RB constrains lineage fidelity and multiple stages of tumour progression and metastasis

Mutations in the Retinoblastoma (RB) tumour suppressor pathway are a hallmark of cancer and a prevalent feature of lung adenocarcinoma(1,2,3). Despite being the first tumour suppressor to be identified, the molecular and cellular basis underlying selection for persistent RB loss in cancer remains unclear(4–6). Methods that reactivate the RB pathway using inhibitors of cyclin-dependent kinases CDK4 and CDK6 are effective in some cancer types and currently under evaluation in lung adenocarcinoma(7–9). Whether RB pathway reactivation will have therapeutic effects and if targeting CDK4/6 is sufficient to reactivate RB pathway activity in lung cancer is unknown. Here, we model RB loss during lung adenocarcinoma progression and pathway reactivation in established oncogenic KRAS-driven tumours in the mouse. We show that RB loss enables cancer cells to bypass two distinct barriers during tumour progression. First, RB loss abrogates the requirement for MAPK signal amplification during malignant progression. We identify CDK2-dependent phosphorylation of RB as an effector of MAPK signalling and critical mediator of resistance to CDK4/6 inhibition. Second, RB inactivation deregulates expression of cell state-determining factors, facilitates lineage infidelity, and accelerates the acquisition of metastatic competency. In contrast, reactivation of RB reprograms advanced tumours toward a less metastatic cell state, but is nevertheless unable to halt cancer cell proliferation and tumour growth due to adaptive rewiring of MAPK pathway signalling, which restores a CDK-dependent suppression of RB. Our study demonstrates the power of reversible gene perturbation approaches to identify molecular mechanisms of tumour progression, causal relationships between genes and the tumour suppressive programs they control, and critical determinants of successful therapy.