RB1 Gene Function
RB Transcriptional Corepressor 1
Overview
RB1 encodes the retinoblastoma protein, the archetypal tumor suppressor that restrains cell cycle progression by sequestering E2F transcription factors. Biallelic loss is required for retinoblastoma and is frequent in many cancers.
Molecular Mechanism
Mechanism Summary
Hypophosphorylated pRb enforces G1 arrest by binding E2F transcription factors and recruiting HDAC complexes to silence S-phase genes. Sequential CDK4/6→CDK2-mediated pRb hyperphosphorylation inactivates pRb and irreversibly commits cells to division.
Step-by-Step Mechanism
In G0/early G1, pRb is hypophosphorylated and binds E2F1/2/3 via its pocket domain, recruiting HDAC1/2 and BRG1 to form a co-repressor complex that silences E2F target genes required for S-phase entry.
E2F1/2/3, now partially active, transcribe cyclin E and CDK2. CDK2–cyclin E hyperphosphorylates pRb at Ser811, Thr821, and Thr826, completing pRb inactivation through electrostatic repulsion of E2F.
Fully hyperphosphorylated pRb releases all bound E2F factors. Free E2F1/2/3 act as transcriptional activators of cyclin A, PCNA, MCM2-7, RRM1/2, and DNA polymerase δ — gene products needed for S-phase progression.
Hyperphosphorylated pRb state is maintained through S, G2, and M phases by sustained CDK2/CDK1 activity. pRb is re-activated by PP1/PP2A-mediated dephosphorylation at mitotic exit, re-establishing G1 repression in daughter cells.
p16INK4a (CDKN2A) inhibits CDK4/6, preventing pRb phosphorylation and locking cells in G1. Loss of RB1 (or CDKN2A deletion, or CDK4 amplification) bypasses the restriction point, explaining why the pRb pathway is disrupted in virtually every human cancer.
Upstream Regulators
Initial pRb phosphorylation at Ser780; first step of restriction point bypass
Hyperphosphorylation completing pRb inactivation; positive feedback loop
Dephosphorylate pRb at mitotic exit, restoring G1 repression
Downstream Targets
Silencing of S-phase gene promoters via HDAC recruitment
Chromatin compaction at E2F targets in G1
Chromatin remodelling to repress E2F gene expression
Key Post-Translational Modifications
Partial HDAC release; E2F partial activation — mid-G1 induction
Complete E2F release; G1/S commitment — irreversible
Reduces E2F binding affinity; facilitates de-repression
Disease Mechanism
Biallelic RB1 inactivation is the founding example of Knudson's two-hit tumour suppressor model. It is essential and sufficient for retinoblastoma development. Somatic RB1 loss is near-universal (~90%) in small cell lung cancer and common in bladder cancer, osteosarcoma, and triple-negative breast cancer. RB1 mutation is the defining biomarker of CDK4/6 inhibitor resistance: without pRb, there is no substrate for the drug to protect.
Database References
Key Pathways
- ·Cell cycle
- ·E2F transcriptional regulation
- ·Senescence
- ·Differentiation
Disease Associations
- ·Retinoblastoma
- ·Osteosarcoma
- ·Small cell lung cancer
- ·Bladder cancer
Research Activity
RB1 is an actively studied target: about 150+ clinical trials that mention it are currently recruiting on ClinicalTrials.gov. Trial activity reflects research interest, not proven benefit — designs, endpoints and populations vary widely.
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Functional Partners
Common Questions About RB1
What does RB1 do?
RB1 encodes pRb, which controls the G1/S restriction point by binding and sequestering E2F transcription factors. Hypophosphorylated pRb actively represses E2F target genes needed for S-phase entry. CDK4/6-cyclin D phosphorylation liberates E2F and commits the cell to division — a point of no return.
What cancers are caused by RB1 mutations?
Biallelic RB1 loss causes retinoblastoma — the first cancer explained by Knudson's two-hit hypothesis. Somatic RB1 inactivation is near-universal in small cell lung cancer (~90%), common in bladder cancer and osteosarcoma, and occurs in approximately 20% of breast and prostate cancers.
Does RB1 status predict response to CDK4/6 inhibitors?
Yes — CDK4/6 inhibitors work by keeping pRb in its active hypophosphorylated state. Tumours with RB1 deletion or mutation are intrinsically resistant to CDK4/6 inhibitors because there is no pRb to protect. RB1 status is therefore a critical predictive biomarker for palbociclib, ribociclib, and abemaciclib.