Physicians might one day be able to treat a disease that destroys brain cells in children using genetically modified cells to transport a "drug" to the site of the dying neural cells (cells that transmit impulses).
This discovery occurred based on results of a laboratory study of the technique published by investigators at St. Jude Children's Research Hospital. A report on this work appears in the prepublication online issue of Blood.
There is currently no cure for such disorders, which are called lysosomal storage diseases (LSDs).
The St. Jude researchers successfully treated a laboratory model of an LSD called GM1 – gangliosidosis using bone marrow cells (BMCs) into which scientists inserted the gene for an enzyme that breaks down a fat molecule called GM1. GM1 is a critical component of normal brain cells. But in GM1 – gangliosidosis, brain cells lack this enzyme--beta-galactosidase--and GM1 accumulates to such a high concentration that it disrupts the proper function of the cell and causes it to self-destruct. BMCs include a population of so-called pluripotent stem cells--cells that give rise to a variety of different cell types that have specific functions, such as the immune cells called monocytes.
After the St. Jude team infused the genetically modified BMCs into the laboratory model, resulting monocytes migrated to the degenerating brain cells that lacked the gene for beta-galactosidase. These cells took in the enzyme released by the monocytes and used it to break down excess GM1, thus correcting the potentially fatal buildup of this molecule.
The monocytes homed in on the brain by following a trail of signaling molecules that were released by cells adjacent to the degenerating neurons, according to Alessandra d'Azzo, Ph.D., a member of the St. Jude department of Genetics and Tumor Cell Biology. Such signaling proteins are called chemokines. Under a disease condition, these brain cells, called astrocytes and microglia, release chemokines in order to trigger a migration of immune cells to areas of the brain that are damaged. d'Azzo is senior author of the article in Blood.
"We used the brain's own signaling molecules to guide the genetically modified monocytes along a concentration gradient to the degenerating brain cells," d'Azzo said. A gradient is a pathway of increasing concentration of a specific substance. In the St. Jude study, the BMCs followed the gradient toward increasing concentrations of the chemokines until they reached the site of the degenerating neurons. Normally, the immune cells recruited to the brain by chemokines would cause inflammation and worsen the damage. But in this case, the genetically modified monocytes restored the beta-galactosidase activity, which in turn decreased the extent of neuron degeneration and chemokine levels.
"We showed that improvement of the disease was directly related to the amount of genetically modified monocytes reaching the degenerating brain cells," said Renata Sano, Ph.D., a researcher in St. Jude Genetics and Tumor Cell Biology and the paper's first author. "The improvement was clearly linked to the ability of the corrected neurons to break down excess GM1 with the enzyme delivered by the monocytes. Overall, our findings suggest that if this technique could be adapted to treat children with this type of LSD we would have an effective therapy."
Key to the promise of curing LSDs in children using this technique is the fact that since BMCs are stem cells, they continually give rise to all the various types of blood cells in the body, including monocytes, d'Azzo said. Therefore, genetically modified BMCs would be able to produce an endless supply of monocytes carrying the gene for beta-galactosidase, ensuring that the enzyme would always be available to affected neurons in the brain of children with GM1 – gangliosidosis. If the BMCs were harvested from the same child who is treated with them after these cells are genetically modified, this would eliminate the potential complications arising from transplanting BMCs from a donor.
d'Azzo previously showed that GM1 – gangliosidosis is an inherited disease in which one of the enzymes in the lysosomes is defective (Molecular Cell, Sept. 10, 2004; www.stjude.org/media/0,2561,453_5297_12276,00.html). Lysosomes are the cell's recycling centers, where proteins, fats and other molecules are broken down into their basic building blocks, which are then reused to make new molecules. LSDs occur when lysosomes lack the enzymes they need to perform their recycling tasks, leading to abnormal accumulation of the molecules the lysosome is supposed to break down. These diseases, including GM1 – gangliosidosis, are responsible for most severe cases of neurodegeneration and mental retardation among children.
The other authors of this paper include Alessandra Tessitore (currently at Telethon Institute of Genetics and Medicine; Naples, Italy) and Angela Ingrassia (St. Jude).