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ANALYTICAL CURRENTS Methylated genes in cancer cells One way that cells can turn off a gene is by adding methyl groups to the gene’s cytosines. Researchers have found that particular genes, such as tumor-suppressor genes, are methylated only in cancer cells. To reliably detect methylated genes in clinical samples, Saraswati Sukumar and colleagues at the Johns Hopkins University School of Medicine have developed a new technique called quantitative multiplex methylation-specific PCR (QMMSP), which measures the degree of methylation on several genes simultaneously. Unlike current methods for assaying methylation, QM-MSP does not require large amounts of starting material, and it is quantitative. First, samples are treated with sodium bisulfite, which converts unmethylated cytosines to uracils. The methylated cytosines remain unchanged. Because PCR copies nucleotides but not methyl groups, this step allows the researchers to differentiate methylated cytosines from those that were unmodified prior to PCR. Next, each gene is
Detecting rare cancer cells Metastatic cancer cells traveling in the bloodstream are present in very low concentrations. In fact, only 1 in 106 or 107 cells in circulation may originate from a tumor. Richard Bruce and colleagues at the Palo Alto Research Center, Synta Pharmaceuticals, the Scripps Research Institute, Cima Potenica, Ltd., and Harvard Medical School have developed fiber-optic array scanning technology (FAST) to detect these rare cells. The technique is 500 faster and is more specific than the conventional technique of automated digital microscopy (ADM), yet it offers similar sensitivity. When the two techniques are combined, the analysis time for one sample is reduced to 1 h, compared with 18 h for ADM alone. In both FAST and ADM, blood samples are scanned for fluorescently labeled tumor cells. In FAST, the researchers increased the field of view to 100 that of ADM by incorporating a fiber-optic array and a sensitive photomultiplier detector instead of a CCD detector. A laser excites the fluorophores in the FAST instrument, whereas the ADM system uses a less efficient mercury light source. To test the sen-
Substrate motion
Rotating mirror
Photomultiplier tube
Collection Light collector Bandpass lens Collimating filter lens LAS ER
A schematic of FAST. (Adapted with permission. Copyright 2004 Institute of Electrical and Electronics Engineers.)
sitivity and specificity of FAST, Bruce and colleagues spiked normal blood with tumor cells. FAST’s enlarged field of view comes at a price—the technique has low resolution. Therefore, the researchers investigated using FAST as a first screen to enrich rare cells prior to rescreening with ADM, which provides higher resolution. Although other enrichment techniques exist, they require additional processing before rescreening. In contrast, FAST samples can be placed directly onto an ADM instrument. (Proc. Natl. Acad. Sci. USA 2004, 101, 10,501–10,504)
copied with gene-specific primers. Finally, a second round of amplification is performed on the products, and the reaction is monitored in real time with primers specific for methylated or unmethylated genes. Sukumar and colleagues tested breasttissue samples from 18 healthy women and 21 women with tumors. Four different genes were assayed by QM-MSP, and significant differences in methylation were observed between the two groups. As few as 1–10 methylated copies were detected in a mixture of 100,000 unmethylated genes. The assay was also 100% specific for methylated DNA. The method can be applied to many types of cancers from other tissues and can be used when the amount of sample is limited. (Cancer Res. 2004, 64, 4442–4452)
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Designer surface plasmons J. B. Pendry of Imperial College London (U.K.) and colleagues at Universidad de Zaragoza and Universidad Autónoma de Madrid (both in Spain) have found a way to engineer surface plasmons with just about any frequency by drilling an array of holes in a highly conducting surface. The ability to tailor-make surface plasmons opens up the door for controlling radiation on surfaces over a wide spectral range and could lead to new applications for surface plasmon optics. Surface plasmons originate from free electrons when light interacts with a metal surface. They have been used in a variety of applications, from producing the black part of a photographic neg-
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ative via light absorption by silver colloids to improving the sensitivity of the Raman signal in surface-enhanced Raman scattering experiments. Metals such as silver are known to support surface plasmons. Pendry and colleagues now suggest that highly conducting surfaces perforated with holes are also governed by a dielectric function of the plasma form. The size and spacing of the holes can be adjusted to create designer surface plasmons of almost any frequency. As long as the spacing between the holes is smaller than the wavelength of the incident light, the holes will mimic surface plasmons and play an analogous role to the real ones on silver. (Science 2004, 305, 847–848)
