By Philip R. Fischer, MD, DTM&H
Professor of Pediatrics, Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN
Dr. Fischer reports no financial relationships relevant to this field of study.
SYNOPSIS: Approximately five years ago, a single gene mutation altered Zika virus, making it able to target neuronal progenitor cells and cause what we now know as congenital Zika syndrome with microcephaly and ocular abnormalities.
SOURCE: Yuan L, Huang XY, Liu ZY, et al. A single mutation in the prM protein of the Zika virus contributes to fetal microcephaly. Science 10.1126/science.aam7120. [Epub ahead of print.]
For 60 years since its discovery in a Ugandan forest, Zika virus seemed “just” to cause infrequent mild infections. During outbreaks on Pacific islands during the past decade, congenital brain malformations of offspring of women infected during pregnancy became prevalent. This led to a public health emergency as the virus spread to the Americas.
There are two strains of Zika virus circulating: the Asian strain in the Pacific and Americas and the African strain elsewhere. The Zika virus includes genomic RNA that encodes for both non-structural and structural proteins, including the prM structural protein.
Yuan and colleagues wondered why a relatively quiescent virus suddenly emerged during recent years as a cause of severe brain malformations. Thus, they undertook experiments involving cultured cells and living mice.
Intracerebral injection of contemporary (from 2015 and 2016) strains of Zika virus into newborn mice was associated with motor weakness, hind limb paralysis, and 100% mortality. Similar injection of a 2010 Zika strain was associated with only 17% mortality. Thus, it seems that currently circulating Zika strains are more virulent in the central nervous system than were strains circulating seven years ago.
The researchers then employed a mouse embryonic microcephaly model. They injected the virus into fetal mouse brains at a point in gestation corresponding to what would be the second trimester of a human pregnancy. The 2016 Zika virus strain led to cortical thinning and marked microcephaly; the 2010 Zika strain was associated with notably less severe findings. Each virus strain targeted neuronal progenitor cells, but the 2016 strain showed significantly enhanced replication in the brain as compared to the 2010 strain.
In a related experiment using cultured mouse neuronal progenitor cells, the 2016 strain caused more apoptosis with more loss of both mature and immature brain cells than did the 2010 strain. The proliferation and distribution of neuronal progenitor cells also was more greatly reduced by the 2016 strain than by the 2010 strain. Thus, the 2016 strain was shown to be more virulent to the mouse brain and to cause more significant microcephaly than did the older 2010 strain.
Yuan and colleagues then sought to identify genetic determinants of varying virulence. Several amino acid substitutions were identified in contemporary strains as compared to older ancestral strains. Specific genetic changes were determined to have occurred at different time points and then to have been maintained in the circulating virus. One mutation and resultant amino acid substitution in particular (S139N) was associated with the most neuronal virulence. And, genetically manipulating a reverse S139N change in an otherwise unaltered 2016 virus reduced the mortality of infection in neonatal mice. With the amino acid substitution, there was enhanced viral replication and increased cell death compared to infection with virus lacking that substitution.
The investigators then studied the impact of the altered virus (with S139N substitution) in the embryonic microcephaly model. Infection with the mutant virus was associated with more severe microcephaly, more cortical thinning, more robust infection of neuronal progenitor cells, and more cell death in the cortical plate.
Thus, a single mutation could cause a specific amino acid substitution (S139N) in the Zika virus. This mutation first emerged in early 2013 and has persisted since. The cell data, mouse results, and clinical experience all are consistent in showing that this newly evolved Zika strain is associated with aggressive and destructive pathology in developing central nervous systems (and, it seems from clinical experience, with changes leading to Guillain-Barré syndrome). Studies in related dengue viruses suggest that this specific amino acid substitution is part of the prM protein that affects the timing of fusion of viral particles to infected cells.
Yuan and colleagues in China seem to have completed a circle of understanding of Zika infections. Clinical observations were consistent with a change in manifestations of Zika infection with both more virulent infections and more congenital brain malformations in recent years. In cellular and animal studies, the researchers showed virus-specific changes over time that correlate directly with clinical observations. And they identified a mutant gene and its altered protein, showing that the mutation was linked with neuropathology, that the absence of mutation was associated with less neuronal pathology, and that the protein plausibly is associated with the resulting pathologic changes. This translational science has linked the cycle from epidemiology to clinical presentations to genetics and to proteomics, and it has prompted a helpful understanding of the history of Zika infection.
The clinical characterization of congenital Zika syndrome is progressing. Although devastating when it occurs, microcephaly is identified only in a small minority of offspring of Zika-infected women.1 In addition, there are reports of twin pregnancies in which only one of the twins develops microcephaly.2 Clearly, there is more to learn about the pathogenesis of congenital infection with Zika virus.
Recently, the ocular findings of congenital Zika infection were characterized in embryonically infected humans.3 Zika antigen was found in iris, retina, choroid, and optic nerve tissue. The eyes manifested pupillary membranes with altered anterior chamber angles, thinning of retinal pigment epithelium, and choroidal inflammation. One can easily speculate that the affinity of current Zika strains for neuronal progenitor cells is what made the virus so harmful to developing retinal and optic nerve cells.
- Coelho AVC, Crovella S. Microcephaly prevalence in infants born to Zika virus-infected women: A systematic review and meta-analysis. Int J Mol Sci 2017;18:pii E1714.
- Linden VV, Linden HV, Leal MC, et al. Discordant clinical outcomes of congenital ZIka virus infection in twin pregnancies. Arg Neuropsiquiatr 2017;75:381-386.
- Fernandez MP, Parra Saad E, Ospina Martinez M, et al. Ocular histopathologic features of congenital Zika syndrome. JAMA Ophthalmol doi:10.1001/jamaophthalmol.2017.3595. [Epub ahead of print.]