X-linked hypophosphatemia: normal renal function despite medullary nephrocalcinosis 25 years after transient vitamin D2-induced renal azotemia

Misdiagnosed metabolic bone abnormality: a case report

BACKGROUND

Metabolic bone disease causes significant morbidity and mortality, especially when misdiagnosed. With genetic testing, multiple disease pathologies can be analyzed.

CASE PRESENTATION

A 5-year and 9-month-old otherwise healthy Yemeni girl presented to her Yemen physician for evaluation of inward bending of her right knee and short stature. After extensive medical testing, she was given a diagnosis of hypophosphatemic rickets and growth hormone deficiency and started on treatment. Despite appropriate treatment, however, her condition continued to progress, prompting her family to pursue additional workup including genetic testing outside of Yemen. Genetic testing ultimately revealed a variation of unknown significance associated with amelogenesis imperfecta.

CONCLUSIONS

Hypophosphatemic rickets secondary to renal tubular acidosis was the working diagnosis. However, the patient's condition did not improve. Further genetic testing revealed a variation of unknown significance associated with amelogenesis imperfecta. We aim to present this case, provide an overview of the causes, and diagnostic metabolic bone health evaluation.

Identification of a Novel Missense Mutation of the PHEX Gene in a Large Chinese Family with X-Linked Hypophosphataemia

X-linked hypophosphataemia (XLH) is an X-linked dominant rare disease that refers to the most common hereditary hypophosphatemia (HH) caused by mutations in the phosphate-regulating endopeptidase homolog X-linked gene (PHEX; OMIM: * 300550). However, mutations that have already been reported cannot account for all cases of XLH. Extensive genetic analysis can thus be helpful for arriving at the diagnosis of XLH. Herein, we identified a novel heterozygous mutation of PHEX (NM_000444.5: c.1768G > A) in a large Chinese family with XLH by whole-exome sequencing (WES). In addition, the negative effect of this mutation in PHEX was confirmed by both bioinformatics analysis and in vitro experimentation. The three-dimensional protein-model analysis predicted that this mutation might impair normal zinc binding. Immunofluorescence staining, qPCR, and western blotting analysis confirmed that the mutation we detected attenuated PHEX protein expression. The heterozygous mutation of PHEX (NM_000444.5: c.1768G > A) identified in this study by genetic and functional experiments constitutes a novel genetic cause of XLH, but further study will be required to expand its use in clinical and molecular diagnoses of XLH.

Incidence of Complications in 25 Adult Patients With X-linked Hypophosphatemia

CONTEXT

Adults with X-linked hypophosphatemia (XLH) present complications other than osteomalacia.

OBJECTIVE

To describe the incidence and severity of comorbidities in adults with XLH.

METHODS

This observational retrospective study included a total of 25 adults with XLH with thorough investigations, including spinal computed tomography scans, x-rays of hip/knee joints and Achilles tendons, abdominal ultrasounds, and audiograms. The index of ossification of the anterior/posterior longitudinal ligament and yellow ligament (OA/OP/OY index) and the sum of OA/OP/OY index (OS index) were utilized to evaluate the severity of spinal ligament ossification. The Kellgren-Lawrence (KL) classification was adopted to evaluate the severity of the hip/knee osteophytes.

RESULTS

The participants consisted of 13 male patients and 12 female patients from 21 families, with a median age of 43 (range, 18-72) years. In all, 20 patients (80%) showed spinal ligament ossification. The median OA/OP/OY/OS indices were 2 (0-22), 0 (0-15), 6 (0-13), and 12 (0-41), respectively. Hip/knee osteophytes were reported in 24 (96%) and 17 cases (68%). The median KL grade was 3 in the hip joint and 2 in the knee joint, and 18 cases (72%) developed enthesopathy in the Achilles tendon. Nephrocalcinosis and hearing impairment were observed in 18 (72%) and 8 (32%) cases.

CONCLUSION

This study revealed a high prevalence and severity of ectopic ossification and disclosed the incidence of nephrocalcinosis and hearing impairment in adults with XLH. In cases with severe spinal ligament ossification or noticeable osteophytes around the hip/knee joints, undiagnosed XLH should be considered as a possible underlying condition.

Biopsychosoziales Betreuungskonzept für Kinder mit X‑chromosomaler Hypophosphatämie (XLH)

Patienten mit einer schweren, seltenen und komplexen Erkrankung benötigen das multiprofessionelle biopsychosoziale Betreuungskonzept eines Sozialpädiatrischen Zentrums für chronisch kranke Kinder und Jugendliche. Am Beispiel der Multiorganerkrankung X‑chromosomale Hypophosphatämie (XLH), der häufigsten angeborenen Rachitisform, wird das Betreuungskonzept dargestellt.

Der Erkrankung liegen inaktivierende Mutationen im „Phosphate-regulating gene with Homologies to Endopeptidases on the X‑chromosome“(PHEX)-Gen zugrunde, die zu einer vermehrten Synthese und Sekretion des Fibroblastenwachstumsfaktors 23 (FGF23) führen. FGF23 hat eine wichtige Funktion in der Phosphathomöostase. Durch die hohen FGF23-Konzentrationen kommt es über den renalen Phosphatverlust zur schweren Hypophosphatämie mit der Folge erheblicher Mineralisationsstörungen des Skelettsystems und der Zähne. Stand bis vor Kurzem nur eine konventionelle medikamentöse Therapie aus Phosphat und aktivem Vitamin D zur Verfügung, so ist nun mit dem neutralisierenden FGF23-Antikörper eine gezielte Therapie der Erkrankung möglich. Das multiprofessionelle Betreuungskonzept umfasst zahlreiche ärztliche Spezialisten und ein psychosoziales Team. Ziel der Betreuung ist es, den Patienten mit ihrer schweren chronischen Erkrankung eine altersentsprechende Partizipation ohne größere Teilhabestörung zu ermöglichen. Die Fortsetzung der Betreuung im Erwachsenenalter muss durch eine implementierte Transition sichergestellt werden.

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.

In this Evidence-Based Guideline on X-linked hypophosphataemia, the authors identify the criteria for diagnosis of this disease, provide guidance for medical and surgical treatment and explain the challenges of follow-up.

FGF23, Hypophosphatemia, and Emerging Treatments

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

Thiazide diuretics arrest the progression of nephrocalcinosis in children with X-linked hypophosphatemia

OBJECTIVE

X-linked hypophosphatemia (XLH) is characterized clinically by rickets, hypophosphatemia, and hyperphosphaturia. Conventional treatment of XLH with oral phosphate and vitamin D is associated with increased urinary calcium excretion and nephrocalcinosis. Thiazide diuretics decrease urinary calcium excretion. The objective of this study was to determine the effect of thiazide diuretics on the clinical and radiologic course of nephrocalcinosis in children with XLH.

METHODS

The effect of hydrochlorothiazide (HCTZ) on clinical and radiologic progression of nephrocalcinosis was evaluated in 11 children with XLH. All patients had been treated previously with vitamin D and oral phosphate and had radiologic evidence of nephrocalcinosis. The average age of the patients at the start of HCTZ was 6.6 +/- 1.0 years. The effect of oral HCTZ at 0.8 +/- 0.1 mg/kg body weight per day given for 3.3 +/- 0.6 years on the progression of nephrocalcinosis and urinary calcium excretion was evaluated.

RESULTS

There was no change in serum phosphorous, calcium, potassium, and chloride after HCTZ therapy. HCTZ therapy increased serum bicarbonate and decreased urinary calcium excretion. The grade of nephrocalcinosis increased from 0.4 +/- 0.2 to 1.5 +/- 0.3 in the 2.3 +/- 0.3 years before initiation of HCTZ therapy, whereas the degree of nephrocalcinosis was stable after 3.3 +/- 0.6 years of HCTZ therapy (1.5 +/- 0.3 vs 3.0 +/- 0.3).

CONCLUSION

HCTZ decreased urinary calcium excretion but did not result in the resolution of nephrocalcinosis. However, when compared with the control period, HCTZ prevented the progression of nephrocalcinosis in children with XLH.

X-linked recessive spondyloepiphyseal dysplasia tarda. Clinical and radiographic evolution in a 6-generation kindred and review of the literature

We characterize the clinical and radiographic evolution of X-linked recessive spondyloepiphyseal dysplasia tarda (SEDT) in a 6-generation kindred from Arkansas (SEDT(AK)). Our observations show the natural progression of SEDT(AK) and enable carrier detection by radiographic study. We find that, SEDT(AK) manifests as a postnatal defect. Affected hemizygous males can have radiographically normal vertebrae at birth. The pathogenesis seems to involve a developmental disturbance in endochondral bone formation that is reflected most dramatically in vertebrae by a radiographically inapparent ring apophysis. This defect leads to distinctive malformation of the anterior margins of the lumbar vertebrae during childhood. Subsequently, there is degeneration of intervertebral discs and destruction of spinal facet joints. In the femur, the head, neck, and distal condyles are abnormally shaped and become distorted so that osteoarthritis of the hip is not uncommon. Obligate carrier females heterozygous for the SEDT(AK) gene defect demonstrate several similar but more subtle skeletal abnormalities beginning in early adult life. These women seem to be troubled frequently by arthralgia by middle age. The cumulative findings in SEDT(AK) implicate a defect in a gene at Xp22.2-22.1 that engenders a relatively mild disturbance in endochondral bone formation, especially in the axial skeleton. Accounts of large, well-characterized SEDT kindreds remain essential to appreciate fully any interfamily variability of disease expression and to understand better the pathogenesis of the SEDT defect on the X chromosome.