Date of Award

5-2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Medical Science)

Program

Biological Science

Track

Genetics, Functional Genomics and Proteomics

Research Advisor

Malak Y. S. Kotb, Ph.D.

Committee

Linda M. Hendershot, Ph.D. Tony N. Marion, Ph.D. David R. Nelson, Ph.D. Robert W. Williams, Ph.D.

Abstract

Clinical outcomes of infectious diseases are controlled by complex interactions between the host and the pathogen. Epidemiological, genetic and molecular studies in my mentor’s laboratory provided evidence that in invasive Group A streptococcal (GAS) infections, genetic variations in both bacteria and patients influenced the severity of GAS sepsis. Allelic variations in class II human leukocyte antigens (HLA) contributed significantly to differences in the severity of group A streptococcal sepsis caused by the same virulent strain of the bacteria. HLA class II molecules present streptococcal superantigens (SAgs) to T cells, and variations in HLA class II molecules can strongly influence SAg responses. However, the bacteria produce a very large number of additional virulence factors that participate in the pathogenesis of this complex disease, and it is likely that host genes besides HLA class II molecules are also participating in modulating the severity of GAS sepsis. The main focus of this Ph.D. project was to identify additional host genes and pathways that may be modulating the severity of GAS sepsis.

To achieve this goal I applied a systems genetics approach, involving genome wide association studies (GWAS) of GAS sepsis in the Advanced Recombinant Inbred (ARI) panel of BXD mouse strains. We used this panel of ARI-BXD strains as a genetically diverse reference population to study differential severity of GAS sepsis as ARI-BXD strains diversity mimics the genetic diversity of human population. We assessed several traits associated with differential host responses to GAS sepsis, and analyzed variations in these traits in the context of mice genotypic variability, using genome-wide scans and the sophisticated analysis tools of WebQTL. This allowed us to map quantitative trait loci (QTL) associated with modulating susceptibility to severe GAS sepsis on chromosome (Chr) 2 and Chr X. The mapped QTLs strongly predicted disease severity (accounting for 25–30% of variance), and harbored highly polymorphic genes known to play important roles in innate immune responses. Based on linkage analyses, gene ontology, co-citation networks, and variations in gene expression, we identified interleukin 1 (IL1) and prostaglandin E (PGE) pathways as prime candidates associated with modulating the severity of GAS sepsis.

To further investigate mechanisms underlying differential host susceptibility, we analyzed genome-wide differential gene expression in blood and spleens of uninfected vs. infected mice belonging to highly resistant or susceptible BXD strains, at selected times post infection. Our transcriptional analyses revealed common pathways between susceptible and resistant strains associated with innate immune response, e.g. Interferon signaling pathway.

Since our data has pointed to a strong association of differential response to GAS with innate immune responses, we explored if differences in the numbers of relevant immune cells among the BXD strains played a role in their differential susceptibility to GAS. We found no significant differences in numbers or percentages of immune cell populations between susceptible and resistant strains under normal, uninfected conditions. However, depletion of neutrophils and/or macrophages significantly increased the severity of GAS sepsis in both resistant and susceptible strains. Taken together, our data suggested that differences in mobilization and /or function of these cells between susceptible and resistant strains might play a role in modulating differential severity of GAS sepsis.

In conclusion, we found that variations in the severity of GAS sepsis have a strong genetic component that is complex and multigenic. Different combinations of genetic variants influenced theonset, progression, and severity of GAS sepsis and disease and ultimate outcome. Our overall approach of systems genetics, where we systematically dissected genetic, molecular, cellular and functional differences that may be associated with differential host susceptibility to GAS provided us with tremendous insight into disease mechanism. The knowledge gained can help the development of better diagnostics and means to predict disease severity based on a set of genetic and prognostic biomarkers to help customize patient care, to apply effective and more targeted therapeutic interventions and improve disease outcomes in septic patients.

DOI

10.21007/etd.cghs.2010.0003

Comments

Two year embargo expired May 2012.

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