Our results indicate that tumor p27 status is a prognostic factor for patients receiving adjuvant chemotherapy following surgery for stage III colon cancer, as patients with p27 deficient tumors treated with either FU/LV or IFL had a 5-year OS that was 12% less than those with p27 intact tumors. When the interaction between p27 status and treatment was examined, we found that loss of p27 did not predict improved response to the irinotecan-containing regimen used in this study. As a result, tumor p27 status, taken alone, is not currently useful in guiding choice of adjuvant chemotherapy. However, low levels of p27 in colorectal cancers result from enhanced ubiquitin-dependent degradation of this tumor suppressor, and recent studies identified the ubiquitin ligase, Pirh2, as a major effector of p27 degradation. These studies confirmed an inverse relationship between p27 and Pirh2 expression in colorectal cancer (26
), suggesting that selective inhibition of Pirh2 would be beneficial in treating patients with p27 deficient tumors.
In an analysis from the Nurses Health Study and the Health Professional Follow-up Study, Ogino, et. al. examined the relationship between tumor MSI and p27 status for 634 cases of colorectal cancer that developed during prospective follow-up (27
). Using a definition of p27 status similar to that employed here, they found that 36.4% of cancers were p27 deficient, and 16.6% exhibited MSI-H (27
). MMR status in this study was determined by tumor genotyping to identify high levels of microsatellite instability (MSI-H), rather than IHC. These investigators found that p27 deficient tumors were more likely to be MSI-H (26.0% p27 deficient/MSI-H, 11.2% p27 deficient/MSS or MSI-L, p<0.001). Our results are consistent with this, as 19.2% of the p27 deficient cases were MMR-D, compared to 9.1% of those with intact p27.
A study of the relationship between treatment outcome and tumor MMR status for this same cohort suggested that MMR-D status predicted improved IFL response, although MMR status was not a significant prognostic factor overall (25
). We therefore performed an exploratory analysis combining a prognostic factor (tumor p27 status) with a predictive factor (tumor MMR status). The p27 deficient tumors in this series showed some similarities to those that were MMR-D, as these categories were both associated with proximal location, poorly differentiated histology, and higher nodal ratio. Despite these similar clinicopathologic characteristics, p27 deficient and MMR-D tumors show distinct clinical behavior. While patients with p27 deficient tumors have a worse outcome, those with MMR-D tumors generally have a better prognosis, or at least do not demonstrate the aggressive behavior typical of poorly differentiated cancers (28
). In this trial, both treatment arms contained 5FU+LV, and we found no difference in outcome for the MMR-D and MMR-I cases, although there was a trend toward improved 5-year DFS in those with MMR-D tumors (0.67 vs 0.60, p=0.18). In addition, we found that only 18% of tumors that were p27 deficient were also MMR-D. Given these results, we predicted that the best outcome would be observed for patients whose tumors were p27 intact and MMR-D, and the worst outcome for those whose tumors were p27 deficient and MMR-I. This was what we observed for the DFS analysis, although the 19.6% difference in DFS between the categories with the best outcome (p27 intact, MMR-D) and worst outcome (p27 deficient, MMR-I) was not statistically significant (logrank p-value for this comparison = 0.219).
This trial allowed us to explore whether a combination of tumor p27 and MMR status predicted better response to IFL. Although the sample size is small, patients whose tumors were both p27 deficient and MMR-D had a DFS that was improved by 72% (logrank p=0.042) and OS that was improved by 35% (logrank p=0.128) when they received the regimen containing irinotecan. This relationship was not observed in tumors that were p27 intact/MMR-D, p27 intact/MMR-I, or p27 deficient/MMR-I. This subset analysis of a small number of samples in the p27 deficient, MMR-D category (N=36) must be interpreted with caution, however several factors make these data intriguing. First, this study was a pre-specified prospective analysis of both MMR and p27 as markers of disease response and treatment outcome, and is therefore free of the bias inherent in retrospective tumor bank studies. Similarities in marker frequency and tumor location between p27 deficient and MMR-D tumors suggest a biological relationship between these factors. It is possible that this yet unidentified relationship is responsible for enhanced tumor sensitivity to irinotecan.
These results provide important insight into the ways in which prognostic and predictive factors can interact, potentially leading to inaccuracy when single factors are used to predict disease behavior. Tumors that are p27 deficient are less likely to respond to adjuvant chemotherapy, an effect that is largely independent of treatment. However, tumors that are p27 deficient are also somewhat more likely to exhibit MMR-D, a condition that appears to convey a differential response to chemotherapy in this colon cancer adjuvant treatment trial. This study, therefore, illustrates the importance of testing prognostic and predictive factors in large randomized trials where the relationships between treatment and multiple tumor-specific variables can be assessed.
Statement of Translational Relevance
The current AJCC staging system for colon cancer is insufficient, as evidenced by a wide variation in adjuvant treatment outcome among patients with stage III disease. In this prospective study of adjuvant chemotherapy following surgery for stage III colon cancer, we showed that loss of tumor p27 expression is associated with post-treatment disease recurrence. This prognostic result allows clinicians to subdivide stage III colon cancer into high- and low-risk groups, and identifies p27 signaling pathway as a potential target for new agent development.