Repair of DNA damage induced by ionizing radiation and benzo[α]pyrene in mammalian cells

Abstract

The biological effects of DNA‐damaging agents are codetermined by the structural characteristics of the lesions, the quality and extent of the local distortion of DNA and chromatin structure, and the mode(s) of damage processing used by a given type of cell. Persistent damage (i.e., damage that is not removed before it is reached by DNA replication) may be mostly responsible for metagenesis and carcinogenesis. To understand the effects of environmental physical and chemical DNA‐damaging agents on human health, the mechanisms of damage processing used by human cells have to be elucidated.

We report our studies of the excision of gamma‐ray products of the 5,6‐dihydroxydihydrothymine type (t^γ _o2) in normal human fibroblasts and in fibroblasts from patients with the hereditary diseases Fanconi's anemia (FA) and ataxia telangiectasia (AT). Both diseases are characterized by chromosomal instability and increased susceptibility for the development of cancer. Decreased capacities of nuclear and whole cell sonicates for the removal of t^γ _o2 from gamma‐irradiated DNA were observed for two FA strains (FA CCL 122 or GM 62 and FA 1802B). Our results support the idea that FA is a “DNA repair disease.” Whole cell sonicates of four AT strains (AT CRL 1312, A T CRL 1343, AT GM 367, and AT 4 Bl) possessed normal capacities for t^γ _o2 excision from exogenous free DNA and isolated normal and AT‐chromatin.

Formation and repair of DNA‐benzo[a]pyrene (DNA‐B[a]P) adducts were studied in baby hamster kidney cells (BHK 21/C13), secondary mouse embryo cells [MEF (C57B1)], and human lymphoma (EHR‐B Ramos). Formation of DNA‐B[a]P‐that is, probably of N₂ ‐(10‐{7α,8β,9β or 9α‐trihydroxy‐7,8,9,10‐tetra‐hydrobenzo[a]pyrene}yl)deoxyguanosine‐was observed only in the rodent lines. Removal of DNA‐B[a]P was investigated in double label experiments using /³ H/benzo[a]pyrene and [¹⁴C]thymidine. BHK cells removed approximately 28% of the adducts from DNA within 24 hr posttreatment incubation, while MEF cells removed only 15% within the same period. The relative persistence of DNA‐B[a]P may explain the high mutagenicity of the 7,8‐dihydroxy‐9,10‐epoxy‐tetrahydro‐benzo[a]pyrene metabolites in rodent cells that has been observed by several investigators.