Doyle Stephen R, Bourguinat Catherine, Nana-Djeunga Hugues C, Kengne-Ouafo Jonas A, Pion Sébastien D S, Bopda Jean, Kamgno Joseph, Wanji Samuel, Che Hua, Kuesel Annette C, Walker Martin, Basáñez Maria-Gloria, Boakye Daniel A, Osei-Atweneboana Mike Y, Boussinesq Michel, Prichard Roger K, Grant Warwick N
Department of Animal, Plant and Soil Sciences, La Trobe University, Bundoora, Australia.
Wellcome Trust Sanger Institute, Hinxton, Cambridge, United Kingdom.
PLoS Negl Trop Dis. 2017 Jul 26;11(7):e0005816. doi: 10.1371/journal.pntd.0005816. eCollection 2017 Jul.
Treatment of onchocerciasis using mass ivermectin administration has reduced morbidity and transmission throughout Africa and Central/South America. Mass drug administration is likely to exert selection pressure on parasites, and phenotypic and genetic changes in several Onchocerca volvulus populations from Cameroon and Ghana-exposed to more than a decade of regular ivermectin treatment-have raised concern that sub-optimal responses to ivermectin's anti-fecundity effect are becoming more frequent and may spread.
METHODOLOGY/PRINCIPAL FINDINGS: Pooled next generation sequencing (Pool-seq) was used to characterise genetic diversity within and between 108 adult female worms differing in ivermectin treatment history and response. Genome-wide analyses revealed genetic variation that significantly differentiated good responder (GR) and sub-optimal responder (SOR) parasites. These variants were not randomly distributed but clustered in ~31 quantitative trait loci (QTLs), with little overlap in putative QTL position and gene content between the two countries. Published candidate ivermectin SOR genes were largely absent in these regions; QTLs differentiating GR and SOR worms were enriched for genes in molecular pathways associated with neurotransmission, development, and stress responses. Finally, single worm genotyping demonstrated that geographic isolation and genetic change over time (in the presence of drug exposure) had a significantly greater role in shaping genetic diversity than the evolution of SOR.
CONCLUSIONS/SIGNIFICANCE: This study is one of the first genome-wide association analyses in a parasitic nematode, and provides insight into the genomics of ivermectin response and population structure of O. volvulus. We argue that ivermectin response is a polygenically-determined quantitative trait (QT) whereby identical or related molecular pathways but not necessarily individual genes are likely to determine the extent of ivermectin response in different parasite populations. Furthermore, we propose that genetic drift rather than genetic selection of SOR is the underlying driver of population differentiation, which has significant implications for the emergence and potential spread of SOR within and between these parasite populations.
通过大规模施用伊维菌素治疗盘尾丝虫病，已在整个非洲以及中美洲/南美洲降低了发病率并减少了传播。大规模药物施用可能会对寄生虫施加选择压力，并且来自喀麦隆和加纳的几个盘尾丝虫种群（这些种群已接受了十多年的常规伊维菌素治疗）的表型和基因变化引发了人们的担忧，即对伊维菌素抗生育力作用的次优反应正变得越来越频繁且可能扩散。
方法/主要发现：采用混合下一代测序（Pool-seq）来表征108条成年雌虫的基因多样性，这些雌虫在伊维菌素治疗史和反应方面存在差异。全基因组分析揭示了能显著区分良好反应者（GR）和次优反应者（SOR）寄生虫的基因变异。这些变异并非随机分布，而是聚集在约31个数量性状基因座（QTL）中，两国之间假定的QTL位置和基因含量几乎没有重叠。在这些区域中基本不存在已发表的伊维菌素SOR候选基因；区分GR和SOR蠕虫的QTL富含与神经传递、发育和应激反应相关的分子途径中的基因。最后，单虫基因分型表明，地理隔离和随时间的基因变化（在有药物暴露的情况下）在塑造基因多样性方面比SOR的进化发挥了显著更大的作用。
结论/意义：本研究是对寄生线虫进行的首批全基因组关联分析之一，并为伊维菌素反应的基因组学和盘尾丝虫的种群结构提供了见解。我们认为伊维菌素反应是一种多基因决定的数量性状（QT），即相同或相关的分子途径而非个别基因可能决定不同寄生虫种群中伊维菌素反应的程度。此外，我们提出遗传漂变而非SOR的遗传选择是种群分化的潜在驱动因素，这对SOR在这些寄生虫种群内部和之间的出现及潜在传播具有重要意义。
