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Research Calls Forensic DNA Technique Into Question

Posted: 03 Mar 2010 02:31 PM PST

A DNA-matching technique often used in forensics has been called to the stand.

Fine-grained analysis of DNA found in cell structures called mitochondria suggests that it can vary widely between tissues, making samples tricky to compare.

"I wouldn't say that it throws other results out the window, but it does throw a curve ball," said Nickolas Papadopoulos, a Johns Hopkins University geneticist and co-author of the study, published March 4 in Nature.

Mitochondria are found by the hundreds in every human cell. They convert glucose to energy, and possess their own tiny genomes, separate and distinct from the organismal genome found in each cell nucleus.

In the mid-1990s, law enforcement added mitochondrial DNA comparison to its forensic genetic toolkit. Because there are so many mitochondria in each cell, readable copies of their genomes can often be found even when the nuclear genome has been damaged. This is especially useful for old, highly degraded biological samples.

Mitochondrial DNA-matching is based on the assumption that it doesn't vary much in an individual: Aside from a few inevitable mutations, mitochondrial DNA are generally supposed to be the same in, say, heart cells and hair cells. But when Papadopoulos' team applied the latest in gene-sequencing technology to mitochondrial genomes from nine tissue types in two people, that's not what they found.

Instead, each person seemed to have a mixture of mitochondrial genotypes. One DNA variant, for example, was found in about 7 percent of a person's skeletal-muscle mitochondria, but 90 percent of their kidney mitochondria. That spread was typical.

"It's more than was thought, and was present in almost every tissue we looked at," said Papadopoulos. Further research into these variations is needed, but forensic specialists should be careful to compare the same types of tissue, he said.

John Planz, associate director of the DNA Identity Laboratory at the University of North Texas Health Science Center, cautioned that further studies are needed. High levels of genetic variation between mitochondria that were found in previous studies turned out to be the result of errors in measurement and analysis, he said.

Mitochondrial DNA analysis is also used in other types of research. Evolutionary family trees are deduced from comparisons of mutations between fossil samples. The same techniques are used to trace the historical flows of human populations.

Those studies involve group patterns and relatively large-scale changes over long periods of time. So they may not be as challenged by the Nature findings as forensic applications are, which try to find perfect matches, said Papadopoulos.

"This requires more study, but it could put a damper on how things have been interpreted to this point," he said.

Image: Mitochondria in the brain tissue of a rat./Indiana University-Purdue University Indianapolis

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Citation: "Heteroplasmic mitochondrial DNA mutations in normal and tumour cells." By Yiping He, Jian Wu, Devin C. Dressman, Christine Iacobuzio-Donahue, Sanford D. Markowitz, Victor E. Velculescu, Luis A. Diaz Jr, Kenneth W. Kinzler, Bert Vogelstein and Nickolas Papadopoulos. Nature, Vol. 463, No. 7285, March 4, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.

How Black Holes Overcome Centrifugal Force to Suck in Gas

Posted: 03 Mar 2010 02:04 PM PST

andromeda_galaxy1

Astronomers have finally gotten a firmer grip on how supermassive black holes in the centers of most galaxies gobble up gas from their surroundings. In a new study, two astronomers neatly explain how stars drag swirling gases toward a galaxy's center, bringing them close enough that the black holes can suck them in like water down a bathtub drain.

Although supermassive black holes wield an enormous tug on their immediate surroundings, astronomers have been uncertain how these astrophysical beasts manage to pull in the large amounts of gas they absorb from their host galaxies. A key problem is that gas swirling rapidly around a black hole has enormous angular momentum, which creates a centrifugal force that can slow or halt the material from edging toward the abyss.

Generally, black holes easily swallow up gas that approaches to less than a third of a light-year from the galactic center, because the black hole's own magnetic field acts like a brake, slowing down the rotational motion of the gas and causing it to fall in. At much larger distances–about 30 to 300 light-years from the center–disturbances from collisions with other galaxies and the gravitational interactions of matter within the galaxy can drive gas toward the central black hole. But that still leaves a critical gap at intermediate distances between about one light-year and 30 light-years from the center, where nothing seems to reduce the rotational motion and centrifugal force of gas enough that the black hole can pull it in.

andromeda_bsar That's where new simulations by Philip Hopkins and Eliot Quataert, both of the University of California, Berkeley, come into play. Their computer models show that at intermediate distances from a supermassive black hole, gas and stars form separate, lopsided disks that are off-center with respect to the black hole. The two disks are tilted with respect to one another, the astronomers say in a paper posted online Feb. 5 at arXiv.org, allowing the stars to exert a drag on the gas that slows its swirling motion and brings it closer to the black hole.

The new work is purely theoretical. However, the researchers note that observers have found evidence that the centers of several galaxies known to house supermassive black holes, notably the Milky Way's sister galaxy Andromeda, sport lopsided disks of elderly stars. The off-center features in Andromeda have puzzled researchers for more than a decade.

Hopkins and Quataert now suggest that these old, off-center disks are the fossils of the stellar disks generated by their models. In their youth, such disks helped drive gas into black holes, they say.

The new study "is interesting in that it may explain such oddball [stellar disks] by a common mechanism which has larger implications, such as fueling supermassive black holes," says Tod Lauer of the National Optical Astronomy Observatory in Tucson. "The fun part of their work," he adds, is that it unifies "the very large-scale black hole energetics and fueling with the small scale."

Off-center stellar disks are difficult to observe because they lie relatively close to the brilliant fireworks generated by supermassive black holes. But searching for such disks could become a new strategy for hunting supermassive black holes in galaxies not known to house them, Hopkins says.

Images: 1) NASA. 2) A. Field, NASA, ESA.
Video: P. Hopkins

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