SUMMARY: ColoRectal Cancer (CRC) is the third most common cancer diagnosed in both men and women in the United States. The American Cancer Society estimates that approximately 151,030 new cases of CRC will be diagnosed in the United States in 2022 and about 52,580 patients are expected to die of the disease. The lifetime risk of developing CRC is about 1 in 23.
Approximately 15-25% of the patients with CRC present with metastatic disease at the time of diagnosis (synchronous metastases) and 50-60% of the patients with CRC will develop metastatic disease during the course of their illness. Advanced colon cancer is often incurable and standard chemotherapy when combined with anti EGFR (Epidermal Growth Factor Receptor) targeted monoclonal antibodies such as VECTIBIX® (Panitumumab) and ERBITUX® (Cetuximab) as well as anti VEGF agent AVASTIN® (Bevacizumab), have demonstrated improvement in Progression Free Survival (PFS) and Overall Survival (OS). The benefit with anti EGFR agents however is only demonstrable in patients with metastatic CRC (mCRC) whose tumors do not harbor KRAS mutations in codons 12 and 13 of exon 2 (KRAS Wild Type). It is now also clear that even among the KRAS Wild Type patient group about 15-20% have other rare mutations such as NRAS and BRAF mutations, which confer resistance to anti EGFR agents. Patients with Stage IV CRC are now routinely analyzed for extended RAS and BRAF mutations. KRAS mutations are predictive of resistance to EGFR targeted therapy.
Approximately 8-15% of all metastatic CRC tumors present with BRAF V600E mutations, which is recognized as a marker of poor prognosis in this patient group. These patients predominantly present with right-sided proximal tumors, tend to have aggressive disease with a higher rate of peritoneal metastasis, and do not respond well to standard treatment intervention. Approximately 30% of the BRAF-mutated population in the metastatic setting has MSI-High tumors, but MSI-High status does not confer protection to this patient group. Further, in striking contrast to patients with melanoma harboring BRAF V600E mutations in whom there is a 70% Objective Response Rate with BRAF inhibitor monotherapy, there is little or no clinical benefit with the same treatment among BRAF V600E mutant CRC patients.
Preclinical studies have shown that inhibiting BRAF in colorectal tumors can transiently reduce Mitogen-Activated Protein (MAP) kinase signaling. However, this can result in feedback upregulation of EGFR signaling pathway, which can then reactivate the MAP kinase pathway. This aberrant signaling can be blocked by dual inhibition of both BRAF and EGFR. In the Phase III BEACON Colorectal Cancer study, a combination of BRAF inhibitor BRAFTOVI® (Encorafenib) and EGFR antagonist ERBITUX® (Cetuximab), with or without concomitant MEK inhibition improved Response Rates, Overall Survival and Progression Free Survival in patients with metastatic CRC with a BRAF V600E mutation. The FDA approved this doublet therapy in 2020 for this patient group. Despite this improved efficacy, a significant percentage of patients do not respond this therapy and among those who respond, the responses noted in CRC are still not as robust as has been in BRAF-mutant metastatic melanomas treated with anti-BRAF therapy. This suggests that there may be other factors modulating treatment response, including molecular determinants, that need to be identified, to optimize clinical management of these patients.
BRAF V600E mutated tumors in CRC are also associated with specific molecular features, including a low frequency of APC mutations and a high rate of mutations in the tumor suppressor gene RNF43 (Ring Finger Protein 43). RNF43 is a E3 ubiquitin ligase which negatively regulates Wnt signaling by inducing degradation of the Wnt receptors. It has been postulated that the a cross-talk between the MAPK and WNT signaling pathways may modulate the antitumor activity of anti-BRAF/EGFR therapy.
The researchers in this study sought to explore which genes were enriched for somatic mutations in responder and non-responder groups, among patients with BRAF V600E mutant CRC, treated with anti-BRAF/EGFR combination therapy. This study included 166 patients (N=166) with BRAF V600E mutant CRC of whom 98 patients received treatment with anti-BRAF/EGFR combination therapy (N=46 in the Discovery cohort and N=52 in the Validation cohort). The Control cohort (N=68) consisted of BRAF V600E mutant CRC patients treated with chemotherapy with or without antiangiogenic therapy, and were not exposed to anti-BRAF therapy. Whole-Exome Sequencing (WES) and/or targeted gene sequencing was performed on baseline tumor and/or plasma cell-free DNA (cfDNA) samples of all included patients, and over 20,000 genes were analyzed.
It was noted that RNF43 mutations were identified in 29% of BRAF V600E-mutated MicroSatellite-Stable (MSS) metastatic CRC tumors, and this finding was strongly associated with a clinical response to anti-BRAF/EGFR-based combination therapy. When compared to BRAF V600E-mutated, MicroSatellite-Stable metastatic CRC patients without the RNF43 mutation (RNF43 wild-type), patients with BRAF V600E-mutated, MicroSatellite-Stable metastatic CRC carrying a RNF43 mutation had a Response Rate of 72.7% versus 30.8% (P=0.03), longer median Progression Free Survival (10.1 months versus 4.1 months, HR=0.30; P=0.01) and longer median Overall Survival (13.6 months versus 7 months, HR=0.26; P=0.008). Conversely, the predictive value of RNF43 mutations seen in MicroSatellite-Stable tumors was not observed in MicroSatellite Instability (MSI)-High tumors.
The researchers concluded that these findings suggest that RNF43 may be a potential stratification biomarker that could help with decision making, in patients with MicroSatellite-Stable, BRAF V600E–mutant metastatic Colorectal cancer. They added that RNF43 gene may be a predictive biomarker of a response to treatment with anti-BRAF/EGFR combination therapy in this patient group.
RNF43 mutations predict response to anti-BRAF/EGFR combinatory therapies in BRAF V600E metastatic colorectal cancer. Elez, E, Ros J, Fernandez J, et al. Nature Medicine 2022;28:2162–2170.
