X-linked hypophosphatemia and growth

Meta-analysis and systematic review: burosumab as a promising treatment for children with X-linked hypophosphatemia

Objective

The aim of this study was to evaluate the effectiveness of burosumab therapy in children with X-Linked Hypophosphatemia (XLH).

Materials and methods

We systematically reviewed literature from PubMed, Web of Science, The Cochrane Library, and Embase up until January 2024, using EndNote Web for study organization. The Newcastle-Ottawa scale guided quality assessment, while Revman software was used for data analysis and visualization. Study selection, quality evaluation, and data aggregation were independently performed by three researchers.

Results

The meta-analysis encompassed ten studies, including eight cohort studies that examined burosumab's impact pre- and post-administration, and two randomized controlled trials comparing burosumab to standard therapy. The evidence from this review suggests burosumab's superiority in managing XLH in pediatric populations, particularly in improving key biochemical markers including 1,25-dihydroxyvitamin D (1,25-(OH)2D), phosphorus, and alkaline phosphatase (ALP), alongside improvements in the renal tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmP/GFR), and significant skeletal improvements as indicated by the rickets severity score (RSS) and the 6-minute walk test (6MWT). However, the long-term safety and effects, including height and quality of life (QOL) data, remains to be elucidated.

Conclusions

Burosumab has shown significant therapeutic effectiveness in treating children with XLH, highlighting its potential as a key treatment option.

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.

Adult height improved over decades in patients with X-linked hypophosphatemia: a cohort study

OBJECTIVES

The aim of this study is to analyze height after cessation of growth (final height [FH]) and its evolution over the last decades in X-linked hypophosphatemia (XLH) patients in France, as the data on natural history of FH in XLH are lacking.

DESIGN

We performed a retrospective observational study in a large cohort of French XLH patients with available data on FH measurements.

MATERIALS AND METHODS

We divided patients into 3 groups according to their birth year: group 1 born between 1950 and 1974, group 2 born between 1975 and 2000, and group 3 born between 2001 and 2006, respectively, and compared their FHs.

RESULTS

A total of 398 patients were included. Mean FHs were the following: for group 1, -2.31 ± 1.11 standard deviation score (SDS) (n = 127), 156.3 ± 9.7 cm in men and 148.6 ± 6.5 cm in women; for group 2, -1.63 ± 1.13 SDS (n = 193), 161.6 ± 8.5 cm in men and 153.1 ± 7.2 cm in women; and for group 3, -1.34 ± 0.87 SDS (n = 78), 165.1 ± 5.5 cm in men and 154.7 ± 6 cm in women. We report a significant increase in mean FH SDS over 3 generations of patients, for both men and women (P < .001). Final height SDS in male (-2.08 ± 1.18) was lower than in female (-1.70 ± 1.12) (P = .002).

CONCLUSION

The FH of XLH patients in France increased significantly over the last decades. Even though men's FHs improved more than women's, men with XLH remain shorter reflecting a more severe disease phenotype. While the results are promising, most patients with XLH remain short leaving room for improvement.

X-Linked Familial Hypophosphatemia: A Case Report of 27-Year Old Male and Review of Literature

X-linked hypophosphatemia (XLH) associated with short stature during childhood are mostly referred to the hospital and diagnosed as vitamin D deficiency rickets and received vitamin D before adulthood. A case is presented with clinical features of hypophosphatemia from childhood who did not seek medical care for diagnosis and treatment, nor did his mother or two brothers, who have short statures, bone pain, and fractures. The patient was assessed for sociodemographic, hematological, and biochemical parameters together with a genetic assessment. A DEXA scan and X-ray were done to determine the abnormalities and deformities of joints and bones despite clinical examination by an expert physician. All imaging, laboratory parameters, and the genetic study confirmed the diagnosis of XLH. A detailed follow-up of his condition was performed after the use of phosphate tablets and other treatments. X-linked hypophosphatemia needs a good assessment, care, and follow up through a complementary medical team including several specialties. Phosphate tablets in adulthood significantly affects clinical and physical improvement and prevention of further skeletal abnormality and burden on daily activity. The patients should be maintained with an adequate dose of phosphate for better patient compliance. More awareness is needed in society and for health professionals when conducting medical checkups during the presence of stress fractures, frequent dental and gum problems, rickets, short stature, or abnormality in the skeleton or walking to think of secondary causes such as hypophosphatemia. Further investigations including a visit to a specialist is imperative to check for the primary cause of these disturbances.

Growth hormone treatment improves final height in children with X-linked hypophosphatemia

BACKGROUND/AIM

Despite optimal conventional treatment (oral phosphate supplements and active vitamin D analogs), about 40-50% of children with well-controlled X-linked hypophosphatemia (XLH) show linear growth failure, making them less likely to achieve an acceptable final height. Here, we studied the hypothesis that rhGH treatment improves final height in children with XLH and growth failure.

METHODS

Two cohorts of children with XLH were included in this retrospective longitudinal analysis: (1) a cohort treated with rhGH for short stature (n = 34) and (2) a cohort not treated with rhGH (n = 29). The mean duration of rhGH treatment was 4.4 ± 2.9 years. We collected the auxological parameters at various time points during follow-up until final height.

RESULTS

In rhGH-treated children, 2 years of rhGH therapy was associated with a significant increase in height from - 2.4 ± 0.9 to - 1.5 ± 0.7 SDS (p < 0.001). Their mean height at rhGH discontinuation was - 1.2 ± 0.9 SDS and at final height was - 1.3 ± 0.9 SDS corresponding to 165.5 ± 6.4 cm in boys and 155.5 ± 6.3 cm in girls. Notably, the two groups had similar final heights; i.e., the final height in children not treated with rhGH being - 1.2 ± 1.1 SDS (165.4 ± 6.8 cm in boys and 153.7 ± 7.8 cm in girls), p = 0.7.

CONCLUSION

Treatment with rhGH permits to improve final height in children with XLH and growth failure, despite optimal conventional treatment. We propose therefore that rhGH therapy could be considered as an option for short stature in the context of XLH.

Impact of X-Linked Hypophosphatemia on Muscle Symptoms

X-linked hypophosphatemia (XLH) is the most common hereditary form of rickets and deficiency of renal tubular phosphate transport in humans. XLH is caused by the inactivation of mutations within the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene and follows an X-dominant transmission. It has an estimated frequency of 1 case per 20,000, and over 300 distinct pathogenic variations have been reported that result in an excess of fibroblast growth factor 23 (FGF23) in the serum. Increased levels of FGF23 lead to renal phosphate loss, decreased serum 1,25-dihydroxyvitamin D, and increased metabolism of 1,25-dihydoxyvitamin D, resulting in hypophosphatemia. Major clinical manifestations include rickets, bone deformities, and growth retardation that develop during childhood, and osteomalacia-related fractures or pseudo-fractures, degenerative osteoarthritis, enthesopathy, dental anomalies, and hearing loss during adulthood, which can affect quality of life. In addition, fatigue is also a common symptom in patients with XLH, who experience decreased motion, muscle weakness, and pain, contributing to altered quality of life. The clinical and biomedical characteristics of XLH are extensively defined in bone tissue since skeletal deformations and mineralization defects are the most evident effects of high FGF23 and low serum phosphate levels. However, despite the muscular symptoms that XLH causes, very few reports are available on the effects of FGF23 and phosphate in muscle tissue. Given the close relationship between bones and skeletal muscles, studying the effects of FGF23 and phosphate on muscle could provide additional opportunities to understand the interactions between these two important compartments of the body. By describing the current literature on XLH and skeletal muscle dysfunctions, the purpose of this review is to highlight future areas of research that could contribute to a better understanding of XLH muscular disability and its management.

Health-related quality of life of X-linked hypophosphatemia in Spain

BACKGROUND

Health-related quality of life (HRQoL) of patients with X-linked hypophosphatemia (XLH) is lower than that of both the general population and the patients with other chronic diseases, mainly due to diagnostic delay, treatment difficulties, poor psychosocial support, and problems with social integration. Early diagnosis and optimal treatment are paramount to control the disease in patients with XLH, avoid complications, and maintain or improve their HRQoL. We, therefore, analyzed the HRQoL of pediatric and adult patients with XLH treated with conventional therapy in Spain.

RESULTS

We used several versions of the EuroQol-5 dimensions (EQ-5D) instrument according to the age of patients with XLH. Then we compared the HRQoL of patients to that of the general Spanish population. Children with XLH (n = 21) had moderate problems in walking about (61.9%), washing or dressing themselves (9.52%), and performing their usual activities (33.33%). They also felt moderate pain or discomfort (61.9%) and were moderately anxious or depressed (23.81%). Adults with XLH (n = 29) had lower HRQoL, with problems in walking (93%, with 3.45% unable to walk independently), some level of pain (86%, with 3.45% experiencing extreme pain), problems with their usual activities (80%) and self-care (> 50%), and reported symptoms of anxiety and/or depression (65%). There were important differences with the general Spanish population.

CONCLUSIONS

XLH impacts negatively on physical functioning and HRQoL of patients. In Spanish patients with XLH, the HRQoL was reduced despite conventional treatment, clearly indicating the need to improve the therapeutic approach to this disorder.

Function of PHEX mutations p.Glu145* and p.Trp749Arg in families with X-linked hypophosphatemic rickets by the negative regulation mechanism on FGF23 promoter transcription

X-linked hypophosphatemic rickets (XLH) is characterized by increased circulating fibroblast growth factor 23 (FGF23) concentration caused by PHEX (NM_000444.5) mutations. Renal tubular resorption of phosphate is impaired, resulting in rickets and impaired bone mineralization. By phenotypic-genetic linkage analysis, two PHEX pathogenic mutations were found in two XLH families: c.433 G > T, p.Glu145* in exon 4 and c.2245 T > C, p.Trp749Arg in exon 22. Immunofluorescence showed that the localization of p.Glu145* and p.Trp749Arg mutant and secretory PHEX (secPHEX) changed, with decreased expression. In a HEK293T cell model co-transfected with PHEX, secPHEX, and FGF23, wild-type PHEX, secPHEX, and FGF23 proteins were distributed in the cell membrane or endoplasmic reticulum, while the mutant was located in the nuclear membrane and cytoplasm. qPCR of p.Glu145* revealed decreased PHEX and secPHEX mRNA expression in cells, with no difference in mRNA expression of p.Trp749Arg. Both mutations decreased intracellular PHEX endopeptidase activity. Western blot analysis showed decrease in mutant and secPHEX protein expression and no FGF23 protein expression in single-transfected PHEX and secPHEX cells. In cells co-transfected with FGF23, PHEX and secPHEX mutation promoted FGF23 expression. Dual-luciferase reporter gene was used to detect the effect of PHEX on FGF23 promoter. The dual-luciferase reporter gene showed that after PHEX overexpression, the activity of mutant firefly luciferase was significantly higher than that of wild type. The regulatory mechanism between PHEX and FGF23 is still unclear, but we found that PHEX is a direct transcriptional inhibitor of FGF23 and affects the expression of FGF23. This study verified the pathogenicity of the two variants and revealed the possible regulatory mechanism between PHEX and FGF23.

Cellular and Molecular Alterations Underlying Abnormal Bone Growth in X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH), the most common form of hereditary hypophosphatemic rickets, is caused by inactivating mutations of the phosphate-regulating endopeptidase gene (PHEX). XLH is mainly characterized by short stature, bone deformities and rickets, while in hypophosphatemia, normal or low vitamin D levels and low renal phosphate reabsorption are the principal biochemical aspects. The cause of growth impairment in patients with XLH is not completely understood yet, thus making the study of the growth plate (GP) alterations necessary. New treatment strategies targeting FGF23 have shown promising results in normalizing the growth velocity and improving the skeletal effects of XLH patients. However, further studies are necessary to evaluate how this treatment affects the GP as well as its long-term effects and the impact on adult height.

Long-term effect of conventional phosphate and calcitriol treatment on metabolic recovery and catch-up growth in children with PHEX mutation

OBJECTIVES

Hereditary hypophosphatemic rickets (HR) is conventionally treated with phosphate and calcitriol. Exploring genotype and phenotypic spectrum of X-linked hypophosphatemic rickets (XLHR), focusing on short-term, long-term, and pubertal impact of conventional treatment was aimed.

METHODS

Sixteen patients from 12 unrelated families with HR were analyzed for phosphate regulating endopeptidase homolog X-linked (PHEX) mutation. Initially Sanger sequencing analysis was performed. If PHEX mutation was not detected, multiplex ligation-dependent probe amplification (MLPA) was performed. If molecular defect was detected, first-degree relatives were analyzed. Thirteen patients (81%) and five first-degree relatives with XLHR were evaluated for genotype-phenotype or gender-phenotype correlation. Clinical characteristics and response to conventional treatment were determined retrospectively.

RESULTS

Nine different PHEX mutations were identified; four splice-site, three point mutations, and two single exon deletions. Four were novel mutations. Despite conventional treatment, median adult height was lower than median height on admission (-3.8 and -2.3 SDS, respectively), metabolic and radiographic recovery were not achieved, adherence was low (30%). Although mean adult height was better in compliant patients than noncompliants (-2.6 vs. -3.7 SDS, respectively), they were still short. Correlation between phenotype and genotype or gender could not be shown. Median phosphate decreased significantly throughout puberty (p=0.014). Median pubertal height was lower than prepubertal height (-4.4 vs. -3.6 SDS; respectively), pubertal growth spurt was not observed. Among five patients with a follow-up longer than five years, three had nephrocalcinosis (60%), two had hyperparathyroidism (40%), 4/6 (33%) required correction osteotomy.

CONCLUSIONS

Conventional treatment appears to have limited effect on metabolic, clinical and radiographic recovery in XLHR. Metabolic control and growth worsened during puberty. Although, long-term adverse effects are yet to be seen, introduction of burosumab as first-line treatment may be an alternative after infancy.

Challenges in the management of tumor-induced osteomalacia (TIO)

Tumor-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare acquired paraneoplastic disease, which is challenging to diagnose and treat. TIO is characterized by hypophosphatemia resulting from excess levels of tumor-secreted fibroblast growth factor 23 (FGF23), one of the key physiological regulators of phosphate metabolism. Elevated FGF23 results in renal phosphate wasting and compromised vitamin D activation, ultimately resulting in osteomalacia. Patients typically present with progressive and non-specific symptoms, including bone pain, multiple pathological fractures, and progressive muscle weakness. Diagnosis is often delayed or missed due to the non-specific nature of complaints and lack of disease awareness. Additionally, the disease-causing tumour is often difficult to detect and localize because they are often small, lack localizing symptoms and signs, and dwell in widely variable anatomical locations. Measuring serum/urine phosphate should be an inherent diagnostic component when assessing otherwise unexplained osteomalacia, fractures and weakness. In cases of hypophosphatemia with inappropriate (sustained) phosphaturia and inappropriately normal or frankly low 1,25-dihydroxy vitamin D, differentiation of the potential causes of renal phosphate wasting should include measurement of FGF23, and TIO should be considered. While patients experience severe disability without treatment, complete excision of the tumour is typically curative and results in a dramatic reversal of symptoms. Two additional key current unmet needs in optimizing TIO management are: (1 and 2) the considerable delay in diagnosis and consequent delay between the onset of symptoms and surgical resection; and (2) alternative management. These may be addressed by raising awareness of TIO, and taking into consideration the accessibility and variability of different healthcare infrastructures. By recognizing the challenges associated with the diagnosis and treatment of TIO and by applying a stepwise approach with clear clinical practice guidelines, patient care and outcomes will be improved in the future.

X-linked hypophosphatemia: The medical expert's challenges and the patient's concerns on their journey with the disease

X-linked hypophosphatemia (XLH) is a rare inheritable disorder of phosphate handling due to loss of function mutations of the PHEX gene, associated with increased production of FGF23 and impaired bone mineralization. In children, the disease's most common manifestations are bowing deformities of the lower limbs, short stature, and spontaneous dental abscesses. In adults, these are osteomalacia, insufficiency fractures, and enthesopathies associated with bone and joint pain. The XLH patient's journey with the disease may be difficult, reflecting concerns and experiences globally common to all patients with rare genetic diseases. Delays in diagnosis often preclude an optimal treatment outcome. Under-treatment is common as treating physicians, particularly those not familiar with the disease, tend to err on the side of caution, often choosing safety over efficacy. Physical abnormalities, pain, diminished function, and impaired mobility tend not only to isolate the XLH patient from his peers but also to have a significant psychological effect, eventually leading to significant impairment in quality of life. Significant advances in understanding the pathophysiology of XLH, the availability of a very comprehensive Evidence-based Guideline for the diagnosis and management of XLH, and the successful development of an effective and safe disease-specific novel therapy for XLH, have paved the way for a significant improvement in the management of this rare disorder of phosphate metabolism, heralding a significant improvement in the disease's outcome measures. Additional data from long-term observational studies and randomized controlled trials are eagerly awaited to consolidate these promising developments in the field of this rare disease.

X-Linked Hypophosphatemic Rickets: Report of a Novel PHEX Mutation and Cinacalcet as Adjuvant Therapy in the Mineral Metabolism Control

X-linked hypophosphatemic rickets (XLH) is a rare disease caused by a mutation in the PHEX gene, located on the X chromosome. This gene encodes the phosphate regulating endopeptidase, and its inactivation leads to increased levels of circulating phosphatonins responsible for renal phosphate loss. The treatment of XLH is still carried out with long-term administration of phosphate and calcitriol, which can be complicated by hyperparathyroidism, nephrocalcinosis, renal failure and hypertension. We describe the case of a four-decades follow-up patient with XLH. When she was diagnosed, at 19, due to bone pain and deformities, she was put on therapy with phosphorus and cholecalciferol. Despite the clinical improvement, serum phosphorus remained difficult to control. At the age of 44, she developed tertiary hyperparathyroidism and was submitted to parathyroidectomy. Five years later, parathyroid hyperfunction recurred. This time, cinacalcet was started, 30 mg alternating with 60 mg/day. Currently, she is 59 years-old and remains with controlled mineral metabolism. The genetic study of this patient revealed a nonsense heterozygous mutation (c.501G> A) in PHEX gene that was not previously described. In this case, the off-label use of cinacalcet resulted in the normalization of serum PTH and phosphorus levels, eliminated recurrent secondary hyperparathyroidism, which aggravates the bone fragility inherent to XLH and prevented a new parathyroidectomy. This report also adds important information to the genetic basis of XLH with the identification of a new nonsense mutation of the PHEX gene.

Clinical and Genetic Characteristics of 153 Chinese Patients With X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is caused by inactivating mutations in the phosphate-regulating endopeptidase homolog, X-linked (PHEX) gene, resulting in an excess of circulating intact fibroblast growth factor-23 (iFGF-23) and a waste of renal phosphate. In the present study, we retrospectively reviewed the clinical and molecular features of 153 Chinese patients, representing 87 familial and 66 sporadic cases with XLH. A total of 153 patients with XLH presented with signs or symptoms at a median age of 18.0 months (range, 9.0 months–26.0 years). Lower-limb deformity was the most frequent clinical manifestation, accounting for 79.1% (121/153). Biochemical screening showed increased serum levels of iFGF23 in patients with XLH, with a wide variation ranging from 14.39 to 730.70 pg/ml. Median values of serum iFGF23 in pediatric and adult patients were 94.87 pg/ml (interquartile range: 74.27–151.86 pg/ml) and 72.82 pg/ml (interquartile range: 39.42–136.00 pg/ml), respectively. Although no difference in circulating iFGF23 levels between these two groups was observed (P = 0.062), the proportion of patients with high levels of circulating iFGF23 (>42.2 pg/ml) was greater in the pediatric group than in the adult group (P = 0.026). Eighty-eight different mutations in 153 patients were identified, with 27 (30.7%) being novel. iFGF23 levels and severity of the disease did not correlate significantly with truncating and non-truncating mutations or N-terminal and C-terminal PHEX mutations. This study provides a comprehensive description of the clinical profiles, circulating levels of iFGF23 and gene mutation features of patients with XLH, further enriching the genotypic spectrum of the diseases. The findings show no evident correlation of circulating iFGF23 levels with the age or disease severity in patients with XLH.

Value-based decision-making for orphan drugs with multiple criteria decision analysis: burosumab for the treatment of X-linked hypophosphatemia

OBJECTIVE

Burosumab is an orphan medicinal product (OMP) approved in Europe for the treatment of X-linked hypophosphatemia (XLH). The aim of this study was to assess the value of burosumab versus conventional therapy for the treatment of paediatric XLH, through a multi-criteria decision analysis (MCDA) framework for health technology assessment (HTA) of OMPs in Portugal.

METHODS

The MCDA framework considered 14 criteria related to disease burden, therapeutic value and economic burden. A multidisciplinary panel of national stakeholders participated in a two-phase exercise. In the first phase, relative weights and part-worth utilities for the criteria and their levels were elicited and a reimbursement likelihood function was calibrated through adaptive conjoint analysis. In the second phase, burosumab and conventional therapy were assessed against the criteria, providing a global value score (0-100) and reimbursement likelihood (0-100%) for both.

RESULTS

Of the 14 criteria, disease burden, therapeutic value and economic burden criteria represented 27.29%, 57.17% and 15.53% of the total weight in the decision, respectively. All disease burden and some therapeutic value criteria, typically not included in traditional HTA, represented 47.88% of the total weight. Burosumab was unanimously considered superior to conventional therapy, with an average (range) global value score of 84.96 (82.48-86.54) against 48.06 (43.37-57.68), and reimbursement likelihood of 97.50% (96.78%-98.32%) against 43.66% (31.48%-68.73%), respectively.

CONCLUSIONS

MCDA represents a powerful tool in HTA decision-making for OMPs. The results of this MCDA acknowledge burosumab as a disease-modifying drug, deemed superior to conventional therapy for the treatment of paediatric XLH.

Outcome of primary tubular tubulopathies diagnosed in pediatric age

BACKGROUND AND OBJECTIVE

Primary tubulopathies are rare and usually present at pediatric age. Recent advances in genetic diagnosis and treatment have changed its natural history. This study provides the clinical spectrum of a series of primary tubulopathies diagnosed in a Pediatric Nephrology Unit and to offer long-term follow-up data regarding growth, estimated glomerular filtration (eGFR) and intercurrent complications.

PATIENTS AND METHODS

Observational study in 53 patients with primary tubulopathies and identified genetic defect: Gitelman syndrome (36%), distal renal tubular acidosis (15%), cystinuria (11%), X-linked hypophosphatemic rickets (7%), Dent-syndrome Lowe (7%), cystinosis (6%), and 1-2 cases of other tubulopathies. Demographic, analytical and clinical data were collected at diagnosis, during evolution and at the time of the study.

RESULTS

The age (median and interquartile range) at diagnosis was 5.08 years (1.33-8.50). The most frequent presentation manifestations were metabolic decompensations associated with intercurrent processes (40%) and short stature (38%). Height (mean ± SD) was -1.39 ± 1.49 at diagnosis and 1.07 ± 1.54 after a follow-up of 18.92 (6.25-24.33) years. Sixteen (32%) developed an eGFR <90 ml/min/1.73 m². Three patients required replacement renal replacement. Eleven patients had metabolic decompensations that required hospitalization, 9 renal colic and/or kidney stones and 10 mental problems. Six of 8 patients with distal renal tubular acidosis developed sensorineural deafness.

CONCLUSIONS

Primary tubulopathies are a heterogeneous group of diseases that cause growth impairment, largely reversible with treatment, risk of eGFR reduction and significant extrarenal complications derived or associated.

Phenotypic characterization of X-linked hypophosphatemia in pediatric Spanish population

Background

X-linked hypophosphatemia (XLH) is a hereditary rare disease caused by loss-of-function mutations in PHEX gene leading tohypophosphatemia and high renal loss of phosphate. Rickets and growth retardation are the major manifestations of XLH in children, but there is a broad phenotypic variability. Few publications have reported large series of patients. Current data on the clinical spectrum of the disease, the correlation with the underlying gene mutations, and the long-term outcome of patients on conventional treatment are needed, particularly because of the recent availability of new specific medications to treat XLH.

Results

The RenalTube database was used to retrospectively analyze 48 Spanish patients (15 men) from 39 different families, ranging from 3 months to 8 years and 2 months of age at the time of diagnosis (median age of 2.0 years), and with XLH confirmed by genetic analysis. Bone deformities, radiological signs of active rickets and growth retardation were the most common findings at diagnosis. Mean (± SEM) height was − 1.89 ± 0.19 SDS and 55% (22/40) of patients had height SDS below—2. All cases had hypophosphatemia, serum phosphate being − 2.81 ± 0.11 SDS. Clinical manifestations and severity of the disease were similar in both genders. No genotype—phenotype correlation was found. Conventional treatment did not attenuate growth retardation after a median follow up of 7.42 years (IQR = 11.26; n = 26 patients) and failed to normalize serum concentrations of phosphate. Eleven patients had mild hyperparathyroidism and 8 patients nephrocalcinosis.

Conclusions

This study shows that growth retardation and rickets were the most prevalent clinical manifestations at diagnosis in a large series of Spanish pediatric patients with XLH confirmed by mutations in the PHEX gene. Traditional treatment with phosphate and vitamin D supplements did not improve height or corrected hypophosphatemia and was associated with a risk of hyperparathyroidism and nephrocalcinosis. The severity of the disease was similar in males and females.

X-Linked Hypophosphatemic Rickets: Multisystemic Disorder in Children Requiring Multidisciplinary Management

X-linked hypophosphatemic rickets (XLH) is the commonest inherited form of rickets. It is caused by an impaired regulation of fibroblast growth factor 23 (FGF23) due to a PHEX gene mutation, which leads to reduced tubular reabsorption of phosphate and renal 1α-hydroxylase activity and increased renal 24-hydroxylase activity. Hypophosphatemia associated with renal phosphate wasting, normal serum levels of calcium, parathyroid hormone, and 25-hydroxyvitamin D represents the main biochemical sign in affected patients. Patients with XLH show rickets and osteomalacia, severe deformities of the lower limbs, bone and muscular pain, stunted growth, and reduced quality of life. However, XLH is a multisystemic disorder requiring multidisciplinary approaches in specialized subdisciplines. Severe complications may occur in patients with XLH including craniosynostosis, hearing loss, progressive bone deformities, dental and periodontal recurrent lesions, and psychosocial distress. Moreover, long-term conventional treatment with active vitamin D metabolites and oral inorganic phosphate salts may cause endocrinological complications such as secondary or tertiary hyperparathyroidism, and adverse events in kidney as hypercalciuria, nephrocalcinosis, and nephrolithiasis. However, conventional treatment does not improve phosphate metabolism and it shows poor and slow effects in improving rickets lesions and linear growth. Recently, some trials of treatment with recombinant human IgG1 monoclonal antibody that targets FGF23 (burosumab) showed significant improvement of serum phosphate concentration and renal tubular reabsorption of phosphate that were associated with a rapid healing of radiologic signs of rickets, reduced muscular and osteoarticular pain, and improved physical function, being more effective for the treatment of patients with XLH in comparison with conventional therapy. Therefore, a global management of patients with XLH is strongly recommended and patients should be seen regularly by a multidisciplinary team of experts.

Development of Enthesopathies and Joint Structural Damage in a Murine Model of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia, caused by inactivating mutations in the Phosphate-regulating endopeptidase homolog X-linked (PHEX) gene. With aging, adult patients develop paradoxical heterotopic calcifications of tendons and ligaments at their insertion sites (enthesophytes), and joint alterations. Understanding the progression of this structural damage that severely affects patients’ quality of life will help to improve the management of XLH. Here, we characterized the occurrence of enthesophytes and joint alterations through a 12 month in vivo micro-CT follow-up in the Hyp mouse, a murine model of XLH (n = 5 mice per group). Similar to adult patients with XLH, Hyp mice developed calcaneal enthesophytes, hip joint alterations, erosions of the sacroiliac joints and periarticular calcifications. These lesions were already present at month 3 and gradually worsened over time. In sharp contrast, no abnormalities were observed in control mice at early time points. Histological analyses confirmed the presence of bone erosions, calcifications and expansion of mineralizing enthesis fibrocartilage in Hyp mice and their absence in controls and suggested that new bone formation is driven by altered mechanical strain. Interestingly, despite a strong deformation of the curvature, none of the Hyp mice displayed enthesophyte at the spine. Peripheral enthesophytes and joint alterations develop at the early stages of the disease and gradually worsen overtime. Overall, our findings highlight the relevance of this preclinical model to test new therapies aiming to prevent bone and joint complications in XLH.

Duodenal Microbiota in Stunted Undernourished Children with Enteropathy

BACKGROUND

Environmental enteric dysfunction (EED) is an enigmatic disorder of the small intestine that is postulated to play a role in childhood undernutrition, a pressing global health problem. Defining the incidence of this disorder, its pathophysiological features, and its contribution to impaired linear and ponderal growth has been hampered by the difficulty in directly sampling the small intestinal mucosa and microbial community (microbiota).

METHODS

In this study, among 110 young children (mean age, 18 months) with linear growth stunting who were living in an urban slum in Dhaka, Bangladesh, and had not benefited from a nutritional intervention, we performed endoscopy in 80 children who had biopsy-confirmed EED and available plasma and duodenal samples. We quantified the levels of 4077 plasma proteins and 2619 proteins in duodenal biopsy samples obtained from these children. The levels of bacterial strains in microbiota recovered from duodenal aspirate from each child were determined with the use of culture-independent methods. In addition, we obtained 21 plasma samples and 27 fecal samples from age-matched healthy children living in the same area. Young germ-free mice that had been fed a Bangladeshi diet were colonized with bacterial strains cultured from the duodenal aspirates.

RESULTS

Of the bacterial strains that were obtained from the children, the absolute levels of a shared group of 14 taxa (which are not typically classified as enteropathogens) were negatively correlated with linear growth (length-for-age z score, r = -0.49; P = 0.003) and positively correlated with duodenal proteins involved in immunoinflammatory responses. The representation of these 14 duodenal taxa in fecal microbiota was significantly different from that in samples obtained from healthy children (P<0.001 by permutational multivariate analysis of variance). Enteropathy of the small intestine developed in gnotobiotic mice that had been colonized with cultured duodenal strains obtained from children with EED.

CONCLUSIONS

These results provide support for a causal relationship between growth stunting and components of the small intestinal microbiota and enteropathy and offer a rationale for developing therapies that target these microbial contributions to EED. (Funded by the Bill and Melinda Gates Foundation and others; ClinicalTrials.gov number, NCT02812615.).

A novel c.2179T>C mutation blocked the intracellular transport of PHEX protein and caused X-linked hypophosphatemic rickets in a Chinese family

Background

X‐linked hypophosphatemic rickets (XLH) is a heterogeneous genetic phosphate wasting disorder that occupies the majority of inheritable hypophosphatemic rickets (HR). XLH is caused by loss‐of‐function mutations in the phosphate‐regulating endopeptidase gene (PHEX) located on the X chromosome.

Method

In this study, we performed whole‐exome sequencing (WES) on the proband to identify the causative gene. The mutations were analyzed by predictive online software, such as PolyPhen‐2. Plasmids containing the wild‐type (WT) and mutant cDNA of the candidate gene were transfected into HEK293, then, the expression, cellular localization, and glycosylation state of the candidate proteins were detected by western blot, immunostaining, and endoglycosidase H digestion. The expression and concentration of related factor were measured by RT‐PCR and ELISA.

Results

We identified a novel missense mutation c.2179T>C in the PHEX that results in the substitution of p.Phe727Leu (F727L). This mutation was predicted to be disease‐causing by all four predictive online software. In vitro studies demonstrated that the F727L substitution hindered the intracellular trafficking of the mutant PHEX, with ~59% of mutant PHEX protein retained in the endoplasmic reticulum (ER) and only ~16% of the mutant protein localized on the cell surface. Endoglycosidase H digestion assay showed that the mutant F727L PHEX protein was not fully glycosylated. The concentration of intact FGF23 in hFOB1.19 cell culture medium collected from the mutant PHEX group was the highest (62.9 pg/ml) compared to the WT group (32.1 pg/ml) and control group (23.5 pg/ml).

Conclusion

Our results confirmed that the mutant PHEX protein was lowly glycosylated and retarded within the ER, the intact FGF23 level in cell culture media caused by the mutant PHEX protein was significantly elevated compared to that of the WT group, which may explain why the single base mutation in the PHEX led to XLH syndrome in this family.

Nationwide Turkish Cohort Study of Hypophosphatemic Rickets

OBJECTIVE

Hypophosphatemic rickets (HR) is a rare renal phosphate-wasting disorder, which is usually X-linked and is commonly caused by PHEX mutations. The treatment and follow-up of HR is challenging due to imperfect treatment options.

METHODS

Here we present nationwide initial and follow-up data on HR.

RESULTS

From 24 centers, 166 patients were included in the study. Genetic analysis (n=75) showed PHEX mutation in 80% of patients. The mean follow-up period was 6.7±2.4 years. During the first 3-years of treatment (n=91), mild increase in phosphate, decrease in alkaline phosphatase and elevation in parathyroid hormone (PTH) levels were detected. The height standard deviation scores were -2.38, -2.77, -2.72, -2.47 at initial, 1^st, 2^nd and 3^rd year of treatment, respectively (p>0.05). On follow-up 36% of the patients showed complete or significant improvement in leg deformities and these patients had similar phosphate levels at presentation with better levels in 1^st and 2^nd years of treatment; even the treatment doses of phosphate were similar. Furthermore, 27 patients developed nephrocalcinosis (NC), the patients showed no difference in biochemical differences at presentation and follow-up, but 3^rd year PTH was higher. However, higher treatment doses of phosphate and calcitriol were found in the NC group.

CONCLUSION

HR treatment and follow-up is challenging and our results showed higher treatment doses were associated with NC without any change in serum phosphate levels, suggesting that giving higher doses led to increased phosphaturia, probably through stimulation of fibroblast growth factor 23. However, higher calcitriol doses could improve bone deformities. Safer and more efficacious therapies are needed.

Genetic Diagnosis of Rare Diseases: Past and Present

Rare diseases are heterogeneous life-threatening or seriously debilitating conditions that affect < 1 in 2000 individuals, and most have a genetic component. The diagnostic process is usually based on classic clinical practices, such as physical examination, personal and family history (inheritance pattern), laboratory tests and image studies, but diagnosis can be delayed several years after the initiation of symptoms. The advances in molecular genetics that have taken place in recent years have led to an important shift in medical practice and in its approach to the diagnosis and treatment of many rare diseases. The objective of this review is to promote a better understanding of the mechanisms underlying genetic diseases in humans and the tools available for their diagnosis. A practical example of X-linked hypophosphataemic rickets is described.

X-Linked Hypophosphataemic Rickets and Growth

X-linked hypophosphataemia (XLH) is the most prevalent form of hereditary rickets characterized by an alteration of phosphate metabolism which frequently leads to the appearance of fractures, bone deformities and growth delay. Although the mechanism of growth impairment in patients with XLH still needs to be clarified, it is known that this alteration is not due to genetic or endocrine factors. A potential explanation for the impairment of growth in this disease is the alteration of the growth plate, a structure responsible for longitudinal growth of bones. Some of the findings in the growth plate of patients with XLH include atypical organization of chondrocytes due to low rates of proliferation and apoptosis and disturbance of chondrocyte hypertrophy, overactivation of the mitogen-activated protein kinase (MAPK) signalling pathway and upregulation of phosphorylated extracellular signal-regulated kinase (pERK). Conventional treatment of XLH (consisting of oral phosphate supplements and active vitamin D analogues) is often insufficient for the longitudinal growth of bone, but other strategies based on recombinant growth hormone or therapies targeting fibroblast growth factor 23 (FGF23) or its receptor, such as burosumab, have shown promising results. This article briefly describes the relationship between XLH and growth retardation, and how to address this alteration in patients with XLH.

New perspectives on the treatment of skeletal dysplasia

The last few decades have been marked by the identification of numerous genes implicated in genetic disorders, helping in the elucidation of the underlying pathophysiology of these conditions. This has allowed new therapeutic approaches to emerge such as cellular therapy, gene therapy, or pharmacological therapy for various conditions. Skeletal dysplasias are good models to illustrate these scientific advances. Indeed, several therapeutic strategies are currently being investigated in osteogenesis imperfecta; there are ongoing clinical trials based on pharmacological approaches, targeting signaling pathways in achondroplasia and fibrodysplasia ossificans progressiva or the endoplasmic reticulum stress in metaphyseal dysplasia type Schmid or pseudoachondroplasia. Moreover, the treatment of hypophosphatasia or Morquio A disease illustrates the efficacy of enzyme drug replacement. To provide a highly specialized multidisciplinary approach, these treatments are managed by reference centers. The emergence of treatments in skeletal dysplasia provides new perspectives on the prognosis of these severe conditions and may change prenatal counseling in these diseases over the coming years.

Familial hypophosphatemic rickets caused by a PHEX gene mutation accompanied by a NPR2 missense mutation

Background Familial hypophosphatemic rickets, which is usually acknowledged as X-linked hypophosphatemic rickets (XLH), is a rare hereditary disease. XLH caused by mutations in the PHEX gene often manifests as growth retardation, skeletal deformities, osteodynia and dental dysplasia. NPR2 mutations are reported to cause disproportionate short stature. Our study was designed to identify the gene mutations of three patients in one family. Case description A 40-year-old Chinese male visited the hospital for continuous osteodynia and presented with bilateral leg bowing, absent teeth and a progressive limp. The age of onset was approximately 2 years old. His 63-year-old mother and 42-year-old brother both shared identical symptoms with him. The laboratory tests were consistent with XLH, which showed decreased levels of blood phosphorus and 1,25-dihydroxyvitamin D3 as well as increased urinary phosphorus excretion. Mutation analysis revealed that the proband as well as his mother and his brother all had a PHEX mutation in exon 14 (c.1543C > T), and the proband also had a NPR2 mutation in exon 21 (c.3058C > T). Conclusions We report the familial hypophosphatemic rickets of three patients in a Chinese family caused by a PHEX gene mutation in exon 14 (c.1543C > T), which had never been reported in Chinese patients. We first report an XLH case together with a NPR2 mutation that had never been reported before.

Diagnosis, treatment-monitoring and follow-up of children and adolescents with X-linked hypophosphatemia (XLH)

Early diagnosis, optimal therapeutic management and regular follow up of children with X-linked hypophosphatemia (XLH) determine their long term outcomes and future quality of life. Biochemical screening of potentially affected newborns in familial cases and improving physician's knowledge on clinical signs, symptoms and biochemical characteristics of XLH for de novo cases should lead to earlier diagnosis and treatment initiation. The follow-up of children with XLH includes clinical, biochemical and radiological monitoring of treatment (efficacy and complications) and screening for XLH-related dental, neurosurgical, rheumatological, cardiovascular, renal and ENT complications. In 2018, the European Union approved the use of burosumab, a humanized monoclonal anti-FGF23 antibody, as an alternative therapy to conventional therapy (active vitamin D analogues and phosphate supplements) in growing children with XLH and insufficiently controlled disease. Diagnostic criteria of XLH and the principles of disease management with conventional treatment or with burosumab are reviewed in this paper.

Positive Response to One-Year Treatment With Burosumab in Pediatric Patients With X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) causes significant burden in pediatric patients in spite of maintained treatment with phosphate supplements and vitamin D derivatives. Administration of burosumab has shown promising results in clinical trial but studies assessing its effect in the everyday practice are missing. With this aim, we analyzed the response to one-year treatment with burosumab, injected subcutaneously at 0.8 mg/kg every 2 weeks, in five children (three females) aged from 6 to 16 years, with genetically confirmed XLH. Patients were being treated with phosphate and vitamin D analogs until the beginning of burosumab treatment. In all children, burosumab administration led to normalization of serum phosphate in association with marked increase of tubular reabsorption of phosphate and reduction of elevated serum alkaline phosphatase levels. Baseline height of patients, from -3.56 to -0.46 SD, increased in the three prepubertal children (+0.84, +0.89, and +0.16 SD) during burosumab treatment. Growth improvement was associated with reduction in body mass index (-1.75, -1.47, and -0.17 SD, respectively), suggesting a salutary effect of burosumab on physical activity and body composition. Burosumab was well-tolerated, mild local pain at the injection site and transient and mild headache following the initial doses of burosumab being the only reported undesirable side effects. No patient exhibited hyperphosphatemia, progression of nephrocalcinosis, worsening of metabolic control or developed hyperparathyroidism. Mild elevation of serum PTH present at the beginning of treatment in one patient 4 was not modified by burosumab administration. These results indicate that in the clinical setting, beyond the strict conditions and follow-up of clinical trials, burosumab treatment for 1 year exerts positive effects in pediatric patients with XLH without major adverse events.

Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is the most common cause of inherited phosphate wasting and is associated with severe complications such as rickets, lower limb deformities, pain, poor mineralization of the teeth and disproportionate short stature in children as well as hyperparathyroidism, osteomalacia, enthesopathies, osteoarthritis and pseudofractures in adults. The characteristics and severity of XLH vary between patients. Because of its rarity, the diagnosis and specific treatment of XLH are frequently delayed, which has a detrimental effect on patient outcomes. In this Evidence-Based Guideline, we recommend that the diagnosis of XLH is based on signs of rickets and/or osteomalacia in association with hypophosphataemia and renal phosphate wasting in the absence of vitamin D or calcium deficiency. Whenever possible, the diagnosis should be confirmed by molecular genetic analysis or measurement of levels of fibroblast growth factor 23 (FGF23) before treatment. Owing to the multisystemic nature of the disease, patients should be seen regularly by multidisciplinary teams organized by a metabolic bone disease expert. In this article, we summarize the current evidence and provide recommendations on features of the disease, including new treatment modalities, to improve knowledge and provide guidance for diagnosis and multidisciplinary care.

Risk of cardiovascular involvement in pediatric patients with X-linked hypophosphatemia

OBJECTIVE

To find out if cardiovascular alterations are present in pediatric patients with X-linked hypophosphatemia (XLH).

STUDY DESIGN

Multicentre prospective clinical study on pediatric patients included in the RenalTube database ( www.renaltube.com ) with genetically confirmed diagnosis of XLH by mutations in the PHEX gene. The study's protocol consisted of biochemical work-up, 24-h ambulatory blood pressure monitoring (ABPM), carotid ultrasonography, and echocardiogram. All patients were on chronic treatment with phosphate supplements and 1-hydroxy vitamin D metabolites.

RESULTS

Twenty-four patients (17 females, from 1 to 17 years of age) were studied. Serum concentrations (X ± SD) of phosphate and intact parathyroid hormone were 2.66 ± 0.60 mg/dl and 58.3 ± 26.8 pg/ml, respectively. Serum fibroblast growth factor 23 (FGF23) concentration was 278.18 ± 294.45 pg/ml (normal < 60 pg/ml). Abnormally high carotid intima media thickness was found in one patient, who was obese and hypertensive as revealed by ABPM, which disclosed arterial hypertension in two other patients. Z scores for echocardiographic interventricular septum end diastole and left ventricular posterior wall end diastole were + 0.77 ± 0.77 and + 0.94 ± 0.86, respectively. Left ventricular mass index (LVMI) was 44.93 ± 19.18 g/m^2.7, and four patients, in addition to the obese one, had values greater than 51 g/m^2.7, indicative of left ventricular hypertrophy. There was no correlation between these echocardiographic parameters and serum FGF23 concentrations.

CONCLUSIONS

XLH pediatric patients receiving conventional treatment have echocardiographic measurements of ventricular mass within normal reference values, but above the mean, and 18% have LVMI suggestive of left ventricular hypertrophy without correlation with serum FGF23 concentrations. This might indicate an increased risk of cardiovascular involvement in XLH.

Hypophosphatemic Rickets

Hypophosphatemic rickets, mostly of the X-linked dominant form caused by pathogenic variants of the PHEX gene, poses therapeutic challenges with consequences for growth and bone development and portends a high risk of fractions and poor bone healing, dental problems and nephrolithiasis/nephrocalcinosis. Conventional treatment consists of PO4 supplements and calcitriol requiring monitoring for treatment-emergent adverse effects. FGF23 measurement, where available, has implications for the differential diagnosis of hypophosphatemia syndromes and, potentially, treatment monitoring. Newer therapeutic modalities include calcium sensing receptor modulation (cinacalcet) and biological molecules targeting FGF23 or its receptors. Their long-term effects must be compared with those of conventional treatments.

Analysis of 2 novel mutations of PHEX gene inducing X-linked dominant hypophosphatemia rickets in 2 families: Two case reports

RATIONALE

X-linked dominant hypophosphatemia rickets (XLH, OMIM 307800) is the most common hereditary hypophosphatemic rickets and characterized by growth retardation, skeletal malformations, dental dysplasia, spontaneous fractures and osteomalacia. PHEX gene was identified for XLH and novel mutations were consistent with loss of function.

PATIENT CONCERNS

Case1: the proband 1 III3 in family 1 was a fourteen-year-old boy with bowing of bilateral legs, obviously enlarged joints, tooth absence and difficulty in walking; X-rays showed bilateral femoral multiple fractures with sclerosis at the fracture edge. Case 2: the proband 2 III2, a five-year-old boy in family 2, showed growth retardation, dental dysplasia, gingiva abscess and bilateral legs malformations; X-rays showed low bone density, delayed bone age, bowing of legs, frayed and widened metaphyses of the distal femurs and proximal tibias. Both of their mothers suffered from skeletal malformations, tooth absence and were performed with osteotomy due to fractures of lower limb. Their biochemical parameters showed hypophosphatemia, elevated alkaline phosphatase.

DIAGNOSES

X-linked dominant hypophosphatemia rickets (XLH).

INTERVENTIONS AND OUTCOMES

Treatment with high doses of phosphate and 1,25-dihydroxyvitamin D3, the height of proband 2 increased 10 cm and femoral Multiple fracture of proband1 almost healed after treatment for 6 months and the patients's PHEX gene was investigated.

LESSONS

Two novel pathogenic PHEX mutations were found: c.497delG in family 1 and c.388G> T in family 2, both of which caused early termination of translation and produced truncated protein. Serum FGF23 concentration in our XLH patients were obviously higher than the normal and may be related to age to some extent. Early initiation of treatment produces better effect.

Genetic analysis of three families with X-linked dominant hypophosphatemic rickets

Background Hypophosphatemic rickets, including familial hypophosphatemic vitamin D-resistant rickets, which commonly manifests in childhood, is generally hereditary. X-linked dominant hypophosphatemic rickets (XLH, MIM307800), caused by inactivating mutations in the PHEX gene, is the most common form. This study aimed to identify the gene mutations responsible for three cases of XLH and its clinical phenotype. Methods We conducted a genetic diagnosis and clinical phenotypic linkage analysis of three pedigrees with XLH. Three probands finally diagnosed as XLH were analyzed by next-generation sequencing (NGS). Sanger sequencing was used for mutation scanning in other family members. Results For the three patients with XLH, the age of onset ranged from 1.5 to 2 years and their heights were less than three standard deviations (SDs) below the median. The patients exhibited curved deformities in both lower limbs, hypophosphatemia, elevated serum FGF23 levels and elevated levels of blood alkaline phosphatase, with normal levels of blood parathyroid hormone (PTH) and calcium. X-ray analysis of the limbs and chest revealed characteristic rickets signs. Three candidate pathogenic mutations were identified in PHEX (NM_000444.5): c.433G>T (p.Glu145*, p.E145*) in exon 4, c.1735G>A (p.Gly579Arg, p.G579R) (rs875989883) in exon 17 and c.2245T>C (p.Trp749Arg, p.W749R) in exon 22. The nonsense mutation (p.E145*) in PHEX is novel and is predicted to cause a truncation of the encoded protein, resulting in loss of function. Conclusions The novel nonsense mutation (p.E145*) in PHEX is possibly involved in inherited XLH.

Genetic Risk Factors for Atypical Femoral Fractures (AFFs): A Systematic Review

Atypical femoral fractures (AFFs) are uncommon and have been associated particularly with long‐term antiresorptive therapy, including bisphosphonates. Although the pathogenesis of AFFs is unknown, their identification in bisphosphonate‐naïve individuals and in monogenetic bone disorders has led to the hypothesis that genetic factors predispose to AFF. Our aim was to review and summarize the evidence for genetic factors in individuals with AFF. We conducted structured literature searches and hand‐searching of conference abstracts/reference lists for key words relating to AFF and identified 2566 citations. Two individuals independently reviewed citations for (i) cases of AFF in monogenetic bone diseases and (ii) genetic studies in individuals with AFF. AFFs were reported in 23 individuals with the following 7 monogenetic bone disorders (gene): osteogenesis imperfecta (COL1A1/COL1A2), pycnodysostosis (CTSK), hypophosphatasia (ALPL), X‐linked osteoporosis (PLS3), osteopetrosis, X‐linked hypophosphatemia (PHEX), and osteoporosis pseudoglioma syndrome (LRP5). In 8 cases (35%), the monogenetic bone disorder was uncovered after the AFF occurred. Cases of bisphosphonate‐naïve AFF were reported in pycnodysostosis, hypophosphatasia, osteopetrosis, X‐linked hypophosphatemia, and osteoporosis pseudoglioma syndrome. A pilot study in 13 AFF patients and 268 controls identified a greater number of rare variants in AFF cases using exon array analysis. A whole‐exome sequencing study in 3 sisters with AFFs showed, among 37 shared genetic variants, a p.Asp188Tyr mutation in the GGPS1 gene in the mevalonate pathway, critical to osteoclast function, which is also inhibited by bisphosphonates. Two studies completed targeted ALPL gene sequencing, an ALPL heterozygous mutation was found in 1 case of a cohort of 11 AFFs, whereas the second study comprising 10 AFF cases did not find mutations in ALPL. Targeted sequencing of ALPL, COL1A1, COL1A2, and SOX9 genes in 5 cases of AFF identified a variant in COL1A2 in 1 case. These findings suggest a genetic susceptibility for AFFs. A large multicenter collaborative study of well‐phenotyped AFF cases and controls is needed to understand the role of genetics in this uncommon condition. © 2017 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Genetic Causes of Rickets

Rickets is a metabolic bone disease that develops as a result of inadequate mineralization of growing bone due to disruption of calcium, phosphorus and/or vitamin D metabolism. Nutritional rickets remains a significant child health problem in developing countries. In addition, several rare genetic causes of rickets have also been described, which can be divided into two groups. The first group consists of genetic disorders of vitamin D biosynthesis and action, such as vitamin D-dependent rickets type 1A (VDDR1A), vitamin D-dependent rickets type 1B (VDDR1B), vitamin D-dependent rickets type 2A (VDDR2A), and vitamin D-dependent rickets type 2B (VDDR2B). The second group involves genetic disorders of excessive renal phosphate loss (hereditary hypophosphatemic rickets) due to impairment in renal tubular phosphate reabsorption as a result of FGF23-related or FGF23-independent causes. In this review, we focus on clinical, laboratory and genetic characteristics of various types of hereditary rickets as well as differential diagnosis and treatment approaches.