ADH is a rare genetic disorder of phosphate homeostasis, caused by heterozygous point mutations at amino acid residues 176 or 179 in fibroblast growth factor 23 (FGF23). ADH is characterised by impaired mineralisation of bone, rickets and/or osteomalacia, suppressed levels of calcitriol (1,25-dihydroxyvitamin D3), renal phosphate wasting, and low serum phosphate. This disease is the less common form of hypophosphatemia, and has an incomplete penetrance and variable age of onset. Fluctuations in FGF23 concentration correlate with disease severity. Recent studies have revealed that iron could be one of the factors contributing to the regulation of FGF23 expression. Low iron levels were shown to correlate negatively with FGF23 levels in ADHR patients.

Treatments for ADH include: daily oral administration of phosphate and calcitriol, which prevents secondary hyperparathyroidism. It is recommended to conduct frequent monitoring of height, calcium, alkaline phosphatase, parathyroid hormone, and phosphate serum levels, urinary calcium and creatinine. In some cases, corrective surgery of skeletal deformities may be necessary.

ARHR is an extremely rare disease, characterised by hypophosphatemia resulting from renal phosphate wasting. It has been described in few families of Middle Eastern and European origins. Clinical manifestations include: short stature, lower-extremity deformities, pathologic fractures, dental defects, and, in some patients, also enthesopathy. Patients also show hypophosphataemia, low levels of serum 1,25-dihydroxyvitamin D, whereas serum calcium, parathyroid hormone, urinary calcium excretion are normal, and high circulating levels of FGF23. ARHR is subdivided in two subtypes: Autosomal recessive hypophosphataemic rickets type 1 (ARHR1) and type 2 (ARHR2).

ARHR1 is caused by homozygous loss-of-function mutations in the DMP1 (Dentin matrix protein 1) gene. DMP1 is a noncollagenous extracellular protein, highly expressed in osteoblasts and osteocytes, in bone and teeth. It plays critical roles in bone mineralisation, phosphate homeostasis and odontogenic differentiation.

ARHR2 is caused by homozygous loss-of-function mutation in the ENPP1 gene. ENPP1 encodes a protein called ectonucleotide pyrophosphatase/ phosphodiesterase 1 (NPP1), which is a major generator of extracellular pyrophosphate (PPi). Because PPi inhibits calcification (hydroxyapatite crystal deposition and growth), homozygous inactivating mutations in ENPP1 gene are also responsible for generalized arterial calcification of infancy (see also GACI). In patients with ARHR2, high circulating levels of FGF23 have been described. FGF23 is a secreted protein, which reduces expression of sodium-phosphate co-transporters, NPT2a and NPT2c, resulting in renal phosphate wasting, diminishes the renal 1α-hydroxylase and increases the 24-hydroxylase activity. Moreover, FGF23 acts at the parathyroid gland to decrease parathyroid hormone synthesis and secretion. Currently, it is unclear how mutations in ENPP1 gene results in high FGF23 levels.

Another rare type of the disorder is known as hereditary hypophosphataemic rickets with hypercalciuria (HHRH). In addition to hypophosphatemia, this condition is characterised by the excretion of high levels of calcium in the urine (hypercalciuria).

Two forms of vitamin D exist: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Both forms of vitamin D need two-step hydroxylation to become biologically active. The first step occurs in the liver where vitamin D is hydroxylated to 25-hydroxyvitamin D 25(OH)D by several hepatic enzymes having 25-hydroxylase activity, and CYP2R1 is the major enzyme. The second step of hydroxylation occurs mainly in the kidney, where 25(OH)D is hydroxylated by the mitochondrial 25-OHvitamin D-1α-hydroxylase to the biologically active hormone 1,25-(OH)2D. The 1,25-(OH)2D plays a central role in calcium homeostasis, bone metabolism, and on cell proliferation and differentiation of a variety of tissues. Alteration of vitamin D metabolism causes defects in the growth plate and bone demineralisation, resulting rickets in children and osteomalacia in adults. 

Hydroxylation-deficient Type 1A (VDDR1A) is a rare autosomal recessive disorder caused by mutations in the CYP27B1 gene, resulting in 1α-hydroxylase enzyme deficiency. Clinical manifestations of VDDR1A include: hypotonia, short stature, muscle weakness, hypocalcaemic seizures in early infancy and rickets. Until now, over 60 mutations have been described in different ethnic groups. Certain mutations are more frequent in certain ethnic groups. 

Hydroxylation-deficient Type 1B (VDDR1B) is due to a defect in vitamin D 25-hydroxylation, and is caused by mutation in the CYP2R1 gene. The synthesis of bioactive vitamin D requires hydroxylation at the 1 α and 25 positions by cytochrome P450 enzymes in the kidney and liver, respectively. The mitochondrial enzyme CYP27B1 catalyzes 1 α-hydroxylation in the kidney, and the CYP2R1 is the major enzyme for hydroxylation of vitamin D to 25-hydroxyvitamin D.

Vitamin D-dependent type 2A (VDDR2A) is an autosomal recessive disorder caused by mutation in the gene encoding the vitamin D receptor (VDR). Signalling via this receptor regulates gene expression in 1,25 OHvitamin D3-responsive cells. It is characterised by hypocalcemia, due to reduced intestinal absorption of calcium, with secondary hyperparathyroidism and hypophosphataemia. Hypocalcemia and hypophosphataemia impair normal bone mineralisation leading to childhood rickets. Other clinical manifestations include muscle weakness and convulsions caused by hypocalcaemia, and in many cases alopecia.

Vitamin D-dependent type 2B (VDDR2B) is an unusual form of Vitamin D-dependent rickets due to abnormal expression of a hormone response element binding protein that interferes with the normal function of the VDR, without mutations in the VDR coding region. Hormone resistance results from constitutive overexpression of heterogeneous nuclear ribonucleoprotein (hnRNP) that competed with a normally functioning VDR-retinoid X receptor (RXR) dimer for binding to the vitamin D response element (VDRE).

Treatment: Consists of administration of large doses of vitamin D and calcitriol. The potential complications of therapy are: nephrocalcinosis, hypercalciuria, and hypercalcemia. Therefore, regular checks monitoring of physical and biochemical examination, and renal ultrasound are required.

TIO is an ultra-rare disease caused by typically benign, slow-growing tumours that produce excess levels of fibroblast growth factor 23 (FGF23), which is involved in phosphate reabsorption. Patients with TIO can experience symptoms including severe hypophosphataemia (low levels of phosphate in the blood), osteomalacia (softening of the bones), muscle weakness, fatigue, bone pain and fractures. There are an estimated 500 to 1,000 people in the United States with TIO, and approximately half of all cases are believed to be inoperable. The tumours can arise in bone or soft tissue, occur from head to toe, and are typically very small in size, locating these tumours is often quite challenging. In patients for whom the tumour or lesion is inoperable, the current treatment consists of oral phosphate and/or active vitamin D replacement. Efficacy of this management is often limited, and its benefits must be balanced with monitoring for potential risks.