High Tumor Mutational Burden Correlates with Longer Survival in Immunotherapy-Naïve Patients with Diverse Cancers.

Higher tumor mutational burden (TMB) has been correlated with response to checkpoint blockade immunotherapy. However, it is unclear whether TMB independently serves as a prognostic biomarker for outcomes in immunotherapy-naïve patients. Here, we evaluated the relationship between TMB and overall survival in 1,415 immunotherapy-naïve patients with diverse advanced malignancies. TMB was studied both as a tiered variable (low ≤5 mutations/Mb, intermediate >5 and <20, high ≥20 and <50, and very high ≥50) and as a continuous variable. Interestingly, we observed a parabolic correlation between TMB and overall survival, in which intermediate-range TMB correlated with decreased survival, whereas low and very high TMB correlated with improved outcomes (median survival: 238, 174, 195, and 350 weeks for low, intermediate, high, and very high TMB, respectively; multivariate P < 0.01). This corresponded to an HR of 1.29 (95% confidence interval, 1.07-1.54; P < 0.01) for intermediate-range TMB on multivariable survival analysis correcting for known confounders, including primary tumor of origin. These results demonstrate that TMB may have utility as a prognostic biomarker in immunotherapy-naïve patients, with a protective effect at higher TMBs, and that studies of survival in immunotherapy-treated patients may need to stratify or randomize by TMB in a nonlinear fashion to account for this confounding.

Prognostic implications of RAS alterations in diverse malignancies and impact of targeted therapies.

RAS alterations are often found in difficult-to-treat malignancies and are considered "undruggable." To better understand the clinical correlates and coaltered genes of RAS alterations, we used targeted next-generation sequencing (NGS) to analyze 1,937 patients with diverse cancers. Overall, 20.9% of cancers (405/1,937) harbored RAS alterations. Most RAS-altered cases had genomic coalterations (95.3%, median: 3, range: 0-51), often involving genes implicated in oncogenic signals: PI3K pathway (31.4% of 405 cases), cell cycle (31.1%), tyrosine kinase families (21.5%) and MAPK signaling (18.3%). Patients with RAS-altered versus wild-type RAS malignancies had significantly worse overall survival (OS; p = 0.02 [multivariate]), with KRAS alterations, in particular, showing shorter survival. Moreover, coalterations in both RAS and PI3K signaling or cell-cycle-associated genes correlated with worse OS (p = 0.004 and p < 0.0001, respectively [multivariate]). Among RAS-altered patients, MEK inhibitors alone did not impact progression-free survival (PFS), while matched targeted therapy against non-MAPK pathway coalterations alone showed a trend toward longer PFS (vs. patients who received unmatched therapy) (HR: 0.79, 95% CI: 0.61-1.03, p = 0.07). Three of nine patients (33%) given tailored combination therapies targeting both MAPK and non-MAPK pathways achieved objective responses. In conclusion, RAS alterations correlated with poor survival across cancers. The majority of RAS alterations were accompanied by coalterations impacting other oncogenic pathways. MEK inhibitors alone were ineffective against RAS-altered cancers while matched targeted therapy against coalterations alone correlated with a trend toward improved PFS. A subset of the small number of patients given MEK inhibitors plus tailored non-MAPK-targeting agents showed responses, suggesting that customized combinations warrant further investigation.

Pancreatic mesenchyme regulates epithelial organogenesis throughout development.

The developing pancreatic epithelium gives rise to all endocrine and exocrine cells of the mature organ. During organogenesis, the epithelial cells receive essential signals from the overlying mesenchyme. Previous studies, focusing on ex vivo tissue explants or complete knockout mice, have identified an important role for the mesenchyme in regulating the expansion of progenitor cells in the early pancreas epithelium. However, due to the lack of genetic tools directing expression specifically to the mesenchyme, the potential roles of this supporting tissue in vivo, especially in guiding later stages of pancreas organogenesis, have not been elucidated. We employed transgenic tools and fetal surgical techniques to ablate mesenchyme via Cre-mediated mesenchymal expression of Diphtheria Toxin (DT) at the onset of pancreas formation, and at later developmental stages via in utero injection of DT into transgenic mice expressing the Diphtheria Toxin receptor (DTR) in this tissue. Our results demonstrate that mesenchymal cells regulate pancreatic growth and branching at both early and late developmental stages by supporting proliferation of precursors and differentiated cells, respectively. Interestingly, while cell differentiation was not affected, the expansion of both the endocrine and exocrine compartments was equally impaired. To further elucidate signals required for mesenchymal cell function, we eliminated ß-catenin signaling and determined that it is a critical pathway in regulating mesenchyme survival and growth. Our study presents the first in vivo evidence that the embryonic mesenchyme provides critical signals to the epithelium throughout pancreas organogenesis. The findings are novel and relevant as they indicate a critical role for the mesenchyme during late expansion of endocrine and exocrine compartments. In addition, our results provide a molecular mechanism for mesenchymal expansion and survival by identifying ß-catenin signaling as an essential mediator of this process. These results have implications for developing strategies to expand pancreas progenitors and ß-cells for clinical transplantation.