An integrated systems analysis implicates EGR1 downregulation in simian immunodeficiency virus encephalitis-induced neural dysfunction

Human immunodeficiency virus (HIV)-associated dementia (HAD) is a syndrome occurring in HIV-infected patients with advanced disease that likely develops as a result of macrophage and microglial activation as well as other immune events triggered by virus in the central nervous system. The most relevant experimental model of HAD, rhesus macaques exhibiting simian immunodeficiency virus (SIV) encephalitis (SIVE), closely reproduces the human disease and has been successfully used to advance our understanding of mechanisms underlying HAD. In this study we integrate gene expression data from uninfected and SIV-infected hippocampus with a human protein interaction network and discover modules of genes whose expression patterns distinguish these two states, to facilitate identification of neuronal genes that may contribute to SIVE/HIV cognitive deficits. Using this approach we identify several downregulated candidate genes and select one, EGR1, a key molecule in hippocampus-related learning and memory, for further study. We show that EGR1 is downregulated in SIV-infected hippocampus and that it can be downregulated in differentiated human neuroblastoma cells by treatment with CCL8, a product of activated microglia. Integration of expression data with protein interaction data to discover discriminatory modules of interacting proteins can be usefully used to prioritize differentially expressed genes for further study. Investigation of EGR1, selected in this manner, indicates that its downregulation in SIVE may occur as a consequence of the host response to infection, leading to deficits in cognition.