Roundworm study uncovers cellular effects of kidney disease mutation

A microscopic worm that has no kidneys is helping Rutgers scientists understand a genetic change suspected of causing a serious inherited kidney disease.

By making a precise change in the roundworm's DNA and following the proteins it produces, researchers discovered the altered protein failed to reach the place where it normally works. But when a healthy version was present in the same animal, that protein continued doing its job.

The findings, published in the journal Genetics, offer clues about how this particular mutation affects cells, the researchers said. They also demonstrate a way to investigate a rising challenge in medicine: understanding what the DNA changes revealed by genetic testing actually mean.

The approach could be applied to other genetic changes affecting protein features that worms and humans share.

"Over time, it could help doctors interpret genetic test results more accurately and help researchers understand how individual parts of a protein contribute to its function," Wang said.

To investigate those questions, Wang and her colleagues turned to C. elegans, a tiny, extensively studied roundworm. Wang is the lead scientist in the laboratory of Maureen Barr, a Distinguished Professor of Genetics at Rutgers.

The researchers focused on a change linked to autosomal dominant polycystic kidney disease, an inherited condition in which fluid-filled sacs grow in the kidneys and can eventually cause kidney failure. Most cases involve changes in one of two genes that make cooperating proteins called polycystin-1 and polycystin-2.

Worms have corresponding proteins. Although they lack kidneys, their cells use these proteins in structures called cilia, tiny projections that act somewhat like antennas, helping cells sense their surroundings.

In male worms, the proteins help nerve cells gather information needed for mating. That gives scientists a way to connect a change inside a cell with an observable effect on an animal's behavior.

Using a gene-editing tool called CRISPR (short for clustered regularly interspaced short palindromic repeats), the team introduced the worm equivalent of a human genetic change classified as likely to cause disease. The change replaced one building block in the worm's version of polycystin-2.

The consequences were substantial.

The amount of the altered protein in the main part of the nerve cell fell to about 15% of normal. It was no longer detectable in the cilia where it normally functions.

Wang compared the problem to a product that remains inside the factory where it was made, unable to reach its workplace.

"They cannot go to their action site," she said.

The damage also affected its partner, the worm's version of polycystin-1. The amount of that protein dropped sharply, and it, too, was absent from the cilia. The findings suggest that disrupting one member of the partnership can undermine the other.

The worms' behavior reflected these cellular problems. Twenty percent of males with the mutation began the expected mating behavior after contacting a partner, compared with all the normal males tested. The researchers tested 60 males in each group.

Next, the team asked what would happen when a worm carried both a healthy and an altered copy of the gene.

They attached different fluorescent labels to the proteins, making the healthy version glow red and the altered version green. This allowed them to distinguish the two inside the same living animal.

"We found that the mutant protein did not disrupt where the healthy protein was located in the cell or prevent it from functioning normally," Wang said.

That distinction matters. Some defective proteins interfere with their healthy counterparts. This one didn't in the worm experiments. One working gene copy was enough to support the functions the researchers measured.

Whether the corresponding human mutation behaves the same way in kidney cells requires further investigation. The researchers did not observe kidney disease developing in worms or test a treatment. Instead, they isolated a protein defect and examined its consequences in a living animal.

"Now it's time to harvest the knowledge," Wang said.

For her, the study illustrates why understanding the smallest details of biology matters, even when a treatment remains a distant goal.

"But if you understand the disease, then you can think of ways to tackle that disease," Wang said.

Other Rutgers scientists on the study included: Barr; Carlos Nava Cruz, a master's student; Inna Nikonorova, an assistant research professor; Jonathan Walsh, a postdoctoral associate; and Elizabeth desRanleau, a senior lab technician. All are members of the Department of Genetics at Rutgers and the Human Genetics Institute of New Jersey. This research was funded by the National Institutes of Health and Polycystic Kidney Disease Foundation.

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Rutgers University

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