Hepatitis B virus genotypes B and C

描述（由申请人提供）：慢性乙型肝炎病毒（HBV）感染大大增加了发生肝细胞癌（HCC）的风险，基因型B和C占全球慢性HBV感染的大多数。尽管这两种基因型通过相同的（垂直）传播方式感染相似的人群（亚洲人），但基因型C感染与活性病毒复制时间延长、HBeAg血清转化延迟以及HCC终生风险增加有关。基因型C也更容易发生核心启动子突变，这与HCC风险独立相关。我们的初步研究发现，当将核心启动子突变引入基因型A和C的克隆时，核心启动子突变上调了基因组复制和核心蛋白的表达。我们还发现，大多数具有野生型核心启动子序列的基因型C分离物产生的前基因组RNA少于相应的基因型B分离物，导致核心蛋白表达和基因组复制较低。然而，它们在病毒粒子分泌方面更有效。考虑到核心蛋白是一种强免疫原，我们推测其基因型C的低表达延长了感染的免疫耐受期并延迟了HBeAg血清转化，而高效的病毒粒子分泌确保了病毒在肝脏中的快速传播，这是建立持续感染所必需的。我们还假设免疫清除机制通过核心启动子突变选择具有增强复制能力的基因型C突变体，并伴随核心蛋白表达的增加，从而引发肝损伤和HCC风险。在这项R21基金申请中，我们将确定决定因素并阐明导致野生型基因型C分离株基因组复制能力较低但病毒粒子分泌效率更高的机制。我们还将测试另一种可能性，即基因型B中核心启动子突变的低患病率是由于它们无法增强该特定基因型的基因组复制。我们的工作假设和提出的研究将揭示C基因型核心启动子突变的较高患病率，并有助于解决长期以来的难题，即为什么C基因型在宿主体内持续时间更长，从而诱发肝硬化和HCC等晚期肝脏疾病。这些研究还将揭示HBV基因组复制和病毒粒子分泌的新控制机制。我们在HBV遗传变异生物学特性表征方面的丰富经验和可靠记录将确保拟议研究的成功完成。

DESCRIPTION (provided by applicant): Chronic infection with hepatitis B virus (HBV) greatly increases the risk to develop hepatocellular carcinoma (HCC), and genotypes B and C account for majority of chronic HBV infection worldwide. Although these two genotypes infect similar human population (Asians) via the same (vertical) mode of transmission, infection with genotype C is associated with prolonged active virus replication, delayed HBeAg seroconversion, and increased lifelong risk for HCC. Genotype C is also more prone to develop core promoter mutations, which have been independently associated with HCC risk. Our preliminary studies revealed that core promoter mutations up regulate genome replication and core protein expression when introduced into clones of genotype A and C. We also found that most genotype C isolates with wild-type core promoter sequence produce less pregenomic RNA than corresponding genotype B isolates, leading to lower core protein expression and genome replication. Nevertheless, they are more efficient at virion secretion. Considering that the core protein is a strong immunogen, we hypothesize that its low expression by genotype C prolongs the immune tolerance phase of infection and delays HBeAg seroconversion, whereas efficient virion secretion ensures rapid virus spread in the liver as required for establishment of persisten infection. We also hypothesize that immune clearance mechanisms select for genotype C mutants with augmented replication capacity through core promoter mutations, with the concomitant increase in core protein expression triggering liver damage and HCC risk. In this R21 grant application we will identify the determinants and elucidate the mechanisms responsible for lower genome replication capacity of wild-type genotype C isolates but more efficient virion secretion. We will also test the alternative possibility that the low prevalence o core promoter mutations in genotype B is due to their inability to enhance genome replication in this particular genotype. Our working hypothesis and proposed studies will shed light on the higher prevalence of core promoter mutations in genotype C, and help solve the longstanding puzzle of why genotype C persists longer in the host to induce advanced liver diseases such as cirrhosis and HCC. These studies will also reveal novel control mechanisms in HBV genome replication and virion secretion. Our rich experience and proven track record in characterizing biological properties of HBV genetic variants will ensure successful completion of the proposed study.