Double strand breaks induced by low doses of γ rays or heavy ions: Quantitation in nonradioactive human DNA

We have developed a method of quantitating low frequencies (0-30 sites/10⁸ base pairs) of double strand breaks in ~1 μg of nonradioactive human DNA. Unirradiated or irradiated DNA is digested with the restriction endonuclease NotI, producing cleavage fragments that include a major group centered at ~1.2-1.3 Mbp. The DNA molecules are separated as a function of size by transverse alternating field electrophoresis. The frequency of double strand breaks is computed directly from the decrease in number average molecular length induced in the 1.2- to 1.3-Mbp cleavage fragment group by ¹³⁷Cs γ or Fe²⁶ (1.1 GeV/nucleon) irradiation vs the corresponding unirradiated DNA samples. The double strand break frequency can be quantitated easily in the dose range of 0-10 cGy of γ rays. The frequency of breaks per unit dose calculated for γ irradiation of DNA in human cells (~4.6 double strand breaks/10⁹ bp/Gy) is within the range of values obtained by others (2-8 sites/10⁹ bp/Gy) who used methods requiring higher doses.