Friday, November 24, 2006

A New Argument Against Pre-Natal Testing

I came across this story entitled "Prenatal Screening not so Accurate as Once Thought – 'Normal' Children Killed as 'Defective'?" from It refers to a story in Canada's Globe and Mail newspaper, "Study turns human genetics on its head." According to the Globe and Mail article, a recent study, published in the journal Nature, has brought to our attention more genetic differences than previously thought possible among human beings.
An international research team has overturned the harmonious message that flowed from the Human Genome Project in 2000 and discovered more DNA differences exist among people than the experts expected.

Using new technology to study the genomes of 270 volunteers from four corners of the world, researchers have found that while people do indeed inherit one chromosome from each parent, they do not necessarily inherit one gene from mom and another from dad.

One parent can pass down to a child three or more copies of a single gene. In some cases, people can inherit as many as eight or 10 copies. In rare instances a person might be missing a gene.

Yet despite these anomalies, they still appear to be healthy -- countering the notion of what doctors have deemed "normal" in genetics.

The work highlights how DNA helps to make each human unique, hinting that a towering basketball player, for example, might boast extra copies of a growth gene or that a daughter really might be more like her dad.
The scientific study of the human genome in the Human Genome Project (1990-2003) argued that "the human genome sequence is almost (99%) exactly the same in all people" (see here for reference). This most recent study suggests that this picture of almost identical human genome sequences is false. This recent
...research finds that the size of at least 12 per cent of the genome -- including 2,900 genes and regions between them -- can differ dramatically between people, and in some cases, between certain ethnic groups.

The size differences are the result of DNA that is either duplicated or deleted or contains unexpected added bits of genetic code. Scientists call the phenomenon "copy number variation" or CNV for short. And it is already reshaping genetic research.

"When we're accounting for what the human genome means, there's not going to be a single human genome map that is going to be useful to one person," said Robert Hegele, a noted genetic scientist at the Robarts Research Institute in London, Ont., who read the study. "It's a huge surprise that there's so much variation of this type . . . that is so common in so many healthy people."
The study suggests that there is quite a bit of variance in the human genetic structure among healthy people in which it is useless to try and pin "normal" on one version of the human genome map and then use that as a basis to determine who fits and does not fit this normal standard. What does this mean for pre-natal testing?
For this reason, scientists agree that doctors looking at less-detailed genetic tests -- such as karyotyping -- might have mistaken unusually-sized bits of DNA as signs of a medical problem.

Patients, or prospective parents receiving results of a prenatal test, for instance, might have been informed that something looked abnormal when, the new work suggests, it isn't.

While the report does not delve into the issue directly, Dr. Scherer [co-author of the study) acknowledged this is a possibility. He offered as an example a genetic test that relies on a "diagnostic probe" to evaluate the length of DNA code near the ends of chromosomes.

Shorter chromosomes, he said, are implicated in developmental delay or mental retardation due to DNA code that might be missing.

"But we found that in a large number of cases (shorter chromosomes) exist in the general population," said Dr. Scherer, who is also director of the Centre for Applied Genomics. "The chromosomes don't necessarily line up evenly . . . so people really need to scrutinize these results more closely before assuming it's pathogenic.

"The bottom line is that there's so much natural variation you have to go back and look closer."

Dr. Hegele agreed that such things might have been misread. "It's always been assumed those big changes would result in some type of disease, that they were rare and would lead to sort of catastrophic conditions," he said, noting that Down syndrome is the result of an extra copy of chromosome 21.
This recent study calls into question the nature of some pre-natal testing and how it identifies abnormalities. If what this study suggests is true, then some pre-natal testing, such as mentioned above in the quote, is useless and nothing but speculation based on false information concerning the human genetic structure and its variance among human beings.

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