Are viruses dead or alive? To answer this question just imagine an apple fallen from the tree and lying on the ground. Its multiplication is only possible under very specific conditions of its environment, which supply sufficient nutrients and the suitable temperature. The virus per se is down to the detail similar to the apple - it has a coat, corresponding to the apple-peel, a nucleus as symmetric as the apple core, in the virus there is even an analogy to the pips, called nucleoproteins. Therein is the nucleic acid, the genetic information of virus or apple with instructions for the descendants. In addition viruses have a surface spices with which they choose in which cell they want to multiply. The AIDS virus chooses cells of the immune system - lymphocytes - thus exactly those cells, which the body needs to attack an intruding virus. Hence the disease, the acquired immune deficiency syndrome AIDS. The virus is named after the disease: Human-Immune-deficiency-Virus, HIV (Fig. 1). A small detail, based on the fact that the hereditary information of these viruses is hot exactly the same as the host cell's, gave them the name retroviruses. In retroviruses the hereditary information does not consist of the much-quoted double helix DNA, but is a single-stranded RNA. For virus replication the virus uses a trick to adjust its RNA to the specific information of the host cell. The virus is able to transform RNA into DNA by means of an enzyme, which it carries along for that very purpose: the reverse transcription. The single-stranded RNA resembles a flexible rope and can be shortened or changed by a splicing mechanism, known to sailors. The single-strandedness of the RNA is of severe consequence for the AIDS virus; it is able to change permanently. All RNA viruses have this capability i.e. the influenza virus, too. The changeability is the major problem for the vaccine production. Not only does the virus escape the immune system by its variability but also the pace of the vaccine producers, or even worse, the imagination of the vaccine producers, who have failed up to now to find a vaccine.
| Fig. 1. Retroviruses can show up in various forms. One leads to AIDS, which is caused by the Human Immunedeficiency Virus HIV. It is replication-competent and leads to a high number of virus progeny. It destroys its host cell, which is responsible for an immune response against viruses. The virus destroys this response. In contrast to HIV cancer viruses exist which cannot replicate but can recombine with the host genetic material and caused cancer. The genes taken up by the virus are modified and become viral oncogenes, V-Onc. The cancer virus create the tumor cell, which is not killed by the virus but malignantly transformed. This process of a tumor virus can be reversed by taking a tumor virus in which the oncogene is replaced by a therapeutic gene (TG). Such a virus can infect a tumor cell revert its phenotype to a normal cell without producing virus particles. This approach is used for gene therapy in many clinical trials in patients. |
The straying around of the viral RNA in the host cell is the explanation of
another manifestation of retroviruses: besides of AIDS they can also cause cancer
(Fig. 1). Cells carry more than 200'000 genes, the virus, however, only more
than 10. As consequence there is a genetic exchange - the virus can recombine
with genetic information of the cell. This new viral gene must supply an advantage
for growth for the host cell in which the virus is located. If, on the contrary,
the virus is stuck in a cell, which divides more slowly than its surroundings,
this cell will be overgrown by its neighbour cells and will perish. For successful
virus replication it must be the other way round: The cell containing the virus
must overgrow the other cells. Thus we come to cancer cells and tumor viruses.
The new viral gene is a growth accelerator for the cell and is called cancer
gene or oncogene. To understand the main problem of struggle against cancer,
just remember that the oncogene originates from the cell and thus is very cognate
with the normal cellular gene. If you find a therapy against the viral oncogene,
you also damage with great probability the normal cell - this is the dilemma
of cancer therapy. Approximately 100 oncogenes captured by retroviruses are
known. They work as accelerators for growth of the host-cells, leading to tumor
growth. However, oncogenes dispose of yet another capability, themed 'if one
strategy is good, two is better'. Oncogenes are capable of blocking the natural
cellular breaks. The breaks of a healthy cell are the tumor suppressor genes.
They always operate whenever the hereditary information is at danger. By slowing
down growth, the cell plays for time in order to repair mistakes in the DNA
- a sort of self-purification process. In the presence of a viral oncogene this
break can be blocked and the naturally occurring mistakes in the genetic information
of a cell accumulates instead of being repaired. Development of cancer is almost
for sure. There is no motive for the virus to leave the host-cell in view of
such an advantage of growth. Furthermore, the virus adjusts to the host-cell,
the viral RNA is being transformed into DNA by means of the reverse transcription
and thus can be built into the chromosome of the cell. The cell carries now
not 30'000 genes, but 30'001, i.e. one oncogene in addition and a few remainder
genes of the virus. The uptake way of a cellular gene by a virus gene back to
the cell is almost always connected with mistakes - so called mutations. Thus
the cell receives in the end a mutated gene - a v-onc-gene, a viral oncogene.
Only a new helper retrovirus super-infecting the cell can help the integrated
tumor virus to leave the host-cell and release its hereditary information. This
helper retrovirus supplies proteins for packaging of the genome into structural
proteins.
Many tumor cells do not produce viruses in contrast to HIV, which releases 1011
virus particles/ml daily into the blood. Cancer patients have no intact viruses
- at the most remainders of viruses (Fig.1). This is the reason why one tries
in the lab to activate slumbering remainders of viruses by over infecting the
tumor cell with helper viruses, which sometimes works. Thus we conclude that
viruses may contribute to human tumor development.
As far as research on the AIDS virus is concerned, our lab has focused on understanding
molecular mechanisms on virus replication, among others on the reverse transcriptase
(Fig. 2). The reverse transcriptase does not only build up the DNA but has yet
another capability, namely to destroy the RNA matrix after having produced the
first DNA copy. In the process from single strand RNA to the double helix DNA
results an intermediate, a hybrid, whose RNA is being disintegrated by an RNase
H (H for hybrid), which we discovered. The analysis of this mechanism on the
molecular level has been a focus of our research for many years and has led
to the development of an inhibitor for reverse transcriptase. We imitated a
piece of DNA, which puts an end to both, the reverse transcriptase and the RNase
H at the same time. This substance has been very effective in the lab and is
presently tested in mice and afterwards in apes, the only appropriate animal
model. The way into the clinic, however, is uncertain and still long.
Furthermore, we are trying to develop vaccines on the basis of naked DNA - as
many other colleagues too. Naked DNA, produced in vitro by means of recombination
nucleic acids, is then being injected into the muscle of an animal. The vaccine
DNA carries only a few of the viral genes - never all of them. Thus stimulates
the immune response but does not allow virus replication (Fig. 2). The viral
genes are translated into proteins and should above all stimulate the cellular
immune response. Such a vaccine approach has been developed a few years ago
in collaboration with an US-company and tested at the University Hospital in
Zurich with HIV-infected individuals. The vaccine DNA showed no adverse effects,
but also showed no effect.
Vaccination with naked DNA by injection can mimick a virus replication, however without integration of the DNA. The vaccine DNA normally only codes for some of the viral proteins not for a whole virus for safety reasons. The DNA can also code for normal proteins or immune modulators to stimulate the immune response against viruses, cancer or other diseases.
Oncogenes are part of the basic research ongoing at the Institute of Medical Virology. Our investigation is focussed on the viral oncogene Raf (Rat fibrosarcoma). We have shown a few years ago that Raf is a protein kinase, an enzyme, which phosphates other proteins and activates a kinase cascade. This leads to a permanent growth stimulus in the cell and transmits the signal for cell division. We have shown that Raf occupies a key position in the network of signal transduction and plays a major role in many different elementary processes, not only in the development of cancer but also in normal cell growth, in differentiation of immature cells. Furthermore, Raf into normal fibroblasts can cause ageing (senescence), moreover Raf can lead to the cell's death (apoptosis) if the cell is suffering from lack of nutrients. Raf also plays a role in signal transduction in the brain and memory.
Recently a PhD student furnished proof of a missing link of the Raf kinase's switch-off mechanism, the existence of which had to be assumed in theory, but could not be found in practice up to now. The PhD student identified the responsible factor.
Cancer viruses as carriers of oncogenes challenge the imagination of the scientists:
is the inversion possible - to transform a cancer virus, carrying an oncogene
into a therapy virus, carrying a therapy gene? As a matter of fact, this proved
to be possible (Fig. 1). Therapy viruses as gene therapy for patients with terminal
illnesses are being tested in the clinic for the last years.
Retroviruses with therapy genes have been used up to now in almost 100 different
therapy concepts including approx. 1'200 patients. This is a very novel approach
and requires great development work. We ourselves try to specialise the therapy
viruses in a manner that they are only active in tumor cells and not in the
normal cells of the surrounding tissue. We also test different therapy genes
in search of the most effective point of application. We were surprised to find
that one therapy gene was not sufficient to destroy a tumor, but it required
two of them, both of which destroyed a single oncogene.
It is a novel approach to tackle the results of the HIV virus not only from
the point of view of basic research but to take also in consideration the possibility
for therapeutic methods or improved diagnostics. The example HIV showed that
the danger of infectious diseases has not been banished yet. In the field of
cancer therapy we need more success too.
Links to the Research databank of the University of Zürich
Improved
methods for virus detection and diagnosis
The
Raf kinase in signal transduction of normal and tumor cells
Immunotherapy
for breast and ovarian cancer; participation in an EU-Project
Naked
DNA as vaccine and therapeutic against viruses and cancer
Clinical trial against cancer
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