Slower synthesis of individual replicons and adjacent replicon clusters in a radiosensitive xeroderma pigmentosum strain with and without X-irradiation.

Two replication parameters, synthesis of individual replicons and adjacent replicon clusters, were measured using DNA fiber autoradiography in a radiosensitive form of group C xeroderma pigmentosum (XP) XP2SP, in group C XP4SP, in group A Cockayne syndrome (CS) CS1SP and two normal human fibroblast strains. The novel observation here is that in non-irradiated XP2SP cells synthesis of individual replicons was significantly retarded as compared with all other cell lines tested and remained unchanged after 5-Gy X-rays. Also the number of simultaneously operating adjacent replicon clusters was uniquely reduced in only non-irradiated XP2SP cells and remained unaltered after 5-Gy irradiation. While the normal, XP4SP and CS1SP cells are radiosensitive to reduction in this replication parameter to a low level seen in both non-irradiated and 5-Gy irradiated XP2SP cells. Thus, non-irradiated XP2SP cells mimic irradiated normal, XP4SP and CS1SP cells. A possible relation of the above abnormalities in individual replicons and adjacent replicon clusters to a high incidence of spontaneous sister-chromatid exchanges and X-irradiation-induced chromosomal aberrations in XP2SP cells is discussed.