Cutaneous skeletal hypophosphatemia syndrome: clinical spectrum, natural history, and treatment

Burosumab, a Transformational Treatment in a Pediatric Patient With Cutaneous-Skeletal Hypophosphatemia Syndrome

Cutaneous-skeletal hypophosphatemia syndrome (CSHS) is a rare disorder characterized by the presence of melanocytic nevi, dysplastic cortical bony lesions, and fibroblast growth factor 23 (FGF23)-mediated hypophosphatemic rickets. Herein, we describe the diagnosis of an 8-year-old girl presenting with short stature, reduced lower limb mobility, and abnormal gait due to muscle weakness and constant pain in the legs. Biochemical parameters demonstrated hypophosphatemia, hyperphosphaturia, slight increase in parathyroid hormone (PTH), high levels of alkaline phosphatase, and elevated FGF23. Burosumab improved phosphate-wasting, serum phosphorus, alkaline phosphatase, and PTH, followed by a significant mineralization in vertebral bodies evidenced by radiographic assessment. Our report shows a long-term follow-up of CSHS with a notable improvement promoted by an anti-FGF23 therapy.

Exploring New Drug Repurposing Opportunities for MEK Inhibitors in RASopathies: A Comprehensive Review of Safety, Efficacy, and Future Perspectives of Trametinib and Selumetinib

The RASopathies are a group of syndromes caused by genetic variants that affect the RAS-MAPK signaling pathway, which is essential for cell response to diverse stimuli. These variants functionally converge towards the overactivation of the pathway, leading to various constitutional and mosaic conditions. These syndromes show overlapping though distinct clinical presentations and share congenital heart defects, hypertrophic cardiomyopathy (HCM), and lymphatic dysplasia as major clinical features, with highly variable prevalence and severity. Available treatments have mainly been directed to target the symptoms. However, repurposing MEK inhibitors (MEKis), which were originally developed for cancer treatment, to target evolutive aspects occurring in these disorders is a promising option. Animal models have shown encouraging results in treating various RASopathy manifestations, including HCM and lymphatic abnormalities. Clinical reports have also provided first evidence supporting the effectiveness of MEKi, especially trametinib, in treating life-threatening conditions associated with these disorders. Nevertheless, despite notable improvements, there are adverse events that occur, necessitating careful monitoring. Moreover, there is evidence indicating that multiple pathways can contribute to these disorders, indicating a current need to more accurate understand of the underlying mechanism of the disease to apply an effective targeted therapy. In conclusion, while MEKi holds promise in managing life-threatening complications of RASopathies, dedicated clinical trials are required to establish standardized treatment protocols tailored to take into account the individual needs of each patient and favor a personalized treatment.

Burosumab for the treatment of cutaneous-skeletal hypophosphatemia syndrome

Cutaneous-skeletal hypophosphatemia syndrome (CSHS) is a rare bone disorder featuring fibroblast growth factor-23 (FGF23)-mediated hypophosphatemic rickets. We report a 2-year, 10-month-old girl with CSHS treated with burosumab, a novel human monoclonal antibody targeting FGF23. This approach was associated with rickets healing, improvement in growth and lower limb deformity, and clinically significant benefit to her functional mobility and motor development. This case report provides evidence for the effective use of FGF23-neutralizing antibody therapy beyond the classic FGF23-mediated disorders of X-linked hypophosphatemia and tumor-induced osteomalacia.

Inherited fibroblast growth factor 23 excess

Syndromes of inherited fibroblast growth factor 23 (FGF-23) excess encompass a wide spectrum that includes X-linked hypophosphataemia (XLH), autosomal dominant and recessive forms of rickets as well as various syndromic conditions namely fibrous dysplasia/McCune Albright syndrome, osteoglophonic dysplasia, Jansen's chondrodysplasia and cutaneous skeletal hypophosphataemia syndrome. A careful attention to patient symptomatology, family history and clinical features, supported by appropriate laboratory tests will help in making a diagnosis. A genetic screen may be done to confirm the diagnosis. While phosphate supplements and calcitriol continue to be the cornerstone of treatment, in recent times burosumab, the monoclonal antibody against FGF-23 has been approved for the treatment of children and adults with XLH. While health-related outcomes may be improved by ensuring adherence and compliance to prescribed treatment with a smooth transition to adult care, bony deformities may persist in some, and this would warrant surgical correction.

Phacomatosis spilosebacea: A new name for a distinctive binary genodermatosis

Phacomatosis pigmentokeratotica (PPK) is defined by the association of papular nevus spilus arranged in a flag-like pattern and sebaceous nevus following Blaschko's lines. A systematic search of the worldwide literature retrieved 95 well-established PPK cases. An additional 30 cases were excluded for a number of reasons. Based on this study, we propose to rename PPK phacomatosis spilosebacea (PSS). Mosaic mutations of the HRAS gene are the only proven cause of PSS. The extracutaneous abnormalities of PSS result from various degrees of intermingling of Schimmelpenning syndrome and papular nevus spilus syndrome. PSS seems to be a condition at particularly high risk of developing basal cell carcinoma, urogenital malignancies, and vitamin D-resistant hypophosphatemic rickets. Extracutaneous abnormalities were detected in approximately 75% of PSS cases.

Murine models of HRAS-mediated cutaneous skeletal hypophosphatemia syndrome suggest bone as the FGF23 excess source

Cutaneous skeletal hypophosphatemia syndrome (CSHS) is a mosaic RASopathy characterized by the association of dysplastic skeletal lesions, congenital skin nevi of epidermal and/or melanocytic origin, and FGF23-mediated hypophosphatemia. The primary physiological source of circulating FGF23 is bone cells. However, several reports have suggested skin lesions as the source of excess FGF23 in CSHS. Consequently, without convincing evidence of efficacy, many patients with CSHS have undergone painful removal of cutaneous lesions in an effort to normalize blood phosphate levels. This study aims to elucidate whether the source of FGF23 excess in CSHS is RAS mutation-bearing bone or skin lesions. Toward this end, we analyzed the expression and activity of Fgf23 in two mouse models expressing similar HRAS/Hras activating mutations in a mosaic-like fashion in either bone or epidermal tissue. We found that HRAS hyperactivity in bone, not skin, caused excess of bioactive intact FGF23, hypophosphatemia, and osteomalacia. Our findings support RAS-mutated dysplastic bone as the primary source of physiologically active FGF23 excess in patients with CSHS. This evidence informs the care of patients with CSHS, arguing against the practice of nevi removal to decrease circulating, physiologically active FGF23.

The efficacy and safety of burosumab in two patients with cutaneous skeletal hypophosphatemia syndrome

Cutaneous skeletal hypophosphatemia syndrome (CSHS) is an ultra-rare mosaic disorder manifesting as skeletal dysplasia and FGF23-mediated hypophosphatemia, with some experiencing extra-osseous/extra-cutaneous manifestations, including both benign and malignant neoplasms. Like other disorders of FGF23-mediated hypophosphatemia including X-linked hypophosphatemia (XLH) and tumor-induced osteomalacia (TIO), patients with CSHS have low serum phosphorus and active 1,25-dihydroxyvitamin D levels. Current treatment options for patients with CSHS include multiple daily doses of oral phosphorus and one or more daily doses of active vitamin D analog to correct the deficits. Recently, the fully human monoclonal antibody against FGF23 burosumab received US approval for the treatment of XLH and TIO, two rare diseases characterized by FGF23-mediated hypophosphatemia leading to rickets and osteomalacia. Given the similarities between the pathobiologies of these disorders and CSHS, we investigated the impact of burosumab on two patients, one pediatric and one adult, with CSHS who participated in separate, but similarly designed trials. In both the pediatric and adult patients, burosumab therapy was well-tolerated and contributed to clinically meaningful improvements in disease outcomes including normalization of phosphorus metabolism and markers of bone health, and improvements in skeletal abnormalities, fractures, and physical function. Reported adverse events were minimal, with only mild injection site reactions attributed to burosumab therapy. Together, these findings suggest that burosumab therapy is a promising therapeutic option for patients with CSHS.

Approach to Hypophosphatemic Rickets

Hypophosphatemic rickets typically presents in infancy or early childhood with skeletal deformities and growth plate abnormalities. The most common causes are genetic (such as X-linked hypophosphatemia), and these typically will result in lifelong hypophosphatemia and osteomalacia. Knowledge of phosphate metabolism, including the effects of fibroblast growth factor 23 (FGF23) (an osteocyte produced hormone that downregulates renal phosphate reabsorption and 1,25-dihydroxyvitamin-D (1,25(OH)2D) production), is critical to determining the underlying genetic or acquired causes of hypophosphatemia and to facilitate appropriate treatment. Serum phosphorus should be measured in any child or adult with musculoskeletal complaints suggesting rickets or osteomalacia. Clinical evaluation incudes thorough history, physical examination, laboratory investigations, genetic analysis (especially in the absence of a guiding family history), and imaging to establish etiology and to monitor severity and treatment course. The treatment depends on the underlying cause, but often includes active forms of vitamin D combined with phosphate salts, or anti-FGF23 antibody treatment (burosumab) for X-linked hypophosphatemia. The purpose of this article is to explore the approach to evaluating hypophosphatemic rickets and its treatment options.

New treatments for rare bone diseases: hypophosphatemic rickets/osteomalacia

Phosphorus is one of the most abundant minerals in the human body; it is required to maintain bone integrity and mineralization, in addition to other biological processes. Phosphorus is regulated by parathyroid hormone, 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], and fibroblast growth factor 23 (FGF-23) in a complex set of processes that occur in the gut, skeleton, and kidneys. Different molecular mechanisms - overproduction of FGF-23 by tumors responsible for oncogenic osteomalacia, generation of an FGF-23 mutant that is resistant to cleavage by enzymes, and impaired FGF-23 degradation due to a reduction in or loss of the PHEX gene - can lead to FGF-23-stimulating activity and the consequent waste of urinary phosphate and low levels of 1,25(OH)₂D₃. Conventional treatment consists of multiple daily doses of oral phosphate salts and vitamin D analogs, which may improve radiographic rickets but do not normalize growth. Complications of the conventional long-term treatment consist of hypercalcemia, hypercalciuria, nephrolithiasis, nephrocalcinosis, impaired renal function, and potentially chronic kidney disease. Recently, burosumab, an antibody against FGF-23, was approved as a novel therapy for children and adults with X-linked hypophosphatemia and patients with tumor-induced osteomalacia. Burosumab showed good performance in different trials in children and adults. It increased and sustained the serum phosphorus levels, decreased the rickets severity and pain scores, and improved mineralization. It offers a new perspective on the treatment of chronic and disabling diseases.

Disorders of phosphate homeostasis in children, part 2: hypophosphatemic and hyperphosphatemic disorders

Phosphorus, predominantly in the form of inorganic phosphate PO₄^-3, has many essential physiological functions. In the skeleton, phosphate and calcium form the mineral component and phosphate is also essential in regulating function of skeletal cells. Considerable advances have been made in our understanding of phosphate homeostasis since the recognition of fibroblast growth factor-23 (FGF23) as a bone-derived phosphaturic hormone. This second part of a two-part review of disorders of phosphate homeostasis in children covers hypophosphatemic and hyperphosphatemic disorders that are of interest to the pediatric radiologist, emphasizing, but not limited to, those related to abnormalities of FGF23 signaling.

Determination of FGF23 Levels for the Diagnosis of FGF23-Mediated Hypophosphatemia

Fibroblast growth factor-23 (FGF23) measurement is a critical tool in the evaluation of patients with disordered phosphate homeostasis. Available laboratory reference ranges for blood FGF23 were developed using samples from normophosphatemic individuals. Reliance on such values can lead to misdiagnosis in patients with FGF23-mediated hypophosphatemia, such as X-linked hypophosphatemia (XLH) and tumor-induced osteomalacia (TIO), in whom pathology-driving FGF23 levels can be in the "normal range." To determine FGF23 levels that are diagnostic for the identification of patients with FGF23-mediated hypophosphatemic disorders, we studied 149 patients with various disorders of FGF23-mediated and FGF23-independent hypophosphatemia and defined cut-off levels for both intact FGF23 (iFGF23) and C-terminal FGF23 (cFGF23) that can accurately distinguish between FGF23-mediated and FGF23-independent hypophosphatemia. In addition, to demonstrate the relationship between FGF23 and phosphate across the spectrum of human physiology, we assessed blood levels of FGF23 and phosphate in 434 patients with various forms of hypophosphatemia, hyperphosphatemia, and normophosphatemia. An intact FGF23 cut point of 27 pg/mL was 100% sensitive and specific in distinguishing FGF23-mediated from FGF23-independent hypophosphatemia, and a cFGF23 cut point of 90 RU/mL was 100% sensitive and specific in distinguishing specifically TIO from FGF23-independent hypophosphatemia. There was overlap in the cFGF23 range of 45-90 RU/mL between genetic forms of FGF23 excess and FGF23-independent hypophosphatemia, substantiating the superiority of iFGF23 over cFGF23 in making the diagnosis of FGF23-mediated hypophosphatemia. In this cohort, using the laboratory upper limit of normal for cFGF23 (180 RU/mL) would result in a misdiagnosis in more than half of patients with FGF23-mediated hypophosphatemia. In this, the largest study of FGF23 in chronic hypophosphatemia to date, we established iFGF23 and cFGF23 cut-off values to assist in the evaluation and diagnosis of hypophosphatemic conditions. © 2022 American Society for Bone and Mineral Research (ASBMR). This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Fibrous dysplasia in cardio-facio-cutaneous syndrome: A case report and review of literature

Cardio-facio-cutaneous (CFC) syndrome (OMIM #:115150, 615278, 615279, 615280) is a rare genetic condition caused by variants in the RAS/mitogen-activated protein kinase (MAPK) signal transduction pathway. Up to 75% of cases are caused by mutations in the BRAF gene, whereas KRAS gene mutation has only been reported in <2% of cases. CFC syndrome is characterized by cardiac abnormalities, distinctive craniofacial dysmorphism, and various cutaneous abnormalities. Musculoskeletal and orthopedic manifestations are also prevalent in patients with CFC syndrome, among which the most common are skeletal deformities and joint laxities. Dysplastic bone disorders, on the other hand, have not been reported in CFC syndrome before. We report on a case of symmetrical polyostotic fibrous dysplasia (FD) in a patient with CFC syndrome with the KRAS(NM_004985.5):c.57G>C; p.Leu19Phe variant. The FDs were incidentally picked up, and patient was conservatively managed and remained asymptomatic on follow-up. The same variant was reported previously in a patient with Oculoectodermal Syndrome (OES), who developed polyostotic non-ossifying fibroma (NOF). This case explores FD as a possible new clinical feature of CFC syndrome, and when linked to the historical case of OES, explores whether the KRAS(NM_004985.5):c.57G>C; p.Leu19Phe mutation may potentially contribute to the development of dysplastic bone lesions in patients with this particular mutation.

Burosumab treatment in a child with cutaneous skeletal hypophosphatemia syndrome: A case report

Cutaneous skeletal hypophosphatemia syndrome (CSHS) is a rare disorder caused by somatic mosaicism for the gain of function RAS mutations . Affected patients have segmental epidermal nevi, dysplastic cortical bony lesions, and fibroblast growth factor-23 (FGF23)–mediated hypophosphatemic rickets. Herein, we describe a case of an Emirati girl with CSHS, whose hypophosphatemic rickets and osteomalcic pseudofractures and dysplastic bony lesions failed to recover due to poor adherence to treatment with oral phosphate supplements and alfacalcidol (conventional treatment). Treatment with burosumab, a fully human immunoglobulin G1 monoclonal antibody against FGF23 for 12 months, led to normalization of serum inorganic phosphate and alkaline phosphatase levels, radiographic healing of rickets, partial healing of pseudofractures, improvement in 6-minute walk test, and the physical scale of the Pediatric Quality of Life Inventory. We conclude that burosumab is effective in treatment of CSHS, however results of the ongoing phase 2 trial in adults (NCT02304367) are awaited.

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Cutaneous skeletal hypophosphatemia syndrome (CSHS) is caused by somatic gain-of-function RAS mutations.

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It is associated with fibroblast growth factor-23 (FGF23) mediated hypophosphatemic rickets.

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Burosumab, a monoclonal antibody to FGF23 may have a role in the treatment of CSHS.

Burosumab for the Treatment of Tumor-Induced Osteomalacia

Tumor-induced osteomalacia (TIO) is caused by phosphaturic mesenchymal tumors producing fibroblast growth factor 23 (FGF23) and is characterized by impaired phosphate metabolism, skeletal health, and quality of life. UX023T-CL201 is an ongoing, open-label, phase 2 study investigating the safety and efficacy of burosumab, a fully human monoclonal antibody that inhibits FGF23, in adults with TIO or cutaneous skeletal hypophosphatemia syndrome (CSHS). Key endpoints were changes in serum phosphorus and osteomalacia assessed by transiliac bone biopsies at week 48. This report focuses on 14 patients with TIO, excluding two diagnosed with X-linked hypophosphatemia post-enrollment and one with CSHS. Serum phosphorus increased from baseline (0.52 mmol/L) and was maintained after dose titration from week 22 (0.91 mmol/L) to week 144 (0.82 mmol/L, p < 0.0001). Most measures of osteomalacia were improved at week 48: osteoid volume/bone, osteoid thickness, and mineralization lag time decreased; osteoid surface/bone surface showed no change. Of 249 fractures/pseudofractures detected across 14 patients at baseline, 33% were fully healed and 13% were partially healed at week 144. Patients reported a reduction in pain and fatigue and an increase in physical health. Two patients discontinued: one to treat an adverse event (AE) of neoplasm progression and one failed to meet dosing criteria (receiving minimal burosumab). Sixteen serious AEs occurred in seven patients, and there was one death; all serious AEs were considered unrelated to treatment. Nine patients had 16 treatment-related AEs; all were mild to moderate in severity. In adults with TIO, burosumab exhibited an acceptable safety profile and was associated with improvements in phosphate metabolism and osteomalacia. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research..

The Causes of Hypo- and Hyperphosphatemia in Humans

Phosphate homeostasis involves several major organs that are the skeleton, the intestine, the kidney, and parathyroid glands. Major regulators of phosphate homeostasis are parathormone, fibroblast growth factor 23, 1,25-dihydroxyvitamin D, which respond to variations of serum phosphate levels and act to increase or decrease intestinal absorption and renal tubular reabsorption, through the modulation of expression of transcellular transporters at the intestinal and/or renal tubular level. Any acquired or genetic dysfunction in these major organs or regulators may induce hypo- or hyperphosphatemia. The causes of hypo- and hyperphosphatemia are numerous. This review develops the main causes of acquired and genetic hypo- and hyperphosphatemia.

Congenital Conditions of Hypophosphatemia in Children

Great strides over the past few decades have increased our understanding of the pathophysiology of hypophosphatemic disorders. Phosphate is critically important to a variety of physiologic processes, including skeletal growth, development and mineralization, as well as DNA, RNA, phospholipids, and signaling pathways. Consequently, hypophosphatemic disorders have effects on multiple systems, and may cause a variety of nonspecific signs and symptoms. The acute effects of hypophosphatemia include neuromuscular symptoms and compromise. However, the dominant effects of chronic hypophosphatemia are the effects on musculoskeletal function including rickets, osteomalacia and impaired growth during childhood. While the most common causes of chronic hypophosphatemia in children are congenital, some acquired conditions also result in hypophosphatemia during childhood through a variety of mechanisms. Improved understanding of the pathophysiology of these congenital conditions has led to novel therapeutic approaches. This article will review the pathophysiologic causes of congenital hypophosphatemia, their clinical consequences and medical therapy.

Rickets in association with skin diseases and conditions: A review with emphasis on screening and prevention

BACKGROUND

Rickets is a common disease worldwide. In the developed world, its prevalence dramatically decreased but still diagnosed in at-risk populations. The skin plays a critical role in vitamin D synthesis. Therefore, several skin diseases, especially keratinization disorders, could lead to impaired vitamin D metabolism and vitamin D deficient rickets.

OBJECTIVE

The article aimed to summarize the current knowledge of skin diseases and conditions associated with rickets.

METHODS

To examine the association between rickets and skin diseases, we performed a systematic review of the literature using PubMed database. The search included studies published from the database inception to August 2019.

RESULTS

A total number of 75 articles were included. Identified conditions associated with rickets were ichthyosis being a more common skin diseases, alopecia, epidermal and melanocytic nevi, xeroderma pigmentosum, mastocytosis, psoriasis, and atopic dermatitis. Three types of rickets were identified: vitamin D-dependent rickets, hypocalcemic vitamin D-dependent rickets type 2, and hypophosphatemic rickets. Cutaneous skeletal hypophosphatemia syndrome is a newly described and under-recognized condition. It is defined by the association of epidermal or melanocytic nevi, hypophosphatemic rickets, and elevated levels of fibroblast growth factor 23. Rickets in patients with ichthyosis was mainly due to impaired ability of ichthyotic skin to synthesize vitamin D, poor UV penetration of the skin caused by keratinocyte proliferation, and dark phototype. The latter may be considered a risk factor for rickets in patients with ichthyosis.

CONCLUSION

Despite its rarity, these associations should be properly recognized by dermatologists. Early diagnosis of rickets is important to prevent growth retardation and skeletal deformities.

Keratinocytic epidermal nevi associated with localized fibro-osseous lesions without hypophosphatemia

Keratinocytic epidermal nevi (KEN) are characterized clinically by permanent hyperkeratosis in the distribution of Blaschko's lines and histologically by hyperplasia of epidermal keratinocytes. KEN with underlying RAS mutations have been associated with hypophosphatemic rickets and dysplastic bone lesions described as congenital cutaneous skeletal hypophosphatemia syndrome. Here, we describe two patients with keratinocytic epidermal nevi, in one associated with a papular nevus spilus, who presented with distinct localized congenital fibro-osseous lesions in the lower leg, diagnosed on both radiology and histology as osteofibrous dysplasia, in the absence of hypophosphatemia or rickets, or significantly raised FGF23 levels but with distinct mosaic HRAS mutations. This expands the spectrum of cutaneous/skeletal mosaic RASopathies and alerts clinicians to the importance of evaluating for bony disease even in the absence of bone profile abnormalities.

Can Tissue Expansion Reconstruction in the Trunk of Children Increase the Risk of Scoliosis?

Introduction

We hypothesize that treatment of significant truncal lesions with truncal tissue expanders and subsequent flap surgery in pediatric patients may increase the risk of scoliosis. This study aims to investigate any relationship between tissue expansion (TE) and scoliosis and to compare the prevalence of scoliosis in our tissue expander population to the general population.

Methods

Health records of patients who underwent truncal TE at BC Children's Hospital between 1997 and 2017 were retrospectively reviewed and analyzed. The cross-sectional component of the study consisted of radiological imaging to establish the presence or absence of scoliosis.

Results

We identified 28 patients who underwent truncal TE over the study period. Ten had a scoliosis X-ray on their chart or as a part of the study. Three (10.7%) patients were identified as having developed scoliosis after TE.

Conclusions

We recommend that pediatric TE patients be made aware of the potential complication of scoliosis and be followed closely in the years during and after their treatment, in order to allow for preventative measures, early diagnosis and early management (if required).

Hypophosphataemic Rickets: Similar Phenotype of Different Diseases

Hypophosphataemic rickets (HR) is a group of rare disorders caused by excessive renal phosphate wasting in which the participation of fibroblast growth factor 23 (FGF23) can be prominent. These diseases pose therapeutic challenges with important consequences for growth and bone development in childhood, with higher risk of fractures and poorer bone healing, dental problems, and nephrolithiasis or nephrocalcinosis. In some cases, the diagnostic delay can be very long; laboratory findings and an exhaustive anamnesis could help distinguish between various pathologies, and FGF23 values-although currently not routinely measured-have implications for the differential diagnosis. Genetic testing is encouraged, especially in sporadic or insidious cases. In this review we discuss the clinical features of HR, with a particular emphasis on the differential diagnosis and the therapeutic implications.

Hypophosphatemic rickets: A rare complication of congenital melanocytic nevus syndrome

We report the case of a child who presented with a giant melanocytic nevus with numerous satellite nevi at birth and developed hypophosphatemic rickets due to excessive secretion of the FGF23 hormone. A NRAS c.182A>G (Q61R) mutation was identified in the lesional skin. The functional outcome was favorable with medical treatment.

DIAGNOSIS OF ENDOCRINE DISEASE: Mosaic disorders of FGF23 excess: Fibrous dysplasia/McCune-Albright syndrome and cutaneous skeletal hypophosphatemia syndrome

Fibrous dysplasia/McCune-Albright Syndrome (FD/MAS), arising from gain-of-function mutations in Gαs, and cutaneous skeletal hypophosphatemia syndrome (CSHS), arising from gain-of-function mutations in the Ras/MAPK pathway, are strikingly complex, mosaic diseases with overlapping phenotypes. Both disorders are defined by mosaic skin and bone involvement, and both are complicated by increased FGF23 production. These similarities have frequently led to mis-diagnoses, primarily in patients with CSHS who are often assumed to have FD/MAS. The intriguing similarities in skeletal involvement in these genetically distinct disorders have led to novel insights into FGF23 physiology, making an understanding of FD/MAS and CSHS relevant to both clinicians and researchers interested in bone and endocrine disorders. This review will give an overview of FD/MAS and CSHS, focusing on the roles of mosaicism and FGF23 in the pathogenesis and clinical presentation of these disorders.

Mosaicism in genodermatoses

Genodermatoses are inherited disorders presenting with cutaneous manifestations with or without the involvement of other systems. The majority of these disorders, particularly in cases that present with a cutaneous patterning, may be explained in the context of genetic mosaicism. Despite the barriers to the genetic analysis of mosaic disorders, next-generation sequencing has led to a substantial progress in understanding their pathogenesis, which has significant implications for the clinical management and genetic counseling. Advances in paired and deep sequencing technologies in particular have made the study of mosaic disorders more feasible. In this review, we provide an overview of genetic mosaicism as well as mosaic cutaneous disorders and the techniques required to study them.

Approach to patients with hypophosphataemia

Phosphate metabolism is an evolving area of basic and clinical research. In the past 15 years, knowledge on disturbances of phosphate homoeostasis has expanded, as has the discovery of new targeted therapies. Hypophosphataemia might be the biochemical finding in several diseases, and its clinical evaluation should initially focus on the assessment of pathophysiological mechanisms leading to low serum phosphate concentrations. Clinical consequences of hypophosphataemia can involve multiple organ systems and vary depending on several factors, the most important being the underlying disorder. This Review focuses on the approach to patients with hypophosphataemia and how underlying pathophysiological mechanisms should be understood in the evaluation of differential diagnosis. We define an algorithm for the assessment of hypophosphataemia and review the most up-to-date literature on specific therapies. Continuous research in this area will result in a better understanding and management of patients with hypophosphataemia.

Clinical Evidence for the Benefits of Burosumab Therapy for X-Linked Hypophosphatemia (XLH) and Other Conditions in Adults and Children

Burosumab (KRN23) is an FGF23 neutralizing antibody that has been the subject of several recent clinical trials principally focused on the treatment of hypophosphatemic rickets in patients with X-linked hypophosphatemia (XLH). Since the first publications in 2014, these trials have demonstrated efficacy with minimal safety concerns in both adult and pediatric cohorts. These studies have used dose-escalation to establish a dosing regimen that is well-tolerated in clinical use. This review summarizes the clinical trial data with respect to burosumab treatment in adults and children as well as noting several clinical trials currently underway. While burosumab appears transformative for the treatment of XLH, long term follow-up studies would be required to allay concerns over the potential for nephrocalcinosis and cardiac calcification. While these do not appear to be problematic in current trials, the effects of chronic or lifelong treatment have yet to be established.

When Low Bone Mineral Density and Fractures Is Not Osteoporosis

PURPOSE OF REVIEW

To review the differential diagnosis of low bone mineral density (BMD).

RECENT FINDINGS

Osteoporosis is the most common cause of low BMD in adults; however, non-osteoporotic causes of low BMD should be considered in the differential diagnosis of patients with low BMD. Mild osteogenesis imperfecta, osteomalacia, and mineral and bone disorder of chronic kidney disease as well as several other rare diseases can be characterized by low BMD. This review summarizes the differential diagnosis of low BMD. It is important to differentiate osteoporosis from other causes of low BMD since treatment regimens can vary tremendously between these different disease processes. In fact, some treatments for osteoporosis could worsen or exacerbate the mineral abnormalities in other causes of low BMD.

Functional analysis of a de novo mutation c.1692 del A of the PHEX gene in a Chinese family with X-linked hypophosphataemic rickets

Objectives

X-linked hypophosphataemic rickets (XLHR) is a disease of impaired bone mineralization characterized by hypophosphataemia caused by renal phosphate wasting. The main clinical manifestations of the disorder are O-shaped legs, X-shaped legs, delayed growth, and bone pain. XLHR is the most common inheritable form of rickets, with an incidence of 1/20 000 in humans. It accounts for approximately 80% of familial cases of hypophosphataemia and serves as the prototype of defective tubular phosphate (PO4³⁺) transport, due to extra renal defects resulting in unregulated FGF23 activity. XLHR is caused by loss-of-function mutations in the PHEX gene. The aim of this research was to identify the genetic defect responsible for familial hypophosphataemic rickets in a four-generation Chinese Han pedigree and to analyze the function of this mutation.

Methods

The genome DNA samples of all members in the pedigree were extracted from whole blood. We sequenced all exons of the PHEX and FGF23 genes, as well as the adjacent splice site sequence with Sanger sequencing. Next, we analyzed the de novo mutation c.1692 del A of the PHEX gene with an online digital service and investigated the mutant PHEX with SWISS-MODEL, immunofluorescence, and protein stability detection.

Results

Through Sanger sequencing, we found a de novo mutation, c.1692 del A, in exon 16 of the PHEX gene in this pedigree. This mutation can make the PHEX protein become unstable and decay rapidly, which results in familial XLHR.

Conclusion

We have found a de novo loss-of-function mutation, c.1692 del A, in exon 16 of the PHEX gene that can cause XLHR.

Cite this article: J. Huang, X. Bao, W. Xia, L. Zhu, J. Zhang, J. Ma, N. Jiang, J. Yang, Q. Chen, T. Jing, J. Liu, D. Ma, G. Xu. Functional analysis of a de novo mutation c.1692 del A of the PHEX gene in a Chinese family with X-linked hypophosphataemic rickets. Bone Joint Res 2019;8:405–413. DOI: 10.1302/2046-3758.88.BJR-2018-0276.R1.

FGF23 and Associated Disorders of Phosphate Wasting

Fibroblast growth factor 23 (FGF23), one of the endocrine fibroblast growth factors, is a principal regulator in the maintenance of serum phosphorus concentration. Binding to its cofactor αKlotho and a fibroblast growth factor receptor is essential for its activity. Its regulation and interaction with other factors in the bone-parathyroid-kidney axis is complex. FGF23 reduces serum phosphorus concentration through decreased reabsorption of phosphorus in the kidney and by decreasing 1,25 dihydroxyvitamin D (1,25(OH)2D) concentrations. Various FGF23-mediated disorders of renal phosphate wasting share similar clinical and biochemical features. The most common of these is X-linked hypophosphatemia (XLH). Additional disorders of FGF23 excess include autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia, and tumor-induced osteomalacia. Treatment is challenging, requiring careful monitoring and titration of dosages to optimize effectiveness and to balance side effects. Conventional therapy for XLH and other disorders of FGF23-mediated hypophosphatemia involves multiple daily doses of oral phosphate salts and active vitamin D analogs, such as calcitriol or alfacalcidol. Additional treatments may be used to help address side effects of conventional therapy such as thiazides to address hypercalciuria or nephrocalcinosis, and calcimimetics to manage hyperparathyroidism. The recent development and approval of an anti-FGF23 antibody, burosumab, for use in XLH provides a novel treatment option.

FGF23, Hypophosphatemia, and Emerging Treatments

FGF23 is an important hormonal regulator of phosphate homeostasis. Together with its co-receptor Klotho, it modulates phosphate reabsorption and both 1α-hydroxylation and 24-hydroxylation in the renal proximal tubules. The most common FGF23-mediated hypophosphatemia is X-linked hypophosphatemia (XLH), caused by mutations in the PHEX gene. FGF23-mediated forms of hypophosphatemia are characterized by phosphaturia and low or low-normal calcitriol concentrations, and unlike nutritional rickets, these cannot be cured with nutritional vitamin D supplementation. Autosomal dominant and autosomal recessive forms of FGF23-mediated hypophosphatemias show a similar pathophysiology, despite a variety of different underlying genetic causes. An excess of FGF23 activity has also been associated with a number of other conditions causing hypophosphatemia, including tumor-induced osteomalacia, fibrous dysplasia of the bone, and cutaneous skeletal hypophosphatemia syndrome. Historically phosphate supplementation and therapy using analogs of highly active vitamin D (eg, calcitriol, alfacalcidol, paricalcitol, eldecalcitol) have been used to manage conditions involving hypophosphatemia; however, recently a neutralizing antibody for FGF23 (burosumab) has emerged as a promising treatment agent for FGF23-mediated disorders. This review discusses the progression of clinical trials for burosumab for the treatment of XLH and its recent availability for clinical use. Burosumab may have potential for treating other conditions associated with FGF23 overactivity, but these are not yet supported by trial data. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Genetic approaches to metabolic bone diseases

Metabolic bone diseases comprise a diverse group of disorders characterized by alterations in skeletal homeostasis, and are often associated with abnormal circulating concentrations of calcium, phosphate or vitamin D metabolites. These diseases commonly have a genetic basis and represent either a monogenic disorder due to a germline or somatic single gene mutation, or an oligogenic or polygenic disorder that involves variants in more than one gene. Germline single gene mutations causing Mendelian diseases typically have a high penetrance, whereas the genetic variations causing oligogenic or polygenic disorders are each associated with smaller effects with additional contributions from environmental factors. Recognition of familial monogenic disorders is of clinical importance to facilitate timely investigations and management of the patient and any affected relatives. The diagnosis of monogenic metabolic bone disease requires careful clinical evaluation of the large diversity of symptoms and signs associated with these disorders. Thus, the clinician must pursue a systematic approach beginning with a detailed history and physical examination, followed by appropriate laboratory and skeletal imaging evaluations. Finally, the clinician must understand the increasing number and complexity of molecular genetic tests available to ensure their appropriate use and interpretation.

Melorheostosis: Exome sequencing of an associated dermatosis implicates postzygotic mosaicism of mutated KRAS

Melorheostosis (MEL) is the rare sporadic dysostosis characterized by monostotic or polyostotic osteosclerosis and hyperostosis often distributed in a sclerotomal pattern. The prevailing hypothesis for MEL invokes postzygotic mosaicism. Sometimes scleroderma-like skin changes, considered a representation of the pathogenetic process of MEL, overlie the bony changes, and sometimes MEL becomes malignant. Osteopoikilosis (OPK) is the autosomal dominant skeletal dysplasia that features symmetrically distributed punctate osteosclerosis due to heterozygous loss-of-function mutation within LEMD3. Rarely, radiographic findings of MEL occur in OPK. However, germline mutation of LEMD3 does not explain sporadic MEL. To explore if mosaicism underlies MEL, we studied a boy with polyostotic MEL and characteristic overlying scleroderma-like skin, a few bony lesions consistent with OPK, and a large epidermal nevus known to usually harbor a HRAS, FGFR3, or PIK3CA gene mutation. Exome sequencing was performed to ~100× average read depth for his two dermatoses, two areas of normal skin, and peripheral blood leukocytes. As expected for non-malignant tissues, the patient's mutation burden in his normal skin and leukocytes was low. He, his mother, and his maternal grandfather carried a heterozygous, germline, in-frame, 24-base-pair deletion in LEMD3. Radiographs of the patient and his mother revealed bony foci consistent with OPK, but she showed no MEL. For the patient, somatic variant analysis, using four algorithms to compare all 20 possible pairwise combinations of his five DNA samples, identified only one high-confidence mutation, heterozygous KRAS Q61H (NM_033360.3:c.183A>C, NP_203524.1:p.Gln61His), in both his dermatoses but absent in his normal skin and blood. Thus, sparing our patient biopsy of his MEL bone, we identified a heterozygous somatic KRAS mutation in his scleroderma-like dermatosis considered a surrogate for MEL. This implicates postzygotic mosaicism of mutated KRAS, perhaps facilitated by germline LEMD3 haploinsufficiency, causing his MEL.

Tumour-induced osteomalacia

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic disorder caused by tumours that secrete fibroblast growth factor 23 (FGF23). Owing to the role of FGF23 in renal phosphate handling and vitamin D synthesis, TIO is characterized by decreased renal tubular reabsorption of phosphate, by hypophosphataemia and by low levels of active vitamin D. Chronic hypophosphataemia ultimately results in osteomalacia (that is, inadequate bone mineralization). The diagnosis of TIO is usually suspected when serum phosphate levels are chronically low in the setting of bone pain, fragility fractures and muscle weakness. Locating the offending tumour can be very difficult, as the tumour is often very small and can be anywhere in the body. Surgical removal of the tumour is the only definitive treatment. When the tumour cannot be located or when complete resection is not possible, medical treatment with phosphate salts or active vitamin D is necessary. One of the most promising emerging treatments for unresectable tumours that cause TIO is the anti-FGF23 monoclonal antibody KRN23. The recent identification of a fusion of fibronectin and fibroblast growth factor receptor 1 (FGFR1) as a molecular driver in some tumours not only sheds light on the pathophysiology of TIO but also opens the door to a better understanding of the transcription, translocation, post-translational modification and secretion of FGF23, as well as suggesting approaches to targeted therapy. Further study will reveal if the FGFR1 pathway is also involved in tumours that do not harbour the translocation.

X-linked hypophosphatemia and growth

X-Linked hypophosphatemia (XLH) is the most common form of hereditary rickets caused by loss-of function mutations in the PHEX gene. XLH is characterized by hypophosphatemia secondary to renal phosphate wasting, inappropriately low concentrations of 1,25 dihydroxyvitamin D and high circulating levels of fibroblast growth factor 23 (FGF23). Short stature and rachitic osseous lesions are characteristic phenotypic findings of XLH although the severity of these manifestations is highly variable among patients. The degree of growth impairment is not dependent on the magnitude of hypophosphatemia or the extent of legs´ bowing and height is not normalized by chronic administration of phosphate supplements and 1α hydroxyvitamin D derivatives. Treatment with growth hormone accelerates longitudinal growth rate but there is still controversy regarding the potential risk of increasing bone deformities and body disproportion. Treatments aimed at blocking FGF23 action are promising, but information is lacking on the consequences of counteracting FGF23 during the growing period. This review summarizes current knowledge on phosphorus metabolism in XLH, presents updated information on XLH and growth, including the effects of FGF23 on epiphyseal growth plate of the Hyp mouse, an animal model of the disease, and discusses growth hormone and novel FGF23 related therapies.

Fibroblast Growth Factor 23 and Hypophosphatemia: A Case of Hypophosphatemia along the Rickets-Osteomalacia Spectrum

Phosphorus is a key component of bone, and a deficiency results in poor mineralization along with other systemic symptoms of hypophosphatemia. Various causes of hypophosphatemia with renal wasting of phosphorus have been identified. These include the Fanconi syndrome, various genetic mutations of fibroblast growth factor 23 (FGF23) handling and the sodium/phosphate cotransporter, and those due to FGF23 secretion by mesenchymal tumors. Depending on the cause, vitamin D metabolism may also be impaired, which may amplify the deficiency in phosphorus and render treatment more challenging. Here, we report a case of hypophosphatemia and multiple stress fractures in a 20-year-old male college student living with chronic bone pain and anxiety about suffering further fractures. We further review the literature regarding this spectrum.