Laura Finn, M.D. University of Washington and Children's Hospital and Regional Medical Center Seattle, Washington
This AGA female infant (XX karyotype) was delivered vaginally to a 19-year-old G1P1 mother at 34 weeks
gestation with Apgars of 71 and 95. The placenta was large and the infant was noted to have widely spaced
cranial sutures and large anterior and posterior fontanelles. She was discharged in good condition on day
12 but presented at 1 month of age with abdominal distension and respiratory distress. Peritoneal fluid and
blood cultures grew Streptococcus pneumoniae. Evaluation revealed hypoalbunemia and nephrotic range
proteinuria (3.37grams/24hr); complement levels were normal; BUN and Cr were 3mg/dl and 0.3mg/dl,
respectively. The kidney size by ultrasound was the upper limit of normal and there was diffuse bilateral
echogenicity. Multiple bouts of sepsis and peritonitis ensued. She was unresponsive to medical management
and underwent a nephrectomy at 4 months and 7.5 months of age.
Light microscopy: The wedge of kidney has ectatic and microcystically dilated proximal tubules surrounded
by mildly fibrotic and minimally inflamed interstitium. Glomerulocystic change is minimal. Most glomeruli
have at least mild mesangial hypercellularity and increased matrix; rare globally sclerotic glomeruli and
immature glomeruli are present. Duplicated capillary basement membranes are not evident. Some arterial
walls are thickened. Immunofluorescent microscopy: 2+ focal mesangial staining with IgM and C3. 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
was not expanded and infrequent proximal tubules were only minimally dilated.
Differential Diagnosis and Discussion
The differential diagnosis of congenital (<3 months) and infantile (4-months to 1 year) nephrotic syndrome
includes diseases of both idiopathic and secondary etiology. Secondary causes such as congenital
infections, toxins and HUS are useful to exclude with a biopsy, as some of these entities may respond to
treatment. Within the former category are congenital nephrotic syndrome (Finnish type) [CNF], diffuse
mesangial sclerosis [DMS] (isolated or in association with malignancies and genital abnormalities
[Denys-Drash syndrome, (DDS)]), minimal change disease, mesangial proliferative glomerulonephritis (GN) and
FSGS. Integrating the clinical and pathologic findings usually allows differentiation among these entities
Based on the light microscopic appearance of the glomeruli, this case could be diagnosed as mesangial
proliferative GN; however unresponsive massive proteinuria is atypical. Tubular dilation is also uncommon
and is possibly the consequence of such proteinuria. Progressive ectasia was evident in this patient.
CNF, an autosomal recessive disorder, is particularly common though not restricted to the Finnish population
and has a characteristic clinical presentation: 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; infants are typically premature and have large placentas; 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. The pathologic hallmark of CNF is dilatation of the proximal tubules, sometimes to the
level of microcyst formation. A variety of glomerular changes may be seen including sclerosis, mesangial
hypercellularity or matrix increase and thickened capillary loops; they may be completely normal. The EM
changes are non-specific and include foot process fusion and subendothelial widening 1.
Diffuse mesangial sclerosis (DMS) may be confused with CNF by its early onset, familial occurrence and
tubular ectasia but the rapid deterioration of renal function distinguishes the two clinically. 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. Mesangial hypercellularity has
been described in DMS but is not a constant feature. Interstitial fibrosis and inflammation may be
These classic descriptions expounded from observations made over 35 years ago 3,4 are well known to most
pediatric pathologist. 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 foot process (Table 1), the outer most component of the three layered glomerular filtration barrier
that also includes the glomerular basement membrane (GBM) and the fenestrated endothelial cells lining the
inside of the glomerular capillary. A zipper-like membrane known as the slit diaphragm bridges the
interdigitating podocyte foot processes (Figure 1).
Nephrin, a transmembrane protein produced by podocytes and localized to the slit diaphragm, is the product
of the NPHS1 gene (19q.13.1); abnormalities in NPHS1 are responsible for CNF5,6,7. Over 50 mutations have
been reported in both Finnish and non-Finnish patients 8. Two common mutations (Finmajor and Finminor) are
present in almost 95% of the cases in Finland and result in truncated non-functional proteins 6; staining
with antibodies directed against the intra- and extra-cellular components of nephrin is negative in this
group. Most mutations found in non-Finnish patients are '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 9. Too little data is available to know if nephrin can be immunohistochemically demonstrated
in these glomeruli; however staining was positive in one patient, heterozygous for Finmajor and a missense
mutation 10. Non-Finnish patients with compound heterozygous mutations may have a milder disease but
currently a phenotypic/genotypic correlation has not been established. 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. Likewise, the large number of missense mutations make their
classification as pathogenic or polymorphism problematic.
One protein that interacts with the intracellular component of nephrin is CD2AP (CD2-associated protein), a
widely expressed protein first identified in T-cells and NK cells, which also localizes to the slit
diaphragm. It was recently found that the mouse knockout of CD2AP dies at 6-7 weeks from proteinuria with
foot process effacement 11. Nephrin (staining) and a slit diaphragm are initially present in these mice,
but disappear in parallel, implying that other intracellular proteins compensate for CD2AP during
development and that there are transitions in podocyte gene expression 12. A renal disease resulting from
the absence of CD2AP has not yet been identified in humans.
Another recently discovered component of the glomerular filtration barrier is podocin, predicted to be an
integral membrane protein though the exact subcellular location is currently unknown 13. Mutations
(nonsense, frameshift and missense) in the gene responsible for podocin, NPHS2 (1q25-q31) were identified in
families with steroid-resistant idiopathic nephrotic syndrome (SRNS), characterized by an autosomal
recessive mode of inheritance, onset between three months and five years of age, biopsy showing FSGS, and
rapid progression to end-stage renal failure; extrarenal disorders were absent 13,14. Podocin, as
demonstrated by in-situ hybridization, appears to be confined to podocytes and is first expressed in the
late S-phase. The structure of podocin suggests that it may act to organize the cytoskeleton and mediate
podocyte signaling through indirect or direct interactions with nephrin and CD2AP 13. NPHS2 mutations were
recently described in 9 of 44 children with clinically and pathologically characteristic non-familial FSGS
15. Though the authors advocate molecular testing, it is remains available only on a research basis.
Another genetic cause of FSGS has been identified in families with a mild type of nephrotic syndrome
(FSGS1); once again a defect in the podocyte is suspect. Patients presented in their teens or early
adulthood and progressed slowly to end-stage renal disease. Dominant mutations were identified in the
actin-binding region of alpha-actinin (ACTN4, 19q13), which is highly expressed in podocytes and functions
by crosslinking actin filaments 16. 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 significance
of a properly organized cytoskeleton to the maintenance of normal foot process function.
The podocyte too may be pivotal in the glomerular injury of DMS/DDS, which are associated with defects in
the Wilms' tumor suppressor gene (WT1; 11p13). WT1 encodes a zinc-finger transcription factor involved in
kidney and gonadal differentiation by modulating (usually repressing) the activity of other proteins such as
PAX2, IGF2, PDGF, EGFR, etc. 17. More than 60 constitutional de novo mutations in WT1 have been identified
in patients with the complete or incomplete form of DDS and a small percentage of patients, usually female,
with isolated DMS18-20. Most are missense changes in either exon 8 or 9, encoding zinc-fingers 2 and 3,
respectively, that interfere with DNA binding. DNA analysis for mutations in WT1 is currently performed in
this country on a research basis only.
The podocytes are responsible for basement membrane production and are the cells morphologically altered in
DMS. The PAX2 transcription factor and WT1 are inversely expressed in early kidney development, as WT1
represses PAX221. In the mature glomerulus, WT1 persists only in the podocytes and PAX2, only in the
parietal epithelium. Abnormal PAX2 expression, in conjunction with diminished WT1, has been identified in
the podocytes of children with DDS and isolated DMS whereas none of the nephrotic control subjects had
altered expression. Though its role has not been thoroughly dissected, WT1 is directly involved in
nephrogenesis and mutations, through a variety of cascades, may lead to defective glomerular structures.
And so the saga continues...
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Table 1 - Congenital, early and late onset nephrotic syndromes