mutations in codons 12 and 13 of kirsten rat sarcoma viral oncogene homolog ( kras ) and b - raf murine sarcoma viral oncogene homolog b1 ( braf ) genes are frequently present in tumors of patients with metastatic colorectal cancer ( crc ) ; they are observed in 30 - 45% and 5 - 20% of cases respectively 1 . less commonly detected crc tumor mutations are codon 61 , 146 of the kras oncogene , and at other sites in the braf gene . kras and braf mutations have been frequently described as mutually exclusive in crc , in that tumors usually have neither , or only one , of these specific mutations . rarely patients may have crc with coincident kras and braf mutations ; herein we report three such cases and review of the literature . although highly uncommon , identification of this dual tumor genotype is important , as current clinical trials are being designed enriching for patients whose tumors harbor either a kras or braf mutation , but haven't addressed those patients with coincident kras and braf mutations . due to the infrequent rate of coincident mutation , it is not known if these patients have a tumor biology distinct from kras or braf mutant tumors . under an institutional review board approved retrospective chart review protocol ( dr-11 - 0113 ) , we reviewed samples tested at a clinical laboratory for tumor kras and/or braf mutations from 2008 - 2011 . dna was extracted from microdissected paraffin - embedded tumor and analyzed by a polymerase chain reaction ( pcr)-based dna sequencing method to examine codons 12 , 13 and 61 of the kras proto - oncogene . the sensitivity of detection of this assay is approximately 1 in 10 mutation - bearing cells in microdissected area . dna was also analyzed by pcr and a pyrosequencing method for codons 595 to 600 of exon 16 of braf oncogene ; the methods for these assays have been described elsewhere 2 . a total 6,633 crc patient tumors were screened for mutation analyses and of those 1,483 case were tested for both kras and braf mutations . 644 were kras - mutant ( 43% ) and 3 ( 0.2% ) of these were bearing a concomitant braf ( v600e ) mutation . the first patient is a 65 year - old woman who presented with small bowel obstruction . the patient underwent a biopsy of the metastatic liver lesion that was sent for kras mutation and braf mutation analysis ; a tumor mutation was detected in codon 12 ( ggt to gat ) of the kras gene that would change the encoding amino acid from glycine to aspartic acid ( g12d ) and codon 600 ( gtg to gag ) in exon 15 of the braf gene that would change the encoding amino acid from valine to glutamine ( v600e ) . the second case identified was a 72 year - old woman with rectal cancer metastatic to the lungs ; similar tumor mutations were detected in kras ( g12d ) and braf ( v600e ) proto - oncogenes . in both cases the tumors were microsatellite stable ; concomitant braf and kras mutations were not identified among patients samples ' whose tumors were microsatellite instable . the third case is 51 year - old man who presented with abdominal pain and was found to have a descending colon cancer with liver metastasis . in specimen from liver lesion , coincident kras and braf mutations appear to be rare entity ( present in 3 out of over 1,928 samples tested ) . due to the infrequent observation of this phenomena , it is not clear whether or not these tumors have a different biology and natural history than kras or braf mutant tumors , or which of the two mutations is the dominant oncogene driving tumor proliferation . a prior study has demonstrated that concomitant tumor mutations have been relatively frequently observed human crc cell line ; in this study , of 24 human crc cell lines , one cell line ( gp2d ) demonstrated mutations in both kras and braf oncogenes ( however the braf mutation was in the less frequently observed at the codon 529 site ) 3 . in an analysis of 250 patients tumor specimens with microsatellite stable disease , kras mutations were identified in 45.2% of primary tumors , and that concomitant kras and braf mutations were found in 10 lymph nodes ( 35.7% ) from a total 28 samples whereas only 3 of primary tumor with negative lymph nodes were positive for coincident mutation ( 2.3%)4 . there was also noted to be a higher proportion of concomitant mutations based on the degree of transmural penetration of the tumor ; 1/36 ( 2.8% ) for t2 tumors and 3/32 ( 9.4% ) for t4 tumors , suggesting activation of both genes is associated with progression of disease . since kras mutation testing became standard - of - care for determining lack of treatment efficacy with antibody anti - epidermal growth factor ( egfr ) monoclonal antibody therapy in patients with advanced crc , large studies involving kras and braf mutation analysis crc screenings have been undertaken ; the results of studies with over 200 patients is demonstrated in table 1 . unlike study by olivera et al 4 , all of these large data sets demonstrated that in crc kras and braf mutations were mutually exclusive , except in one case 5 . in one other series , two patients had tumors with two distinct kras mutations 6 . molecular profiling demonstrates that kras and braf mutant tumors have very different gene signatures suggesting different signaling pathways are activated 12 . in addition early data suggest outcomes with the use of anti - egfr monoclonal antibody therapy appear to be different depending on the site of kras mutation 13 . at this time it is not known which gene expression profile pattern of concomitant kras and braf tumors is , and whether it more represents a kras tumor gene signature or braf signature . given fact of tumor heterogeneity and the large variety of mutations observed in this disease , as well as epigenetic changes observed , it is likely that in the future , tumor gene expression profiling will provide a better indication of the activation of tumor signaling , and be a better biomarker of treatment efficacy . strengths of our study are screened large patient population and available detailed clinical and pathological information . the main limitation , however , were retrospective nature of study and possible artefactual mutation detection . further multi - center prospective studies are necessary to understand true frequency and the role concomitant kras and braf mutations . concomitant kras and braf tumor mutations are rare enough to be considered virtually ( albeit not entirely ) mutually exclusive . since kras and microsatellite instability analysis are the only validated negative biomarkers of therapy efficacy in crc , routine analysis for braf mutations in kras wild type tumors is not recommended . however if patients are being considered for clinical trial , especially those in which the eligibility require the presence of a kras mutation , braf mutation analysis would be highly advisable . future enriched clinical trials for kras or braf should specifically address eligibility of patients whose tumors harbor a concomitant kras and braf mutation .
kras mutations occur frequently in colorectal cancers ( crc ) and predict lack of response to anti - epidermal growth factor receptor ( egfr ) monoclonal antibody therapy . 
 crc braf mutations , most commonly at v600e , occur less than 10% of the time , and occur usually in kras wild - type tumors , and more frequently in microsatellite instable tumors . 
 concomitant kras and braf mutant crcs are rare ( occurring in 0.001% ) ; braf mutations should not be routinely tested in patients with kras mutant tumors , unless the patients is participating in a clinical trial enriching for the presence of a kras or braf tumor . 
 clinical trials treating patients with either kras or braf mutant tumors should address eligibility of patients with concomitant kras and braf mutations .