The current study is limited by its sample size and the validity of the questionnaires that were employed in the analyses. The trends observed in the results for concordance rates and heritability estimates of indicators of oral malodor nevertheless merit discussion. To our knowledge, this is the first report in the literature on the contributions of genetic and environmental factors to indicators of oral malodor that employed the twin study model.
The presence of a tongue coating is a common problem, and studies employing large cohorts have determined its prevalence to be about 45% (Quirynen et al., 2009
). The papillary structure of the dorsum is a unique ecosystem in the oral cavity, offering a large surface area that favors the accumulation of oral debris and bacteria. The morphology of the dorsum of the tongue provides additional irregularities such as fissures, grooves and depapillated areas that may serve as retention areas that harbor bacteria (Krespi et al., 2006
; Roldan et al., 2003a; Roldan et al., 2003b; van den Broek et al., 2008
). The high concordance rates for tongue coating in MZ twins (67%) and low rates for DZ twins (11%; ) suggest that some mechanism is operating that may govern the accumulation of bacterial mats on the tongue surface. Whether this is related to the MHC (major histocompatibility complex) and related HLA (human leukocyte antigen) genes involved in immunological modeling of microbial colonization of the oral cavity including the tongue surface remains to be determined.
The next set of indicators of oral malodor (self-reported) that had marked differences in concordance rates between MZ and DZ twins are related to gland and mucosal function, i.e., dry mouth, unusual sweating and sinus infection (). The reports of the mouth feeling dry and of unusual sweating are contrasting, but they indicate that body and salivary gland functions may be influenced by genetic factors. Similarly, the presence of sinus infections indicates a breakdown in mucosal defenses against bacteria and viruses, which may also be mediated by genetic factors. Indeed, animal studies suggest that genetic background is involved in responses to bacterial sinusitis (Kirtsreesakul et al., 2006
In these sets of twins, we found that genetic factors make a significant contribution to the levels of VSCs in exhaled breath. The heritability estimates for intraoral air were 0.28±0.17 (NS), whereas for exhaled air they were 0.50+0.20 (p = .0207). This means that the variation in intraoral breath values was essentially modulated by the environment (presumably volatile microbial by-products) as expected and, that variation (50%) in exhaled breath values had a significant genetic contribution. One can speculate that a number of non-oral factors may be under genetic influence, including the regulation of metabolic conditions involving enzymatic and transport pathways (Preti et al., 2006), leading to the systemic production of volatile compounds in exhaled air. This genetic contribution is population-specific because it focuses on latent omnibus genetic effects in population variation and not on effects of a specific identified genotype in individuals.
The quantification of trace gases present in exhaled air is timely because its potential for clinical diagnosis is being realized. Exhaled breath gases are often used to indicate early-stage metabolic diseases (??Spanel et al., 2007
). In our case, our population was ostensibly healthy, and gases detected by the halimeter may only be indicative of oral malodor in a combination of intraoral breath and exhaled breath. Halimeter measurements have been shown to correlate with organoleptic rating scores, that is, with correlation coefficients ranging from r
= 0.42 to r
= 0.64 (Kazor et al., 2003
). These correlations indicated that the VSCs measured by the halimeter account for only 18 to 41% of the organoleptic rating score, indicating that there are other important compounds, such as volatile fatty acids, cadaverine and other compounds present in exhaled breath that contribute to the organoleptic rating score but are not detected by the halimeter. This explains the anomalous finding that sometimes malodor can be detected by the examiner, but the VSC levels are in the low range (Hartley et al.; 1996
; Miyazaki et al., 1995
). This further suggests that additional studies involving an increased number of compounds in intra-oral and exhaled (expired) breath should be evaluated in the twin study model as well as the inclusion of organoleptic measures to validate these findings.
Although our results are preliminary, taken together they suggest that studies in larger cohorts of twins should be conducted to dissect the relative contribution of genetic and environmental factors to indicators of oral malodor. In addition, the contributions of these factors to objective parameters of malodor (VSCs and other markers) should be further evaluated in larger cohorts of twins with subsequent genomic analyses.