July 18, 2013
Switching Off The Extra Chromosome Provides Major Breakthrough In Down’s Syndrome Research
Susan Bowen for redOrbit.com - Your Universe Online
Scientists at the University of Massachusetts Medical School have made a huge breakthrough in our understanding of Down's syndrome. They reported in the online science journal Nature, that in a lab, they have switched off the extra chromosome that causes the disorder in humans. This is the proof-of-principal that opens the doorway for exciting new discoveries about this condition.
Down's syndrome is the world's leading genetically caused mental disorder, affecting around one in 600 live births in the United States. It is caused when people are born with an extra copy of the 21st chromosome. The condition is called trisomy 21. The presence of this extra chromosome causes a whole host of problems including cognitive impairment, heart defects, thyroid problems, and early-onset Alzheimer's.
Scientists have made huge progress in correcting disorders caused by faulty versions of a single gene. However, the obstacles to finding a therapy that can be used on a whole chromosome are daunting.
Jeanne B. Lawrence, PhD, is a professor of cell and developmental biology at UMass Med School and lead author of the study. She states, "The last decade has seen great advances in efforts to correct single-gene disorders, beginning with cells in vitro and in several cases advancing to in vivo and clinical trials. By contrast, genetic correction of hundreds of genes across an entire extra chromosome has remained outside the realm of possibility. Our hope is that for individuals living with Down syndrome, this proof-of-principal opens up multiple exciting new avenues for studying the disorder now, and brings into the realm of consideration research on the concept of 'chromosome therapy' in the future."
The new procedure uses a naturally occurring gene normally occurring on the X chromosome. The gene, called XIST (pronounced 'exist'), shuts down one of the two X chromosomes found in female mammals. If both X chromosomes were working at the same time, they would over-function and create problems. To prevent this, the XIST gene produces a large piece of the RNA molecule which coats one of the genes and condenses it into a dense, inaccessible bundle, basically turning it off.
Jun Jiang at UMass Medical was able to transfer an existing XIST gene, using a process called genome editing, to the extra chromosome 21 in Down's syndrome cells grown in a laboratory. The XIST gene did the same thing with the extra chromosome 21 that it had done with the unneeded X chromosome.
Jiang used enzymes called zinc finger nucleases to cut the DNA in just the right spot to insert the XIST gene. The cells, from a boy with Down's syndrome, were reprogrammed into a stem cell-like state. The genes on that copy of the chromosome were almost completely deactivated. To ensure that XIST only shuts down one of the chromosomes, Jiang adjusted its concentration. Future experiments might target the gene to sequences that are only found on one copy of the extra chromosome.
The researchers noticed some promising signs after the chromosome was de-activated. The treated cells grew more quickly into early stage brain cells and produced larger colonies. They were also far better at creating neuron-making cells.
Jiang also found something else. Though XIST evolved to shut down the X chromosome, it works on all of them. Therefore it must be acting on something found on all chromosomes. She thinks that it may recognize repetitive bits of DNA that, up until now, seemed to serve no purpose. They are not used to make proteins. XIST is classified as a "long, non-coding RNA" or lncRNA. Mitchell Guttman from Cal Tech finds this aspect of the research exciting. He told National Geographic, "The field will surely build upon this in the future as it continues to dissect the roles of other lncRNAs and learns more about the principles governing their localization and function."
Though using XIST to silence the extra chromosome seemed obvious, there were a lot of problems that had to be resolved if there was to be any hope of success. First of all, XIST is a huge gene, far larger than any gene previously inserted into a genome. Would the team be able to get the gene into the right place? Would it actually silence chromosome 21 without killing the cell? Would it silence all three copies instead of just one? "None of these challenges made the project impossible, but collectively they made it pretty improbable," Lawrence told National Geographic reporter Ed Yong. "We didn't know if we'd spend years not getting anything to work."
The next step will be to silence the extra chromosome in mice bred with an extra chromosome 21, rather than in a dish of cells. The team hopes to have results within a year. They will insert the gene into early-stage embryos. "That would correct the whole mouse, but it's not really practical in humans," Lawrence told Guardian reporter Ian Sample.
The Guardian report expresses some of the problems associated with using a similar therapy on humans. "A chromosome therapy for humans would be fraught with practical and ethical difficulties. To prevent Down's syndrome, the genome editing would have to be performed on an embryo or fetus in the womb, and correct most, if not all, of the future child's cells. That is far beyond what is possible, or allowed, today."
What is more within reach is the possibility of using XIST to study the effects of Down's syndrome in different organs and tissue types. This could be done by splitting altered cells into two batches - one with the extra chromosome turned on, and one with it off - and watch what happens.
There is one possible drawback to using XIST. Turning it on may not block all action by the extra chromosome, which could muddle test results. Still, it is a much more promising research tool than any previous method of manipulating chromosomes.
This research could lead to therapies to treat some of the many symptoms of Down's syndrome. For example, the risk of leukemia might be reduced by introducing XIST into bone marrow cells. In addition, if the chromosome could be turned off in adults, it might decrease the likelihood of them developing dementia. The research could also be used to test drugs designed to alleviate some of the symptoms. Similar therapies might be applicable to other disorders caused by an excess number of chromosomes in addition to number 21.