Researchers discover novel way to get proteins into tumoursBMJ 1998; 316 doi: https://doi.org/10.1136/bmj.316.7143.1477l (Published 16 May 1998) Cite this as: BMJ 1998;316:1477
Scientists have discovered a novel way to get cytotoxic proteins into the heart of tumours. Their discovery relies on a structural component produced by the herpes simplex virus which seems to invade cells with great ease. By fusing cytotoxic proteins to this component, the researchers hope to piggyback proteins directly into tumour cells, achieving a “direct hit.”
Peter O'Hare and his team at the Marie Curie Institute in Surrey observed that the protein known as VP22, which is produced by the herpes simplex virus, is able to spread from a single cell, gaining rapid access through neighbouring cells.
Under the microscope VP22 is seen to create a “halo effect” around the central cell as it extends outwards, and within 40 hours a whole layer of single cells will contain the protein in their nuclei. Not only does this property seem to be unique to this particular protein, but according to Dr O'Hare, it is intuitively difficult to understand.
“It doesn't make sense for one protein to leave a cell, and then want to enter other cells,” he said. “It goes against the dogma of conventional protein trafficking which usually depends on the process of endocytosis—VP22 appears to get in and out of cells even when endocytosis is blocked.” So far the function of VP22 remains elusive, but one suggestion is that VP22 may be involved in the replication of herpes simplex virus—acting as an “advance scout” by spreading into cells and waiting for the virus to invade.
Tumour cells are notoriously difficult to penetrate with drugs because cancerous lesions tend to have an extremely poor blood supply. Dr O'Hare believes that VP22's ability to spread into neighbouring cells may provide a novel alternative: “Only a small proportion of cells will be hit by conventional gene or drug delivery, but by linking genes or drugs with VP22, although the same number of cells will be hit, we hope that the protein will act as a vehicle, carrying the drug or gene as its cargo.”
To test this idea, the researchers fused VP22 with the p53 gene (which causes apoptosis or cell suicide). About half of all cancers develop in cells that are either missing the p53 gene or contain a mutant version of it. The result is that appropriate cell death does not occur, and cellular damage is not checked.
Dr O'Hare chose to fuse VP22 with the p53 gene partly because the gene causes cell death but also to see if the process of fusion changed the way either VP22 or p53 works. His team showed that the proteins from the fusion of VP22 with p53 not only spread easily through a human osteosarcoma cell line but also that cell death was induced (Nature Biotechnology 1998;16:440-3). More importantly, the VP22-p53 fusions induced greater cell death than p53 alone. Showing that this complex, high profile gene can be successfully linked, and that its function remains intact, offers hope that other drugs and genes could be delivered by a similar route.
In addition, Dr O'Hare believes it may be possible to incorporate other signals which could target fusion proteins to other specific components of the cell, such as the cytoplasm. This would be of particular interest for those involved in developing DNA vaccines, where naked DNA must be held in the cytoplasm in order to be presented to the immune system. If it works, the DNA could be transported to the appropriate site within the cell, as well as achieving a greater degree of spread than it would otherwise have done on its own. Yet another possibility is that drugs which have been consigned to the dustbin because of a failure of transportation in animal models may now be offered a new lease of life.