August 22, 2003

An Emphasis on Compassion

Continued from page 2

By Thea Singer

The remains from 9/11, however, were often so compromised that few of even the minimal 13 regions could be profiled. In response, Cash's team developed what he calls "virtual profiles." If after multiple attempts only partial data could be extracted, the company would take whatever results it had and combine them, as Cash puts it, "to make a sample that never really existed." For example, one cutting from a recovered bone might yield a specific number of repeats for a given region. An attempt with a second cutting might yield a partial reading on another region. M-FISys would combine the values to provide a profile with a greater possibility of matching to other test results. "I've not seen any other program that will actually put together a virtual profile," says the British Forensic Science Service's Maguire.

In fact, half the victim samples recovered did not yield enough information for identification by short tandem repeat analysis alone. So the medical examiner's office turned to analysis of DNA from cellular components called mitochondria as an adjunct. Such analysis is typically used for studies of human evolution. Mitochondrial DNA analysis is not nearly as precise as short tandem repeat analysis (the most common pattern of the mitochondrial sequence is shared by about 7% of the Caucasian population), but it has two advantages: mitochondrial DNA is 500 times more plentiful than chromosomal DNA (of which everyone has just two copies). And it is much shorter and therefore hardier, which made it more likely to survive three months near the 1,000-plus-degree Celsius heat of burning jet fuel.

Unlike chromosomal DNA, mitochondrial DNA is inherited from the mother alone. And instead of being three billion bases long, each copy consists of just 16,569 letters. There are two regions in mitochondrial DNA, called hypervariable-one and hypervariable-two, where the sequence of bases varies a great deal between people. Together the regions total only about a thousand bases. Mitochondrial DNA analysis tracks just the letters in the hypervariable regions that differ from those outlined in a reference sequence called the Anderson Sequence. The differences are recorded in M-FISys, which matches them up against the patterns from other victim samples, personal effects, and DNA samples from maternal relatives. Mitochondrial DNA analysis alone is not enough to identify a victim, but it may help narrow the possibilities.

For the most degraded samples, investigators are starting to examine single nucleotide polymorphisms, or SNPs (pronounced "snips")-locations along the genome where just one letter varies. With SNP analysis, which was developed to diagnose a genetic predisposition to certain diseases, a piece of chromosomal DNA that's very short-perhaps only 60 letters long-can be used for an experiment. The GeneScreen division of Orchid Biosciences, in Dallas, TX, has developed population statistics to project the likelihood of any particular individual inheriting certain SNP pairs at specific genetic locations. In M-FISys, those probabilities are added to the test results from the mitochondrial and short tandem repeat analyses to further diminish the number of possible matches.