Our study shows that high SLFN11 expression is an independent factor for good prognosis in ESCC patients treated with dCRT. Our data suggest that examination of SLFN11 level in biopsy samples may be useful for treatment selection (surgery or dCRT), and that further investigation of SLFN11 as a biomarker of treatment response in clinical settings is warranted.
SLFN11 level and ESCC patient prognosis after dCRT
We observed a significant correlation between SLFN11 expression level in biopsy tissue before treatment and overall survival post-dCRT in ESCC patients. While dCRT combines three treatments, nedaplatin, 5-fluorouracil and irradiation, our study (Figure S1) and a report by Mu et al. (21) indicate that the improved outcome in tumors expressing high levels of SLFN11 is derived from the cells’ response to nedaplatin and irradiation but not 5-fluorouracil.
SLFN11 expression and age
Although we found no significant correlation between SLFN11 expression and other clinicopathological variables (Table 3), SLFN11 expression levels tended to be lower in older patients. A previous report suggested that high levels of methylation of SLFN11 were significantly associated with older age (32). Bioinformatics analysis using CellMinerCDB (https://discover.nci.nih.gov/cellminercdb/) demonstrated a strong inverse correlation between SLFN11 methylation in promoter regions and its mRNA expression level (r = -0.750, p = 0.00013; Figure S2) (30, 31). Therefore, high levels of methylation of SLFN11 may explain the tendency for SLFN11 expression to be lower in older patients in our study.
Translation of SLFN11 to the clinical practice
Our data showed that the prognosis of stage I (cT1b) patients was relatively good irrespective of SLFN11 status. In the USA, clinical stage IVA and IVB (cM1-lym) are indications for palliative therapy, and dCRT is not provided as a treatment option. In contrast, clinical stage II and III are generally indications for surgery or dCRT (as a non-surgical option). Importantly, the prognosis of patients treated with surgery is comparable to that of patients treated with dCRT (11) (33). In our study setting, we showed that the SLFN11-dependent good outcome is more evident in clinical stage II and III patients than in those at other stages. Therefore, examination of SLFN11 expression level may be particularly useful for clinical stage II–III ESCC patients who wish to choose dCRT (instead of surgery) to preserve esophageal function. Moreover, this subset of patients is also a candidate for trimodal treatment, namely neoadjuvant concurrent chemoradiotherapy followed by surgery.
Taken together, our results suggest that the examination of SLFN11 expression might be useful in optimal treatment selection.
Our data should be interpreted in the context of several limitations.
First, because our study was retrospective, we could not address all confounding biases. In an attempt to minimize the influence of confounding biases, we recruited as many patients as possible (n = 597) and adopted a solid outcome endpoint (i.e. overall survival).
Second, from among these 597 patients, 73 were enrolled in accordance with the inclusion/exclusion criteria. Even in a study of this sample size, SLFN11 expression level was an independent prognostic factor, probably because of its strong correlation with DNA-damaging treatment sensitivity. Similar future studies should be conducted to confirm our conclusions.
Third, we provisionally defined an H-score ≥ 51 as indicating high SLFN11 expression.
This threshold was selected from an ROC curve of SLFN11 expression level versus 3-year overall survival after dCRT (data not shown). Therefore, the threshold may differ according to different settings such as the antibody used for immunohistochemistry, specimen size, and/or cancer type.
Fourth, we have used a low-dose nedaplatin + 5-fluorouracil regimen in dCRT for renal protection since 2003. Nedaplatin is used as an alternative to cisplatin, especially in Japan, as it reportedly has fewer side-effects than cisplatin (34). According to national data from 2016 (Japanese Ministry of Health, Labor and Welfare), nedaplatin (instead of cisplatin) was used at a frequency of 12% (at 1 to 7.4 ratio) in chemo-/chemoradiotherapy in esophageal cancer patients (35). Although cisplatin + 5-fluorouracil is the global standard combination regimen for dCRT in ESCC patients, the treatment performance of cisplatin + 5-fluorouracil with concomitant radiotherapy and low-dose nedaplatin + 5-fluorouracil with concomitant radiotherapy is reportedly similar in esophageal cancer patients (6, 34). In this setting, as shown above, cancer cell lines with high SLFN11 expression had better therapeutic sensitivity to nedaplatin than those with low SLFN11 expression (Figure S1). Given that our aim was to investigate the effect of SLFN11 on the reactivity of ESCC to a platinum derivative and radiation, we do not think that enrolling patients treated with a low-dose nedaplatin + 5-fluorouracil regimen with radiation affected our conclusion.
Fifth, among the types of esophageal cancer, adenocarcinoma is the most prevalent in western countries, while ESCC is the most frequent (90.5%) in Japan (36). Therefore, the frequency of different pathological types of esophageal cancer may differ according to ethnicity.
Sixth, although, SLFN11 expression was an independent prognostic factor in our own study cohort, external validation of this finding is warranted. Confirmation in a larger study may allow the drawing of firm conclusions.
Finally, in our vitro study, we investigated the association between SLFN11 expression and sensitivity to these anticancer agents using leukemia cell lines, as in our previous report, to allow timely reporting (18). Additional evaluation in esophageal cancer cell lines is desirable.