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Background

Fibronectin glomerulopathy (FNG) is a rare autosomal dominant glomerulopathy that manifests at various ages in both sexes [1]. Common clinical features are mild proteinuria and varying degrees of hematuria, hypertension and slow progression to end-stage renal disease over 15–20 years [1]. Decline of kidney function over time is variable. Serum fibronectin levels are usually normal, and systemic manifestations have not been reported. Recurrence in the transplant may occur [2].

Currently, there is no specific treatment for fibronectin glomerulopathy.

 

Case report

A 22-year-old Italian female presented to our department for evaluation of sub-nephrotic proteinuria without edema. She didn’t take any medications and her medical history was indifferent. Her blood pressure was 120/80 mmHg, heart rate 85bpm. Blood chemistry tests showed BUN 17 mg/dL, serum creatinine 0,6 mg/dL. Her estimated glomerular filtration rate (eGFR) was 120 mL/min/1.73m², as calculated using the CKD-EPI equation; urinalysis revealed proteinuria and microscopic hematuria in dipstick; the 24-hour urine collection showed urine total protein of 1700 mg/day (Table 1).

Blood test
White Blood Cell	9400 /μL
Hemoglobin	12.3 g/dL
Platelet	303×103/μL
Total protein	6.3 g/dL
Albumin	3.6 g/dL
Urea nitrogen	17 mg/dL
Creatinine	0.6 mg/dL
eGFR	120 mL/min/1.73m2
Cholesterol	194 mg/dL
HDL/LDL	58/120 mg/dL
Triglycerides	154 mg/dL
C-reactive protein	0.05 mg/dL

 

Immunoserology
IgG	996 mg/dL
IgA	346 mg/dL
IgM	230 mg/dL
Complement 3	136 mg/dL
Complement 4	23 mg/dL
ANA	neg
Anti-GBM antibody	neg
MPO-ANCA	neg
PR3-ANCA	neg
PLA2R	neg
HBs-Ag	–
HCV-Ab	–

 

Urinalysis
specific gravity	1.020
pH	6.2
Red Blood Cell	5-10/high power
Protein	1700mg/24h

Table 1. Laboratory data from initial admission.

Renal ultrasound showed kidneys with normal size and increased parenchymal echogenicity.

A renal biopsy was performed: immunofluorescent staining revealed mild glomerular deposition of IgA, C3 and Lambda while the results for IgG, IgM, C4, Kappa and Fibrinogen were negative. Light microscopy examination detected thirteen glomeruli one of which was sclerotic. The other glomeruli had normal size, exhibited significant mesangial expansion and lobular accentuation, normal glomerular basement membranes, thickening of Bowman’s capsule without extracapillar proliferation. The renal tubular epithelial cells were vacuolated, and there were no pathological changes in the renal interstitium, it appeared edematous with diffuse inflammatory infiltrates. (Figure 1, 2, 3 e 4)

Congo-red staining was negative for amyloid; electron microscopy showed mesangial and subendothelial electron-dense deposits (EDD) with high density, mostly granular with focal fibrillary substructure (Figura 5).

Immunohistochemistry for CD3 showed strong positive staining in the interstitial inflammatory infiltrate while immunohistochemistry for CD20 and CD138 were negative. Also immunohistochemistry for DNAJB9 was negative.

Genetic testing of whole-blood samples (using the Sanger method) revealed an FN1 gene mutation with a thymine changed to adenine at nucleotide 5773 of the complementary DNA (c.5773T>A) causing a substitution in which the tryptophan at amino acid 1925 is replaced by arginine (W1925R).

The conclusive diagnosis was Fibronectin glomerulopathy (FNG).

Family history was investigated. The patient’s brother had never undergone nephrological evaluation before. Following sister’s diagnosis of the disease, he underwent urinalysis, which revealed proteinuria and microscopic hematuria too – in normal renal function; consequently, genetic testing was performed, confirming the same pathogenic mutation identified in his sister (W1925R).

None of the patient’s parents showed renal disease or proteinuria; her father died young without apparent cause.

The patient started conservative therapy with ACE-I and low salt diet. Three years after kidney biopsy, serum creatinine was 0.5 mg/dL (eGFR 125 mL/min/1.73m²) and proteinuria reduced to 0.7g/24h, chemistry panel was unchanged.

 

Discussion

In this report, we present a case of FNG with a pathological diagnosis of Membranoproliferative Glomerulonephritis (MPGN). The diagnosis was confirmed by Electron Microscopy (EM) and genetic analysis – patient’s parents did not have a history of kidney disease – with later genetic diagnosis confirmed also in the patient’s brother as well.

FNG is caused by mutations in the fibronectin 1 gene (FN1) on chromosome 2 [3] and is featured by massive deposition of mutant fibronectin in the mesangium and along capillary walls [2, 4, 5].

Molecular Background

Fibronectin (FN) is a high-molecular-weight glycoprotein component of the extracellular matrix. It is normally produced by the liver and renal mesangial cells [2]. FN is present in plasma as a soluble form (pFN) or deposited in extracellular matrix as insoluble organized fibrils (cellular FN) [6]. The Hep-II and -III domains play a main role in regulating FN assembly into organized fibrils in extracellular matrix, through complex FN–FN and FN–cell surface proteoglycan interactions [7–10].

The pathogenic mechanism of fibronectin accumulation is not completely understood but may involve the production of a fibronectin variant that cannot be cleared, or the formation of a variant fibronectin formed by attachment of a circulating factor. The deposits consist predominantly of the soluble plasma-derived form of fibronectin, rather than the insoluble cellular form. Another proposed mechanism is a defect in the catabolism of fibronectin [2].

Fibronectin 1 (FN1) gene mutation is usually detected in about 40% of patients in Castelletti study group [3] and is believed to be responsible for the occurrence of the disease. However, no specific treatment is currently available. [3, 11, 12]. The W1925R variant identified in the present case was reported in literature to cause glomerulopathy with fibronectin deposits [3]: in this study W1925R variant was identified in all affected by FG and was not found in any of 100 healthy subjects [3].

They sequenced the FN1 in 15 unrelated pedigrees and found three heterozygous missense mutations, the W1925R, L1974R, and Y973C, that cosegregated with this glomerulopathy [3].

Mutations in the FN1 gene have been implicated in a variety of collagen-related diseases due to fibronectin’s essential role in the extracellular matrix and its interactions with collagen. Studies have shown that alterations in FN1 have been linked to early-onset osteoarthritis, where impaired interactions between fibronectin and collagen type II affect cartilage integrity [13]. Furthermore, skeletal dysplasias have been associated with FN1 mutations, as fibronectin is critical for the correct formation of collagen fibers in bones [14]. In the context of Ehlers-Danlos syndrome, a connective tissue disorder primarily caused by defects in collagen, FN1 mutations may exacerbate collagen dysfunction, further destabilizing the extracellular matrix and contributing to the clinical manifestations of the disease [15].

Our W1925R mutation introduces a basic amino acid in the Hep-II hydrophobic core; this mutation could theoretically increase the Hep-II affinity for heparin, by providing additional cationic charge to the domain; however, it could also alter the folding of the domain and impair its function. So they suggest that GFND-associated mutations in FN1 impair the control of the assembly of FN into fibrils and the balance between soluble and insoluble FN, which could explain the abnormal incorporation of nonfibrillary pFN in the glomerular matrix that has been documented in renal biopsy [3].

Pathophysiology

Usually, the biopsy shows by light microscopy lobular accentuation with mesangial expansion with minimal hypercellularity and variable expansion of glomerular basement membranes by strongly periodic acid-Schiff-positive and silver-negative material. Congo red stain is negative. There are nonspecific tubulointerstitial and vascular changes with increased fibrosis with progression of disease. Immunofluorescence/immunohistochemistry microscopy is usually negative, but may show nonspecific staining for immunoglobulins and C3 [2]. Electron microscopically, fibronectin deposition is shown as finely granular or fibrillary substructures with randomly arranged 12–16-nm fibrils [2, 4, 5] and we could testify it with the help of electron microscope.

Clinical Management

Ti Zhang et al. Reported a case series of 19 patients with FNG diagnosis that were treated with renin-angiotensin system blockade, including 11 patients who were treated with Tripterygium Wilfordii Hook (TWHF), and 4 patients with corticosteroid therapy in combination with immunosuppressive therapies, including 2 with mycophenolate mofetil (MMF) and 2 with tacrolimus. The mean follow-up duration was 78 months (range 14–147 months, median 87). At last follow-up, 7 patients progressed to ESRD despite supportive therapy and required initiation of dialysis, 2 of whom received renal transplantation [16]. In most of cases reported in literature, corticosteroid therapy doesn’t help to reduce or prevent kidney disease progression [17, 18]. Steroid therapy has been tested, but its effectiveness is controversial. Prednisolone treatment decreased proteinuria in some patients with nephrotic-level proteinuria [19] but did not yield a clear treatment response in other patients [17, 18] and is commonly attempted in cases with a histological diagnosis of MPGN [20].

Currently angiotensin-converting enzyme inhibitors and ARBs are generally used for renal protection. The initiation of steroid therapy for FNG should be carefully considered on an individual basis [17].

 

Conclusion

We encountered a case of FNG in a 22-year-old female patient presenting with sub-nephrotic range proteinuria, supported by both histological features of membranoproliferative glomerulonephritis on kidney biopsy and subsequently confirmed by genetic testing. Following the patient’s diagnosis, genetic screening was also extended to first-degree relatives (brother), with a positive result for the disease.

We therefore decided to use a conservative therapy, because of the steroids side effects and the absence of nephrotic syndrome in our patient, with a normal kidney function.

After three years kidney function remained stable and proteinuria reduced to 0.7 g/die.

 

Bibliography

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