The bacterial pathogen Borrelia burgdorferi (Bb) is the causative agent of Lyme disease, the most prevalent tick-borne disease in the United States. This pathogen is capable of long-term infection of humans that can lead to severe arthritis and neurological complications, even in healthy people with intact immune systems. Many patients continue to have debilitating symptoms even after extensive antibiotic treatment, but the reasons behind these symptoms remain unclear. Consistent with this, our proposal directly addresses the TDBRP focus area, to understand underlying mechanisms of persistent symptoms associated with Lyme disease.

We propose the novel hypothesis that when cells are infected by Bb, they respond by turning on genes involved in inflammatory responses, and that these genes are in turn “marked” or “tagged” by small chemical tags called epigenetic marks that allow cells to “remember” the infection. Our genetic makeup, i.e., DNA, can be considered as the hardware, the blueprint for who we are. Epigenetic mechanisms can be considered as the software, that tells the hardware what to do – i.e., determine which genes are turned on or off in different cell types, at distinct times in our lifespan, and in response to environmental stresses that we encounter. Epigenetics is an additional layer of control that sits on top of the genome, (hence the prefix epi-, which means above), and refers to changes in gene activity that do not involve alterations to the genetic code, but still get passed down to at least one successive generation.

Epigenetic “marks” are small chemical tags (methyl, acetyl, phosphate, etc.) that can be attached to the histone proteins around which DNA is wrapped. Different marks or sets of marks can influence whether a particular gene is ON or OFF. It is through epigenetic marks that environmental factors like infection, diet, stress, and other such factors can directly impact genes and how they are turned on or off, and these changes are passed from one cell to its daughter cells when it divides, to the next generation. Although it is a fundamental mechanism that affects several cell types and diseases, very little research has been done on the role of epigenetics in Lyme disease thus far.

We suggest that infection with Bb results in changes in both gene activity (i.e., turning different sets of genes on or off) and also epigenetic marks. Although the gene activity might return to normal once the infection is removed, the epigenetic marks can persist years after the infection is treated, similar to a bookmark, to “remember” that these inflammatory genes were once turned on. Subsequent inflammatory events unrelated to infection might then lead to a rapid and more robust activation of these epigenetically marked genes, resulting in the debilitating symptoms of refractory Lyme arthritis (RLA). Our proposal aims to identify (1) which genes are “memorized” in response to Bb infection, and (2) the genome-wide location of two epigenetic “memory” marks we have identified that can be triggered and then remembered by exposure to the Bb pathogen and whether we can remove these marks using commercially available epigenetic inhibitor drugs to prevent RLA symptoms. This would further address a second Tick-Borne Disease Research Program focus area in the future (treatment and prevention of Lyme disease).

Applicability of the research:

This project has the potential to impact the health and welfare of both military Service members and the American public in the future. In addition to being an economic burden and threat to the public, Lyme disease is a continuing threat to military personnel operating in tick-rich locations, underscoring the need to find effective treatments. This project will lay the groundwork for understanding which genes are involved in driving RLA symptoms, as well as whether these genes are marked by epigenetic memory marks. We predict that epigenetic memory marks might be driving Lyme disease symptoms in at least some patients. Understanding what these marks are and how they are introduced and maintained would help to better manage and treat the disease.

The good thing about epigenetic changes is that they are reversible, unlike gene mutations. As several drugs targeting epigenetic proteins currently exist in the clinic and/or in clinical trials, reversing these marks to treat the disease is highly feasible. Although we are not proposing clinical trials, our work will lay the groundwork for future studies to test feasibility of epigenetic inhibitors using first a mouse model, and then clinical trial for treatment of Lyme disease. Further, identifying epigenetic changes may also lead to the development of biomarkers that could help with both the initial diagnosis of Lyme disease, as well as to monitor disease progression and resolution. Finally, epigenetic mechanisms are widespread, and this work could easily be extended to other tick-borne diseases as well.