The promise of lentiviral gene therapy for liver cancer

Abstract

The liver remains a prominent target for gene therapy, not only because of the importance of liver-directed gene therapy for metabolic and deficiency states, but for liver cancer as well. Hepatocellular carcinoma (HCC) accounts for over 90% of all primary liver cancers and leads to nearly 1 million deaths annually. This will increase further in the future as the incidence of HCC is rising throughout the world in association with large reservoirs of chronic hepatitis B and C. Metastatic disease constitutes another leading cause of liver-related mortalities. Only a very small fraction of patients with HCC or other types of malignant liver disease qualifies for surgical resection. Far fewer patients with liver cancer undergo orthotopic liver transplantation (OLT), although OLT can produce excellent outcomes in early HCC and more patients are receiving OLT following the adoption of new criteria for organ allocation. Nonetheless, the vast majority of liver cancer patients remain untreated, since other therapies are mostly palliative, and novel therapies are desperately needed. Although gene therapy could be an exciting approach for liver cancer, it has largely been confined to preclinical and experimental settings [1]. Over the past decade, despite extensive work by numerous groups, results of gene therapy in people have been mixed, with few successes and notable complications, e.g. adenoviral and retroviral vector-related toxicities in patients with genetic deficiency disorders [2,3]. These results focused further efforts in the areas of vector development, limiting toxicities of viral vectors, including genomic integrations and activation of deleterious endogenous genes, organor cell type-specific gene targeting, and induction of regulatable or sustained gene expression. Fortunately, significant advances in gene therapy have also been made over the past several years, including in the development of effective gene transfer vectors [4]. In particular, lentiviral vectors (LV) are beginning to overcome the challenges of efficient delivery and long-term expression of transgenes without toxicity. Most commonly, replication-defective hybrid LV particles containing core proteins and enzymes of human immunodeficiency virus 1 (HIV-1) are pseudotyped with the G protein of the vesicular stomatitis virus (VSV-G). Pseudotyping refers to the use of an envelope from an unrelated virus to obtain recombinant vector particles. The tropism of vectors can be modulated in this fashion to transduce a broad range of cell types, including hematopoietic cells, neurons and hepatocytes, in the case of VSV-G pseudotyped LV [5]. Moreover, use of robust viral envelopes offers ways to produce LV particles in high titers. LV are well suited for gene replacement therapy because these vectors efficiently integrate into the genome of nondividing cells and thus provide stable transgene expression in the long-term. Newer generations of LV can deliver intron-containing cassettes using only exogenous elements to regulate gene expression, which is advantageous [6,7]. Also, biosafety concerns raised by HIV-derived vectors have been alleviated by the findings that LV can be produced by a minimal set of viral genes. As much of the HIV sequences have been eliminated from constructs used to generate late versions of LV, these vectors should be particularly safe. Moreover, generation of self-inactivating (SIN) vectors containing a deletion in the downstream long terminal repeat (LTR) viral sequences, which transcriptionally inactivates upstream LTR following transduction into cells, substantially decrease the risk of vector mobilization and recombination [8]. However, the ability of LV to transduce hepatocytes in vivo can be debated. For instance, an initial report by Kafri et al. documented that LV efficiently transduced the liver in vivo and produced sustained transgene expression [9]. On the other hand, Park et al. reported that intraportal delivery of LV could transduce only a few hepatocytes, whereas