A new study
published in PLOS Pathogens
to modify an effective live attenuated Severe Acute
Respiratory Syndrome (SARS)
vaccine to make it genetically stable.
vaccine is developed by reducing the harmful quality of a pathogen, but still
keeping it alive. Luis Enjuanes, from the National Center for Biotechnology in
Madrid, Spain, and his research team had previously brought a SARS coronavirus
(SARS-CoV) into use lacking the envelope (or E) gene as a promising vaccine candidate.
‘To prevent such compensation and reversal to virulence, scientists initiated small deletions in the E gene that did not eliminate its PDZ binding motif. Such mutants stayed reduced in virulence but appear to no longer be ready for the incorporation of novel protein domains into the virus genome.’
The vaccine is
known as SARS-CoV-ΔE, and scientists had demonstrated that it was attenuated in
different animal models, intimating that the E protein is imperative for the
virus' ability to cause disease. They had also showed that the vaccine
protected mice against challenges with virulent SARS-CoV that is lethal to
unvaccinated mice, proving that it is an effective vaccine.
In the current
study, scientists addressed the concern of stability of the vaccine candidate.
In order to do this, the investigators propagated the SARS-CoV-ΔE virus for
some generations in cell lines and mice. They found that the virus accumulated
mutations and reverts to a virulent phenotype.
Focusing on a
collection of mutants, they were able to tell the molecular basis of the
reversion. The analysis says that the E protein carries a motif called a PDZ
binding motif or PBM that controls cellular pathways, which is significant for
, dissemination in the host, and pathogenesis. And all the reverted viruses had incorporated into the genome a functional PBM, seemingly to compensate for the removal of this motif with deletion of the E protein.
To prevent such
compensation and reversal to virulence, the scientists initiated small
deletions in the E gene that did not eliminate its PBM, instead of destroying
the entire gene. Such mutants stayed reduced in virulence but appear to no
longer be ready for the incorporation into the virus genome and so avoid the
reversion to the strong form.
In order to
create an extra safeguard, the scientists took mutations into another SARS-CoV
gene known as nsp1. This gene is usually found at a distant site from that of
the E gene in the viral genome. This position makes it less likely that a
single mutational event can restore both the nsp1 gene and the E gene to their
un-attenuated sequences and thereby reinstate virulence. This is the reason the
scientists picked up nsp1 as a second attenuation target.
came to know that small deletions within the nsp1 gene alone brought up an
attenuated virus that could not cause disease but defended vaccinated mice
against challenge with the virulent parental virus. And when they examined the
new vaccine that includes small attenuating mutations in both the E and nsp1
genes, they saw that it maintained its attenuation after prolonged propagation
in both inside a living organism and a test tube and provided full protection
against the virulent original SARS-CoV.
conclude that "understanding the molecular mechanisms leading to
pathogenicity and the in vivo evaluation of vaccine genetic stability
contributed to a rational design of a promising SARS-CoV vaccine." They
also suggest that "understanding how an attenuated SARS-CoV reverted to
virulence could also be useful for vaccine development against other relevant
coronaviruses, such as the MERS
SARS is the
first emerging epidemic of this century. It is too early to get to a conclusion
that how SARS will evolve in the future, but the world has learned several
Jose M. Jimenez-Guarde˝o, Jose A. Regla-Nava, Jose L. Nieto-Torres, Marta L. DeDiego,
Carlos Casta˝o-Rodriguez, Raul Fernandez-Delgado, Stanley Perlman, Luis
Enjuanes. Identification of the Mechanisms Causing Reversion to Virulence in an
Attenuated SARS-CoV for the Design of a Genetically Stable Vaccine. PLOS
Pathogens, 2015; 11 (10): e1005215 DOI: 10.1371/journal.ppat.1005215