FGF23 analysis of a Chinese family with autosomal dominant hypophosphatemic rickets

Molecular Diagnoses of X-Linked and Other Genetic Hypophosphatemias: Results From a Sponsored Genetic Testing Program

X-linked hypophosphatemia (XLH), a dominant disorder caused by pathogenic variants in the PHEX gene, affects both sexes of all ages and results in elevated serum fibroblast growth factor 23 (FGF23) and below-normal serum phosphate. In XLH, rickets, osteomalacia, short stature, and lower limb deformity may be present with muscle pain and/or weakness/fatigue, bone pain, joint pain/stiffness, hearing difficulty, enthesopathy, osteoarthritis, and dental abscesses. Invitae and Ultragenyx collaborated to provide a no-charge sponsored testing program using a 13-gene next-generation sequencing panel to confirm clinical XLH or aid diagnosis of suspected XLH/other genetic hypophosphatemia. Individuals aged ≥6 months with clinical XLH or suspected genetic hypophosphatemia were eligible. Of 831 unrelated individuals tested between February 2019 and June 2020 in this cross-sectional study, 519 (62.5%) individuals had a pathogenic or likely pathogenic variant in PHEX (PHEX-positive). Among the 312 PHEX-negative individuals, 38 received molecular diagnoses in other genes, including ALPL, CYP27B1, ENPP1, and FGF23; the remaining 274 did not have a molecular diagnosis. Among 319 patients with a provider-reported clinical diagnosis of XLH, 88.7% (n = 283) had a reportable PHEX variant; 81.5% (n = 260) were PHEX-positive. The most common variant among PHEX-positive individuals was an allele with both the gain of exons 13-15 and c.*231A>G (3'UTR variant) (n = 66/519). Importantly, over 80% of copy number variants would have been missed by traditional microarray analysis. A positive molecular diagnosis in 41 probands (4.9%; 29 PHEX positive, 12 non-PHEX positive) resulted in at least one family member receiving family testing. Additional clinical or family member information resulted in variant(s) of uncertain significance (VUS) reclassification to pathogenic/likely pathogenic (P/LP) in 48 individuals, highlighting the importance of segregation and clinical data. In one of the largest XLH genetic studies to date, 65 novel PHEX variants were identified and a high XLH diagnostic yield demonstrated broad insight into the genetic basis of XLH. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Autosomal Dominant Hypophosphatemic Rickets: A Case Report and Review of the Literature

Autosomal dominant hypophosphatemic rickets (ADHR) is an extremely rare form of genetic rickets caused by mutations in the fibroblast growth factor 23 gene. ADHR is characterized by hypophosphatemia secondary to isolated renal phosphate wasting. Only a few cases of ADHR have been reported in the literature to date. We describe the case of a 17-month-old girl who presented with severe failure to thrive (length: −4.08 standard deviation (SD), weight: −2.2 SD) and hypotonia. Hypophosphatemia, decreased tubular phosphate reabsorption (69%), and rachitic lesions were found. Genetic analysis showed the heterozygous variant c.536G>A (NM_020638.3:c.536G>A) in exon 3 of the FGF23 gene, leading to the diagnosis of ADHR. She was treated with phosphate salts and oral alfacalcidol. After 4 years of treatment, at 5 years of age, the patient’s ADHR resolved spontaneously. Considering the lack of knowledge regarding ADHR, we reviewed the literature to describe the features of this rare and poorly understood disease. Eleven ADHR pediatric cases have been described thus far, with cases tending to be more common in females than males. Similar to the general population, two groups of patients with ADHR can be described depending on the mutations present: patients with an R179 and R176 mutation have early-onset of disease and higher frequency of rickets, and a milder and late-onset of disease, respectively. Symptoms and disease severity may fluctuate. Spontaneous remission may occur during the pediatric age.

Earlier Onset in Autosomal Dominant Hypophosphatemic Rickets of R179 than R176 Mutations in Fibroblast Growth Factor 23: Report of 20 Chinese Cases and Review of the Literature

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare hereditary disorder characterized by variant onset ages and diverse phenotypes. Our aim is to explore the genotype-phenotype correlations between ADHR patients with R176 and R179 mutations in FGF23 gene. Clinical manifestations, laboratory examinations, and genetic analyses were collected from 20 patients in six Chinese ADHR kindreds in our hospital. Previously published ADHR literatures were reviewed. Among 20 Chinese ADHR mutation carriers, 11 patients revealed overt symptoms. 10/11 (90.9%) of which were females. Patients with R179 mutations presented with earlier onset than those with R176 mutation [1.3 (1.0, 37.0) years vs. 28.5 (19.0, 44.0) years]. More patients with R179 mutations had a history of rickets with lower extremity deformity [3/4 (75%) vs. 1/7 (14.3%), p < 0.05]. The serum phosphate, i-FGF23 and c-FGF23 levels of patients with R179 and R176 mutations were 0.47 ± 0.14 mmol/L versus 0.57 ± 0.17 mmol/L, 79.6 ± 87.0 pg/mL versus 79.9 ± 107.4 pg/mL, and 33.4 ± 3.0 RU/mL versus 121.3 ± 177.6 RU/mL, respectively. 7/11 of patients had iron deficiency at onset of disease. When combined with previously reported seven ADHR families, difference was observed in the age of onset among symptomatic patients with R179 and R176 mutations [1.0 (0.9, 37.0) years vs. 24.5 (1.2, 57.0) years, p < 0.05]. Patients with R179 mutation were more likely to have rickets than R176 mutation (11/13, 84.6% vs. 5/20, 25.0%, p < 0.01) and lower extremity deformity (10/13, 76.9% vs. 6/19, 31.6%, p < 0.01). ADHR patients with R179 mutations had earlier onset age and more rickets compared to those with mutations in R176, which partially explained the clinical heterogeneity of ADHR.

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.

Phosphate homeostasis disorders

Our understanding of the regulation of phosphate balance has benefited tremendously from the molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the glycosyltransferase GALNT3, the endopeptidase PHEX, and the matrix protein DMP1, and molecules that function as downstream effectors of FGF23 such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted treatment of FGF23-dependent hypophosphatemic conditions, but also provide clinically relevant observations related to the dysregulation of mineral ion homeostasis in health and disease.

Fibroblast Growth Factor 2 and Its Receptors in Bone Biology and Disease

The fibroblast growth factor (FGF) regulatory axis is phylogenetically ancient, evolving into a large mammalian/human gene family of 22 ligands that bind to four receptor tyrosine kinases for a complex physiologic system controlling cell growth, differentiation, and metabolism. The tissue targets for the primary FGF function are mainly in cartilage and in bone for morphogenesis, mineralization, and metabolism. A multitude of complexities in the FGF ligand-receptor signaling pathways have made translation into therapies for FGF-related bone disorders such as osteomalacia, osteoarthritis, and osteoporosis difficult but not impossible.

The rachitic tooth

Teeth are mineralized organs composed of three unique hard tissues, enamel, dentin, and cementum, and supported by the surrounding alveolar bone. Although odontogenesis differs from osteogenesis in several respects, tooth mineralization is susceptible to similar developmental failures as bone. Here we discuss conditions fitting under the umbrella of rickets, which traditionally referred to skeletal disease associated with vitamin D deficiency but has been more recently expanded to include newly identified factors involved in endocrine regulation of vitamin D, phosphate, and calcium, including phosphate-regulating endopeptidase homolog, X-linked, fibroblast growth factor 23, and dentin matrix protein 1. Systemic mineral metabolism intersects with local regulation of mineralization, and factors including tissue nonspecific alkaline phosphatase are necessary for proper mineralization, where rickets can result from loss of activity of tissue nonspecific alkaline phosphatase. Individuals suffering from rickets often bear the additional burden of a defective dentition, and transgenic mouse models have aided in understanding the nature and mechanisms involved in tooth defects, which may or may not parallel rachitic bone defects. This report reviews dental effects of the range of rachitic disorders, including discussion of etiologies of hereditary forms of rickets, a survey of resulting bone and tooth mineralization disorders, and a discussion of mechanisms, known and hypothesized, involved in the observed dental pathologies. Descriptions of human pathology are augmented by analysis of transgenic mouse models, and new interpretations are brought to bear on questions of how teeth are affected under conditions of rickets. In short, the rachitic tooth will be revealed.

FGF23 and Phosphate Wasting Disorders

A decade ago, only two hormones, parathyroid hormone and 1,25(OH)2D, were widely recognized to directly affect phosphate homeostasis. Since the discovery of fibroblast growth factor 23 (FGF23) in 2000 (1), our understanding of the mechanisms of phosphate homeostasis and of bone mineralization has grown exponentially. FGF23 is the link between intestine, bone, and kidney together in phosphate regulation. However, we still do not know the complex mechanism of phosphate homeostasis and bone mineralization. The physiological role of FGF23 is to regulate serum phosphate. Secreted mainly by osteocytes and osteoblasts in the skeleton (2,3), it modulates kidney handling of phosphate reabsorption and calcitriol production. Genetic and acquired abnormalities in FGF23 structure and metabolism cause conditions of either hyper-FGF23 or hypo-FGF23. Hyper-FGF23 is related to hypophosphatemia, while hypo-FGF23 is related to hyperphosphatemia. Both hyper-FGF23 and hypo-FGF23 are detrimental to humans. In this review, we will discuss the pathophysiology of FGF23 and hyper-FGF23 related renal phosphate wasting disorders (4).

Hypophosphatemia and growth

Over the last decade the discovery of fibroblast growth factor 23 (FGF23) and the progressive and ongoing clarification of its role in phosphate and mineral metabolism have led to expansion of the diagnostic spectrum of primary hypophosphatemic syndromes. This article focuses on the impairment of growth in these syndromes. Growth retardation is a common, but not constant, feature and it presents with large variability. As a result of the very low prevalence of other forms of primary hypophosphatemic syndromes, the description of longitudinal growth and the pathogenesis of its impairment have been mostly studied in X-linked hypophosphatemia (XLH) patients and in Hyp mice, the animal model of this disease. In general, children with XLH have short stature with greater shortness of lower limbs than trunk. Treatment with phosphate supplements and 1α vitamin D derivatives heals active lesions of rickets, but does not normalize growth of XLH patients. Patients might benefit from recombinant human growth hormone (rhGH) therapy, which may accelerate the growth rate without increasing body disproportion or correcting hypophosphatemia. These clinical data as well as research findings obtained in Hyp mice suggest that the pathogenesis of defective growth in XLH and other hypophosphatemic syndromes is not entirely dependent on the mineralization disorder and point to other effects of hypophosphatemia itself or FGF23 on the metabolism of bone and growth plate.

Autosomal dominant hypophosphatemic rickets in an 85 year old woman: characterization of her disease from infancy through adulthood

BACKGROUND

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare genetic disorder of phosphate homeostasis characterized, when severely expressed, by osteomalacia, suppressed levels of calcitriol, and renal phosphate wasting due to elevated levels of fibroblast growth factor 23 (FGF23). The disease is caused by heterozygous FGF23 mutations at the RXXR site that prevent cleavage of the intact hormone.

OBJECTIVES

An FGF23 mutation was identified in the proband an 85-year-old woman with elevated FGF23 levels, and her clinical course was characterized. Medical records revealed she was treated for rickets as an infant. She was then asymptomatic until soon after her 4th pregnancy, when she suffered incapacitating bone pain and weakness, age 37. Symptoms remitted with brief treatment.

RESULTS

The proband and one son, but not other family members, were found to be heterozygous for the R176Q mutation in FGF23. Expression of this germ line mutation was strikingly different in both individuals in terms of skeletal health, FGF23 levels and disease activity.

CONCLUSIONS

The identified FGF23 mutation in two members of this family raises questions about molecular mechanisms that have led to intermittent increases in FGF23 synthesis and secretion, and disease expression.

Can features of phosphate toxicity appear in normophosphatemia?

Phosphate is an indispensable nutrient for the formation of nucleic acids and the cell membrane. Adequate phosphate balance is a prerequisite for basic cellular functions ranging from energy metabolism to cell signaling. More than 85% of body phosphate is present in the bones and teeth. The remaining phosphate is distributed in various soft tissues, including skeletal muscle. A tiny amount, around 1% of total body phosphate, is distributed both in the extracellular fluids and within the cells. Impaired phosphate balance can affect the functionality of almost all human systems, including muscular, skeletal, and vascular systems, leading to an increase in morbidity and mortality of the involved patients. Currently, measuring serum phosphate level is the gold standard to estimate the overall phosphate status of the body. Despite the biological and clinical significance of maintaining delicate phosphate balance, serum levels do not always reflect the amount of phosphate uptake and its distribution. This article briefly discusses the potential that some of the early consequences of phosphate toxicity might not be evident from serum phosphate levels.