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Can the community of bacteria living in our gut change the course of CCM disease?

Data from mouse studies at the University of Pennsylvania suggest that yes, modulating the type of naturally-occurring bacteria in our gut can block the development of CCM lesions.

The team at U Penn has worked for several years with genetic models of CCM1 and CCM2 diseased mice. This is an early onset model that uses a genetic trick to turn off both copies of a CCM gene specifically within endothelial (blood vessel) cells, just after the mouse is born. In this model, mice develop lots of CCM lesions within a week after birth. To turn the gene off, an injection of the drug tamoxifen is given, which works with engineered sequences in the mouse DNA to selectively turn off genes.

This story of discovering the gut connection to CCM began after moving the mouse colony to a new animal facility on campus. The research team noticed a subset of the mice suddenly stopped developing lesions. These resistant mice continued to breed and for generations, no CCMs were seen. Mostly. A few mice in this new CCM-resistant group did develop an aggressive course of CCM. Interestingly, these mice with CCM lesions also had developed bacterial infections at the tamoxifen injection site. This observation of bacterial infections being associated with lesion formation led the team to further investigate the natural bacteria living in the mice, their microbiome.

The team hypothesized that a specific type of bacteria, Gram-Negative Bacteria, accelerates lesion formation. Several experiments provided evidence toward this hypothesis:

  • Bacterial infection – Using resistant mice (those from the portion of the colony that have the genetic mutations and should develop lesions, but do not), the team injected gram-negative bacteria to cause the mice to develop an infection. Indeed, 9 out of 16 previously-resistant mice developed CCM lesions. This experiment showed that the bacterial signal traveled through the blood to stimulate lesion development in the brain.
  • Sterile C-Sections – To assess the impact of germs in the environment, the team aimed to raise mice from the original colony (that develop CCM lesions) in sterile, germ-free environments. Mice were delivered by sterile C-section and raised by mothers in a standard or germ-free environment. Amazingly, 7 of 8 mice that were raised germ-free had no lesions!
  • Fecal analysis – As the last step, the team analyzed mouse fecal matter and found one specific type of bacterial especially prevalent in the mice that develop lesions and absent from those lesion-free mice.

How can these findings translate to therapies for human CCM patients? In associated studies of the signaling molecules, the team identified the molecule TLR4 as that which receives a signal from the gram-negative bacteria and stimulates downstream events to cause lesion development. The details of the chemical signal are described in more detail in the paper, Endothelial TLR4 and the microbiome drive cerebral cavernous malformations. The UPenn Study team identified two potential druggable targets: TLR4 (the signal receiver) and gut bacteria (which sends signals through the blood), both of which were tested in mice with exciting results.

The following model is how bacteria trigger TLR4 to stimulate further signaling and eventually CCM lesion development. Disrupting the signaling pathway at either the point of the bacteria or the TLR4 receptor has shown to inhibit lesion development in mice.

Microbiome Illustration

 Blocking TLR4 with a small molecule inhibitor (drug) decreases lesion volume in mice by 80%. Also, treating parent mice with antibiotics specific for the gram-negative bacteria resulted in mouse offspring with a 95-100% reduction in lesions.

These results show us that targeting either the bacteria themselves or the TLR4 molecule that receives the bacterial signal can have profound effects on lesion development. The next step in the research process is to determine whether these findings translate to the human condition. Investigation of human CCM patient microbiome is underway - a pilot study is complete and a larger study has been proposed.

 

-- Amy Akers PhD

 

Last updated 12.27.2018