The toxicity and mutagenicity of the aflatoxin B₁ formamidopyrimidine DNA adduct

Abstract

Aflatoxin B1 (AFB1) is a fungal metabolite that contaminates the food supply in certain areas of the world. It is produced by Aspergillusflavus and related fungi that grow on improperly stored foods such as corn, rice, and peanuts. Epidemiological studies have shown a correlation between exposure to AFBI and incidence of hepatocellular carcinoma (HCC). Mutations in p53 are observed in over 50% of the HCC samples studied, and a unique mutational hotspot occurs at the third position of codon 249 in this gene, yielding almost exclusively GC to TA transversions. It is of interest to evaluate the mutagenic properties of specific chemical structures of AFBI adducts in order to determine which of these may be responsible for the mutations that may play a role in the formation of HCC. The primary DNA adduct formed by the epoxide of AFB is the 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 (AFBI-N7-Gua) adduct, which can lead to two secondary lesions, an apurinic site or a ring opened formamidopyrimidine (FAPY) adduct, which itself has two rotameric forms. This study focuses on of the determination of how well cells tolerate each of the AFB1-FAPY rotamers and of the type and frequency of mutations caused by the persistent AFB I-FAPY adduct in a site specifically modified M13 viral vector transfected into E. coli. Four major results were concluded from this work. First, one of the rotamers of AFBI-FAPY is a strong block to DNA replication, even when bypass polymerases are employed by the cell.

(cont.) Second, the G to T mutation frequency of the AFB I-FAPY adduct is at least six fold greater than that observed for the AFBi-N7-Gua adduct. Third, a spectrum of mutations that is unique to the AFBI-FAPY adduct was observed. Fourth, cell strains expressing different bypass polymerases responded differently when challenged with the AFB1-FAPY and AFBI-N7-Gua DNA adducts. These results show that AFB I-FAPY is the most toxic and mutagenic species of aflatoxin adduct studied to date.