Posts tagged trisomy

Researchers at Johns Hopkins and the National Institutes of Health have identified a compound that dramatically bolsters learning and memory when given to mice with a Down syndrome-like condition on the day of birth. As they report in the Sept. 4 issue of Science Translational Medicine, the single-dose treatment appears to enable the cerebellum of the rodents’ brains to grow to a normal size.

The scientists caution that use of the compound, a small molecule known as a sonic hedgehog pathway agonist, has not been proven safe to try in people with Down syndrome, but say their experiments hold promise for developing drugs like it.

“Most people with Down syndrome have a cerebellum that’s about 60 percent of the normal size,” says Roger Reeves, Ph.D., a professor in the McKusick-Nathans Institute of Genetic Medicine at the Johns Hopkins University School of Medicine. “We treated the Down syndrome-like mice with a compound we thought might normalize the cerebellum’s growth, and it worked beautifully. What we didn’t expect were the effects on learning and memory, which are generally controlled by the hippocampus, not the cerebellum.”

Reeves has devoted his career to studying Down syndrome, a condition that occurs when people have three, rather than the usual two, copies of chromosome 21. As a result of this “trisomy,” people with Down syndrome have extra copies of the more than 300 genes housed on that chromosome, which leads to intellectual disabilities, distinctive facial features and sometimes heart problems and other health effects. Since the condition involves so many genes, developing treatments for it is a formidable challenge, Reeves says.

For the current experiments, Reeves and his colleagues used mice that were genetically engineered to have extra copies of about half of the genes found on human chromosome 21.
The mice have many characteristics similar to those of people with Down syndrome, including relatively small cerebellums and difficulty learning and remembering how to navigate through a familiar space. (In the case of the mice, this was tested by tracking how readily the animals located a platform while swimming in a so-called water maze.)
Based on previous experiments on how Down syndrome affects brain development, the researchers tried supercharging a biochemical chain of events known as the sonic hedgehog pathway that triggers growth and development. They used a compound — a sonic hedgehog pathway agonist — that could do just that.

The compound was injected into the Down syndrome-like mice just once, on the day of birth, while their cerebellums were still developing. “We were able to completely normalize growth of the cerebellum through adulthood with that single injection,” Reeves says.

But the research team went beyond measuring the cerebellums, looking for changes in behavior, too. “Making the animals, synthesizing the compound and guessing the right dose were so difficult and time-consuming that we wanted to get as much data out of the experiment as we could,” Reeves says. The team tested the treated mice against untreated Down syndrome-like mice and normal mice in a variety of ways, and found that the treated mice did just as well as the normal ones on the water maze test.

Reeves says further research is needed to learn why exactly the treatment works, because their examination of certain cells in the hippocampus known to be involved in learning and affected by Down syndrome appeared unchanged by the sonic hedgehog agonist treatment. One idea is that the treatment improved learning by strengthening communication between the cerebellum and the hippocampus, he says.

As for the compound’s potential to become a human drug, the problem, Reeves says, is that altering an important biological chain of events like sonic hedgehog would likely have many unintended effects throughout the body, such as raising the risk of cancer by triggering inappropriate growth. But now that the team has seen the potential of this strategy, they will look for more targeted ways to safely harness the power of sonic hedgehog in the cerebellum. Even if his team succeeds in developing a clinically useful drug, however, Reeves cautions that it wouldn’t constitute a “cure” for the learning and memory-related effects of Down syndrome. “Down syndrome is very complex, and nobody thinks there’s going to be a silver bullet that normalizes cognition,” he says. “Multiple approaches will be needed.”

(Source: newswise.com)

Borrowing a trick from nature, researchers have switched off the extra chromosome that causes Down syndrome in cells taken from patients with the condition.

Though not a cure, the technique, reported July 17 in Nature, has already produced insights into the disorder. In the long run it might even make the flaw that causes Down syndrome correctable through gene therapy.

“Gene therapy is now on the horizon,” says Elizabeth Fisher, a molecular geneticist at University College London. “But that horizon is very far away.”

Down syndrome, also called trisomy 21, occurs when people inherit three copies of chromosome 21 instead of the usual two. It is the most common chromosomal condition, affecting around one in every 700 babies born in the United States. People with the disorder typically have both physical and cognitive complications of having an extra chromosome.

“Down syndrome has been one of those disorders where people say, ‘Oh, there’s nothing you can do about it,’ ” says Jeanne Lawrence, a chromosome biologist and genetic counselor at the University of Massachusetts Medical School in Worcester, who led the study with colleagues Lisa Hall and Jun Jiang.

The researchers decided to see whether they could shut down the extra chromosome by drawing on a biological process called X inactivation. Women have two X chromosomes and men have only one X and a Y. To halve the amount of X chromosome products, female cells shut down one copy. Cells do that using a chunk of RNA called XIST, which is made by one X chromosome but not the other. The RNA works by pulling in proteins that essentially board up the chromosome like an abandoned building. The other X stays on by making a different RNA.

Lawrence and Hall thought that if they put XIST on another chromosome, it might shut that one down too. So Jiang put the gene for XIST onto one of the three copies of chromosome 21 carried by stem cells grown from a man with Down syndrome. That copy of the chromosome got switched off.

“It’s kind of surprising that it wasn’t done before. I’m smacking my own forehead and saying, ‘duh,’ ” says Roger Reeves, a geneticist at Johns Hopkins University.

One idea about why an extra chromosome 21 causes cognitive problems is that it may slow down the growth of brain cells. Jiang grew nerve cells from the Down patient’s stem cells to see how cells with one shut-down chromosome developed compared with cells bearing three active copies. The cells with only two working chromosomes grew faster, forming clusters of neurons in a day or two, while the uncorrected cells needed four or five days.

The work is an enormous step forward in Down syndrome research, Fisher says, and “may take us much closer to understanding the molecular basis of the disorder.” The technique could allow researchers to figure out which genes are involved in Down syndrome and how extra copies affect cells and ultimately the body, she says.

Reeves wants to use the technology in animal experiments, a critical step in determining whether it could find use as gene therapy for people with Down syndrome. He plans to work with Lawrence’s group to switch off the extra chromosome in mice engineered to have a disorder that simulates some features of Down syndrome.

But Reeves doubts that scientists could use the method to switch off the extra chromosome in every cell in the body. Doing so would probably require gene therapy at a very early stage of pregnancy, something scientists don’t know how to do. “I just don’t see how we would get there from where we are today,” Reeves says.

Such universal silencing of the extra chromosome may be necessary to forestall developmental problems. But other problems associated with Down syndrome might be prevented or reversed by shutting down the extra chromosome after birth. For instance, people with Down syndrome are at high risk of developing childhood leukemia and of getting Alzheimer’s disease. Gene therapy to turn off the extra chromosome in the bone marrow or the brain might prevent those problems.

Therapeutic possibilities are still far in the future and may never pan out, says William Mobley, a neurologist and neuroscientist at the University of California, San Diego. “We have to move cautiously and deliberately and not say that a cure for Down syndrome is on the horizon,” he says. “It’s not true, but gosh is there excitement that progress is being made.”

(Source: sciencenews.org)

23 JUILLET 2012

Children with trisomy 13 or 18, who are for the most part severely disabled and have a very short life expectancy, and their families lead a life that is happy and rewarding overall, contrary to the usually gloomy predictions made by the medical community at the time of diagnosis, according to a study of parents who are members of support groups published today inPediatrics. The study was conducted by Dr. Annie Janvier of the Sainte-Justine University Hospital Center and the University of Montreal with the special collaboration of the mother of a child who died from trisomy 13, Barbara Farlow, Eng, MSc as the second author.

Source : Wikimedia Commons

The study interviewed 332 parents who live or have lived with 272 children with trisomy 13 or 18. It turns out that their experience diverges substantially from what healthcare providers said it would be, according to which their child would have been “incompatible with life” (87 %), would have been “a vegetable” (50 %), would have led “a life of suffering” (57 %) or would have “ruin their family or life as a couple” (23 %).

It should be noted that trisomies 13 and 18 are rare chromosome disorders that are most often diagnosed before birth and sometimes after. Children who have received these diagnoses generally do not survive beyond their first year of life, while some who do have severe disabilities and a short life. When trisomy 13 or 18 is diagnosed before birth, many parents decide to interrupt the pregnancy, whereas others choose to carry it to term and in such cases miscarriages are common.

As children with trisomies 13 or 18 generally receive palliative care at birth, some parents who opt to continue the pregnancy or desire life-prolonging interventions for their child encounter the prejudices of the medical system. In this regard, the parents interviewed in the study consider that caregivers often view their child in terms of a diagnosis (“a T13”, “a lethal trisomy”) rather than a unique baby.

“Our study points out that physicians and parents can have different views of what constitutes quality of life,” states Dr. Annie Janvier, a neonatologist and co-founder of the Master’s program in Pediatric Clinical Ethics at the University of Montreal. In fact, over 97% of the parents interviewed considered that their child was happy and its presence enriched the life of their family and their life as a couple regardless of longevity. “In the medical literature on all handicaps, disabled patients – or their families – rated their quality of life as being higher than caregivers did,” adds Dr. Annie Janvier.

Parents who receive a new diagnosis of trisomy 13 and 18 and join a parental support group often acquire a more positive image of these diagnoses than the predictions made by the medical profession. In fact, according to the parents interviewed, belonging to a support group helped them view their experience positively. “Our research reveals that some parents who chose a path to accept and to love a disabled child with a short life expectancy have experienced happiness and enrichment. My hope is that this knowledge improves the ability of physicians to understand, communicate and make decisions with these parents,” concludes Barbara Farlow.

Given the rarity of trisomy 13 or 18 cases (one case out of approximately every 10,500 births), the parents were recruited through online support groups that parents often join after receiving the physicians’ diagnosis. Dr. Annie Janvier and Barbara Farlow sometimes give joint talks on the subject of trisomies 13 and 18.

Source: Université de Montréal