RERF Report No. 12-15

Paper published in Mutation Research/Genetic Toxicology and Environmental

Mutagenesis §“Relationship between spontaneous γH2AX foci formation and progenitor
functions in circulating hematopoietic stem and progenitor cells among
atomic-bomb survivors”
Junko Kajimura, Seishi Kyoizumi, Yoshiko Kubo, Munechika Misumi, Kengo Yoshida,
Tomonori Hayashi, Kazue Imai, Waka Ohishi, Kei Nakachi, Nan-ping Weng, Lauren F.
Young, Jae-Hung Shieh, Malcolm A. Moore, Marcel R.M. van den Brink, Yoichiro
Kusunoki
Mutat Res Genet Toxicol Environ Mutagen 2016 (May); 802:59-65(doi: 10.1016/j.mrgentox.2016.03.007)

Study Findings
The self-renewability of hematopoietic stem cells has been suggested to decline inatomic bomb (A-bomb) survivors exposed to higher radiation doses and who had more
DNA damage in their hematopoietic stem and progenitor cells (HSPCs), which areresponsible for the production of red, white, and other blood cells.
Explanation
1. Study Purpose
Accumulation of DNA damage in hematopoietic stem cells is known to be one factor
in the functional decline of human hematopoiesis associated with aging. Not known,
however, is whether an association exists between radiation exposure and hematopoietic
decline due to accumulated DNA damage. This study examined the association between
radiation exposure and hematopoietic function by measuring DNA damage frequency inthe circulating HSPCs of A-bomb survivors.
2. Study Methods
Using peripheral blood collected from 229 consenting A-bomb survivors in Hiroshima
who participated in the Adult Health Study (AHS) from 2011 to 2013, we evaluated DNA
damage frequency using γH2AX foci formation* as a marker. Also assessed washematopoietic function including self-renewability of HSPCs, which can be detected by
the cobblestone area-forming cell** assay in CD34-positive/lineage marker-negative(CD34+Lin-) HPSCs. In addition, we measured the length of granulocyte telomeres*** as amarker for the assessment of hematopoietic system aging.

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RERF Report No. 12-15*γH2AX foci formation: In the presence of DNA damage such as DNA double-strandbreaks, γH2AX and other DNA repair proteins form a mass called a “focus” at thebreak. By detecting such foci and ascertaining their numbers, DNA damage
frequency can be determined.
**Cobblestone area-forming cell: When a cell-culture environment is created in vitro
similar to one in which hematopoietic stem cells self-renew in the bone marrow(culturing HSPCs in the absence of hematopoietic factors and in the presence ofstromal cells), hematopoietic stem cells form an undifferentiated cobblestone-likecolony, allowing assessment of how many hematopoietic stem cells haveself-renewability.
***Length of granulocyte telomeres: Cellular telomere length is a marker used toassess aging because length of a telomere, a repeat base sequence located at the endof a chromosome, is shortened at each cell division. The length of granulocyte
telomeres is a marker to assess aging of the entire hematopoietic system, because
granulocytes have a short lifecycle of a few days in vivo and are continuously
produced from hematopoietic progenitor cells.
3. Study Results(1) Frequency of DNA damage in HSPCs in association with radiation dose
DNA damage frequency (γH2AX foci formation) gradually decreased withincreasing radiation dose up to 1.5 Gy. At higher doses, an increase in foci was observed.
Because DNA damage detected on the basis of γH2AX foci can be repaired within a few
days, detected DNA damage may have been induced by oxidative stress in vivo over theperiod of a few days before blood drawing rather than by A-bomb radiation. Theassociation observed in this study with radiation dose may indicate lower levels of DNA
damage stress or more effective DNA repair among A-bomb survivors exposed to 1.5 Gyor less. Such details are unclear at this point in time.
(2) Frequency of DNA damage in HSPCs in association with granulocyte telomere length
Frequency of γH2AX foci formation increased with decreasing length of
granulocyte telomeres, which suggests an association between DNA damage in HSPCs
and aging of the hematopoietic system. A similar association has been reported by otherstudies. In this study, the association with aging was also confirmed in A-bomb
survivors.
(3) An interaction effect between radiation dose and frequency of DNA damage
A negative interaction effect between radiation dose and γH2AX foci formation
frequency was indicated in the cobblestone formation assay. This result suggests thatthe self-renewability of hematopoietic stem cells may have decreased among survivors
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RERF Report No. 12-15

who had a higher radiation dose and more DNA damage.
Study SignificanceMany years after radiation exposure and as the A-bomb survivors continue to age,
their hematopoietic functions as a group show normal aging, but this study suggests thatamong A-bomb survivors with large quantities of accumulated DNA damage in HPSCs,
reduced self-renewability of hematopoietic stem cells due to aging may be modified byA-bomb radiation exposure.
The Radiation Effects Research Foundation has studied A-bomb survivors and their offspring in Hiroshima
and Nagasaki for nearly 70 years. RERF’s research achievements are considered the principal scientific basis forradiation risk assessment by the United Nations Scientific Committee on the Effects of Atomic Radiation(UNSCEAR) and for recommendations regarding radiation protection standards by the InternationalCommission on Radiological Protection (ICRP). RERF expresses its profound gratitude to the A-bomb survivors
and survivors’ offspring for their cooperation in our studies.

Mutation Research-Genetic Toxicology and Environmental Mutagenesis is a section of Mutation
Research (peer-reviewed journal that publishes research papers in the area of mutation research§with a focus on fundamental mechanisms underlying the phenotypic and genotypic expression ofgenetic damage). This section publishes papers in the field of genetic toxicology, focusing on
genotoxicity testing of specific agents, in vitro or in vivo, and assessment of health effects
resulting from genotoxic exposures in human populations. (Impact factor in 2015: 1.83)

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