GENETICS

Dr. mohammed aslam vDepartment of orthodontics

CONTENTS Introduction. History Genetic principles and terminologies Human genetics. Mutations Modes of inheritance. Role of homeobox genes Heritability of malocclusion Genetic influence on external root resorption. Genetic influence on tooth number, size, morphology, position and

eruption.

CONTENTS Genetics and dental arch form Does knowing the heritability matter in treatment? Human Genome project and beyond. Conclusions. References.

Several questions need to be answered before a complete understanding can be gained about how genetic factor influence a feature or disorder this include,

How important are genetic factors on human differences?

What kind of action and interaction occur between gene products in the pathways between genotype and phenotype?

Are the genetic effects on a trait consistent across sexes?

Are there some genes that have particularly outstanding effect when compared to others?

INTRODUCTION The relative contribution of genes and the environment to the

etiology of malocclusion has been a matter of controversy throughout the 20th century.

Genetic mechanisms are clearly predominant during craniofacial morphogenesis but environment is also thought to influence dentofacial morphology postnatally

The key to the determination of the etiology of malocclusion lies in the ability to differentiate the effect of genes on the craniofacial skeleton in a particular individual.

Orthodontists may be interested in genetics to help understand why a patient has a particular malocclusion and if the problem is genetic they may be limited in what they can do.

HISTORY Early Greek philosophers – Aristotle & Hippocrates

1750’s – “ spontaneous generation”

Linnaeus (1707-1778) – the founder of systematics – “ fixity of species”

Pasteur (1822-1895) & Tyndall (1820-1893) – “continuity of life”

“Every living thing from a living thing” (Omne vivum e vivo)

Preformationism Graaf, Maupertuis – 1700s

hereditary particles one from each parent Lamarck (1744-1829) proposed that characters acquired by

individuals of one generation are transmitted to those in the next generation

Pangenesis and Inheritance of Acquired Characters – Charles Darwin (1809-1882) – “Gemmules”

TERMINOLOGYGenetics – The branch of biology which deals

primarily with the principles of heredity & variation & secondarily with the role of environmental factors as they interact with genes in the development of an individual.

Genes – Danish botanist Johannsen – Units of genetic information. About 100,000

Genotype – of an individual is his genetic constitution

Phenotype – is the expression of the genotype as a morphological, biochemical or physiological trait

Genome – the full DNA content of the chromosome set

Karyotype – Characteristic chromosome constitution of a species e.g. 44XY

Linked – Genes on the same chromosome

Homozygous – An individual who has the same factors for a particular characteristic. (Eg. – TT or yy)

Heterozygous – Individual with different factors (Tt or Yy) character that is manifested – dominant and the other – recessive

Allele - The genes responsible for contrasting (alternative) characters are called alleles

Locus – Fixed position in a chromosome occupied by a gene

Linkage – The tendency of genes to stay together during inheritance

Transcription – Production of RNA molecule complementary to a DNA molecule which serves as the template for RNA polymerase, the enzyme that catalyzes transcription

Translation – Synthesis of a polypeptide in association with a ribosome and directed by an mRNA molecule with the help of tRNA and several highly specific enzymes

He made his discoveries by analysis of results after crossing varieties of Garden pea (Pisum sativum)

round or wrinkled seeds tall or dwarf plant yellow or white flowers

He crossed varieties differing only in one pair of these

characteristics

MENDELISM – GREGOR JOHANN MENDEL

TT tt

Tt

TT Tt Tt tt

All dominant Same as F2 All recessive

F1

F2

F3

Mendel’s 1st law / The Law of Segregation 2 factors for a specific character parent transmits only one to

offspring matter of chance as to which unite

Law of Independent assortment Members of different gene pairs assort to the gametes (sex

cells) independently of one another

Law of Unit Inheritance Characteristics of one parent may not appear in one

generation (F1) but may reappear in the next generation (F2)

HUMAN GENETICS

The earliest diseases to be studied were albinism. Polydactyly in early 1700s

The effects of Nature and Nurture was studied by GALTON and he coined the term eugenics in 1800s

1900s Sir Garrod

Alkaptonuria – dark urine

Children were usually normal, but the disorder could reappear later in the descendents.

Mendelian recessive type of inheritance

CHROMOSOMES

During the cell division thechromatin network in the nucleus becomes condensed to form the chromosomes

These are thread like structures.

Each species has a specific number of chromosomes

Each chromosome is composed of two parallel and identical filaments chromatids held together at a narrowed region, the primary constriction, within which is a pale staining region, the centromere. It has 2 arms, P arm (short arm) and the Q arm (long arm)

Classified on the basis of centromere position1. Telocentric2. Acrocentric3. Submetacentric4. Metacentric

CHROMOSOME STRUCTURE

20

21

22

Cell Division23

CELL DIVISIONMITOSIS

MEIOSIS

MITOSIS24

Mitosis is a process of cell division which results in

the production of two daughter cells from a single

parent cell

25

MEIOSIS26

Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced.

Reduction division

27

Reorganization of genes among the chromosomes hence increases genetic variability.

Chiasmata sites of cross over2 chromatids take part in any crossover. But all 4 chromatids of the bivalent may be simultaneously involved in crossovers at different sites.

CROSSING OVER

MUTATIONS

A mutation is defined as an alteration or change in genetic material.

Mutations are due to Mutagenic factors – radiation,chemicals etc.

Spontaneous errors in DNA replication & repair.

Mutations of the somatic cells cannot be transmitted, if it occurs in gamete cells it can be transmitted.

Mutations can be further divided into - length mutations & point mutations. In a point mutation a single nucleotide base is replaced

by a different nucleotide base. Transition – purine to purine pyrimidine to pyrimidine

Transversion – purine to pyrimidine or vice versa

Synonymous – Mutations that do not alter the polypeptide product of gene, also called silent.

Non synonymous – Mutations that lead to alterations in the encoded polypeptide.

Missense – A simple base pair substitution that can result in coding for a different amino acid and synthesis of an altered protein.

Nonsense – A substitution that leads to the generation of stop codon which will result in premature termination of translation of polypeptide chain.

Deletion is the loss of 1 or more nucleotides

Insertion is the addition of 1 or more nucleotides

Results in FRAME SHIFT mutation if these are not in a multiple of 3

The majority of mutations are likely to cause reduced fitness ,a reduced ability of the resulting zygote to contribute progeny to next generation, in this way harmful genes tend be eliminated from the population

A balance between the production of disadvantageous alleles through mutation & their elimination by selection results in the presence of harmful alleles in the population at a low frequency

MODES OF INHERITANCE Population genetics deals with the study of

mode of inheritance of traits and distribution of genes in populations.

A trait is a particular aspect or characteristic of the phenotype.

A trait can be- Monogenic Polygenic / Multifactorial

All human beings normally have 22 homologous pairs of chromosomes called autosomes that are numbered by size and other characteristics.

Also, one pair of sex chromosomes may be homologous (X, X) in females or only partly homologous (X, Y) in males.

Genes at the same locus on a pair of homologous chromosomes are alleles.

Exception to this – gametes contain only single representative of each pair of chromosomes.

When 2 gametes join at fertilization- new individual with paired genes, one from father & other from mother is formed.

MONOGENIC INHERITANCE

Traits that develop because of the influence of a single gene locus are monogenic.

Also called as mendelian traits.

E.g.-blood group/hemophilia

Also called as – Discrete or Qualitative (yes or no)

If they are present, these traits still may be variable and quantifiable in some cases.

Autosomal dominant traits and penetrance

If having only one particular allele of the two alleles on a homologous pair of autosomes (heterozygosity) is sufficient to lead to the production of the trait, the effect is autosomal dominant.

If production of the trait does not occur with only one particular allele of the two alleles on an autosome but does occur when both alleles are the same (homozygosity), then the effect is autosomal recessive.

penetrance and expressivity

Penetrance is a statistical term and indicates the proportions of individuals carrying certain gene who can be detected. As our ability to detect the expression of gene improves, the penetrance increases.

Expressivity refers to the degree of expression of a gene in an individual.

The presence of one or two of these findings comprises partial expressivity while the absence of all four ocassionally found in carriers of this gene is zero expressivity.

For example, osteogenesis imperfecta involving type I collagen

abnormalities. (1) multiple fractures (2) blue sclera (3) dentinogenesis imperfecta (4) hearing loss.

Variation occurs among the different clinical types of osteogenesis imperfecta

AUTOSOMAL RECESSIVE TRAITS

If production of the trait does not occur with only one particular allele of the two alleles on an autosome but does occur when both alleles are the same (homozygosity), then the effect is autosomal recessive.

The concept of a gene carrier is used with autosomal recessive traits.

The carrier is heterozygous for a recessive gene that has only subtle, if any, expression of that single gene.

Parents of a child with the autosomal recessive trait are typically heterozygous (carriers) and most often are diagnosed as normal.

In autosomal recessive traits, the following three gene pairs are found –

AA – homozygous, not showing the trait or being a carrier for the trait.

Aa – heterozygous, not showing the trait but being a carrier of the trait

aa – homozygous, showing the trait.

X LINKED TRAITS ANY LYONISATION

Most of the genes on the X and Y chromosomes are not homologous and are unequally distributed to males and females.

Males are hemizygous for X-linked genes, meaning that they have only half (or one each) of the X-linked genes.

Because females have two X chromosomes, they may be homozygous or heterozygous for X-linked genes, just as with autosomal genes.

A normally functioning homologous allele is not present on another chromosome, recessive genes on the one male X chromosome express themselves phenotypically as if they were dominant genes.

However, X-linked recessive genes must be present at the same (homologous) locus in females to express themselves fully.

THE LYON HYPOTHESIS

In the female all or most of one X chromosome is genetically inactive and forms Barr body.

Decision whether maternally derived Xm or paternally derived Xp is inactive is made early in embryonic life and is random for each cell.

All cells subsequently have the same inactive X chromosome-Xm or Xp.

MULTIFACTORIAL INHERITANCE

There are phenotypes that "run in families" but do not adhere to patterns of Mendelian inheritance.

These are referred to as complex or common diseases.

They have greater incidence compared with monogenic phenotypes.

These are the traits influenced by polygenic factors.

Multifactorial inheritance – Trait is determined by the interaction of a number of genes at different loci, each with a small but additive effect, together with environmental factors.

Many congenital malformations are inherited as multifactorial traits & categorized as – Continuous or discontinuous.

Multifactorial = Polygenic + Environmental

A change in phenotype depends on the result of the genetic and environmental factors present at a given time.

Thus compared with monogenic traits, polygenic traits are more amenable to change following environmental (treatment) modification.

Cleft lip and palate is a congenital malformation inherited as a multifactorial trait.

HOMEOBOX GENES Edward Lewis was the first person to identify the homeotic

genes in the fly, which help in controlling the developmental response of groups of cells along the body’s antero-posterior axis.

Proteins encoded by these homeobox genes are transcription factors which control transcription of RNA from the DNA template within cell nucleus.

Transcription factors can switch genes on and off by activating or repressing gene expression.

Four homeobox gene clusters (HOXA, HOXB, HOXC, and HOXD) that comprise a total of 39 genes have been identified in humans.

Each cluster contains a series of closely linked genes. In each HOX cluster there is a direct linear correlation between the position of the gene and its temporal and spatial expression.

These observations indicate that these genes play a crucial role in early morphogenesis.

Lower number HOX genes are expressed earlier in development and more anteriorly and proximally than are the higher number genes.

Homeobox gene clusters in humans

HOX Cluster

Number of genes

Chromosome location

HOXA(=HOX1)

11 (1-7, 9-11, 13) 7p

HOXB(=HOX2) 10 (1-9, 13) 17q

HOXC(=HOX3) 9 (4-6, 8-13) 12q

HOXD(=HOX4)

9 (1, 3, 4, 8-13) 2q

The sub-families of the HOX genes which are of

particular interest in craniofacial patterning and

morphogenesis, include –

      Muscle segment (Msx)     Distal-less (Dlx)       Goosecoid (Gsc)       Otx gene (Orthodontical)       Shh gene (Sonic hedgehog)      

Some of the important regulatory molecules in the mesenchyme, through which homeobox genes information is expressed at the cellular level are –

  Fibroblast Growth Factor (FGF)   Epidermal Growth Factor (EGF),   Transforming Growth Factors (TGFα, TGFβ)   Bone Morphogenetic Proteins (BMPs) .

TWIN STUDIES Twin studies are useful in study of population genetics. To determine the role of genetic and environmental factors.(nature vs nurture) Twins can be dizygotic or monozygotic

Various methods have been used to differentiate –

Hair & eye color Ear form Teeth morphology Phenylthiocarbamide taste sensitivity Blood groups Serum proteins (gamma globulins) Dermatoglyphics

CONCORDANCE AND DISCORDANCE

Twins are concordant if they both show a discontinuous trait and discordant if only one shows the trait.

As twins usually share a similar family environment it may be difficult to separate the relative extent of environmental (nurture) and genetic contributions (nature) to a multifactorial trait

Monozygotic twins have similar genotypes

Dizygotic twins are like siblings.

If a condition has no genetic component concordance would be expected to be similar for both types of twins.

For a single-gene trait or a chromosomal disorder the monozygotic concordance rate will be 100%, whereas the dizygotic rate will be less than this and equal to the rate in siblings.

For discontinuous multifactorial traits with both genetic and environmental contributions, the rate in monozygotic twins, although less than 100%, will exceed the rate in dizygotic twins

In cleft studies, the monozygotic twin concordance rate for cleft lip and palate and cleft palate is 35 and 26 per cent, respectively. And for dizygotic twins 5 and 6 per cent, respectively. (Connor and Ferguson-Smith. 1993).

This reflects the heritability of the condition,higher the monozygotic concordance more the genetic contribution.

Genetic influence on tooth number, size, morphology,position & eruption

Twin studies have showed that tooth crown dimensions are strongly determined by hereditary.

Genetic variations for mesiodistal and buccolingual crown dimension of the permanent teeth ranged from 56 to 92 pc.

Larger discordance in dizygotic twins than in monozygotic twins provides strong evidence for the existence of genetic control of individual buccolingual and mesiodistal dimensions

The lower first premolar has an extremely wide morphologic variability. Kraus and Furr were able to indicate 17 genetically different independently inherited traits only for this tooth.

Crown dimensions were largely under genetic control. Genetic determination of maxillary and mandibular dentition was independent

of each other. Wide range of genetic factors influenced the mandibular than the maxillary teeth.

Hypodontia is to a great degree, genetically determined, and transmitted by an autosomal dominant inheritance with incomplete penentrance and variable expression.

The Hox7 and Hox8 genes being responsible for the stability in dental patterning are known to play a role.

In addition to this familial tendency, environment also plays a role as suggested by the Evolutionary theory.

As dietary habits in humans adapt from a hunter/gatherer to a refined food culture evolutionary selection pressures are tending to reduce tooth volume, which is manifested in the 3rd molar , 2nd premolar and lateral incisor “fields” ( Butler`s field theory)

Clinical evidence suggest that congenital absence of teeth and reduction in tooth

size are related. ex hypodontia and hypoplasia of the maxillary lateral incisors present simultaneously

Gruneberg suggested that a tooth germ must reach a critical size during a particular stage of development or the structure will regress. It is apparent from all evidence in this respect that tooth size fits the polygenic multifactorial threshold model.

Genetics and dental arch form

Studies by Harris et al have shown that genetic variation has a major effect on arch width and arch length. Arch length is more inheritable than arch breadth.

Also the maxillary arch being more inheritable than the mandibular arch.

About 60 pc of the variation in measurements of arch size and shape is attributable to hereditary.

On the other hand only about 10 pc of the variation in overjet, overbite, crowding and molar relations result from genetic causes

Overjet was most highly heritable with a 3/1 ratio of genetic to nongenetic variance (75 pc heritability).

Arch-width at Pl and Ml, buccal segment relation (sagittal overjet), and overbite were less heritable.

Study by Corrucini and Sharma (EJO 1986) on Punjabi twins revealed significant genetic control for dental arch and palate dimensions, but environmental influences seemed important for occlusal traits.

Thus environmental determination of occlusal variation is roughly twice as important as earlier thought and orthodontic researchers should consider environmental correlates of malocclusion more vigorously

HERETABILITY OF MALOCCLUSION Since there is evidence that orofacial structures are

under genetic control and are significant in craniofacial development they must be considered in the etiology of malocclusion. It is well established that craniofacial abnormalities have a Multifactorial inheritance.

The same is true of malocclusion and best evidence can be seen from twin studies and family studies.

Twin and family studies help in distinguishing the effect of genetics and environment on the development and structure of the dentofacial structure.

Malocclusion is a manifestation of genetic and environmental interaction on the orofacial region.

Malocclusion could be produced by inherited characteristics in two major ways –

Inherited disproportion between the size of teeth and the size of the jaws: causing either crowding or spacing.

Inherited disproportion between the size or the shape of the upper and lower jaws: causing improper occlusal relationships.

CLASS II DIV I MALOCCLUSION

The mandible is significantly more retruded than in Class I patients, with the body of the mandible smaller and overall mandibular length reduced. Higher correlation between the patient and his immediate family than among random pairings of unrelated siblings. This supports the concept of Polygenic Inheritance for Class II division 1 malocclusion

Environmental factors can also contribute to the etiology e.g. lip/tongue contact for an anterior oral seal during swallowing can encourage the lower lip to retrocline the lower incisors and the protruding tongue to procline the upper incisors.

Digit sucking

Lip incompetence

CLASS II DIV II MALOCCLUSION Distinct clinical entity which may be considered as a syndrome.

Unique combination of deep overbite, retroclined incisors, Class II skeletal discrepancy, high lip line with strap-like activity of the lower lip, and active mentalis muscle.

Often accompanied by a poorly developed cingulum on the upper incisors and a characteristic crown root angulation.

Peck et al. (1998) described characteristically smaller than average teeth when measured mesio-distally.

Hilton found these teeth to be significantly thinner in the labiolingual direction.

Tendency to a forwardly rotating mandibular development which in turn, has an influence on the position of the lower lip relative to the upper incisors, and an increase in masticatory muscle forces.

Familial occurrence of Class II division 2 has been documented in several published reports including twin and triplet studies e.g. Korkhaus (1930), Markovic (1992), Peck et al. (1998).

Markovic (1992) carried out a clinical and cephalometric study of 114 Class II division 2

malocclusions (48 twin pairs and six sets of triplets)

Of the monozygotic twin pairs, 100 per cent demonstrated concordance for the Class II division 2 malocclusion, while almost 90 per cent of the dizygotic twin pairs were discordant.

This is strong evidence for genetics as the main etiological factor. Genetic influence is probably

Autosomal dominant with incomplete penetrance and variable expressivity

It could also be explained by a Polygenic model with a simultaneous expression of a number of genetically determined

morphological traits acting additively rather than being the effect of a single controlling gene for the entire occlusal malformation.

Controversy regarding the etiology arises from a failure to appreciate the synergistic effects of genetics and environment

CLASS III MALOCCLUSION A Class III malocclusion may result from deficiency in

maxillary growth, excessive mandibular growth, or a combination of both.

The relative contribution of genetic and environmental factors to class III has been the subject of many studies.

Familial studies of mandibular prognathism are suggestive of hereditary in the etiology of this condition.

oThe most famous example of a genetic trait passing through several generations is the pedigree of the so called Hapsburg jaw. (Hungarian /Austrian dual monarchy)

Strohmayer (1937) concluded from pedigree analysis of the Hapsburg family line that the mandibular prognathism was transmitted as an Autosomal dominant trait

Studies by Suzuki (1961) showed a incidence of 34.3 pc in family members of the index cases as compared to families of individuals with normal occlusion.

Schulze and Weise (1965) reported six times higher concordance rate in monozygotic twins than in dizygotic twins. Both of the above studies report a Polygenic hypothesis as the primary cause for mandibular prognathism.

Various models have been suggested to explain the inheritance of class III malocclusion.

Litton (1970) carried out an analysis of the literature to determine a possible mode of transmission and ruled out autosomal dominant and recessive transmission as equal number of males and females were affected.

Edwards ( 1960) put forward a polygenic multifactorial threshold model

They also made the suggestion that different modes

of transmission might be operating in different families or different populations.

A wide range of environmental factors have also been suggested as contributory to the development of prognathism. e.g. enlarged tonsils, nasal blockage, congenital anatomic defects, hormonal disturbances, endocrine imbalances, posture, trauma/disease, including premature loss of the first permanent molars.

To conclude…

There appears to be a strong familial tendency in the development of class II and III malocclusions.

The hereditary pattern must thus be taken into consideration in the diagnosis and treatment of patients with these classes of malocclusion

GENETIC FACTORS AND EXTERNAL ROOT RESORPTION

The degree and severity of external apical root resorption associated with orthodontic treatment is multifactorial, involving host and environmental factors.

Genetic variation accounts for 50% to 64% of the variation in EARR of the maxillary incisors.

Variation in the interleukin-lb gene (IL-l B) in orthodontically treated individuals accounts for 15% of the variation in EARR.

Persons in the orthodontically treated sample who were homozygous for IL-1B allele "1" were estimated to be 5.6 times more likely to experience EARR of 2 mm or more than those who were heterozygous or homozygous for allele "2“(Hartsfield and Everett AJO 2003)

Practical and clinical implications In clinical orthodontics it must be appreciated

that each malocclusion occupies its own distinctive slot in the genetic /environmental spectrum. The greater the genetic component to the malocclusion, the worse the prognosis for a successful outcome by means of orthodontic intervention.

The difficulty, of course is that it is seldom possible to determine the precise contribution from hereditary and environment in a particular case.

For ex- In case of mouth breathing where the influence of habit and posture is very much dependant on the genetically determined craniofacial morphology on which it is superimposed, and the reason for the habit developing may well be dependant on the morphology in the first place.

This is a classical example of the interaction of genes and environment and ultimately success of treatment will depend on the ability to ascertain the relative contribution of each.

There is also, currently a lack of evidence to show that orthopedic appliances can influence the growth of skeletal bases significantly beyond their innate genetic potential.

Human studies to date tend to support the genetic determination of craniofacial form with a lack of evidence to show any significant long term influence on maxillary and mandibular dental bases using orthopaedic appliances.

Human genome project and beyond

Gregor Mendel ( Father of Genetic Sciences) created a method to investigate hereditary and presented his theories in 1865.

In 2001 the Human genome project was developed and since then technological advances in dentistry have been accomplished.

The HGP has assisted significantly in the investigation of dental anomalies and tooth agenesis.

Application of HGP methodology identified a defective gene in a family with autosomal dominant agenesis of second premolars and third molars on chromosome 4p

The critical locus included MSX1 gene in which a missense mutation in the homeodomain had occurred. The gene had become inactive and reduced amount of the gene lead to the particular phenotyope.

SYNDROME CHROMOSOME NUMBER

Van der Woude Syndrome Chr 1q32 (deletion)Chediak – Higashi Syndrome Chr 1q42-43Dentinogenesis Imperfecta DSPP on Chr 4Cherubism Chr 4p16.3Treacher Collins Syndrome Chr 5q32Gardener Syndrome Chr 5Cleidocranial Dysplasia Chr 6p21Apert Syndrome Chr10q25-q26 (missense

mutation)Cowden Syndrome Chr 10Craniosynostosis Syndrome Chr 10qMarfan Syndrome Chr 15q

SYNDROME CHROMOSOME NUMBER

Neurofibromatosis type-1 Chr 17q 11.2Peutz Jegher Syndrome Chr 19p13.3Fibrous Dysplasia Chr 20q13.2Neurofibromatosis type-2 Chr 22Amelogenesis Imperfecta

Chr Xp22

Downs Syndrome Trisomy 21Patau syndrome Trisomy 13Edward Syndrome Trisomy 18Klienfelter Syndrome XXYTurner Syndrome XO

Conclusion We are used to describing the humans in an

anatomic manner. The time has come when a number of genes will describe us. Physical characteristics (phenotype) may matter less than the set of genes one carries (genotype)

Classification of characteristics will soon be based on genetic analyses. Orthodontists may have a difficult time changing their morphologic classifications. Eg. Angle`s classification.

The advent of diagnostic techniques in molecular genetics would make it possible to identify relevant morphogenes and genetic markers or to influence the development of malocclusion eg. Eliminating crowding through the selective manipulation of homeobox gene responsible for initiation of tooth formation and patterning.