DENTAL CARIES: A DENT ON DOGMA.

PART 4

By

Dr John Gabrovsek

Received: April 1997

 

Dental Caries Part 1 Dental Caries Part 2 Dental Caries Part 3 Dental Caries Part 5

 

The Genetic Role in Dental Caries Development.

At present not much attention is given to the host and to the genetic factors which may have an effect on the development of dental caries.Evidence of the involvement of hereditary factors in susceptibility or resistance to the development of dental caries is derived from two sources: human observations and animal experimental studies.

Human studies: In dental caries literature of the 30's, 40's, and 50's we can find studies which explored the frequency of dental caries development in related individuals. In all of these studies the existence of familial resemblances in dental caries experiences were reported (Klein et al,1938)(Klein,1946)(Klein,1947)(Book et al,1953). Some studies of twins also suggest that there maybe genetic contribution to dental caries development (Fairpo,1979)(Conry et al,1993)

Animal studies:It is evident that experiments conducted on animals can be more strictly controlled than is the case with the human observation studies. Hunt and Hoppert from Michigan State College started already in 1937 planning and later executing an original noteworthy and enormous experiment. They placed over one hundred albino rats (Mus norvegicus) on a cariogenic diet which they devised The surviving animals developed dental caries in 28 to 209 days. The most susceptible rats from the first generation were crossed to start a caries susceptible strain and the more caries resistant rats were crossed to begin the resistant line. In this fashion, they developed genetically resistant and susceptible strains of rats. The strains were studied for many generations and included about 9,800 rats altogether. The average time for inducing the first carious lesion in the lower molars of the susceptible rats decreased from 57 days in the second generation to 35 days in the twenty-fifth generation. The average time to induce carious lesions in the resistant strain increased from 116 days for the second generation to 505 days for the seventeenth generation (Hunt,et al,1944) (Hunt,et al,1944)(Hunt,et al,1955)

Shaw and Griffiths used the Harvard strains with high and low susceptibility of dental caries development. In the experiment they transferred the newborn of the resistant strain to the mothers of the susceptible strain to be nursed, and vice versa. The characteristic level of dental caries development for particular strain prevailed in spite of cross nursing (Shaw,et al,1960). Rosen, Hunt, and Hoppert (Rosen,et al,1961) experiments are in agreement with those of Shaw and Griffiths. In another cross breeding experiment and study the authors concluded that both parents exert equal influences on the caries activity of the offspring, and that nursing mothers had little, if any, effect (Shaw,et al,1961).

Keyes reported that dental caries can be induced in caries-inactive animals by exposing them to caries-active animals, and this may imply that caries "resistant" animals do not develop dental caries simply because they were not infected with caries producing bacteria (Keyes,1960).

To further substantiate the genetic factors in dental caries development Rosen, on the basis of Keyes' observation, performed another experiment. The susceptible strain of rats showed an increase in caries activity after depression of the penicillin-sensitive flora with penicillin and reinoculation with feces. The resistant strain did not show a significant increase in caries activity after depression of the penicillin-sensitive flora and reinoculation with the same kind of feces. These experiments demonstrated that dental caries can be transmitted, provided that the animals are genetically predisposed to development of dental caries. If the animals are genetically resistant, however, even massive infection with faecal material containing cariogenic flora will not cause a significant increase in dental caries development (Rosen,et al,1961).

Larson and Sims demonstrated that dental caries is appreciably more active in Osborn-Mendel (O-M) strain than in NIH black rats (B-R) when both were exposed to identical diets (Larson,et al,1965). In another experiment the same researchers used (O-M) female rats and mated them with both (O-M) and NIH black rats (B-R). The litters contained both (O-M) white and crossbreed gray to black offspring and both groups developed significantly different dental caries status.The (O-M) and NIH black crossbreeds had significantly lower caries activity than pure (O-M) stain. This phenomenon occurred even though the rats were exposed to identical environmental conditions (Larson,et al,1965).

Variations may exist in the oral microflora harbored by different strains of rats. It could be concluded that these variations may be the reason for the difference in caries level observed in these two groups. In order to investigate further, Grenby and Owen set up an experiment under strictly gnotobiotic conditions. Two different strains of rats Osborne-Mendel caries susceptible and Wistar much less caries susceptible were infected with the same oral microorganism. According to the authors: "The experiment was planned with precise time-mating to derive simultaneously germ-free young of both strains, which were fostered onto germ-free lactating female of a third strain. At weaning they were inoculated with an absolutely standardized culture of a known caries-inducing oral microorganism, after which they were fed on a standard cariogenic diet for a fixed length of time under strictly uniform conditions. At the end of the experiment, their teeth were examined to see whether the usual interstrain difference in the level of caries had persisted, so as to determine whether caries susceptibility was either of hereditary genetic characteristic, or whether the apparent strain difference would disappear when the oral bacterial flora in both strains was standardized. The findings demonstrated that under both conventionalized and gnotobiotic conditions, a significant difference was maintained between the two strains of rats. Osborne-Mendel rats were consistently more caries active than the Wistar rats. The difference was highly significant in the gnotobiotic rats in terms of caries score and the total number of lesions per rat. Another important finding was the uniformity between the animals within the experimental group as seen in the results as low standard error (Grenby,et al,1980) Thus, according to the authors, heredity rather than the nature of the oral microflora is the dominant factor in determining the different levels of caries activity in the two strains of rats.

This short pamphlet does not allow to quote all publications. Nevertheless all animal experiments designed to study genetic involvement in dental caries development strongly confirm that there is such an involvement! In other words, just like in other diseases the host's response plays an important role in the development of dental caries

References:

Book,JA,Grahnen,H.(1953) Clinical and Genetic Studies of Dental Caries. II. Parents and Sibs of Adult Highly Resistant Propositi. Odontol.Revy.4,1-53.

Conry,JP,Messer,LB,Boraas,JC,Aeppli,DP,Bouchard,TJ,Jr.(1993) Dental Caries and Treatment Characteristic in Human Twins Reared Apart.Arch. Oral Biol.38,937-43.

Fairpo,CG.(1979) Total Caries Experience in Monozygotic and like-sexed Dizygotic Twins of Caucasoid Origin Aged 5 to 15 Years. Archs.Oral Biol.24,491-494.

Grenby,TH,Owen,D.(1980) A Gnotobiotic Study to Distinguish between Heredity and the Oral Microflora as Transmitters of Dental Caries Activity in Laboratory Rats.Caries Res. 14,434-440.

Keyes,PH.(1960) The Infectious and Transmissible Nature of Experimental Dental Caries. Arch.Oral Biol.3,247-257

Klein,H,Palmer,CE (1938) Studies on Dental Caries. V. Familial Resemblance in Caries Experience in Siblings. Publ. Health Rep. 53,1353-1364.

Klein,H.(1946) The Family and Dental Desease. IV.Dental Disease (DMF) Experience in Parents and Offspring. JADA.33,735-743.

Klein,H.(1947) The Family and Dental Disease. Publ.Health Rep.62,1247-1253.

Larson,RH,Simms,ME.(1965) Genetic and Environmental Influences on Dental Caries in the Osborne-Mendel and the NIH Black Rat. Arch.Oral Biol. 10,663-668.

Larson,RH,Simms,ME.(1965) Double Mating: Its Use To Study Heritable Factors in Dental Caries. Science. 149,982-983.

Rosen,S,Hunt,HR,Hoppert,CA.(1961) Hereditary Limitations of the Infectious and Transmissible Nature of Experimental Dental Caries. Arch. Oral Biol.5,92-97.

Rosen,S,Hunt,HR,Hoppert,CA.(1961) The Importance of the Genotype on Susceptibility to Dental Caries in the Rat.J.Dent.Res.40,352-354

Shaw,JH,Griffiths,D.(1960) Evaluation of the Degree of Caries Susceptibility in Strains of Rats.Arch.Oral Biol.3,15-27.

Shaw,JH,Griffiths,D.(1961) Studies on the Inheritance of Dental Caries in the Harvard Strains of Caries-Susceptible and Caries-Resistant Rats. Arch.Oral.Biol.3,247-257.

 

 

John Gabrovsek can be contacted via email at: GABROVJ@cesmtp.ccf.org



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