Abstract
Mouse nuclear protein NP95 is colocalized with PCNA in early and mid-S phase cell nuclei and visualized as foci in proliferating cells. It apparently endows cells with the resistance against DNA damaging agents including X-rays and replication arrests incurred by hydroxyurea. As homozygously Np95-inactivated embryonic stem (ES) cells show an extremely high rate of spontaneous mutation, it is of interest if Np95-nulligous cells are also susceptible to X ray-induced mutagenesis. Whereas spontaneously-arising and 6 Gy (10 % survival)-induced TG-resistant mutation frequencies were 5 x 10e(-8) and 6.2 x 10e(-7), respectively, for wild-type E14 cells, unirradiated control and 3 Gy (10% survival) exposure brought about the Hprt mutations at 5.6 x 10e(-4) and 3.0 x 10e(-3), respectively, in Np95-nulligous 19.4 cells. Heterozygously Aprt-inactivated FM3A cells were transfected with anti-sense Np95 cDNA to see if LOH type mutation was affected by NP95 functions. The dose response of radiation-induced DAP-resistant mutations was higher in the parental Aprt(+/-) SR1 cells than in its Np95-inactivated derivative #15D, and the proportions of LOH mutant clones resulting from homologous recombination, those from deletion and those from point mutation were remarkably different between the parental cells and the Np95-inactivated subclones, implicating some critical roles for NP95 in LOH type mutagenesis induced by ionizing radiation.
