title: CDK4 Amplification Reduces Sensitivity to CDK4/6 Inhibition in Fusion-Positive Rhabdomyosarcoma.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
authors: Cancer Molecular Pathology Section, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
authors: Mary E
authors: ME
authors: Olanich
authors: Cancer Molecular Pathology Section, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
authors: Wenyue
authors: W
authors: Sun
authors: Tissue Array Research Program and Applied Molecular Pathology Laboratory, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
authors: Stephen M
authors: SM
authors: Hewitt
authors: Chromosome Pathology Unit, Laboratory of Pathology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
authors: Zied
authors: Z
authors: Abdullaev
authors: Chromosome Pathology Unit, Laboratory of Pathology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland.
authors: Svetlana D
authors: SD
authors: Pack
authors: Cancer Molecular Pathology Section, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland. barrfg@mail.nih.gov.
authors: Frederic G
authors: FG
authors: Barr
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D002471:Cell Transformation, Neoplastic / Q000235:genetics; D002881:Chromosomes, Human, Pair 12; D051358:Cyclin-Dependent Kinase 4 / Q000037:antagonists & inhibitors* / Q000235:genetics*; D051361:Cyclin-Dependent Kinase 6 / Q000037:antagonists & inhibitors*; D004195:Disease Models, Animal; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D050684:E2F Transcription Factors / Q000378:metabolism; D005260:Female; D059585:G1 Phase Cell Cycle Checkpoints / Q000235:genetics; D005784:Gene Amplification; D017353:Gene Deletion; D015870:Gene Expression; D055785:Gene Knockdown Techniques; D006801:Humans; D051379:Mice; D015514:Oncogene Proteins, Fusion / Q000235:genetics*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D000090063:Proto-Oncogene Mas; D034622:RNA Interference; D016160:Retinoblastoma Protein / Q000378:metabolism; D012208:Rhabdomyosarcoma / Q000188:drug therapy / Q000235:genetics* / Q000473:pathology; D047368:Tumor Burden / Q000187:drug effects / Q000235:genetics; D023041:Xenograft Model Antitumor Assays
chemical_list: D050684:E2F Transcription Factors; C000715470:MAS1 protein, human; D015514:Oncogene Proteins, Fusion; D047428:Protein Kinase Inhibitors; D000090063:Proto-Oncogene Mas; D016160:Retinoblastoma Protein; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6