Nigel Fraser, Ph.D.

Professor of Microbiology

Office Address:
Department of Microbiology
Perelman School of Medicine
University of Pennsylvania
319 Johnson Pavilion
3610 Hamilton Walk
Philadelphia, PA 19104-6076

TEL 215-898-3847
LAB 215-898-3846
FAX 215-898-3849
nfraser@mail.med.upenn.edu

RESEARCH SUMMARY

Using the techniques of molecular biology, we have studied HSV-1 latency in a mouse model system. My group has shown that the physical state of the latent viral DNA is episomal and not integrated, and that it is harbored in the trigeminal ganglia and brain stem of latently infected mice. Furthermore, the latent DNA appears to be coated with nucleosomes and thus is similar to cellular chromatin. We think that this is most significant as it suggests that gene expression from the latent viral DNA is under the control of cellular transcription factors. In situ hybridization has allowed us to visualize latently infected cells in peripheral nervous system (trigeminal ganglia) and the central nervous system (brain) tissue of mice and humans. The viral RNA can be shown to map to about 10% of the viral genome - the repeat long region - and there is a very strong region of hybridization corresponding to 1/3 of this region. The in situ data has shown that the latently infected cells are neuronal cells, that the viral RNA expressed is mainly in the nucleus of the latently infected cells - rather than in the cytoplasm as is seen for cellular mRNA's. Recently the use of in situ PCR technology has allowed us to detect latent viral genomes.

Using the techniques of molecular genetics, we have obtained mutant viruses that do not produce LAT transcripts. These mutants reactivate more slowly from latency and thus we believe that the LAT genes are involved in reactivation. Another mutant, deficient in the virion transactivating function (Vmw65), has been shown not to initiate an acute infection although a does form a latent infection. This mutant has shown the importance of Vmw65 in the decision of the virus to form a lytic or latent infection and we are presently examining molecular models of latency in which Vmw65 plays a critical role.

The role of the immune system in limiting the spread of HSV has also been studied. In SCID mice, in the absence of functional B and T Iymphocytes, HSV expression in neurons is still limited indicating that some other components of the immune system are active in this respect.

We are presently developing vector systems for use in gene therapy in the nervous system. In collaboration with a group working on mucopolysaccharidosis type Vll (a lysosomal storage disease which is caused by a defect in the enzyme b-glucuronidase), we have inserted the b-glucuronidase gene into HSV-1 and detected expression in latently infected mice. We will use our HSV vectors to study b-glucuronidase in neuronal cells of glucuronidase-negative mice, as a model for human lysosomal storage disease in the nervous system. Recently, the potential of HSV strains that do not replicate in the nervous system but do replicate in transformed tissue culture cells in cancer therapy has been realized. We have been studying the potential of Vmw65 and g34.5 mutants to destroy experimental brain tumors in mice.