In this study, we did not observe significant main effects of the SNPs in the promoter regions of NOXA
genes on the risk of SCCHN. However, all tested polymorphisms showed effect modification on risk of HPV-associated SCCHN, particularly for oropharyngeal cancer patients who were never smokers, never drinkers, and younger subjects (aged
year). Although these data suggest a potential joint effect between HPV16 infection and NOXA
polymorphisms, larger studies are still needed to validate our findings.
The deregulation of apoptosis is known as an important cause of neoplasia. In the intrinsic apoptotic pathway, apoptosis is usually mediated by cooperation among proapoptotic members (such as NOXA) and antiapoptotic members (such as MCL1) of the Bcl-2 family proteins. Differing from other BH3-only members, such as PUMA, NOXA usually executes its proapoptotic function by exclusively binding to MCL1, A1 or Bcl-XL
, but with higher priority and affinity to MCL1 [2
]. Studies from multiple cancer cells have shown that the NOXA inhibition or MCL1 overexpression can dramatically reduce apoptosis and consequently promote the tumor viability [36
], which suggests a possible effect of NOXA-MCL1 axis on susceptibility to cancers. In consistence with this scenario, we found in current study that, to some extent, the risk of HPV16-associated SCCHN, particular oropharyngeal cancer, can be modified by genetic variants of NOXA
The decrease of SCCHN incidence in recent years accompanied by decline of tobacco use has not been observed for all head and neck cancer sites. However, the results from the America and European cohort studies suggested that the oropharyngeal cancer incidence (especially in younger white populations) has steadily increased and shown to be associated with the involvement of HPVs, particularly the high risk type 16 [17
]. The HPV oncoproteins E6 and E7 may play decisive roles in HPV16-associated SCCHN by E6-mediated p53 degradation or E7-mediated pRb-E2F degradation and disruption of the p53-related or pRb-related pathway [23
are targets of p53 and E2F, and p53 can affect the expression of its instant downstream target NOXA
by directly binding to the promoter response element. In addition, other common transcription factors, such as E2F and p73, may also transcriptively regulate the expression of NOXA
as well as MCL1
, through respective response elements in the promoter regions of NOXA
], thereby altering susceptibility to SCCHN. It is therefore mechanically conceivable and biologically plausible that HPV16 infection and the functional SNPs in the promoters of NOXA
may jointly affect apoptosis induction, accordingly altering susceptibility to SCCHN. In agreement with that, we did observe a pronounced modification effect of these genetic variants on the risk of SCCHN, especially oropharyngeal cancer, associated with HPV16 seropositivity.
Some other studies provided evidence for such kind of joint effects. In a very recent study, by blocking HPV E6-mediated p53 degradation to activate the transcription of NOXA
, RITA (a small-molecule reactivation of p53 and induction of tumor cell apoptosis) induced apoptosis in multiple cancer cells containing HPV16 and substantially suppressed the growth of cervical carcinoma xenografts in vivo [29
]. Not only the E6 oncoprotein, but the E7 oncoprotein can also interact with NOXA
. By disrupting pRb-E2F complexes, the expression of HPV16 E7 protein up-regulates the NOXA
expression, whereas, the binding of E2F to the NOXA
promoter results in a significant reduction of NOXA
-mediated apoptosis [42
], suggesting that HPV E7 may indeed deregulate the NOXA
expression. In addition, HPV E6- or E7-transfection can significantly up-regulate the expression of MCL1,
therefore promoting lung tumor cell progression through PI3K pathway [30
]. These data suggest a possibility that, by deregulating apoptosis and inferring tumor growth, HPV16 infection may alter the progress of NOXA-MCL1 axis-mediated carcinogenesis through both p53- dependent and independent mechanisms. Indeed, we observed in current study that the risk associated with the joint effects between HPV16 seropositivity and variants of NOXA
dramatically increased for SCCHN at the oropharyngeal (HPV-related) site but not at non-oropharyngeal (HPV-unrelated) sites. Although our study implied significant trends of effect modification, we did not have adequate power to detect the interaction.
In consistence with our previously published studies in which HPV16 and genetic variants of p53 pathway genes, such as p53
, may have interactive effects on risk of oropharyngeal cancer, particularly in never smokers and never drinkers [27
], we also found in current study more pronounced modification effects of NOXA
variants on the risk associated with HPV16 seropositivity for oropharyngeal cancer, particularly in never smokers and never drinkers. In addition, when the subjects were dichotomized by the mean age of the controls, we observed more apparent joint effects in younger, than older subjects. Though the mechanism is unclear, many researchers recently documented an apparent increase of incidence of HPV-related oropharyngeal cancer, particularly in young adults, perhaps due to distinct changes in sexual behaviors [43
]. To the best of our knowledge, the current study is the first to evaluate the association between polymorphisms in the promoter region of NOXA
and risk of SCCHN, as well as the joint effects between the genetic variants and HPV16 infection in the etiology of SCCHN.
However, there are several limitations in our present study. Firstly, although our results suggest a potential effect on SCCHN risk associated with HPV16, the underlying mechanism of NOXA and MCL1 in the development of SCCHN is still unclear. Thus future studies are needed to focus on how these polymorphisms affect functional changes of the two genes. Secondly, the HPV16 serology assay used in our study could test the pre-exposure status but could not define the accurate infected organs or determine if the viral exposure took place before or after tumor development. Therefore, with this uncertainty applied to both the cases and controls, possible false-negative HPV16 cases might result in misclassification of the HPV16 status. However, the use of serology assay allowed for the inclusion of a cancer-free control group. Additionally, the relative small sample size could not provide enough statistical power for further stratified analysis or interaction analysis; thus we cannot exclude the possibility that our results could be by chance. Finally, we cannot deduce similar conclusions with certainty to other ethnic populations, because our study included only non-Hispanic white participants.