Fibrillary glomerulonephritis (FGN) is a rare disease with unknown aetiopathogenesis and poor outcome. Electron microscopy has been the only method to establish diagnosis. However, now, DNAJB9 can help with diagnosis of FGN with 99 to 100% sensitivity. Here, Dr. NK Hase writes on the novel tissue biomarker.

Until recently, diagnosis was mainly based on electron microscopic (EM) demonstration of randomly oriented straight fibril10-30 nm thickness which lack hollow centre. They are Congo red negative and stain with antisera to immunoglobulin. EM is not available in all laboratories even in developed countries.

A novel proteomic tissue biomarker for FGN namely DNAJB9 has been discovered by researchers. DNAJB9 immuno- histochemistry is highly sensitive (98 to 100%) and specific (99 to 100%) for FGN. Availability of this novel immune histochemical biomarker DNAJB9 into clinical practice will now allow for more rapid and accurate diagnosis of FGN. Discovery of DNAJB9 can be considered as a landmark discovery in the diagnosis of glomerular diseases.

In 1977 Rosemann and Eliakim first reported a FGN in a patient who presented with nephrotic syndrome. The histopathology showed an amyloid material in glomeruli but on EM fibrils appeared shorter than amyloid fibrils. In 1983, Duffy et al reported 8 patients presenting with haematuria, proteinuria, hypertension, and renal insufficiency with histopathology showing Congo red negative deposits with EM findings of fibrils wider than amyloid with 20 to 1500 nm in length. They labeled it as ‘Renal fibrillosis’. Finally Alpers et al introduced the term Fibrillary Glomerulonephritis (FGN).

Diagnosis of FGN

For the diagnosis of FGN, EM is mandatory. More than four decades since the first reporting of this disease there are several limitations of current means of diagnosis of FGN.

1. Diagnosis of FGN remains a challenge.
2. No single feature on clinical history, light microscopy IF or EM is diagnostic of this disease.
3. Patients with FGN present with nephritic, nephrotic syndrome, and rapidly progressive renal failure.
4. Renal prognosis is poor with 50% of patient’s progress to end stage within four years of diagnosis.
5. The 10-year patient and renal allograft reported were 100% and 67% respectively compared to other patients with ESRD.
6. FGN recurs in 50% of transplanted kidneys but recurrence is often benign.

The LM findings of mesangial expansion and hypercellularity can be lacking in early stages of the disease and can also be seen in several other glomerular diseases like IgA nephropathy, lupus nephritis, and infection related GN. The nodular sclerosis can mimic amyloid, diabetic nephropathy, immunotactoid, fibrinectin glomerulopathy, monoclonal gammopathy (light chains deposition disease) and collagenofibrotic glomerulopathy.

IF findings can also overlap with other form of immune-complex mediated GN such as lupus nephritis, membranous nephropathy and immunotactoid glomerulopathy. On EM, FGN can be difficult to distinguish from other glomerular disease that exhibit fibrils or small microtubules like amyloidosis and fibrinonectin glomerulopathy. The recent discovery through use of laser microdissection assisted liquid chromatography- tandem mass spectrometry for DNAJB9, a novel biomarker for FGN has almost resolved this diagnostic puzzle .

DNAJB9 biomarker

DNAJB9 (DNAJ homologue sub-family B member 9) belongs to a family of proteins that acts as ‘co-chaperons’ to heat shock proteins 70 (HSP 70). The molecular chaperons are important for proper folding, unfolding, translocations or degradation of proteins. It is also known as micro- vascular endothelial differential gene 1 protein or endoplasmic reticulum, DNAJ domain containing protein 4. DNAJB9 is composed of 223 amino acids with molecular weight of 25.5 kDa. It is expressed in all healthy tissues and localised to the endoplasmic reticulum (ER). DNJB9 is induced by ER stress,-activated macrophages, nitric oxide and other inflammatory stimuli.

1. Lee et al demonstrated that DNAJB 9 is induced by p53. This is primarily mediated through Ras/Raf/ERk pathways. There are several functions of DNAJB9 that have been described. It protects against cell death by inhibiting the pro-apoptic functions of p53, maintains B cell development function and integrity of hematopoietic stem cells during ER stress.
2. Dasari et al from Mayo Clinic analysed protein contents of glomeruli in patients with fibrillary GN and discovered DNAJB9 as 4th most abundant molecule in this group and it was absent in glomeruli of patients with amyloidosis, non-fibrillary GN and healthy cohort.
3. Nasr SH et al developed immune histochemistry (IHC) for DNAJB9 and tested in 84 cases of FGN, 21 cases of renal amyloidosis, 98 cases of non-FGN glomerular disease and 11 normal subjects. DNAJB9 IHC had 98% sensitivity and 99% specificity for FGN.
4. A research team from University of Washington, Seattle detected DNAJB9 in 7 of 8 FGN samples by laser micro dissection assisted liquid chromatography-tandem mass spectroscopy. They also demonstrated co-localisation of DNAJB9 with Ig G by IF and identified DNAJB9 in 11 of 11 cases of FGN but not in 31 non FGN cases by IHC.

The findings from these two groups established DNAJB9 as the first biomarker for FGN. DNAJB9 IHC positivity in FGN is characterised by strong smudgy glomerular extracellular staining, mesengial and glomerular capillary walls which corresponds to smudgy IgG and IgG4 IF staining. The pathogenesis of FGN is unknown. DNAJB9 has been proposed to be an auto-antigen in FGN. It is conceivable that a misfolded DNAJB9 molecule is formed and deposited in glomeruli. This theory is yet to be proved; circulating anti DNAJB9 auto antibodies have not been identified.

DNAJB9 has been confirmed as an excellent tissue biomarker for FGN. Recently an immunoprecipitation- based multiple reaction method to measure serum/urine level of DNAJB9 has been developed. It showed a 4 fold higher level of serum DNAJB9 in patients with FGN than in controls. This method is complex and awaits validation.

Conclusion

DNAJB9 is an excellent tissue biomarker for diagnosis of FGN with high specificity and sensitivity. DNAJB9 IHC provides prompt and early diagnosis of FGN which can be used by most laboratories throughout the world. Now, further studies are required to prove its role in pathogenesis and whether serum or urinary level can serve as a non-invasive marker of FGN. Finally therapies targeting DNAJB9 should be investigated. Theoretically, since the induction of DNAJB9 is mediated through Ras/Raf/ERK pathways, inhibitors of pathway like sorafenib may be beneficial for the treatment.

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

The author Dr. NK Hase is a Director clinical Nephrology & Transplant working at Jupiter Hospital, Thane.