Neural stem cells derived from human embryonic stem cells carry abnormal gene expression

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Researchers from the Institute for Stem Cell Biology and Medicine at UCLA coaxed cells from the federally approved line to differentiate into neural stem cells, a process that might one day be used to grow replacement cells to treat such debilitating diseases as Parkinson's and Alzheimer's. However, the neural stem cells expressed a lower level of a metabolic gene called CPT 1A, a condition that causes hypoglycemia in humans.

The study may shed new light on better ways to grow neural and other stem cells in the lab so they mirror normal cells and promote normal functioning, said Guoping Fan, an assistant professor of human genetics and a researcher in UCLA's stem cell institute. The study appears this week in an early online edition of the journal Human Molecular Genetics.

"This study is a very important first step in looking at the differentiation process in neural stem cells," said Fan, senior author of the study. "Now we have a direct measurement of the types of cells that eventually, we hope, will be used for transplantation. We can tell, are they normal or not. Understanding why these cells under-expressed CPT 1A is the first step in a comprehensive understanding of cells obtained from human embryonic stem cells."

The study, Fan said, deals with one of the most important aspects in stem cell biology-potential abnormalities in cells derived from human embryonic stem cells. Stem cells with abnormalities may not effectively treat the diseases they were created to treat, or they may result in secondary problems such as hypoglycemia, Fan said.

UCLA researchers also compared the neural stem cells they grew to cancer cells to ensure that the neural stem cells did not have any abnormalities in a DNA modification associated with gene silencing. The abnormal DNA modification is characteristically a hallmark of cancer cells. The good news, Fan said, is that the neural stem cells in their study did not share any abnormal characteristics associated with cancer. The means, theoretically, that a patient undergoing transplantation with these neural stem cells would not later develop a malignancy.

In the three-year study, researchers compared the neural stem cells grown in the lab from human embryonic stem cells to neural stem cells that already had differentiated and were derived from donated fetal tissue. The question: would the cell lines be the same and mirror the normal neural stem cells found in humans or would one cell line be superior to the other?

"Compared to the normal cells derived from the fetal tissue, the level of gene expression in the neural stem cells grown in the lab is lower," Fan said. "Proper levels of gene expression are essential for normal cell function. This study suggests that the differentiation procedure used in the lab needs to be improved so all genes are properly regulated in the stem cells we grow."

Fan and his colleagues now are studying what may have gone awry in the process they used to coax the human embryonic stem cells to differentiate into neural stem cells that may have resulted in the under-expression of the CPT 1A gene. They're also planning to repeat their work on other federally approved stem cell lines to see if the abnormality was an aberration found only in this one stem cell line. Fan and other UCLA researchers said the abnormality found in the federally approved stem cell line reinforces the need for other embryonic stem cells lines on which to conduct research.

To compare the neural stem cells, researchers extracted DNA fragments and used high throughput micro array technology to study the pattern of DNA cytosine methylation. They also monitored for levels of gene expression that are necessary for cell function as well as abnormalities that might be problematic.

"Any stem cells that might one day be used for transplantation have to be as close as possible to normal stem cells," Fan said. "The next step is to see if we can improve the way we grown these cells. I think we learned an important lesson with this study."

University of California-Los Angeles