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The role of chromatin modifier protein hDaxx and its interaction partners in intrinsic resistance to virus infection

Final Report Summary - CELLULAR ANTIVIRALS (The role of chromatin modifier protein hDaxx and its interaction partners in intrinsic resistance to virus infection.)

The herpes simplex viruses (HSV-1 and -2) cause one of the most common sexually transmitted diseases, with 20,000 new cases annually referred to UK clinics. Immunocompromised patients, such as transplant recipients and neonates, and individuals who endure frequent HSV reactivations, can suffer considerable morbidity. Infections in the eye can cause blindness, active genital herpes necessitates caesarean delivery, and herpes encephalitis is usually fatal.
The key property of herpesvirus infections in general is the establishment of a latent state after a primary infection, which usually occurs at an early age. Thereafter, infected individuals carry the virus for life, since latent virus cannot be eliminated. Because the ability to establish, maintain and reactivate from latent infections is crucial for the clinical importance and evolutionary success of HSV, these topics are being studied intensively in a number of laboratories worldwide.
The purpose of this project was to study in detail a cellular protein known as hDaxx and its role in HSV-1 infection. hDaxx is recruited to the viral genomes soon after they enter the infected cell nucleus. The evidence suggests that this reflects a cellular defence mechanism that represses HSV-1 replication and reactivation.
To identify the function of a protein in a biological process such as a virus infection, the protein can be depleted from the cells to observe any resulting changes in infection. If hDaxx is part of a repressive mechanism to HSV-1 infection, then depletion of hDaxx should lead to an increase in infection.
By mutating certain regions within hDaxx, light can be shed on the mechanism of its repressive action, and the cellular and viral interaction partners involved can be identified.
During the course of the study a novel depletion approach was developed to analyse the influence of hDaxx, and also two other cellular repressors, PML and Sp100, on HSV-1 infection. We found that each of them individually is able to repress HSV-1 to a certain degree, but that all three act cooperatively to repress HSV-1 extensively. When hDaxx, PML and Sp100 were depleted, more cells developed a progressing HSV-1 infection, and more cells were able to escape repression into an inactive latent state. A panel of hDaxx mutants was analysed for their behaviour in HSV-1 infection, and we have found several regions within hDaxx that are essential for its repressive function, of which a number have not been previously identified.
These findings will contribute to the understanding of the mechanisms that regulate the infection of not only HSV-1, but also several other herpesviruses and potentially DNA viruses in general, providing novel targets for the treatment of these viral diseases. Our work has stimulated interest in a number of other laboratories, to whom we have contributed several research reagents. The technologies developed in this project will enhance research in a number of fields, since hDaxx itself is a protein that is studied in many diverse contexts, and the mutant forms of hDaxx are of interest to a variety of researchers.