P.falciparum var gene diversity and malaria control

描述（由申请人提供）：恶性疟原虫疟疾的负担是巨大的，每年有多达100万人死亡，数十亿人感染持续数月。这些大多无症状的慢性感染构成了感染库，并助长了疟疾的持续传播。传统上，通过血液涂片中寄生虫的显微镜可视化来测量人群调查中的感染库。然而，这种方法不敏感，也不能捕捉到物种内的变异，以准确评估疟疾流行地区的个体携带多个不同的寄生虫基因组的恶性疟原虫的真正宿主。var多基因家族编码恶性疟原虫红细胞膜蛋白-1 (PfEMP1)，这是一种被无性寄生虫和传播阶段感染的红细胞表面表达的分子。恶性疟原虫基因组中有50-60个变异基因，这些基因的差异表达是克隆抗原变异的基础。这种转换表面抗原变体的能力使疟原虫能够逃避宿主的免疫反应，并建立慢性感染，从而优化向蚊子的传播。不同的寄生虫基因组中存在不同的var基因库。这种多样性被认为允许重复感染在暴露的宿主中建立、持续和传播。因此，var基因多样性提供了一种特定的诊断措施，可以衡量流行地区感染库的大小以及持续或持续时间的潜力。本研究的总体目标是了解var基因的进化，以提高恶性疟原虫的分子监测。为了实现这一目标，我们需要在三个全球环境中收集基线var多样性数据：撒哈拉以南非洲、美拉尼西亚和南美洲。由于媒介生物学、传播强度和种群历史等方面的差异，每种环境都显示出不同的疟疾流行病学。我们将使用一个标准化的分子流行病学框架，其中包含由Day实验室在过去6年中开发的定制统计和生物信息学，以描述var基因的群体遗传学。此外，我们将分析微卫星多样性作为当地寄生虫种群结构的标志。这一重要的假设——相对于全球水平，var多样性的地理差异存在于地方层面——将得到检验，因为这种种群结构将影响寄生虫种群之间的交叉免疫。此外，这种结构将大大影响制定和评估消除和根除战略的方法，以比现有方法更复杂的方式减少疟疾感染的蓄水池。这是一项重要的“跨学科”自然历史实验，类似于为流感疾病监测定义血凝素分子的变异，除了var遗传学更为复杂，因为每个基因组有多个高度多样化的var拷贝和重组的可能性。通过测量变异多样性来确定感染库是在消灭疟疾的新时代控制疟疾的关键步骤。

DESCRIPTION (provided by applicant): The burden of Plasmodium falciparum malaria is enormous with up to a million deaths per annum and billions of infections that persist for many months in the human population. These chronic infections, largely asymptomatic, constitute the reservoir of infection and serve to fuel continued malaria transmission. Traditionally, the reservoir of infection has been measured in population surveys by microscopic visualization of the parasite in blood smears. This method, however, is not sensitive and does not capture within species variation to accurately assess the true reservoir of P. falciparum where individuals in malaria endemic areas carry multiple distinct parasite genomes. The var multigene family encodes P. falciparum Erythrocyte Membrane Protein-1 (PfEMP1), a molecule that is expressed on the surface of erythrocytes infected by asexual parasite stages and transmission stages. There are 50-60 var genes in a P. falciparum genome and differential expression of these genes underlies clonal antigenic variation. This ability to switch surface antigen variants allows the malaria parasite to evade the host immune response and establish chronic infection to optimize transmission to the mosquito. Diverse repertoires of var genes exist in different parasite genomes. This repertoire diversity is proposed to allow a superinfection to establish, persist, and transmit in an exposed host. Thus, var gene diversity gives a specific diagnostic measure of the size as well as the potential for persistence or duration of the reservoir of infection in an endemic area. The overall goal of this proposal is to understand the evolution of var genes in order to improve molecular surveillance of P. falciparum. To achieve this goal, we need to collect baseline var diversity data in three global settings: sub- Saharan Africa, Melanesia, and South America. Each of these settings displays a distinct epidemiology of malaria due to differences including vector biology, transmission intensity and population history. We will use a standardized molecular epidemiology framework with custom made statistics and bioinformatics developed over the past 6 years by the Day laboratory to describe the population genetics of var genes. In addition, we will analyze microsatellite diversity as a marker for local parasite population structure. The important hypothesis--that geographic variation in var diversity exists at a local level relative to global level--will be tested as such population structure would influence cross-immunity among parasite populations. Moreover, such structure would dramatically influence the approach to developing and assessing elimination and eradication strategies to reduce the reservoir of malaria infection in a more sophisticated manner than existing methods. This is an important "interdisciplinary" natural history experiment, akin to defining variation in the hemagglutinin molecule for influenza disease surveillance, except that the var genetics is more complex due to the multiple, highly diverse var copies per genome and possibilities for recombination. Defining the reservoir of infection by measuring var diversity is a critical step for malaria control in the new era of malaria eradication. PUBLIC HEALTH RELEVANCE: Malaria infections caused by the parasite Plasmodium falciparum can persist in humans for hundreds of days. Conventional measures of this reservoir of infection are underestimates as they do not account for multiple infections in individuals or genetic diversity within parasites. Using novel genomic methods to measure the reservoir of infection, we will define the diversity and geographic differences in malaria populations to optimize planning and evaluation of malaria control.