Hand-holding molecules: X-rayed chaperones

Rare treatment

 

X-ray crystallography has been used to identify two new small chaperone molecules. The targets may prove useful in developing the first pharmaceutical treatment for the rare but debilitating inherited metabolic disorder Schindler/Kanzaki disease.

Schindler disease, also known as Kanzaki disease and alpha-N-acetylgalactosaminidase deficiency is one of more than fifty related disorders that affect 7000 to 8000 infants each year leading to symptoms at age 8 to 15 months, although only a few individuals with each form of the condition have been identified.

Babies born with it have a defect in each of their copies of the gene that codes for the alpha-N-acetylgalactosaminidase (alpha-NAGAL) enzyme, one of the cell's "recycling" machines that clean up used, toxic molecules, or substrate. When it works normally, alpha-NAGAL breaks down a sugar-containing substrate in the cell's recycling centre, the lysosome. If alpha-NAGAL underperforms or fails, patients have neuromuscular problems such as seizures and muscle weakness.

It seemingly reverses development and earlier learning, leading to muscle loss, learning disability and seizures. Infants with the condition tend to die before they are four years old. There is a milder adult form that afflicts individuals more commonly in their 30s (this is Type II or Kanzaki disease) causing coarsening of facial features and some intellectual impairment. A third form leads can lead to varying levels of seizures, learning difficulties, behavioural problems or an autistic type condition.

The disease is a lysosomal storage disorder caused by a deficiency in alpha-NAGAL due to a mutation on chromosome 22, the result of which is excessive accumulation of glycoproteins in lysosomes, glycosphingolipids accumulate throughout the body. The condition was first reported by Detlev Schindler who published details in 1988. Hiro Kanzaki outlined additional aspects of the biochemistry and molecular biology in 2006.

Researchers at the Universities of Oxford, UK and Massachusetts Amherst, USA, Nathaniel Clark, Scott Garman and their colleagues report details of the work supported by the US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and the National Institute of General Medical Sciences (NIGMS) in the journal Proceedings of the National Academy of Sciences. Garman adds that for children with the disease, "Some substrates are very toxic and children born with these diseases are really, really sick, many only living a short time."

The faulty gene causes problems by misfolding proteins, which leads to an unstable and poorly functioning form of alpha-NAGAL enzyme. Researchers hope that one way treat the disease might involve somehow stabilizing alpha-NAGAL by using "pharmacological chaperones." Garman and colleagues have now identified two such iminosugar chaperones and have successfully tested how they can maintain alpha-NAGAL enzyme. The work demonstrates biochemically and crystallographically and through cellular experiments how chaperones can guide the proper re-folding of defective alpha-NAGAL enzyme. Their data also reveal precisely how these small molecules bind to the enzyme to provide this stability, which could lead the way to developing other chaperones.

"People had hypothesized that this approach, using DGJNAc to treat Schindler disease, would work," Garman says. "Now we have shown for the first time that it does. These experiments show wThe next step would be to see if the compounds work not just in cells but in organisms. Ultimately we would like to improve treatments for diseases by understanding the molecular interactions at the root of a disease. e can add DGJNAc to cells and increase the amount of the Schindler enzyme." The team was the first to define the enzyme's structure in 2009 and this work takes them an important step forward as they reveal the first small molecules that can bind and stabilize it.

"The next step would be to see if the compounds work not just in cells but in organisms," Garman told SpectroscopyNOW. "Ultimately we would like to improve treatments for diseases by understanding the molecular interactions at the root of a disease." Meanwhile obtaining approval for "compassionate" use of these or related compound in trials is one aim. DGJ itself has been demonstrated as safe in humans and is itself now in Phase III clinical trials for Fabry disease, another lysosomal storage disorder; it may now be possible to extend the scope of trials to Schindler disease too.