Identification of genetic mutations associated with attenuation and changes in tropism of Urabe mumps virus

Attenuation and Degeneration of SARS-CoV-2 Despite Adaptive Evolution

The evolution of severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) has followed similar trends as other RNA viruses, such as human immunodeficiency virus type 1 and the influenza A virus. Rapid initial diversification was followed by strong competition and a rapid succession of dominant variants. Host-initiated RNA editing has been the primary mechanism for introducing mutations. A significant number of mutations detrimental to viral replication have been quickly purged. Fixed mutations are mostly diversifying mutations selected for host adaptation and immune evasion, with the latter accounting for the majority of the mutations. However, immune evasion often comes at the cost of functionality, and thus, optimal functionality is still far from being accomplished. Instead, selection for antibody-escaping variants and accumulation of near-neutral mutations have led to suboptimal codon usage and reduced replicative capacity, as demonstrated in non-respiratory cell lines. Beneficial adaptation of the virus includes reduced infectivity in lung tissues and increased tropism for the upper airway, resulting in shorter incubation periods, milder diseases, and more efficient transmission between people.

Animal and human RNA viruses: genetic variability and ability to overcome vaccines

RNA viruses, in general, exhibit high mutation rates; this is mainly due to the low fidelity displayed by the RNA-dependent polymerases required for their replication that lack the proofreading machinery to correct misincorporated nucleotides and produce high mutation rates. This lack of replication fidelity, together with the fact that RNA viruses can undergo spontaneous mutations, results in genetic variants displaying different viral morphogenesis, as well as variation on their surface glycoproteins that affect viral antigenicity. This diverse viral population, routinely containing a variety of mutants, is known as a viral ‘quasispecies’. The mutability of their virions allows for fast evolution of RNA viruses that develop antiviral resistance and overcome vaccines much more rapidly than DNA viruses. This also translates into the fact that pathogenic RNA viruses, that cause many diseases and deaths in humans, represent the major viral group involved in zoonotic disease transmission, and are responsible for worldwide pandemics.

Mechanisms of temperature sensitivity of attenuated Urabe mumps virus

Temperature sensitivity is a phenotype often associated with attenuation of viruses. Previously, we purified several mumps variants from an incompletely attenuated Urabe strain live attenuated vaccine. Here we characterize one isolate that is sensitive to growth at high temperature. This virus was attenuated in a small animal model of mumps virulence, and we identified unique coding substitutions in the hemagglutinin-neuraminidase (HN), the viral polymerase (L) gene, and a non-coding substitution close to the anti-genome promoter sequences. At the non-permissive temperature, transcription of viral mRNAs and production of the replication intermediate were reduced compared to events at the permissive temperature and to a non-ts virulent Urabe virus. As well, synthesis of viral proteins was also reduced at the higher temperature. While the actual sequence substitutions in the ts virus were unique, the pattern of substitutions in HN, L and genome end sequences is similar to another attenuated Urabe virus previously described by us.

Investigation of a Possible Link Between Vaccination and the 2010 Sheep Pox Epizootic in Morocco

Sheep pox is endemic in most parts of Northern Africa and has the potential to cause severe economic problems. Live attenuated vaccines are used in Morocco, and in many other countries, to control the disease. Sheep pox virus (SPPV) re-appeared in 2010 causing a nodular clinical form previously not observed in Morocco. The severe clinical signs observed during the course of this outbreak and initial reports citing similarity in nucleotide sequence between the Moroccan vaccine strain and field isolates warranted a more in depth analysis of this epizootic. In this study, sequence analysis showed that isolates obtained from four provinces of eastern Morocco were identical, demonstrating that a single SPPV strain was responsible for the 2010 epizootic. In addition, the genome fragments sequenced and phylogenetic analyses undertaken as part of this study showed significant differences between field isolates and the Moroccan vaccine strain. New PCR methods were developed to differentiate between wild-type isolates and vaccine strains of SPPV. Using these methods, no trace of wild-type SPPV was found in the vaccine and no evidence was found to suggest that the vaccine strain was causing clinical disease.

Amino acid substitution at position 464 in the haemagglutinin-neuraminidase protein of a mumps virus Urabe strain enhanced the virus growth in neuroblastoma SH-SY5Y cells

Mumps virus (MuV) infects various organs including central nervous system (CNS). However, the molecular basis of the neural cell specificity of MuV is not well understood. We found that the Hoshino vaccine strain rescued from cDNA replicated moderately in neuroblastoma SH-SY5Y cell line, while an Urabe strain (Ur89-250) isolated from a post-vaccination aseptic meningitis case replicated efficiently in the same cells. In order to examine the contribution of individual genes of Ur89-250 to the growth in SH-SY5Y cells, recombinant Hoshino vaccine strains in which each gene(s) was replaced with corresponding gene(s) of Ur89-250 were generated. A recombinant virus possessing the small hydrophobic and haemagglutinin-neuraminidase (HN) genes of Ur89-250 grew as efficiently in SH-SY5Y cells as Ur89-250. Further analysis indicated that an amino acid substitution at position 464 in the HN protein was most important for efficient growth. Thus, single amino acid substitution in the HN protein could affect neural cell specificity of mumps virus.