NEUTRON-INDUCED DNA DAMAGE AND CELLULAR LETHALITYIN CULTURED MAMMALIAN CELLS
Kazuo Sakai 1 ~ Shozo Suzuki 2 ~ and Shigefumi Okada1 ) 3 )1) Faculty of Medicine, University of Tokyo
2) Institute of Medical Scierice, University of Tokyo
3) Radiation Biology Division, Institute of Physicaland Chemical Research
The induction and repair of DNA lesions induced by fast neutrons aswell as their killing effect were compared to those by low LET radiations
in mouse leukemic L5178Y cells.
Fast neutrons (1.43 Gy/min) wereobtained from a cyclotron at the Institute of Medical Sience (IMS), Universityof Tokyo by bombarding a Be target with
15 MeV deuterons. Gamma-rays (0.9 Gy/min) were from 60co source in IMS. Xrays were obtained from a deep therapy
machine operated at 230 kV, 22mA. Doserate was 3.5 Gy/min. Irradiation of thecells with fast neutrons, gamma-rays, or
X-rays was carried out at 0°C.
After exposure to fast neutrons orgamma-rays, surviving fractions of thecells were estimated as colony forming
ability in soft agar medium. As is shownin Fig.l, killing effect of fast neutrons
was greater than that of gamma-rays.
Initial number of DNA lesions weredetermined immediately after irradiation
using alkaline separation techniquel~Fig. 2 shows that the fast neutrons
induced smaller extent of DNA breaks inspite of their higher efficiency in cellkilling than gamma-rays.
In order to study repair profile ofDNA damage, L5178Y cells were irradiatedwith 18 Gy of gamma-rays or 58 Gy ofneutrons, which induced similar amountof DNA breaks. After irradiation thecells were incubated at 37°C, and thenumber of DNA lesions was measured after
various periods of incubation (Fig. 3).
The breaks induced by gamma-rays wererepaired quickly, while those induced byneutrons showed long-lived fraction evenafter 8 hours of post-irradiation incubation time.
Previous study 1 ) indicated that DNA
breaks induced by low LET radiation (230kV X-rays) were grouped into three: fastreparable, slow-reparable, and non-reparable. This was also the case for fastneutron-induced damage. The three typesof lesions were compared in the Table.
-187-Dose (Gy)
1 2 3 40
1. 0 +.~:----'----'--.._____,__\'p\c::·-0'o y-rays• neutrons·Cl= 0\\ 8,Q"'-

= 8u\\': 8\0Lt0,·-·::; 0.1c::~a\5lf)o,Fig. 1: Dose-survival curve ofL5178Y cells exposed to gammarays or fast neutrons~ 10001~liDO.g

Q_g!

~~ 600~ 1.00/9'l5~200~v/gamrr:;,~~-'(+//neutrons~/0 • -,----,-..--.---..--,,---02"6810 12Dose (Gy)Fig. 2: Dose-response curveof initial breaks induced by
gamma-rays or fast neutrons

It should be noted that the rejoining rates of fast-repara ble andslow-repara ble breaks were the same whether they were induced by X-rays
or fast neutrons, and that neutrons induced smaller extents of fast- andslow-repara ble damage but greater amount of non-reparab le damage.
It is also obvious from Fig. 4 that non-reparab le damage is induced
more efficiently by fast neutrons than by gamma-rays, paralleling theefficiency in cell killing.
These results indicate that greater cell killing caused by fastneutrons would be explained not by the fast-repara ble, nor the slowreparable damage but by the non-reparab le lesions on DNA molecules.

• neutrons

100[J500a neutrons 58 Gy "l:c gamma rays 18 Gy <:c gamma rays

III

£::!

§

400~

60~ 300
C>......
II

II.!2200tJ

I
I
I~RlI

.....~ 20J?
& 0.....

!~ +'JT'.
10..-!-••\,

II..--9 --~0~I24100.Q.l~)-·-+-·-+- ~ 0 a ~yQ-.,--.,--

68Time of Incubation ( hr)Fig. 3: Rejoining profile of DNA
breaks induced by 58 Gy of neutrons
or 18 Gy of gamma-rays

Dr----r--r- --.,r---r--- r--

0 20 40 60 80 100 120
Dose (Gy)Fig. 4: Dose-respon se curve of
non-reparab le DNA damage inducedby neutrons or gamma-rays
fastreparable
damageslowreparable
damagenonreparable
damageX-Rayshalf time of rejoining
breaks/10 12 dalton DNA

5 min
4700

70min
480

90

Neutrons

half time of rejoining
breaks/l0 12 dalton DNA

5min
1500

70 min430

160

Table:Reference1)

Comparison of the three types of DNA damage in L5178Y cells
exposed to 58 Gy of X-rays or fast neutrons
Sakai, K. and Okada, S.: An Alkaline Separation Method forDetection of Small Amount of DNA Damage. J. Radiat. Res.
~. 415-424 (1981)-188-