Genetic control of susceptibility to Porphyromonas gingivalis-induced alveolar bone loss in mice

Abstract

Periodontal disease affects a large percentage of the human population. Resorption of the alveolar bone of the jaw is a pivotal sequela of periodontal disease, because this bone is the attachment site for the periodontal ligaments that anchor the teeth. Using a murine model in which alveolar bone loss is induced by oral infection with Porphyromonas gingivalis, a gram-negative bacterium associated with human adult periodontal disease, we provide evidence suggesting that susceptibility to such bone loss is a genetically determined trait. AKR/J, DBA/2J, and BALB/cByJ or BALB/cJ mice were highly susceptible, while A/J, A/HeJ, 129/J, SJL/J, and C57BL/6J mice were much more resistant. When susceptible BALB/cJ and BALB/cByJ mice were crossed to resistant strains, two patterns were observed. (BALBc/ByJ x C57BL/6J)F(1) offspring were susceptible, suggesting C57BL/6J has recessive resistance alleles, while (BALB/cJ x A/J)F(1) mice were all resistant, suggesting that A/J mice have dominant resistance alleles. These results suggest a tractable genetic basis for P. gingivalis-induced alveolar bone loss and open the possibility of exploiting the mouse model to identify loci important for host susceptibility and resistance to periodontal disease.

FIG. 1

Alveolar bone loss caused by oral infection with P. gingivalis. Measurement of bone levels is made comparing the distance from the CEJ to the ABC at seven buccal sites on the three molars on the left side and seven sites on the right side of the maxilla (shown by the arrows in panels A and C). (A) Sham-infected BALB/cByJ mouse. (B) P. gingivalis-infected BALB/cByJ mouse. As alveolar bone resorbs in infected mice, the distance from the CEJ to the ABC increases, becoming greater than in sham mice. (C) Sham-infected C57BL/6J mouse. (D) P. gingivalis-infected C57BL/6J mouse. When there is no bone loss, the distances from the CEJ to the ABC are the same in sham-infected and infected mice.

FIG. 2

Effect of P. gingivalis oral infection on alveolar bone levels in different strains of SPF mice. The values shown are the total millimeters of change at 14 sites (mean ± standard error). Negative values of millimeter change in bone indicate bone loss. Infected AKR/J, DBA2/J, BALB/cByJ, and BALB/cJ mice lost bone compared to sham controls (P = 0.03 for AKR/J, P = 0.04 for DBA/2J, P = 2 × 10⁻⁷ for BALB/cByJ, and P = 0.006 for BALB/cJ). In the other mouse strains, bone levels were not different in sham controls and infected mice (P > 0.05). n, number of mice tested. An equivalent number of P. gingivalis-infected and sham mice were tested per strain. When n is greater than 8, results are pooled data from 8 to 10 infected and control mice per experiment.

FIG. 3

Bone loss in P. gingivalis-infected F₁ mice from two different crosses. Each symbol represents one infected mouse. Data from sham-infected mice are not shown. Open symbols and solid symbols are results from independent experiments. Squares with brackets represent the mean ± standard error of the respective experiments. (A) BALB/cJ × A/J cross. Infected mice of one parent strain (BALB/cJ) lost bone (different from BALB/cJ sham controls at P = 0.02 in one experiment P = 0.01 in the other, and P = 0.0006 for pooled data); infected mice of the other parent strain (A/J) and infected F₁ mice (CAF₁/J) did not lose bone (not different from sham controls; P > 0.05). Values from infected BALB/c mice differed from those from infected A/J mice (P = 0.031 in one trial, P = 0.028 in the other, and P = 0.0024 for pooled data). Infected BALB/cJ mice also differed from infected CAF₁ mice (P = 0.0049 in one experiment, P = 0.0041 in the other, and P = 6 × 10⁻⁶ for pooled data). (B) BALB/cByJ × C57BL/6J cross. Infected mice of one parent strain (BALB/cByJ) and infected F₁ mice (CByB6F₁/J) lost bone. (Infected BALB/cByJ mice were different from BALB/cByJ sham controls at P = 7 × 10⁻⁶ in one experiment, P = 0.01 in the other, and P = 9 × 10⁻⁷ for pooled data; infected CByB6F₁ mice were different from CByB6F₁ sham controls at P = 5 × 10⁻⁷ in one trial, P = 0.04 in the other, and P = 2 × 10⁻⁵ for pooled data.) Infected mice of the other parent strain (C57BL/6J) mice did not lose bone (not different from sham controls, P > 0.05). Values from infected BALB/c mice differed from those from infected C57BL/6J mice (P = 2 × 10⁻⁷ in one trial, P = 0.04 in the other, and P = 3 × 10⁻⁷ for pooled data). Infected CByB6F₁/J mice also differed from infected C57BL/6J mice (P = 3 × 10⁻⁸ in one trial, P = 0.05 in the other, and P = 3 × 10⁻⁶ for pooled data).

FIG. 4

Viable anaerobic microflora from various strains of mice immediately prior to infection with P. gingivalis. Duplicate bars for the same mouse strain are from two different experiments. Each pattern or tone of gray is a different bacterial species; the proportion of the bar shows the mean percentage of the total flora represented by that species.

FIG. 5

Recovery of P. gingivalis from different mouse strains. P. gingivalis is shown as a percentage of the total anaerobic flora at the termination of the experiments. The first three mouse strains (BALB/cJ, A/J, and CAF₁/J) correspond to the strains in Fig. 3A, while the second three (BALB/cByJ, C57BL/6J, and CByB6F₁/J) are from the experiment shown in Fig. 3B. Bars represent the means from 10 mice ± standard error. No groups were significantly different (P > 0.05).