Genetics evolved as a field of science in the 1900s and reached the stage of decoding the whole human genome in 2004 under the Human Genome Project. Earlier experiments on fruit flies, bacteria and viruses helped lay the foundation of genetics. ^, The role of individual genes or groups of genes became clearer with newer experimental methods, such as transgenic and knockout mouse models. ^, Many breakthroughs like next-generation sequencing, genetic engineering and the Human Genome Project helped discover genes associated with various diseases of genetic origin. Starting in 2010, there was a significant emphasis on epigenomics, which involves studying environmental factors that impact the regulation of gene expression. This field of study explains how external factors influence gene expression and how this can impact various biological processes. ^,

Great leaders like Kingsley and Farrar hinted that genes influence occlusion and malocclusion. In 1911, during the famous ‘Great Extraction Debate’, Dr. Calvin Case emphasised that tooth extraction is required to address the inherited disparity between tooth size and jaws. ^, According to the consensus among many researchers, it was believed that the sizes of the maxilla and mandible were inherited independently from each parent. The intermingling of ethnic and racial groups was thought to potentially give rise to malocclusion. During the period spanning 1930 to 1970, there was a focus on the impact of environmental factors such as tonsils and pressure from soft tissues like lips and cheeks. It was acknowledged that hereditary factors were deemed paramount, with local environmental factors serving as secondary influencers to the growth and development of the jaws and teeth. The predominant genetic theory was further supported by twin studies, helping to differentiate between genetic and environmental contributions. The balance between heredity and environment had again shifted towards the environment with the introduction of Melvin Moss’s functional matrix theory. Melvin Moss, a distinguished craniofacial biologist from Columbia University, is credited with this significant shift. At the beginning of the 21st century, research shifted towards specific molecular pathways that regulate dentofacial development.

The most significant leap in understanding the contribution of genetics in orthodontics can be attributed to studies after the 2000s, which led to the identification of growth factors, cytokines and transcription factors that control the growth of the condylar cartilage and craniofacial tissues. Many candidate genes controlling these tissues and contributing to face deformities were identified, enhancing understanding of tissue response to orthodontic treatment, genetic predisposition to temporomandibular joint (TMJ) pain and root resorption.

Current research in genetics related to facial development growth and facial deformities is focused on the role of gene variants, such as single nucleotide polymorphisms (SNPs), complex groups of genes, genome-wide association studies (GWAS) and regulatory molecules, such as microRNAs.

The cell is the basic unit of life. It comprises the cell membrane, cytoplasm, nucleus and various cellular organelles. The nucleus contains chromosomes that encode for the genetic material ( Fig. 16.1 ). Humans have 23 pairs of chromosomes, of which 22 are autosomes and the 23rd is a sex chromosome (X and Y). Each pair contains two chromosomes, one derived from the father and the other from the mother.

The cell contains the genetic material (chromosomes) in its nucleus.

The genetic information is stored in the double helix of DNA. This genetic information is transmitted from DNA to mRNA (transcription) and from mRNA into protein (translation). This systematic and well-regulated process of transcription and translation is known as the Central Dogma .

Genetic information stored in DNA and RNA mainly helps transfer and express information. The hallmark of DNA is the double-stranded helical structure and hydrogen bonding between the bases from opposite strands ( Fig. 16.1 ). Adenine pairs with thymine with two H-bonds and guanine with cytosine via three H-bonds, making one strand complementary to the other, and the DNA replicates semi-conservatively. The process whereby genetic information is transmitted from DNA to mRNA is called transcription and from mRNA into protein is called translation ( Fig. 16.1 ).

Gene is a segment of DNA that codes for mRNA. During transcription, one of the strands of DNA acts as a template to direct the synthesis of complementary RNA. In eukaryotes, the gene is an interrupted gene that consists of introns and exons. The coding sequences, known as exons, are interrupted by non-coding sequences called introns. Introns are removed and exons are spliced together to produce functional RNA.

The mRNA contains base sequences that are read in combination with three bases to code for amino acids, referred to as genetic code. The mRNA then migrates out of the nucleus into the cytoplasm, which becomes associated with the ribosomes, the site of protein synthesis. The translation process occurs in the ribosome, which binds to mRNA and provides a platform for joining amino acids. In the cytoplasm, transfer RNA (tRNA) transfers the amino acids once the helix has unzipped. The ribosome then holds the tRNA and matches the tRNA with mRNA till the new chain is complete. The ribosome then releases the tRNA and the newly formed protein.

Transcription and translation must be regulated as they consume much energy. Some genes are not concerned or involved with the synthesis of proteins but control the functioning of structural genes. These genes are called control genes, operons or operator genes. The operator gene, in turn, is controlled by a regulator gene, which is not necessarily close to the operator gene. This whole process leads to the regulation of the genetic code and the mechanism of inheritance.

The inheritance of a trait can be either monogenic (governed by one gene), polygenic (governed by multiple genes) or through multifactorial inheritance. Apart from this nuclear inheritance, there is a small amount of mitochondrial DNA inheritance. Since the mitochondrial DNA is in the cytoplasm, not the nucleus, it is inherited exclusively through mothers. Mendel’s laws of inheritance, law of dominance and segregation, and independent assortment are the general inheritance rules ( Fig. 16.2 ).

• 1.

  Monogenic traits:

  • Phenotypic characteristics that manifest under the influence of a single gene are denoted as monogenic traits. In a homologous pair of chromosomes, the genes at the same locus are called alleles. Alleles are homozygous when both members of alleles are identical for that locus and are heterozygous when alleles at a specific locus are different.

  • A family tree, also known as a pedigree, is a valuable and time-honoured tool used to trace and examine the inheritance of traits across multiple generations within a family. The symbols used to represent sex are a square for males and a circle for females. Monogenic traits can be inherited by any of the following mechanisms:

    • A.

      Autosomal dominant trait:

      • Autosomal dominant inheritance occurs when having just one copy of a specific allele on a pair of autosomes is enough to express the trait. Subsequent generations show vertical inheritance and 50% of children show equal sex predilection.

    • B.

      Variable expressivity:

      • The severity of a disease/trait in different individuals varies due to variable expressivity. Even with an autosomal dominant gene, the disease expression can vary, making one patient look different to others. For example, variations of expression are observed in osteogenesis imperfecta (OI), showing up in four different clinical types of diseases: OI may present with different combinations and severity of one of the traits from multiple fractures, blue sclera, dentinogenesis imperfecta and hearing loss. Similarly, identical mutations in the fibroblast growth factor (FGF) receptor 2 gene result in three different clinical syndromes with similar features found in Crouzon, Pfeiffer and Jackson–Weiss syndromes.

    • C.

      Autosomal recessive trait:

      • In an autosomal recessive trait, the carrier/parent is heterozygous for a recessive trait and is called a gene carrier. In such an inheritance pattern, the transmission is horizontal, with only a 25% chance of offspring having the trait. In consanguineous marriages, the children are more likely to be affected by the disease as two recessive genes will become dominant and express the disease.

    • D.

      X-linked recessive traits:

      • The sex chromosome in males comprises XY, and in females, it comprises XX. The genes on Y chromosomes mainly govern the development of the male reproductive system. In males, because the allele on the Y chromosome is absent, the recessive gene on one of the X chromosomes expresses itself dominantly, whereas in females, it remains recessive. Thus, some traits/diseases, like haemophilia A, are only restricted to males in the family.

• 2. Complex (polygenic/multifactorial trait):

  • In the early years, genetic studies focused only on monogenic traits. Unfortunately, many traits, like diabetes and hypertension, are multifactorial, meaning multiple genes and the environment decide how the trait will be expressed. These multifactorial traits do not follow the Mendelian pattern of inheritance. Traits with multifactorial inheritance and of concern to orthodontists include non-syndromic cleft lip–palate, mandibular prognathism, lengths of the mandible and maxilla.

Modes of inheritance.

Several genes are responsible for controlling the development of neural crest cells into prespecified tissues of the face or dentition. These neural crest cells are highly pluripotent and migrate all over the embryo into four main areas: cephalic, cardiac, trunk and sacral regions. The head develops from cephalic neural crest cells of the posterior midbrain and the hindbrain regions. Periodic bulges develop in the hindbrain, known as rhombomeres. These neural crest cells from the lateral margin of the neural plate migrate from the rhombomeres to the branchial arches and develop the mesenchyme of orofacial structures.

Homeotic genes are a group of genes that give characteristic structure to a segment. They encode transcription factors that control the downstream gene activity and have a highly conserved 180-basepair sequence called the homeobox. Homeobox genes are a group of genes found in insects and vertebrates and are responsible for the individuality of a particular segment. Mutations in this gene will transform one segment into another type of segment. Similar to the Drosophila homeobox are the Hox genes in vertebrates that decide the diversity in the branchial arches. The 39 Hox genes in the human genome are arranged in four clusters (instead of one in the fly) on four different chromosomes: HOX A–D in man ( Fig. 16.3 ). HOX genes, particularly HOX2 , control the patterning in the hindbrain segment from the central nervous system to the branchial arches.

Diagram representing the migration of neural crest cells from rhombomeres to branchial arches and the expression of Hox code (39 Hox genes; HoxA-D in man).

Adapted from Cobourne MT. Construction for the modern head: current concepts in craniofacial development. J Orthod. 2000 Dec;27(4):307–14. doi: 10.1093/ortho/27.4.307. PMID: 11099568.

The migration of the cranial neural crest and the patterns of HOX2 expression in the branchial region are illustrated in Fig. 16.3 . In the first branchial arch, neural crest cells migrate from rhombomeres 1 and 2, and in the second and third arches, they migrate from rhom bomeres 4 and 6, respectively. Rhombomeres (2, 4 and 6) also contain the exit points for cranial nerves V, VII and IX, which will innervate branchial arches 1, 2 and 3. Thus, an axial-level specific code exists that is highly conservative, which means it is transferred and preserved in species and decides which part of the face will develop, such as the eye, ear and other parts of the face.

Other homeobox-containing genes, such as MSX1, MSX2, DLX1–6 and BARX1 , encode homeodomain-containing transcription factors, are also expressed in the face and jaw. The skull bones comprise the neurocranium, which surrounds the brain and sense organs, and the viscerocranium, which forms the bones of the facial skeleton. Skeletal tissue originates from the neural crest from the cranial portion of the neural plate. Evidence has shown that the same family of genes is responsible for patterning in vertebrates and arthropods. So, the same family of genes present in a single copy in Amphioxus undergoes several rounds of duplication to evolve in the human brain. Essentially, there is a significant overlap between the genes responsible for developing limbs and those involved in craniofacial development. Therefore, when clinicians suspect a craniofacial syndrome in a patient, examining the limbs is crucial because abnormalities or variations in limb development can often indicate underlying craniofacial issues. This integrated approach enhances diagnostic accuracy in clinical practice. Mutations in various human genes, such as FGF receptor gene, TWIST, sonic hedgehog ( SHH ) and ALX4, have been linked to malformations of the limbs and/or the skull.

The Msx homeobox gene family represents a pivotal group of proteins that operate to repress transcription in the intricate orchestration of craniofacial development. The MSX family consists of three members: MSX1, MSX2 and MSX3. Among these three, MSX1 and MSX2 exhibit robust expression within the developing craniofacial region. The Msx genes interact directly with a key protein called the TATA-binding protein (TBP) to execute transcription repression. Both MSX1 and MSX2 are detected in the forming skull and meninges, in the suture mesenchyme and dura mater, the distal aspects of the facial primordia, the associated sense organs and teeth. MSX1 expression, unlike MSX2, continues in the postnatal stages of skull morphogenesis and the palatal mesenchyme of the anterior palate.

In early embryonic development, MSX2 is expressed in the maxilla, mandible, Meckel’s cartilage and tooth germs. The expression of the MSX gene is regulated by growth factor regulation, antisense ‘quenching’, retinoids and complementary/antagonistic interactions with other transcription factors. The development of the face through MSX expression is regulated at multiple levels of transcription, translation and protein function. FGF4, BMP4 and bi-functional zinc finger protein YY1 additions have been found to influence sutural mesenchyme induction and closure. ^,

During the development of the face, MSX1 and BARX1 show complementary expression, and Msx and Dlx show overlapping expression, which specifies patterning events. Mutations in the genes that control independent head organisers during early mammalian development lead to midline malformations and cranial truncation. Some of the key genes for this function are LHX1 (LIM homeobox protein 1), HESX1 (homeobox gene expressed in embryonic stem cells), OTX2 (orthodenticle homologue 2), PCSK6 (proprotein convertase subtilisin/kexin type 6), SIL (Tal1 interrupting locus) and SHH (sonic hedgehog). Exencephaly is a rare neural tube deformity characterised by projecting brain tissue and the absence of a calvarium. It occurs due to mutations in Tcfap2α (transcription factors AP2α), PAX3 (paired box gene 3), TWIST , GLI3 (GLI-Kruppel family member 3) and CART1 (cartilage homeoprotein 1). Sensory organ defects are caused by mutations in BMP7 (bone morphogenetic protein 7), CHX10 (homeobox protein CHX10), CHRD (chordin), PAX2, PAX6 and RAX (retina and anterior neural fold homeobox). ^, They are controlled by DLX (distal-less), endothelin pathway, GSC (goosecoid), MSX (Msh-like), PRRX (paired-related) and PAX3 homeodomain families. Abnormalities in this control can lead to neural crest defects. RUNX2 (runt-related transcription factor 2) regulates osteoblast differentiation and essential parts of the extracellular matrix, such as COL11A1 (collagens), COL2A1, CRTL1 (cartilage link protein 1) and HSPG2 (perlecan). Its mutations can lead to abnormal skeletal discrepancy. JAG2 (Jagged 2), LHX8 and TGFβ3 (transforming growth factor-β3) knockout mice develop isolated clefts of the secondary palate, thus pointing to their role in palatal shelf elevation or fusion. ^,

The hedgehog gene secretes the sonic hedgehog ( SHH ), which determines the midline facial structures. Thirty-seven per cent of structural anomalies in familial cases are intragenic mutations of SHH . ^, Apart from these, three more genes, sine oculis homeobox 3 ( SIX3 ), TG interacting factor ( TGIF ) and zinc-finger protein of the cerebellum 2 ( ZIC2 ), may also lead to holoprosencephaly.

Branchio-oto renal (BOR) and Treacher Collins syndromes develop due to single gene disorders and can have major facial abnormalities and problems such as deafness, abnormal external ears and branchial fistulas. These syndromes happen due to heterozygous mutations of EYA1 and Treacher Collins–Franceschetti syndrome 1 ( TCOF1 ), respectively.

Two genes that have been associated with syndromic clefts are MSX1 and PVRL1 . ^, Syndromic CL/P has also been associated with alterations in MID1 (encoding midline 1 ring finger) and TP63 (tumour protein p63). ^, Mutations in HOXA1 and HOXA2 are associated with isolated clefts of the palate. TGFβ 3 controls the fusion of the midline epithelial seam and thus controls the secondary palate development. ^, Dominant mutations in one of the three FGF receptor genes ( FGFR1 , FGFR2 and FGFR3 ) or the transcription factor TWIST account for 20% of cases of craniosynostosis leading to higher intracranial pressure, problems associated with breathing, eyesight, hearing and facial deformity. ^{, ,}

The genes mentioned above play a dominant role in skeletal development. The genes responsible for tooth development play a role in two stages. The first is initiation, followed by development from the bud stage to the cap stage. Problems in any of the two stages would lead to either agenesis/supernumerary tooth formation or malformed tooth. Before the bud stage, the potential to initiate tooth formation lies in the epithelium, which is later shifted to mesenchyme. Many protein signalling molecules and growth factors regulate this epithelium mesenchymal interaction. The first step in tooth development is the prespecification of neural crest cells.

Tooth development and patterning happen through the first branchial arches, controlled by homeobox genes, whereas HOX genes are not expressed in the first branchial arch. Three essential genes ( MSX , DLX and BARX1 form the homeobox code) regulate the odontogenic potential of different regions of the jaw.

Before odontogenesis, both MSX1 and MSX2 exhibit horseshoe-shaped mesenchymal expression. MSX1 expression extends further to the sites of future epithelium thickening. With tooth development, the expression of MSX1 and MSX2 becomes localised in the mesenchymal cells of the dental follicle and dental papilla. MSX2 is also expressed strongly in the enamel organ. DLX1 and DLX2 expression is restricted to the area of development of future molars, and BARX1 is only expressed in the mesenchymal tissues of molars. Hence, each region expresses a particular gene set, specifying which teeth to develop in that area. Targeted mutations of MSX1 in mice led to failed incisor development, and molar development ceased in the late bud stage. Similarly, targeted mutations in either DLX1 or DLX2 resulted in defects in skeletal components derived from first branchial arches, and mutations in both DLX1 and DLX2 resulted in missing molars.

The major signalling pathways that control tooth development are BMP, FGF, SHH and WNT. Bone morphogenetic proteins, namely BMP2 and BMP4, are a group of proteins that regulate the MSX1 and MSX2 expression. The early tooth development from the bud to cap phase is controlled by Msx1, which regulates the Wnt signalling pathway by suppressing the expression of DKK2 and SFRP2. SOSTDC1, also known as Wise, a Wnt antagonist, regulates tooth formation. Hence, its deletion would lead to forced activation of the Wnt pathway, leading to supernumerary teeth formation. Around 46% of tooth agenesis occurs due to MSX1, WNT10A or AXIN2 mutations.

FGF is another group of proteins that regulate the signalling. FGF8 and FGF9 are responsible for tooth initiation, and FGF4 and FGF9 are responsible for coronal morphology. Sonic Hedgehog (SHH) is vital in tooth initiation and development and determining cuspal morphology. Downregulation of BMP4 and FGF3 leads to the arrest of early molar development. Msx1 and PAX9 interact to initiate the expression of FGF3 and FGF10 in early incisor development. These exciting leaps in understanding the molecular basis of tooth development through fundamental research will help to discover therapies to improve oral health through regenerative approaches.

Class II malocclusion is a complex trait resulting from the interplay of transcription and growth factors that act on bone, teeth and skeletal muscles, as well as environmental influences, including nutrition. Clinically, it appears as mandibular deficiency, maxillary excess or a combination of both. Five distinctive types of class II phenotypes have been identified based on variations in mandibular rotation, maxillary incisor angulation, and mandibular length.

Evidence suggests that genetics and environmental factors interact to regulate skeletal class II malocclusions in individuals who exhibit variable craniofacial morphology.

NOGGIN gene, which is essential for late events in mandibular development, and the SNAIL gene, which is vital in epithelial-to-mesenchymal transitions (EMT) and contribute to the formation of the mesoderm and the neural crest, are associated with mandibular hypoplasia. In addition, the following genes positively correlate with skeletal class II malocclusion. These are FGFR2 , MSX1 , MATN1 , MYOH1 , ACTN3 , GHR , KAT6B , HDAC4 and AJUBA .

Abnormal hard tissue and soft tissue abnormalities influenced by genetics define class II malocclusion. Phenotype–genotype correlations of facial width and height proportions (facial index) in patients with class II malocclusion are correlated with SNPs rs7924176 (ADK), rs17101923 (HMGA2) and rs997154 (AJUBA). Morphology and masseter muscle function are significant factors in determining the craniofacial form; analysing the type of fibres, expression of genes and relation to malocclusion’s phenotype provides crucial information. The myosin heavy chain (MHC) protein is the primary protein responsible for the contraction velocity of muscle fibres. Additionally, cytoskeletal muscle proteins such as α-actinin-2 and α-actinin-3 are also involved. While α-actinin-2 appears in all types of muscle fibres, α-actinin-3 is limited to fast-contracting muscle fibres of type II, which enhance muscle strength.

Class II malocclusion involves polymorphisms of the ACTN2 and ACTN3 genes. R577X is a common gene polymorphism in ACTN3 that deletes α-actinin-3. This leads to the formation of smaller-diameter type II fibres and an increase in ENPP1 expression, which is a negative regulator of mineralisation. An increase in type II fibres is associated with a decrease in vertical dimension frequently found in class II malocclusions.

Class II division 2 has a stronger genetic aetiology than division 1. The mode of inheritance of Class II division 2 is autosomal dominant, with incomplete penetrance and variable expressivity documented through twin and triplet studies. It could be explained that there is a simultaneous and additive expression of several traits controlled by multiple genes, not a single gene determining the occlusal features.

Class II division 2 is often associated with dental anomalies of peg laterals and missing teeth, which may suggest a shared genetic aetiology between dental anomalies and maxillomandibular skeletal size discrepancy. The CYP19A1 gene, located at chromosome 15q21.2, encodes aromatase that regulates the conversion of androgen to oestrogen. Oestrogen is associated with bone modelling, and hence, this gene has been suggested as the determinant for male sagittal jaw growth, which could be responsible for a deficiency of up to 2.5 mm/year in these class II patients during adolescence.

Skeletal class III malocclusion, especially mandibular prognathism, runs in families, though it might vary in the severity with which it affects the individual member (variable expressivity). Phenotype mandibular prognathism may sometimes not affect a generation due to incomplete penetrance.

• Mode of inheritance : Class III patterns of mandibular prognathism have an autosomal dominant mode of inheritance, variable expressivity and incomplete penetrance. Class III malocclusion is largely polygenic in inheritance. Exceptionally monogenic inheritance with a single mutated gene, which follows the Mendelian inheritance pattern, is known in the noble European family, showing a class III skeletal pattern. ^, Environmental factors might influence the penetration of the trait and thus affect the severity of malocclusion.

• Genes involved : Genome-wide linkage scans identify the genetic marker inherited by all family members affected by the trait but not by any unaffected family members. With these linkage analyses, chromosomal loci 1p36, 12q23 and 12q13 have been identified to possess genes that make one predisposed to class III malocclusion.

  • In case-control studies, the role of candidate gene erythrocyte membrane protein band 4.1 (EPB4.1) is reported in mandibular prognathism. In familial studies, genetic mutations within four genes have been identified in five families. ARHGAP21 (chromosome 10), FGF23 (chromosome 12), DUSP6 (chromosome 12) and ADAMTS1 have been identified in linkage and association studies of class III malocclusion.

• Genes and condyle : Genes that encode growth factors that influence the condylar cartilage under mechanical strain and thus influence the growth of the mandible are the Indian hedgehog homologue (IHH), parathyroid hormone–like hormone (PTHLH), insulin-like growth factor-1 (IGF-1) and vascular endothelial growth factor (VEGF). Thus, they play an essential role in the aetiology of class III malocclusion. As in the vertical malocclusions, the masseter muscle fibres and proteins are somewhat under genetic control in class III malocclusion. Type IIA and IIX MHC proteins are expressed in the masseter muscles of patients with mandibular prognathism.

• Genes and growth hormone : Growth hormone influences the size and angulation of craniofacial structures. Growth hormone receptors are present in the mandibular condyle. GHR gene mutations or deficiency might lead to short mandibular ramal height, and hence, GHR is considered a possible genetic marker for mandibular ramus height.

Genetic factors significantly influence the configuration and dimensions of facial features. A graphical representation in Fig. 16.4 succinctly presents the genes correlated with facial components’ phenotype and dimensional attributes. Great work done by Richmond is further adapted for the simplicity of graphic representation.

Gene association with regionalised facial features in normal populations.

Source: Adapted from Richmond S, Howe LJ, Lewis S, Stergiakouli E, Zhurov A. Facial genetics: a brief overview. Front Genet. 2018 Oct 16;9:462.

Some intra-arch traits, mainly the maxillary arch width, length, shape and crowding of the mandibular anterior teeth, are found to be under the influence of heritability. However, interact traits such as overjet, posterior crossbite and sagittal molar relations are not genetically determined. Ectodysplasin A (EDA) and X-linked ectodermal dysplasia receptor (XEDAR) genes decide the trait of dental crowding among the Hong Kong Chinese population. However, this might not be true for other ethnic groups.

The stability of dental patterning is attributed mainly to HOX7 and HOX8 (presently MSX1 and MSX2 ) genes. Supernumerary teeth in the premaxillary region show autosomal dominant inheritance with incomplete penetrance with possible sex-linked inheritance. Familial tooth agenesis, except for third molars, can result from a single dominant gene defect or recessive or X-linked. Transcription factor (PAX9) and growth factor-related genes play a vital role in tooth agenesis. Mutations in MSX1 and BMP4, axis inhibitor 2 (AXIN2) and PAX9 are associated with hypodontia or oligodontia. Non-syndromic primary eruption failure is mainly related to parathyroid hormone receptor 1 (PTH1R), which regulates the progenitor cells and the development of alveolar bone and periodontal ligament at eruption.

The pathways influencing orthodontic tooth movement (OTM) and external apical root resorption (EARR) are the ATP/P2X7R/IL-1β inflammatory signalling pathway and the RANKL/RANK/OPG bone modelling and remodelling pathway. A balance between IL-1β and IL-1RA synthesis for the bone modelling and remodelling processes is involved in OTM.

At diagnosis, the clinician should elucidate the relative contribution of genes and environment through a detailed history and evaluation of patients and siblings. The greater the genetic component, the poorer the chance of success of orthodontic treatment or the higher the chance of relapse after correction in the growing phase.

Genetic factors govern the growth of the craniofacial skeleton, with little evidence to support the contribution of orthopaedic appliances in influencing the maxillary or mandibular growth in the long term. The TMJ in the mandible, maxillary suture in the maxilla, periodontal ligament and alveolar process of teeth are the structures that majorly react to this mechanical loading. Maxillary sutures can cause bone deposition and resorption in response to mechanical loading of orthodontic or orthopaedic forces. Interleukin-1, RANKL, RANK and osteoprotegerin (OPG) systems control bone and craniofacial development. Under tensile mechanical stress, extracellular matrix proteins such as osteopontin (OPN) regulate the osteocyte function and bone formation. The control of bone morphology is by a sequence of three genetic mechanisms: (1) growth and ischaemic factors, (2) vascular induction and invasion and (3) mechanically induced inflammation. Two physical influences of the genetic mechanism are the limitation of diffusion to maintain viable osteocytes and the history of mechanical loading.

Identifying the genes involved in late mandibular growth would help predict which cases would relapse after class III correction. This would also help determine if early camouflage treatment would be stable or if one should wait for correction with orthognathic surgery later.

‘Epigenetic’ is a term that defines a heritable change in phenotype without involving the underlying DNA sequence change. In other words, they are the factors that control the gene activity, from condensation of chromosomes to DNA replication, repair, transcription to protein synthesis. These factors decide when, where and which gene to express. DNA methylation, microRNA-based mechanisms (non-coding RNA) and histone modifications are the three main mechanisms that contribute to epigenetics ( Fig. 16.5 ). Among these, histone modification, which comprises histone acetylation and histone deacetylation, epigenetically controls the chromatin. The histone code regulates the condensation of chromosomes, DNA replication, repair and transcription. The external environment (orthopaedic forces) can directly or indirectly change the expression of genes and, thus, function.

Mechanisms of epigenetic regulation:

Epigenetic factors are those factors that regulate the gene activity. The main mechanisms by which epigenetic factors regulate (activate and supress) gene activity are shown in the figure.

• 1.

  DNA methylation occurs by addition of methyl group to the cytosine nucleotide in cytosine-guanine sequences (CpGs). These methylated CpGs inhibit DNA transcriptase factor and prevent activation of transcription.

• 2.

  Histones are proteins that make up the nucleosome (basic repeating subunits of chromatin, which consists of DNA wrapped around a core of histone protein). These histone proteins have tails that can be modified in the post translation phase through methylation, acetylation, deacetylation, phosphorylation, ubiquitination and sumoylation. This modification affects spatiotemporal activation or suppression of neural crest gene regulatory networks. This can affect neural crest proliferation, specification, migration and differentiation.

• 3.

  Non-coding RNAs are functional RNA molecules that are transcribed but not translated into protein associated with gene silencing. Non-coding RNAs participate in DNA methylation, histone modification, formation of heterochromatin and gene silencing.

• 4.

  Large protein complexes, such as polycomb repressive complex or ATP-dependent chromatin remodelling complex, affect the structure and position of chromatin. EZH2 is a protein component of polycomb repressor complex (PRC2) and represses the HOX gene family and activates the bone formation.

Source: Adapted from Singh A, Gill G, Jakhu H. Epigenetics and its implications for oral health. J Oral Biosci. 2018 Jun 1;60(2):41–8; and Shull LC, Artinger KB. Epigenetic regulation of craniofacial development and disease. Birth Defects Res. 2024 Jan;116(1):e2271.

The epigenetic control in class II malocclusion occurs via histone modification. This process involves Myosin Heavy Chain (MyHC) genes, which control the type of masseter muscle fibres. In class II malocclusions, a deep bite is usually found, which is due to an increase in type II muscle fibres, epigenetically controlled by histone modification.

Gene expressions for fast isoforms of myosin and contractile regulatory proteins and KAT6B and HDAC4 were severalfold greater in masseter muscles from a patient with a deep bite than compared to one with an open bite. Fig. 16.5 illustrates the three main mechanisms of epigenetic regulation and the epigenetic control of craniofacial development.

Prenatal genetic testing is an option to screen the population for disease carriers and at-risk pregnancies or as a diagnostic test to confirm a suspected disease. This screening test helps in the early identification of fatal diseases, which may warrant timely termination of pregnancy, thus reducing the burden of genetic diseases in the population.

Diseases like beta-thalassaemia, Down syndrome and neural tube defects require population-based screening programmes. Earlier invasive techniques like amniocentesis or chorionic villus sampling were used for screening. Genetic testing from cell-free foetal DNA (cffDNA) in maternal plasma is a non-invasive method of prenatal testing (NIPT). However, positive NIPT findings must be confirmed by an invasive method such as amniocentesis, although this confirmation is not required for monogenic disorders.

Apart from blood, saliva, which carries DNA sufficient for genetic profiling, is being increasingly explored. Direct-to-consumer kits are available to get a personalised report from cancer risk to earwax type.

The laws for genetic screening vary by country. The American College of Medical Genetics and Genomics mandates screening a panel of 57 genes that may be associated with medical conditions such as cancer, hypercholesterolaemia and cardiac disorders, regardless of the disease being tested for. Any mutations in these genes should be informed to the clinician for further action. However, the UK Association of Genetics Nurses and Counsellors does not recommend opportunistic testing of children for adult-onset conditions.

A dentist can be the first healthcare worker to diagnose some genetic conditions like hypophosphatasia (HPP), X-linked hypophosphatemic (XLH) rickets, tooth agenesis and ectodermal dysplasia (ED).

From an orthodontic perspective, the heritability of skeletal characteristics such as class III malocclusion and long face patterns is very high and runs in families. Cleft lip and palate, skeletal deep bite, class II division 2 malocclusions and facial morphology associated with obstructive sleep apnoea also have a genetic component.

Among the dental traits, supernumerary teeth, hypodontia, agenesis, primary failure of eruption, crowding, especially of the mandibular arch, spacing, midline diastema with a thick frenal attachment, bilateral canine impactions with mislocated root apices, and peg or missing lateral incisors seem to be under genetic control ( Table 16.1 ). If a trait is gentically controlled it will be expressed bilaterally, that is both on the left and right side of the jaw. That is why bilateral canine impactions, that too with mislocated root apices, are considered exclusively hereditary. Since knowledge of genetic diseases is essential for every dentist, some countries have proposed inclusion in the undergraduate dental curriculum.

Some genetic diseases can be cured by introducing genes into the cell. Once introduced, the nucleotides reinstate the protein’s normal function.

At this point, gene therapy for use in orthodontics is limited to in vitro experiments. Gene therapy has been experimented with OTM, preventing craniosynostosis, influencing the growth of the mandible and alleviating orthodontic pain. ^,

Two main pathways that regulate OTM and apical root resorptions are the interleukin-1β signalling pathway and the RANKL/RANK/OPG pathway. OPG binding at RANKL, causing RANK inhibition, interrupts the synthesis of osteoclasts and, hence, bone resorption. In vivo experiments have shown that OPG gene transfer leads to a slower rate of tooth movement, whereas RANKL gene transfer leads to accelerated tooth movement.

Gene therapy has also been experimented to alleviate the pain related to orthodontic treatment. Gene therapy with an antagonist for the transient receptor potential vanilloid 1 (TRPV1) gene, responsible for both inflammatory and neuropathic pain, has shown some promise in this context.

Bone morphogenetic proteins (BMP2, 4 and 7) have been used to induce new bone formation.

Craniosynostosis, which can result in facial deformity, is the early fusion of single or multiple cranial sutures. Syndromic craniosynostosis often displays mid-facial hypoplasia, anterior mandibular rotation, increased ramal length, decreased corpus length and sometimes, a class III malocclusion. Glypican (GPC 1 and GPC3) specifically inhibit the genes for BMP2, BMP4 and BMP7. The downregulation of GPC1 and GPC3 found in the cranial suture synostosis is hypothesised to cause early fusion. Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (PIN1) controls the transcriptional activity of the RUNX2 gene and regulates bone development. Hence, inhibition of PIN1 can prevent early fusion of coronal suture and may turn into a non-surgical alternative for the treatment of craniosynostosis.

Recombinant adeno-associated virus (rAAV)-mediated VEGF gene transfer, which helps in neovascularisation, has been found to have the potential to increase the size of the mandibular condyle and promote mandibular condylar development. Besides this, important tissue growth regulators, such as the basic fibroblast growth factor (bFGF) in synergy with low-intensity pulsed ultrasound (LIPUS), promote mandibular condylar growth.