Parvoviruses such as H-1PV have been shown to selectively attack and destroy human cancer cells. However, they are unable to replicate in healthy human cells. A preliminary clinical trial is currently being carried out to assess the suitability of parvoviruses for treating malignant brain cancer. Scientists from the German Cancer Research Center (DKFZ) have now discovered the cause of the selective effect of H-1PV. The finding helps identify cancer patients who might benefit from parvovirus therapy.
Parvoviruses are among the smallest known viruses (Latin: parvus = small). They have a single-stranded linear DNA whose genome is around 5,000 nucleotides in size. Their genome only carries the information for a handful of proteins, including the polypeptides of the viral capsid as well as a few regulatory proteins such as NS1. Such proteins are essential for viral replication and destruction of the host cell. Prof. Dr. Jean Rommelaere, director of the Department of Tumour Virology at the German Cancer Research Center (DKFZ) in Heidelberg, has been researching parvoviruses since 1992.
The main reason for the scientific interest in these tiny pathogens is that parvoviruses such as H-1PV, which are common in rodents, can infect normal human cells but without being able to replicate and cause disease symptoms. That said, the viruses mature and multiply rapidly in cancer cells, destroying the cells as they are released from them.
Rommelaere and his team are seeking to exploit the viruses' selectivity for cancer therapy. Their main focus is glioblastoma, the most malignant of all human brain tumours. The minuteness of the viruses is a decisive advantage for the intended purpose: they can easily cross the blood-brain barrier and can therefore be administered through the bloodstream.
Five years ago, the researchers showed in the animal model that parvovirus H-1PV led to the regression of human glioblastomas in rats' brains (see article entitled "Viruses against cancer: complete regression of malignant brain tumours after treatment with parvoviruses").
A preliminary clinical trial has been ongoing since 2011 at Heidelberg University Neurosurgery Hospital to evaluate the safety of treating cancer patients with the parvovirus H-1PV. The study involves patients with advanced glioblastoma, who are given gradually increasing doses of the virus. The results are promising as Dr. Jürgen Nüesch from the DKFZ's Department of Tumour Virology says optimistically: "We have already seen that the viruses are able to find the glioblastoma cells, even when they were injected into an arm vein." Adverse effects have so far not been observed, not even at doses as high as 109 infectious units per patient. Whether the viruses also help prolong survival time will have to be clarified in a larger trial.
Molecular details behind the viruses' selectivity for cancer cells have not been understood until now. "As the viruses might soon play a role in cancer medicine, it is important to know why they replicate exclusively in tumour cells in humans," says Nüesch. Nüesch and his colleagues Séverine Bär and Jean Rommelaere have recently published a paper in the journal PLOS Pathogen that explains the viruses' selectivity.
In order to replicate in infected cells, destroy the cell membrane and produce the next generation of viruses, the viruses depend on the activity of a cellular enzyme called 3-phosphoinositide-dependent kinase 1 (PDK1). This enzyme acts as a main switch for numerous signalling chains that control cell growth and cell proliferation. PDK1 activates other enzymes by way of phosphorylation and is itself also activated by way of phosphorylation. Normally, the enzyme is activated from the outside by growth factors that interact with certain phospholipids (phosphatidylinositol phosphates) in the cell membrane.
The scientists from Heidelberg have discovered in mouse cells - natural hosts for parvovirus H-1PV - an internal PDK1 activation pathway independent of external growth factors. They found an enzyme complex called PKCη/Rdx that transfers a phosphate group to a serine building block at position 138 in the PDK1 molecule, and then activates it. This finding explains why parvoviruses can complete their life cycle in rodent cells, but not in human cells: the activation of the PDK1 enzyme is required for the proper function of the viral protein NS1, which is involved in the regulation of viral DNA and the expression of viral genes. NS1 also induces cytotoxicity and the processes of cell lysis.
Nüesch and his colleagues examined numerous tissue samples of human glioblastomas and found a PDK1 enzyme in many of them. This enzyme is phosphorylated at position 135 (position 138 in mice) from the start, and is therefore permanently activated. Permanently activated "PDK1phosphoS135" seems to be characteristic of advanced brain tumours. It also appears to be the reason why parvoviruses are able to replicate in cancer cells, but not in healthy cells. When the researchers equipped healthy human cells (in which the viruses are normally unable to replicate) in the Petri dish with permanently activated PDK1, they observed that more viral DNA was synthesised in the cells and that NS1 induced the death of the infected cells.
Jürg Nüesch explains that for the cancer cells, permanent activation of PDK1 is biologically useful because it allows the cells to be independent of growth factors. "The parvoviruses in turn exploit this for their own purposes. Thus, we have found for the first time a molecular cause for the viruses' natural selectivity for cancer cells." In addition, phosphorylation of PDK1 (PDK1phosphoS135) in human brain tumours enables the researchers to predict whether therapy with parvoviruses can be effective in a particular tumour.
Séverine Bär, Jean Rommelaere, and Jürg P.F. Nüesch: PKCη/Rdx-driven Phosphorylation of PDK1: A Novel Mechanism Promoting Cancer Cell Survival and Permissiveness for Parvovirus-induced Lysis. Plos Pathogen 2015, DOI: 10.1371/journal.ppat.1004703