Scientists from the University of Surrey have made an important breakthrough in the fight against the flesh-eating tropical skin disease Buruli ulcer, by their discovery that the bacteria causes a blood clot in patients’ skin, similar to those that cause deep vein thrombosis (DVT). The new findings mean that, like DVT, the clots may respond to anticoagulant medicines, heal more quickly and with fewer side effects than with antibiotics alone.
The team hopes this major discovery will accelerate the development of a cure for this chronic debilitating disease which affects poor communities in West Africa, and can lead to permanent disfigurement and disability. The World Health Organisation considers Buruli ulcer to be an emerging threat to public health.
“This is a huge breakthrough in our understanding of the disease,” said lead author Dr Rachel Simmonds from the University of Surrey. “Buruli ulcer is an emerging tropical disease, which is caused by infection with Mycobacterium ulcerans, an organism which belongs to the family of bacteria that causes tuberculosis and leprosy. Around 5,000 cases are recorded each year, the majority in poor rural communities in West Africa, Australia and Southeast Asia where the infection is thought to occur when people bath in slow running water.
“While antibiotics are currently used to treat Buruli ulcer, they take a long time to work and few people with the disease can afford to pay for extended stays in hospital. The ulcers are often painless, and as a result, early signs of infection are ignored, or thought to be a ‘curse’. Infected people, often children, are treated by traditional healers rather than modern medicine.
“We hope our research will now enable better treatment combinations that will reduce the lifetime deformity patients have to bear.”
A well-known histopathological feature of diseased skin in Buruli ulcer (BU) is coagulative necrosis caused by the Mycobacterium ulcerans macrolide exotoxin mycolactone. Since the underlying mechanism is not known, we have investigated the effect of mycolactone on endothelial cells, focussing on the expression of surface anticoagulant molecules involved in the protein C anticoagulant pathway. Congenital deficiencies in this natural anticoagulant pathway are known to induce thrombotic complications such as purpura fulimans and spontaneous necrosis. Mycolactone profoundly decreased thrombomodulin (TM) expression on the surface of human dermal microvascular endothelial cells (HDMVEC) at doses as low as 2ng/ml and as early as 8hrs after exposure. TM activates protein C by altering thrombin’s substrate specificity, and exposure of HDMVEC to mycolactone for 24 hours resulted in an almost complete loss of the cells’ ability to produce activated protein C. Loss of TM was shown to be due to a previously described mechanism involving mycolactone-dependent blockade of Sec61 translocation that results in proteasome-dependent degradation of newly synthesised ER-transiting proteins. Indeed, depletion from cells determined by live-cell imaging of cells stably expressing a recombinant TM-GFP fusion protein occurred at the known turnover rate. In order to determine the relevance of these findings to BU disease, immunohistochemistry of punch biopsies from 40 BU lesions (31 ulcers, nine plaques) was performed. TM abundance was profoundly reduced in the subcutis of 78% of biopsies. Furthermore, it was confirmed that fibrin deposition is a common feature of BU lesions, particularly in the necrotic areas. These findings indicate that there is decreased ability to control thrombin generation in BU skin. Mycolactone’s effects on normal endothelial cell function, including its ability to activate the protein C anticoagulant pathway are strongly associated with this. Fibrin-driven tissue ischemia could contribute to the development of the tissue necrosis seen in BU lesions.