RNA virus populations are complex distributions of closely related but nonidentical variant genomes termed quasispecies. The large heterogeneity in quasispecies is due to the low fidelity of viral RNA polymerases and the lack of error-repair activities during replication. Studies on genome replication with high error rates predict the existence of an error threshold above which the maintenance of the genetic information would not be possible. The quasispecies structure would be lost when the error rate of replication crosses the error threshold, in a process that has been termed entry into error catastrophe. This fact opens interesting possibilities for the design of a new antiviral strategy based on inducing viral error catastrophe by forcing viruses to replicate in the presence of mutagens. The term lethal mutagenesis is referred to as viral extinction through error catastrophe. A critical question for the application of error catastrophe is whether mutations can occur that render viruses resistant to mutagenic agents. The main objective of this Ph. D. Thesis is to understand the mechanisms underlying resistance of RNA viruses to mutagenic agents and the evolutionary implications of quasispecies dynamics. The nucleoside analogue ribavirin (R) is mutagenic for foot-and-mouth disease virus (FMDV). We have selected FMDV with amino acid substitution M296I in the viral polymerase (3D) by passaging FMDV in the presence of increasing concentrations of R. Measurements of progeny production and viral fitness with chimeric viruses in the presence and absence of R documented that 3D substitution M296I conferred FMDV a selective replicative advantage in the presence of R but not in the absence of R. Purified mutant polymerase with I296 showed a decreased capacity to use ribavirin triphosphate (RTP) as substrate as compared with the wild type enzyme. The results suggest that M296I has been selected because it attenuates the mutagenic activity of R on FMDV. Replacement M296I is located within a highly conserved stretch in picornaviral polymerases, which includes residues that interact with template-primer and with the incoming nucleotide, according to the three-dimensional structure of FMDV 3D. Given that a 3D substitution, distant from M296I, was associated with resistance to R in poliovirus, the results indicate that picornaviral polymerases include different sites that can alter the interaction of the enzyme with mutagenic nucleoside analogues. Implications for lethal mutagenesis are discussed.

El trabajo presentado en esta Tesis Doctoral ha sido realizado en el Centro de Biología Molecular “Severo Ochoa”, bajo la dirección del Dr. Esteban Domingo Solans y financiado por una beca Predoctoral de Formación de Profesorado Universitario (FPU) del Ministerio de Educación y Ciencia.

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología Molecular. Fecha de lectura 13-07-2007

Peer reviewed