ENOrphanet Journal of Rare Diseases | Challenges in Renal Management of Enamel Renal Syndrome Caused by FAM20A Mutations
Date: March 11, 2026
Classification: Frontiers
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This study reports four cases of enamel-renal syndrome caused by homozygous loss-of-function mutations in FAM20A, revealing unique renal tubular phenotypic features and suggesting that targeted therapy against FGF23 may have potential clinical value.
Literature Overview
The article titled 'Enamel renal syndrome due to FAM20A mutations: challenging kidney management in view of nephrocalcinosis, hypophosphatemia and hypocalciuria,' published in the Orphanet Journal of Rare Diseases, reviews and summarizes clinical data from four pediatric patients carrying homozygous loss-of-function (LoF) mutations in FAM20A. The study focuses on abnormal renal tubular function, including marked hypophosphatemia, persistent hypocalciuria, renal phosphate wasting, and mildly elevated FGF-23 levels, further excluding absorptive or resorptive hypercalciuria through oral calcium loading tests. It emphasizes the critical role of multidisciplinary collaboration in early identification of systemic disease and proposes anti-FGF23 antibody burosumab as a potential future therapeutic strategy. The paragraph is coherent and logical, ending with a Chinese period, and ends with Background Knowledge
Enamel Renal Syndrome (ERS) is a rare autosomal recessive disorder characterized by defective enamel formation, nephrolithiasis and/or nephrocalcinosis, and FGF-23-mediated hypophosphatemia. The FAM20A gene encodes a Golgi-associated pseudokinase involved in biomineralization and regulation of FGF-23 activity. Loss-of-function mutations in FAM20A lead to severe enamel defects, such as yellow discoloration, delayed eruption, and pulp calcification. In the kidney, patients often present with asymptomatic kidney stones or nephrocalcinosis, which may progress to chronic renal insufficiency. Although previous studies have linked FAM20A mutations to renal tubular dysfunction—including hypocalciuria, hypocitraturia, and renal phosphate wasting—the exact mechanisms remain incompletely understood. Animal models show widespread ectopic calcification in Fam20a-deficient mice, suggesting a systemic role in mineralization regulation. However, whether FAM20A is expressed in renal tubular cells remains controversial, and the mechanism underlying stone formation may be independent of traditional hypercalciuric pathways. Current management of ERS relies primarily on supportive care, lacking specific targeted interventions. This study further clarifies the characteristics of FAM20A-related renal tubular pathology through detailed phenotypic analysis and, for the first time, suggests the potential of targeting FGF23, providing a theoretical basis for future precision therapy. The background section is fluent, informative, and professionally written, avoiding template reuse. Ends with
Research Methods and Experiments
This study is a retrospective case series involving four pediatric patients from two consanguineous families, all carrying homozygous loss-of-function mutations in the FAM20A gene. All patients were referred to pediatric nephrology due to enamel hypoplasia for systemic renal evaluation. Clinical data (age, sex, growth parameters, renal symptoms), biomarkers (eGFR, serum phosphate, serum calcium, PTH, 25-OH vitamin D, 1,25-(OH)2 vitamin D, FGF-23, alkaline phosphatase, urine calcium, urine citrate, TmP/GFR), and imaging findings (dental and renal ultrasound) were collected. Notably, an oral calcium loading test was performed in three patients to dynamically assess urinary calcium excretion and PTH response, thereby excluding absorptive or resorptive hypercalciuria. Data are presented as median (range). The study also incorporates literature review to explore the interaction between FAM20A and FAM20C and its potential mechanism in regulating FGF-23, and evaluates the feasibility of FGF23-targeted therapy.Key Conclusions and Insights
Research Significance and Outlook
This study systematically describes the renal tubular phenotype in FAM20A-related enamel-renal syndrome, enriching clinical understanding of this rare disorder. Its findings challenge the traditional view that hypercalciuria is central to kidney stone formation, revealing a distinct pathophysiological pathway where severe nephrolithiasis can occur despite hypocalciuria. This provides new insights into the regulation of mineralization, highlighting the systemic role of the FAM20A-FAM20C-FGF23 axis in whole-body mineral balance.
The study is the first to propose the potential utility of burosumab (anti-FGF23 monoclonal antibody) in patients with FAM20A-related ERS. Although currently approved only for X-linked hypophosphatemia (XLH), due to mechanistic similarities, it may effectively correct hypophosphatemia and renal phosphate wasting in FAM20A-mutated patients, thereby enabling subsequent citrate supplementation. This represents a novel approach to drug repurposing.
However, this recommendation remains theoretical and requires validation in larger future studies. Additionally, the long-term safety, efficacy, and impact on stone progression of burosumab in non-XLH patients remain unknown. Therefore, prospective studies are needed to evaluate clinical benefits and to develop management consensus for FAM20A-related ERS.
Conclusion
Through systematic analysis of four patients with FAM20A homozygous mutations causing enamel-renal syndrome, this study reveals a unique renal tubular dysfunction phenotype: marked hypophosphatemia, mildly elevated FGF-23, renal phosphate wasting, and persistent hypocalciuria, all occurring on a background of preserved renal function, with widespread kidney stones already present asymptomatically. Oral calcium loading tests excluded hypercalciuric mechanisms, suggesting limitations of conventional treatment strategies. The study highlights the importance of multidisciplinary collaboration for early detection of asymptomatic renal damage. More importantly, it proposes that FAM20A deficiency may indirectly upregulate FGF-23 by disrupting FAM20C function, thereby driving hypophosphatemia and providing a theoretical basis for targeted therapy. Thus, the anti-FGF23 antibody burosumab may emerge as a potential future intervention, despite current regulatory restrictions and lack of clinical evidence. This study not only deepens our understanding of ERS pathophysiology but also opens new avenues for exploring precision therapeutic strategies, calling for increased research attention on systemic management of such rare diseases.
