Researchers have shown that dangerous cysts, which form over time in polycystic kidney disease (PKD), can be prevented by a single normal copy of a defective gene.
This means the potential exists that scientists could one day tailor a gene therapy to treat the disease. They also discovered that a type of drug, known as a glycoside, can sidestep the effects of the defective gene in PKD. The discoveries could set the stage for new therapeutic approaches to treating PKD, which affects millions worldwide. The study, partially funded by the National Institutes of Health (NIH), is published in Cell Stem Cell.
Scientists used gene editing and 3-D human cell models known as organoids to study the genetics of PKD, which is a life-threatening, inherited kidney disorder in which a gene defect causes microscopic tubes in the kidneys to expand like water balloons, forming cysts over decades. The cysts can crowd out healthy tissue, leading to kidney function problems and kidney failure. Most people with PKD are born with one healthy gene copy and one defective gene copy in their cells.
To better understand the genetic reasons cysts form in PKD, researchers sought to determine if 3-D human mini-kidney organoids with one normal gene copy and one defective copy would form cysts. They grew organoids, which can mimic features of an organ’s structure and function, from induced pluripotent stem cells, which can become any kind of cell in the body.
To generate organoids containing clinically relevant mutations, the researchers used a gene editing technique called base editing to create mutations in certain locations on the PKD1 and PKD2 genes in human stem cells. They focused on four types of mutations in these genes that are known to cause PKD by disrupting the production of polycystin protein. Disruptions in two types of the protein – polycystin-1 and polycystin-2 – are associated with the most severe forms of PKD.
They then compared cells with two gene copy mutations in organoids to cells with only one gene copy mutation. In some cases, they also used gene editing to correct mutations in one of the two gene copies to see how this affected cyst formation. They found organoids with two defective gene copies always produced cysts and those that carried one good gene copy and one bad copy did not form cysts.
According to senior study author Benjamin Freedman, Ph.D., the cells with one healthy gene copy make only half the normal amount of polycystin-1 or polycystin-2, but that was sufficient to prevent cysts from developing. He added that the results suggest the need for a second trigger and that preventing that second hit might be able to prevent the disease.
The organoid models also provided the opportunity to study the effectiveness of a class of drugs known as eukaryotic ribosomal selective glycosides on PKD cyst formation.
Freedman’s team found that the drugs could restore the ability of genes to make polycystin, increasing the levels of polycystin-1 to 50% and preventing cysts from forming. Even after cysts had formed, adding the drugs slowed their growth.
