This AGA male infant (XY karyotype) was delivered at 36 weeks to a 31-year-old G3P3 mother who received
no prenatal care. The placenta was large and the infant had widely spaced cranial sutures, large
anterior and posterior fontanelles, mild periorbital and moderate peripheral edema. He developed
respiratory distress and an infectious etiology was excluded. Evaluation revealed low serum albumin
(<1.0 g/dl) and protein (2.6 g/dl) with nephrotic-range proteinuria; BUN and Cr were 12 mg/dl and 0.2
mg/dl, respectively. Renal sonogram showed enlarged kidneys with diffuse bilateral echogenicity. An
open biopsy was performed. The infant failed to thrive and several bouts of sepsis followed. His
proteinuria was unresponsive to drug management but his renal function remained intact. A unilateral
nephrectomy was performed at 4 months of age (slide); persistent heavy proteinuria prompted removal of
the second kidney at 7.5 months.
Case 2 - Figure 1 - At 4 months, there is patchy interstitial fibrosis and moderate tubular dilatation. Subcapsular glomeruli resemble those in the deeper cortex.
Case 2 - Figure 2 - Representative glomeruli adjacent to dilated proximal tubules have minimal mesangial hypercellularity without mesangial sclerosis.
Case 2 - Figure 3 - A prior biopsy at 6 weeks of age showed minimal fibrosis, mildly thickened arteries and mild mesangial matrix and cell increase. Only a few tubules are dilated.
Case 2 - Figure 4 - Podocyte foot process fusion is obvious by electron microscopy. Electron dense deposits are not identified. The basement membrane is appropriately thin for age.
Light microscopy: The kidney sample from the first nephrectomy has
multifocal dilatation of proximal tubules more pronounced in the deep cortex with occasional microcysts
containing eosinophilic hyalinized material. There is mild interstitial fibrosis and inflammation. Some
glomeruli have mild mesangial hypercellularity and increased matrix; duplicated capillary basement
membranes are not evident. Infrequent globally sclerotic glomeruli are present and scattered superficial
glomeruli appear immature. Glomerulocystic change is minimal. Several arterial walls are thickened. At
7.5 months the kidney had more advanced tubular ectasia with scattered microcysts. Immunofluorescent microscopy: The glomeruli were negative for the following:
IgG, IgA, IgM, C3, C1q, C4, fibrinogen and albumin. Electron microscopy:
Foot process effacement was widespread. The mesangial matrix was focally increased but electron dense
deposits were not seen.
A wedge biopsy performed at 6 weeks of age contained primarily superficial cortical tissue. The
glomeruli were largely normocellular with prominent visceral epithelial cells as expected for age. The
interstitium had a few small clusters of immature hematopoietic cells but was not expanded by fibrosis.
Infrequent proximal tubules were only minimally dilated. No nephrin immunoreactivity could be
Diagnosis: Congenital nephrotic syndrome of the Finnish type.
Differential Diagnosis and Discussion
Early onset nephrotic syndrome, whether congenital (traditionally, presentation <3 months) or
infantile (4-months to 1 year) includes diseases of both idiopathic and secondary etiology 1.
Nephrotic syndrome in the first year of life may be associated with lesions that occur in older patients
such as minimal change disease, diffuse mesangial proliferation and focal segmental glomerulosclerosis
(FSGS); these, however, are rarely present at birth. Secondary causes include such diverse entities as
toxins and HUS, but typically, congenital infections (syphilis, toxoplasmosis, CMV) are the culprit.
Integration of the clinical and pathologic data often distinguishes between these disorders, which is
worthwhile as most patients with idiopathic nephrotic syndrome respond to steroid therapy. A subgroup,
though, is treatment resistant and may represent one of the entities unique to this age group, diffuse
mesangial sclerosis (DMS) or congenital nephrotic syndrome of the Finnish type (CNF).
Based solely on the light microscopic appearance of the glomeruli, the diagnosis of diffuse mesangial
proliferation might be considered. However, unresponsive massive proteinuria is atypical in that
idiopathic entity and tubular dilation is also uncommon, and likely the consequence of such proteinuria.
Progressive ectasia was striking in this patient over the 6-month interval between the first biopsy and
The pathologic hallmark of CNF is dilatation of the proximal tubules with microcyst formation.
Tubular dilatation in CNF typically begins deep and sampling limited to the superficial cortex could
result in a delayed or incorrect diagnosis. A variety of glomerular changes may be seen including
sclerosis, mesangial hypercellularity or matrix increase and thickened capillary loops; glomeruli may be
completely normal. The EM changes include podocyte foot process fusion, as in most cases of massive
proteinuria, and subendothelial expansion1. Unfortunately, all of these findings are
CNF, an autosomal recessive disorder, is the most frequent single cause of congenital nephrotic
syndrome, and particularly common though not restricted to the Finnish population. Its characteristic
clinical presentation includes the following: (1) nephrotic syndrome with massive proteinuria is
normally obvious within the first few days of life and almost always by 3 months; however, renal function
remains normal through the first six months; (2) infants are typically premature and have large
placentas; (3) maternal and amniotic fluid alpha-fetoprotein (AFP) levels are often elevated reflecting
fetal proteinuria. By 5 to 8 years of age there is end-stage renal failure but most usually die from
sepsis, not renal failure, within the first year 1, 2.
This latter point helps to distinguish clinically CNF from DMS in which renal function rapidly
deteriorates. Similarities abound and include early onset, familial occurrence and tubular ectasia. The
histology typically emphasized in DMS besides the obvious mesangial sclerosis, is podocyte hypertrophy
and zonal differentiation, with the deepest glomeruli being the least affected. The subcapsular region
often contains only diminutive and simplified glomeruli amongst primitive and atrophic tubules.
Mesangial hypercellularity has been described in DMS but is not a constant feature. Interstitial
fibrosis and inflammation may be significant 1, 3. Glomerular changes of DMS are common in
Denys-Drash syndrome (DDS; nephrotic syndrome, Wilms tumor and genital abnormalities) and may be seen in
Galloway-Mowat syndrome (nephrotic syndrome and CNS abnormalities).
These classic descriptions expounded from observations made over 35 years ago 2,4 are well
known to most pediatric and nephro-pathologists. Today evidence suggests that the unresponsive,
early-onset massive proteinuria of congenital nephrotic syndrome is primarily associated with defects in
the structure and function of the podocyte and in particular, the slit
diaphragm that bridges the interdigitating podocyte foot processes. Molecular approaches have
allowed recent identification of several key components of the slit diaphragm.
Abnormalities in the NPHS1 gene (19q.13.1) that encodes nephrin are
responsible for CNF 5, 6, 7 . Nephrin is a transmembrane adhesion molecule produced by
podocytes that localizes to the slit diaphragm and is involved in cell-cell or cell-matrix interactions.
It is proposed that strands of nephrin project from opposing foot processes to form the interlocking
"teeth" of the slit diaphragm, resembling a zipper 8. Over 50 mutations have been reported
in both Finnish and non-Finnish patients and affect both extracellular and intracellular domains
9. Two common mutations (Fin major and Fin minor) account for almost
95% of the cases in Finland and result in truncated non-functional proteins 6. Antibodies
directed against the intra- and extra-cellular components of nephrin are negative in this group.
It would seem that nephrin staining would be a simple diagnostic modality but barriers exist, the most
significant being that the antibody is currently not commercially available. Likewise, little data exist
about nephrin staining in non-Finnish patients, most of whom have 'private' missense mutations that lead
to misfolding of the mutant nephrin and its retention in the endoplasmic reticulum rather than transport
to the cell membrane 10. Interestingly, staining was positive in one patient, heterozygous
for Fin major anda missense mutation 11 and other non-Finnish patients with
compound heterozygous mutations 9.
Mutational analysis would therefore seem to be the better diagnostic approach but is unfortunately
only available on a research basis. Non-Finnish patients with compound heterozygous mutations, likely
de novo events, may have a milder disease but currently a
phenotypic/genotypic correlation has not been established 9. Current data would suggest that
some patients with congenital nephrotic syndrome without Fin major and Fin minor
mutations and whose kidneys express nephrin might respond to ACE inhibitor and indomethacin treatment
11. Though accurate DNA testing is theoretically available, no NPHS1 mutations were
identified in some cases of classic CNF, implying that mutations may be present in regulatory elements of
the gene (introns or flanking regions) or that proteins that interact with nephrin may be altered
12. Likewise, the large number of missense mutations make their classification as pathogenic
or polymorphism problematic.
This issue is operative in prenatal testing as well. Direct mutational analysis of Fin
major and Fin minor for parental carrier status, yielded no false negative results
in 1183 Finnish pregnancies 13. However, this relatively simple and inexpensive approach of
screening for Fin major and Fin minor mutations is not applicable to other
populations that harbor a diverse range of often unique gene abnormalities 13.
Even fetal renal pathology may not be diagnostic. Equally high maternal serum and amniotic fluid
alpha-fetoprotein (AFP) concentrations as well as similar proteinuric features of renal tubular
microcysts were demonstrated in both homozygous and heterozygous (carrier) fetuses, even though the
latter are not destined to suffer disease 14. Reduced nephrin synthesis and transient
dysfunction of the slit diaphragm with protein leakage were hypothesized.
How does the practicing pathologist integrate this knowledge into his diagnoses? From a practical
standpoint, the answer is that he must wait until molecular, immunoelectron microscopic and even
immunohistochemical testing become available outside of a research setting. The classic three pronged
light, immunofluorescent (or immunoperoxidase) and ultrastructural examination combined with a thorough
clinical history remains the best approach to nephrotic syndrome.
As our understanding of the genetic basis of nephrotic syndrome grows, targeted therapeutic approaches
may develop. Currently, however, the only cure for CNF is renal transplant and though function and
survival is generally good, recurrent nephrotic syndrome has been documented 15. Analogous to
the development of de novo anti GBM disease in transplant recipients with Alport's disease, anti-nephrin
antibodies were detected in some patients transplanted for CNF who were thus primarily exposed to nephrin
16, 17. Recurrent nephrotic syndrome was only in patients with homozygous Fin
major mutationsand notably, some responded to cyclophosphamide 17.
This raises the interesting notion that some patients with idiopathic nephrotic syndrome, like FSGS,
may have anti-nephrin antibodies. This question remains to be answered, as does the definitive role of
nephrin in other acquired proteinuric kidney disease. Investigations to date have varied results.
Though no significant differences were observed in nephrin mRNA or protein expression in proteinuric
pediatric patients studied by in situ hybridization or immunoperoxidase staining, respectively 18
, immunofluorescent staining showed reduced and redistributed nephrin in a group of adult patients
with proteinuria 19. Immunoelectronmicroscopy, (perhaps a more suitable investigative method)
in kidneys from patients with glomerulonephritis, demonstrated lower nephrin expression only in regions
of foot process effacement 20. Whether this observation is the cause or consequence of
podocyte disruption remains to be proven. What is becoming well established, however, is the importance
of nephrin's interactions with other podocyte-associated proteins.
CD2AP (CD2-associated protein), a widely expressed adapter molecule first
identified in T-cells and NK cells, is critical to podocyte function and also localizes to the slit
diaphragm where it appears to interact with the cytoplasmic tail of nephrin 21. It was found
that the mouse knockout of CD2AP dies at 6-7 weeks with proteinuria and foot process effacement
22. A renal disease resulting from the absence of CD2AP has not yet been identified in
Podocin also localizes to the slit diaphragm and is predicted to be an
integral membrane protein acing to organize the cytoskeleton 23 ,24.
Mutations (nonsense, frameshift and missense) in the gene for podocin, NPHS2 (1q25-q31) are
responsible for autosomal recessive steroid-resistant idiopathic nephrotic syndrome (SRNS)
23. Cellular activation through nephrin expression is greatly enhanced by podocin and
together, nephrin and podocin may form a signaling complex that supports the functional and structural
integrity of podocytes 25. Moreover, podocin not only binds to nephrin but it can directly
bind to CD2AP, which may facilitate the interaction of the former proteins 24.
Alpha-actinins, proteins that crosslink actin filaments, are highly
expressed in podocytes, and are involved in cytoskeleton organization. Mutations in the actin-binding
region of the alpha-actinin-4 gene (ACTN4, 19q13) have been identified in
patients with an autosomal dominant form of FSGS (FSGS1) 26. The mutant protein was shown
in vitro, to have an increased affinity for actin; this interaction may be
altered in vivo rendering the podocyte susceptible to damage. While the
pathogenesis of this variety of FSGS requires further elucidation, the discovery reinforces the impact
that cytoskeleton perturbation can have on normal foot process function. This is underscored by recent
demonstrations that nephrin, CD2AP and podocin are linked to the actin cytoskeleton and that
depolymerization of actin filaments in cultured cells leads to the loss of cell membrane localization of
these proteins 27, 28.
The discovery of the genetic defect in CNF has led to an explosion of knowledge about the glomerular
filtration barrier. Implementing this new molecular data with pathology will hopefully lead to useful
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