Congenital nephrotic syndrome (CNS) is a hallmark of glomerular disease and the result of a defect in specialized cells (podocytes) within the kidney structures that filter the blood (glomeruli). CNS is characterized by massive proteinuria and infants present clinical symptoms early in life, or even before birth. CNS survival prognosis is poor. Patients typically develop end-stage kidney disease between 2 and 8 years old and kidney transplantation is the only effective treatment option available. Podocytes play an important role in the glomerular filtration barrier. Podocytes possess interdigitating foot processes that are bridged by a protein complex called the slit diaphragm, which contains proteins such as nephrin (NPHS1) and podocin (NPHS2). The slit diaphragm is essential for proper blood filtration as it allows the passage of small waste solutes into the pro-urine, whereas larger and essential molecules such as proteins are retained in the blood. Malfunctioning of podocytes causes massive proteinuria, resulting in nephrotic syndrome. Mutations in nephrin and podocin are the dominating causes of CNS.
In close collaboration between the Smeets and Schreuder lab, erythroblasts from a CNS patient diagnosed with compound heterozygous NPHS2 (podocin) mutations were successfully reprogrammed into induced pluripotent stem cells (iPSC). Jitske Jansen, assistant professor in both labs, differentiated these iPSC into kidney organoids containing podocytes lacking podocin expression. Using CRISPR/Cas9 the researchers successfully repaired the genetic defect in NPHS2 and showed a restored filtration slit (DOI 10.1242/dev.200198). Moreover, the researchers demonstrated the need for accurate 3D organoid models, as they showed that 2D (iPS-derived) podocyte models lack the expression of filtration slit proteins and are not suitable to model CNS in vitro. At the moment, a next-generation organoid on a chip is being developed to enable active filtration through the slits in vitro. The next big leap forward would be to treat CNS patients early in life. Therefore, the potency of a novel mRNA therapy is being tested in vitro, and the first promising results have been established.
Here we refer to a recent video created by the Dutch Kidney Foundation in which Prof dr Michiel Schreuder explains to a pediatric kidney patient how we use the kidney organoids in our labs to study kidney disorders and to develop future treatments.
Left to right: Jitske Jansen, Bart Smeets and Michiel Schreuder