Compound Fixes Problem Seen in Rare Kidney Disease

Mutant AGT (red) and peroxisomes (green) appear in different places in untreated cells (left), but together (shown in yellow) in the cells treated with DECA (right).Image by the researchers, courtesy of PNAS

Primary hyperoxaluria is a rare condition characterized by excess oxalate (also called oxalic acid). There are many causes, but the end result is that excess oxalate combines with calcium in the kidneys to form calcium oxalate, the main component of kidney stones. Calcium oxalate deposits can damage the kidneys and other organs and lead to kidney failure. Researchers haven’t found an effective treatment besides organ transplants.

Type 1 primary hyperoxaluria (PH1), the more common form of the condition, is caused by problems with an enzyme called alanine:glyoxylate aminotransferase (AGT). This enzyme normally converts the compound glyoxylate to alanine. When it malfunctions, amassed glyoxylate gets converted to oxalate instead. PH1 is estimated to affect 1 to 3 of every million people.

More than 75 AGT mutations can lead to PH1. In some patients, the enzyme is still functional but goes to the wrong place. AGT operates in a cellular compartment called the peroxisome. A targeting sequence at one end of the enzyme tells the cell’s sorting system to send it there. Some mutations, including one called AGT^P11LG170R, generate a targeting sequence that directs the enzyme into mitochondria instead. This mutation is thought to account for a third of all PH1 cases.

A team led by Dr. Carla Koehler at the University of California, Los Angeles, reasoned that compounds that interfere with mitochondrial targeting might be used to treat these forms of PH1. They thus developed a way to identify molecules that interfere with mitochondrial targeting. Using a robotic system, they screened a library of about 2,000 FDA-approved compounds in yeast cells. The work was funded by NIH’s National Institute of General Medical Sciences (NIGMS). Results appeared online on September 18, 2014, in the Proceedings of the National Academy of Sciences.

One compound, dequalinium chloride (DECA), proved particularly effective in the yeast screen. DECA is already approved as an antibacterial for treating oral and vaginal infections. In cell models, DECA blocked import of AGT^P11LG170R into mitochondria and restored import into peroxisomes, reducing the accumulation of oxalate in the cells.

“In many mutations that cause diseases, the enzyme doesn’t work,” Koehler says. “In PH1, the enzyme does work, but it goes to the wrong part of the cell. We wanted to use DECA in a cell model to block AGT from going to the wrong address and send it back to the right address. DECA blocks the mitochondria ‘mailbox’ and takes it to the peroxisome address instead.”

The researchers call the compound MitoBloCK-12, or MB-12. They are now testing other small molecules, which they call MitoBloCKs, for their ability to combat cellular defects seen in other diseases. These discoveries may lead to clinical therapies.

—by Harrison Wein, Ph.D.