Attenuated reovirus displays oncolysis with reduced host toxicity

Mammalian REO (respiratory enteric orphan) virus is a small, non-enveloped icosahedral virus that contains segments of double-stranded RNA as its genome. In humans, reovirus targets the upper respiratory and gastrointestinal tracts to generate a relatively benign and often asymptomatic infection (Tyler, 2001). Of interest is its ability to infect and kill many types of transformed cells (Hashiro et al, 1977; Duncan et al, 1978), making reovirus one example among a variety of replication competent oncolytic viruses that potentially serve as anti-cancer therapies. Such oncolytic viruses have, in principle, two main advantages over conventional chemotherapy and radiotherapy. First, they generally target cancer cells because of their reduced ability, whether innate or engineered, to replicate in normal cells. Second, in comparison with replication-incompetent vectors, they can propagate from initially infected cancer cells to surrounding or distant cancer cells, thereby achieving a large volume of distribution and potent anti-cancer effects. Despite the above, ongoing challenges in this field include, ensuring that (a) all cancer cells are efficiently targeted, including those that have metastasised to distant sites; (b) damage to normal tissues and proliferating cells is minimized and (c) natural host-immune responses do not prematurely block viral eradication of tumour cell populations. Reovirus Type 3 Dearing (T3D) is a naturally occurring oncolytic virus that preferentially targets Ras-transformed cells both in vitro and in vivo (Coffey et al, 1998; Strong et al, 1998; Norman et al, 2004). Constitutively activated Ras contributes to reovirus oncolysis by increasing the efficiency of (1) reovirus disassembly during entry, (2) infectious virion production and (3) apoptosis and virion release (Marcato et al, 2007). As Ras gene mutations are observed in over 30% of all human cancers (Duursma and Agami, 2003), these findings have led to the current use of reovirus in phase I, II and III clinical trials (Oncolytics Biotech Inc., 2010). However, in immunocompromised hosts, such as newborn and severe combined immunodeficiency (SCID) animals; wild-type reovirus shows significant pathogenicity, especially to neural and cardiac muscle tissues (Sabin, 1959; Weiner et al, 1977; Baty and Sherry, 1993; Loken et al, 2004). Occasionally, this pathogenesis is observed in immunocompetent hosts, but mainly at high viral loads (Hirasawa et al, 2003; Terheggen et al, 2003). Clinical trials have reported no severe toxicity when utilising wild-type reovirus in combination with radiation or chemotherapy (Thirukkumaran and Morris, 2009). Even so, a less virulent reovirus that displays reduced pathogenicity to healthy tissues and cells while retaining potent oncolytic activity could still be beneficial by providing a potentially higher number of treatments or dosage. One way of generating less virulent reovirus is through the establishment of persistently infected cells. Persistent reovirus infection is occasionally induced in various cell types in vitro (Dermody, 1998). Although the underlying basis of reovirus persistent infection is not well understood, it has been speculated that interactions between the virus and host cell can be fundamentally altered such that a modified cytopathic virus–host relationship is established (Wetzel et al, 1997). Importantly, such infections can give rise to distinct reovirus variants (Ahmed and Fields, 1982; Wilson et al, 1996; Wetzel et al, 1997); however, the potential utility of these variants for reovirus oncolysis has yet to be examined. In this report, we describe a mutant reovirus with a modified S1 gene that has been isolated from persistently infected cells. The modified reovirus contains a truncated σ1-cell-attachment protein and displays significantly reduced pathogenic potential to healthy cells while maintaining its oncolytic activity. This attenuated virus (AV) may provide a desirable option in future oncolytic cancer therapy.