Investigation of the impact chromosomal integration of human herpes virus 6 (HHV6) has on telomere molecular biology.

Human chromosomes are capped by telomeres, which are DNA sequences bound by proteins and they are essential for the stability of the human genome. It is now well known that telomeres play important roles in cellular senescence, which contributes to ageing, and in cancer progression thus it is important to identify and characterise factors that influence telomeres. The human herpesvirus 6 (HHV-6) occurs in two subtypes, A and B, and unlike other human herpes viruses, HHV-6 can insert itself into a chromosome. When it does this it always inserts in or next to a telomere but it is not known if this affects telomere function or whether it contributes to an increased risk of disease. It has been estimated that 1% of the UK population carry an integrated copy of the HHV-6 virus (CI-HHV-6) that is often inherited from a parent and so it is present in all cells of the body. In this project we will determine the nature of the chromosomal insertion site and use the information to understand how the virus inserts and how frequently it achieves this in the germ-line. We will investigate how the human cell controls expression of the viral genes and we will also try to determine whether the presence of CI-HHV-6 affects telomere function in somatic cells and in the germ-line.

Telomere maintenance and length regulation is central to genome stability and cell immortality and plays important roles both in cancer and ageing. It is therefore important to understand factors that influence telomere function, as deleterious affects can contribute to disease risk. The human herpesvirus 6 can be subdivided into two types, A and B. These viruses often cause a mild illness in early childhood and then persist lifelong in a latent form in some cells. Reactivation of the latent virus often occurs in immuno-compromised patients where it is can be pathogenic. Unlike other human herpesviruses both HHV-6A and 6B can integrate into the telomeric region of chromosomes and the chromosomally integrated form (CI-HHV-6) can be inherited. About 1% of the UK population are carriers for a full-length integrated copy at one chromosome end but the rate and mechanism of germ-line integration are poorly understood. Moreover it is not known whether the presence of CI-HHV-6 influences disease risk in carriers. It is reasonable to hypothesis that the insertion of the ~160kb HHV-6 genome into or near a telomere may affect chromatin organisation and also telomere function. We will conduct experiments to gain insight into the rate of viral insertion and the mechanism by which HHV-6 integrates. This will be achieved in part by the isolation and characterization of viral sequences at the insertion sites from unrelated individuals. If we find evidence of common ancestry it will indicate that the viral insertion rate in the germ-line is low. To determine whether the presence of an integrated virus affects telomere function, we will compare the length of the telomere associate with the CI-HHV-6 to other telomeres in somatic cells and in the germ-line. We will explore the chromatin organisation across the integrated virus to determine whether it is silenced by epigenetic modifications and how these compare to epigenetic modification at another telomere-adjacent region. We will also seek to determine whether components of the Shelterin protein complex bind to the telomere-like repeats present within the HHV-6 genome as this may also interfere with telomere function at this chromosome end.