Mutation of a Single Envelope N-Linked Glycosylation Site Enhances the Pathogenicity of Bovine Leukemia Virus

Abstract

Viruses have coevolved with their host to ensure efficient replication and transmission without inducing excessive pathogenicity that would indirectly impair their persistence. This is exemplified by the bovine leukemia virus (BLV) system in which lymphoproliferative disorders develop in ruminants after latency periods of several years. In principle, the equilibrium reached between the virus and its host could be disrupted by emergence of more pathogenic strains. Intriguingly but fortunately, such a hyperpathogenic BLV strain was never observed in the field or designed in vitro. In this study, we sought to understand the role of envelope N-linked glycosylation with the hypothesis that this posttranslational modification could either favor BLV infection by allowing viral entry or allow immune escape by using glycans as a shield. Using reverse genetics of an infectious molecular provirus, we identified a N-linked envelope glycosylation site (N230) that limits viral replication and pathogenicity. Indeed, mutation N230E unexpectedly leads to enhanced fusogenicity and protein stability. There is no satisfactory treatment for HAM/TSP, and the prognosis for ATLL is still poor despite improved therapies (2). BLV is responsible for major economic losses in cattle due to export limitations, carcass condemnations, and reduction in milk production. BLV infection also correlates with a significant morbidity resulting from opportunistic infections and a decrease in longevity due to tumor development (3). In a proportion (i.e., about one third) of infected cattle, BLV induces a lymphoproliferative disease called persistent lymphocytosis. After a latency period of several years, BLV infection also leads to leukemia and/or lymphoma in a minority (5%) of the infected animals. However, the majority of BLV carriers remain clinically healthy and acts as asymptomatic carriers for viral spread (4). Besides its natural hosts (i.e., cattle, zebu, and water buffalo), BLV can be experimentally transmitted to sheep and goats, where leukemia/lymphoma develops after shorter latency periods (5-7).Retroviral envelope glycoproteins play an important role in the viral life cycle: they contain the recognition site required for entry and mediate cell fusion (8). The BLV envelope is composed of two glycoproteins: a surface (SU) protein, Gp51, and a transmembrane (TM) protein, Gp30, derived from the proteolytic cleavage of a common precursor (gpr72) encoded by the env gene (9-11). The SU protein is N-glycosylated in the rough endoplasmic reticulum by covalent attachment of oligosaccharide chains (12). Although the role of BLV SU-linked glycans is currently unknown, it is expected that N-glycosylation is required for protein folding, stability, or solubility (13). Furthermore, N-glycans are also likely involved in transport of BLV envelope proteins to the cell membrane, binding to cellular receptors and cell-to-cell fusion (14-18) similarly to glycans of the human immunodeficiency virus (HIV) envelope glycoprotein that modulate fusogenicity ...

Identification of an Atypical Enzootic Bovine Leukosis in Japan by Using a Novel Classification of Bovine Leukemia Based on Immunophenotypic Analysis

“…First, a mutation of the virus genome can affect the incubation period until tumor development, because in a previous study, the percentage of EBL cases associated with the L233-Tax protein was significantly higher than that associated with the P233-Tax in young cattle (26). Moreover, a mutation in an N-linked envelope glycosylation site (N230E) resulted in high provirus loads during experimental infection in sheep, which led to an accelerated pathogenesis and shortening of the incubation period (30). Second, it has been reported that certain alleles of the major histocompatibility complex class II DRB3 gene are involved in susceptibility and resistance against BLV infection (31,32).…”

Reducing the global burden of HTLV-1 infection: An agenda for research and action

“…There is also a potential threat associated with the emergence of hyperpathogenic strains, as described in related viruses (de Brogniez et al, 2015). …”

Enzootic bovine leukosis in a two-month-old calf

“…The replacement of asparagine with glutamic acid at codon 230 of the Env protein reportedly enhances the pathogenicity of BLV through the alteration of the N-linked glycosylation status (de Brogniez et al, 2015). However, the BLV strain in the calf in this study maintained asparagine at this site.…”