We report here that specific T-cell receptor rearrangements were observed in fractionated-X-irradiation-induced murine leukemias. Consistent gamma-chain rearrangements, limited beta-chain rearrangements, and no detectable alpha-chain rearrangements were observed. Gene expression studies revealed that, in comparison with normal thymus tissue, expression of alpha T-cell receptor genes was lower in the thymomas, beta expression was much higher but approximately equal to that of normal thymocytes, and gamma expression was significantly increased. After coupling these data with those from analyses using reagents against other surface markers, such as Lyt-2, L3T4, H-2, IL-2R and MEL-14, we concluded that the target T cells for fractionated-X-irradiation-induced transformation resemble fetal thymocytes from days 15 and 16 of gestation.

Our laboratory has focused on defining, localizing, and understanding the mode of action of genes involved in fractionated x-irradiation (FXI) leukemia in susceptible and restraint mouse strains. We have described the genetic and molecular evidence suggesting the existence of multiple independent loci involved in FXI-induced leukemogenesis. These studies indicated that one of these, Ril-1, a locus on the distal portion of chromosome 15, is the major locus influencing susceptibility to the disease. Our data unequivocally place Ril-1 in the gene complex Ly-6--Ril-1--Sis--H-30--Pol-5. Ril-1 appears to be closest to Ly-6 and Sis. We report that in FXI-induced leukemias there are hypomethylation changes in the Ly-6 region as compared to normal thymocytes. In contrast, Sis was found to be hypermethylated and not expressed. In addition, we have noted DNA rearrangements in the Ly-6--Pol-5 region in the majority of tumors examined using the Ly-6 and spleen focus-forming virus (SFFLV) molecular probes. Increased expression of Ly-6 and other surface markers encoded in this region has been noted in FXI-induced thymomas.

Positioning of nucleosomes was examined in a reconstituted system using a plasmid DNA and histones from normal human and xeroderma pigmentosum, complementation group A (XPA), lymphoblastoid cells. The present studies indicate that the arrangement of nucleosomes, composed of normal human histones, in a region near the SV40 origin of replication on the plasmid DNA, is nonrandom. The alignment of nucleosomes in this region was not affected by the presence of histone H1. No difference in nucleosome positioning was observed when the nucleosomes were composed of histones from XPA cells.