Congenital Nephrotic Syndrome

Shanthi S. Balani

Sarah J. Kizilbash

INTRODUCTION

Nephrotic syndrome is a heterogeneous group of disorders classically characterized by a triad of massive proteinuria, hypoalbuminemia, and edema. Based on the age of onset, it has been classified into three types: congenital nephrotic syndrome (CNS) manifesting in utero to 3 months of age, infantile nephrotic syndrome presenting between 3 months to 1 year of age, and childhood nephrotic syndrome occurring after 1 year of age.

This distinction is based on clinical observations of a more severe course in children with CNS. They have a worse prognosis and rapidly progress to end-stage kidney disease. Since the identification of NPHS1 mutation in 1998, a growing number of genetic defects involving the glomerular filtration barrier (GFB) have surfaced, revolutionizing our understanding of CNS pathogenesis. Although the genetic defects may manifest at any age without a clear genotype-phenotype correlation, this distinction helps guide the evaluation and management of CNS.¹

ETIOLOGY/PATHOGENESIS

Urinary losses of macromolecules, specifically albumin, caused by the dysfunction of the GFB is central to the pathophysiology of CNS. The GFB is composed of three interacting complex layers: the glomerular fenestrated endothelium, the glomerular basement membrane (GBM), and the podocytes with their interdigitating foot processes interconnected by slit diaphragms. Maintaining the integrity of slit diaphragm, which is a highly specialized intercellular junction between podocytes, is crucial for maintaining the size and charge selectivity of the GFB.²,³ Most genes implicated in CNS encode proteins related to the podocytes and slit diaphragms (Figure 7.1). Understanding the genetics has helped unravel the central role of GFB and podocytes in the pathogenesis of nephrotic syndrome.

PRIMARY/GENETIC CONGENITAL NEPHROTIC SYNDROME

Genetic defects are the most common cause of GFB dysfunction. Some of these defects present as syndromes including characteristic extrarenal manifestations. A monogenic cause is reported in 69% of children with CNS and 50% of children with infantile forms of nephrotic syndrome. Although CNS has been linked to more than 55 genes (Table 7.1), mutations in five genes account for >80% of the cases: NPHS1 (39%-61%), NPHS2 (15%-39%), LAMB2 (2%-4%), WT1 (˜2%), or PLCE1 (˜2%).⁴,⁵

NPHS1 (Nephrin)

NPHS1 encodes nephrin, which is the principal component of the slit diaphragm and is integral for its functioning. It is an immunoglobulin superfamily
transmembrane protein that forms heterodimers essential for connecting adjacent foot processes and regulating podocyte polarity. It also transduces phosphorylation-mediated signals that play a role in actin polymerization.⁶

FIGURE 7.1: Schematic of podocyte and glomerular filtration barrier (GFB). The GFB is made up of the podocytes, the glomerular basement membrane (GBM), and the fenestrated endothelial cells. The podocyte possesses a unique actin cytoskeleton composed of F-actin and nonmuscle myosin including nonmuscle myosin heavy chain 9 (MYH9) and myosin 1E (MYO1E). It also has actin-binding proteins such as synaptopodin and α-actinin 4 (ACTN4) and actin polymerization regulatory protein inverted formin 2 (INF2). Wilms tumor 1 gene, a nuclear transcription factor, is important in podocyte gene development. The cell-cell junction of the podocyte (the slit diaphragm) is composed of nephrin, podocin, CD2-associated protein, and NEPH1. Podocin is associated with lipid rafts, a signaling domain of the slit diaphragm. It recruits nephrin and NEPH1 to form a signaling complex with other molecules such as transient receptor potential cation channel 6 (TRPC6), growth factor receptor-bound protein 2, and phospholipase C ε-1 (PLCE1) at the slit diaphragm. The apical membrane of the podocyte is formed by negatively charged molecules such as podocalyxin, podoplanin, podoendin, and glomerular epithelial protein-1. RhoA-activated Rac1 GTPase-activating protein (ARHGAP24) and its regulator RhoGDIα (ARHGDIA) are critical for maintaining podocyte cell shape and membrane dynamics. COQ6 and other regulators of the coenzyme Q10 biosynthesis pathway are important in podocyte energy regulation. The basal part of the podocyte contains α3β1 integrin and α- and β-dystroglycans that anchor the podocyte to the GBM. Talin, paxillin, and vinculin (TPV) interact with different laminins in the GBM, especially laminin β2. Mutations in many of the aforementioned proteins have been associated with nephrotic syndrome. (From Rheault MN, Gbadegesin RA. The genetics of nephrotic syndrome. J Pediatr Genet. 2016;5(1):15-24. doi:10.1055/s-0035-1557109. © Georg Thieme Verlag KG.)

NPHS1 mutation was first identified in Finnish families in 1998. It is the most prevalent cause of CNS. The Fin-major (p.L41fsX91—frameshift and stop codon mutation in exon 2) and Fin-minor (p.R1109X—nonsense mutation in exon 26) account for 78% and 16% of the mutated alleles, respectively, among Finnish patients.⁷ More than 200 different NPHS1 mutations have been identified thus far and involve all 29 exons of NPHS1.⁸

TABLE 7.1 Genes Implicated in the Pathogenesis of CNS

Gene/s	Locus	Protein	Protein Function	Phenotype
Autosomal dominant
WT1	11p13	Wilms tumor 1	Podocyte transcription factor	DMS, DDS, Frasier syndrome, WAGR, ISRNS
LMX1B	17q11	LIM homeobox transcription factor 1, β	Podocyte transcription factor	Nail-patella syndrome
INF2	14q32.33	Inverted formin 2	Actin regulator	FSGS
CD2AP	6p12	CD2-associated protein	Podocyte slit-diaphragm protein	FSGS (adult)
Autosomal recessive
NPHS1	19q13.1	Nephrin	Podocyte slit-diaphragm protein	CNS of Finnish type
NPHS2	1q25-31	Podocin	Podocyte slit-diaphragm protein	Idiopathic CNS, SRNS
LAMB2	3p21	Laminin β2	GBM protein	Pierson syndrome
LAMB3	1q32	Laminin β3	GBM protein	Herlitz junctional epidermolysis bullosa
PLCE1	10q23	Phospholipase Cε1	Podocyte signaling protein	SRNS, DMS
PDSS2	6q21	Prenyl diphosphate synthase subunit 2	Mitochondrial coenzyme Q10 synthesis	Leigh syndrome
ITGA3	17q25.3	Integrin, α3	Adhesion molecule between podocytes and GBM	NS with interstitial lung disease
Autosomal recessive
ARHGDIA	17q25.3	Rho GDP dissociation inhibitor	Podocyte cytoskeletal regulation	Idiopathic CNS
OCRL1	Xq26.1	Inositol polyphosphate-5-phosphatase	Cytoskeletal regulation	Lowe syndrome
COQ2, COQ6, ADCK4	4q21-q22		Mitochondrial coenzyme Q10 synthesis	Some extrarenal manifestations (deafness, seizures, ataxia) May respond to COQ10 supplementation
SCARB2	4q21.1	Lysosomal membrane protein 2	Lysosomal scavenger function	Action myoclonus-renal failure syndrome
LAGE3, OSGEP, TP53RK, TPRKB	Multiple		KEOPS complex (kinase endopeptidase and other proteins of small size)	Galloway-Mowat syndrome
CNS, congenital nephrotic syndrome; FSGS, focal segmental glomerulosclerosis; GBM, glomerular basement membrane; GDP, guanosine diphosphate.

NPHS2 (Podocin)

NPHS2 encodes for podocin, a transmembrane protein that functions on the podocyte’s cytoplasmic side and serves as a vital connection between the slit diaphragm and the podocyte cytoskeleton. It recruits nephrin to the slit diaphragm and therefore plays a vital role in stabilizing nephrin to actin cytoskeleton. More than 100 different mutations in NPHS2 have been described. They are typically associated with milder phenotypes and are more commonly responsible for childhood-onset steroid-resistant nephrotic syndrome. However, severe mutations causing complete loss of function may cause CNS.⁶

LAMB2 (Laminin β2)

Laminins are matrix proteins that are found in GBM. They act as the fundamental framework for GBM and play a function in cell adhesion. LAMB2 encodes Laminin β2, a principal component of Laminin-521 (α5, β2, and γ1), the most important β2 laminin isoform and is specifically expressed in the GBM. It is also expressed in some other sites, like intraocular muscles, and is responsible for the phenotype of Pierson syndrome (Table 7.2).⁶

TABLE 7.2 Clinical Features Associated With Various Causes of Congenital Nephrotic Syndrome

Finnish-type CNS (NPHS1 encoding nephrin)	Prototype of CNS High incidence in Finland 1:8,200 live births Associated with premature birth Large placenta (>25% of birth weight) Proteinuria in utero, severe presentation with onset of edema in the first week of life Progressive kidney failure and ESKD by 2-3 yr of age Large and hyperechoic kidneys on ultrasound, with eventual loss of corticomedullary differentiation Biopsy: irregular microcystic dilatation of tubules Electron microscopy: complete absence of slit diaphragms and podocyte effacement
WT1 (Wilms tumor suppressor gene)⁹	Autosomal dominant inheritance A variable spectrum of phenotypes: Denys-Drash syndrome Early onset with diffuse mesangial sclerosis and rapid progression to ESKD Male pseudohermaphroditism A propensity for tumors such as Wilms tumor or gonadoblastomas WAGR: Wilms tumor, aniridia, genitourinary anomalies, and mental retardation Isolated diffuse mesangial sclerosis Frasier syndrome: gonadal dysgenesis and male pseudohermaphroditism, gonadoblastoma, FSGS
Pierson syndrome LAMB2 (Laminin B2)¹⁰,¹¹	Microcoria-congenital nephrosis syndrome CNS with diffuse mesangial sclerosis Ocular malformations: microcoria, abnormal lens including cataracts, retinal abnormalities Neurologic: neurodevelopmental retardation, hypotonia
Galloway-Mowat syndrome¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷	Microcephaly, intellectual disability, hiatal hernia, skeletal anomalies Biopsy: minimal change or FSGS Numerous genes implicated
Herlitz junctional epidermolysis bullosa¹⁸,¹⁹,²⁰	Recurrent blister formation due to structural fragility of the skin secondary to involvement of genes in collagen, integrins, or laminin synthesis
Congenital syphilis²¹,²²	Kidney involvement variable Presents as CNS when severe, around 2-3 mo of age Other stigmata of congenital syphilis: hepatosplenomegaly, persistent rhinitis, hepatitis, skin lesions, neurosyphilis Biopsy: membranous nephropathy and mesangial proliferation Penicillin treatment results in resolution.
Congenital toxoplasmosis²³	Rarely manifests as CNS Other stigmata of toxoplasmosis: ocular and neurologic Biopsy: mesangial proliferation with or without FSGS Resolution with antimicrobials
CNS, congenital nephrotic syndrome; ESKD, end-stage kidney disease; FSGS, focal segmental glomerulosclerosis.

PLCE1 (PLCε1) (Phospholipase Cε1)

PLCε1 catalyzes the hydrolysis of phosphoinositides to generate two second messengers—inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG). These second messengers play an important role in signaling and initiating intracellular pathways of cell growth and differentiation. PLCε1 is abundantly expressed in embryonic glomeruli and is essential for their development. Its absence causes glomerular growth to halt at the capillary loop stage, with a significant reduction in nephrin and podocin expression. In mature glomeruli, it plays a role in cell junction and signaling events.⁶

WT1 (Wilms Tumor Suppressor Gene)

WT1 is a transcription factor that binds to the DNA and regulates the expression of several genes. This tumor suppressor gene is crucial for gonadal and kidney development in utero, where it is widely expressed and later localized to podocytes of mature kidneys and plays a role in podocyte stabilization.

SECONDARY CONGENITAL NEPHROTIC SYNDROME

Secondary CNS may be caused by congenital infections (Table 7.3). However, the possibility of a genetic mutation should be considered if proteinuria persists despite infection treatment.²⁴ CNS has been reported in the setting of cytomegalovirus (CMV) infection where genetic mutations were discovered when patients failed to respond to anti-CMV therapy.²⁵,²⁶

Other rare causes of secondary CNS include maternal lupus, mercury poisoning, and fetomaternal antineutral endopeptidase alloimmunization.²⁹

CLINICAL FEATURES

Depending on the extent of proteinuria, CNS encompasses a spectrum of clinical phenotypes ranging from minimal symptoms to critical illness with significant anasarca and hemodynamic instability. Additional clinical features of CNS may vary depending on the underlying cause. Table 7.3 presents clinical characteristics of common mutations and congenital infections associated with CNS.

DIAGNOSIS

The diagnosis is often based on clinical features of proteinuria, hypoalbuminemia, and edema. History and physical examination findings may provide clues to the underlying etiology. The recommended evaluation for children with CNS is outlined in Table 7.4 and Figure 7.2.

TABLE 7.3 Secondary Causes of CNS

Infectious Causes

Cytomegalovirus

Congenital syphilis

Toxoplasmosis

Malaria

Rubella

Hepatitis B

Hepatitis C

HIV²⁷

Other Causes

Maternal SLE²⁸