Mass Spectrometry-Based Method to Measure Aflatoxin B₁ DNA Adducts in Formalin-Fixed Paraffin-Embedded Tissues

Abstract

Aflatoxin B 1 (AFB 1 ) is a potent human liver carcinogen produced by certain molds, particularly Aspergillus flavus and Aspergillus parasiticus, which contaminate peanuts, corn, rice, cottonseed, and ground and tree nuts, principally in warm and humid climates. AFB 1 undergoes bioactivation in the liver to produce AFB 1 -exo-8,9-epoxide, which forms the covalently bound cationic AFB 1 −N7-guanine (AFB 1 −N7-Gua) DNA adduct. This adduct is unstable and undergoes base-catalyzed opening of the guanine imidazolium ring to form two ring-opened diastereomeric 8,9dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)-9-hydroxy-aflatoxin B 1 (AFB 1 −FapyGua) adducts. The AFB 1 formamidopyrimidine (Fapy) adducts induce G → T transversion mutations and are likely responsible for the carcinogenic effects of AFB 1 . Quantitative liquid chromatography−mass spectrometry (LC-MS) methods have shown that AFB 1 −N7-Gua is eliminated in rodent and human urine, whereas ring-opened AFB 1 −FapyGua adducts persist in rodent liver. However, fresh frozen biopsy tissues are seldom available for biomonitoring AFB 1 DNA adducts in humans, impeding research advances in this potent liver carcinogen. In contrast, formalin-fixed paraffin-embedded (FFPE) specimens used for histopathological analysis are often accessible for molecular studies. However, ensuring nucleic acid quality presents a challenge due to incomplete reversal of formalin-mediated DNA crosslinks, which can preclude accurate quantitative measurements of DNA adducts. In this study, employing ion trap or high-resolution accurate Orbitrap mass spectrometry, we demonstrate that ring-opened AFB 1 −FapyGua adducts formed in AFB 1 -exposed newborn mice are stable to the formalin fixation and DNA de-cross-linking retrieval processes. The AFB 1 −FapyGua adducts can be detected at levels comparable to those in a match of fresh frozen liver. Orbitrap MS 2 measurements can detect AFB 1 −FapyGua at a quantification limit of 4.0 adducts per 10 8 bases when only 0.8 μg of DNA is assayed on the column. Thus, our breakthrough DNA retrieval technology can be adapted to screen for AFB 1 DNA adducts in FFPE human liver specimens from cohorts at risk of this potent liver carcinogen.