DNA damaging agents such as platinum-based compounds and radiation therapy constitute an essential part of the armamentarium against HNSCC. However, there is a high frequency of tumor resistance to these therapies, with approximately 50% local treatment failure at five years (5
). These therapies cause substantial morbidity, with up to 82% of patients experiencing severe side effects (34
). Patients that are likely to fail DNA damaging therapy could be better served by alternative treatment regimens, such as surgery or a taxane-based regimen. As new therapeutic options emerge, it is increasingly important for clinicians to be able to predict in advance whether a tumor is likely to respond to a particular type of treatment. Thus, a biomarker or panel of biomarkers predicting a high chance of failing DNA damaging therapy would be extremely valuable.
SNPs are attractive for use as biomarkers, as they can be reliably detected using a simple blood test, and the technique does not require tumor tissue or sophisticated histochemical techniques. SNPs in XPF
could mark altered DNA repair capacity by modifying XPF expression or function. For this reason, SNPs in XPF
have previously been evaluated for their association with cancer risk (35
). To our knowledge, no report has yet found SNPs in XPF
to be a biomarker for clinical outcome. We identified four allelic variants of XPF
that associate marginally with worse clinical prognosis. This is consistent with previous reports that polymorphisms in ERCC1
predict worse outcome in HNSCC (28
). However, in the absence of external validation it is possible that these results are due to type I error resulting from multiple testing. The mechanistic implications of these SNP findings remains incompletely understood. One might hypothesize that SNPs in XPF
might correlate with a change in protein expression, activity level or function, which in turn could impact response to treatment. However, none of the SNPs we analyzed correlated with XPF protein expression. However, it is also possible that the SNPs mark a more complex haplotype, which predict disease progression through an unrelated mechanism, for example, perhaps through distant regulation of other gene products near the XPF genetic locus, or through non-coding RNA such as microRNA. The four SNPs that we identified may prove useful in the development of prognostic blood tests for HNSCC patients, and deserve further clinical investigation in a prospective setting after validation in a second study population.
In contrast to SNPs, several tumor protein biomarkers have previously been identified that predict tumor response in HNSCC. For instance, EGFR overexpression correlates with shorter disease-free survival (40
). In addition, cell-cell adhesion receptor E-cadherin and its partner beta-catenin (which regulate epithelial-mesenchymal transition), amphiregulin and epiregulin (markers for response to EGFR antagonist in colorectal cancer) (42
), the anti-apoptotic gene BCL-XL (43
) and TP53 (43
) have all been recommended for further investigation as biomarkers in the design of clinical trials on HNSCC (44
). In DNA repair pathways, Ku80, which participates in DSB repair, has recently been identified as a marker of outcome in HNSCC treated with radiotherapy (46
). ERCC1 has been previously identified as a biomarker that predicted outcome in HNSCC, but recent reports challenge the earlier conclusion (15
), possibly because a non-specific antibody (clone 8F1) was used in these studies (19
Herein, we sought to determine if XPF, the essential binding partner of ERCC1, could be several mucosal sites including the oral cavity, pharynx and larynx. Third, despite strict inclusion criteria the study is a retrospective analysis of prospectively collected samples in which the initial cohort was not prospectively accrued to test XPF level specifically. Finally, patients were not enrolled in a single clinical trial, so treatment regimens varied. However, despite these limitations, we detected a significant association between XPF expression and PFS after adjusting reliably measured in tumors and used as a biomarker to predict clinical outcome. Importantly, our study, like most, has several limitations. First, there was a limited quantity of tumor tissue available from individual patients, preventing a comparison of XPF expression by multiple methods (e.g., immunohistochemistry and immunoblot), in normal tissue vs. tumor, and in tumors before and after chemoradiation therapy. Second, the cohort included HNSCC tumors from for confounding clinical and pathological factors.
Robust mechanistic data explains how expression of XPF could influence clinical outcome in tumors treated with DNA damaging agents. XPF, with its partner ERCC1, play important roles in NER, ICL and DSB repair pathways used to correct the genotoxicity of platinum compounds and radiation. Experiments in cells from XPF-deficient patients and animal models of their disease demonstrate that low XPF expression is associated with an exquisite sensitivity to DNA damaging agents (8
). Similarly, tumors expressing a low level of XPF are more likely to be sensitive to genotoxic agents such as cisplatin and radiation. Our result that tumors with low levels of XPF have a better clinical outcome, is consistent with this mechanism.
Biochemical analysis of tissue lysates revealed that XPF levels were higher in tumors than in adjacent normal tissue, in six out of seven paired specimens. This raises the possibility that XPF expression might be induced during tumorigenesis. Cisplatin induces increased expression of ERCC1 in some ovarian cancer cell lines (48
). This is a particularly important line of investigation because it argues for the need for repeated measurement of XPF levels in tumors during the course of treatment to predict changes in treatment response and to minimize unnecessary side effects if drug resistance emerges.
The observation that XPF expression is higher in tumors originating in the oral cavity compared to other sites is interesting. While the reason for this difference is unclear it may explain why cancers from the oral cavity are typically treated primarily with surgery, reserving chemoradiation for adjuvant therapy. Importantly, since XPF expression was higher in oral cavity tumors, we asked if our analysis was biased. Theoretically it is possible that the patients with low XPF expression could have a longer PFS simply because this group has an over-representation of oropharyngeal and laryngeal tumors, which may have a better prognosis, irrespective of XPF expression. Two arguments make this possibility unlikely. First, univariate analysis showed that PFS did not strongly associate with tumor sites (p=0.157). Second, multivariate analysis revealed that in this cohort XPF is an independent factor associated with PFS, irrespective of tumor site. XPF is therefore a bona fide predictor of clinical outcome.
In summary, this is the first report, in any cancer, demonstrating that the expression level of XPF associates with clinical outcome and that XPF may represent a biomarker predicting success of DNA-damaging therapy. These findings will inform the design of prospective clinical trials providing personalized treatment for HNSCC as a function of the anticipated susceptibility of the tumor to DNA damaging agents, in hopes of minimizing unnecessary treatment-related toxicities and improving outcomes.