 SEX AND MENTAL DEFECT
Mutations in a key section of a human sex chromosome appear to cause mental retardation
Nanoscale structures made from DNA
PARIS
Best known as the blueprint for life, DNA is also a marvel of architecture that can be used to build three-dimensional structures measured in billionths of a meter. A team of scientists in the United States has shown in experiments how to construct complex, spherical objects with tiny strings of DNA that assemble by themselves.
DNA nanotechnology uses the building blocks of living organisms not as a repository for biological data, but as a structural material instead. These molecular-scale biomaterials hold tremendous promise in fields ranging from robotics to electronics to computation, scientists say.
One of the ultimate goals is the self-assembling biochips for nanocomputers, according to New York University chemist Nadrian Seeman, a leading expert on DNA-based technology.
Whether synthetic or natural, DNA strands often display properties that cannot be duplicated in conventional organic or inorganic chemistry. Such biomaterials have the added advantage of being a renewable resource and, by definition, biodegradable.
The DNA double helix structure consists of two intertwined spirals of sugar and phosphate molecules linked by pairs of nucleotides, the basic building blocks of all life. In nature, the double helix is about two nanometers wide, and varies in length depending on the organism whose genetic codes it contains. In humans, the DNA ladder consists of some three billion "rungs" or base pairs that would stretch out nearly a meter in length if unfolded.
Some two-dimensional DNA nanostructures have already been made by coaxing DNA molecules to interact and lock together in such a way as to produce a desired pattern. But larger and three-dimensional forms have been harder to make using existing methods because of the need to manipulate hundreds of unique DNA strands.
A team of researchers led by Chengde Mao of Perdue University in Illinois overcame this problem by programming the DNA to first fold into a basic, "prefab" structural unit shaped like a three-pointed star.
This made it easier for these uniform units to coalesce into five- or 12-sided geometric forms, or even "buckyballs," molecules with 32 sides composed of 20 hexagons and 12 pentagons.
"We expect that our assembly strategy can be adapted to allow the fabrication of a range of relatively complex three-dimensional structures," the researchers concluded.
New syndrome linked to mental retardation
PARIS
Scientists in the United States have identified a genetic defect that appears to cause mental retardation and epilepsy as well as malformations of the face and hands. The still unnamed syndrome, linked to a tiny segment of missing DNA code, accounts for one out of 330 cases of retardation of unknown origin and may affect one person out of 40,000 in the general population, according to a recent study.
Evan Eichler of the University of Washington School of Medicine led a team of 33 researchers from the United States, Italy, and Britain in screening the entire genomes of 757 individuals with mental retardation. Two unrelated persons with very similar symptoms were found to be lacking a string of 1.5 million base pairs of genetic code, located on chromosome 15 and spanning six different genes.
There are approximately three billion base pairs-chemical "rungs" assembled from four different compounds-in the human genome. One of those genes, known as CHRNA7, is responsible for a protein critical for transmitting messages across brain cells, and has also been associated with epilepsy and schizophrenia.
Once they knew which part of the genome to explore, Eichler then screened 1,040 other individuals with mental retardation using data from the Greenwood Genetic Center in South Carolina, half of European ancestry, half of them African-American. Seven other individuals were found with precisely the same telltale genetic defect, and shared many of the same symptoms. Of the nine cases found, all showed mild to moderate retardation, and seven had epilepsy seizures or abnormal readings for electrical activity in the brain. All also shared certain abnormal facial characteristics, and seven of the nine had joint defects.
The newly identified condition occurs with the same frequency as three other syndromes related to mental retardation: Williams, Angelman, and Prader-Willi. The researchers predicted that other minor syndromes are likely to emerge through high-resolution scans for "submicroscopic" deletions in the human genetic code.
Researchers get closer to safe stem-cell treatments
CHICAGO
Researchers came a step closer to finding a safe way to use stem cells in clinical treatment when a team of Japanese scientists announced they found a way to induce stem cells without triggering tumors.
Stem-cell research is highly controversial because, until recently, viable human embryos were destroyed in the process of extracting the most flexible stem cells. Two groups of scientists recently bypassed this problem by transforming human-skin cells into stem cells that had the same properties as embryonic stem cells. This offered the promise of treatments tailored to the specific genetic code of a patient-anything from blood transfusions to transplantable organs grown in a petri dish-without the need for harmful drugs to prevent rejection. But the skin cells were regressed using a method that caused tumors and the potential for genetic mutation, which meant those induced pluripotent stem cells or iPS were not safe for clinical use.
One of those teams has now managed to induce stem cells without causing tumors and in a way that may not cause as much genetic disruption. They again used a retrovirus to introduce four genes, this time experimenting with adult-mouse-liver and stomach-lining cells.
Mice implanted with these iPS remained tumor free after six months, according to the study published online in Science Express. And the researchers showed that the retrovirus did not need to burrow into the adult cell genome at a specific site. This could help scientists avoid viral integration at sites prone to trigger tumors.
While the results are promising, there is still much work to be done, said lead researcher Shinya Yamanaka of Kyoto University. "It still takes years of basic research before we become able to use iPS cells to treat patients," Yamanaka said. "We are doing our best to bring it to clinics as quick as possible."
Fluorescent pig passes on trait to offspring
BEIJING
A pig genetically modified in China to make it glow has given birth to fluorescent piglets, proving such changes can be inherited. The sow was one of three pigs who had fluorescent green protein injected into their embryos when they were bred in December 2006 by scientists in northeast China, Xinhua news agency said. The pigs glow green when placed under an ultraviolet light.
After mating the sow with an ordinary pig, two of the resulting 11 piglets inherited the feature, Liu Zhonghua, a professor at Northeast Agricultural University in the city of Harbin, was quoted as saying. "The mouths, trotters, and tongues of the two piglets glow green under ultraviolet light, which indicates the technology to breed transgenic pigs via cell nuclear transfer is mature," said Liu.
Scientists in Taiwan had bred fluorescent pigs in January 2006, but Liu said the birth of the fluorescent Chinese piglets proved such changes could be passed on to offspring, which expands scientific and medical possibilities. "This technology promises to breed excellent transgenic pigs and even raise special pigs to provide organs for human transplant operations in the future," he said.
His teams used somatic-cell-nuclear-transfer technology previously employed in the cloning of animals by US, South Korean, and Japanese scientists.
Mutation in sex chromosome linked to mental defects
SYDNEY
Mutations in a key section of a human sex chromosome appear to be able to cause mental retardation. A change involving duplication of a small part of the X chromosome was discovered in six families out of more than 550 in a worldwide study of mental-health problems, Australian geneticist Jozef Gecz said, adding that although the figure sounded relatively low, because of the complexity of the change the number was actually "very high."
Another three families where a gene had been lost from the same section of the chromosome also suffered mental retardation, said Gecz of the University of Adelaide.
The study, conducted with Katholieke Universiteit Leuven in Belgium and the Wellcome Trust Sanger Institute in Cambridge, England, is published in February's edition of the American Journal of Human Genetics.
The X chromosome is one of two chromosomes that determine gender. Women normally have two, while men have one plus a smaller Y chromosome. As a result, problems with the X chromosome appear more frequently in boys than girls, although women may be silent "carriers" who can pass the defective genes on to their sons. Mental disability is about 30-percent more common in men than in women.
The finding adds weight to arguments that the X chromosome plays a key role in human-brain function, Gecz said. Around 30 individuals were affected, mostly boys or men, he said. They appeared normal, but showed subtle deficits. "To some extent they can dress and feed themselves. They don't necessarily require 24/7 care but they still need supervision," he added. "Visually these people look exactly like you and I. They just don't have the learning and memory working properly to allow them to lead a normal life."
The duplicated part of DNA contained two genes known by the numbers HSD17B10 and HUWE1, the latter a so-called "tumor-suppressor gene" whose roles include regulating the renewal of nerve cells.
"Clearly the amount you produce of this gene has to be just right," Gecz said. "We are making an educated guess that this is in control of growth of cells in the human brain."
A test has now been developed that will allow women to find out before becoming pregnant if they carry the defective DNA. Researchers also plan to look into possible future treatments. So far 85 genes have been discovered on the X chromosome that are involved in various forms of mental disability. There are about 800 genes on the chromosome, Gecz said.
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