Smoking modified the association between XRCC1 polymorphisms and HNSCC risk for the HPV16 seronegative but not the HPV16 seropositive. Among the HPV16 seronegative, those who smoked 20 or more pack-years were at increased risk of HNSCC if they carried a variant polymorphism for XRCC1. For light or never smokers who were HPV16 seronegative, XRCC1 polymorphisms may result in a reduced risk of HNSCC. Among the HPV16 seropositive, there was no relationship between either XRCC1 polymorphisms or smoking and HNSCC risk. The XRCC1 polymorphisms tagged for unique haplotypes. The individual haplotypes containing 399Gln or 194Trp were associated with HNSCC risk among heavier smokers without HPV16 exposure. The data suggested that this interaction among the HPV16 seronegative may be most relevant to tumors of the oral cavity and pharynx. However, the site-specific analyses were preliminary as our power to examine this question was low. Overall, we cannot dismiss the role that chance may have had in these results, yet the data further support the literature indicating that HNSCC pathways may differ by HPV status.
Without accounting for exposure to HPV16, the association between XRCC1
haplotypes, smoking, and HNSCC risk would have been missed. That there was no interaction between HPV16 and XRCC1
is not surprising given that HPV16 carcinogenesis is not directly associated with DNA damage, but rather, the HPV16 proteins E6 and E7 interfere with the tumor suppressors pRb and p53.41, 42
The lack of an interaction between XRCC1
polymorphisms and HPV16 serology suggests that impaired DNA repair does not influence HPV16 carcinogenesis. Similarly, to our knowledge, no DNA damaging agent has been found to enhance the carcinogenicity of HPV16 in HNSCC. For example, it has been previously reported that smoking, which leads to the formation of DNA adducts, does not further increase risk of HNSCC risk among the HPV16 positive.32
Among the HPV16 seronegative, we found that smoking modified an association between XRCC1
polymorphisms and HNSCC risk. DNA adducts from smoking are thought to lead to mutations in daughter cells, and these mutations may deactivate tumor suppressors including p53.43, 44
If the XRCC1
variant alleles result in poorer repair, this increases the likelihood that the DNA adducts would persist, leading to greater risk of HNSCC, consistent with our data among heavy smokers with variant alleles. However, variant alleles for the light/never smokers may be associated with a reduced risk of HNSCC. Previous studies of HNSCC and XRCC1
polymorphisms have also observed this interaction where the variant allele was associated with a reduced risk in the low exposure category but an elevated risk in the high exposure category, when the exposure was either smoking18
or alcohol consumption.21
Researchers who have studied other types of cancer and found reduced risks associated with XRCC1
polymorphisms have attributed this to persistent DNA damage in cells containing the variant allele which leads to more mutations, making the cell more likely to undergo apoptosis.45-47
On the other hand, heavy smokers with variant alleles thus leading to reduced repair capacity may harbor a greater burden of mutations and an increased likelihood of compromising genes involved in apoptosis (e.g., the deactivation of p53), thereby preventing cell death. Thus, the cell and the DNA damage persist, resulting in greater HNSCC risk.
Our estimation of XRCC1
haplotypes and their distribution was similar to other studies.15, 19
In addition, our observation of no overall association between HNSCC risk and XRCC1
polymorphisms was consistent with previous research,15, 18, 19, 23
as was the finding of an interaction with smoking.16, 18, 22
However, as stated earlier, not all previous studies were in agreement with these results. Our results indicate that the discrepancies in the literature may be due to differences in the distribution of smoking and exposure to HPV16 across populations, and that consideration of HPV16 status may be necessary to appropriately evaluate the smoking-XRCC1
relationship in HNSCC.
The allele frequencies we observed were comparable to the frequencies reported in other white populations in the United States,18, 21-23
however, reported frequencies for the 194Trp and 280His alleles tended to be slightly higher and the 399Gln allele frequency lower in Korean15
and Indian17, 48
populations. The majority of the non-whites in our study population were black, and their allele frequencies did not vary greatly from that of whites. This similarity has been reported in studies of XRCC1
where both groups were represented,18, 49
although not always.50
When we compared results that included subjects from all backgrounds and controlled for race with models that restricted to whites, the results were unchanged.
The use of serology to determine exposure to HPV16 has its limitations. For example, serology is not site-specific. Thus, being seropositive is not indicative of where in the body infection occurred, although previously we have demonstrated that increasing HPV titer was associated with presence of HPV DNA in HNSCC tumors.35
In addition, HPV serology has been found to have strong specificity but weaker sensitivity.51
In particular, investigators have reported that seroconversion is less likely with transient infections than with persistent ones.52
As a result, it is possible that we are underestimating past HPV16 exposure. Potential misclassification of HPV16 exposure may have reduced the observed associations, although we cannot say with certainty what impact this misclassification may have had on the observed associations and interactions.
A limitation of this study was the lower participation rate in the controls, a widespread challenge in population-based case-control studies. Among those who did participate, the majority also provided blood samples from which both XRCC1
genotypes and HPV16 seropositivity were determined. Thus, for bias to influence our results, controls would have had to preferentially participate by XRCC1
genotype or by HPV16 exposure. This appears unlikely given that subjects were not informed of the study hypotheses. In contrast, smoking was self-reported, and it has been reported that subjects who decreased their smoking tended to underestimate their past cigarette consumption.53
Therefore, if cases had recently decreased their smoking due to disease, it is possible that they may underreport their past cigarette use. If this was the case, the association with smoking may be underestimated.
XRCC1 polymorphisms may confer susceptibility to HNSCC in the context of smoking, among those who are HPV16 seronegative. Our analysis provides support to the growing recognition that HPV16-related HNSCC is a distinct disease and future research into HNSCC susceptibility and other HNSCC risk factors should examine their disease impact separately by HPV16 status.