Abstract
Excess risk of chronic myeloid leukemia (CML), acute lymphoid leukemia (ALL) and acute myeloid leukemia (AML) reached the peak in 1950ies and seemed disappeared. However, preliminary analysis by RERF suggests a persistence of AML risk and epidemiological study of Nagasaki University 21Century COE program indicated that along with aging of the atomic bomb survivors, myelodysplastic syndromes (MDS) are developing recent two decades, with a relative risk of 4 between proximally (less than 1.5 km) exposed group and distal (3.0 km or more) group. Approximately 25% of MDS cases eventually developed into AML. Dose response study is now being conducted for MDS cases within RERF-LSS cohort. If proved positive, MDS/AML is persistently occurring when the young survivors at the time of bombing became elderly, suggesting a life-long effect of atomic bomb radiation on human body. CML is characterized by a simple chromosomal translocation t(9;22) that includes BCR/ABL fusion gene. On the other hand, MDS and AML from MDS are characterized by complicated chromosome abnormalities. From the recent advance in hematology, it is clearly elucidated that CML and MDS, and most AML also, occur in a multi-potent hematopoietic stem cell. The research on the atomic bomb-induced leukemia suggests that there are two major processes in the malignant transformation of hematopoietic stem cells; CML type with short latency and MDS type with extremely long latency. Among proximally exposed healthy survivors, it is still possible to observe chromosome abnormalities in blood cells; some of them are clonal. Whether these abnormalities are mere consequences of radiation injury to chromosomes in 1945 or representing chromosomal instability reflecting leukemogenic process in stem cells is an important question of scientific importance.
