In experiments with human and mouse cell lines in the lab, the researchers found that the BBS4 protein normally transports molecules that help guide the cell's internal highway system -- a network of so-called microtubules along which tiny motors push and pull proteins, cellular packages and even chromosomes. When the BBS4 gene doesn't work correctly, the highway system falls apart, cell division halts and the cell dies.
"But our experiments also revealed something really interesting about pleiotropy -- genetic diseases that severely impact only a smattering of tissues," says Nicholas Katsanis, Ph.D., head of the team's contingent from the McKusick-Nathans Institute of Genetic Medicine at Johns Hopkins. "Once we knew faulty BBS4 prevented correct microtubule construction and led to cell death, the big question was how do people survive when every cell contains these mutations?"
The key is that the BBS4 protein acts through another protein, called PCM1, or pericentriolar material 1 protein. The two proteins are found together, or "co-localized," only in certain cell types in a specific subset of tissues, so it is only in those cells that BBS4 mutations can lead to cell death, the researchers report.
"There is very specific co-localization of the two proteins in specific cells in the retina and in certain brain cells, as well as small areas of other tissues," Katsanis says, describing the team's analysis of tissues from mice and mouse embryos.
Based solely on where the two proteins are found in mice, Katsanis can suggest a few reasons why o
Contact: Joanna Downer
Johns Hopkins Medical Institutions