Rickets, the continuing challenge

Shedding Light on Vitamin D Status and Its Complexities during Pregnancy, Infancy and Childhood: An Australian Perspective

Ensuring that the entire Australian population is Vitamin D sufficient is challenging, given the wide range of latitudes spanned by the country, its multicultural population and highly urbanised lifestyle of the majority of its population. Specific issues related to the unique aspects of vitamin D metabolism during pregnancy and infancy further complicate how best to develop a universally safe and effective public health policy to ensure vitamin D adequacy for all. Furthermore, as Australia is considered a “sunny country”, it does not yet have a national vitamin D food supplementation policy. Rickets remains very uncommon in Australian infants and children, however it has been recognised for decades that infants of newly arrived immigrants remain particularly at risk. Yet vitamin D deficiency rickets is entirely preventable, with the caveat that when rickets occurs in the absence of preexisting risk factors and/or is poorly responsive to adequate treatment, consideration needs to be given to genetic forms of rickets.

Brief Review on Metabolic Bone Disease

Metabolic bone disease (MBD) is a broad term that describes a clinically heterogeneous group of diseases that are only united by a common denominator of an aberrant bone chemical milieu leading to a defective skeleton and bone abnormalities. From a forensic pathologist's perspective, MBDs create a challenging diagnostic dilemma in differentiating them from child abuse, particularly when the victim is an infant. Through this brief narrative review on MBD, bone pathophysiology and two relatively challenging pediatric MBDs will be discussed.

X-linked hypophosphatemic rickets and sagittal craniosynostosis: three patients requiring operative cranial expansion: case series and literature review

PURPOSE

A defect in a phosphate-regulating gene leads to the most common form of rickets: X-linked hypophosphatemic rickets (XLH) or vitamin D-resistant rickets (VDDR). XLH has been associated with craniosynostosis, the sagittal suture being the most commonly involved.

METHODS

We present three patients with rickets and symptomatic sagittal suture craniosynostosis all of whom presented late (>2 years of age). Two had a severe phenotype and papilledema, while the third presented with an osseous bulging near the anterior fontanel and experienced chronic headaches.

RESULTS

All underwent successful cranial vault expansion.

CONCLUSIONS

Rachitic patients with scaphocephaly should be screened for craniosynostosis.

An anadysplasia-like, spontaneously remitting spondylometaphyseal dysplasia secondary to lamin B receptor (LBR) gene mutations: further definition of the phenotypic heterogeneity of LBR-bone dysplasias

We describe a boy who has an anadysplasia-like spondylometaphyseal dysplasia. By whole exome sequencing he was shown to have compound heterozygous mutations of LBR that codes for the lamin B receptor. He shares many similarities with a case previously described, but in whom the early natural history could not be established [Borovik et al., 2013]. Thus, in addition to Greenberg dysplasia (a perinatal lethal disorder), homozygosity or compound heterozygosity of mutations in LBR can result in a mild, spontaneously regressing bone dysplasia.

X-linked hypophosphatemic rickets and craniosynostosis

Bone mineralization is possible via complex interactions among fibroblast growth factor 23 (FGF23), phosphate-regulating gene with homologies to endopeptidases on the X-chromosome (PHEX), and matrix extracellular phosphoglycoprotein. A loss-of-function mutation in PHEX disrupts this interaction leading to hypophosphatemic rickets. X-linked hypophosphatemic (XLH) rickets is the most common form of metabolic rickets, and there have been reports linking XLH rickets to craniosynostosis. A clinical report of a patient with XLH rickets and craniosynostosis is presented with a review of literature. A review of physiology of bone mineralization reveals that, at high levels, there is cross-binding of FGF23 with FGF receptors 2 and 3 at the cranial sutures. This may be the reason for the common association of craniosynostosis and XLH rickets. There are complex interactions of proteins required for mineralization, proteins inhibiting mineralization, bone remodeling, and bone-renal phosphate homeostasis. Clarification of this pathway and reproducibility in a mouse model may pave the way for medical prevention of craniosynostosis in rickets.

Osteogenesis imperfecta type VI: a form of brittle bone disease with a mineralization defect

Osteogenesis imperfecta (OI) is a heritable disease of bone in which the hallmark is bone fragility. Usually, the disorder is divided into four groups on clinical grounds. We previously described a group of patients initially classified with OI type IV who had a discrete phenotype including hyperplastic callus formation without evidence of mutations in type I collagen. We called that disease entity OI type V. In this study, we describe another group of 8 patients initially diagnosed with OI type IV who share unique, common characteristics. We propose to name this disorder "OI type VI." Fractures were first documented between 4 and 18 months of age. Patients with OI type VI sustained more frequent fractures than patients with OI type IV. Sclerae were white or faintly blue and dentinogenesis imperfecta was uniformly absent. All patients had vertebral compression fractures. No patients showed radiological signs of rickets. Lumbar spine areal bone mineral density (aBMD) was low and similar to age-matched patients with OI type IV. Serum alkaline phosphatase levels were elevated compared with age-matched patients with type IV OI (409 +/- 145 U/liter vs. 295 +/- 95 U/liter; p < 0.03 by t-test). Other biochemical parameters of bone and mineral metabolism were within the reference range. Mutation screening of the coding regions and exon/intron boundaries of both collagen type I genes did not reveal any mutations, and type I collagen protein analyses were normal. Qualitative histology of iliac crest bone biopsy specimens showed an absence of the birefringent pattern of normal lamellar bone under polarized light, often with a "fish-scale" pattern. Quantitative histomorphometry revealed thin cortices, hyperosteoidosis, and a prolonged mineralization lag time in the presence of a decreased mineral apposition rate. We conclude that type VI OI is a moderate to severe form of brittle bone disease with accumulation of osteoid due to a mineralization defect, in the absence of a disturbance of mineral metabolism. The underlying genetic defect remains to be elucidated.

A trial of growth hormone therapy in well-controlled hypophosphataemic rickets

OBJECTIVE

Conventional therapy of hypophosphataemic rickets (HR) with oral phosphate and calcitriol does not always result in normal linear growth. Recombinant human growth hormone (rhGH) offers theoretical advantages as an adjunctive therapy. We aimed to determine the effects of adjunctive rhGH therapy in children with well-controlled HR.

PATIENTS

In this report, 5 prepubertal children (aged 3.5-10.9 years) with well-controlled HR on conventional therapy were given adjunctive standard dose rhGH therapy for one year.

DESIGN AND MEASUREMENTS

Height, growth velocity, metabolic markers of calcium and phosphate metabolism, body composition, bone mineral density, wrist and knee X-rays, and renal sonography were assessed at regular intervals. Height and growth velocities were also calculated 12 months after ceasing rhGH therapy.

RESULTS

After 12 months therapy with rhGH, no significant biochemical or radiological benefits were observed. A significant increase in height SD score was observed (P = 0.023), but this was not associated with any increase in the growth velocity SD score and appears to have been due to catch-up growth caused by conventional therapy alone. When rhGH therapy was ceased, no significant decreases in mean height SD or growth velocity SD scores were observed.

CONCLUSIONS

In well-controlled hypophosphataemic rickets patients receiving conventional therapy, adjunctive therapy with standard dose rhGH offers no benefits in linear growth or rachitic disease markers.

The role of the PHEX gene (PEX) in families with X-linked hypophosphataemic rickets

For over a hundred years, the bane of rickets (a disease of bone), has been prominent in those countries that have participated in, and seeded, the industrial revolution. Industrialisation had major effects of the demography of populations, and many people moved to dark, heavily industrialised cities to find work. It soon became apparent that rickets could be cured by supplementing the diet with cod liver oil and exposure to sunlight. This in turn led to the discovery that photoactivation of 7-dehydrocholesterol was required to produce vitamin D, an indispensable regulator of bone mineral metabolism. Although inadequate exposure to light and poor dietary intake are the main causes of rickets and osteomalacia, recent research has confirmed the role of familial, and tumour forms of the disease. This review will describe the recent advances in our knowledge of the molecular defects in X-linked hypophosphataemic rickets (HYP), and oncogenic hypophosphataemic osteomalacia (OHO). Although HYP and OHO have different primary defects, both diseases have similarities that suggest a linked or overlapping pathophysiology. Also, without doubt, the recent cloning of the gene defective in HYP (the PHEX gene), has given researchers a new reagent to explore the molecular regulation of bone and its links to kidney endocrine function. The fact that the PHEX gene codes for a Zn metallopeptidase raises new and intriguing questions, and adds new momentum to the research on diseases of bone mineral metabolism.

Osteomalacia

Osteomalacia is a generalized bone disorder characterized by impairment of mineralization, leading to accumulation of unmineralized matrix or osteoid in the skeleton. The classical clinical features of osteomalacia include musculoskeletal pain, skeletal deformity, muscle weakness and symptomatic hypocalcaemia. In childhood the features of osteomalacia are accompanied by rickets, with widening of the epiphyses and impaired skeletal growth. The major cause of osteomalacia is vitamin D deficiency, which is most often due to reduced cutaneous production of vitamin D in housebound elderly people, immigrants to Northern countries and women who adopt strict dress codes which prohibit exposure of uncovered skin. Vitamin D deficiency osteomalacia may also occur with malabsorption, liver disease and anticonvulsant therapy. Less commonly, osteomalacia may result from abnormal vitamin D metabolism, resistance to the action of vitamin D, hypophosphataemia or toxic effects on osteoblast function.

Laboratory assessment of nutritional metabolic bone disease in infants

OBJECTIVES

There are numerous laboratory investigations available for the assessment of an infant with suspected metabolic bone disease (MBD); thus, comprehensive laboratory investigations on every aspect of MBD would impose unnecessary stress to the infant and the costs involved would be prohibitive. An overview of the assessment of an infant with suspected MBD, in particular, nutrition-related bone disease, is presented. Our objectives include an understanding of: 1. the importance of appropriate information from history and physical examination to guide the laboratory investigations; 2. relevance and limitations of specific laboratory investigations: a. radiologic studies include diagnostic radiographs and quantitative bone mass determination by dual energy x-ray absorptiometry, b. biochemical measurements to determine mineral homeostasis and bone turnover, c. vitamin (vitamin D metabolites) and hormonal (parathyroid hormone and calcitonin) measurements; with respect to diagnosis and monitoring of the natural progress or response to therapy.

CONCLUSION

Relevant information from clinical history and physical examination, and an understanding of the role and limitations of various laboratory investigations, would allow the optimal utilization of laboratory tests in the assessment of an infant with MBD.

Autonomous hyperparathyroidism in X-linked hypophosphataemia

Four patients with familial hypophosphataemic rickets developed significant hypercalcaemia which persisted after discontinuation of vitamin D therapy. They had increased PTH levels and were operated for hyperparathyroidism at the ages of 18, 20, 24 and 45 years, respectively. Three of the patients had previously received phosphate treatment and one patient developed hyperparathyroidism 7 years after treatment with calcitriol. Histological evaluation revealed different degrees of parathyroid hyperplasia in all patients, with persistently increased PTH and/or calcium levels after surgery. The possibility of autonomous hyperparathyroidism should be evaluated in the follow-up of patients with X-linked hypophosphataemic rickets.