Assessment of Correlation of Growth Hormone Receptor Gene with Tooth Dimensions: A CBCT and Genotyping Study.

AIM: Assessment of root morphology, size, and amount of bone around tooth is essential before starting the orthodontic treatment. The present study aimed to assess the relationship between tooth dimensions with two gene variants of growth hormone (GH), namely rs6184 and rs6180.
MATERIALS AND METHODS: This study was conducted on 218 subjects (males: 104, females: 114) requiring orthodontic treatment. All underwent cone beam computed tomography (CBCT) scan for orthodontic treatment planning with Kodak CBCT machine. In all teeth, crown height (CH), root length (RL), and crown-root ratio were evaluated. Two growth hormone receptor (GHR) variants (rs6184 and rs6180) were genotyped using the TaqMan genotyping assay.
RESULTS: The mean CH and RL of all teeth, that is, maxillary and mandibular central incisors, lateral incisor, canine, first premolar, second premolar, first molar, and second molar, were measured. There was no significant difference in males and females (P > 0.05). Allele frequencies of GHR variants for rs6180 and rs6184 were 48.1% and 8.92%, respectively. Multiple regression analysis showed GHR rs6184 association with maxillary central incisor CH, maxillary canine RL, mandibular canine CH, and mandibular first premolar RL (P < 0.05).
CONCLUSION: There was correlation of CH of maxillary and mandibular canine and RL of maxillary canine and mandibular first premolar with GHR rs6184.

INTRODUCTION

Facial esthetics is of great concern not only in young but in old too. With the advancement in the field of orthodontics, various techniques have been emerged, leading to correction of skeletal and dental anomalies. For the application of orthodontic forces, there should be adequate crown structure. Type of malocclusion determines the type of force as well as area from which force has to be given.[1]

A significantly acceptable crown–root ratio is necessary for obtaining good results. Assessment of root morphology, and size and amount of bone around tooth are essential before starting the orthodontic treatment. Studies have revealed genetic and environmental factors as causative factors for tooth and skeletal anomalies.[23]

A study conducted by Kimura et al.[4] has demonstrated common variation in EDAR as a genetic determinant of shovel-shaped incisors in study group. Genetics has been considered as one of the important factors of dental and skeletal anomalies. Similarly, another study by Kimura et al.[5] suggested the role of polymorphisms in WNT10A in tooth morphology as well as hair shape.

The presence of hypoconulid in mandibular second molar, accessory cusp in mandibular second premolar, and cusp of carabelli in maxillary first molar is considered as a result of genetic predisposition. Growth hormone (GH) is responsible for postnatal growth with GH/GHR/insulinlike growth factor 1 signaling axis. Van et al.[6] showed that GH is required for normal tooth eruption and maturation. Thus, there is great need of such studies that may depict the role of genetics in assessing dental anomalies. This study was conducted to assess the relationship between tooth dimensions with two gene variants of GH, namely rs6184 and rs6180.

MATERIALS AND METHODS

This study was conducted in the Department of Pedodontics and Preventive Dentistry, SMBT Dental College and Hospital, Sangamner, Maharashtra, India. It comprised 218 subjects (males: 104, females: 114) requiring orthodontic treatment. All participants were informed regarding the study and written consent was obtained. Ethical clearance was obtained prior to commencement of study from ethical committee.

Inclusion criteria included patients aged 14–22 years and patients without history of trauma. Exclusion criteria were patients with congenital tooth and skeletal anomalies, orthodontic history, and poor-quality radiographs.

General information such as name, age, and gender was recorded in case history pro forma. All underwent cone beam computed tomography (CBCT) scan for orthodontic treatment planning with a Kodak CBCT machine operating at 80 kVp, 5 mA, and 18 second exposure time. Multiplanar reconstruction was performed and all the planes such as axial, coronal, and sagittal were obtained [Figures 1–3]. For assessment of crown height (CH), a line joining the mesial and distal cementoenamel junction was measured. The length was measured between crown tip and this line. For root length (RL), the measurement between cervical line and root tip, was considered [Figures 4 and 5]. Measurements of all teeth were performed and overall tooth dimension was calculated by adding CH with RL. Crown–root ratio was evaluated by dividing CH with RL.

Figure 1

Axial plane

Figure 3

Sagittal plane

Figure 4

Measurements (CH, canine height; RL, root length)

Figure 5

Measurements (CH, canine height; RL, root length)

Coronal plane

Two independent oral and maxillofacial radiologists performed all the measurements and average of both findings was considered to eliminate bias. All the measurements were performed with OsiriX CBCT software.

For genotyping, unstimulated saliva of subjects was collected in a saliva collector and stored at room temperature. Genomic DNA was extracted from the saliva samples. Two growth hormone receptor (GHR) variants (rs6184 and rs6180) were genotyped using the TaqMan genotyping assay. Results thus obtained were subjected to statistical analysis. P value less than 0.05 was considered significant.

RESULTS

Table 1 and Graph 1 show that there are 104 (47.7%) males and 114 (52.3%) females. Table 2 and Graph 2 show mean CH and RL of all teeth, that is, maxillary central incisor, lateral incisor, canine, first premolar, second premolar, first molar, and second molar. There is no significant difference between males and females (P > 0.05).

Table 1

Distribution of subjects

Males	Percentage	Females	Percentage
104	47.7	114	52.3

Graph 1

Distribution of subjects

Table 2

Measurements of maxillary teeth

Tooth	Mean CH (mm)		Mean RL (mm)
	Male	Female	Male	Female
Maxillary central incisor	10.12	10.11	13.24	13.20
Maxillary lateral incisor	10.04	10.02	12.18	12.19
Maxillary canine	11.25	11.21	15.42	15.32
Maxillary first premolar	9.02	9.04	11.31	11.25
Maxillary second premolar	8.76	8.65	11.20	11.23
Maxillary first molar	8.54	8.51	10.08	10.16
Maxillary second molar	8.34	8.41	10.12	10.19

CH = canine height, RL = root length

Graph 2

Measurements of maxillary teeth

Table 3 and Graph 3 show mean CH and RL of all teeth, that is, mandibular central incisor, lateral incisor, canine, first premolar, second premolar, first molar, and second molar. There is no significant difference in males and females (P > 0.05).

Table 3

Measurements of mandibular teeth

Tooth	Mean CH (mm)		Mean RL (mm)
	Male	Female	Male	Female
Mandibular central incisor	8.14	8.01	12.10	12.23
Mandibular lateral incisor	9.12	9.23	12.05	12.21
Mandibular canine	10.21	10.14	15.88	14.36
Mandibular first premolar	9.32	8.04	13.34	12.21
Mandibular second premolar	8.24	8.15	13.22	13.25
Mandibular first molar	8.44	8.58	10.28	10.12
Mandibular second molar	7.29	8.47	11.85	11.22

CH = canine height, RL = root length

Graph 3

Measurements of mandibular teeth

Table 4 shows that allele frequencies of GHR variants for rs6180 and rs6184 are 48.1% and 8.92%, respectively. Table 5 shows that multiple regression analysis indicates association of GHR rs6184 with maxillary central incisor CH, maxillary canine RL, mandibular canine CH, and mandibular first premolar RL (P < 0.05).

Table 4

Single nucleotide polymorphism

rs no.	chr position	mRNA position	Alleles	Genotype frequency			Derived allele frequency
				AA	AD	DD
rs 6180	chr5: 42719137	A1822C	A/C	65	95	58	48.1%
rs 6184	chr5: 42719242	C1927A	C/A	141	35	42	8.92%

CH = canine height, RL = root length

Table 5

Association tests using multiple regression analyses

Trait	Single nucleotide polymorphism	B	Standard error	P value
Maxillary central incisor CH	rs 6184	−0.142	0.214	0.01
Maxillary canine RL	rs 6184	−0.411	0.328	0.05
Mandibular canine CH	rs 6184	−0.387	0.217	0.03
Mandibular first premolar RL	rs 6184	−0.210	0.645	0.012

CH = canine height, RL = root length

DISCUSSION

In orthodontics, the size of tooth, that is, crown length and RL, plays an important role. GH is essential for tooth eruption and maturation. It has been found that in dwarfism, there is a small-sized tooth whereas in the case of gigantism, there is an enlarged tooth.[7] Campbell et al.[8] revealed that GH mutation is the leading cause of delayed tooth eruption and maturation. In present study, we assessed the relationship between tooth dimensions with two gene variants of GH, namely rs6184 and rs6180.

In this study, we included 218 subjects requiring orthodontic treatment of both genders. In all subjects, genotyping of two GHR variants (rs6184 and rs6180) was performed using the TaqMan genotyping assay. CBCT scan of all teeth was conducted.

Tomoyasu et al.[9] performed a study to find out the association between mandibular height and the GHR gene in study population. The authors found a strong correlation between mandibular ramus height and GHR gene. Similarly, a study performed by Kang et al.[10] found an association of the GHR gene polymorphisms with mandibular height. The distance between the left and right condyles and coronoid process was assessed to find its relation with GHR.

In this study, we evaluated CH and RL as well as crown–root ratio. We observed that there was no significant difference in CH, RL, and crown–root ratio of maxillary and mandibular teeth.

It has been postulated that the formation and eruption of teeth are greatly affected by cell sensitivity of GH and the site of GH action.[11] At sites of new matrix formation, cementoblasts and odontoblasts displayed expression specifically against GHR, although cementocytes and mature odontoblasts at later stages of tooth development did not. The functional mechanism by which the GHR gene variant identified may be responsible for tooth length is still unclear.[12]

We found that mean CH of maxillary central incisor in males was 10.12 mm and in females was 10.11 mm; the RL was 13.24 mm in males and 13.20 mm in females. In lateral incisors, the mean CH was 10.04 and 10.02 mm in males and females, respectively, whereas RL was 12.18 and 12.19 mm, respectively. In canine, CH was 11.12 and 11.21 mm and RL was 15.42 and 15.32 mm in males and females, respectively. Our results are in agreement with those of Smith et al.[13]

Maxillary first premolar showed mean CH as 9.02 mm in males and 9.04 mm in females, and RL as 11.31 mm in males and 11.25 mm in females; second premolar showed CH as 8.76 mm in males and 8.65 mm in females, and RL as 11.20 mm in males and 11.25 mm in females. Maxillary first molar showed CH as 8.54 mm in males and 8.51 in females, and RL as 10.08 in males and 10.16 mm in females. Maxillary second molar showed CH as 8.34 mm in males and 8.41 mm in females whereas RL was 10.12 and 10.19 mm in males and females, respectively.

We found that mean CH of mandibular central incisor in males was 8.14 mm and in females was 8.01 mm; the RL was 12.10 mm in males and 12.23 mm in females. In lateral incisors, the mean CH was 9.12 and 9.23 mm in males and females, respectively, whereas RL was 12.05 and 12.21 mm. In canine, CH was 10.21 and 10.14 mm, and RL was 15.88 and 14.36 mm in males and females, respectively.

Mandibular first premolar showed mean CH as 9.32 mm in males and 8.04 mm in females, and RL as 13.34 mm in males and 12.21 mm in females; second premolar showed CH as 8.24 mm in males and 8.15 mm in females, and RL as 13.22 mm in males and 13.25 mm in females. Mandibular first molar showed CH as 8.44 mm in males and 8.58 mm in females, RL as 10.28 mm in males and 10.12 mm in females. Mandibular second molar showed CH as 7.29 mm in males and 8.47 mm in females whereas RL was 11.85 and 11.22 mm in males and females, respectively. This is similar to the results of the study by Pentinpuro et al.[14]

In this study, we found correlation of GHR rs6184 with maxillary central incisor CH, maxillary canine RL, mandibular canine CH, and mandibular first premolar RL. This is in agreement with the results of the study by Hikita et al.[15] The authors found relationship of GHR variant rs6184 with RLs and tooth length for the upper and lower lateral incisors and upper canines.

We found allele frequencies of GHR variants for rs6180 and rs6184 as 48.1% and 8.92%, respectively. Similarly, Hikita et al.[15] found 46.6% and 8.1% allele frequencies of GHR variants for rs6180 and rs6184.

In this study, CBCT was selected as a radiological tool because it offers better results in terms of providing three dimensions and accurate measurements compared to conventional two-dimensional radiographs. It offers less patient exposure than CT and hence useful in this study. A limitation of this study was limited patient size.

CONCLUSION

We found correlation of CH of maxillary and mandibular canine and RL of maxillary canine and mandibular first premolar with GHR rs6184. Large-scale future studies are required to support the results of the present study. GHR gene assessment may predict tooth maturation and development.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.