X-linked hypophosphataemia (XLH) is an X-linked dominant disorder caused by mutations in PHEX (located at Xp22.1), which encodes a cell-surface-bound protein-cleavage enzyme (phosphate-regulating neutral endopeptidase PHEX), predominantly expressed in osteoblasts, osteocytes and teeth (odontoblasts and cementoblasts). XLH is the most common cause of inherited phosphate wasting, with an incidence of 3.9 per 100,000 live births and a prevalence ranging from 1.7 per 100,000 children to 4.8 per 100,000 persons (children and adults) 1-3. Although the pathogenesis of XLH is not fully understood, animal studies indicate that loss of Phex function results in enhanced secretion of the phosphaturic hormone fibroblast growth factor 23 (FGF23), with osteocytes being the primary source of FGF23 production 4. These effects explain most of the characteristic features of the disease, including renal phosphate wasting with consequent hypophosphataemia, diminished synthesis of active vitamin D (1,25(OH) 2 vitamin D), rickets, osteomalacia, odontomalacia and disproportionate short stature 4-6. Patients usually develop clinical symptoms during the first or second year of life. Early treatment with oral phosphate supplementation and active vitamin D heals rickets, limits dental abscess formation and prevents progressive growth failure, but in a substantial proportion of patients treatment is unsuccessful and/or associated with adverse effects (for example, hyper parathyroidism and nephrocalcinosis) 7,8. Up to two-thirds of children with XLH require lower limb surgery 9-12. Conventional therapy further stimulates FGF23 levels and thereby renal phosphate wasting, resulting in a vicious circle, which might limit its efficacy 6,13-15. Adult patients with XLH are at risk of complications such as early osteoarthritis, enthesopathies, spinal stenosis, pseudofractures and hearing loss, which might limit quality of life 16-18. In 2018, burosumab, a fully human monoclonal IgG1 antibody neutralizing FGF23, was approved by health authorities for the treatment of patients with

In children, hypophosphatemic rickets (HR) is revealed by delayed walking, waddling gait, leg bowing, enlarged cartilages, bone pain, craniostenosis, spontaneous dental abscesses, and growth failure. If undiagnosed during childhood, patients with hypophosphatemia present with bone and/or joint pain, fractures, mineralization defects such as osteomalacia, entesopathy, severe dental anomalies, hearing loss, and fatigue. Healing rickets is the initial endpoint of treatment in children. Therapy aims at counteracting consequences of FGF23 excess, i.e. oral phosphorus supplementation with multiple daily intakes to compensate for renal phosphate wasting and active vitamin D analogs (alfacalcidol or calcitriol) to counter the 1,25-diOH-vitamin D deficiency. Corrective surgeries for residual leg bowing at the end of growth are occasionally performed. In absence of consensus regarding indications of the treatment in adults, it is generally accepted that medical treatment should be reinitiated (or maintained) in symptomatic patients to reduce pain, which may be due to bone microfractures and/or osteomalacia. In addition to the conventional treatment, optimal care of symptomatic patients requires pharmacological and non-pharmacological management of pain and joint stiffness, through appropriated rehabilitation. Much attention should be given to the dental and periodontal manifestations of HR. Besides vitamin D analogs and phosphate supplements that improve tooth mineralization, rigorous oral hygiene, active endodontic treatment of root abscesses and preventive protection of teeth surfaces are recommended. Current outcomes of this therapy are still not optimal, and therapies targeting the pathophysiology of the disease, i.e. FGF23 excess, are desirable. In this review, medical, dental, surgical, and contributions of various expertises to the treatment of HR are described, with an effort to highlight the importance of coordinated care.

All PRKAR1A and PDE4D patients present similar bone dysplasia characterizing acrodysostosis. Phenotypic differences, including the presence of resistance to GPCR-cAMP signaling hormones in PRKAR1A but not PDE4D patients, indicate phenotype-genotype correlations and highlight the specific contributions of PRKAR1A and PDE4D in cAMP signaling in different tissues.