BY THOMAS R. COLLINS
CHICAGO—A new mouse model for studying congenital Zika virus shows that the virus downregulates genes involved in embryonic brain development and neural differentiation and inhibits cell-cycle progression in a way that's dependent on the Zika viral load, according to findings presented here at the Child Neurology Society annual meeting.
The findings give new clarity to the ways in which congenital infection with Zika leads to microcephaly, said Ganeshwaran Mochida, MD, MMSc, assistant professor of pediatrics at Harvard Medical School and a staff physician in the genetics and genomics division at Boston Children's Hospital.
The model was designed to keep the immune system intact.
"The benefit of our model is that we used the intraplacental infection to circumvent the resistance of mice to Zika virus infection, successfully creating infection in the embryos," Dr. Mochida said. "Also, this model allows us to infect the mouse embryos at an early stage."
Zika was administered at embryonic day 10.5 when cortical development of the brain begins.
The infected embryos showed reduced survival and smaller size and weight. The layered cortical structure was largely preserved, but the layers were thinner, Dr. Mochida said.
Researchers found several important genes that were downregulated at both the RNA and protein level in Zika-infected embryonic brains.
"We were struck by the fact that many of these genes were actually transcription factors that are already known to be important in human brain development," Dr. Mochida said. They include TBR2, NEUROD2, DLX2 and ARX, which have roles in intermediate cell proliferation, terminal differentiation, and cerebral interneuron development.
The researchers did not find genes with roles in neural progenitor cycles, but on histological review they found that a marker for neural progenitor cycles was decreased in the Zika brains.
When they looked at protein abundance and abundance of Zika viral protein — using five brains, each with different levels of virus — they found cell-cycle mitosis genes and chromosomal organization genes that were lower when the amount of Zika viral protein was higher.
"These results show that Zika virus inhibits cell-cycle progression in a dose-dependent manner," Dr. Mochida said.
He said the analysis will likely help identify potential therapeutic targets for congenital Zika infection.
"This (proteogenomic) approach may be applied to other congenital infections such as cytomegalovirus, herpes virus, and a wide variety of neurodevelopmental and neurodegenerative conditions."
Barry Kosofsky, MD, PhD, chief of pediatric neurology at Weill Cornell Medicine, said that with a single injection to the placenta some of the variability in the disease presentation in the mouse embryos may relate to the variability of the injection and its perfusion. This variability was noted in the recognition of the dose response, he said.
Dr. Kosofsky said that an important question that remains to be answered is the role of the immune response. In the data presented, investigators focused on genes that were downregulated, but analysis of genes that were upregulated — which Dr. Mochida said were mostly immune-related — is ongoing.
"What's the individual response on the immune side when you bring that back in in terms of interactions of immune competence with expression of the phenotype?" he asked. "If you're more immune-responsive, is that going to give you a better or worse outcome?"
Drs. Mochida and Kosofsky had no disclosures.
LINK UP FOR RELATED INFORMATION:
Moore CA, Staples JE, Dobyns WB, et al. Characterizing the pattern of anomalies in congenital Zika syndrome for pediatric clinicians. JAMA Pediatr 2017; 171(3):288-295.