title: The effects of G2-phase enrichment and checkpoint abrogation on low-dose hyper-radiosensitivity.
abstract: OBJECTIVE
  An association between low-dose hyper-radiosensitivity (HRS) and the "early" G2/M checkpoint has been established. An improved molecular understanding of the temporal dynamics of this relationship is needed before clinical translation can be considered. This study was conducted to characterize the dose response of the early G2/M checkpoint and then determine whether low-dose radiation sensitivity could be increased by synchronization or chemical inhibition of the cell cycle.
  
  
  METHODS
  Two related cell lines with disparate HRS status were used (MR4 and 3.7 cells). A double-thymidine block technique was developed to enrich the G2-phase population. Clonogenic cell survival, radiation-induced G2-phase cell cycle arrest, and deoxyribonucleic acid double-strand break repair were measured in the presence and absence of inhibitors to G2-phase checkpoint proteins.
  
  
  RESULTS
  For MR4 cells, the dose required to overcome the HRS response (approximately 0.2 Gy) corresponded with that needed for the activation of the early G2/M checkpoint. As hypothesized, enriching the number of G2-phase cells in the population resulted in an enhanced HRS response, because a greater proportion of radiation-damaged cells evaded the early G2/M checkpoint and entered mitosis with unrepaired deoxyribonucleic acid double-strand breaks. Likewise, abrogation of the checkpoint by inhibition of Chk1 and Chk2 also increased low-dose radiosensitivity. These effects were not evident in 3.7 cells.
  
  
  CONCLUSIONS
  The data confirm that HRS is linked to the early G2/M checkpoint through the damage response of G2-phase cells. Low-dose radiosensitivity could be increased by manipulating the transition of radiation-damaged G2-phase cells into mitosis. This provides a rationale for combining low-dose radiation therapy with chemical synchronization techniques to improve increased radiosensitivity.
authors.affiliation: Department of Radiation Oncology, William Beaumont Hospital, Royal Oak, Michigan 48073, USA.
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Krueger
authors.forename: George D
authors.initials: GD
authors.lastname: Wilson
authors.forename: Evano
authors.initials: E
authors.lastname: Piasentin
authors.forename: Michael C
authors.initials: MC
authors.lastname: Joiner
authors.forename: Brian
authors.initials: B
authors.lastname: Marples
mesh_terms: D000818:Animals; D064007:Ataxia Telangiectasia Mutated Proteins; D002453:Cell Cycle / Q000502:physiology / Q000528:radiation effects; D018797:Cell Cycle Proteins / Q000378:metabolism; D002460:Cell Line; D002470:Cell Survival; D000071877:Checkpoint Kinase 1; D064447:Checkpoint Kinase 2; D003114:Colony-Forming Units Assay / Q000379:methods; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair; D004268:DNA-Binding Proteins / Q000378:metabolism; D004307:Dose-Response Relationship, Radiation; D016195:G2 Phase / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D006657:Histones / Q000032:analysis; D018499:Linear Energy Transfer; D016014:Linear Models; D008938:Mitosis / Q000502:physiology / Q000528:radiation effects; D010766:Phosphorylation; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D011829:Radiation Dosage; D011836:Radiation Tolerance / Q000502:physiology*; D051381:Rats; D016196:S Phase / Q000187:drug effects / Q000502:physiology; D013936:Thymidine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D025521:Tumor Suppressor Proteins / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D006657:Histones; D025521:Tumor Suppressor Proteins; D011494:Protein Kinases; D064447:Checkpoint Kinase 2; D064007:Ataxia Telangiectasia Mutated Proteins; D000071877:Checkpoint Kinase 1; C000605871:Chek1 protein, rat; C578632:Chek2 protein, rat; D017346:Protein Serine-Threonine Kinases; D013936:Thymidine