A DNA DOUBLE-STRAND BREAK DEFECTIVE FIBROBLAST CELL-LINE (180BR) DERIVED FROM A RADIOSENSITIVE PATIENT REPRESENTS A NEW MUTANT PHENOTYPE

The 180BR cell line was derived from an acute lymphoblastic leukemia p atient who overresponded to radiation therapy and died following radia tion morbidity. 180BR cells are hypersensitive to the lethal effects o f ionizing radiation and are defective in the repair of DNA double-str and breaks (DSBs). The levels and activity of the proteins of the DNA- dependent protein kinase complex are normal in 180BR cells. To facilit ate a measurement of V(D)J recombination, we have characterized 180BRM , a SV40-transformed line derived from 180BR, 180BRM retains the radio sensitivity and defect in DSB repair characteristic of 180BR. The acti vities associated with DNA-dependent protein kinase are also normal in 180BRM cells. The ability to carry out V(D)J recombination is compara ble in 180BRM and a reference control transformed human cell line, MRC 5V1. These results show that 180BR and 180BRM differ from the rodent m utants belonging to ionizing radiation complementation groups 4, 5, 6, and 7 and, therefore, represent a new mutant phenotype, in which a de fect in DNA DSB rejoining is not associated with defective V(D)J recom bination. Furthermore, we have shown that 180BR can arrest at the G(1) -S and G(2)-M cell cycle checkpoints after irradiation. These results confirm that 180BR can be distinguished from ataxia telangiectasia.