Role of Base Excision Repair in Limiting Hepatocellular Carcinomas

总结 长期饮食暴露于真菌毒素、黄曲霉毒素B1（AFB 1）和B型肝炎病毒（HBV） 感染与早发性肝细胞癌（HCC）的风险显著增加相关， 生活在东亚、中美洲和南美洲以及撒哈拉以南非洲的数百万人。虽然饮食 暴露于黄曲霉毒素构成了癌症发展的第二大全球环境风险因素， 关于致癌作用的分子机制仍存在重大问题。深入 了解这些机制对于识别影响个体的遗传风险因素至关重要。 HBV感染者和AFB 1暴露者的易感性。在这方面，由于AFB 1致癌作用是 由高频率的G到T颠换驱动，启动和完成修复的DNA修复途径， 持续的AFB 1诱导的DNA加合物和HBV诱导的炎症引起的碱基损伤具有强烈的 生物学意义这些途径定义了靶组织中的致突变负荷，并最终限制了 细胞发展为癌症。虽然核苷酸切除修复（NER）途径已被证明， 修复AFB 1 DNA加合物，本文提供小鼠数据证明DNA碱基切除的敲除 由DNA糖基化酶NEIL 1启动的修复（BER）途径比NER重要得多 相对于高致突变性AFB 1-Fapy-dG加合物的去除。因此，我们的数据表明， NEIL 1的缺乏可能导致人类HCC的发生。为了最大限度地与人类相关， 健康，NEIL 1的所有已知变体将从世界上黄曲霉毒素摄入的地区进行表征， 和HBV感染是普遍的。所有变体将表征其相对于以下的生物化学性质： WT NEIL 1，并在细胞和转基因小鼠中表达，以了解催化- NEIL 1的受损变体改变对黄曲霉毒素暴露的敏感性。这些目标将考验 假设NEIL 1的致癌和其他催化受损变体的表达可以 有效地阻断修复并促进增加的诱变和致癌作用。表征 NEIL 1的表型显性致癌变异体将提供设计NEIL 1的分子基础。 在高危人群和早发HCC队列中进行的人类流行病学研究。此外，本申请 提出建立分子机制，通过这些机制， B型肝炎表面抗原和DNA修复缺陷可以协同驱动AFB 1诱导的 诱变和致癌作用。总的来说，这些研究与人类健康直接相关， 通过识别遗传风险因素和生物化学，了解全球环境健康问题 先前未被认为与AFB 1诱导的致癌作用密切相关的途径。

Summary The combination of chronic dietary exposure to the fungal toxin, aflatoxin B1 (AFB1), and hepatitis B viral (HBV) infection is associated with a significant increased risk for early onset hepatocellular carcinomas (HCCs) in millions of people living in East Asia, Central and South America, and sub-Saharan Africa. Even though dietary exposures to aflatoxins constitute the second largest global environmental risk factor for cancer development, there are still significant questions concerning the molecular mechanisms driving carcinogenesis. In-depth knowledge of these mechanisms is critical for the identification of genetic risk factors that affect individual susceptibility for people who are HBV infected and AFB1 exposed. In this regard, since AFB1 carcinogenesis is driven by high frequency G to T transversions, the DNA repair pathways that initiate and complete repair of persistent AFB1-induced DNA adducts and base damage from HBV-induced inflammation have strong biological significance. These pathways define the mutagenic burden in the target tissues and ultimately limit cellular progression to cancer. Although the nucleotide excision repair (NER) pathway has been shown to repair AFB1 DNA adducts, murine data presented herein demonstrate that knockout of the DNA base excision repair (BER) pathway, initiated by the DNA glycosylase NEIL1, is significantly more important than NER relative to the removal of the highly mutagenic AFB1-Fapy-dG adducts. Thus, our data suggest that deficiencies in NEIL1 could contribute to the initiation of HCCs in humans. To maximize relevance to human health, all known variants of NEIL1 will be characterized from regions of the world where aflatoxin ingestion and HBV infection are prevalent. All variants will be characterized for their biochemical properties relative to WT NEIL1 and expressed in cells and transgenic mice to understand the potential for catalytically- compromised variants of NEIL1 to alter susceptibility to aflatoxin exposures. These aims will test the hypothesis that expression of the oncogenic and other catalytically-compromised variants of NEIL1 can efficiently block repair and promote increased mutagenesis and carcinogenesis. Characterization of phenotypically dominant oncogenic variants of NEIL1 will provide the molecular basis from which to design human epidemiological studies in at-risk populations and early onset HCC cohorts. Further, this application proposes to establish the molecular mechanisms by which the combination of chronic inflammation driven by the hepatitis B surface antigen and deficiencies in DNA repair could synergistically drive AFB1-induced mutagenesis and carcinogenesis. Overall, these studies have direct human health relevance pertaining to understanding a global environmental health problem by identifying genetic risk factors and biochemical pathways previously not recognized as germane to AFB1-induced carcinogenesis.