MicrobiologyBytes

Stomach ulcers are caused by chronic infection with the bacterium Helicobacter pylori, which is also the leading risk factor for stomach cancer. One reason for the cancer risk could be that the pathogen creates breaks in the DNA molecules of infected cells:

Carcinogenic bacterial pathogen Helicobacter pylori triggers DNA double-strand breaks and a DNA damage response in its host cells. PNAS USA 108: 14944–14949 (2011)
The bacterial pathogen Helicobacter pylori chronically infects the human gastric mucosa and is the leading risk factor for the development of gastric cancer. The molecular mechanisms of H. pylori-associated gastric carcinogenesis remain ill defined. In this study, we examined the possibility that H. pylori directly compromises the genomic integrity of its host cells. We provide evidence that the infection introduces DNA double-strand breaks (DSBs) in primary and transformed murine and human epithelial and mesenchymal cells. The induction of DSBs depends on the direct contact of live bacteria with mammalian cells. The infection-associated DNA damage is evident upon separation of nuclear DNA by pulse field gel electrophoresis and by high-magnification microscopy of metaphase chromosomes. Bacterial adhesion (e.g., via blood group antigen-binding adhesin) is required to induce DSBs; in contrast, the H. pylori virulence factors vacuolating cytotoxin A, γ-glutamyl transpeptidase, and the cytotoxin-associated gene (Cag) pathogenicity island are dispensable for DSB induction. The DNA discontinuities trigger a damage-signaling and repair response involving the sequential ataxia telangiectasia mutated (ATM)-dependent recruitment of repair factors—p53-binding protein (53BP1) and mediator of DNA damage checkpoint protein 1 (MDC1)—and histone H2A variant X (H2AX) phosphorylation. Although most breaks are repaired efficiently upon termination of the infection, we observe that prolonged active infection leads to saturation of cellular repair capabilities. In summary, we conclude that DNA damage followed by potentially imprecise repair is consistent with the carcinogenic properties of H. pylori and with its mutagenic properties in vitro and in vivo and may contribute to the genetic instability and frequent chromosomal aberrations that are a hallmark of gastric cancer.

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During the past three decades, research using the transforming proteins of small DNA tumor viruses such as human adenoviruses (HAdvs), SV40 and human papillomaviruses (HPVs) has illuminated crucial cellular pathways that control proliferation and oncogenic transformation of mammalian cells. Owing to their small genome size, these viruses are heavily dependent on the host cell machineries to express their genes and replicate their DNA. Because these viruses generally replicate in terminally differentiated quiescent target cells, the viral genes expressed during the early phase of the virus life cycle subvert the host cell cycle, inducing transient proliferation of the infected cells to generate a permissive S-phase state to facilitate viral replication. In non-permissive cells, these viruses express only their early genes, which induce cell proliferation, resulting in abortive infection. A fraction of the infected cells recover, and assume oncogenic properties as a result of continued expression of a subset of viral early genes, referred to as ‘transforming genes’. Transduction of subgenomic DNA fragments containing the transforming genes also achieves transformation of the target cells in significant numbers. Defined viral mutants and transduction of isolated genes have been widely used for molecular genetic analysis of viral transforming genes.

Researchers have used biochemical approaches to identify cellular proteins associated with viral proteins to elucidate the mechanisms through which the transforming proteins of small DNA tumor viruses subvert the cell cycle and elicit oncogenic cell transformation. This approach revealed the interaction of viral transforming proteins with tumor-suppressor proteins such as p53 and the retinoblastoma (pRb) protein. The transforming proteins of HAdvs, SV40 and HPVs share common mechanisms of cell transformation, because they target the same cell-cycle regulatory proteins, such as p53 and the pRb family proteins. This paper discusses the role of adenovirus E1A protein in transformation.

Opposing oncogenic activities of small DNA tumor virus transforming proteins. Trends Microbiol. Feb 15 2011 doi: 10.1016/j.tim.2011.01.003
The E1A gene of species C human adenovirus is an intensely investigated model viral oncogene that immortalizes primary cells and mediates oncogenic cell transformation in cooperation with other viral or cellular oncogenes. Investigations using E1A proteins have illuminated important paradigms in cell proliferation and about the functions of cellular proteins such as the retinoblastoma protein. Studies with E1A have led to the unexpected discovery that E1A also suppresses cell transformation and oncogenesis. Here, I review our current understanding of the transforming and tumor-suppressive functions of E1A, and how E1A studies led to the discovery of a related tumor-suppressive function in benign human papillomaviruses. The potential role of these opposing functions in viral replication in epithelial cells is also discussed.

Related:

• MicroRNA regulation of tumor-killing viruses
• Hepatitis B and C Viruses and Hepatocellular Carcinoma
• Vaccines To Prevent Infections by Oncoviruses

Endogenous retroviruses (ERVs) have spread through mammalian genomes throughout evolution, resulting in thousands of copies or fragments of ERVs, encompassing an estimated 7% of the human genome. However, despite their abundance, specific functions could only rarely be assigned to ERVs. The LTRs of such viruses can serve as strong promoters/enhancers, boosting the transcription of viral or adjacent cellular genes. In a recent discovery of a pathologic example in humans, the derepression of an LTR was found to induce the driver proto-oncogene CSF1R in Hodgkin’s lymphoma. The promoter activity of LTRs can display various tissue specificities and is sometimes limited to germ cells. For example, an LTR from the human endogenous retrovirus 9 (ERV9) predominantly transcribes in testis.

Maintaining genomic integrity in the germ line is a fundamental prerequisite for the evolutionary stability of a species. To achieve this, germ cells with damaged DNA need to be efficiently eliminated. This elimination is particularly important when an individual’s fertility is maintained over several decades, as is the case in humans and great apes (Hominidae). In these species, the already long time frame of fertility in females is even more extended in males. However, only very limited knowledge exists on how the genomic integrity of the male germ line is controlled and preserved in humans.

p63 is a homolog of the tumor suppressor p53. In somatic cells, p53 is the quintessential mediator of apoptosis in response to DNA damage, thus acting as the guardian of the genome. Tumor suppressor p53 binds and transcriptionally activates multiple proapoptotic genes. Moreover, p53 directly associates with mitochondria, and by interacting with anti- and proapoptotic members of the Bcl2 family of outer membrane permeability regulators, triggers the release of cytochrome c, jumpstarting apoptosis. This paper shows that unique isoforms of p63 are highly and specifically expressed in human testis as a result of an upstream insertion of an ERV9 LTR with strong promoter activity that occurred 10 to 15 million years ago during primate evolution at the branching point to long-lived Hominidae. Upon DNA damage, the resulting germ cell-associated, transcriptionally active p63 suppresses proliferation and induces apoptosis. Conversely, GTAp63 expression is frequently lost in human testicular cancers.

Endogenous retrovirus drives hitherto unknown proapoptotic p63 isoforms in the male germ line of humans and great apes. PNAS USA February 7 2011. doi: 10.1073/pnas.101620110

Related:

• Phoenix from the ashes: The 5 million year old virus
• T Cell Responses to Human Endogenous Retroviruses in HIV-1 Infection

I spent 10 years working on Human T Lymphotropic Virus (HTLV), so it’s dear to me, although it’s fallen out of fashion recently, which is why it’s good to see this excellent review paper from my former UCLA colleague Pat Green:

Human T Lymphotropic Virus Type 1 (HTLV-1): Molecular Biology and Oncogenesis. (2010) Viruses 2(9): 2037-2077. doi:10.3390/v2092037
Human T lymphotropic viruses (HTLVs) are complex deltaretroviruses that do not contain a proto-oncogene in their genome, yet are capable of transforming primary T lymphocytes both in vitro and in vivo. There are four known strains of HTLV including HTLV type 1 (HTLV-1), HTLV-2, HTLV-3 and HTLV-4. HTLV-1 is primarily associated with adult T cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is rarely pathogenic and is sporadically associated with neurological disorders. There have been no diseases associated with HTLV-3 or HTLV-4 to date. Due to the difference in the disease manifestation between HTLV-1 and HTLV-2, a clear understanding of their individual pathobiologies and the role of various viral proteins in transformation should provide insights into better prognosis and prevention strategies. In this review, we aim to summarize the data accumulated so far in the transformation and pathogenesis of HTLV-1, focusing on the viral Tax and HBZ and citing appropriate comparisons to HTLV-2.

Related:

• Viruses and Human Cancer
• Viruses and Lymphomas

Mouse mammary tumor virus (MMTV), which was discovered as a milk‑transmitted, infectious cancer-inducing agent in the 1930s, has been used since that time as an animal model for the study of human breast cancer. Like other complex retroviruses, MMTV encodes a number of accessory proteins that both facilitate infection and affect host immune response. In vivo, the virus predominantly infects lymphocytes and mammary epithelial cells. High level infection of mammary epithelial cells ensures efficient passage of virus to the next generation. It also results in mammary tumor induction, since the MMTV provirus integrates into the mammary epithelial cell genome during viral replication and activates cellular oncogene expression. Thus, mammary tumor induction is a by-product of the infection cycle. A number of important oncogenes have been discovered by carrying out MMTV integration site analysis, some of which may play a role in human breast cancer.

Because MMTV has existed as an infectious virus in mice for millions of years, it has evolved to take advantage of its host’s biology, using host genes from transcription factors to immune regulatory molecules, to establish infection. Although it causes mammary tumors, this does not occur until relatively late in life and thus the virus has persisted, since infected mothers are able to transmit virus to offspring. The lack of acute MMTV-induced pathogenesis is most likely due to different host means of limiting virus infection, including factors that operate at the cellular level like intrinsic restriction factors and immune response genes. As additional host-antiviral genes are discovered, MMTV will continue to serve as an important model for testing the ability of these factors to function in vivo. In addition to serving as an important means for studying virus infection, MMTV has provided a number of critical models for understanding human breast cancer. Finally, the use of the MMTV LTR to direct oncogene expression to murine epithelial cells has resulted in the creation of numerous transgenic mouse strains that serve as critical models for understanding human breast cancer. It is likely that such transgenic mice will continue to be a critical tool as additional human breast cancer genes are identified through large-scale human genetic studies.

Mouse Mammary Tumor Virus Molecular Biology and Oncogenesis. (2010) Viruses 2010 2(9): 2000-2012 doi:10.3390/v2092000

Related:

• No HMTV in human breast cancer samples?
• Vaccines To Prevent Infections by Oncoviruses
• The search for infectious causes of human cancers

Hepatocarcinogenesis (liver cancer) is, and will continue to be a major worldwide health problem. With chronic HBV and HCV infections being responsible for a significant proportion of HCC cases, the development of new and relevant cell culture and animal models to study the interactions of HBV and HCV with their host and the development of efficient means to combat chronic infections will remain major tasks to tackle. This publication gives an overview of our current state of knowledge in respect to the basic biology of these viruses, as well as the clinical and therapeutic options that have been, and are being developed, and highlights the major current technical and biological limitations that the field needs to overcome.

Hepatitis B and C Viruses and Hepatocellular Carcinoma. (2010) Viruses 2(8): 1504-1509 doi:10.3390/v2081504

Related:

• Hepatitis B virus integration and hepatocarcinogenesis
• The search for infectious causes of human cancers
• Liver flukes and cancer

The dense bacterial consortium called the “microbiota” that inhabits the intestinal tract is recognized increasingly as playing a major role in human health and disease. The microbiota generally influences the host in a beneficial fashion by shaping gastrointestinal and immune functions, exerting protection against pathogens, and contributing to metabolic pathways. Escherichia coli is a consistent member of the humanintestinal microbiota, colonizing the intestine within a few days after birth and persisting throughout the life of the host. The E. coli strain population can be categorized in at least four major phylogenetic groups, each group being more specifically associated with certain ecological niches. E. coli strains belonging to group B2 are recovered from the environment less frequently but can persist longer in the colon than other groups and represent 30–50% of strains isolated from the feces of healthy humans living in high-income countries.

Up to 34% of commensal E. coli strains of the phylogenetic group B2 carry a conserved genomic island named the “pks island”. This gene cluster codes for genes that allow production of a putative hybrid peptide-polyketide genotoxin, Colibactin. In vitro infection with these strains induces DNA double-strand breaks (DSBs) in cultivated human cells, but the pks island was not proved to cause DNA damage in vivo. In this study, the authors explore whether those bacteria were able to induce genetic damage in vivo on the colonic mucosa and to characterize the consequences of this damage on mammalian cells in relation with the number of infecting bacteria. They report that pks+ E. coli induced DSBs in vivo. In addition, infection of various mammalian cells with pks+ E. coli induced, at very low multiplicity of infection, reversible DNA damage response that did not repair all DSBs, leading to chronic mitotic and chromosomal aberrations together with increased frequency of gene mutation and anchorage-independent growth. Taken together, these findings strongly suggest that these pks+ strains are genotoxic in vivo and provide insights into mechanisms by which common E. coli strains may contribute to cellular transformation and possibly sporadic colorectal cancer tumorigenesis.

Escherichia coli induces DNA damage in vivo and triggers genomic instability in mammalian cells. PNAS USA June 7 2010 doi: 10.1073/pnas.100126110
Escherichia coli is a normal inhabitant of the human gut. However, E. coli strains of phylogenetic group B2 harbor a genomic island called “pks” that codes for the production of a polyketide-peptide genotoxin, Colibactin. Here we report that in vivo infection with E. coli harboring the pks island, but not with a pks isogenic mutant, induced the formation of phosphorylated H2AX foci in mouse enterocytes. We show that a single, short exposure of cultured mammalian epithelial cells to live pks+ E. coli at low infectious doses induced a transient DNA damage response followed by cell division with signs of incomplete DNA repair, leading to anaphase bridges and chromosome aberrations. Micronuclei, aneuploidy, ring chromosomes, and anaphase bridges persisted in dividing cells up to 21 d after infection, indicating occurrence of breakage–fusion–bridge cycles and chromosomal instability. Exposed cells exhibited a significant increase in gene mutation frequency and anchorage-independent colony formation, demonstrating the infection mutagenic and transforming potential. Therefore, colon colonization with these E. coli strains harboring the pks island could contribute to the development of sporadic colorectal cancer.

Related:

• Spread of Pathogenicity Islands in Escherichia coli
• The continuing evolution of E. coli O157:H7
• Global functional atlas of Escherichia coli proteins

So we have this vaccine that prevents people dying of cancer. Should we use it?

Even when financial and healthcare barriers are removed, some parents remain hesitant to have their daughters receive the HPV vaccine. As a result, policymakers must develop and implement strategies to ensure optimal HPV vaccine uptake, says new research in this week’s PLoS Medicine. Researchers surveyed parents of grade 6 girls (age 11) in a publicly funded school-based program in British Columbia, Canada, to determine the level of uptake of the first dose of the HPV vaccine, and to examine the factors involved in their decision to allow receipt of the vaccine. Sixty five percent of the 2,025 parents who completed the survey had consented to their daughter receiving the first dose of HPV vaccine. By contrast, more than 85% of the parents reported to have consented to hepatitis B and meningitis C vaccinations for their daughters. Of those who did not consent, almost a third of the parents said concern about the vaccine’s safety was their main reason and one in eight said they had not been given sufficient information to make an informed decision. The authors report that a positive parental attitude towards vaccination and a parental belief that HPV vaccination had limited impact on sexual practices increased the likelihood of a daughter receiving the HPV vaccine. Having a family with two parents or three or more children and having well-educated parents decreased the likelihood of a daughter receiving the vaccine.

Sigh.

A Population-Based Evaluation of a Publicly Funded, School-Based HPV Vaccine Program in British Columbia, Canada: Parental Factors Associated with HPV Vaccine Receipt. PLoS Med 7(5): e1000270. doi:10.1371/journal.pmed.1000270
Background: Information on factors that influence parental decisions for actual human papillomavirus (HPV) vaccine receipt in publicly funded, school-based HPV vaccine programs for girls is limited. We report on the level of uptake of the first dose of the HPV vaccine, and determine parental factors associated with receipt of the HPV vaccine, in a publicly funded schoolbased HPV vaccine program in British Columbia, Canada.
Methods and Findings: All parents of girls enrolled in grade 6 during the academic year of September 2008–June 2009 in the province of British Columbia were eligible to participate. Eligible households identified through the provincial public health information system were randomly selected and those who consented completed a validated survey exploring factors associated with HPV vaccine uptake. Bivariate and multivariate analyses were conducted to calculate adjusted odds ratios to identify the factors that were associated with parents’ decision to vaccinate their daughter(s) against HPV. 2,025 parents agreed to complete the survey, and 65.1% (95% confidence interval [CI] 63.1–67.1) of parents in the survey reported that their daughters received the first dose of the HPV vaccine. In the same school-based vaccine program, 88.4% consented to the hepatitis B vaccine, and 86.5% consented to the meningococcal C vaccine. The main reasons for having a daughter receive the HPV vaccine were the effectiveness of the vaccine (47.9%), advice from a physician (8.7%), and concerns about daughter’s health (8.4%). The main reasons for not having a daughter receive the HPV vaccine were concerns about HPV vaccine safety (29.2%), preference to wait until the daughter is older (15.6%), and not enough information to make an informed decision (12.6%). In multivariate analysis, overall attitudes to vaccines, the impact of the HPV vaccine on sexual practices, and childhood vaccine history were predictive of parents having a daughter receive the HPV vaccine in a publicly funded school-based HPV vaccine program. By contrast, having a family with two parents, having three or more children, and having more education was associated with a decreased likelihood of having a daughter receive the HPV vaccine.
Conclusions: This study is, to our knowledge, one of the first population-based assessments of factors associated with HPV vaccine uptake in a publicly funded school-based program worldwide. Policy makers need to consider that even with the removal of financial and health care barriers, parents, who are key decision makers in the uptake of this vaccine, are still hesitant to have their daughters receive the HPV vaccine, and strategies to ensure optimal HPV vaccine uptake need to be employed.

Related:

• Should we cure cancer?
• HPV vaccines: prospects for eliminating ano-genital cancer
• Dumb and Dumber

It has been estimated that viruses are etiological agents in approximately 12% of human cancers. Most of these cancers can be attributed to infections by human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), Epstein-Barr virus (EBV), and Kaposi’s sarcoma-associated herpesvirus (KSHV). Prophylactic vaccines against other pathogenic viruses have an excellent record as public health interventions in terms of safety, effectiveness, and ability to reach economically disadvantaged populations. These considerations should prompt efforts to develop and implement vaccines against oncoviruses. Safe and effective HBV and HPV vaccines, based on virus-like particles, are commercially available, and the major focus is now on vaccine delivery, especially to low-resource settings. HCV and EBV vaccines are under active development, but few clinical trials have been conducted, and none of the candidate vaccines has proven to be sufficiently effective to warrant commercialization. Efforts to develop KSHV vaccines have been more limited.

Vaccines To Prevent Infections by Oncoviruses. Annu Rev Microbiol. Apr 26 2010

Related:

• Viruses and Human Cancer
• The search for infectious causes of human cancers
• Prostate Cancer Caused By a Virus?