Surprises in the human genome

March 12, 2001

Heather Heilman

Last month the first full maps of the human genome were unveiled to a lot of fanfare and a little head-scratching by non-scientists confronted by the string of letters and numbers that makes up the sequence.

"The genetic code is a string of several billion copies of the same four letters. And a gene is a sequence of those letters that gives instructions. It's kind of like a paragraph in a book," explained Prescott Deininger, associate director and Zimmerman Professor at the Tulane Cancer Center.

He has been working on the genome for 30 years, since the days when the tools for doing so were just being invented. He discovered about 15 percent of the genome:sort of.

"It would be tongue-in-cheek to say I sequenced the first 15 percent of the human genome," said Deininger. "Twelve years ago we discovered Alu repeats, which are virus-like elements that are spread all around our DNA. There's a million and a half copies in every cell in your body, which is about 15 percent of your DNA."

Alu repeats are "junk DNA." One of the surprises of the genome is that the majority of it is made up of junk, genes that are lying around in the genome but aren't expressed and don't really do much of anything. Deininger compares them to fossils. But even though they're not active, they still have an impact on the genome.

"Junk DNA is one of the causes of instability in the genome, of recombination going awry and messing up genes. So I think a lot of cancer is caused by these elements," he said. Promoters and terminators, elements that mark where a particular gene starts and ends, punctuate the genome's long sequence. "Every cell in your body has the same genes, but they don't all do the same thing," Deininger said. "A liver cell is not a brain cell. The promoter elements and other punctuation pick out which genes are going to be expressed, which lead to the function of particular cells."

Another surprising thing about the genome is the relatively small number of genes it contains. Researchers had expected to find between 70,000 and 100,000 genes. Instead, they found about 30,000, which is only about twice as many as roundworms have.

"There's some question about whether they have found all the genes. They're probably missing some, but it's hard to imagine that they're missing half of them," Deininger said. "But we've also learned that a single gene might be making a series of related proteins, and proteins might be doing multiple functions. Nature's been extremely efficient about reusing the same information and evolving it to do different things."

Even though there might still be a few kinks to be worked out in the current models of the genome, the information can already be put to use. Research is moving in several directions. Many scientists are at work on cures for different diseases. Others are sequencing the genome of the rhesus monkey in order to compare it to the human genome.

Another large project will compare key genes from hundreds of different individuals in order to look at variations in the genome and the effects of that variation. This information could be used to help predict an individual's risk for a disease or to model the way a disease works and develop different ways to intervene in the process for different individuals.

Among Tulane scientists who are putting the genome to work are Melanie Ehrlich, Erik Flemington and Laura Levy at the cancer center, Jess Thoene and other professors in the Haywood Genetics Center at the medical school, as well as Darwin Prockop and his team at the new Center for Gene Therapy, which had its official opening March 7.

"All disease involves genetics in one way or another," Deininger said. "One of the really appealing things about gene therapy is that in theory one should be able to use this information about how the genome works to cure almost any disease. If you're creative enough you can at least imagine a way to do it."

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