Radiosensitivity and repair of the inactive X-chromosome. Insights from FISH and immunocytogenetics.
The inactive X-chromosome provides a unique opportunity to study the role of transcriptional activity and chromatin condensation in the repair of chromosome damage. We induced chromosome breakage in human lymphocytes with X-rays (1 or 2 Gy) in either G0 and G1 phase of the cell cycle, and in the presence or absence of an inhibitor of double strand break repair, adenine 9-beta-D-arabinofuranoside (Ara-A). Chromosomal aberrations involving the X-chromosome were detected by means of fluorescence in situ hybridization with an X-chromosome specific red painting probe. The activation status of the X-chromosomes involved in the chromosomal aberrations was determined by simultaneous immunocytogenetics with FITC-conjugated antibodies against BrdUrd incorporated at late S-phase to distinguish the late replicating inactive X-chromosome in green-yellow. This multicolor approach allowed us to study and compare breakage and the extent of repair in the active and inactive X-chromosome. Our data indicate that both chromosomes responded with a similar radiosensitivity. This observation was consistent at both X-ray doses and at the two stages of the cell cycle analyzed. However, the number of chromosomal aberrations involving the inactive X-chromosome was increased after repair inhibition with Ara-A. The differential sensitivity to repair inhibition was observed in G0 after 1 Gy and in G1 after 2 Gy. Thus, the activation status of the X-chromosome might be a source of heterogeneity in breakage and repair. These observations suggest that there is heterogeneous repair when the active and the inactive X-chromosomes are compared and that the observed fragility is the result of a compromise between the actual number of breaks induced in each chromosome and their differential processing.
Surrallés
J
J
Department of Radiation Genetics and Chemical Mutagenesis, Leiden University, Leiden, Netherlands. jordi.surralles@blues.uab.es
Natarajan
A T
AT
D001973:Bromodeoxyuridine / Q000276:immunology; D002453:Cell Cycle; D002843:Chromatin; D002869:Chromosome Aberrations; D004260:DNA Repair; D004303:Dosage Compensation, Genetic; D017404:In Situ Hybridization, Fluorescence; D008214:Lymphocytes / Q000648:ultrastructure; D014158:Transcription, Genetic; D014740:Vidarabine / Q000494:pharmacology; D014960:X Chromosome / Q000528:radiation effects*
D002843:Chromatin; D014740:Vidarabine; D001973:Bromodeoxyuridine