In sporadic colorectal cancer (CRC), KRAS are alternative to BRAF mutations and occur, respectively, in 30 and 10% of cases. Few reports addressed the association between KRAS–BRAF mutations and tumour progression specifically in sporadic microsatellite-stable (MSS) CRC. We screened KRAS and BRAF in 250 MSS primary CRC and 45 lymph node (LN) metastases and analysed the pathological features of the cases to understand the involvement of KRAS–BRAF activation in progression and metastasis. Forty-five per cent of primary MSS CRCs carried mutations in at least one of these genes and mutations were associated with wall invasion (P=0.02), presence and number of LN metastases (P=0.02 and P=0.03, respectively), distant metastases (P=0.004) and advanced stage (P=0.01). We demonstrated that KRAS and BRAF are alternative events in Tis and T1 MSS CRC and, KRAS rather than BRAF mutations, contributed to the progression of MSS CRC. The frequency of KRAS and/or BRAF mutations was higher in LN metastases than in primary carcinomas (P=0.0002). Mutated LN metastases displayed KRAS associated or not with BRAF mutations. BRAF mutations were never present as a single event. Concomitant KRAS and BRAF mutations increased along progression of MSS CRCs, suggesting that activation of both genes is likely to harbour a synergistic effect.
KRAS and BRAF are members of the MAP kinase (MAPK) pathway, which is hyperactivated in approximately 30% of all cancers (Hoshino et al., 1999). The identification of mutationally activated KRAS and BRAF alleles in several tumour models supports the importance of this signalling pathway in cancer progression (Davies et al., 2002; Rajagopalan et al., 2002). The RAS/RAF/MAPK pathway regulates cell proliferation, differentiation, senescence and apoptosis. In addition, several reports have shown that MAPK activation, owing to oncogenic RAS and BRAF mutations, is likely to be involved in promoting cellular invasiveness in different tumour models (Fugimoto et al., 2001; Sumimoto et al., 2004; Melillo et al., 2005). Moreover, it has been shown that G12V RAS mutation has a 50-fold higher transforming and oncogenic activity in NIH3T3 cells than V600E mutation of BRAF. By itself, BRAF V600E mutation shows a 138-fold transforming and oncogenic activity over wild-type BRAF (Davies et al., 2002).
In sporadic colorectal cancer (CRC), oncogenic mutations affecting KRAS and BRAF occur in about 30 and 10% of the cases, respectively (Rajagopalan et al., 2002; Yuen et al., 2002; Brink et al., 2003; Wang et al., 2003; Oliveira et al., 2003, 2004, 2005). KRAS mutations have been observed in colorectal tumours independently of their microsatellite instability (MSI) status. In sporadic MSI CRCs, KRAS mutations are inversely associated to the oncogenic BRAFV600E mutation, the latter occurring in about 40% of the cases (Rajagopalan et al., 2002; Yuen et al., 2002; Lipton et al., 2003; Oliveira et al., 2003; Wang et al., 2003; Domingo et al., 2004; Fransen et al., 2004; Koinuma et al., 2004), suggesting that each mutation can induce similar cellular effects and signal through the same pathway. However, the recent report by Solit et al. (2006), using MEK (a downstream effector of KRAS and BRAF) inhibitors showed that BRAF mutant cell lines responded differently than KRAS mutant ones, raising the possibility that KRAS and BRAF mutant cancer cells might be differentially dependent on signalling mechanisms that involve MEK.
Although BRAF mutations have been observed mainly in sporadic MSI CRC tumours, approximately 5% of microsatellite stable (MSS) CRC cases also show mutations within BRAF gene (Rajagopalan et al., 2002; Yuen et al., 2002; Lipton et al., 2003; Oliveira et al., 2003, 2005; Wang et al., 2003; Fransen et al., 2004; Koinuma et al., 2004). In contrast to sporadic MSI CRC, data on the presence of both KRAS and BRAF oncogenic mutations in sporadic MSS CRC and their relationship with tumour progression are scarce.
In order to understand the putative involvement of alterations in these two genes in the progression of MSS sporadic colorectal carcinoma, we screened KRAS and BRAF mutations in a series of 250 MSS CRCs and 45 lymph node (LN) metastases (from 28 distinct cases), and studied the pathological features of these cases.
We found mutations in at least one of the genes (KRAS–BRAF) in 45.2% (113/250) primary MSS CRCs, which is in accordance with what has been described previously (KRAS–BRAF: 30–50%; KRAS: 27–45%; BRAF: 2–6%) (Oliveira et al., 2003; Deng et al., 2004; Fransen et al., 2004; Nagasaka et al., 2004; Ince et al., 2005; Lubomierski et al., 2005; Samowitz et al., 2005; Velho et al., 2005).
We studied the association between pathological parameters of MSS CRCs and the presence of KRAS–BRAF oncogenic mutations (Table 1).
We observed an association between the presence of KRAS–BRAF mutations and wall invasion (P=0.02). We found that Tis and T1 MSS CRC had either single KRAS or BRAF mutations and never displayed concomitant mutations (Figure 1a). It was previously shown that KRAS activation occurs in the first steps of colorectal carcinoma progression, along the adenoma–carcinoma sequence (Vogelstein et al., 1988; Fearon and Vogelstein, 1990). According to the literature, BRAF mutations were more frequently found in premalignant colon polyps and early rather than in advanced colorectal carcinomas (Rajagopalan et al., 2002; Yuen et al., 2002; Ikehara et al., 2005). Similar observations have been made in other tumour models, namely activating BRAF mutations have been detected in a high proportion of naevi and benign melanocytic skin lesions (Pollock et al., 2003; Yazdi et al., 2003), although in this specific model, activating mutations of BRAF have also been identified in approximately 90% of melanomas (Davies et al., 2002; Kumar et al., 2003). Our results confirm that KRAS and BRAF mutations alone are frequent and alternative in CRCs with no extension through the muscularis propria (Tis and T1), as previously demonstrated for MSI CRC (Rajagopalan et al., 2002; Oliveira et al., 2003). Presumably, in these tumour stages, BRAF mutations do not occur concomitantly with KRAS mutations because their combined signalling is incompatible with proliferation, as an excess of extracellular signal-regulated protein kinase (ERK) signalling could lead cells to stop cycling and differentiate or to entry senescence (Marshall, 1995; Sewing et al., 1997; Woods et al., 1997; Kerkhoff and Rapp, 1998).
The frequency of concomitant KRAS and BRAF mutations increased along with the depth of wall invasion: T2 – 2.8% (1/36), T3 – 3.5% (6/173) and T4 – 9.4% (3/32).
In T2, T3 and T4 CRC, the frequency of KRAS mutations increase either owing to the acquisition of KRAS mutations in BRAF-negative CRC or to the accumulation of KRAS and BRAF mutations. KRAS activation is likely to confer tumour cells a more invasive behaviour. This relationship between the presence of KRAS mutations and increased ability of tumour cells to invade and progress through the wall may be explained by the putative capability of mutant KRAS to (i) disrupt epithelial cell polarity both by destabilizing adherens junctions and by remodelling cell–matrix interactions through the modulation of integrin expression, maturation and activity (Hughes et al., 1997; Yan et al., 1997; Schramm et al., 2000, ii) promote the passage of tumour cells through the epithelial basement membrane by stimulating the expression and/or activation of MMPs and (iii) increase cell motility in stromal tissue through the activation of RHO family of small-GTPases (Yamamoto et al., 1995; Thiery, 2002; Liao et al., 2003). The number of cases with concomitant KRAS and BRAF mutations also increases in advanced carcinomas, suggesting that activation of both genes may cooperate in tumour progression. As for BRAF alone, as its activation is more prominent in early stages rather than in advanced stages of MSS CRC, we can hypothesize that BRAF activation alone is not sufficient to induce cancer progression in a high frequency of MSS CRC. Its activation in few advanced cases suggests that, in these CRCs, BRAF activation may contribute to tumour progression by protecting cells from apoptosis as suggested by Hingorani et al. (2003) and Ikehara et al. (2005).
A significant association was found between the presence of LN metastases and KRAS–BRAF mutations (P=0.02) (Table 1 and Figure 1b). The frequency of KRAS mutations alone was higher in MSS CRC LNpos (44.9% – 53/118) as compared to MSS CRC LNneg (30.3% – 40/132). The number of carcinomas with BRAF mutations alone was low and similar in MSS CRC LNneg (3.8% – 5/132) and MSS CRC LNpos (4.2% – 5/118). When comparing the frequency of cases with concomitant KRAS and BRAF mutations in MSS CRC LNneg (2.3% – 3/132) and MSS CRC LNpos (5.9% – 7/118), we verified that the latter showed a 2.6-fold increase in the frequency of cases with both mutations, suggesting that concomitant activation of BRAF and KRAS may have a synergistic effect in promoting LN metastasis.
The association between the presence of LN metastases and increased mutation frequency was also verified concerning the number of nodes affected (P=0.03). The frequency of concomitant mutations of KRAS and BRAF was higher in N1 (5.2%) and N2 (6.7%) when compared with N0 (2.3%) carcinomas.
Significant associations were also found between the presence of KRAS/BRAF mutations and positivity for distant metastases (P=0.004), owing to an increased frequency of KRAS mutations in cases with distant metastases. These two observations relate KRAS mutations with colorectal tumour invasion and suggest that KRAS activation is likely to be crucial to render tumour cells the ability of moving and invading not only LNs but also distant organs (Vogelstein et al., 1988; Fearon and Vogelstein, 1990; Pretlow, 1995; Pollock et al., 2005). In contrast to what has been observed for KRAS, the frequency of BRAF mutations alone was not different in primary MSS CRC without and with distant metastases.
The frequency of mutated cases (either KRAS or BRAF or both KRAS and BRAF mutations) in LN metastases (82.1% 23/28) was higher in comparison to primary carcinomas (P=0.0002). The detailed analysis of LN metastases showed that all mutated metastases showed KRAS mutations. Our results suggest that the majority of MSS carcinomas need KRAS activation, through mutation, to be able to metastasize and this activation is crucial for neoplastic cells to acquire invasive potential (Schmidt-Kittler et al., 2003; Campbell and Der, 2004; Carter et al., 2004). In contrast, none of the mutated LN metastasis had BRAF mutations as a single event (Figure 2). These results emphasize the role of KRAS activation in metastasis and argues against BRAF activation by itself, as a pivotal genetic event in promoting MSS CRC metastasis.
As 10 of 28 (35.7%) LN metastases harbour concomitant KRAS and BRAF mutations, we can assume that ‘the lethality of this combination’ in Tis and T1 carcinomas can be suppressed by dominant survival factors or subsequent oncogenic activations in more advanced cancers.
In 43.4% (10/23) of the mutated LN metastasis mutations of both genes were detected, reinforcing the idea that activation of both genes is likely to play a synergistic role in LN metastasis. This observation is supported by the results of Solit et al. (2006) which showed that KRAS and BRAF may signal through different signalling pathways and whereas BRAF mutant cells are preferentially reliant on MEK–ERK signalling, KRAS mutant cells have multiple other targets, such as phosphatidylinositol 3′-kinase (PI(3)K) and RalGDS, reducing the requirement for MEK–ERK activation.
In five of 10 LN metastases with concomitant KRAS and BRAF mutations, a similar picture was observed in respective primary tumours; in the remaining five cases, the mutation pattern in LN metastases was different from primary tumours: one case acquired a BRAF mutation (16), two acquired KRAS mutations (17,18), and two acquired mutations in both genes (19,20) (Figure 2).
In five LN metastases (cases 24–28), no KRAS or BRAF mutations were identified, suggesting that alternative pathways are also responsible for colon cancer metastasis (Figure 1b and 2). In cases 24 and 25, KRAS mutations were identified in the corresponding primary tumours. Extra LN metastases for these two cases (three for case 24 and four for case 25) were analysed and found to be also wild type for both KRAS and BRAF in accordance with what was found in the first ones. This observation suggests that populations of carcinoma cells heterogeneous with respect to wild-type and mutant KRAS were probably present in the primary carcinoma, but the metastatic clone derived from a KRAS negative population, as reported previously (Al-Mulla, 1998).
In cases 6, 13 and 22, we analysed multiple metastases per case. Within each case, the same pattern of mutations was observed in the different LNs analysed. In cases 6 and 13, we demonstrated that all independent metastases, as well as the primary tumours, displayed a G12D KRAS mutation demonstrating that a mutation in KRAS occurred in the primary tumour and was maintained in all metastases. In case 22, the primary tumour did not display a KRAS mutation, but the study of five LN metastases showed that all harboured the same G13D KRAS mutation, suggesting that this alteration was acquired before LN invasion and was pivotal for metastasis. Our data is in accordance to Bernards and Weinberg (2002), who suggested that important components of the genotype of metastasis are already implanted early in tumorigenesis, in small primary tumour cells populations that have the ability to dispatch metastatic pioneers to distant sites in the body.
In the present series, we were unable to detect a specific profile of KRAS and BRAF mutations, namely codon affected (KRAS codons 12 and 13; and BRAF codons 600 and 601) nor amino-acid change associated with tumour progression or metastasis, although specific KRAS mutations have been previously correlated with more aggressive tumour phenotypes (Finkelstein et al., 1993; Moerkerk et al., 1994; Span et al., 1996; Andreyev et al., 2001).
Overall, the results obtained in the present study are supported by the report of Ince et al. (2005), which show additional evidence that KRAS and BRAF mutations are related to disease severity and bad prognosis in CRC patients and demonstrate very elegantly that patients with CRC displaying either KRAS (35%) or BRAF (5.6%) or both KRAS and BRAF (0.4%) mutations had worse prognosis, shorter median survival and shorter overall survival than those with wild-type KRAS and BRAF genotype.
The main conclusions of our work are (1) BRAF and KRAS mutations are alternative in Tis and T1 MSS CRCs, (2) During MSS CRC progression and metastasis concomitant mutations at KRAS and BRAF increase, suggesting that activation of both genes is likely to harbour a synergistic effect, (3) None of the metastases harbour BRAF mutations alone, suggesting that KRAS is the pivotal gene in this process.
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Supported by Grants: Portuguese Fundação para a Ciência e Tecnologia (POCTI/SAU-OBS/56921/2004; REEQ/218/SAL/2005); Novartis- Portugal; Spanish Fondo de Investigaciones Sanitarias (FIS 05/0304); SV, CM, AF and AP were supported by fellowships from the Portuguese Fundação para a Ciência e Tecnologia.
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Oliveira, C., Velho, S., Moutinho, C. et al. KRAS and BRAF oncogenic mutations in MSS colorectal carcinoma progression. Oncogene 26, 158–163 (2007). https://doi.org/10.1038/sj.onc.1209758
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