HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part I. HSV-1 structure, replication and pathogenesis

Abstract

The design of effective gene therapy strategies for brain tumors and other neurological disorders relies on the understanding of genetic and pathophysiological alterations associated with the disease, on the biological characteristics of the target tissue, and on the development of safe vectors and expression systems to achieve efficient, targeted and regulated, therapeutic gene expression. The herpes simplex virus type 1 (HSV-1) virion is one of the most efficient of all current gene transfer vehicles with regard to nuclear gene delivery in central nervous system-derived cells including brain tumors. HSV-1-related research over the past decades has provided excellent insight into the structure and function of this virus, which, in turn, facilitated the design of innovative vector systems. Here, we review aspects of HSV-1 structure, replication and pathogenesis, which are relevant for the engineering of HSV-1-based vectors.

Figure 1

HSV-1 genome structure. The virus genome is a linear, double-stranded DNA of ∼152 kb which encodes more than 80 genes. The genome is composed of unique long (U_L) and unique short (U_S) segments, which are flanked by inverted repeats (R). IR_L, internal repeat of the long segment; TR_L, terminal repeat of the long segment; IR_S, internal repeat of the short segment; TR_S, terminal repeat of the short segment. The repeats of the L component are designated ab and b′a′; those of the S segment are a′c′ and ca. Pac signals are contained in the a sequences located at the junction between the long and short segments and at both termini. The HSV-1 genome contains two different origins of DNA replication, ori_S and ori_L. Ori_S is duplicated because it is located within the inverted repeats flanking U_S between the promoters of the IE3 and IE4/5 genes. Ori_L is located within U_L and is flanked by transcriptional start sites of two E genes, which encode the single-stranded DNA binding protein, ICP8 (U_L29), and the DNA polymerase (U_L30). Approximately half of the genes are essential for virus replication in cell culture. The other half encode accessory functions, which contribute to the virus life cycle in specific tissues or cell types, e.g., postmitotic neurons. However, it can be assumed that the genes known to be dispensable for growth in cultured cells may be important for both optimal lytic replication and replication in vivo, contributing to pathogenesis, host range, latency, or spread in neurons. Genes underlined mark IE genes or genes which are relevant in certain recombinant HSV-1 mutants (see Part II).

Figure 2

Structure of the HSV-1 virion. The HSV-1 virion has a diameter of ∼120–300 nm and consists of an envelope, the tegument, the capsid, and a core containing the virus genome. The capsid has a diameter of ∼100 nm and is surrounded by tightly adhering tegument proteins. The envelope consists of a lipid membrane, containing glycoprotein spikes on the surface, which vary in number and relative amounts.

Figure 3

Electronmicrograph of an HSV-1 virion. Envelope (▲) and capsid (△) are clearly delineated (a kind gift from Drs. Elisabeth Schraner and Peter Wild, University of Zürich).

Figure 4

Lytic and latent HSV-1 infection. The lytic HSV-1 life cycle takes ∼18 hours and the steps include: 1) attachment to heparan sulfate and cell-surface receptors; 2) fusion of the virion envelope with the plasma membrane; 3) release of the capsid into the cytoplasm and 4) active transport along microtubules to the nuclear pores with 5) release of the virion DNA into the nucleus; 6) α-TIF mediated induction of transcription of IE genes; 7) transcription of IE and E genes; 8) viral DNA synthesis; 9) transcription of L genes; 10) capsid assembly; 11) DNA packaging into preformed capsids; 12) capsid envelopment; and 13) virion egress. In sensory neurons, HSV-1 may enter a state of latency 14), which is characterized by the persistence of the HSV-1 genome as a concatemeric or circular molecule bound by nucleosomes that does not express viral genes other than the latency associated transcripts. Host cell, viral and external factors play a role in establishment and reactivation 15) of HSV-1 from latency.