Down Syndrome Abstract
of the Month: Mar 2002
Neuronal target genes of the neuron-restrictive silencer factor in neurospheres derived from fetuses with Down's syndrome: a gene expression study.
Bahn S, Mimmack M, Ryan M, Caldwell MA, Jauniaux E, Starkey M, Svendsen CN, Emson P
Lancet 2002 Jan 26;359(9303):310-5
Department of Neurobiology, Babraham Institute, Cambridge, UK
Abstract:
BACKGROUND: Identification of genes and characterization of their function is an essential step towards understanding complex pathophysiological abnormalities in Down's syndrome. We did a study to investigate abnormalities in gene expression in human neuronal stem cells and progenitor cells from Down's syndrome and control post-mortem human fetal tissue. METHODS: Indexing-based differential display PCR was done on neuronal precursor cells derived from the cortex of a fetus with Down's syndrome, and findings were compared with those of two control samples. Findings were validated against neurosphere preparations from three independent Down's syndrome fetuses and five independent controls by real-time quantitative PCR. FINDINGS: Results of differential display PCR analysis showed that SCG10--a neuron--specific growth-associated protein regulated by the neuron-restrictive silencer factor REST-was almost undetectable in the Down's syndrome sample. This finding was validated by real-time PCR. We also found that other genes regulated by the REST transcription factor were selectively repressed, whereas non-REST-regulated genes with similar functions were unaffected. Changes in expression of several key developmental genes in the Down's syndrome stem-cell and progenitor-cell pool correlated with striking changes in neuron morphology after differentiation. INTERPRETATION: Our findings suggest a link between dysregulation of the REST transcription factor and some of the neurological deficits seen in Down's syndrome. Experimental REST downregulation has been shown to trigger apoptosis, which could account for the striking and selective loss of neurons in the differentiated Down's syndrome cell preparations.
My comments: This is an extremely important paper: the first human stem cell research on Down syndrome cells, and it gives us new, useful information as well. I'll try to simplify it here.
First, the concept of "stem cells." Every fetus begins as one cell which then divides over and over again. From that one cell, we get skin cells and brain cells and muscle cells and...well, you get the idea. Certain signals occur in the cells that cause it to develop into specific cells, and these signals arise from the DNA in those cells. Embryonic stem cells are often collected at the stage that they are about to become a specific cell line; in the case of this study, the researchers used stem cells that are the precursors to neural cells, and are called "neurospheres." The researchers here used neurospheres from post-mortem fetuses with and without Down syndrome.
The first thing these researchers did was to extract the RNA proteins from the neurospheres and compare them to the RNA from normal neurospheres. (DNA directs the synthesis and sequence of RNA; RNA directs the synthesis and sequence of proteins. The final proteins are called "gene products.") Through some pretty technical tricks, the researchers found that one specific protein was missing: the one made by the SCG10 gene. So the SCG10 gene is significantly reduced in the DS neurospheres. They also noted through the same technique that certain other genes were also underexpressed: L1, Synapsin and ß4-tubulin.
What these genes have in common is that they are associated with neuronal growth and "plasticity," which is the characteristic of being able to change from one type of cell into another type. After discovering that these genes' products were reduced, the researchers decided to look at the abilities of the DS neurospheres to become neurons. There were significant differences between the DS neurospheres and the normal neurospheres: the neurons made by the DS neurospheres were smaller in size and number, and had smaller and misshapen neurites (stalks shooting from the main body of the neuron).
We know that DS is a condition of extra genes, or gene overexpression. So the researchers wondered why these four genes were underexpressed. The answer appears to be that they are all are regulated by another protein, called the "Repressor Element Silencing Transcription" factor, or the REST factor (thus the title of the paper). This REST factor serves to repress or turn off certain genes. So could an overexpression of the REST factor cause an oversuppression of the SCG10, L1, Synapsin and ß4-tubulin genes? That sounds reasonable, except the researchers did not find an overexpression of the REST factor in the neurospheres studied (this point isn't covered in the abstract). So we don't have an answer yet as to why these genes are underproducing.
So the bottom line is that this stem cell research discovered a severe and selective derangement of the production of neurons that begins in the earliest stages of formation of the embryo with Down syndrome, due to a disruption of expression of certain genes of the neuron. This needs to be confirmed by measurement of the actual gene product proteins rather than just the RNA levels, but this is very promising cutting-edge research. As the editors of the Lancet said in an accompanying editorial: "Further investigations should indicate whether members of this group of molecules constitute reasonable targets for drug or gene therapies."
Related Websites:
Stem Cell Research -- the University of Wisconsin's Facts Page
National Institute of Health's Page on Stem Cells