Cyclin D-CDK4/6-RB Pathway Dependency
trigger
The tumor's proliferation depends on the cyclin D-CDK4/6-INK4-RB axis to license the G1/S transition. Cyclin D, induced by lineage-specific mitogenic input (estrogen receptor signaling in HR-positive breast cancer, MAPK signaling in melanoma, CCND1 translocation in mantle cell lymphoma), partners with CDK4 or CDK6 to phosphorylate the retinoblastoma protein, releasing E2F transcription factors and committing the cell to S phase. Loss of the INK4 brake (CDKN2A/p16 deletion) or cyclin D/CDK4-6 amplification deepens this dependency. A tumor that has retained functional RB and depends on this axis is the population in which CDK4/6 inhibition is expected to work; this is the module trigger.
Used by disorders
Chordoma
as Cell-Cycle Dysregulation via CDKN2A Loss
Downstream
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Pharmacologic CDK4/6 Inhibition and G1 Arrest
Because proliferation is routed through CDK4/6, the axis is druggable and selective inhibitors impose a G1 block on dependent tumor cells.
Show evidence (2 references)
"Deregulation of the cyclin D-CDK4/6-INK4-RB pathway leading to uncontrolled cell proliferation, is frequently observed in breast cancer."
Establishes deregulation of the cyclin D-CDK4/6-INK4-RB axis as the recurrent lesion that creates the dependency this module is triggered by. Evidence source is OTHER because this is a review article.
"Cyclin-dependent kinase 4 (CDK4) and CDK6 are critical mediators of cellular transition into S phase and are important for the initiation, growth and survival of many cancer types."
Confirms that CDK4/CDK6 control the S-phase transition across many tumor types, supporting the cross-cancer scope of the trigger node. Evidence source is OTHER because this is a review article.
Pharmacologic CDK4/6 Inhibition and G1 Arrest
amplifier
A selective CDK4/6 inhibitor (palbociclib, ribociclib, or abemaciclib) occupies the ATP pocket of CDK4 and CDK6, preventing cyclin D-CDK4/6 from phosphorylating RB. RB stays hypophosphorylated, E2F remains sequestered, and sensitive tumor cells arrest in G1. This is the therapeutic response node — the intended, on-target effect — and it is also the selective pressure that shapes everything downstream in this module.
Downstream
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Cell-Cycle Bypass Lesion Selection
Sustained drug pressure selects for pre-existing or newly acquired lesions in the cell-cycle machinery downstream of the drug target.
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Upstream Bypass Signaling Reactivation
Sustained drug pressure also selects for signaling rewiring that restores cyclin-CDK activity without requiring the inhibited CDK4/6 kinases.
Show evidence (1 reference)
"As expected, CDK4/6 inhibitors arrest sensitive tumour cells in the G1 phase of the cell cycle."
States the on-target pharmacodynamic effect (G1 arrest of sensitive cells) that defines this node. Evidence source is OTHER because this is a review article.
Cell-Cycle Bypass Lesion Selection
effector
Under CDK4/6-inhibitor pressure the tumor selects lesions in the cell-cycle machinery that make the drug target dispensable. Three recurrent classes are documented: (1) loss-of-function alteration of RB1 itself, which removes the substrate the drug protects and is the only class so far confirmed to be positively selected in a randomized clinical trial; (2) cyclin E1 (CCNE1) overexpression, which activates CDK2 to phosphorylate RB independently of CDK4/6 and is a validated clinical resistance biomarker; and (3) amplification/overexpression of CDK6 itself, which raises the target above achievable drug exposure. Conforming disorder entries substitute whichever lesion their tumor type acquires.
Downstream
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RB Pathway Bypass and E2F-Driven S-Phase Re-entry
RB1 loss, CDK2/cyclin E1 activity, or excess CDK6 all restore RB phosphorylation or make it unnecessary, releasing E2F despite the drug.
Show evidence (3 references)
"These observations support the emergence of RB1 aberrations being acquired or selected under pressure from palbociclib, but only in a minority of patients (6/127, 4.7%)."
Paired baseline/end-of-treatment circulating-tumor-DNA sequencing in the randomized PALOMA-3 trial demonstrates positive selection of RB1 loss-of-function under palbociclib, establishing this node clinically — but supports only PARTIALLY, because the same analysis shows the event is subclonal and confined to a small minority, so RB1 loss cannot be curated as the dominant clinical resistance mechanism.
"Addition of palbociclib to fulvestrant demonstrated efficacy in all biomarker groups, although high CCNE1 mRNA expression was associated with relative resistance to palbociclib."
Prospective gene-expression analysis of the randomized PALOMA-3 trial, independently validated in the POP trial, links high cyclin E1 expression to relative palbociclib resistance, supporting the CDK2/cyclin E1 bypass lesion class.
"Knockdown of CDK6 restored drug sensitivity, while enforced overexpression of CDK6 was sufficient to mediate drug resistance."
Gain- and loss-of-function experiments in chronically abemaciclib-exposed breast cancer cell lines establish CDK6 amplification/overexpression as a sufficient bypass lesion. Evidence source is IN_VITRO because the experiments were performed in cultured cell lines.
Upstream Bypass Signaling Reactivation
effector
In parallel with lesions in the core cell-cycle machinery, tumors rewire upstream signaling so that cyclin-CDK activity is restored without the inhibited kinases. Loss of the FAT1 tumor suppressor deregulates Hippo signaling, allowing YAP/TAZ to accumulate on the CDK6 promoter and markedly elevate CDK6; alterations in other Hippo components act the same way. PI3K-AKT-mTOR, FGFR, and RAS-MAPK reactivation similarly sustain cyclin D and D-type-cyclin-independent proliferative drive. Reviews of the field group these with the cell-cycle lesions as "upstream response" versus "downstream bypass" mechanisms.
Downstream
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RB Pathway Bypass and E2F-Driven S-Phase Re-entry
Restored upstream drive raises cyclin-CDK activity above the level the inhibitor can suppress, re-phosphorylating RB.
Show evidence (3 references)
"These findings uncover a tumor suppressor function of Hippo signaling in ER+ breast cancer and establish FAT1 loss as a mechanism of resistance to CDK4/6i."
Conclusion of a genomic analysis of 348 estrogen-receptor-positive breast cancers treated with CDK4/6 inhibitors, establishing FAT1 loss as a clinically observed resistance mechanism acting through Hippo signaling.
"The induction of CDK6 was mediated by the Hippo pathway with accumulation of YAP and TAZ transcription factors on the CDK6 promoter."
Defines the mechanism by which FAT1 loss produces CDK4/6-inhibitor resistance — Hippo-pathway deregulation driving YAP/TAZ-dependent CDK6 induction — which is the archetype of upstream bypass signaling in this node. Evidence source is IN_VITRO because the promoter-occupancy and rescue experiments were performed in cell lines and xenografts.
"The cell signaling pathways involved in therapy resistance are divided into two groups: upstream response mechanisms and downstream bypass mechanisms."
Supports the module's two-branch architecture — upstream signaling reactivation and downstream cell-cycle bypass — as the accepted organizing framework for CDK4/6-inhibitor resistance. Evidence source is OTHER because this is a review article.
RB Pathway Bypass and E2F-Driven S-Phase Re-entry
central effector
The convergent, rate-limiting node of the module. Whether the tumor lost RB1, gained CDK2/cyclin E1 activity, amplified CDK6, or rewired upstream signaling, the common endpoint is the same: the RB brake that the drug was holding in place is bypassed, E2F transcriptional activity is restored, and cells re-enter S phase despite continued CDK4/6 inhibition. This is the key conformance target — a conforming disorder node should describe its own tumor-specific route into this state, not a new endpoint.
Downstream
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Acquired CDK4/6 Inhibitor Resistance and Tumor Progression
Resumed proliferation under therapy is expressed clinically as loss of disease control and radiographic progression.
Show evidence (2 references)
"Overactivation of the cyclin-CDK-Rb axis related to uncontrolled cell proliferation is the main cause of CDK4/6 inhibitor resistance; however, the underlying mechanisms need to be clarified further."
Identifies reactivation of the cyclin-CDK-RB axis as the convergent cause of CDK4/6-inhibitor resistance, which is exactly the rate-limiting node modeled here. Evidence source is OTHER because this is a review article.
"These results from the unbiased search support and confirm the critical role of E2F transcriptional activity, and CCNE1 mRNA in particular, in determining the relative clinical efficacy from the addition of palbociclib to fulvestrant."
Panel-wide expression analysis of a randomized trial shows E2F transcriptional output determines clinical benefit from CDK4/6 inhibition, supporting restored E2F-driven S-phase entry as the node that decides response versus resistance.
Acquired CDK4/6 Inhibitor Resistance and Tumor Progression
consequence
The clinical consequence: tumor cells proliferate despite continued CDK4/6 inhibition, progression-free survival benefit is lost, and the disease progresses. Innate (de novo) resistance produces the same endpoint without an initial response. This node is where conforming disorder entries attach their disease-specific progression phenotypes and post-progression treatment strategies.
Show evidence (1 reference)
"The efficacy of this strategy is, however, limited by innate or acquired resistance mechanisms and its application to other tumor types is still uncertain."
States the clinical endpoint of the module — innate or acquired resistance limiting the efficacy of CDK4/6 inhibition — and flags the uncertain extension beyond breast cancer. Evidence source is OTHER because this is a review article.