Previous studies have demonstrated that CUBN mutations can cause IGS, the common clinical features of IGS include megaloblastic anemia and secondary neurological symptoms, with or without proteinuria. Isolated proteinuria caused by CUBN mutation is rare, only a few cases have been reported in the world so far. Boger et.al  identified a missense mutation (I2984V) in the CUBN gene which is associated with albuminuria in both the general population and in individuals with diabetes. In 2011, Ovunc et.al  revealed that cubilin mutation as a single-gene cause of proteinuria by exome sequencing. As for the underlying mechanism of proteinuria, it’s speculated that mutated CUBN might affect protein reabsorption in the proximal tubules. As a ligand of cubilin, albumin binds to it at the proximal tubule, so there is almost no albumin in the normal urine. Therefore, when cubilin is defective, the uptake of albumin by the proximal tubule cells will be significantly reduced, resulting in proteinuria. Amsellem et.al  suggested that selective daily albumin excretion was increased approximately six-folds in cubilin-deficient mice. The cause of albuminuria is still controversial. Our cases demonstrate the possibility that CUBN geme mutations may pathologically present with pathological changes of podocytes and FSGS. The albuminuria was probably not only originated from renal proximal tubule loss but also from podocyte disfunction. As a podocyte-targeting immunosuppressive agent, tacrolimus could significantly reduce and even normalize the levels of albuminemia, which further supports the important role of podocyte in the pathogenesis of proteinuria in CUBN mutated patients.
Due to the scarcity of cases with CUBN gene mutation, kidney biopsy has been performed in only a few patients so far. In addition to isolated proteinuria, the renal pathology of our patients showed abnormal changes of podocytes. Until now, focal segmental glomerulosclerosis has not been reported in patients with CUBN gene mutation . In 2012, Prabakaran et.al  found cubilin expression in rat podocytes and human podocytes. In rats, cubilin was expressed in the plasma membrane, vesicles and multivesicular bodies. Gianesello et.al  proved that cubilin mediates albumin endocytosis in human podocytes, CUBN gene mutation may lead to the dysfunction of cubilin, thus affect albumin endocytosis. The development of proteinuria is usually related to podocyte damage, such as podocyte foot process effacement and cell loss . Compared to megalin, cubilin is thought to have higher binding affinity for albumin . Megalin can function as a sensor of albumin to determine the effect on cell survival. No or only a small amount of albumin binding to megalin can inhibit podocyte apoptosis, however, a large quantity of albumin binding to megalin can promote cell apoptosis . It’s thought that the CUBN mutation leads to reduction of binding capacity of cubilin to albumin, which makes free albumin available to bind with megalin. Consequently, it promotes podocyte apoptosis via the pI-3K/PKB pathway, leading to a decreased number of podocytes. As a kind of terminal differentiated cell, podocyte can’t proliferate to compensate the loss of podocyte, thereafter glomerular basement membrane become naked, followed by FSGS. Bedin et.al  reported the renal pathologies of the patients with CUBN mutations, most of them were minimal change disease or no lesions. But there were two patients whose lesions tended to be in the early stage of FSGS , which is consistent with our finding. Interestingly, those pathologically manifested with FSGS were unanimously have at least a relatively serious mutation in one allele, such as nonsense mutation, insertion, deletion or mutation in splice sites which may lead to frameshift or protein truncation. Since renal biopsy was rarely performed in patients with CUBN gene mutation, even less for electron microscopy (EM) examination. Therefore the pathological changes of podocytes were not noted on EM in the past. The obvious podocyte abnormalities in our cases provide evidence for its involvement in the pathogenesis of albuminuria in CUBN gene mutated patients.
Bedin et.al  firstly reported that biallelic CUBN mutations could cause isolated proteinuria, and proteinuria-associated CUBN mutations were localized to C-terminal CUBN domains. Our patients showed obvious proteinuria, but without megaloblastic anemia. Except for one intron splice-site mutation, all other mutations are located in highly conserved sites at the C-terminal, CUB19, CUB23 and CUB27 domain respectively, which is consistent with the results of Bedin. All our cases had at least one relatively serious mutation and severe pathological changes accordingly.
In this study, four novel pathogenic mutations of CUBN gene were identified in three prominent proteinuric children. The results demonstrate that, in addition to previous phenotypes, the novel serious CUBN gene mutations may cause pathological changes of podocytes and FSGS, which was not previously reported. Our cases extend the spectrum of renal manifestation and genotype of CUBN gene mutation.