13 - The Self-Sustained Sequence Replication Reaction and Its Application in Clinical Diagnostics and Molecular Biology

In the past decade, the detection and quantification of viruses such as human immunodeficiency virus type 1 (HIV-1), human T- cell lymphoma/leukemia virus types 1 and 2 (HTLV-1 and -2), and cytomegalovirus (CMV) have been challenging problems in clinical diagnostics. Traditional culture methods, complemented by nucleic acid hybridization and sequencing techniques, found early applications in the detection of such low copy number pathogens. Improvements in sensitivity over radioisotopic-hybridization detection methods were made with signal amplification strategies using enzyme reporter molecules. Although signal amplification systems are rapid compared with culture methods for detecting viruses or microbial agents, widespread adoption of these technologies in the clinical setting failed to occur because of the limited sensitivities of the detection systems. The observed threshold of detection of 103–104 target molecules for these systems, principally imposed by the nonspecific background signal amplification of the assays, is frequently higher than the viral titers of HIV-1 and CMV encountered in clinical samples. The advent of in vitro nucleic acid target amplification techniques (Saiki et al., 1985; Lizardi et al., 1988; Kwoh et al., 1989; Wu and Wallace, 1989; Guatelli et al., 1990; Barany, 1991) ushered in a profound change in the molecular diagnostics arena. The discovery of the polymerase chain reaction (PCR) in 1985 (Saiki et al., 1985) made it possible to amplify a single copy gene over a million-fold, thereby simplifying the task of detection by hybridization detection systems