Researchers have developed a new fluorescent label that gives a clearer picture of how DNA architecture is disrupted in cancer cells. The findings could improve cancer diagnoses for patients and classification of future cancer risk. Published today in Science Advances
, the study found that the DNA-binding dye performed well in processed clinical tissue samples and generated high-quality images via superresolution fluorescence microscopy.
“My lab is focused on developing microscopy techniques to visualize the invisible,” said senior author Yang Liu, Ph.D., associate professor of medicine and bioengineering at the University of Pittsburgh
Inside the cell’s nucleus, DNA strands are wound around proteins like beads on a string. Pathologists routinely use traditional light microscopes to visualize disruption to this DNA-protein complex, or chromatin, as a marker of cancer or precancerous lesions.
“Although we know that chromatin is changed at the molecular scale during cancer development, we haven’t been able to clearly see what those changes are. This has bothered me for more than 10 years,” said Liu, who is also a member of the UPMC Hillman Cancer Center.
“To improve cancer diagnosis, we need tools to visualize nuclear structure at much greater resolution.”
In 2014, the Nobel Prize-winning invention of superresolution fluorescence microscopy was a major step towards making Liu’s vision reality. A molecule of interest is labelled with a special fluorescent dye that flashes on and off like a blinking star. Unlike traditional fluorescence microscopy, which uses labels that glow constantly, this approach involves switching on only a subset of the labels at each moment. When several images are overlayed, the complete picture can be reconstructed— at a much higher resolution than previously possible.
Until now, the problem was that fluorescent dyes didn’t work well on DNA or in processed clinical cancer samples. So, Liu and her team formulated a new label called Hoechst-Cy5 by combining the DNA-binding molecule Cy5 and a fluorescent dye called Hoechst with ideal blinking properties for superresolution microscopy.
After showing that the new label produced higher resolution images than other dyes, the researchers compared colorectal tissue from normal, precancerous and cancerous lesions. In normal cells, chromatin is densely packed, especially at the edges of the nucleus. Condensed DNA glows brightly because a higher density of labels emits a stronger signal, while loosely packed chromatin produces a dimmer signal.
The images show that as cancer progresses, chromatin becomes less densely packed, and the compact structure at the nuclear border is severely disrupted. While these findings indicate that the new label can distinguish normal tissue from precancerous and cancerous lesions, Liu said that superresolution microscopy is unlikely to replace traditional microscopes for such routine clinical diagnoses. Instead, this technology could shine in risk stratification.
Xu Jianquan, Sun Xuejiao, Kim Kwangho, et al. Ultrastructural visualization of chromatin in cancer pathogenesis using a simple small-molecule fluorescent probe. Sci Adv
. 8(9):eabm8293. doi: 10.1126/sciadv.abm8293
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