Hyperphosphatemic pseudotumoral calcinosis due to FGF23 mutation with secondary amyloidosis

Abstract

A 44 years old man was admitted for nephrotic syndrome and rapidly progressive renal failure. Two firm, tumour-like masses were localized around the left shoulder and the right hip joint. Since the age of 8 years old, the patient had a history of metastatic calcification of the soft tissues suggesting hyperphosphatemic pseudotumoral calcinosis. Despite treatment for a long time with phosphate binders the metastatic calcinosis had to be removed with several surgeries. The patient had also a history of recurrent fever associated with pain localized toward the two masses and underwent multiple antibiotic courses. Laboratory findings at admission confirmed nephrotic syndrome. S-creatinine was 2.8 mg/dl. Calcium was 8.4 mg/dl, Phosphorus 8.2 mg/dl, PTH 80 pg/ml, 25 (OH)VitD 8 ng/ml. Serum amyloid A was slightly increased. We performed renal biopsy and we found AA amyloid deposits involving the mesangium and the tubules. The bone marrow biopsy revealed the presence of AA amyloid in the vascular walls. During the next two months renal failure rapidly progressed and the patient started hemodialysis treatment. We performed genetic analysis that confirmed homozygous mutation of the FGF23 gene. After 14 months on hemodialysis, the patient’s lesions are remarkably and significantly reduced in dimension. The current phosphate binder therapy is based on sevelamer and lanthanum carbonate. Serum amyloid A is persistently slightly increased as well as C reactive protein. Proteinuria is in the nephrotic range without nephrotic syndrome.

Keywords: pseudotumoral calcinosis, tumoral calcinosis, FGF23, AA amyloid, renal failure, dialysis

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Introduzione

La calcinosi pseudotumorale iperfosfatemica è una rara condizione dovuta a deficit o resistenza all’azione del fibroblast growth factor 23 (FGF23). Dal punto di vista genetico, essa è associata a varianti patogenetiche a trasmissione autosomica recessiva, nei geni codificanti per FGF23 [21] e GALNT3. Quest’ultimo, a sua volta, codifica per una proteina responsabile della glicosilazione di FGF 23 [2123] e KL, regolatrici di KLOTHO, noto co-recettore fondamentale per la trasmissione intracellulare di FGF23 [3]. Ricordiamo che l’azione di FGF23 (Klotho-dipendente) si manifesta in modo simile a quella del paratormone (PTH) a livello del tubulo prossimale, dove inibisce i cotrasportatori sodio-fosforo IIa e IIc, con conseguente effetto fosfaturico. Sull’attivazione della Vit D3, invece, l’azione è opposta a quella del PTH, infatti FGF23 inibisce l’attività della 1-alfa idrossilasi e, quindi, causa una riduzione della 1-25 (OH) Vit D con conseguente inibizione, regolata dal cotrasportatore sodio-fosforo IIb, dell’assorbimento intestinale sia di calcio che di fosforo. In corso di calcinosi pesudotumorale iperfosfatemica, dunque, le mutazioni portano alla perdita della funzione fosfaturica dell’FGF23 con incremento del riassorbimento tubulare del fosforo e dell’1,25 diidrossi Vitamina D; il calcio di solito è normale-elevato e i livelli di PTH sono di solito normali o bassi. 

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Comparison between Creatinine Clearance and eGFRcyst-crea: a real-life experience

Abstract

Introduction: The evaluation of renal function is computed using the estimated glomerular filtration rate methods or the measured glomerular filtration rate. Cystatin C has been well studied as marker of renal function compared to serum creatinine, but only few studies compare Glomerular Filtration Rates estimated including both creatinine and cystatin (eGFRcyst-crea) to creatinine clearance (CrCl). This cross-sectional study compares CrCl and eGFRcyst-crea with eGFRcrea and searches for correlation with comorbidities.
Methods: This cross-sectional study consists of 78 patients hospitalized for acute and/or chronic renal disease. We performed the concordance correlation coefficient analysis between the eGFRcrea and the CrCl and eGFRcyst-crea in the whole sample and in the various subgroups.
Results: Steiger’s comparison of correlations from dependent samples showed a correlation coefficient between C-reactive protein and eGFRcyst-crea stronger than between C-reactive protein and CrCl (Z: 2.51, p=0.012). Similar results were showed with the association with procalcitonin (Z: 5.24, p<0.001), serum potassium (Z: -3.13, p=0.002), and severe CKD (Z: -2.54, p=0.011). The concordance correlation coefficient test showed major differences between diagnostic methods compared to eGFR-crea in diabetic subgroup, severe CKD, and in procalcitonin higher than 0.5ng/ml. Discussion: The demonstration of a strong concordance between the eGFRcrea and the eGFRcyst-crea allows us to diagnose and to stage CKD better than creatinine clearance in patients with high inflammatory status. Furthermore, this information opens new research scenarios, and further, larger studies are needed to confirm these hypotheses.
Keywords: Phosphorus, Hemoglobin, Anemia, Chronic Kidney Disease, FGF23, Generalized estimating equation

Introduction

The association of cystatin C with renal function has been studied for more than 25 years. Cystatin C has been described to have better diagnostic performance than creatinine for assessing renal function, particularly in detecting small reductions in glomerular filtration rate (GFR).

Since cystatin C is a low-molecular-weight protein produced by all nucleated cells, it is less influenced by variables such as age, body weight or diet and it has been proposed as a more reliable marker of kidney function than serum creatinine. 

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