CHROMOSOME ABERRATIONS IN ONE OF A PAIR OF IDENTICAL TWINS AFTER ROENTGEN IRRADIATION OF THE SPINE

Among 13 patients exposed to therapeutic x irradiation, only one gave good chromosome preparations from leukocyte cultures. This 5-yr-old boy was one of identical twins. Since his twin brother had never been irradiated, comparative studies were made of the chromosome preparations in an attempt to establish a specific effect of radiation on genetic material. The patient received a total dose of l80kv x radiation of 4000 rad given over 44 days for treatment of a brain tumor. Leukocytes were cultured 4 days after the last treatment. It was estimated that the spine received 2100 rad. Chromosomes of the irradiated twin differed strikingly from the control in several ways: considerably wider spread of chromosome numbers around the normal human number was evident; karyotypes showed many structurally abnormal chromosomes; and the frequency of polyploid cells was higher. The irradiated brother showed chromosome number variation from 45-- 52, 18% of the cells deviated from the number 46; 29% contained abnormal chromosomes. Some cells contained one or more dicentrics and other deviations, such as acentrics and new chromosome types. Often different deviations were present in the same cell. The incidence of polyploidy was 0.6% in the healthy twin and 5.3% in the treated one.more » Structural changes were much more frequent in the polyploid cells than in the diploid cells. In the latter, 29% had structural changes, and 14% of these had dicentrics, while in the polyploids 54% had 1--6 dicentrics. This difference is probably due to the fact that polyploid cells in which each chromosome is repeated at least four times are less radiosensitive than diploid cells. On the polyploid level, therefore, cells even with extensive structural changes are capable of survival and of entering mitosis. The results show that irradiation of the bone marrow may result in multiple changes of the chromosomes. All damaged cells except one contained 46 centromeres, and most of the cells retained normal total chromosome length. This indicates that loss of chromosome material will prevent cells from reentering mitosis. It also shows that the affected cells were in their first mitosis after the irradiation, The fact that the breaks observed were always of the chromosome type suggests that the leukocytes were exposed to irradiation while in resting stage and reacting as single strands, (BBB)« less