Oncogenic viruses

Maria Shkalikova, "Biomolecule" 

Over the past three decades, it has become increasingly clear that some viruses play a key role in the development of human malignancies. In recent years, there has been an increase in the number of patients suffering from chronic recurrent herpesvirus infections. According to various authors, currently up to 80-90% of the world's population is infected with the Epstein-Barr virus (EBV). This virus is associated with the occurrence of African Burkitt lymphoma (LB), undifferentiated nasopharyngeal cancer (nRNG), B-cell lymphomas in patients with immunodeficiency of various origins. The epidemiological significance of EBV is determined by the wide circulation of the virus among the population. The urgency of the problem is due to the high degree of damage to people all over the world, often the latent course of the disease and the lifelong persistence of the virus. According to WHO, diseases caused by herpesvirus infection occupy the second place after influenza among all infectious pathologies.

Research in the field of the origin of malignant tumors, which annually claim millions of human lives, has been conducted since the XIX century. Working in the field of molecular virology, Varmus, together with J. M. Bishop, made a discovery in the research of the 1970s, which in a new way illuminated the long-debated problem of the etiology of tumors in humans and animals. According to the results obtained, the uncontrolled growth of cells forming a tumor is caused not only by an oncovirus penetrating into the cell from the outside, but also by internal processes in the cell itself. Varmus proved that normal cell growth genes, due to random spontaneous mutations under the influence of chemical carcinogens or, sometimes, the aging process, can change their molecular structure and thus turn into oncogenic protoviruses. For the discovery of the cellular origin of oncogenic protoviruses, Varmus and J. M. Bishop were awarded the Nobel Prize in Physiology or Medicine in 1989 [1].

Also in the early 1970s, the German researcher Harald zur Hausen [2] successfully used the then new in situ hybridization technique for virus identification. Scientists have long assumed that cervical cancer and other genital cancers are caused by some sexually transmitted virus, such as the herpes virus. However, nothing happened with Hausen's herpes: the DNA of the herpes virus in the sample of the cervical tumor could not be detected. Therefore, he suggested that cervical cancer is not caused by the herpes virus, but by another agent – the human papillomavirus. The assumption is unexpected, since the papilloma virus was considered quite harmless at that time. Zur Hausen also suggested that the DNA of the papilloma virus is embedded in the genome of the "host" cell, that is, tumor cells store the genetic information of the virus. Moreover, the papilloma virus does not reproduce its copies in the cell, as it usually happens with a viral infection, but rather "sleeps" inside infected cells. Subsequently, in 1983, he and his colleagues were able to confirm this hypothesis and establish that most cases of cervical cancer are caused by one of two types of these viruses: HPV-16 and HPV-18. Cells infected with such viruses are quite likely to become cancerous sooner or later, and a malignant tumor develops from them. In 2008, Harald zur Hausen was recognized as the winner of the Nobel Prize in Physiology or Medicine "for the discovery of the human papillomavirus as a cause of cervical cancer" [3]. Harald zur Hausen also studied EBV and tried to find the relationship of the virus with other diseases. As a result, he succeeded: he proved that EBV contributes to the occurrence of Burkitt's lymphoma [4, 5].

One of the most common viruses in the human population – the Epstein-Barr virus (Fig. 1) – was discovered and described in 1964 by two English virologists: Michael Epstein and Yvonne Barr. Epstein-Barr virus (EBV) is a member of the herpesvirus family [6, 7]. In infected cells, viral DNA, as a rule, is not embedded in the cellular genome, but is located in the nucleus in the form of a closed ring (episome). The biological significance of the integration of EBV into the cell genome remains unclear. It is suggested that episomal DNA is necessary for the implementation of full-fledged replication of EBV, culminating in the formation of viral particles [8].

Here it reproduces most actively, and therefore the main way of infection with the virus is kissing (here we have arrived). The largest number of viral particles is located in the epithelial cells near the salivary glands, and a large number of them are released with saliva. It is not surprising that infectious mononucleosis, the most common disease caused by the Epstein–Barr virus, is also called kissing disease [9].

The entrance gates of infection are the cells of the oral cavity and nasopharynx [10]. EBV penetrates into the B-lymphoid tissue of the oropharynx, and then it spreads throughout the lymphatic system of the body. The DNA of the virus enters the cell nucleus, while the EBV proteins provide infected B-lymphocytes with the ability to multiply continuously in culture. The virus can make cells immortal [11].

After the first meeting of a person with EBV, the virus persists in a small amount in the host's body throughout life. However, if any element of the immune response is disrupted, even a small number of EBV-infected cells can multiply enormously [12].

Infected B cells can stay in the tonsils for a considerable time, which allows the virus to be released into the external environment with saliva. With infected cells, EBV spreads to other organs. Two types of development are possible in virus-affected cells: lytic, which leads to the destruction of the host cell, and latent (the cell is infected, but nothing gives away the presence of the virus), when the number of viral copies is small and the cell is not destroyed. EBV can remain in B-lymphocytes, nasopharyngeal epithelium and salivary glands for a long time. In addition, it is able to penetrate into other cells: T-lymphocytes, NK cells, macrophages, neutrophils, vascular epithelial cells (Fig. 2a, 2b) [13].

Lymphomas are divided into two main groups: Hodgkin's lymphoma (20-30% of all lymphomas) and non-Hodgkin's lymphomas (about 70%) [14, 15].

Hodgkin's lymphoma (LH) is an independent disease in which characteristic altered (cancerous) cells, known as Hodgkin and Reed–Sternberg cells, are scattered among the surrounding inflammatory cells and make up only about 2% of the tumor mass volume [16]. After the initial infection, the virus remains in the memory B cells in a "dormant" state for a long time throughout the host's life. In healthy individuals infected with EBV, the innate host immunity (inherited from mom and dad) has an inhibitory effect on the reproduction of the virus [17]. Consequently, the increased risk of developing LH in patients with immunodeficiency can be explained by the loss of immune control over the viral infection.

Non–Hodgkin's lymphomas are a set of neoplasms in the occurrence of which various agents are involved [18]. The first group is viruses that transform lymphocytes and other cells (EBV, HHV-8). The second group is represented by factors of various nature that cause immunodeficiency conditions. These factors primarily include HIV (human immunodeficiency virus), which causes an infected person to suppress immunity as a result of depletion of the CD4+ T-lymphocyte pool and the occurrence of AIDS. The third group includes some infections (for example, H. pylori), which increase the risk of lymphomas against the background of their chronic stimulation of the immune system and constant activation of lymphocytes [17].

Recent studies indicate that the harmful LMP1 - latent membrane protein 1 encoded by the gene of the same name (LMP1) plays an extremely important role in the pathogenesis of EBV–associated pathologies. It has the properties of an oncoprotein and functions as a constantly active pseudoreceptor. It is capable of altering human B-lymphocytes [19].

There is a hypothesis that the virus "stole" this gene from human cells in the process of evolution. In addition, it is suggested that this protein may play a role in the development of autoimmune diseases [20].

There is an assumption that amino acid substitutions accumulating in LMP1 also seem to contribute to the occurrence of tumors. The mechanism of this process has not been definitively established, but it is assumed that the enhanced transformative effect of mutated LMP1 may represent an important component of this process. It has been shown that differences in the sequence of the LMP1 gene can determine an aggressive geographically localized EBV genotype [21].

Of the known mechanisms of action of LMP-2 (the second of the brothers of the LMP family) located at the opposite end of the linear genome, only the ability of these proteins to jointly increase signal transduction in EB (+) cells is mentioned [19].

According to Harald zur Hausen, the link between the virus and cancer is considered established when determining the following criteria:

Despite the fact that the products encoded by the virus are capable of causing the proliferation of infected cells, leading to the appearance of lymphomas in patients with immunodeficiency, these clinically aggressive tumors are quite often polyclonal and undergo regression when the immune response to EBV is restored. Tumors such as Burkitt's lymphoma (LB) and Hodgkin's lymphoma (LH) are found not only in EBV-associated, but also in EBV-unassociated variants, which suggests that the pathogenesis of these neoplasms is not only associated with EBV. In addition, malignant cells of patients with LB and LC differ phenotypically from LCL cells obtained under the influence of EBV in vitro, and do not express a number of proteins necessary for transforming growth. These findings suggest that tumor cells may also arise under the influence of factors of non-viral origin, as well as depend on various stimuli that enhance cell growth [17].

Laboratory diagnostics of EBV infection is based on cytological examination of blood or bone marrow, serological studies and PCR. Using the PCR method, it is possible to determine the DNA of the virus in plasma before the clinical manifestations of the disease, and the replication of the virus in the body is an indication for antiviral therapy and a criterion for the effectiveness of the treatment. The material for the study is saliva or oral and nasopharyngeal mucus, scraping of epithelial cells of the urogenital tract, blood, cerebrospinal fluid, tumor tissue and bone marrow. Both EBV patients and carriers can have a positive result in PCR. Therefore, in order to differentiate them, a quantitative PCR analysis is performed to determine the number of copies of the viral genome. In young children (up to 1-3 years old) due to insufficiently formed immunity, antibody diagnosis is difficult, therefore, in this group of patients, PCR comes to the rescue. However, due to the fact that PCR analysis is informative only when the virus multiplies (replicates), there is also a certain percentage of false negative results (up to 30%) associated with the lack of replication at the time of the study. At the same time, it is important to compare the results of clinical, serological and molecular examinations in determining EBV infection as the cause of the existing disease [1].

Specific prevention (vaccination) against EBV has not been developed, but clinical trials are being conducted. The main problem in the development of a vaccine is a large difference in the protein composition of the virus at different phases of its existence. However, a vaccine is currently being developed that contains the recombinant gp350 surface antigen. After vaccination, the primary infection proceeds subclinically, but the actual infection of a person is not prevented. In addition, the neutralizing antibodies produced do not affect the course of various forms of latent infection, including tumors. Preventive measures are reduced to strengthening immunity, hardening children, precautions when a patient appears in the environment, compliance with personal hygiene rules.

Like most tumors of a different viral nature, additional factors play an important role in the pathogenesis of EBV-associated neoplasms, and EBV alone is not enough to cause a tumor. EBV only initiates the proliferation of cells infected with it, and subsequent events affect the histopathological spectrum of emerging neoplasias. One of the most important factors that largely determine the occurrence of EBV-associated tumors is pronounced immunosuppression (congenital, iatrogenic or induced by any viral infection, primarily HIV), leading to the loss of the function of immune recognition of cells infected with EBV.

Thus, despite the long-term study of the connection of EBV with human tumors, the question of the role of the virus in their occurrence has not been fully studied. Uncovering the mechanism of malignant transformation by a virus that persists in a latent state in more than 90% of the world's population is an extremely difficult task. However, the technical achievements of recent years, which have significantly increased the specificity of research, allow us to hope that the details of EBV-associated carcinogenesis will be clarified.

24.11.2015