A group of researchers from University of Toronto Engineering and SickKids Hospital have developed a new way to deliver molecules that target specific genes within cells. Their platform, which uses a modified form of a bacterial toxin, has been shown to downregulate critical genes in cancer cells, and could be used for other genetic diseases as well. The team, led by professors Molly Shoichet and Roman Melnyk of SickKids Hospital, found inspiration from an unexpected source: diphtheria toxin. Scientists looking to place molecules inside cells have a number of existing tools to choose from, but most suffer from the same drawback — while the molecule gets inside the cell, it remains trapped in a kind of bubble called an endosome. If the goal is to deliver therapeutics that will interact with the cell’s DNA, breaking out of the endosome is critical. A major challenge in the field of drug delivery is most therapeutic vehicles cannot escape the acid environment of the endosome once they get into the cell. The diphtheria toxin platform as a delivery vehicle effectively solves that.

As a natural defense mechanism, bacteria such as Corynebacterium diptheriae produces a protein toxin that enters surrounding cells, eventually killing them. Critically, this toxin is known to be capable of escaping from endosomes, which led to the idea of re-engineering it as a delivery platform. Diphtheria toxin induces inhibition of protein synthesis by ADP-ribosylation of eukaryotic elongation factor 2 (eEF2). The precursor (pro-HB-EGF) of heparin-binding epidermal growth factor like growth factor (HB-EGF) anchored on the cell surface functions as a receptor for the toxin. After this, DTx enters into cells via endocytosis and passes into cytosol. Inhibition of protein synthesis, destruction of actin cytoskeleton and activation of nuclease are followed by induction of apoptotic process in the cell. However some mutant forms of diphtheria toxin cannot inhibit protein synthesis. Among these there is CRM197, which is used as a carrier molecule in pediatric conjugated vaccines. Because of its inactivating mutation in the catalytic site (E52G substitution), the enzymatic activity is lost.

Melnyk’s lab specializes in bacterial toxins and invented a non-toxic version of the diphtheria toxin (known as attenuated diptheria toxin). This new molecule has the capacity to enter the cell and efficiently escaping the endosome – and thus excels as a delivery vehicle without any of the toxic effects of diphtheria toxin, especially on cellular protein synthesis. To prove that the concept would work, the researchers used the system to deliver molecules that they believed would be effective against glioblastoma, a form of brain cancer that is extremely aggressive and highly invasive and patients have a very short life expectancy after initial diagnosis. The group first targeted glioblastoma neural stem cells, which are thought to be resistant to chemo drugs. Between the highly heterogeneous cell populations in this type of cancer, the most high immaturity of course belong to the staminal (embryonal) component. The more the immaturity, the higher is chemo drug resistance. And endosome signaling is used to recycle many surfece receptors, many of whose are for cytokine or growth factors used to keep stemness.

Backing to the study, the researchers focused on delivering silencing RNA (siRNA) against two genes. The first is integrin beta 1 (ITGB1), which is associated with the highly invasive nature of glioblastoma (and other cancers). ITGB1 is involved in cancer cell migration, which contributes to glioblastoma’s invasion into healthy brain tissues, The second gene is eukaryotic translation initiation factor 3 subunit b (eIF-3b), which is an essential survival gene since it drives cellular protein synthesis. By eliminating this invasive trait, the researchers could potentially limit progression in diseases like cancer. The research team used an innovative 3D culture system to significantly reduce cell invasion after treatment with their siRNA-attenuated diptheria toxin system, which suggests that it may be effective in slowing disease progression. To demonstrate the breadth of this platform, the researchers also delivered a different nucleic sequence that knocks down eIF-3b, which participates in the ‘survival pathway’ of cancer cells. The group is planning on using in the future this delivery vehicle to treat other diseases as well.

• Edited by Dr. Gianfrancesco Cormaci, PhD; specialist in Clinical Biochemistry.

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