PCR Expands, Creates Revolution

Polymerase chain reaction (PCR) is a relatively new laboratory technique that produces large amounts of specific DNA sequences in vitro, using only a small sample of genetic material. Researchers are using PCR to diagnose genetic disorders and to detect pathogens from viruses associated with AIDS, adult T-cell leukemia, and cervical cancer, as well as many other infectious diseases. The impact of PCR has, in fact, been so great that this field was identified as the second hottest area in all of science, according to an unpublished survey recently conducted by the Institute for Scientific Information (ISI) in Philadelphia. In 1988, only the field of high-temperature superconductors exceeded PCR in immediacy, a measure ISI uses to gauge activity and interest by the scientific community. News of PCR’s ability to produce quickly (in a few hours) more than 1 million copies. from DNA samples previously too small for standard amplification was first published four years ago by scientists at biotech giant Cetus Corp. in Emeryvile, Calif. (R.K. Saiki, et al., Science, 230:1350, Dec. 20, 1985). Three years later, the group published a modification to the original technique, which involves repeated cycles of heat denaturation of the DNA sample (R.K. Saiki, et al., Science, 239:487, Jan. 29, 1988). This improvement introduced a heat-resistant enzyme called Taq polymerase to the reaction. Taq not only simplified the procedure but also enabled it to produce even greater quantities of the target DNA strands. According to Henry A. Erlich and colleagues at Cetus, PCR “can accomplish in a ... three- to four-hour reaction what might otherwise take days or weeks of biological growth and biochemical purification” (Nature; 331:461-2, Feb 4, 1988). Both the groundbreaking 1985 paper and the 1988 paper on the modification to the original technique appear in a group of a dozen papers that are consistently cited by some 353 journal articles indexed in the ISI database in 1988. Together, these citing papers and the group of cited papers, called “core” documents, constitute a 1988 research front, or specialty area, on polymerase chain reaction DNA amplification. The 1985 paper from the Cetus group is the most frequently cited core document in this research front: 168 citations from nearly half of the 1988 citing papers. The second “hottest” core paper is the 1988 report on the modification using Taq polymerase enzyme. That this 1988 paper became a core document in a research front of the same year indicates that its contents were timely and immediately put to use by other researchers. In fact, three of the 12 core documents in this specialty area were published in 1988, which gives it a one-year immediacy rating of 25%. Immediacy is a measure of how quickly a paper becomes a fundamental or standard reference in a field; a high degree of immediacy is a sign of a rapidly changing area of research. The three-year immediacy of the 1988 PCR research front (the percentage of core papers published in 1986, 1987, or 1988) is 83%—an impressive indication of the revolutionary impact that this technique has had on researchers. The table on page 14 illustrates where the majority of work using PCR DNA amplification was performed last year. This ranking of in stitutions derives from an analysis of the authors’ addresses listed on the 353 citing papers from 1988 in ISI’s PCR specialty area. The value to the right of each is the percentage of papers, calculated on a fractional basis, contributed by that institution. Although 34 countries are repre sented by the 353 papers, U.S. scientists contributed more than 50% of the total. As might be expected, the largest share was held by Cetus Corp., which fielded 5.4% of the citing papers. The other top players were: Johns Hopkins University; the University of California, San Francisco; and Baylor University, all of which contributed more than 2% of the 1988 papers. Two months ago, Cetus scientists Shirley Kwok and Russell Higuchi (Nature, 339:237-8, May 18, 1989) issued a set of guidelines that they developed to prevent the occurrence of false positives or mistyping in DNA amplification experiments. These problems may occur when amplified DNA sequences carry over into, and contaminate, subsequent amplifications of the same target sequence. In other words, DNA molecules may be produced from an amplified sequence rather than from the originally intended sample. A comprehensive sourcebook to PCR technology is soon to appear. Written by Henry Erlich, senior scientist and director of human genetics at Cetus, PCR Technology: Principles and Applications for DNA Amplification (New York: Stockon Press, forthcoming) will review PCR’s development, describe the method itself and examine its research and diagnostic applications. Abigail Grissom is a freelance science writer based in Philadelphia.

PCR Expands, Creates Revolution

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Abigail Grissom

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July 1989

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