CDK4 Amplification Reduces Sensitivity to CDK4/6 Inhibition in Fusion-Positive Rhabdomyosarcoma

Clin Cancer Res. 2015 Nov 1;21(21):4947-59. doi: 10.1158/1078-0432.CCR-14-2955. Epub 2015 Mar 25.


Purpose: Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma and includes a PAX3- or PAX7-FOXO1 fusion-positive subtype. Amplification of chromosomal region 12q13-q14, which contains the CDK4 proto-oncogene, was identified in an aggressive subset of fusion-positive RMS. CDK4/6 inhibitors have antiproliferative activity in CDK4-amplified liposarcoma and neuroblastoma, suggesting CDK4/6 inhibition as a potential therapeutic strategy in fusion-positive RMS.

Experimental design: We examined the biologic consequences of CDK4 knockdown, CDK4 overexpression, and pharmacologic CDK4/6 inhibition by LEE011 in fusion-positive RMS cell lines and xenografts.

Results: Knockdown of CDK4 abrogated proliferation and transformation of 12q13-14-amplified and nonamplified fusion-positive RMS cells via G1-phase cell-cycle arrest. This arrest was mediated by reduced RB phosphorylation and E2F-responsive gene expression. Significant differences in E2F target expression, cell-cycle distribution, proliferation, or transformation were not observed in RMS cells overexpressing CDK4. Treatment with LEE011 phenocopied CDK4 knockdown, decreasing viability, RB phosphorylation, and E2F-responsive gene expression and inducing G1-phase cell-cycle arrest. Although all fusion-positive cell lines showed sensitivity to CDK4/6 inhibition, there was diminished sensitivity associated with CDK4 amplification and overexpression. This variable responsiveness to LEE011 was recapitulated in xenograft models of CDK4-amplified and nonamplified fusion-positive RMS.

Conclusions: Our data demonstrate that CDK4 is necessary but overexpression is not sufficient for RB-E2F-mediated G1-phase cell-cycle progression, proliferation, and transformation in fusion-positive RMS. Our studies indicate that LEE011 is active in the setting of fusion-positive RMS and suggest that low CDK4-expressing fusion-positive tumors may be particularly susceptible to CDK4/6 inhibition.

Publication types

  • Research Support, N.I.H., Intramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Line, Tumor
  • Cell Proliferation
  • Cell Transformation, Neoplastic / genetics
  • Chromosomes, Human, Pair 12
  • Cyclin-Dependent Kinase 4 / antagonists & inhibitors*
  • Cyclin-Dependent Kinase 4 / genetics*
  • Cyclin-Dependent Kinase 6 / antagonists & inhibitors*
  • Disease Models, Animal
  • Drug Resistance, Neoplasm / genetics*
  • E2F Transcription Factors / metabolism
  • Female
  • G1 Phase Cell Cycle Checkpoints / genetics
  • Gene Amplification*
  • Gene Deletion
  • Gene Expression
  • Gene Knockdown Techniques
  • Humans
  • Mice
  • Oncogene Proteins, Fusion / genetics*
  • Protein Kinase Inhibitors / pharmacology*
  • Proto-Oncogene Mas
  • RNA Interference
  • Retinoblastoma Protein / metabolism
  • Rhabdomyosarcoma / drug therapy
  • Rhabdomyosarcoma / genetics*
  • Rhabdomyosarcoma / pathology
  • Tumor Burden / drug effects
  • Tumor Burden / genetics
  • Xenograft Model Antitumor Assays


  • E2F Transcription Factors
  • MAS1 protein, human
  • Oncogene Proteins, Fusion
  • Protein Kinase Inhibitors
  • Proto-Oncogene Mas
  • Retinoblastoma Protein
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinase 6