Mutations in four genes that encode components of the outer ring subunits of the nuclear pore complex (NPC) are associated with steroid-resistant nephrotic syndrome (SRNS), say researchers.

Podocytes are the critical site of injury in SRNS and although >50 monogenic causes have been identified, many cases are unexplained. Previous studies have shown that mutations in NUP93 and NUP205, which encode proteins of the inner ring subunit of the NPC, cause SRNS. “NPC proteins are ubiquitously expressed and play an essential role in every eukaryotic cell, so it was surprising that germline mutations in genes that are expressed in every cell of the human body give rise to a cell-specific phenotype,” says Friedhelm Hildebrandt, an author on the new paper.

In their latest study, Braun et al. further investigated the functional link between alterations in NPC proteins and podocyte injury. They performed whole-exome sequencing in 160 families with SRNS. They excluded mutations in known SRNS genes and used homozygosity mapping in consanguineous families to identify mutations in four genes encoding components of the outer ring subunits of the NPC — NUP107, NUP85, NUP133 and NUP160 — that cause SRNS.

The researchers used co-immunoprecipitation experiments to show that particular pathogenic alleles of NUP107, NUP85, NUP160 and NUP133 weaken the protein–protein interactions between components of the NPC.

The researchers used the CRISPR–Cas9 system to generate stable zebrafish lines carrying different alleles of NUP107 and NUP85 and found that fish with two null mutations died early in development, while fish with at least one hypomorphic mutation did not.

Using Xenopus tropicalis as a model system, the researchers showed that knockdown of NUP107, NUP85 or NUP133 disrupted glomerulogenesis. Hildebrandt and co-workers also found that CRISPR–Cas9 knockout of NUP107, NUP85 or NUP133 in podocytes increased the level of active Cdc42, which is an important effector of SRNS pathogenesis.

it was surprising that germline mutations in genes that are expressed in every cell of the human body give rise to a cell-specific phenotype

“Our genetic data provide strong evidence for a functional link between alterations in different structural components of the NPC and development of podocyte injury,” notes Hildebrandt. “However, the specific molecular pathogenesis is yet to be fully understood. We seek to investigate which distinct properties of podocytes convey the specific vulnerability to alterations in NPC proteins as compared to other cell types.”