Autosomal recessive hypophosphatemic rickets type 2 (ARHR2) due to ENPP1-deficiency

Genetic causes of hypophosphatemia

Phosphate is a key component of mineralized tissues and is also part of many organic compounds. Phosphorus homeostasis depends especially upon intestinal absorption, and renal excretion, which are regulated by various hormones, such as PTH, 1,25-dihydroxyvitamin D, and fibroblast growth factor 23. In this review we provide an update of several genetic disorders that affect phosphate transporters through cell membranes or the phosphate-regulating hormones, and, consequently, result in hypophosphatemia.

Diagnosis, treatment, and management of rickets: a position statement from the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology

Rickets results from impaired mineralization of growing bone due to alterations in calcium and phosphate homeostasis. Clinical signs of rickets are related to the age of the patient, the duration of the disease, and the underlying disorder. The most common signs of rickets are swelling of the wrists, knees or ankles, bowing of the legs (knock-knees, outward bowing, or both) and inability to walk. However, clinical features alone cannot differentiate between the various forms of rickets. Rickets includes a heterogeneous group of acquired and inherited diseases. Nutritional rickets is due to a deficiency of vitamin D, dietary calcium or phosphate. Mutations in genes responsible for vitamin D metabolism or function, the production or breakdown of fibroblast growth factor 23, renal phosphate regulation, or bone mineralization can lead to the hereditary form of rickets. This position paper reviews the relevant literature and presents the expertise of the Bone and Mineral Metabolism Group of the Italian Society of Pediatric Endocrinology and Diabetology (SIEDP). The aim of this document is to provide practical guidance to specialists and healthcare professionals on the main criteria for diagnosis, treatment, and management of patients with rickets. The various forms of rickets are discussed, and detailed references for the discussion of each form are provided. Algorithms to guide the diagnostic approach and recommendations to manage patients with rare forms of hereditary rickets are proposed.

Insight into small-molecule inhibitors targeting extracellular nucleotide pyrophosphatase/phosphodiesterase1 for potential multiple human diseases

Extracellular nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) has been identified as a type II transmembrane glycoprotein. It plays a crucial role in various biological processes, such as bone mineralization, cancer cell proliferation, and immune regulation. Consequently, ENPP1 has garnered attention as a promising target for pharmacological interventions. Despite its potential, the development of clinical-stage ENPP1 inhibitors for solid tumors, diabetes, and silent rickets remains limited. However, there are encouraging findings from preclinical trials involving small molecules exhibiting favorable therapeutic effects and safety profiles. This perspective aims to shed light on the structural properties, biological functions and the relationship between ENPP1 and diseases. Additionally, it focuses on the structure-activity relationship of ENPP1 inhibitors, with the intention of guiding the future development of new and effective ENPP1 inhibitors.

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.

Natural history of ENPP1 deficiency: Nationwide Turkish Cohort Study of autosomal-recessive hypophosphataemic rickets type 2

OBJECTIVE

Autosomal-recessive hypophosphataemic rickets type 2 (ARHR2) is a rare disease that is reported in survivors of generalized arterial calcification of infancy (GACI).

DESIGN, PATIENTS AND MEASUREMENT

The objective of this study was to characterize a multicenter paediatric cohort with ARHR2 due to ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) deficiency and with a diagnosis of GACI or GACI-related findings. The clinical, biochemical and genetic characteristics of the patients were retrospectively retrieved.

RESULTS

We identified 18 patients from 13 families diagnosed with ARHR2. Fifteen of the patients had an ENPP1 variation confirmed with genetic analyses, and three were siblings of one of these patients, who had clinically diagnosed hypophosphataemic rickets (HRs) with the same presentation. From nine centres, 18 patients, of whom 12 (66.7%) were females, were included in the study. The mean age at diagnosis was 4.2 ± 2.2 (1.6-9) years. The most frequently reported clinical findings on admission were limb deformities (66.6%) and short stature (44.4%). At diagnosis, the mean height SD was -2.2 ± 1.3. Five of the patients were diagnosed with GACI in the neonatal period and treated with bisphosphonates. Other patients were initially diagnosed with ARHR2, but after the detection of a biallelic variant in the ENPP1 gene, it was understood that they previously had clinical findings associated with GACI. Three patients had hearing loss, and two had cervical fusion. After the treatment of HRs, one patient developed calcification, and one developed intimal proliferation.

CONCLUSION

ARHR2 represents one manifestation of ENPP1 deficiency that usually manifests later in life than GACI. The history of calcifications or comorbidities that might be associated with GACI will facilitate the diagnosis in patients with ARHR2, and patients receiving calcitriol and phosphate medication should be carefully monitored for signs of calcification or intimal proliferation.

Pseudoxanthoma elasticum veiled as vasculitis: shedding light on an uncommon disorder and an in-depth review of the literature

Pseudoxanthoma Elasticum (PXE) is a rare genetic disorder caused by an autosomal recessive mutation in the ABCC6 gene. It manifests with distinctive clinical symptoms impacting the skin, eyes, and cardiovascular system, along with an elevated risk of cardiovascular diseases. We present a case of a 34-year-old male patient who was initially referred to the rheumatology clinic for evaluation due to suspected large vessel vasculitis. The patient's primary complaint was severe hemifacial pain radiating to the neck and upper limb. Radiological imaging studies unveiled substantial vascular narrowing and collateral vessel formation, prompting further investigation to exclude systemic vasculitis. Intriguingly, the patient also exhibited cutaneous manifestations, which were later confirmed via skin biopsy as consistent with PXE. An ophthalmological examination further revealed the presence of the classic PXE findings of angioid streaks. Given the rarity of PXE and its multifaceted clinical presentation, it can be particularly challenging to diagnose and manage. As such, cases like the one presented here may necessitate a referral to a rheumatologist for evaluation of potential systemic involvement. To provide a comprehensive perspective on PXE, we conducted a systematic review of case reports published in the past decade in English, collected from PubMed, Scopus, and the Directory of Open Access databases. The analysis of these cases will be discussed to shed light on the diversity of PXE's clinical features and the diagnostic and management dilemmas it poses and to facilitate ongoing exploration and research into this intricate condition, ultimately leading to improved care for individuals affected by PXE.

Case report: Multiple arterial stenoses induced by autosomal-recessive hypophosphatemic rickets type 2 associated with mutation of ENPP1: a case study

Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1)-related multiple arterial stenoses is a rare clinical syndrome in which global arterial calcification begins in infancy, with a high probability of early mortality, and hypophosphatemic rickets develops later in childhood. The vascular status of an ENPP1-mutated patient when they enter the rickets phase has not been thoroughly explored. In this study, we presented a case of an adolescent with an ENPP1 mutation who complained of uncontrolled hypertension. Systematic radiography showed renal, carotid, cranial, and aortic stenoses as well as random calcification foci on arterial walls. The patient was incorrectly diagnosed with Takayasu's arteritis, and cortisol therapy had little effect on reducing the vascular stenosis. As a result, phosphate replacement, calcitriol substitution, and antihypertensive medication were prescribed, and the patient was discharged for further examination. This research presented the vascular alterations of an ENPP1-mutanted patient, and while there is less calcification, intimal thickening may be the primary cause of arterial stenosis.

Alkaline phosphatase in clinical practice in childhood: Focus on rickets

Serum alkaline phosphatase (ALP) and its isoenzymes reflect bone metabolism: ALP increases the ratio of inorganic phosphate to pyrophosphate systemically and facilitates mineralization as well as reduces extracellular pyrophosphate concentration, an inhibitor of mineral formation. On the contrary, low ALP activity is associated with reduction of bone turnover. ALP includes four isoenzymes depending on the site of tissue expression: intestinal ALP, placental ALP, germ cell ALP and tissue nonspecific ALP or liver/bone/kidney ALP. The bone isoenzyme (B-ALP) is involved in bone calcification and is a marker of bone turnover as a result of osteoblastic activity. ALP and its isoenzymes are crucial in the diagnostic process of all the forms of rickets.The most common cause of rickets is vitamin D nutritional deficiency. The aim of this review is to update on the role played by ALP serum concentrations as a relevant marker in thediagnosis and treatment of rickets. Indeed, the diagnosis of rickets is based on its clinical, radiological and laboratory characteristics. An elevated ALP level is one of the markers for the diagnosis of rickets in children, though it is also associated with bone formation process. ALP is also useful for the differentiation between rickets and other disorders that can mimic rickets because of their clinical and laboratory characteristics, and, together with other biochemical markers, is crucial for the differential diagnosis of the different forms of rickets. Age, severity and duration of rickets may also modulate ALP elevation. Finally, ALP measurements are useful in clinical and therapeutic follow-up.

Phosphatonins: From Discovery to Therapeutics

OBJECTIVE

Phosphate is crucial for cell signaling, energy metabolism, nucleotide synthesis, and bone mineralization. The gut-bone-parathyroid-kidney axis is influenced by parathyroid hormone, 1,25-dihydroxyvitamin D, and phosphatonins, especially fibroblast growth factor 23 (FGF23). These hormones facilitate maintenance of phosphate homeostasis. This review summarizes current knowledge regarding the phosphate homeostasis, phosphatonin pathophysiology, and clinical implications of FGF23-related hypophosphatemic disorders, with specific focus on burosumab treatment.

METHOD

A focused literature search of PubMed was conducted.

RESULTS

Phosphatonins including FGF23, secreted frizzled-related protein 4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor 7 play a pathogenic role in several hypophosphatemic disorders. Excess FGF23 inhibits sodium-dependent phosphate cotransporters (NaPi-2a and NaPi-2c), resulting in hyperphosphaturia and hypophosphatemia. Additionally, FGF23 suppresses 1,25-dihydroxyvitamin D synthesis in the proximal renal tubule, and thus, it indirectly inhibits intestinal phosphate absorption. Disorders of FGF23-related hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia/McCune-Albright syndrome, and tumor-induced osteomalacia (TIO). Complications of conventional therapy with oral phosphate and vitamin D analogs comprise gastrointestinal distress, hypercalcemia, nephrocalcinosis, and secondary/tertiary hyperparathyroidism. In both children and adults with XLH and TIO, the anti-FGF23 antibody burosumab exhibits a favorable safety profile and is associated with healing of rickets in affected children and improvement of osteomalacia in both children and adults.

CONCLUSION

The treatment paradigm for XLH and TIO is changing based on data from recent clinical trials. Research suggest that burosumab is effective and safe for pediatric and adult patients with XLH or TIO.

Hereditary Rickets: A Quick Guide for the Pediatrician

With the increased discovery of genes implicated in vitamin D metabolism and the regulation of calcium and phosphate homeostasis, a growing number of genetic forms of rickets are now recognized. These are categorized into calciopenic and phosphopenic rickets. Calciopenic forms of hereditary rickets are caused by genetic mutations that alter the enzymatic activity in the vitamin D activation pathway or impair the vitamin D receptor action. Hereditary forms of phosphopenic rickets, on the other hand, are caused by genetic mutations that lead to increased expression of FGF23 hormone or that impair the absorptive capacity of phosphate at the proximal renal tubule. Due to the clinical overlap between acquired and genetic forms of rickets, identifying children with hereditary rickets can be challenging. A clear understanding of the molecular basis of hereditary forms of rickets and their associated biochemical patterns allow the health care provider to assign the correct diagnosis, avoid non-effective interventions and shorten the duration of the diagnostic journey in these children. In this mini-review, known forms of hereditary rickets listed on the Online Mendelian Inheritance in Man database are discussed. Further, a clinical approach to identify and diagnose children with hereditary forms of rickets is suggested.

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.

Mutation update: Variants of the ENPP1 gene in pathologic calcification, hypophosphatemic rickets, and cutaneous hypopigmentation with punctate keratoderma

ENPP1 encodes ENPP1, an ectonucleotidase catalyzing hydrolysis of ATP to AMP and inorganic pyrophosphate (PPi), and an endogenous plasma protein physiologically preventing ectopic calcification of connective tissues. Mutations in ENPP1 have been reported in association with a range of human genetic diseases. In this mutation update, we provide a comprehensive review of all the pathogenic variants, likely pathogenic variants, and variants of unknown significance in ENPP1 associated with three autosomal recessive disorders-generalized arterial calcification of infancy (GACI), autosomal recessive hypophosphatemic rickets type 2 (ARHR2), and pseudoxanthoma elasticum (PXE), as well as with a predominantly autosomal dominant disorder-Cole disease. The classification of all variants is determined using the latest ACMG guidelines. A total of 140 ENPP1 variants were curated consisting of 133 previously reported variants and seven novel variants, with missense variants being the most prevalent (70.0%, 98/140). While the pathogenic variants are widely distributed in the ENPP1 gene of patientsgen without apparent genotype-phenotype correlation, eight out of nine variants associated with Cole disease are confined to the somatomedin-B-like (SMB) domains critical for homo-dimerization of the ENPP1 protein.

The regulation of FGF23 under physiological and pathophysiological conditions

Fibroblast growth factor 23 (FGF23) is an important bone hormone that regulates phosphate homeostasis in the kidney along with active vitamin D (1,25(OH)₂D₃) and parathyroid hormone (PTH). Endocrine effects of FGF23 depend, at least in part, on αKlotho functioning as a co-receptor whereas further paracrine effects in other tissues are αKlotho-independent. Regulation of FGF23 production is complex under both, physiological and pathophysiological conditions. Physiological regulators of FGF23 include, but are not limited to, 1,25(OH)₂D₃, PTH, dietary phosphorus intake, and further intracellular and extracellular factors, kinases, cytokines, and hormones. Moreover, several acute and chronic diseases including chronic kidney disease (CKD) or further cardiovascular disorders are characterized by early rises in the plasma FGF23 level pointing to further mechanisms effective in the regulation of FGF23 under pathophysiological conditions. Therefore, FGF23 also serves as a prognostic marker in several diseases. Our review aims to comprehensively summarize the regulation of FGF23 in health and disease.

Case Report and Review of Literature: Autosomal Recessive Hypophosphatemic Rickets Type 2 Caused by a Pathogenic Variant in ENPP1 Gene

Autosomal recessive hypophosphatemic rickets type 2 (ARHR2) is a rare form of hereditary rickets, which is characterized by defective bone mineralization and renal phosphate wasting due to a loss-of-function variant in the ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) gene. Although pathogenic variant of ENPP1 has been known to manifest other phenotypes including arterial calcification, hearing loss, ossification of posterior longitudinal ligament, or pseudoxanthoma elasticum, there have been few reports including systematic examination in individuals diagnosed with ARHR2 to date. Herein, we report a case of ARHR2 with a bi-allelic pathogenic variant of ENPP1, in which the patient presented with gait abnormalities with severe genu varum at 26 months of age. Targeted gene panel sequencing was performed to investigate the genetic cause of rickets, and a homozygous nonsense variant in ENPP1, c.783C>G (p.Tyr261*), was identified. The patient was treated with oral phosphate and active vitamin D supplements and underwent corrective osteotomy for varus deformity. His phenotype was limited to rickets. A periodic systematic evaluation is needed to identify any comorbidities in ARHR2 patients since ENPP1 variants may present phenotypes other than rickets and symptoms may evolve or change over time.

Purinergic signaling in systemic sclerosis

Systemic sclerosis (SSc) is a chronic autoimmune rheumatic disease that involves numerous organs and presents major management challenges. The histopathologic hallmarks of SSc include vasculopathy, fibrosis and autoimmune phenomena involving both innate and adaptive immune systems. Purinergic signalling is a pathway that may be implicated in the pathophysiology of several of these disease manifestations. Extracellular purines are potent signalling mediators, which have been shown to be dysregulated in SSc. As examples, purines can exacerbate vasculopathy and provoke platelet dysfunction; as well as contributing to immune dysregulation. Elements of purinergic signalling further promote organ and tissue fibrosis in several disease models. Here, we provide an overview of extracellular purine metabolism in purinergic signalling and link disorders of these to the molecular pathology of SSc. We also discuss targeting the purinergic signalling and explore the translational applications for new therapeutic options in SSc.