Genetics and malocclusion /certified fixed orthodontic courses by Indian dental academy


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Genetics and malocclusion /certified fixed orthodontic courses by Indian dental academy

  1. 1. Role of genetics in the etiology of malocclusion
  2. 2. INDIAN DENTAL ACADEMY Leader in continuing dental education
  3. 3. Introduction The relative influence of genetics and environmental factors in the etiology of malocclusion has been a matter of discussion, debate & controversy in the orthodontic literature.
  4. 4. Genetic mechanisms are clearly predominant – embryo, but environment is also thought to influence dento facial morphology post natally. The key to determination of etiology of malocclusion & its treatibility lies in the ability to differentiate the effect of genes & environment in the cranio facial skeleton in a particular individual.
  5. 5. Our ability to do this is limited by our lack of knowledge on the genetic mechanisms, that control growth and lack of scientific evidence for the influence of environmental factors on human cranio facial morphogenesis.
  6. 6. Basic terminologies Genotype – The genetic make up of an organism. Phenotype – The external visible appearance of an organism. Alleles/Allelomorphs – A pair of genes controlling the same character and located at the same locus in the homologous chromosomes.
  7. 7. Homozygote – Individual carrying identical genes for a particular character ( HH / hh ) Heterozygote – Individual carrying non identical genes for a particular character ( Hh )
  8. 8. Dominant – If a trait or disease manifests itself when the affected person carries only one copy of the gene responsible, along with one normal allele –dominant ( Dd ) Recessive – If two copies of the defective gene are required for expression of the trait – recessive ( ee )
  9. 9. Complementary genes – Two or more dominant genes interact with one another to produce a character ; but one cannot produce the character in the absence of the other. Supplementary genes – Two independent pair of dominant genes, which interact in such a way that each dominant gene produces its effect when the other is not present. When the two join - ?
  10. 10. Pleiotropism – The production of many characters by a single pair of genes. Polygenes – Two or more non allelic genes controlling single quantitative character ( skin ) in a cumulative fashion. Genic interaction – Expression of a character by interaction of more than one pair of genes-Allelic or Non Allelic
  11. 11. Sex linked gene – The genes controlling body characters located on the sex chromosomes. Most are recessive. Continuous variation – One that is not separable into discrete classes. Determined by multi genic inheritance. Discontinuous variation – Discrete classes are easily recognized.
  12. 12. Twin studies Why were they carried out ? Concordant and Discordant twins?
  13. 13. Twins 1. Identical Twins Similar character Single zygote 2. Fraternal Twins Different genotype Dizygotic 3. Siamese Twins First time in Siam Monozygotic Physically joined
  14. 14. The frequency of MZ twins – 3.5 – 4 per thousand DZ twins – 3.5 – 18 per thousand DZ & not MZ twinning – genetically determined. Inheritance of DZ twinning – confined to the female line
  15. 15. Determination of zygosity The oldest way – Facial appearance Based on hair colour & type, ear form, tongue roll & phenylthiocarbamide sensitivity. Tests on facial similarity & hematological investigations usually agreed.
  16. 16. To determine zygosity at birth; Sex Placental morphometry Protein enzyme analysis Histocompatability tests.
  17. 17. For single gene trait – Monozygotic concordance rate – 100 %. For DZ twins - < this & equal to that in siblings. For multifactorial traits – Rate in MZ twins although less than 100% exceeds rate in DZ twins.
  18. 18. In cleft studies ; MZ - Concordance rate CL – 35%, CP – 26% DZ ; CL – 5%, CP – 6% What does this indicate ?
  19. 19. Population genetics It tries to identify the genetic component of disease, its mode of inheritance and its distribution among a population. It enables the study of genetic linkage (?) & association (?)
  20. 20. Penetrance – The percentage of individuals expressing the character for a particular genotype ( complete / incomplete ). Expressivity – The variation in degree of expression of a particular gene ( partial & zero expressivity ).
  21. 21. Class II div 1 Malocclusion Investigations – Mand is significantly retruded & overall mand length reduced. Higher correlation b/w patient & immediate family members. Environmental factors ; Soft tissue – L incisor. Digit sucking. Lip incompetence.
  22. 22. Class II div 2 Markovic – 1992 -48 twin pairs MZ – 100% concordance rate. DZ – 90% were discordant. This is a strong evidence to quote genetics as a main etiologic factor .
  23. 23. Class III Famous example –Hapsburg jaw. Strohmayer – 1937 – autosomal dominant trait. Schulze & Weize –concordance rate in MZ twins 6 times higher than DZ . Polygenic hypothesis as the primary cause of mand prognathism.
  24. 24. A wide range of environmental factors – Enlarged tonsils Nasal blockage Congenital anatomic defects Hormonal disturbances Posture etc
  25. 25. Influence on tooth no & size Osborne et al – 1958 – twin studies – crown dimensions – heredity Clinical evidence – congenital absence & tooth size reduction – explanation? Tooth size fits the polygenic multi factorial trait.
  26. 26. Super numerary tooth Most frequently seen – premaxillary region – male predilection They are most commonly seen in parents & siblings of patients. Sugaku – 1963 – multi factorial.
  27. 27. Hypodontia Study of children with missing teeth – half of their siblings & parents also had missing teeth. Markovic – 1982 – high rate of concordance for MZ, while DZ were discordant. Single autosomal dominant gene with incomplete penetrance.
  28. 28. Abnormal tooth shape Alvesalo & Portin – 1969 – missing and malformed laterals – common gene defect. Peg shaped to missing teeth – familial trends, female preponderance & association with other dental anomalies – misssing teeth, ectopic canines, transposition – polygenic.
  29. 29. Ectopic maxillary canines Various studies – genetic tendency. Peck – 1994 – concluded palatally ectopic canines were an inherited trait.
  30. 30. Submerged primary molars Most often in mand arch. Siblings– likely to be affected in 18% of cases, high concordance in MZ twins – indicating a genetic component. Associated with other anamolies – may encompass diff manifestation of same syndrome – incomplete penetrance & variable expressivity
  31. 31. The clinical significance of the inheritance of certain dental anomalies is that clinicians should be vigilant in the expectation that clinical or radiographic evidence of one anomaly should alert them to the possibility of other defects in the same individual or other family members.
  32. 32. In clinical orthodontics it must be appreciated that each maloccusion occupies its own distinctive slot in the genetic/environmental spectrum and therefore the diagnostic goal is to determine the relative contribution of genetics and environment
  33. 33. The greater the genetic component, the worse the prognosis for a successful outcome by means of orthodontic intervention. If dento facial structure & malocclusion are primarily genetic e.g. severe mand prognathism or endogenous tongue thrust, then treatment wll either be palliative or surgical
  34. 34. Recent advances …….
  35. 35. Homeobox genes Are known to play a role in patterning the embryonic development. These can be regarded as master genes of the head and face controlling patterning, induction, programmed cell death & epithelial mesenchymal interaction during development.
  36. 36. Of particular interest – Hox group, Msx 1 & Msx 2, Dlx (distaless), Otx(orthodontical), Shh(sonic hedgehog). The proteins encoded – transcription factors (?) At cellular level – The growth factor family, the steroid/thyroid/retinoic acid super family.
  37. 37. Molecular genetics in dental development. Mesenchymal molecules and their receptors – mediators. Bone morpho genetic proteins BMP2, 4 & 7 mRNAs shift. Tooth development and shape is regulated by FGF 8 & FGF 9 via downstream factors MSX 1 & PAX 9.
  38. 38. Msx 1 & Msx 2 - mesenchymal interactions – initiation, developmental position (Msx1) & further development (Msx2). Pax 9 – tooth morphogenesis. BMPs – growth factor family ( BMP 5 – endochondral osteogenesis, BMP 7 – dentinogenesis )
  39. 39. Amelogenesis imperfecta Genetically heterogenous disorder. Two amelogenes – AMGX & AMGY. It is likely that mutations in several genes may be involved in the etiology of diff forms of autosomally inherited AI
  40. 40. Conclusion While phenotype is inevitably the result of both genetic & environmental factors, there is irrefutable evidence for a significant genetic influence in many dental & occlusal variables.
  41. 41. The influence of genetics however varies according to the trait under consideration and in general remains poorly understood. More precise research tools and methods are required to improve knowledge & understanding, which in turn is a pre requisite to the appreciation of the potential for genetic and environmental manipulation in orthodontic therapy.
  42. 42. The fight between genetics and environment as a causative factor continues…….
  43. 43. Thank you !!! Leader in continuing dental education