Abstract
Background: K-ras mutation in a tumour is a powerful negative predictor for treatment success. Identifying tumour K-ras mutation is complex, and could be simplified by an appropriate blood test. Clinical studies were identified in which K-ras mutation status was assessed in both blood and tumour to ascertain whether blood K-ras mutation is predictive of tumour K-ras mutation. Between 29% and 100% of patients with a tumour K-ras mutation in 11 studies presented the same mutation in peripheral blood. Only 5/272 patients presented blood K-ras mutation in the absence of the same tumour mutation, possibly due to sampling errors. K-ras mutation in blood appears to indicate K-ras mutation in tumour, while the absence of blood K-ras mutation does not prove lack of mutation in the tumour. This suggests that a blood test for the detection of tumour K-ras may be possible, and could direct cancer treatment strategies.
With the number of cases of cancer estimated to double globally between 2000 and 2020, and to nearly triple by 2030 (1), the need to develop targeted treatment strategies for the efficient use of costly cancer therapies has never been greater. Under such a strategy, the ineffective use of costly treatments would be reduced by directing drug regimens at those patients who are most likely to benefit, in particular, those with a similar genetic profile to other patients who have been successfully treated in this way.
The ras gene carries a mutation in up to 80-90% of cancer patients, usually at codon 12 or 13 of the K-ras gene (2, 3). The presence of a K-ras mutation has been shown to be a powerful negative predictor of treatment success with epidermal growth factor receptor (EGFR) inhibitors such as panitumumab and cetuximab in certain types of cancer, including prostate cancer and non-small cell lung cancer (NSCLC) (4-19). This opens up the possibility of increasing the efficiency of treatment allocation based on the assessment of a patient's K-ras mutation status.
K-ras mutation is currently usually determined by tissue biopsy, which is invasive, costly and potentially subjective. Furthermore, it can be difficult to obtain suitable tumour samples for mutational analysis from all cancer patients. These concerns create a strong case for the development of an alternative method to detect mutant K-ras from more readily accessible patient samples than the tumour itself.
A non-invasive technique for the determination of the K-ras mutation status would offer several advantages, including greater speed and reduced cost. It would also be technically easier for the health care professional to perform, would cause less distress to the patient, could be performed without a tumour specimen, and would allow repeated assessments to be made for the same patient before and after anticancer treatments. Repeated tests after surgery could provide an early warning for recurrence of disease.
Several studies have investigated K-ras levels in non-tumour tissues as well as in the tumour tissue. We reviewed all reported studies investigating K-ras mutation status in the plasma, serum or urine in addition to the tumour to determine whether a blood or urine test for the presence of K-ras mutation would be a viable alternative to a tissue biopsy. If a correlation between tissue and non-tissue K-ras mutation could be shown, the possibility of a simple blood test to identify candidates for anticancer treatment with EGFR inhibitors comes a step closer.
Study Identification and Selection
Relevant published trials were identified through a literature search of the PubMed database, conducted during the period July–December 2009. We limited the review to studies published in the English language. In order to identify studies which assessed circulating levels of the mutated K-ras gene in cancer patients, the combinations of key-words shown in Table I were used. Relevant articles not identified by this strategy, but referenced in the bibliographies of articles which were located in this way, were also included.
Results
A total of 11 studies were identified which investigated K-ras mutation status in the tumour tissue as well as the plasma or the serum in colorectal cancer and pancreatic cancer (Table II). No relevant studies were identified which assessed K-ras mutation status within the urine and tumour.
Colorectal cancer. Mulcahy et al. reported a study of 14 patients with colorectal cancer, in which the presence of K-ras mutations in the plasma DNA was assessed using polymerase chain reaction (PCR) techniques (20). Eleven healthy subjects were also included as controls. K-ras mutations were found in the tumour tissue of 7 out of the 14 colorectal cancer patients. Of these patients, 6 (86%) also displayed the same mutation in the DNA extracted from plasma samples. No mutations were detected in the plasma of patients whose primary tumour was negative for K-ras or among the 11 healthy controls.
In 2009, Lefebure et al. published the results of a study in which serum K-ras mutation and RASSF2A methylation status were assessed using real-time PCR performed in the presence of a peptide nucleic acid specific of the wild-type sequence in 29 patients presenting an unresectable metastatic colorectal cancer treated by chemotherapy (21). Among 12 patients presenting a K-ras mutation in their primary tumour, 7 (58%) presented the same mutation in the serum. Six patients were found to have no K-ras mutation detectable in the primary tumour, and absence of K-ras mutation in the serum was confirmed in all of these patients.
In a study conducted by Trevisiol et al., K-ras mutations were examined in DNA samples extracted from the serum of 86 patients with colorectal cancer and were compared with the K-ras status of their primary tumours. K-ras mutations were found in tissue samples of 28 patients (33%), of whom, 10 (36%) were also found to have serum K-ras mutations (22). Among the 58 patients whose primary tumours were negative for K-ras mutations, one had a K-ras mutation in the DNA serum sample. The authors suggest that this apparently discrepant case may be explained either by the presence of multiple intratumoural clones or multiple foci of tissue with mutated genes or by a sampling error of the tumour specimen.
Ryan et al. conducted a prospective study of 94 patients who underwent putative curative resection for colorectal carcinoma (23). K-ras mutations were analysed in matched tumour and serum samples. K-ras mutation was found in 41/78 (53%) tumours, of which 31 (76%) had an identical mutation detectable in the serum. A further serum mutant K-ras-positive result was found in 1 out of 37 (3%) tumour K-ras mutation-negative cases. Again, the authors suggest sampling error of the paraffin-embedded tumour specimen may account for the failure to detect a mutation within the tumour in this discrepant case. The authors in this study also reported that repeated post-surgical testing for serum K-ras mutation indicated that patients in whom a serum K-ras mutation could be detected after surgery had a highly significantly greater chance of cancer recurrence than patients with no detectable serum K-ras mutation during follow-up (odds ratio, 71.6; 95% confidence interval, 7.7-663.9; p<0.0001).
Summary of search terms used in PubMed search.
In 2009, Yen et al. published results of a study in which K-ras mutation status in the peripheral blood of 76 metastatic colorectal cancer patients receiving chemotherapy was analysed using membrane-arrays, and that in tumours was analysed by DNA sequencing (24). Among 76 patients with metastatic colorectal cancer, K-ras mutations were identified in the tumour of 33, of whom 28 also presented positive K-ras mutations in the peripheral blood. Two patients with no detectable K-ras in the tumour presented a positive K-ras mutation in the peripheral blood. A highly statistically significant correlation between K-ras mutations in the tumour and the peripheral blood was observed (p<0.0001).
Ryan et al. reported the results of a trial in which K-ras mutations at codon 12 were detected using an enriched PCR-restriction fragment length polymorphism (RFLP) technique in 123 patients with colorectal cancer (Dukes' stages A-D) or dysplastic colorectal adenoma (25). Among 76 patients evaluated prior to tumour resection, 49% had K-ras mutation in the primary tumour, with a serum mutant K-ras was detected in “almost all” of these patients (86% correlation). Among patients evaluated post-surgery, 62% were K-ras mutation-positive in the primary tumour, and serum mutant K-ras was detected in 29% of these patients. The exact number of patients presenting with K-ras mutations in both tumour and serum is not reported, nor whether any patients had a K-ras mutation in the serum but not in the tumour.
In a summary conducted by Sorenson of published results of assays used to detect mutated Kras-2 sequences in plasma/serum samples from patients with colorectal cancer, 131 suitable patients investigated by five different groups were identified (26). Forty-four of these patients had tumours that demonstrated a K-ras mutation, 55% of whom also presented positive plasma/serum assay for mutated K-ras. The presence of any positive plasma/serum K-ras mutation in the absence of a corresponding tumour mutation is not reported.
Summary of studies in which K-ras mutation was assessed in the tumour tissue and in the plasma or serum.
Lecomte et al. conducted a study of the prognostic value of free-circulating tumour-associated DNA in the plasma of colorectal cancer patients (27). In this study, patients with a tumour presenting K-ras mutations, as detected by the mutant allele-specific amplification method, were selected for plasma screening. Of the 58 tumours analysed, 22 (38%) were mutated at K-ras, and an identical alteration was detected in 10 (45%) out of the 22 corresponding plasma samples. It is not reported whether any serum samples positive for K-ras mutation were associated with an absence of K-ras mutation in the tumour.
Pancreatic cancer. In 2009, Olsen et al. published the results of a phase I trial in 12 patients with locally advanced pancreatic carcinoma treated with gefitinib, paclitaxel and three-dimensional conformal radiation. The authors reported the detection of plasma K-ras mutations using a two-stage RFLP-PCR assay on patients' plasma both before and after therapy (28). Mutations were confirmed by direct sequencing. K-ras mutations were detected in the pre-gefitinib plasma of 5/11 patients and in the matched tumour tissue of 3/4 patients.
In a study of 21 patients with pancreatic cancer reported by Mulcahy et al., the presence of K-ras mutations in the plasma DNA was assessed using PCR (20). Biopsy tissues were available for 10 patients, and plasma and tumour DNA alterations were reported to correspond in every case.
Summary of K-ras mutation status in the tumour and plasma/serum of studies in this review.
In a summary of results of the detection of mutated K-ras sequences in the plasma/serum of 144 patients with pancreatic carcinoma published by Sorenson (26), the tumour tissue in 79 patients was found to contain mutated K-ras gene. Of these patients, 51% were also found to have a positive assay in the plasma or serum for K-ras mutation.
Discussion
A series of 11 studies was identified in which K-ras mutation status was assessed in the tumour tissue, as well as the plasma or serum of cancer patients. No studies were identified in which K-ras mutation status in the urine was assessed. In each of the studies identified, patients already had a confirmed diagnosis of cancer (colorectal or pancreatic). Across all studies, the proportion of patients with a positive K-ras mutation in the tumour who also had a positive K-ras mutation in the serum or plasma ranged from 29% to 100%. While these figures indicate that the absence of a K-ras mutation in the plasma or serum does not prove absence of the mutation in the tumour, it is interesting to investigate whether a positive K-ras mutation in the plasma or serum can predict a tumour K-ras mutation. A summary of the findings relating to the presence of absence of K-ras mutations in the tumour and plasma/serum in the 11 papers discussed in this review is given in Table III. From this table we can see that a K-ras mutation was observed in the plasma or serum of just 5 patients where it was not found to exist in the tumour, out of a minimum total of 272 patients examined. Of necessity, this calculation excludes those studies in which the plasma/serum K-ras mutation status was not investigated in patients previously identified as having no tumour K-ras mutation.
The very small number of patients presenting with a plasma or serum K-ras mutation in the absence of a similar tumour mutation may potentially be explained by sampling errors, as acknowledged by the authors themselves (22, 23). Indeed, the very rationale for this review was to seek alternatives to the currently clinically impractical and unreliable methods for the detection of K-ras mutation in tumour tissue.
These findings allow us to tentatively draw the conclusion that the presence of a plasma or serum K-ras mutation strongly indicates a tumour K-ras mutation, while the absence of a plasma or serum K-ras mutation does not necessarily prove the absence of a tumour K-ras mutation. In other words, a positive blood test for K-ras mutation would strongly suggest K-ras mutation was present in the tumour, while a negative blood test would require further investigation. This knowledge opens the possibility of a blood test in which a positive result for K-ras mutation would preclude treatment with EGFR inhibitors, while a negative blood test would require further investigation. While not definitive in itself, the development of such a test could have important implications for the practical management of cancer cases according to their response to treatment.
One study investigated in this review also showed that postoperatively, K-ras mutation is a strong predictor of disease recurrence, stronger even than Dukes' stage of disease (23). Thus, repeated testing for K-ras mutation via a blood test in postoperative patients may prove an inexpensive and convenient method for the early detection of disease recurrence. Clearly, such a test as this is not possible through tumour tissue sampling.
The number of studies available for this review is clearly relatively small and interpretation of the findings should be made with caution. Moreover, due to the diverse nature of the studies, no statistical analysis is possible. Because of the potential value of these findings in the practical management of cancer patients, a larger dedicated, prospective trial should ideally be instigated to confirm the findings discussed in this review.
Conclusion
From a review of studies in which the K-ras mutation status was assessed in both the tumour tissue and the plasma or serum, it is concluded that a positive K-ras mutation in the plasma or serum suggests a K-ras mutation in the tumour, while the absence of K-ras mutation in the plasma or serum does not necessarily prove a lack of similar mutation in the tumour tissue. This finding could open the way for a clinically useful blood test for the detection of tumour K-ras and the targeting of chemotherapeutical agents. Future trials are also needed to determine the optimal method of testing for K-ras mutations in the blood.
- Received March 1, 2010.
- Revision received June 6, 2010.
- Accepted June 11, 2010.
- Copyright© 2010 International Institute of Anticancer Research (Dr. John G. Delinassios), All rights reserved
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