Cytomegalovirus (CMV) is a major cause of sensorineural hearing loss and other serious disabilities in young children, responsible for more long-term problems and childhood deaths than fetal alcohol syndrome, Down syndrome, and neural tube defects, according to the Centers for Disease Control and Prevention. In the United States, one in 150 children is born with CMV infection, but little is known about how the virus attacks a developing fetus's central nervous system.
Now, a new study published by PLoS ONE demonstrates the varying toll CMV exacts as immature stem cells develop into brain cells (2012;7:e49700). According to the research, human stem cells appear to resist CMV infection, becoming more susceptible to the virus as they mature into neurons.
“What we are finding is that different cells in this pathway have different levels of resistance to the virus,” said Vishwajit Nimgaonkar, MD, PhD, professor of psychiatry and human genetics at the University of Pittsburgh School of Medicine and senior author of the report.
It is the first published study to use human induced pluripotent stem cells (iPSCs) to examine the effects of human CMV, and the first to show that these primitive cells can resist human CMV infection.
INDUCED PLURIPOTENT STEM CELLS
Unlike embryonic stem cells, which are harder to come by and pose ethical issues, iPSCs are readily derived by reprogramming human fibroblasts, a common type of cell found in connective tissue. Previous studies examining the effects of CMV have used animal models or neural progenitor cells cultured from fetal brain tissue, the study authors noted.
Dr. Nimgaonkar and colleagues obtained fibroblasts from human skin biopsies taken from the shoulders of two control individuals. The researchers then cultured and harvested those cells for iPSCs. Induced pluripotent stem cells were coaxed to mature into neural progenitor cells (NPCs) and then into neurons.
“What we were able to do is look at the virus and its effects on all these different cells,” Dr. Nimgaonkar explained.
An earlier study conducted by Elizabeth (Lee) Ann Fortunato, PhD, an associate professor of biological sciences at the University of Idaho in Moscow, ID, and colleagues (J Virol 2008;82:9994-10007) provided the impetus for Dr. Nimgaonkar's efforts.
Using fetal brain tissue from premature newborns who died of natural causes unrelated to CMV, Dr. Fortunato showed that neural progenitor cells are susceptible to CMV infection and remain so even as they age. Dr. Nimgaonkar wanted to see if he could replicate those results using the iPSC model.
Currently, Dr. Fortunato is conducting studies using iPSCs, and she is pleased that Dr. Nimgaonkar's findings parallel her own work, she said.
“I would say this study lines up pretty well with what everybody's seeing.”
CMV is a strictly species-specific virus, so observations from animal models may or may not be relevant to humans, explained Mark R. Schleiss, MD, professor of pediatrics and director of pediatric infectious diseases and immunology at the University of Minnesota Medical School in Minneapolis.
“One of the positive things about [Dr. Nimgaonkar's study] is it's a study of human cytomegalovirus in human cells, so I think that's a step forward, but all of these models have things to contribute and help us learn more about why babies' brains are injured by the virus,” he said.
One distinct advantage of iPSCs over neonatally derived NPCs is that they “replicate like crazy,” producing massive numbers of cells and yielding “tons of neurons” when differentiated, Dr. Fortunato explained.
“And that, in and of itself, is what we need to be able to do more large-scale studies,” she said. On the downside, the drugs used in the lab to derive and differentiate the iPSCs are exceedingly expensive, she added.
PREVENTION AND TREATMENT
Although early hearing loss detection is considered crucial for normal speech and language development, healthy newborns in the United States are not routinely screened for CMV. Congenital CMV infection is responsible for an estimated 15 to 20 percent of cases of bilateral moderate to profound sensorineural hearing loss in children, the authors of a literature review concluded (J Clin Virol 2008;41:57-62)
But the toll could be greater than people realize, Dr. Fortunato cautioned.
“If you counted all the kids who perhaps were born with a very, very low-level of hearing loss and no real serious signs of infection, that number could be a lot higher,” she said.
Mass CMV screening of newborns has been stymied by the lack of a quick, reliable, and inexpensive screening method, as well as the absence of a treatment that works without serious negative side effects. Efforts are under way on both fronts.
In the National Institute on Deafness and Other Communication Disorders CMV and Hearing Multicenter Screening (CHIMES) study, researchers reported that a simple saliva test identified about 97% of infected newborns (N Engl J Med 2011; 364:2111-2118; HJ Dec. 2011, p. 13).
And while vaccine research is progressing slowly, a number of new drugs are in various stages of testing, “and they're much better tolerated than ganciclovir,” the antiviral currently given to babies with symptoms of infection, Dr. Schleiss noted.
“I look for these to become available in the clinic in the next few years,” he said.
Dr. Nimgaonkar is currently working on optimizing cell culture assays. Once that is done, his colleagues in pharmacology will assess the effectiveness of various antiviral agents in CMV-infected cells.
Of course, there's no guarantee that the effort will produce a safe and effective drug. “That's a huge gamble,” he acknowledged.