New research provides proof for the hypothesis that there is a genetic or epigenetic influence on susceptibility to Zika syndrome (CZS) and microcephaly. The study, done by geneticists at the Human Genome and Stem Cell Research Center from the University of São Paulo, was published last week in Nature Communications.
It’s been observed that only about 6% to 12% of babies born from mothers infected with the Zika virus during pregnancy will have CZS. To better understand this, the researchers recruited 9 sets of twins born during the 2016 Zika outbreak. Two pairs were identical twins who both had microcephaly, one was non-identical with both affected, and the remaining six were non-identical and discordant, meaning only one twin was affected.
Three pairs of the non-identical, discordant twins had their blood samples collected and used to generate human induced pluripotent stem cells (hiPSC) that were transformed into neural progenitor cells (NPCs). The NPCs were infected with a Brazilian Zika virus strain and after four days, the plaques from the cells from the affected babies had significantly fewer cells than the non-affected ones. A control culture of non-infected cells from affected and non-affected derived NPCs presented no difference in growth.
Using fluorescence imaging, researchers observed larger amounts of Zika virus in cells derived from affected babies compared to non-affected ones. The fact that the cell culture tests mimicked the twins’ real-world infection pattern points to non-random differences in susceptibility, indicating a genetic influence.
The team also analyzed gene sequences from eight pairs of twins and ten other babies that developed CZS and compared them to normal controls. Exome analysis did not identify one gene variation capable of determining the susceptibility to the infection by itself.
Excluding the possibility of a single genetic component, the researchers next performed RNA sequencing. The RNA test from the six discordant twins indicated a group of genes capable of distinguishing the more susceptible genes from the more resistant ones. The largest difference was in the DDIT4L gene, which is expressed 12.6 times less expressed in the affected cells. DDIT4L’s related protein is an inhibitor of the mTOR signaling pathway, which is related to cell growth and death, and has been associated with Zika virus replication on prior studies. Other differences include a significantly lowered expression of FOXG1 and LHX2 genes in cells from affected babies. These genes participate in the brain regionalization process.
These findings shed light on the ways in which genetics affect Zika susceptibility and could potentially be used to identify candidates for receipt of vaccines.
Image: Computed tomography scans from two pairs of twins participant in the study show typical abnormalities (second and forth images) associated with congenital Zika syndrome and microcephaly. Image courtesy of HUG-Cell.