RET fusions in a small subset of advanced colorectal cancers at risk of being neglected.

Background: Recognition of rare molecular subgroups is a challenge for precision oncology and may lead to tissue-agnostic approval of targeted agents. Here we aimed to comprehensively characterize the clinical, pathological and molecular landscape of RET rearranged metastatic colorectal cancer (mCRC). Patients and methods: In this case series, we compared clinical, pathological and molecular characteristics of 24 RET rearranged mCRC patients with those of a control group of 291 patients with RET negative tumors. RET rearranged and RET negative mCRCs were retrieved by systematic literature review and by taking advantage of three screening sources: (i) Ignyta's phase 1/1b study on RXDX-105 (NCT01877811), (ii) cohorts screened at two Italian and one South Korean Institutions and (iii) Foundation Medicine Inc. database. Next-generation sequencing data were analyzed for RET rearranged cases. Results: RET fusions were more frequent in older patients (median age of 66 versus 60 years, P = 0.052), with ECOG PS 1-2 (90% versus 50%, P = 0.02), right-sided (55% versus 32%, P = 0.013), previously unresected primary tumors (58% versus 21%, P < 0.001), RAS and BRAF wild-type (100% versus 40%, P < 0.001) and MSI-high (48% versus 7%, P < 0.001). Notably, 11 (26%) out of 43 patients with right-sided, RAS and BRAF wild-type tumors harbored a RET rearrangement. At a median follow-up of 45.8 months, patients with RET fusion-positive tumors showed a significantly worse OS when compared with RET-negative ones (median OS 14.0 versus 38.0 months, HR: 4.59; 95% CI, 3.64-32.66; P < 0.001). In the multivariable model, RET rearrangements were still associated with shorter OS (HR: 2.97; 95% CI, 1.25-7.07; P = 0.014), while primary tumor location, RAS and BRAF mutations and MSI status were not. Conclusions: Though very rare, RET rearrangements define a new subtype of mCRC that shows poor prognosis with conventional treatments and is therefore worth of a specific management.

Tracking a CAD-ALK gene rearrangement in urine and blood of a colorectal cancer patient treated with an ALK inhibitor.

BACKGROUND: Monitoring response and resistance to kinase inhibitors is essential to precision cancer medicine, and is usually investigated by molecular profiling of a tissue biopsy obtained at progression. However, tumor heterogeneity and tissue sampling bias limit the effectiveness of this strategy. In addition, tissue biopsies are not always feasible and are associated with risks due to the invasiveness of the procedure. To overcome these limitations, blood-based liquid biopsy analysis has proven effective to non-invasively follow tumor clonal evolution. PATIENTS AND METHODS: We exploited urine cell-free, trans-renal DNA (tr-DNA) and matched plasma circulating tumor DNA (ctDNA) to monitor a metastatic colorectal cancer patient carrying a CAD-ALK translocation during treatment with an ALK inhibitor. RESULTS: Using a custom next generation sequencing panel we identified the genomic CAD-ALK rearrangement and a TP53 mutation in plasma ctDNA. Sensitive assays were developed to detect both alterations in urine tr-DNA. The dynamics of the CAD-ALK rearrangement in plasma and urine were concordant and paralleled the patient's clinical course. Detection of the CAD-ALK gene fusion in urine tr-DNA anticipated radiological confirmation of disease progression. Analysis of plasma ctDNA identified ALK kinase mutations that emerged during treatment with the ALK inhibitor entrectinib. CONCLUSION: We find that urine-based genetic testing allows tracing of tumor-specific oncogenic rearrangements. This strategy could be effectively applied to non-invasively monitor tumor evolution during therapy. The same approach could be exploited to monitor minimal residual disease after surgery with curative intent in patients whose tumors carry gene fusions. The latter could be implemented without the need of patient hospitalization since urine tr-DNA can be self-collected, is stable over time and can be shipped at specified time-points to central labs for testing.