Introduction
Imerslund-Gräsbeck Syndrome (IGS) is a rare autosomal recessive disease caused by mutations of
CUBN or
AMN gene [
1,
2]. The clinical features of IGS include selective intestinal vitamin B12 malabsorption resulting in megaloblastic anemia, failure to thrive, recurrent infections, neurological damage, with or without proteinuria and normal renal function [
3] . To date, only 48 different mutations in
CUBN and 32 different variants in
AMN have been reported in the “Human Gene Mutation Database” (HGMD) (
http://www.hgmd.cf.ac.uk/ac/index.php).
CUBN gene encodes a large 460-kDa glycosylated extracellular protein called cubilin, which is composed of 27 CUB domains [
4]. Cubilin expresses in both renal proximal tubular cells [
4] and podocytes [
5]. It contains eight Ca
2+-binding epidermal growth factor repeats and a N-terminal region involved in trimerization of the protein [
6]. The CUB domains of cubulin can act as ligand-binding sites for intrinsic factor-B12 complex [
7], albumin [
8,
9], vitamin carrier proteins, lipoproteins, other carriers, immune- and stress-related proteins and drugs [
10]. Among them, the intrinsic factor-B12 complex binds to CUB domains 5–8 [
11], but the binding sites for albumin remain unclear. Cubilin has no transmembrane domain in the structure, so it is dependent on additional factors for membrane anchoring and for endocytosis of the receptor-bound ligands [
6]. As an endocytic receptor, cubilin mediates the uptake of proteins and protein-bound substances both in intestine and kidney [
6]. The majority of filtrated albumin is reabsorbed through cubilin-mediated endocytosis in the proximal tubule, resulting in very low level of albumin in final urine.
In addition to Imerslund-Gräsbeck Syndrome,
CUBN gene mutations have been reported in only a few cases with isolated proteinuria. The proteinuria is thought from tubular loss without glomerular involvement and the prognosis is benign [
12‐
14]. In this paper, we report 3 patients with
CUBN gene biallelic pathogenic variants presented as isolated proteinuria and FSGS in renal biopsy. Our cases extend the spectrum of renal manifestation and genotype of
CUBN gene mutation.
Discussion
Previous studies have demonstrated that
CUBN gene mutations can cause IGS that typically manifests as megaloblastic anemia and secondary neurological symptoms, with or without proteinuria. Isolated proteinuria caused by
CUBN gene mutations is rare, only a few cases have been reported in the world so far. Boger et.al [
18] found a missense mutation (I2984V) of
CUBN gene which is associated with albuminuria in general population and in individuals with diabetes. In 2011, Ovunc et.al [
3] identified a homozygous frameshift mutation in
CUBN gene in two siblings of consanguineous patients with intermittent nephrotic-range proteinuria, which indicated that cubilin mutation may be considered as a rare single-gene cause of nephropathy. Recently Bedin et al. [
14] identified 39 patients with biallelic
CUBN gene mutations from three cohorts, and found that they had sub-nephrotic proteinuria and normal renal function. Our cases add more evidence that
CUBN gene mutations may cause moderate proteinuria.
Cubilin works as a receptor for albumin, its defect may significantly reduce albumin reabsorption in renal proximal tubular cells and leads to the occurrence of proteinuria. Amsellem et.al [
19] found that selective daily albumin excretion was increased approximately six-folds in cubilin-deficient mice. It is assumed that albuminuria come from renal tubular malabsorption in
CUBN mutated patients, but whether it is also resulted from glomerular loss remains to be elucidated. Although there was no direct evidence to support glomerular origin, our cases showed pathological changes of podocytes and FSGS and their proteinuria gradually attenuated after administration of tacrolimus. Those suggest that albuminuria was probably not only originated from renal proximal tubule malabsorption, but may also from podocyte disfunction in
CUBN-mutated patients.
Due to the scarcity of cases with
CUBN gene mutations, kidney biopsy has been performed in only a few patients so far. Until now, focal segmental glomerulosclerosis has just been reported in 1 of 4 patients in genetic kidney disease cohort II with
CUBN gene mutations [
14]. In 2012, Prabakaran et.al [
5] found cubilin expression in rat podocytes and human podocytes. Gianesello et.al [
20] proved that cubilin mediates albumin endocytosis in human podocytes,
CUBN gene mutations may lead to the dysfunction of cubilin, thus affect albumin endocytosis in podocytes. The development of proteinuria is usually related to podocyte damage, such as podocyte foot process effacement and cell loss [
21]. Compared to megalin, cubilin is thought to have higher binding affinity for albumin [
22]. 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 [
5]. It’s thought that the
CUBN gene mutations lead to reduction of cubilin-binding ability with albumin, which make more free albumin available to bind with megalin. Consequently, it promotes podocyte apoptosis via the PI-3 K/PKB pathway, leading to a decreased number of podocytes. As a kind of terminal differentiated cell, podocyte can’t proliferate to compensate its loss, thereafter glomerular basement membrane become naked, followed by development of FSGS. Bedin et.al [
14] reported the renal pathologies in 19 patients with
CUBN gene 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 [
14]. Interestingly, those pathologically manifested with FSGS unanimously had at least a relatively serious mutation in one allele, such as nonsense mutation, insertion, deletion or mutation in splice sites which led to frameshift or protein truncation. Since renal biopsy was rarely performed in patients with
CUBN gene mutations, even less for electron microscopy (EM) examination. Therefore, the pathological changes of podocytes were not noted on EM in the past. The observation of obvious podocyte abnormalities with EM in our cases provide evidence for its involvement in the pathogenesis of albuminuria in
CUBN gene-mutated patients.
Bedin et.al [
14] firstly noted that all proteinuria-associated
CUBN mutations were localized to C-terminal CUBN domains. Our patients showed that except for one intron splice-site mutation, other mutations are located in highly conserved sites at the C-terminal, CUB19, CUB23 and CUB27 domain respectively. All our cases had at least one relatively serious mutation and severe pathological changes accordingly.
In this study, four biallelic pathogenic variants of CUBN gene were identified in three isolated proteinuric children. The results demonstrate that CUBN gene mutations may cause isolated proteinuria pathologically presented as FSGS. Our cases extend the spectrum of renal manifestation and genotype of CUBN gene mutation.
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