
craniofacial morphology in an Iranian study sample.
Received in revised form
Setting and sample population: The department of orthodontics at Islamic Azad Medical
23 December 2008
University and Iranian Twin Centre. Fifty pairs of twins were selected (25 monozygotic,
Accepted 5 January 2009
MZs and 25 dizygotic, DZs). The mean age of subjects in MZ and DZ groups were 16.2 (13.4–
Published on line 20 February 2009
19.8) and 16.6 (13.8–20.1) years, respectively. Methods and materials: A cross-sectional twin study carried out using lateral cephalograms.
Keywords:
The subjects were required to pass their pubertal growth spurts and received no previous
Iranian twin study
orthodontic treatment. Thirty-three linear and angular cephalometric variables were iden-
Cephlometric variables
tified and used. The heritability assessments were undertaken according to the Path
Heritability
Analysis model and also using the Holzinger’s equation. For each cephalometric variables Pearson’s intra-pair correlation coefficients were calculated for MZ and DZ twin pairs. The estimate of heritability (h2) and coefficient of cultural heritability (c2) were then calculated for cephalometric variables. Results: Overall vertical variables showed higher heritability than horizontal variables. The anterior cranial base (S-N), saddle angle (NSBa), total anterior facial height (N-Me), lower anterior facial height (ANS-Me), SNA, SNB, SNPog, Gonial angle, SN-GoGn angle and SNMaxillary plane angle showed high heritability. Heritability was low to moderate for the dento-alveolar variables. Conclusions: Vertical variables (in particular total anterior facial height, TAFH and lower anterior facial height, LAFH) showed more heritability than horizontal ones. Heritability seems to be expressed more anteriorly than posteriorly. The lower third of the face seems to be under strong genetic control. # 2009 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.
1.
Introduction
Craniofacial morphology has long captured the interest and imagination of orthodontists. The aetiology of malocclusion
thought to involve both major and minor genetic influences with variable interactions from environmental factors [1,2]. The separation of these factors in the contribution of severity of malocclusion is significant for clinical orthodontics. Facial
* Corresponding author. E-mail address: [email protected] (F. Amini). 1344-0241/$ – see front matter # 2009 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved. doi:10.1016/j.odw.2009.01.001
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bone sizes and their growth potential would be determined genetically. However according to the Moss’s functional matrix theory, environment also plays a part and these bones are subjected to related functional muscle attachments and oro-naso-pharyngeal function, which determines their eventual characteristics [3–7]. The prognosis of orthodontic treatment is determined largely by therapeutic environmental interventions. In this context, orthodontists have used different functional appliances, headgears and expanders to correct the anterior–posterior, vertical, and transverse discrepancies of the jaws and the facial skeleton [8–13]. The ‘‘heritability’’ means the strength of the genetic control and ‘‘high heritability’’ means small room for environmental intervention. Heritability is a ratio. It ranges from 0 to 1, indicating the relative importance of genetics compared with environment on a given trait’s variability. Orthodontists usually assume that genetic factors can significantly affect the development of malocclusion. There is now a considerable body of evidence that genes play a dominant role in the aetiology of malocclusion [14]. Studies of monozygotic (MZ) and dizygotic (DZ) twins offer a powerful method of partitioning genetic and environmental sources of covariance of quantitative traits. MZ twins are genetically identical, whereas DZ twins share their genes similar to siblings (i.e. fraternal). When only trait values, not genotypic information, are available, variance-component estimation can also be used to estimate heritability of a quantitative trait. The trait differences in MZ twins are considered to be due to environmental differences and those in DZ twins due to both genetic and environmental factors. Therefore if a trait has no genetic component, for example due to chance or trauma, concordance rates would be expected to be similar for MZ and DZ twins. For a single-gene trait or a chromosomal disorder the MZ concordance rate will be 100%, whereas the DZ rate will be less than this and equal to the rate in siblings. For discontinuous multi-factorial traits with both genetic and environmental contributions, the rate in MZ twins, although less than 100%, will exceed the rate in DZ twins. Previous studies investigated the heritability of malocclusion and craniofacial morphology among siblings and the twin study method is one of the most commonly used methods available for investigating genetically determined variables in orthodontics [16–24]. We know from previous studies that measurements of facial height are more genetically determined and more difficult to alter than measurements of facial depth [16–24]. An expansion of the ethnic variety of twin studies away from the predominantly Euro-American whites would be of particular interest. The objective of this study was to estimate the heritability of different cephalometric parameters, both skeletal and dento-alveolar, between MZ and DZ healthy twins in an Iranian study sample and compare it with previous studies.
2.
Materials and methods
2.1.
Study sample
The ethical approval for the present study was given by the Research Ethics Committee, School of Dentistry, Islamic
Table 1 – Age descriptive statistics (in years) for MZ and DZ twins. Groups studied
Mean age
Minimum
Maximum
MZ pairs Male (n = 13) Female (n = 12)
16.2 16.6 14.8
13.4 15.0 13.4
19.8 19.8 19.0
DZ pairs Male (n = 13) Female (n = 12)
16.6 15.9 15.1
13.8 14.9 13.8
20.1 20.1 19.8
Azad Medical University, Tehran, Iran. Subjects for this study were selected from the pool of patients who registered with the Iranian Twin Center based in Tehran, Iran. A total of 53 pairs of twins were identified and then allocated to MZ and DZ groups. Initially 28 pairs of DZ twins were selected, however 3 DZ pairs showed extreme discordance and were excluded from the study as an outlier. Finally, 25 pairs of MZ twins and 25 pairs of DZ twins were selected for this study (50 pairs). It was decided to select pairs who have passed their peak pubertal growth spurts therefore subjects who were younger than 13 years old were excluded from this study. The sample’s age characteristics are shown in Table 1. Twin zygosity was initially determined in Iranian Twin Center, by means of evaluating the concordance or discordance of various physical characteristics (standing height, finger print, tooth size, and hair and eye colour). Subsequently all subjects were genotyped and zygosity reconfirmed by means of hematological studies. Informed consent was obtained after the objective of the study was fully explained. All participants were offered free orthodontic consultation and as a part of initial examination and standardized lateral cephalographs were taken. They were healthy and had no previous orthodontic treatment or permanent dental extractions. None of the participants received any facial trauma that could have resulted in bony fracture.
2.2.
Cephalometric measurements
Cephalographs were obtained by the Brodbent cephalometric technique which positions the subject’s head in a cephalostat oriented to the Frankfort horizontal plane. An anode-to-object distance of 5 feet was used [25]. Following landmarks were identified on each lateral X-ray film: Sella turcica (S), sphenoethmoidale (Se), nasion (N), basion (Ba), orbitale (Or), anterior nasal spine (ANS), posterior nasal spine (PNS), porion (Po), point A (A), point B (B), pogonion (Pog), gnathion (Gn), gonion (Go), menton (Me) and articulare (Ar), incisor inferior (Ii), incisor superior (Is). Overall 33 linear and angular cephalometric measurements were identified and later digitized and processed using Dentofacial Planner software (Dentofacial Software, Toronto, Ontario, Canada) (Table 2, Figs. 1–3). Linear parameters were either actual lengths or projected lengths (projected lengths on the FH plane for horizontal variables and FH-vertical plane for vertical variables) (Fig. 3). The variables were divided into four groups: cranial base/skeletal variables, maxillary skeletal variables, mandibular skeletal variables and dentoalveolar variable.
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Table 2 – Cephalometric linear and angular variables used in the study. Cranial base/skeletal variables Linear (actual): S-N (Anterior cranial base), S-Ar, S-Go (TPFH) Linear (projected): N-Me (TAFH), ANS-Me (LAFH) Angular: NSBa (saddle angle), SN-GoMe, GoMe-Occl.Plane, GoMe-ANS.PNS, SN-ANS.PNS Maxillary skeletal variables Linear (actual): S-A, ANS-PNS Linear (projected): N-ANS (UAFH), Se-PNS (UPFH), A horizontal Angular: SNA Mandibular skeletal variables Linear (actual): Go-Gn (mandibular body length), Ar-Go (ramus height) Linear (projected): B Horizontal, Pog Horizontal Angular: SNB, SNPog, ArGoMe (Gonial angle), SArGo Dento-alveolar variables Lineara: UI ? ANS.PNS, UM ? ANS.PNS, LI ? GoMe, LM ? GoMe Linear (projected): UI horizontal, LI horizontal Angular: UI-SN, LI-GoMe, UI-LI (interincisal angle) a
Perpendicular distance from ANS.PNS and GoMe plane.
Fig. 2 – Cephalometric planes and angles used in the study.
own pair mean), d is the difference between the first and second recordings, and n is the number of radiographs replicated. In addition to the above method, Paired t-test analysis was also used to compare the original and repeated data.
Fig. 1 – Cephalometric landmarks used in the study.
2.3.
Measurement error
To avoid inter-examiner error, one investigator (FA) traced and digitized all radiographs. To determine the measurement error, all cephalometric measurements were repeated on 20 randomly selected radiographs 1 week apart. The cephalograms were retraced and re-evaluated 1 week apart. The method error values calculated with the following formula described by Dahlberg [26] Se = H(Sd2/2n). Where, Se is the method error (or standard deviation of the difference of each of the paired measurements from its
Fig. 3 – Linear measurements used in the study. Projected horizontal variables are parallel to Frankfurt horizontal plane. Projected vertical variables are perpendicular to Frankfort horizontal plane.
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orthodontic waves 68 (2009) 72–79
2.4.
Statistical analysis
All analysis was carried out with the SPSS for Windows statistical package (Version 14.0, SPSS Inc., Chicago, IL). We used the estimate of heritability (h2) to assess the heritability. It expresses the proportion of total phenotypic variance that is attributable to the average effect of inherited genes. Theoretically, h2 value should range from zero for no heritability to 1.0 for a completely inherited trait. This is based on the assumption that MZ twins are genetically identical (share 100% of their genes) and differences between them can be due only to environmental factors, whereas DZ twins share on average half of their segregating genes and differences between them can be due to a combination of both environmental and genetic factors. To measure the resemblances between the twins, for each cephalometric variable Pearson’s intra-pair correlation coefficients (rMZ and rDZ) were first calculated for MZ and DZ twin pairs. The estimates of heritability (h2) or coefficient of cultural heritability (c2) was then calculated for cephalometric variables. To compare our findings with previous works we calculated heritability estimates according to the Dr Lundstro¨m’s approach [19], the Path Analysis model [27]. However, this approach does not always yields h2 values between 0 and 1.0. Therefore we also used the Holzinger equation [28]. Path Analysis calculates the proportion of the total variance explained by additive genes (heritability), and by the common and specific environment. The estimate of heritability (h2) and coefficient of cultural heritability (c2) are based on intra-pair correlation coefficients calculated from the MZ and DZ twins with the following formulas: h2 = 2(rMZ rDZ), c2 = 2rDZ rMZ. As mentioned earlier, to compare the results with previous studies we also calculated the Holzinger heritability estimate [28] using the following formula: Holzinger’s heritability estimate ¼
3.
Table 3 – Dahlberg’s error of method (Se) for repeatability of the angular and linear measurements and paired ttest results. Cephalometric variables S-N (mm), anterior cranial base S-Ar (mm) S-Go (mm), TPFH N-Me (mm), TAFH ANS-Me (mm), LAFH NSBa (8), saddle angle SN-GoMe(8) GoMe-Occl.Plane (8) GoMe-ANS.PNS (8) SN-ANS.PNS (8) S-A (mm) ANS-PNS (mm) N-ANS (mm), UAFH Se-PNS (mm), UPFH A horizontal (mm) SNA (8) Go-Gn (mm), mandibular body length Ar-Go (mm), ramus height B Horizontal (mm) Pog Horizontal (mm) SNB (8) SNPog (8) ArGoMe (8), Gonial angle SArGo (8) UI ? ANS-PNS (mm) UM ? ANS-PNS (mm) LI ? GoMe (mm) LM ? GoMe (mm) UI horizontal (mm) LI horizontal (mm) UI-SN (8) LI-GoMe (8) UI-LI (8), interincisal angle
Se 0.29 0.30 0.36 0.31 0.37 0.43 0.50 0.45 0.52 0.48 0.33 0.35 0.38 0.39 0.39 0.51 0.36 0.29 0.30 0.31 0.39 0.41 0.54 0.50 0.34 0.32 0.35 0.39 0.34 0.36 0.41 0.47 0.42
t-Test NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS
Se = error of method = H(Sd2/2n). NS; not significant.
rMZ rDZ 1 rDZ
Results
The measurement error values for angular and linear measurements on the lateral cephalograms did not exceed 0.548 and 0.39 mm (Table 3). None of the angular or linear measurements showed any statistical difference (P < 0.05) between the original and repeated values. Table 4 summarizes the Pearson’s intra-pair correlation coefficients, estimates of genetic heritability (h2), cultural inheritance (c2) and Holzinger’s heritability values between MZ and DZ twins. Overall in MZ twin pairs, intra-pair correlations were considerably higher than DZ twin pairs. Excluding the TPFH and UAFH, all angular and linear vertical variables showed considerably higher intra-pair correlation coefficients (r) in MZ pairs when compared with DZ pairs. Heritability estimate for linear measurement of the anterior cranial base (S-N) was high (h2 = 0.80). The flexure of cranial base (saddle angle) presented with one of the highest estimates of heritability (h2 = 1). Relative sagittal position of maxilla to the anterior cranial base (SNA) and its linear measurement (S-A) showed high h2 value compared to the variables representing the vertical
position of maxilla (UPFH). Upper anterior facial height (UAFH) showed one of the lowest h2 values (h2 = 0.18). Conversely lower anterior facial height (LAFH) showed a very high heritability. Reviewing different mandibular variables; Gonial angle presented with a considerably high h2 value (h2 = 1.62). Relative sagittal position of mandible to the anterior cranial base (SNB, SNPog) also showed a high coefficient of heritability. Mandibular body length showed a very low heritability. Overall heritability estimates for variables representing the vertical dimension were higher than those representing the horizontal variables. Dental variables in general showed low to moderate heritability. Facial plane angles (SN-ANS.PNS, GoMe-ANS.PNS and SN-GoMe) presented with high h2 values. Reviewing the Holzinger values, they ranged from 0.20 (linear measurements of UAFH and LM ? GoMe) to 0.93 (Gonial angle).
4.
Discussion
Heritability estimation is usually a first step in genetic studies, because it provides an estimate of how much phenotypic variation is attributable to genetic effects. Present study used Dr. Lundstro¨m’s approach, the Path Analysis model [27], and
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Table 4 – Intra-pair correlation coefficients (r) followed by estimates of genetic heritability (h2) and cultural inheritance (c2) according to the Path Analysis model [27] and Holzinger [28] values are listed in the last column. Cephalometric variables
rMZ
rDZ
h2
c2
Holzinger, h 2
Cranial base/skeletal variables S-N (mm), anterior cranial base S-Ar (mm) S-Go (mm), TPFH N-Me (mm), TAFH ANS-Me (mm), LAFH NSBa (8), saddle angle SN-GoMe (8) GoMe-Occl.Plane (8) GoMe-ANS.PNS (8) SN-ANS.PNS (8)
0.82 0.89 0.87 0.94 0.90 0.95 0.90 0.88 0.82 0.80
0.42 0.68 0.78 0.40 0.36 0.45 0.29 0.67 0.21 0.19
0.80 0.42 0.18 1.08 1.08 1.00 1.22 0.42 1.22 1.22
0.02 0.47 0.69 0.14 0.18 0.05 0.32 0.46 0.40 0.42
0.69 0.66 0.41 0.90 0.84 0.91 0.86 0.64 0.77 0.75
Maxillary skeletal variables S-A (mm) ANS-PNS (mm) N-ANS (mm), UAFH Se-PNS (mm), UPFH A horizontal (mm) SNA (8)
0.75 0.67 0.64 0.88 0.82 0.92
0.40 0.50 0.55 0.62 0.60 0.49
0.70 0.34 0.18 0.52 0.44 0.86
0.05 0.33 0.46 0.36 0.38 0.06
0.58 0.34 0.20 0.68 0.55 0.84
Mandibular skeletal variables Go-Gn (mm), mandibular body length Ar-Go (mm), ramus height B Horizontal (mm) Pog Horizontal (mm) SNB (8) SNPog (8) ArGoMe (8), Gonial angle SArGo (8)
0.80 0.83 0.87 0.81 0.83 0.81 0.94 0.59
0.67 0.58 0.665 0.58 0.34 0.32 0.13 0.46
0.26 0.50 0.41 0.46 0.98 0.98 1.62 0.26
0.54 0.33 0.46 0.35 0.15 0.17 0.68 0.33
0.39 0.60 0.61 0.55 0.74 0.72 0.93 0.24
Dento-alveolar variables UI ? ANS.PNS (mm) UM ? ANS.PNS (mm) LI ? GoMe (mm) LM ? GoMe (mm) UI horizontal (mm) LI horizontal (mm) UI-SN (8) LI-GoMe (8) UI-LI (8), interincisal angle
0.90 0.92 0.88 0.88 0.73 0.87 0.80 0.93 0.87
0.60 0.52 0.50 0.85 0.45 0.52 0.71 0.45 0.70
0.60 0.80 0.76 0.06 0.56 0.70 0.18 0.96 0.34
0.30 0.12 0.12 0.82 0.17 0.17 0.62 0.03 0.53
0.75 0.83 0.76 0.20 0.51 0.73 0.31 0.87 0.57
h2 = 2(rMZ rDZ), c2 = 2rDZ rMZ; Holzinger heritability estimate = (rMZ rDZ)/(1 rDZ).
Holzinger equation [28] in an attempt to investigate the heritability of craniofacial variables within pairs of MZ and DZ young, healthy twins. Craniofacial morphology is the result of complex interaction between inheritance and environmental factors and is therefore multi-factorial. The final product, the phenotype, represents the combination of genetic and environmental influences. Path Analysis model calculates the proportion of the total variance explained by additive genes (heritability), and by the common and specific environment. This is a common method previously used in twin studies [19,24]. The overall pattern of craniofacial development (short or long facial height) is established early and on average does not change with age [29]. It is also acknowledged that facial maturity develops in females between 10 and 13 years and 2 years later for males. Interestingly faces of pre-pubertal boys and girls may appear very similar [30]. Harris and Johnson [23] studied craniofacial dimensions using cephalometric radiographs obtained at regular intervals during the age period
from 4 to 20 years. They reported that heritability estimates of craniofacial variables increased with age. Therefore comparison of hereditary characteristics is more valid in the postadolescent period when growth is almost completed and in the present study, subjects who were younger than 13 years old were excluded from the study sample. Overall many skeletal cephalometric variables showed high heritability, including the total anterior facial height and facial plane angles (SN-ANS.PNS, GoMe-ANS.PNS and SNGoMe). As previously shown [24,31–33]; we also observed higher heritability estimates for many vertical linear measurements, compared with horizontal linear measurements. However this is in contrast to the findings of Lundstro¨m and McWilliam [19]. They did not find significant differences between the horizontal and vertical measurements. The present study is consistent with the findings of previous studies [15,24,34] and showed high genetic determination for total anterior facial height (TAFH) and for its lower component (LAFH).
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Using the present findings, we agree with Manfredi et al. [24] in that, heritability seemed to be more influential on the anterior vertical variables of the face than its posterior vertical variables. In the present study, the UAFH and posterior facial height (S-Go) showed low heritability. Ramus height (Ar-Go) also showed a moderate heritability (h2 = 0.50). In reviewing the sagittal maxillary variables, we found high heritability estimate for SNA angle and also its linear measurement (S-A). However heritability values for vertical variables (UAFH and UPFH) are suggestive of more environmental influence. This is consistent with the findings of Manfredi et al. [24] and again it may suggest that there is more likelihood of changing the vertical position of maxilla than its sagittal position. From a clinical perspective, it seems that the vertical position of the maxilla is much less under the control of genetics than its sagittal position. This result is also consistent with the earlier findings of Lundstro¨m and McWilliam [19]; they reported lower heritability for UAFH than other vertical dimensions. However, heritability is a function of one trait, in one sample, in one population, at one point in time. To be more precise, heritability is a population concept the concept of h2 is irrelevant to the individual. It would be misleading to suggest that features with low heritability might be more amenable to prevention or treatment in individuals .The ability to move the teeth or have an orthopedic effect on bone involves a complex mix of biomechanical properties of bone and other biologic tissues, the direction, strength, and duration of the mechanical force. The ability of the individual to respond to changes in the environment (including treatment) will define the amount of treatment effect. This is not measured or determined by estimating heritability. Therefore using the present findings it is difficult to conclude that orthopedic treatment is more successful in changing the vertical position of maxilla or in altering the posterior facial complex. Carels et al. [32] using the model fitting approach [35] calculated the heritability estimates. Although there are some inconsistencies with our results but the overall pattern is in agreement with our findings. The calculated heritability estimates were higher for the vertical variables compared to the horizontal variables. In contrast to our findings, Carels et al. [32] reported higher heritability value for the UAFH. However when they calculated the heritability estimate for UAFH, using the Path Analysis, their result was comparable to our findings. In the present study the linear measurement of anterior cranial base (S-N) showed high genetic determination. Horowitz [15], Dudas and Sassouni [34], Carels et al. [32] and Nakata et al. [36] also reported similar findings. The saddle angle revealed one of the highest heritability estimates. Johannsdottir et al. [37] investigated the heritability of different cephalometric parameters between parents and their offspring. They reported high heritability estimate for saddle angle that increases with age. This is in agreement with our results. However, Carels et al. [32], using the model fitting approach [35] reported a moderate heritability for the saddle angle. We found one of the highest heritability values for the Gonial angle. This is consistent with the findings of Manfredi et al. [24]. In contrast to our result Carels et al. [32] and Johannsdottir et al. [37] reported a moderate heritability estimate for the Gonial angle. Similar to the work of Nakata
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et al. [36], we found low heritability for mandibular body length (h2 = 0.26) and moderate heritability estimate for ramus height (h2 = 0.50). Dudas and Sassouni [34] and Carels et al. [32] reported greater genetic determination for mandibular length. Carels et al. [32] suggested that there are no dominant genes involved for determining the anterior and posterior cranial bases, whereas mandibular length seems to be determined by dominant genes. The design of the study, unfortunately, did not allow investigating the influence of genetic dominance. As Neale and Cardon [35] stated, it is difficult to investigate the influence of genetic dominance in studies of twins raised together unless the sample size is very large. We found high heritability estimates for SNA and SNB angles. This is consistent with the findings of Harris and Johnson [23]. However this is in contrast with the work of Lobb [20] and Carels et al. [32]. According to Carels et al. [32], only an environmental influence explains the variations in SNA and SNB angles which is different to the concept of Path Analysis. This can partially explains the differences between the results. In addition, twin pairs, especially MZ ones, can give the same reply to similar environmental factors. This can lead to exaggerated h2 values. Previous studies showed that heritability estimates for inter-arch variables such as overbite and overjet were considerably lower than skeletal variables [18,31,38]. In reviewing the dento-alveolar variable there was some inconsistency. Some variables showed high heritability (linear measurement of upper molars and lower incisors, SNA and SNB angles). However other dento-alveolar variables showed low to moderate heritability (interincisal angle, linear measurement of lower molars, horizontal linear measurements of point A and B). Environmental factors like lips, tongue and cheeks, active muscles and certain functions (breathing and mastication) play an important part in the development of tooth position and the occlusion [3–7,39]. It is frequently assumed that environmental factors are exogenous (e.g., trauma), however, variety of endogenous events leading to alterations in cellular development and differentiation may also lead to differences in phenotypic expression between MZ twin pairs [40,41]. Examples include variations in expression of missing, peg-shaped, diminutive and normally formed upper lateral incisors within pairs of MZ twins [40,42]. This study was the first attempt to assess the heritability of different cephalometric variables in an Iranian study sample and further research using model fitting approaches are recommended, so that various models can be fitted to the data and goodness-of-fit assessed statistically. Although new findings are not many, it is still useful to validate the evidence of previous studies and to verify contradictory findings in other studies. This is important to understand that h2 values can only imply the extent to which genetic affects the trait variation and it would be wrong to say ‘‘how much’’ of a trait’s size is genetic. Instead, heritability estimates the extent to which genetics affects a trait’s variation in a specific population at a specific time. There are potential limitations that need to be considered in interpreting the current findings. MZ twins can give the same reply to the similar environmental factors and they tend to be more similar than DZ twins, not just because they are identical genetically, but also because society treats them more similarly and they spend more time
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together [43,44]. The fundamental precept of the twin model is that the environment affects MZ and DZ twins equally. When this assumption fails, h2 estimates can be exaggerated [45]. As Harris [46] stated there are numerous sources of environmental covariation, most of which cause family members’ phenotypes to converge, and consequently produce an inflated h2 values. Environmental covariation can lead to positive and sometimes substantial h2 estimates in the absence of any genetic contribution at all [45]. The inaccuracies introduced by the unequal variances in MZ and DZ groups can also result in distorted h2 value. The fact that some c2 estimates are negative could also imply some type of environmental divergence. Corruccini et al. [47] based on the earlier work of Christian et al. [48,49] showed that with appropriate corrections, the heritability for some dental variables like overjet is close to zero. Finally no formula for calculating h2 is without its legitimate critics because none accounts for all sources of bias inherent in the required simplifying assumptions [50–54]. Therefore full understanding of the interplay between environmental and genetic patterns involved in the development of craniofacial complex is required for better interpretation of our results and values presented here might be taken as conservative estimates for the heritable values.
5.
Conclusion
Our results suggest that many skeletal cephalometric variables investigated are under strong genetic control. Vertical variables (in particular TAFH and LAFH) showed more heritability compared to horizontal variables. Heritability seems to be expressed more anteriorly than posteriorly. The lower third of the face seems to be under strong genetic control.
Acknowledgements We sincerely thank the patients, their families and the Iranian Twin Center for making this study possible. We would like to thank the staff at the Islamic Azad Medical University Dental School for their assistance.
references
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