NEIL1 protects against aflatoxin-induced hepatocellular carcinoma in mice

Abstract

Global distribution of hepatocellular carcinomas (HCCs) is dominated by its incidence in developing countries, accounting for >700,000 estimated deaths per year, with dietary exposures to aflatoxin (AFB 1 ) and subsequent DNA adduct formation being a significant driver. Genetic variants that increase individual susceptibility to AFB 1 -induced HCCs are poorly understood. Herein, it is shown that the DNA base excision repair (BER) enzyme, DNA glycosylase NEIL1, efficiently recognizes and excises the highly mutagenic imidazole ring-opened AFB 1 -deoxyguanosine adduct (AFB 1 -Fapy-dG). Consistent with this in vitro result, newborn mice injected with AFB 1 show significant increases in the levels of AFB 1 -Fapy-dG in Neil1 −/− vs. wild-type liver DNA. Further, Neil1 −/− mice are highly susceptible to AFB 1 -induced HCCs relative to WT controls, with both the frequency and average size of hepatocellular carcinomas being elevated in Neil1 −/− . The magnitude of this effect in Neil1 −/− mice is greater than that previously measured in Xeroderma pigmentosum complementation group A (XPA) mice that are deficient in nucleotide excision repair (NER). Given that several human polymorphic variants of NEIL1 are catalytically inactive for their DNA glycosylase activity, these deficiencies may increase susceptibility to AFB 1 -associated HCCs.aflatoxin | base excision repair | ring-fragmented purines | liver cancer | environmental toxicant L iver cancers pose an international public health concern as the second leading cause of cancer-related deaths worldwide, with >700,000 estimated deaths per year (1-3). This mortality approaches its annual incidence throughout the world, highlighting the need for development of effective treatments and early diagnostic tools. HCCs represent the major histological subtype among liver cancers. The global distribution of HCCs is dominated by its incidence in developing countries, especially in eastern Asia and Africa, where two major chronic etiological factors drive this disease: (i) routine dietary exposures to grains and nuts that are contaminated with molds, Aspergillus flavus and Aspergillus parasiticus, which produce aflatoxins, and (ii) extremely high rates of hepatitis B (HBV) and C viral infections. In geographical regions of China where aflatoxin contamination of human food products is highest, there is a large shift in not only the age of onset of HCCs, but also in the incidence rate. Within Qidong, a significant number of HCCs occur in males beginning in their early 20s, with the frequency of HCCs peaking between the ages of 40 and 50 (4). These data are in contrast to HCC frequencies in portions of China, such as Beijing, where aflatoxin exposures are minimal. The kinetics of HCC formation in aflatoxin-affected areas are similar to that observed in early onset breast and ovarian cancers in women who are carriers (heterozygotic) for inactivating mutations in BRCA1 or 2.Although there are several different aflatoxin structures, AFB 1 has been demonstrated to be the most potent hepatoc...

Chronic and Acute Toxicities of Aflatoxins: Mechanisms of Action

“…For example, hypermethylation of the promoter of NEIL1 (Nei Like 1) gene coding for a DNA glycosylase (NEIL1), which plays a key role in BER, was recently shown to reduce the excision efficiency in AFB1-FAPy adducts by transcriptional repression of the gene [39]. The repair of AFB1-FAPy lesions may be further restricted in humans due to the widespread polymorphic variants producing catalytically inactive NEIL1 enzyme [40]. Polymorphism in other human DNA repair genes, such as XPC, XPD, XRCC1, XRCC3, XRCC4, XPD and XRCC7, has been reported to be an additional factor that increases the risk of aflatoxin-induced HCC in high exposure environments [38,[41][42][43][44].…”

Cellular Responses to Aflatoxin-Associated DNA Adducts

“…In mice, the NEIL1 gene has been isolated and knock-out of the gene leads to higher levels of AFB1-associated DNA adducts and AFB1-associated HCC [121]. The NEIL1 enzyme recognizes and excises AFB1-Fapy-dG adducts in "bubble" DNA structures, such as the one described by Brown et al [70,111].…”

“…One idea is that AFB1-Fapy-dG adducts may stably intercalate in the helix and be recognized by NEIL1-dependent BER pathway but not by the NER pathway; the repair pathway may thus depend on the DNA sequence context of the AFB1-Fapy-dG adduct. Knock-out of NEIL1 in mice leads to an increase of AFB1-associated tumors and an accumulation of Fapy-adducts [121]. Vartanian et al [121] assert that the AFB1-associated carcinogenicity in Neil1 −/− mice is as high if not higher than that observed in Xpa −/− mice, noting that both the size and number of tumors are greater in the Neil1 −/− mice compared to the Xpa −/− mice.…”

“…Knock-out of NEIL1 in mice leads to an increase of AFB1-associated tumors and an accumulation of Fapy-adducts [121]. Vartanian et al [121] assert that the AFB1-associated carcinogenicity in Neil1 −/− mice is as high if not higher than that observed in Xpa −/− mice, noting that both the size and number of tumors are greater in the Neil1 −/− mice compared to the Xpa −/− mice. However, the investigators indicate that spontaneous tumors arise at a much higher frequency in Xpa −/− mice, so that the increase in AFB1-associated tumors were measured until the mice were 11 months in age and not when the mice were 15 months in age.…”

“…Intercalation of the AFB1-Fapy-dG in duplex DNA. The adduct is in lighter tone; adapted from Ref [121]…”