Extended Data Fig. 10: RB controls multiple barriers to tumour progression and is repressed by multiple pathways that will require multiple pharmacological interventions to reverse.

a, In the KP model, adenomas transit through an ‘early barrier’ that limits progression to the carcinoma state by amplifying the MAPK signalling cascade (red). The ‘late barrier’ limits the onset of metastatic ability and is characterized by the loss of lineage fidelity marked by lost expression of lineage-specific transcription factors, NKX2-1 (blue) and FOXA2 (purple), and the differentiation marker of alveolar type 2 cells, SPC (green). Loss of these lineage commitment factors precedes the derepression of the embryonic-restricted chromatin factor HMGA2 that functionally drives metastasis and marks the metastatic cell state (yellow). Downregulation of p16^INK4a expression is associated with the metastatic cell state (grey). b, The additional suppression of Rb1 in the KP;Rb^TR/TR model alters the molecular trajectory of these tumours by first abrogating the early barrier through the elimination of the requirement for amplification of the MAPK signal (lack of red) and then by facilitating loss of lineage fidelity to overcome and blur the late barrier. Carcinomatous KP;Rb^TR/TR tumours can rapidly derepress HMGA2 (yellow) and lose the lineage identity marker SPC (green); however, loss of lineage fidelity is unlinked from NKX2-1 and FOXA2, which normally enforce lung cell identity in these tumours. Notably, metastatic primary tumours and distant metastases can sometimes maintain expression of NKX2-1 (blue) and FOXA2 (purple). Expression of p16^INK4a is maintained in RB-deficient metastatic cell states (grey). c, In lung adenomas that maintain RB, RB tumour suppressor activity blocks progression to carcinomatous stages and the onset of metastatic cell states and enforces lineage fidelity. d, In lung adenocarcinomas that maintain RB expression, amplification of the MAPK signal activates CDK2-dependent hyperphosphorylation of RB to promote carcinoma progression. e, Inactivation of RB removes early barriers that limit carcinoma progression, removes constraints that reinforce lineage fidelity and disrupts late barriers that suppress metastatic competency. f, Reactivation of RB in tumours that lack Rb pathway function highlights a need for a multi-pronged approach to inhibit CDK4/6 as well as CDK2 and/or MAPK pathway signalling (for example, through MEK inhibition) to fully reactivate RB-mediated tumour suppression. These data emphasize the need for the development of selective CDK2 inhibitors.