Morphologic changes suggesting abnormal renal differentiation in congenital nephrotic syndrome

Glomeruli from patients with nephrin mutations show increased number of ciliated and poorly differentiated podocytes

BACKGROUND

Podocytes are postmitotic, highly specialized cells which maintain the glomerular filtration barrier (GFB). Their injury is characterized by foot processes effacement and change in protein expression leading to proteinuria and end-stage kidney disease.

METHODS

Our study focuses on the morphological and immunohistochemical changes of human podocytes during normal development and postnatal period, compared to congenital nephrotic syndrome of the Finnish type (CNF). Kidney tissues taken from 17 human conceptuses 8th-38th weeks old, two healthy and three CNF kidneys were embedded in paraffin for immunohistochemical or double immunofluorescence methods, or were embedded in resin for electron microscopy. Paraffin sections were stained with markers for proliferation (Ki-67), proteins nephrin and nestin, and alpha-tubulin. Quantification of positive cells were performed using Mann Whitney and Kruskal-Wallis test.

RESULTS

Tissue analysis showed that proliferation of podocytes gradually decreased during development and disappeared in postnatal period. Decrease in number of ciliated glomerular cells and visceral podocytes (from 47% to 3%), and parietal epithelial cells (from 32% to 7%) characterized normal development. Nestin and nephrin co-expressed in developing podocytes in different cellular compartments. During development, nephrin expression increased (from 17% to 75%) and postnatally changed its pattern, while nestin positive glomerular cells decreased from 98% to 40%. CNF glomeruli displayed increased number of immature ciliated podocytes (6%) and parietal epithelial cells (9%).

CONCLUSION

Changes in cytoplasmic alpha-tubulin expression and reduced nephrin expression (20%) indicating association of incomplete podocyte maturation with failure of GFB function and appearance of prenatal proteinuria in CNF patients.

NPHS1 gene mutations confirm congenital nephrotic syndrome in four Brazilian cases: A novel mutation is described

AIM

Autosomal recessive mutations in NPHS1 gene are a common cause of congenital nephrotic syndrome (CNS). The disorder is characterized by massive proteinuria that manifests in utero or in the neonatal period during the first 3 months of life. NPHS1 encodes nephrin, a member of the immunoglobulin family of cell adhesion molecules and the main protein expressed at the renal slit diaphragm. Currently, there are approximately 250 mutations described in the NPHS1 gene distributed among all nephrin domains. The main objective of this study was to perform the analysis of the NPHS1 gene in patients with congenital nephrotic syndrome in order to determine the molecular cause of the disease.

METHODS

Direct sequencing of NPHS1 gene in four children was performed.

RESULTS

Each patient was heterozygous for two pathogenic mutations disclosing the molecular cause of the disease in 100% of the cases. We identified six different mutations, consisting of one in-frame deletion, one frameshift, and four missense substitutions. The p.Val736Met mutation that is described here for the first time was considered pathogenic by different mutation predictive algorithms. Regardless of the type of mutation, three patients had a bad outcome and died

CONCLUSIONS

Despite the small size of the cohort, this study contributed to the increasing number of deleterious mutations in the NPHS1 gene by describing a new mutation. Also, since we identified NPHS1 pathogenic mutations as the cause of the disease in all cases analyzed, it might be a frequent cause of CNS in the South Eastern region of Brazil, although the analysis of a larger sample is required to obtain more indicative epidemiological data.

Mechanism of cystogenesis in nephrotic kidneys: a histopathological study

BACKGROUND

Nephrotic syndrome (NS) is pathological condition characterized by heavy proteinuria. Our study investigates hypothesis that change in cell proliferation of proximal tubules influences primary cilia structure and function and promotes cystogenesis in congenital nephrotic syndrome of the Finnish type (CNF) and focal segmental glomerulosclerosis (FSGS).

METHODS

CNF kidneys were analyzed genetically. Proliferation (Ki-67), apoptosis (caspase-3), and primary cilia (α-tubulin) length and structure were analyzed immunohistochemically and ultrastructurally in healthy, CNF and FSGS kidneys. Cyst diameters were measured and correlated with proliferation index.

RESULTS

Proximal tubules cells of healthy kidneys did not proliferate. In nephrotic kidneys, tubules with apparently normal diameter covered by cuboidal/columnar epithelium (PTNC) contained 81.54% of proliferating cells in CNF and 36.18% in FSGS, while cysts covered with columnar epithelium (CC) contained 37.52% of proliferating cells in CNF and 45.23% in FSGS. The largest cysts, covered with squamous epithelium (CS) had 11.54% of proliferating cells in CNF and 13.76% in FSGS. Increase in cysts diameter correlated with changes in proliferation index, tubular cells shape, primary cilia formation and appearance of apoptotic cells.

CONCLUSIONS

We present a novel histopathological data on the structure and possible changes in function of tubular cell in NS kidneys during cystogenesis. We suggest existence of common principles of cystogenesis in CNF and FSGS kidneys, including serious disturbances of tubular cells proliferation and apoptosis, and faulty primary cilia signaling leading to deterioration of proteinuria in NS kidneys.

Familial forms of nephrotic syndrome

The recent discovery of genes involved in familial forms of nephrotic syndrome represents a break-through in nephrology. To date, 15 genes have been characterized and several new loci have been identified, with a potential for discovery of new genes. Overall, these genes account for a large fraction of familial forms of nephrotic syndrome, but they can also be recognized in 10-20% of sporadic cases. These advances increase diagnostic and therapeutic potentials, but also add higher complexity to the scenario, requiring clear definitions of clinical, histopathological and molecular signatures. In general, genetic forms of nephrotic syndrome are resistant to common therapeutic approaches (that include steroids and calcineurin inhibitors) but, in a few cases, drug response or spontaneous remission suggest a complex pathogenesis. Finally, syndromic variants can be recognized on the basis of the associated extra-renal manifestations. In this educational review, clinical, histological and molecular aspects of various forms of familial nephrotic syndrome have been reviewed in an attempt to define a rational diagnostic approach. The proposed model focuses on practical and economic issues, taking into consideration the impossibility of using genetic testing as starting diagnostic tool. The final objective of this review is to outline a diagnostic flow-chart for clinicians and geneticists and to generate a rational scheme for molecular testing.

Clinical features and long-term outcome of nephrotic syndrome associated with heterozygous NPHS1 and NPHS2 mutations

BACKGROUND AND OBJECTIVES

Mutations in nephrin (NPHS1) and podocin (NPHS2) genes represent a major cause of idiopathic nephrotic syndrome (NS) in children. It is not yet clear whether the presence of a single mutation acts as a modifier of the clinical course of NS.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We reviewed the clinical features of 40 patients with NS associated with heterozygous mutations or variants in NPHS1 (n = 7) or NPHS2 (n = 33). Long-term renal survival probabilities were compared with those of a concurrent cohort with idiopathic NS.

RESULTS

Patients with a single mutation in NPHS1 received a diagnosis before those with potentially nongenetic NS and had a good response to therapies. Renal function was normal in all cases. For NPHS2, six patients had single heterozygous mutations, six had a p.P20L variant, and 21 had a p.R229Q variant. Age at diagnosis and the response to drugs were comparable in all NS subgroups. Overall, they had similar renal survival probabilities as non-NPHS1/NPHS2 cases (log-rank chi(2) 0.84, P = 0.656) that decreased in presence of resistance to therapy (P < 0.001) and in cases with renal lesions of glomerulosclerosis and IgM deposition (P < 0.001). Cox regression confirmed that the only significant predictor of dialysis was resistance to therapy.

CONCLUSIONS

Our data indicate that single mutation or variant in NPHS1 and NPHS2 does not modify the outcome of primary NS. These patients should be treated following consolidated schemes and have good chances for a good long-term outcome.

Glomerular structural factors in progression of congenital nephrotic syndrome

The glomerular structural factors associated with progression of congenital nephrotic syndrome of Finnish type (CNF) are incompletely understood. We studied glomerular volume and the proportion of various glomerular subtypes in seven CNF patients (aged 4 months to 3 years). Glomerular tubular connections (GTC) were analyzed in two patients early and late in the disease spectrum (creatinine 0.2 and 2.8 mg/dl, respectively). The proportions of "normal" glomeruli decreased (r=-0.91, P=0.003) and microcystic glomeruli increased (r=0.85, P=0.015), while the fetal and sclerosed glomeruli remained stable (r=0.073 and 0.08 respectively, P=NS for both) with age. The "normal" glomeruli were larger in CNF than in six age-matched minimal change nephrotic patients (P=0.009). GTC analyses in the mild disease showed that 75% of the 12 "normal" glomeruli had normal tubules, and 25% were atubular or were attached to atrophic tubules, while 88% of the 12 fetal glomeruli were either atubular or were attached to atrophic tubules. In the advanced disease, 67% of 12 "normal" glomeruli had normal tubules, and 33% were atubular or atrophic, while none of the fetal or microcystic glomeruli had "normal tubules". Thus atubular glomeruli may play an important role in disease progression in CNF, as it is associated with a progressive decrease in hypertrophied "normal" glomeruli and an increase in largely atubular microcystic glomeruli.

Recent insights into the structure and functions of heparan sulfate proteoglycans in the human glomerular basement membrane

As the first barrier to be crossed on the way to urinary space, the glomerular basement membrane (GBM) plays a key role in renal function. The permeability of the GBM for a given molecule is highly dependent on its size, shape and charge. As early as 1980, the charge-selective permeability was demonstrated to relate to the electrostatic properties of covalently bound heparan sulfates (HS) within the GBM. Since the identification of perlecan as a heparan sulfate proteoglycan (HSPG) of basement membranes, the hypothesis that perlecan could be a crucial determinant of GBM permselectivity received considerable attention. In addition to perlecan, the GBM also contains other HSPG species, one of which was identified as agrin. The high local expression of agrin in the GBM, together with the presence of agrin receptors at the cell matrix interface, suggests that this HSPG contributes to glomerular function in multiple ways. Here, we review the current knowledge regarding the structure and functions of HSPGs in the GBM, and discuss how these molecules could be involved in various glomerular diseases. Possible directions for future investigation are suggested.

The idiopathic nephrotic syndrome

The nephrotic syndrome is defined by edema, hypoalbuminemia, proteinuria, and hyperlipemia with elevated cholesterol and triglyceride concentrations in serum or plasma. It may be primary or secondary to systemic disorders. Because of its incidence--between 2.0 and 7.0 cases per 100,000 children per year--this disorder is not uncommon in a busy pediatric practice, and it forms the basis of the referrals evaluated by a pediatric nephrology service. Over the 2 years since this subject was reviewed, several articles have appeared that have provided insights into the clinical features, etiology, pathology, pathogenesis, complications, and therapy of the idiopathic primary nephrotic syndrome. Nevertheless, a remarkable feature of this condition remains that the fundamental cause and pathogenesis of the most common form of primary nephrotic syndrome, minimal lesion nephrotic syndrome of childhood, is largely unknown. Hence, a huge challenge remains for researchers in the area of pediatric nephrology.