Clinical applications involving CNS gene transfer

Abstract

Diseases of the central nervous system (CNS) have traditionally been the most difficult to treat by traditional pharmacological methods, due mostly to the blood-brain barrier and the difficulties associated with repeated drug administration targeting the CNS. Viral vector gene transfer represents a way to permanently provide a therapeutic protein within the nervous system after a single administration, whether this be a gene replacement strategy for an inherited disorder or a disease-modifying protein for a disease such as Parkinson's. Gene therapy approaches for CNS disorders has evolved considerably over the last two decades. Although a breakthrough treatment has remained elusive, current strategies are now considerably safer and potentially much more effective. This chapter will explore the past, current, and future status of CNS gene therapy, focusing on clinical trials utilizing adeno-associated virus and lentiviral vectors.

Keywords: AAV; CNS; Canavan disease; Clinical trial; Gene therapy; Lentivirus; Parkinson's disease; Retrovirus; Vector.

Figure 2.1

Ex vivo Gene Transfer Using a Retroviral Vector. Hematopoietic stem cells isolated from a patient's bone marrow are transduced ex vivo with lentiviral vector. Transduced cells then reinfused back to the patient expressing the therapeutic protein.

Figure 2.2

HIV-1 Structure. HIV-1 is a complex retrovirus that contains two copies of RNA molecules embedded into nucleocapsid proteins (small open beads at the center). protease (Pr), integrase (Int), and reverse transcriptase (RT) enzymes are shown surrounded by the viral capsid (oval-shaped beads). Matrix proteins (shown as circles) enclosed by the viral envelopes consist of two components: transmembrane, gp41 (closed triangle), and surface, gp120 (ovals), embedded into the lipid membrane.

Figure 2.3

Generations of Lentivirus Cassettes. The first generation of the lentivirus (expression) cassette carried all accessory genes, vif, vpu, vpr, and nef, and regulatory genes, tat and rev, flanked by unmodified 5′- and 3′- LTRs. In addition, it harbors cis-acting elements, including a primer binding site (PBS); a splice donor (SD) and acceptor (SA); central polypurine tract (cPPT) and polypurine tract (PPT); Rev response element (RRE); and a packaging signal, psi (Y). Second generation retroviral vectors are characterized by a deletion of all accessory genes of HIV. The woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) is incorporated into the viral cassette. In a third generation, the cytomegalovirus (CMV) promoter replaced a parental HIV promoter located in the 5′-LTR. The self-inactivated (SIN) vector represents a fourth generation. SIN is completely devoid of HIV-parental enhancer/promoter sequences, located in the U3′ of the 3′-LTR (deletion is shown). (B) A deletion introduced in the 3′LTR translocated to the 5′LTR during reverse transcription as shown (see also in the text).