The laboratory has two objectives. One is to study the frequency of chromosome aberrations in the lymphocytes of A-bomb survivors, along with the intensity of the ESR signals from their tooth enamel and then to examine the relationship between the results obtained and DS86 dose estimates. The other is to study the chromosomes of the children of A-bomb survivors and see whether the frequency of individuals carrying chromosome aberrations has increased in the offspring of A-bomb survivors due to parental exposure.

Study of chromosomes of A-bomb survivors

It is well known that radiation is capable of inducing chromosome aberrations in cells. The incidence of chromosome aberrations increases with radiation dose. The frequency of chromosome aberrations in human blood lymphocytes can therefore be a reliable indicator of the radiation dose received by the body. The study, conducted so far on about 3,000 A-bomb survivors in Hiroshima and Nagasaki, shows that lymphocytes with chromosome aberrations–induced by radiation released from the atomic bombs detonated in 1945–have continued to exist for several decades and the proportion of cells having these aberrations increased with the radiation dose.

The Giemsa staining method employed for the chromosome study detects about 2/3 of all chromosome aberrations. However, the chromosome aberration data for A-bomb survivors show relatively wide variations when compared with the dose estimates of individual survivors. The statistical “overdispersion” may have originated from errors made during interviews concerning location and shielding conditions at the time of the bombing, or it may be due to errors in the physical estimation of radiation doses based on the D86 system. Individual differences in radiation response may also be attributable to genetic factors, age at the time of exposure, gender, and lifestyle differences including smoking. These differences can cause variations in chromosome aberration data. In addition, there may be errors in the cytogenetic examinations. To study the variety of possible factors, a means to estimate radiation dose using biological dosimetry other than chromosome aberrations has been pursued.

The electron spin resonance (ESR) method–by which the radicals (unpaired electrons) in tooth enamel can be detected–has been used by many research institutions as another indicator of previous radiation exposure. In 1995, an ESR detector was installed in the Cytogenetics Laboratory and has been used to examine about 100 of the more than 300 tooth samples donated by A-bomb survivors. A close relationship between ESR measurements and the chromosome aberration data for tooth donors has been found, which shows the ESR method is a powerful method for estimating individual radiation doses. The findings obtained by the ESR method also corroborate A-bomb survivors’ chromosome data.

In addition to the ESR technique on tooth enamel to assess radiation exposure, the Cytogenetics Laboratory now utilizes fluorescence in situ hybridization (FISH) to study the chromosomes of A-bomb survivors. In a multiple-step process that involves the painting of chromosomes different colors, aberrations become apparent after abnormal color sequencing of chromosome segments takes place. The FISH technique has enabled a more accurate and rapid identification of chromosomal aberrations than previous methods employed.

Study of the chromosomes of the children of A-bomb survivors

An extensive chromosome study using the traditional staining method was conducted on about 16,000 children born to A-bomb survivors (8,000 children born to parents, one or both of whom were proximally exposed; and 8,000 born to parents, both of whom were distally exposed). The results of this study showed no difference in the number of children carrying chromosome aberrations for both proximally and distally exposed groups. The results were similar to those of other chromosome studies on newborn babies conducted in various parts of the world. Since chromosome aberrations (especially structural aberrations) in children frequently include inherited cases, examinations of parents are necessary to determine whether such abnormalities are newly arisen. The parents of more than half of the children with chromosome aberrations have also been studied. One case each of new chromosome aberration has been found in the proximally and distally exposed groups. Therefore, no radiation effect on the germ cells was detected.