To study these minute genetic differences further, Dr. Pandolfi and his colleagues developed two groups of mice: One group harbored the PML-RAR gene fusion, while a second group had the PLZF-RAR gene fusion. Both groups of mice developed APL similar to that seen in humans. In normal cells, certain proteins called transcription factors -- such as RAR -- turn on the expression of other genes with specific functions, such as controlling cell growth and maturation. Using advanced analytical techniques, Dr. Pandolfi's group showed that unlike RAR , proteins produced by the PML-RAR and PLZF-RAR gene fusions turned off, or "repressed," the expression of the cell's genes that normally prompt a cell to mature into a healthy, functioning white blood cell. Because these genes were turned off, the cells didn't mature, and leukemia developed. Moreover, the investigators found that PML-RAR and PLZF-RAR were acting as transcription repressors because of their ability to interact with other proteins. In PML- RAR , these interactions were broken apart by RA, while in PLZF-RAR they were insensitive to RA, explaining why retinoic acid works in some APL patients but not others.
Armed with this new knowledge, the MSKCC researchers set out to find
ways to undo this chain of events. They knew that genetic material in a
chromosome is tightly held together by proteins called histones. However,
histones prevent individual gene expression. Previous studies of histones have
shown that transcription repressors may work by favoring the place
Contact: Kelli Stauning
Memorial Sloan-Kettering Cancer Center