Molecular analysis of the genetic determinants that contribute to virulence in lineage 2 West Nile virus
Date
2017
Authors
Romo, Hannah Elizabeth, author
Ebel, Greg, advisor
Brault, Aaron, advisor
Blair, Carol, committee member
Bowen, Richard, committee member
Huyvaert, Kate, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The ability of arboviruses to impart significant global disease burdens is related to the corresponding capacity of arboviruses to emerge in naïve environments or re-emerge in endemic environments. The introduction of West Nile virus (WNV) into North America was marked by rapid spread across the continent, high rates of neuroinvasive disease in humans and horses, and subsequent displacement by newer evolved genotypes. In the last 12 years, an underrepresented lineage of WNV, lineage 2 (L2) has similarly emerged from sub-Saharan Africa into areas of Europe and Russia, causing widespread neurological disease and recurrent enzootic transmission. Given the potential for further geographic spread of L2 WNV and to understand mechanisms that drive emergence events for WNV, I sought to characterize L2 WNV in a comprehensive and comparative manner by investigating potential molecular mechanisms of pathogenesis in mosquitoes, birds, and mice (as models for human disease). A more thorough understanding of the mechanisms that dictate rapid dispersal and endemic maintenance of arboviruses will improve our ability to predict emergence events, increase the effectiveness of surveillance mechanisms, and develop effective intervention strategies. Within lineage 1 (L1) WNV, the role of the NS3-249P amino acid in modulating severe virogenesis in American Crows (AMCRs) has been well established and is predicted to be involved in facilitating the emergent capacity of L1 WNV. The evolution of a proline at the same NS3-249 locus in L2 WNV was initially observed during the first L2 WNV associated outbreak in Europe. However, no bird mortality was observed during the NS3-249P associated L2 WNV outbreak, and the extent of L2 WNV pathogenesis in birds is unclear. In this aim, I examined the viremia titers and mortality profiles of North American AMCRs and house sparrows following infection with African and European L2 WNV strains with and without amino acid mutations at the NS3-249 locus. Our results demonstrate that L2 WNV strains can elicit severe virogenic and fatal outcomes in AMCRs and HOSPs. Additionally, I found that the NS3-249 locus is modulating AMCR viremia titer outcomes, similar to what has been previously observed for the NS3-249 locus in L1 WNV strains. I also demonstrated the 3' UTR of NS10 reduces viremia titers of AMCRs at later time points. The vast majority of our understanding regarding the vector competence of Culex mosquitoes for WNV originates from studies performed with L1 WNV strains, and as such, little information is available regarding the competency of Culex mosquitoes for L2 WNV. To remediate this, I assessed the vector competence phenotypes of two different North American Culex mosquito species for multiple L2 WNV strains. Our results demonstrate that Culex pipiens and Culex quinquefasciatus mosquitoes can effectively transmit L2 WNV. I also identified a L2 strain harboring an NS3-249P mutation (NS10) that limited infection to the midgut of Culex pipiens mosquitoes. The competence of North American Culex mosquitoes to transmit L2 WNV taken together with the ability of AMCRs and HOSPs to serve as reservoir hosts for L2 WNV demonstrates the capacity for L2 WNV transmission in the Western Hemisphere. Previous studies generated in this dissertation demonstrated that high viral titers in AMCRs were modulated by the NS3-249P mutation in the NS10 L2 WNV strain and that this same strain also generated lower infection rates in Culex pipiens compared to other L2 WNV strains, suggesting that the NS3-249 locus might be involved in concurrently modulating vector competence in Culex pipiens and viral titers in AMCRs. To conclusively determine the role the NS3-249P mutation in facilitating emergence of L2 WNV, I examined the phenotype of NS3-249P and NS3-249H L2 WNV mutations in a transmission cycle inclusive manner. Specifically, I found that the NS3-249P mutation was directly involved in decreasing fitness in Culex pipiens. Furthermore, I found that following infection in mice, the NS3-249 residue did not modulate neuroinvasive disease phenotypes and suggests that the emergence of L2 WNV in Greece was potentially facilitated by increases in force of transmission related to the occurrence of the NS3-249P mutation, rather than the emergence of a more neuroinvasive L2 genotype.
Description
Rights Access
Subject
lineage 2
West Nile virus