Education
- Post-doc, Onderstepoort Veterinary Institute (South Africa)
- PhD, Purdue University
- BA, Texas A&M
Research
My research focuses on hemoparasitic disease employing comparative and functional genomics and transcriptomics and molecular methods. I work in the areas of transmission biology, pathogen persistence and vaccine discovery.
Comparative genomics of Anaplasma marginale: I completed the first genome sequence for A. marginale (Brayton et al., 2005) prior to the high throughput era which detailed the metabolic capacity of the pathogen and revealed that the surface proteome was smaller than expected and skewed to two families of surface proteins, Msp2 and Msp1. Subsequent genome sequences were used in comparative genomics strategies that have defined a list of genes involved in tick transmission (Dark et al., 2009; Aguilar Pierlé et al., 2012). Comparative analyses have shown that A. marginale has a closed core genome (individual strains do not have unique genes), contrasted with a moderately high degree of single nucleotide polymorphisms (SNPs) There is a high degree of genome synteny (conservation of gene order), which has been exploited to analyze the msp2 gene repertoire, showing conservation of alleles throughout the U.S., and hinting at a mechanism for evolution of the msp2 repertoire. A recent collaboration with the Tick Fever Centre in Australia allowed for comparison of a virulent, tick transmissible strain with a non-transmissible, apathogenic strain. These comparisons revealed 1) markers to distinguish between the low virulence strain and Australian field isolates and 2) that Australian isolates appear to be more closely related as they have many fewer SNPs than do U. S. strains (Aguilar Pierlé et al., 2014).
Functional genomics of A. marginale: The lack of a manipulable genetics system for this pathogen makes functional genomics a real challenge (This organism has only been transformed twice). We continue to work to develop a transformation system for this pathogen so that we can functionally test candidates identified through our comparative genomics studies. In the absence of these tools, I have developed (in collaboration with Shira Broschat) an algorithm for the prediction of effectors of the type IV secretion system (T4SS) (Lockwood et al., 2011). These molecules are secreted into and interact with host cell machinery. We have developed a transfection system for ISE6 tick cells where we can directly express the bacterial effector in the host cell to study the effect of these proteins.
Transcriptomics of A. marginale: The development of high throughput sequencing technologies has recently allowed for transcriptional analysis of this obligate intracellular organism on a genome-wide scale. I have employed these tools to 1) examine transcriptional differences between a tick transmissible and non-transmissible strain (Aguilar Pierlé et al., 2012), 2) examine the remodelling of surface of A. marginale when it transitions between the tick vector and the mammalian host (Hammac et al., 2014), and 3) examine the altered transcription in a transformed strain of A. marginale which exhibits a slow growth phenotype (Aguilar Pierlé et al., 2013). In these studies it has been shown that all but twenty bases of the genome are transcribed. We are currently analyzing the transcriptome in a strand specific manner to confirm this result as it will have enormous implications for protein expression.
Vaccine discovery: My work combined with that of Wendy Brown, Susan Noh and Guy Palmer has elucidated a list of vaccine candidates (between 8 and 20, depending on screening criteria; Palmer et al., 2012) for A. marginale that are currently in various stages of testing. If not for the genomics, we would never have identified several of these candidates. I have tested the slow growing transformant (mentioned above) as a potential vaccine platform (Hammac et al., 2013), which has elicited interest from several quarters.
Tick transmission: In addition to identifying pathogen genes involved in transmission as mentioned above, I have worked on the tick side of transmission. I developed a tick microarray to assess transcriptional changes in the tick during infection with the pathogen, A. marginale. This study was the first to document that relatively few tick genes are differentially transcribed in response to pathogen infection, which has now been seen with several other tick-pathogen pairs (Mercado-Curiel et al., 2011). We are studying the bacterial microbiome of Dermacentor andersoni ticks to determine 1) how stable it is over time, 2) whether we can manipulate the microbiome to affect transmission of pathogens. This work is in the early stages, but we have seen that the microbiome is not particularly stable, that we can shift it, and we are currently assessing whether altering the microbiome composition affects transmission of A. marginale. In collaboration with Susan Noh, I will also use Francisella novicida in future transmission studies.
Publications