Molecular Genetics Laboratory
Since 1948, there have been several large-scale studies of how parental exposure to radiation affects their children (genetic effects of radiation). Studies have been conducted to see whether the frequencies of stillbirths, malformations, perinatal deaths, as well as the chromosome aberration and protein level mutations increased with dose among children whose parents (one or both) were exposed to A-bomb radiation. No effect of parental exposure was observed. Although no effects of radiation exposure on the mortality of the children after reaching adulthood has been observed, epidemiological studies continue.
These studies were conducted based on the latest knowledge and state-of-the-art technology at the time. However, judging from the present level of knowledge, to our regret, not all of the technologies used for those studies were appropriate for detecting the genetic effects of radiation.
With the recent rapid progress in molecular biology, it has become possible to study genes (DNA) directly. A plan is being developed to use this new technique for elucidating the genetic effects of radiation. At the 1984 Genetics Study Conference, immortalization of blood cells (lymphocytes) of father-mother-child trios was recommended. Cells collected from 1,000 families are now cryopreserved in liquid nitrogen at -200ºC. DNA from those cells will be used for a large-scale molecular genetic study in the future.
At the Human Germline Mutagenesis Workshop in 1991, initiation of a pilot study using cells from 100 families (50 families with the highest parental exposure dose and 50 families with the least parental exposure dose) was recommended to identify the DNA sequence most suitable for examining radiogenic mutations and to investigate the optimal method to study a large number of individuals.
Following this recommendation, a pilot study was conducted on the loci of microsatellites and minisatellites (repeated base sequences scattered over the human genome. As the number of repetitions depend on the individual, there is no distinction between normal and abnormal. Background mutation rate is high.). Although the number of study subjects is limited, no effect of parental radiation exposure has been detected to date.
Deletion is the type of mutation most frequently caused by radiation. In case of autosomal genes, deletion in full or in part occurs as loss of one of the two genes normally seen.
At present, two procedures are under development to efficiently detect such mutations. One is a microarray-based comparative genomic hybridization (array CGH). With array CGH, differentially fluoresce-labeled test and reference genomic DNA are co-hybridized to an array to which cloned DNA fragments are immobilized as targets. The fluorescence ratios at arrayed DNA fragments provide a target-to-target measure of DNA copy-number variation. This method can detect single-copy decrease and increase from normal diploid, and can examine a large number of DNA fragments on a single slide-glass. The other technique requires that an autoradiogram be created via two-dimensional electrophoresis after DNA fragments are labeled with a radioisotope. The autoradiograhic spots obtained on the X-ray film are then quantitatively analyzed using a computer. The advantage of this technique is that it allows us to obtain information on thousands of genes, both known and unknown, at one time. A pilot study is being conducted to investigate the utility of these techniques.