Bardet-Biedl syndrome (BBS), characterized by
obesity, learning disabilities and eye and kidney problems, is caused by genetic mutations in the BBS family of
genes. Now, researchers who've long studied the condition have discovered that genetic mutations in one of those genes, called BBS4, lead to cell death by disrupting the cells' internal "highway" system.
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 obesity may be common in people with the disorder, including improperly controlled or targeted neurons, improper hormone release, and improper growth of fat cells, all of which may short-circuit normal appetite controls. However, experiments with genetically engineered mice will be necessary to know for sure, he says.
Although the work is far from revealing an anti-obesity "magic bullet," the researchers, also from Simon Fraser University in Canada and the University College London, say it does point to microtubule failure as a primary mechanism for the problems seen in BBS, particularly the disorder's obesity, diabetes and retinal degeneration. Moreover, their discovery adds to evidence that the cells' highway system could be a major factor in other multisystem disorders.