Blanc Inter SVSE 6 - Blanc international - Sciences de la vie, de la santé et des écosystèmes : Génomique, génomique fonctionnelle, bioinformatique, biologie systémique

Deciphering the role of Human OAS gene family in the pathogenesis of arbovirus infection – ARBOAS

Role of human 2’-5’ oligoadenylate synthetase genes in mosquito-borne viral infection

The 2’-5’ oligoadenylate synthetase (OAS) is an interferon-induced protein that plays important roles in defense mechanism against viral infection. On the other hand, this protein can be harmful and cause severe diseases. We will try to understand the role of this protein in protection and/or severity enhancement of diseases caused by viruses transmitted by insects.

Does OAS gene family protect mosquito-borne viral infection or enhance disease severity?

Mosquito-borne viral infections are a major public health problem in tropical countries and are spreading to countries in temperate zone due to climatic change. Dengue, Chikungunya and Japanese encephalitis virus are important mosquito-borne human pathogens causing hemorrhagic, febrile, and severe encephalitic illnesses. Dengue virus causes diseases ranging from a mild illness to life-threatening symptom. There were estimated 50 to 100 million cases of dengue disease and several hundred thousand cases of severe dengue annually around the world. To date, there is no vaccine or specific treatment for dengue infection. It is urgent to understand immune defense mechanism and how the virus causes the disease and transmits. Japanese encephalitis viral infection causes human epidemic encephalitis, which is reported with an estimated 10,000 to 15,000 annual mortality in the South and Southeast Asia. Although there is an effective vaccine, there is no specific treatment.<br />Innate antiviral mechanisms mediated by interferons are potentially the most important pathways of host cell defense limiting viral replication. We propose to study the role of OAS gene family, important genes in this innate immunity pathway in Dengue, Chikungunya and Japanese encephalitis viral infection. The mechanisms underlying the different effects of the OAS gene family members and their isoforms against the viruses need full investigation. In addition, the role of these enzymes in natural human infections of Dengue and Chikungunya is not known.<br />

We used a human genetic association approach to identify variants of OAS genes involved in dengue disease severity and perform functional study to confirm the results. During 2000-2006, we recruited 681 dengue patients from three hospitals in Thailand. We collected DNA samples from 696 blood donors from the same hospitals to represent the gene frequency in the general population. We screened variants of OAS gene family by sequencing in 25 Dengue Hemorrhagic Fever patients and 32 healthy controls.
We tested for association of the genes on the risk of severe dengue. One variant of OAS3 had highly significantly reduced risk of severe dengue (by half). We would like to confirm our genetic study results in newly collected patients from Thailand and other countries.
We study the anti-dengue effect of the variants and other variants that we will indentify in the future using a cellular system developed Dr Yi-Ling Lin in Taiwan. We wish to perform functional studies using the same system with other mosquito-borne viruses. We will investigate the mechanism underlying antiviral activity and subsequent consequences, such as immune response, cell death etc, which may lead to severe disease. In addition, we will investigate the mechanism viral escape from OAS activity by growing a virus in cell system highly expressing OAS gene. We will grow the virus until the virus mutates to escape the effect of OAS gene. We will then study the underlying mechanism.

We found that some variants of OAS gene products but not the other members exhibited anti-dengue activity. These variants are likely to contribute to host defense against dengue viral infection and play a role in determining dengue disease severity. Difference from dengue virus, none of these human OAS proteins showed anti-Japanese encephalitis effect.
We identified variants of OAS gene family which are implicated in development of severe dengue. This knowledge is important for OAS based treatment which is under development. Our results indicated that a stronger antiviral effect might be a double-edged sword that could not only block dengue viral replication but also produce small ribosomal nucleic acids that activate the cell death and cytokine release which could result in severe dengue.
We are currently conducting this program to investigate the variants of OAS genes with a team of immunologists from the Institute of Biomedical Sciences, Taiwan and a team of human geneticists, molecular virologists and biochemists from the Pasteur Institute in Paris, France
In addition, we obtained a grant from the European Union to investigate a role of asymptomatic dengue infection in transmission of the virus. This question is important for controlling dengue epidemics and prevention of spreading dengue virus to uninfected area including Europe. There are 14 partners from Europe, South East Asia and Caribbean.

We are collaborating with a company in United Kingdom who discovered a molecule which can enhance an effect of OAS gene product. This molecule showed anti dengue activity in a cell culture model. We will further test this molecule for its antiviral effect and toxicity in a mouse model. As our results showed that high OAS activity could lead to severe disease, we will use our genetic markers to predict the outcome of dengue viral infection with an OAS based treatment.

We discovered one genetic marker of OAS gene family which can predict severe dengue. Further investigation needs to be performed to obtain predictive positive and predictive negative values of this marker together with other factors related to severe dengue.

Innate immunity plays a major role in the earliest stages of arbovirus infection. IFN-a/ß constitute the very first lines of antiviral innate immune response against arboviral infection. IFN-a/ß trigger the activation of IFN-stimulated genes such as members of IFN-induced 2’, 5’-Oligoadenylate (OAS) family that play a critical role in the establishment of an antiviral state against RNA viruses such as flaviviruses (Dengue [DENV], West Nile [WNV] and Japanese encephalitis [JEV] viruses) and alphaviruses (Chikungunya [CHIKV] virus). DENV, WNV, JEV, and CHIKV are emerging zoonotic arboviruses of medical concern in France and Taiwan.
OAS play important roles in the antiviral IFN pathway through the action of activated latent endoribonuclease, RNase L. The 1b isoform of mouse Oas1 has been identified as a flavivirus resistance gene in mice. The Human OAS gene family, is comprised of 4 genes, and following alternative splicing can encode 10 different isoforms. In this ANR-NSC joint call grant proposal, we propose a collaborative project between a French team consisting of 3 groups (molecular virology, biochemistry, and human genetics) at the Institut Pasteur, Paris (IPP) and a team of immunologists from the Institute of Biomedical Sciences (IBS), Taiwan. All teams have been working on the OAS gene family and have published several articles on this subject.
The Flavivirus-Host Molecular Interactions (FHMI) group at IPP led by Dr DESPRES discovered a non-sense mutation in mouse Oas1b gene resulting in increased susceptibility to WNV. In vitro study showed that cells expressing Oas1b but not the C-terminally truncated Oas1b efficiently inhibited WNV replication. Genetic knock-in of the Oas1b resistance allele into a susceptible mouse strain confers mouse with resistance against Yellow Fever virus. Recently, FHMI in collaboration with the team at IPP led by Dr SAKUNTABHAI, observed that the large form of human OAS (OAS3) exerts RNase L-independent antiviral activity in infected human cells against the wild-type virulent strain of CHIKV. Cells expressing a genetic variant , found at 2% allele frequency in Caucasians, lacks 20% of the C-terminus and were less resistant to CHIKV, raising the question of the role of OAS3 genetic polymorphisms in human susceptibility to alphavirus infection.
The antiviral effect of human OAS family members against Dengue virus serotype 2 (DENV-2) has been studied by Dr LIN’s laboratory, IBS Taiwan, who found that OAS1 p42, OAS1 p46, and OAS3 p100, but not the other gene family members, exhibited anti-DENV activity; these antiviral effects were largely lost in cells deprived of RNase L expression. Moreover, RNase L activity indicated that the human OAS1 p42, OAS1 p46, and OAS3 p100 triggered activation of RNase L during DENV replication. Thus, OAS1 p42/p46 and OAS3 p100 are likely to contribute to host defense against DENV and play a role in determining the outcome of DENV infection.
Human OAS gene family was recently demonstrated in vivo and in vitro to contribute to susceptibility to WNV. Clinical evidence from WNV-seropositive patients showed that "A" allele at a SNP of OAS1, that generates OAS1 p48 and p52 but not p46, is a risk factor for the initial infection of WNV, supporting our data that OAS1 p42/p46 but not OAS1 p44/p48/p52 mediate a potent anti-DENV-2 activity and may contribute to host defense against DENV infection.
We propose to study further the role of OAS in DENV, CHIKV and JEV infection. The mechanisms underlying the differential antiviral effects of the OAS gene family members and their isoforms require further investigation. In addition, the role of these enzymes in natural human infections of DENV and CHIKV is not known. We, therefore, propose a program that combines expertise from the IBS, Taiwan and IPP, France with the aim of understanding the mechanisms underlying the differential anti-viral activity of the human OAS gene family and their role in natural infections.

Project coordinator

Monsieur Anavaj Sakuntabhai (INSTITUT PASTEUR) –

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.



Help of the ANR 206,200 euros
Beginning and duration of the scientific project: - 36 Months

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