Metastases are the major cause of death from melanoma, a skin cancer which has the fastest rising incidence of any malignancy in the Western world. Molecular pathways that drive melanoblast migration in development are believed to underpin the movement and ultimately the metastasis of melanoma. Here we show that mice lacking P-Rex1, a Rac-specific Rho GTPase guanine nucleotide exchange factor (GEF), have a melanoblast migration defect during development evidenced by a white belly. Moreover, these P-Rex1−/− mice are resistant to metastasis when crossed to a murine model of melanoma. Mechanistically, this is associated with P-Rex1 driving invasion in a Rac-dependent manner. P-Rex1 is elevated in the great majority of human melanoma cell lines as well as tumor tissue. We conclude that P-Rex1 plays an important role in melanoblast migration and cancer progression to metastasis in mice and humans.

Summary During embryogenesis, melanoblasts proliferate and migrate ventrally through the developing dermis and epidermis as single cells. Targeted deletion of Rac1 in melanoblasts during embryogenesis causes defects in migration, cell cycle progression and cytokinesis. Rac1 null cells migrate markedly less efficiently, but surprisingly, global steering, crossing the dermal/epidermal junction and homing to hair follicles occur normally. Melanoblasts navigate in the epidermis using two classes of protrusion: short stubs and long pseudopods. Short stubs are distinct from blebs and are driven by actin assembly, but are independent of Rac1, Arp2/3 complex, myosin or microtubules. Rac1 positively regulates the frequency of initiation of long pseudopods, which promote migration speed and directional plasticity. Scar/WAVE and Arp2/3 complex drive actin assembly for long pseudopod extension, which also depends on microtubule dynamics. Myosin contractility balances the extension of long pseudopods by effecting retraction and allowing force generation for movement through the complex 3D epidermal environment.

Melanoma cells break down lysophosphatidic acid from the environment, creating a chemotactic gradient that the tumor cells then follow; this provides an explanation for the rapid metastasis of melanoma.

KRAS is the most frequent oncogene in non-small cell lung cancer (NSCLC), a molecular subset characterized by historical disappointments in targeted treatment approaches such as farnesyl transferase inhibition, downstream MEK inhibition, and synthetic lethality screens. Unlike other important mutational subtypes of NSCLC, preclinical work supports the hypothesis that KRAS mutations may be vulnerable to immunotherapy approaches, an efficacy associated in particular with TP53 co-mutation. In this review we detail reasons for previous failures in KRASmutant NSCLC, evidence to suggest that KRAS mutation is a genetic marker of benefit from immune checkpoint inhibition, and emerging direct inhibitors of K-Ras which will soon be combined with immunotherapy during clinical development. With signs of real progress in this subgroup of unmet need, we anticipate that KRAS mutant NSCLC will be the most important molecular subset of cancer to evaluate the combination of small molecules and immune checkpoint inhibitors (CPI).

We validate that the baseline presence of ≥5 total CTCs in advanced NSCLC confers a poor prognosis. CTCs from EGFR-mutant NSCLC express epithelial-mesenchymal transition characteristics, not seen in CTCs from patients with KRAS-mutant adenocarcinoma.