LightCycler technology in molecular diagnostics

Abstract

LightCycler technology combines rapid-cycle polymerase chain reaction with real-time fluorescent monitoring and melting curve analysis. Since its introduction in 1997, it is now used in many areas of molecular pathology, including oncology (solid tumors and hematopathology), inherited disease, and infectious disease. By monitoring product accumulation during rapid amplification, quantitative polymerase chain reaction in a closed-tube system is possible in 15 to 30 minutes. Furthermore, melting curve analysis of probes and/or amplicons provides genotyping and even haplotyping. Novel mutations are identified by unexpected melting temperature or curve shape changes. Melting probe designs include adjacent hybridization probes, single labeled probes, unlabeled probes, and snapback primers. High-resolution melting allows mutation scanning by detecting all heterozygous changes. This review describes the major advances throughout the last 15 years regarding LightCycler technology and its application in clinical laboratories.

Figure 1

Monitoring PCR in real time using DNA dyes, hydrolysis probes, and hybridization probes. The top row shows data collected once each PCR cycle, and the bottom row shows data collected continuously (five times per second) during all PCR cycles. Adapted from Wittwer and Kusukawa with permission of the publisher.

Figure 2

Rapid-cycle, real-time PCR. Momentary denaturation and annealing allows amplification in 10 to 20 minutes. Fluorescence is acquired once each cycle and is used for detection and quantification. Melting analysis is performed immediately after PCR and is used for genotyping, product identification, or heterozygote scanning. The real-time data were obtained on a Roche LightCycler 1.5 by amplifying a 250-bp fragment of exon 2 of PIGA from human genomic DNA in the presence of 1× LCGreen Plus dye. After a 5-second denaturation at 95°C, 45 cycles of 95°C for 0 second, 60°C for 0 second, and 72°C for 2 seconds with a 2°C/second ramp between annealing and extension temperatures was performed. Fluorescence was acquired at the end of each extension step. Temperature cycling required just longer than 15 minutes, whereas melting analysis at 0.2°C/second required another 4 minutes.

Figure 3

Genotyping by melting analysis using dual hybridization probes (A), single hybridization probes (B), unlabeled probes (C), and snapback primers (D). Within each panel, a conceptual diagram of the assay is on the top and resulting derivative melting curves are on the bottom. Dotted lines indicate the DNA template, arrows represent primers, thick line segments indicate probes, Pi indicates 3′-blocking with phosphate, asterisks indicate covalent fluorescent dyes, and a shallow v in the template indicates the position of variation. Three samples of each genotype are shown in each panel, including matched homozygous wild-type (light gray), heterozygous (dark gray), and mismatched homozygous variant (black). A: The dual hybridization probe design was introduced with the original LightCycler. Two adjacent probes are each labeled with one fluorophore, selected so that they form a resonance energy transfer pair. Long wavelength fluorescence is generated when both the anchor and mutation probe are hybridized. The probe with an exposed 3′-end is blocked with phosphate to prevent extension. B: The single hybridization probe design includes only one probe with a single fluorescent label. It may be 3′-labeled (as shown) or 5′-labeled with the 3′-end blocked. The G indicates a deoxyguanosine base, either in the template or the probe that influences fluorescence. Depending on the configuration, either quenching or dequenching of fluorescence occurs on hybridization. No PCR product or amplicon melting curves are detected with hybridization probes. C: The unlabeled probe design requires a 3′-blocked probe without any covalently attached fluorophore. Fluorescence is generated by a saturating DNA dye (shaded area) that only fluoresces when bound to duplex DNA. Asymmetric PCR produces both full-length PCR product (amplicon) and product/probe duplexes (probe). D: The snapback primer design only requires two primers, one with a 5′ extension that is complementary to its own extension product. Asymmetric PCR overproduces one strand, so that both full-length PCR product (amplicon) and an excess of one strand are formed. The excess single strand snaps back on itself (dotted line) so that its complementary regions (light gray lines) anneal to form a hairpin stem. Melting of the hairpin results in probe melting peaks. Both unlabeled probe and snapback primer designs typically have amplicon melting peaks (that can be use for heterozygote scanning) as well as probe melting peaks (that are used for genotyping).