One half-century of advances in the evaluation and management of disorders of bone and mineral metabolism in children and adolescents

Abstract

The past 50 years of research in pediatric bone and mineral metabolism have led to remarkable progress in the identification and characterization of disorders that affect the developing skeleton. Progress has been facilitated through advances in both technology and biology and this paper provides a brief description of some but not all of the key findings, including identification of the calcium sensing receptor and the polypeptides parathyroid hormone and parathyroid hormone-related protein as well as their shared receptor and signal generating pathways; the elucidation of vitamin D metabolism and actions; discovery of fibroblast growth factor 23 (FGF23), the sodium-phosphate co-transporters and the other components that regulate phosphate metabolism. Moreover, the past half-century of research has led to the delineation of the molecular bases for genetic forms of hypoparathyroidism, pseudohypoparathyroidism, and primary hyperparathyroidism as well as the determination of the genetic causes of osteogenesis imperfecta, osteopetrosis, hypophosphatasia, and other disorders of mineral/bone homeostasis. During the next decade we expect that many of these fundamental discoveries will lead to the development of innovative treatments that will improve the lives of children with these disorders.

This manuscript describes a variety of the extraordinary scientific advances in the biology of bone and mineral metabolism that have occurred in the last half-century and have led to improvements in the evaluation and management of children and adolescents with these disorders. Progress in this field has been facilitated through new knowledge gained by remarkable technological advances that have enabled characterization and measurement of key hormones and metabolites, the identification of novel genes, proteins, steroids, and discovery of biological pathways that affect bone and mineral metabolism. In many instances, new knowledge in these fields has arisen through application of the tools of molecular biology including: genomics/genetics – the identification of the composition and sequence of the units of heredity termed genes which contain the blueprints for synthesizing ribonucleic acid (RNA) and protein molecules based on the precise sequence of four nucleotide bases (adenine, cytosine, guanine and thymine) that are attached to a backbone made of alternating deoxyribose molecules and phosphate groups; transcriptomics – the highly regulated and cell-specific process through which genes are transcribed into messenger RNA molecules that specify the assembly of proteins through the orderly linkage of members of a set of only twenty amino acids, each of which has a unique side chain that can bond with one another to hold a length of protein in a designated shape or configuration; proteomics – the study of all the proteins (proteome) produced by an individual cell or multicellular organism; and metabolomics – the study of the substrates and products and the chemical/physiological reactions (metabolism) within a cell that enable the cell to live and reproduce. Collectively, these fields of study have provided detailed information about the mechanisms that regulate mineral and skeletal homeostasis including the identification of genes, proteins, carbohydrates, and fats specifically associated with disorders of calcium, phosphate, and skeletal homeostasis [1, 2].

The metabolism of calcium and phosphorus/phosphate and skeletal homeostasis are affected by the oral intake and intestinal absorption of these ions, their deposition into and reabsorption from bone, and their renal glomerular filtration and excretion and reabsorption by the renal tubules. These processes are, in turn, regulated by parathyroid hormone (PTH), calcitonin, calcitriol (1,25-dihydroxyvitamin D3 – the biologically active form of vitamin D3), fibroblast growth factor 23 (FGF23), and bone alkaline phosphatase (TNSALP = Tissue non-specific alkaline phosphatase) (Table 1) [2]. Among the most important accomplishments in the fields of mineral and bone metabolism in the past half-century have been:

Table 1:

Genetic regulation of mineral and bone metabolism.

Gene	Gene product	Gene site	OMIM
Calcium/phosphorus
CASR	Calcium sensing receptor	3q13.33-q21.1	601199
PHEX	Phosphate-regulating endopeptidase, homolog, X-linked	Xp22.11	300550
SLC34A1	Solute carrier family 34, member 1 (NPT2a)	5q35.3	182309
SLC34A3	Solute carrier family 34, member 3 (NPT2c)	9q34.3	609826
TNSALP	Tissue non-specific alkaline phosphatase	1p36.12	171760
PTH/PTHRP/calcitonin
PTH	Parathyroid hormone	11p15.3	168450
PTHLH	Parathyroid-like hormone (PTH related protein - PTHrP)	12p11.22	168470
PTH1R	Parathyroid hormone receptor	3p21.31	168468
CALCA	Calcitonin/calcitonin-related polypeptide, alpha	11p15.2	114130
Vitamin D
CYP2R1	Cytochrome P450, subfamily IIR, polypeptide 1 (Vitamin D 25-hydroxylase)	11p15.2	608713
CYP27B1	Cytochrome P450, subfamily XXVIIB, polypeptide 1 (25-hydroxyvitamin D3–1-alpha hydroxylase)	12q14.1	609506
CYP3A4	Cytochrome P450, subfamily IIIA, polypeptide 4 (25-hydroxyvitamin D3–23-hydroxylase)	7q22.1	124010
CYP24A1	subfamily A, polypeptide 1	20q13.2	126065
VDR	Vitamin D receptor	12q13.11	601764
Fibroblast growth factor 23/receptor
FGF23	Fibroblast growth factor 23	112p13.2	605380
FURIN	Furin, paired basic amino acid cleaving enzyme	15q26.1	136950
FGFR1	Fibroblast growth factor receptor 1 c	8p11.23	136350
KL	Klotho	13q13.1	604824
PPARGC1A	Peroxisome proliferator-activator receptor-gamma, coactivator, alpha 1	4p15.2	604517
TRPV	Transient receptor potential cation channel, subfamily V, member 5	7q34	606679
Bone
CSF1	(Macrophage) colony-stimulating factor	1p13.1	120420
BMP1	Bone morphogenetic protein 1	8p21.3	112264
SOX9	SRY-Box 9	17q24.3	608160
WNT1	Wingless-type MMTV integration site family, member 1	12q13.12	164820
CTNNB1	Catenin, beta 1	3p22.1	116806
COL1A1	Collagen, type 1, alpha-1	17q21.33	120150
COL1A2	Collagen, type 1, alpha-2	7q21.3	120160
P4HB	Procollagen-proline, 2-oxoglutarate-4-dioxygenase, beta subunit, Prolyl-4-hydroxylase	17q25.3	176790
PLOD1	Procollagen-lysine, 2-oxoglutarate-5-dioxygenase, Lysyl hydroxylase	1p36.2	153454
SERPINH1	Serpin peptidase inhibitor, Clade H, Member 1 Heat shock protein 47	11q13.5	600493
DMP1	Dentin matrix acidic phosphoprotein 1	600980	600980
BGLAP	Gamma-carboxyglutamic acid protein, bone (Osteocalcin)	1q22	112260
TNFRSF11B	Tumor necrosis factor receptor superfamily, member 11B (Osteoprotegerin)	8q24.12	602643
TNFSF11	Tumor necrosis factor superfamily member 11 (Osteoprotegerin ligand, RANK ligand)	13q14.11	602462
TRPV5	Transient receptor potential cation channel, subfamily V member 5	7q34	606679
SP7	Transcription factor Sp 7 (Osterix)	12q13.13	606633
SPP1	Secreted phosphoprotein1 (Osteopontin)	4q22.1	166490
Other
GNAS	Guanine nucleotide-binding protein, alpha-stimulating	20q13.32	139320
GNA11	Guanine nucleotide binding protein, alpha 11	19p13.3	139313
STX16	Syntaxin 16	20q13.32	603666
GALNT3	UDP-N-acetyl-alpha- D-galactosamine: Polypepide N-acetylgalactosaminyl-transferase 3	2q24.3	601756
KDELR2	KDEL endoplasmic retention protein retention receptor 2	7p22.1	609024
NFAM1	NFAT activating protein with ITAM motif 1 [Immunoreceptor tyrosine-based activation motif (ITAM)-mediated co-stimulatory signaling]	22q13.2	608740

1. The discovery of the calcium sensing receptor (CASR) and the development of assays for the measurement of total and ionized calcium and its regulatory components;

2. Recognition of FGF23, α-Klotho, and Phosphate-regulating endopeptidase, homolog, X-linked (PHEX) as key regulators of phosphorus homeostasis; identification of the roles of FGF23 and α-Klotho in the control of the renal type II sodium-phosphate cotransporter channels (NPT2a – encoded by SLC34A1) and NPT2c (encoded by SLC34A3);

3. Isolation of PTH, development of assays for its measurement, identification of its cellular receptor, and delineation of the corresponding guanine nucleotide-binding protein G protein signal transduction pathways; identification of PTH-related protein (PTHrP), an important regulator of skeletal development and calcium homeostasis in the fetus as well as postnatally;

4. Recognition of the molecular bases for genetic forms of hypoparathyroidism, pseudohypoparathyroidism, and primary hyperparathyroidism; analysis of the complexity of GNAS encoding the α subunit of the stimulatory G protein (Gα_s);

5. Elucidation of the metabolism of vitamin D and identification of its bioactive form – 1,25-dihydroxyvitamin D (calcitriol), and of its nuclear receptor (VDR);

6. Identification of the genetic causes of osteogenesis imperfecta, osteopetrosis, hypophosphatasia, and other disorders of mineral/bone homeostasis.

In this manuscript, the authors have selected a limited number of topics concerning advances in mineral and bone metabolism over the past 50 years upon which to focus.

Calcium and the calcium-sensing receptor

Among the most important accomplishments in the fields of mineral and bone metabolism in the past half-century has been the development of accessible and accurate methods for the measurement of both total and ionized serum calcium concentrations. Calcium is present in the serum in three fractions: that linked to proteins – primarily albumin (45 percent); that complexed to anions such as phosphate or citrate (7 percent); and that in the free or ionized state (Ca²⁺) (48 percent), the latter being the biologically active form of this cation [3, 4]. The total serum calcium concentration is measured colorimetrically; it may also be determined by atomic absorption spectrophotometry. Measurement of total serum calcium serves as a useful proxy for Ca²⁺ determination, if total serum calcium levels are not extreme and concentrations of serum proteins are normal. Although a variety of mathematical algorithms have been developed to “adjust” the total serum calcium value when serum levels of proteins, particularly albumin, diverge from normal, none of these corrections is entirely satisfactory. One of the most welcome technological advances for this field was the development of ion-selective electrodes that enabled the direct measurement of Ca²⁺. The accuracy of this method requires strict adherence to the technique of sample acquisition and preparation and rapid transport of the chilled, anerobic serum sample to the analytical laboratory.

A significant advance in our field was the identification and cloning of the biological receptor that enables cells to detect (sense) extracellular concentrations of Ca²⁺ (CASR). The CASR is a heptahelical transmembrane protein that is expressed on the surface of parathyroid, bone, and renal tubular cells as well as many other cells throughout the body. The CASR is a member of a large family of receptor proteins that transduce extracellular signals by interacting with heterotrimeric (α, β, and γ subunits) GTP-binding proteins (G-proteins). Ligand-binding to receptor leads to activation of GTP and dissociation of the α subunit from the βγ complex. The free α subunit regulates downstream effector targets. G protein-coupled signal transduction offers a number of advantages over other forms of transmembrane signal transduction. Firstly, heptahelical receptors often activate multiple classes of G proteins, thereby eliciting diverse signals from one ligand. Secondly, G protein-coupled signal transduction amplifies receptor signals as one activated Gα molecule can interact with multiple downstream effectors, thereby achieving exquisite sensitivity. Thirdly, there is endogenous control of the duration of signaling through the rate of GTP to GDP (guanosine diphosphate) hydrolysis which provides a molecular timer that limits the duration of signaling action. Hydrolysis of GTP to GDP facilitates reassociation of the α subunit with the βγ complex - resetting the G protein for a new cycle of signal transduction events. Thus, G protein-coupled signaling enables precise control of enzymes, ion channels, and transporter proteins that regulate intracellular messaging that controls diverse cellular processes.

The CASR binds Ca²⁺ with high affinity but can also bind other divalent cations such as magnesium. In the parathyroid cell, increasing concentrations of extracellular Ca²⁺ bind to the CASR leading to dissociation of G-protein α subunits from G11 and Gq with consequent activation of phospholipase Cβ (PLCβ). PLCβ hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into diacylglycerol (DAG) and 1,4,5-trisphosphate (IP3) which serve as second messengers that activate pathways that suppress PTH synthesis and release. In the kidney, activation of CASR signaling reduces tubular reabsorption of calcium and thus increases its urinary excretion, thereby lowering serum Ca²⁺ concentrations. Heterozygous variants of CASR that result in reduced receptor activity cause type 1 familial hypocalciuric hypercalcemia (FHH1) (OMIM 145980) – which is often clinically silent and manifested primarily by mild elevations in serum PTH and calcium with low urine calcium excretion. Patients are generally asymptomatic, although pancreatitis or chondrocalcinosis may develop in some affected adults. By contrast, biallelic variants cause the more severe disorder neonatal severe hyperparathyroidism (OMIM 239200) – typified by asymptomatic, and often life-threatening hypercalcemia (i.e., failure to thrive, skeletal demineralization). CASR variants that result in gain-of-function cause autosomal dominant hypocalcemia type 1 (OMIM 601198) – a form of hypoparathyroidism that is characterized by markedly increased renal excretion of calcium. Mutations in genes encoding other components of the CASR signaling pathway, with loss-of-function variants in GNA11 and AP2S1 causing FHH type 2 and type 3, respectively, while gain-of-function variants in GNA11 cause autosomal dominant hypocalcemia type 2 (OMIM 615361).

Phosphorus/phosphate metabolism

Phosphorus is an abundant element that is widespread in its distribution and is a major intracellular anion. In vivo, phosphorus is complexed with oxygen to form phosphate. Approximately 85 percent of total body phosphorus is in the skeleton in the form of the bone mineral hydroxyapatite – Ca₅(PO₄)₃(OH); the remaining 15 percent is present in soft tissues. Most of intracellular phosphorus in the body is in the organic form as a complex with carbohydrates, lipids, and proteins, while nearly all the phosphorus found in the extracellular fluid space is in the form of inorganic phosphate [5]. Serum inorganic phosphate reflects only a very minor percentage of total body phosphorus, but this form is easily measurable and provides an indication of total body phosphorus stores. Phosphorus plays a critical biochemical role through its involvement in cellular and extracellular metabolism, as an integral component of nucleic acids, cell membranes, and high-energy compounds [e.g., adenosine triphosphate (ATP)] utilized in metabolism, and through regulating the activity of many enzymes. Phosphorus is also an important component of the hydroxyapatite crystal, which provides mechanical strength to mineralized tissues and participates in maintaining the proper pH of extracellular fluids. Phosphorus is readily absorbed by the intestinal tract and reabsorbed from glomerular filtrate in the proximal renal tubule. Exploration of the mechanisms that regulate renal phosphate reabsorption has revealed the essential roles played in this process by PTH as well as FGF23, the latter synthesized primarily by bone osteoblasts and osteocytes, and α-Klotho (KL, OMIM 604824), a molecule whose extracellular domain serves with FGF receptor 1c (FGFR1, OMIM 136350) as a co-receptor for FGF23 [6–10]. FGF23 (OMIM 605380) expression is stimulated by Ca²⁺, phosphate, calcitriol, and PTH. Transcription of FGF23 in osteocytes is inhibited by dentin matrix protein 1 (DMP1, OMIM 600980) interacting with the X-linked phosphate regulating endopeptidase (PHEX, OMIM 300550) (vide infra) [11, 12].

The FGF23 gene encodes the 251 amino acid polypeptide which includes a 24-amino acid signal peptide. FGF23 is stabilized by N-acetyl glycosylation of amino acid THr 178 by UDP-Gal-NAc transferase 3, a member of the GalNAc-transferases family that is encoded by GALNT3 (OMIM 601756). After cleavage of the signal peptide, biologically active “intact” FGF23 is secreted. Intact FGF23 protein contains the arginine-X-X-arginine (RXXR) motif that is a protease recognition site for furin (FURIN, OMIM 136950), a preprotein convertase. When proteolytically cleaved between Arg179 and Ser180 the intact FGF23 (~23 kDa) forms biologically inactive – and C-terminal (~12 kDa) fragments.

α-Klotho is a single-pass transmembrane glycoprotein of 1,012 amino acids that is highly expressed in the kidney and brain and is also present in the parathyroid glands. There are three primary isoforms of the α-Klotho protein – a full length transmembrane form (mKl), a shed soluble form consisting of the extracellular domain of the parent protein (sKl), and a secreted truncated form that is generated by alternative splicing of KL mRNA. Membrane-bound α-Klotho associates with FGFR1c forming a pocket that serves as the receptor for FGF23 [9]. This complex then dimerizes initiating intracellular transduction of the FGF23 signal. Synthesis of α-Klotho is enhanced by calcitriol and ligands for peroxisome proliferator-activated receptor-γ (PPARγ, PPARGC1A, OMIM 604517) and inhibited by FGF23.

In the proximal renal tubule, FGF23 reduces transcellular reabsorption of phosphate by increasing the rates of internalization and degradation of the apically-sited sodium phosphate co-transporters - NPT2a and NPT2c [encoded by SLC34A1 (OMIM 182309) and SCL34A3 (OMIM 609826), respectively] [7]. FGF23 increases reabsorption of calcium in the renal distal convoluted tubule via α-Klotho, which stabilizes the calcium transporter TPRV5 (encoded by TRPV5, OMIM 606679). FGF23 antagonizes some of the actions of vitamin D and PTH by repressing expression of CYP27B1 (OMIM 609506, encoding 25-hydroxyvitamin D-1-α-hydroxylase) thus decreasing synthesis of calcitriol in proximal renal tubules and by inactivating vitamin D metabolites via induction of CYP24A1 (OMIM 126065, encoding cholesterol-24-hydroxylase). In bone, FGF23 depresses mineralization of hydroxyapatite. In the rat parathyroid gland, FGF23 inhibits the synthesis of PTH.

Circulating levels of FGF23 are increased in patients with autosomal dominant hypophosphatemic rickets (OMIM 193100), X-linked hypophosphatemic rickets (OMIM 307800), and the autosomal recessive forms of hypophosphatemic rickets 1 and 2 (OMIM 241520 and 613312, respectively), as well as fibrous dysplasia and tumor induced osteomalacia – the latter due to excessive synthesis of FGF23 by abnormal cells in the bone lesion or tumor, respectively. The secretion of FGF23 is also increased in patients with chronic renal disease and may be pathogenetically associated with some of the adverse consequences of this state such as myocardial hypertrophy. Serum concentrations of intact FGF23 are decreased in patients with familial hyperphosphatemic tumoral calcinosis type 1 (OMIM 211900) due to mutations in GALNT3 that impair activity of the encoded glycosyl-transferase needed for stabilization of FGF23 via O-glycosylation of Thr178, as well as in subjects with familial hyperphosphatemic tumoral calcinosis type 2 (OMIM 617993) due to loss of function mutations in FGF23 itself.

X-linked hypophosphatemic rickets (XLHR, OMIM 307800) and other forms of FGF23-dependent hypophosphatemic rickets are characterized by suboptimal mineralization of bone that, in the growing child, leads to deformation of the long cartilagenous bones (resulting in genu varum and/or genu valgus) and decreased resistance to external pressure of membranous bones (craniotabes) as well as craniosynostosis. XLHR is due to inactivating variants of phosphate-regulating endopeptidase homolog, X-linked (PHEX, OMIM 300550); the PHEX protein is an enzyme that normally degrades osteopontin (SPP1, OMIM 166490), a protein that inhibits bone mineralization [13].

XLHR is also associated with high circulating levels of FGF23 resulting in hyperphosphaturia and hypophosphatemia and decreased synthesis of calcitriol, thus impairing intestinal absorption of calcium. The conventional management of this disorder has consisted of administration of active forms of vitamin D plus multiple daily doses of phosphate; lack of compliance with dosing regimens complicates management of the disease. The treatment of XLHR has changed dramatically with the development of burosumab, a humanized FGF23-neutralizing monoclonal antibody [14–16]. Subcutaneous administration of burosumab every 14 days to children with XLHR and every 28 days to adults with this disease results in normalization of serum concentrations of phosphate, calcitriol, and alkaline phosphatase with clinical and radiological improvement in rickets and bone deformities but without amelioration of the associated dental defects [17, 18]. Burosumab has also been effective in patients with ARHR1 due to inactivating variants of dentin matrix acidic phosphoprotein 1 (DMP1, OMIM 600980) with normalization of serum phosphate concentrations, amelioration of bone pain, and healing of pseudo-fractures [19].

Parathyroid hormone and PTH-related protein (PTHrP)

PTH is synthesized and secreted by the parathyroid glands via a precise mechanism that is based on the ability of the CASR to sense extracellular concentrations of Ca²⁺; as Ca²⁺ values decline, synthesis and secretion of PTH increase. PTH increases serum levels of calcium and decreases serum levels of phosphate via interaction with the type 1 PTH receptor (PTH1R, OMIM 168468) which is coupled via the G protein – Gs – to activation of adenylyl cyclase [20]. When administered intermittently, PTH enhances bone formation, in part by inhibiting apoptosis of osteoblasts. PTH-related protein (PTHLH, OMIM 168470) is a cytokine essential in utero where it increases the linear growth of endochondral bone and postnatally as it contributes to bone remodeling; PTHrP is synthesized by keratinocytes, endothelial and smooth muscle cells, and pancreatic islet cells. The common receptor for PTH and PTHrP – PTH1R (PTH1R, OMIM 168468) – acts principally through G_s to stimulate adenyl cyclase and to generate the second messenger cyclic AMP which activates specific protein kinase A signal transduction pathways that ultimately produce the physiologic actions of PTH or PTHrP. Because PTH and PTHrP share significant amino acid sequence homology within the first 13 amino acids of their protein sequences, they can bind to the common PTH1R and effect similar actions.

The stimulatory guanine nucleotide-binding protein, G_s

The discovery that cyclic AMP was the principal second messenger for PTH action was a landmark observation and quickly led to studies to determine the basis for PTH resistance in patients with pseudohypoparathyroidism type 1A (PHP1A, OMIM 103580) in which biochemical hypoparathyroidism (i.e., hypocalcemia and hyperphosphatemia) is due to resistance to PTH rather than its absence. Most of these patients also manifest features of Albright’s hereditary osteodystrophy (AHO, OMIM 103580), a constellation of developmental defects characterized by short stature, brachydactyly, and subcutaneous ossifications. Early studies had shown that these patients failed to manifest the normal phosphaturic response to PTH administration, while subsequent studies demonstrated that PTH infusion failed to elicit an increase in urinary excretion of nephrogenous cyclic AMP. The recognition that PTH1R action required the heterotrimeric G_s protein for activation of adenylyl cyclase led to studies in accessible cells that demonstrated a 50 percent reduction in the levels of its alpha subunit (Gα_s) in patients with PHP1A. These patients also demonstrated resistance to a variety of hormones whose receptors require Gα_s for activation of adenylyl cyclase as well as early-onset obesity and neurocognitive delays. [PHP1A has also been designated “inactivating PTH/PTH Related Protein (PTHrP) signaling disorder (iPPSD)”]. By contrast, other patients with PHP1 who lacked major features of AHO and in whom hormone resistance was principally to PTH had normal levels of Gα_s in accessible cells, a disorder now termed PTH1B (OMIM 603223). Patients with pseudopseudohypoparathyroidism (PPHP, OMIM 612463) manifest clinical features of AHO but do not have hormone resistance.

GNAS is a complex, reciprocally imprinted gene with alternative first exons that generate a variety of transcripts in addition to Gα_s (Figure 1) [21, 22]. Gα_s is expressed biallelically (i.e., from both the maternal and paternal alleles) in most tissues. However, specific cells in the proximal renal tubule, pituitary, thyroid, gonads, hypothalamus, and brown adipose tissue express Gα_s transcripts preferentially from the maternal allele. Hormone resistance in patients with PHP1A is the consequence of mutations that affect exons 1–13 of the maternally derived GNAS allele and which reduce expression or function of Gα_s protein. By contrast, the features of AHO arise as a consequence of a 50 percent reduction of Gα_s in non-imprinted tissues. PPHP patients carry a mutation on the paternal GNAS allele that do not affect Gα_s expression in cells in which GNAS is imprinted, because the paternal allele is not expressed in these cells; hence, these patients have AHO but do not manifest hormone resistance [21–23].

Figure 1:

Functional organization of the GNAS locus on chromosome 20q13.32 and variations associated with pseudohypoparathyroidism (PHP) and pseudopseudohypoparathyroidism (PPHP). Exons 1–13 of GNAS encode the α subunit of the G-stimulatory protein (Gαs) requisite for intracellular signal transduction. In most tissues, GNAS is expressed by both maternal and paternal alleles; however, in the proximal renal tubule, brown adipose tissue, anterior pituitary, thyroid, and gonads, GNAS is expressed only by the maternal allele. Deleterious variants of maternal GNAS result in pseudohypoparathyroidism (PHP), while variants occurring in the paternal GNAS allele lead to pseudopseudohypoparathyroidism (PPHP). Centromeric to exon 1 of the GNAS chromosomal site are: the A/B transcript, a very large form of Gαs – XLαs, the neuroendocrine secretory protein 55 (NESP55), and the antisense (AS) transcript. The A/B, XLαs, and NESP transcripts are first exons/promoters that bypass exon 1 of GNAS and attached directly to exons 2–13. Maternal variants of GNAS exons 1–13 result in Albright’s hereditary osteodystrophy and PHP1A; while paternal variants of GNAS exons 1–13 lead to PPHP or progressive osseous heteroplasia (POH). Further centromeric of GNAS is STX16 – variants of which lead to PHP1B (isolated renal tubular resistance to PTH). [Reproduced with permission from [21] Juppner H. Molecular definition of pseudohypoparathyroidism variants. J Clin Endocrinol Metab 2021;106:1541–1552. doi: 10.1210/cline/dgab060].

Patients with PHP1B lack typical features of AHO except for mild brachydactyly. PHP1B (OMIM 603223; also termed iPPSD3) is primarily a disorder of imprinting [24]. All subjects with PHP1B described to date show complete loss of methylation (LOM) of the maternal exon A/B differentially methylated region (DMR) which leads to biallelic expression of exon A/B transcripts and suppression of Gα_s transcription in those tissues in which Gα_s is normally expressed preferentially from the maternal allele. Approximately 15–20 percent of patients with PHP1B demonstrate autosomal dominant transmission through a maternal GNAS allele. Subjects with familial PHP1B usually have loss of methylation that is limited to the A/B differentially methylated region and carry microdeletions within the maternal STX16 allele approximately 220 kb upstream of GNAS that correspond to 3 kb (missing exons 4–6) or 4.4 kb (missing exons 2–4), although larger deletions have been rarely identified. Other mutations that lead to loss of methylation that is limited to exon A/B include a few duplications/triplications involving the region centromeric of exon A/B. Some patients with familial PHP1B have additional methylation defects that affect other differentially methylated regions (DMR) and that are associated with microdeletions that include sequences within NESP and/or the paternally expressed antisense transcript of AS of the GNAS complex locus. The genetic bases for most cases of sporadic PHP1B remain unknown, however. These patients often have global epigenetic defects in methylation that affect all four GNAS DMRs, perhaps related to failure of remethylation of GNAS during oogenesis [25]. In some patients, partial or complete paternal uniparental disomy for chromosome 20 has been identified.

The McCune-Albright syndrome (OMIM 174800) of polyostotic fibrous dysplasia, irregularly bordered café-au-lait macules (CALM) of the skin, and multiple endocrinopathies – the most frequent of which is peripheral isosexual precocious puberty – is the consequence of post-conceptual, somatic, gain-of-function variants of GNAS [26, 27]. The most common activating variants of GNAS are Arg201Cys or His in exon 8 and result in increased activity of Gα_s and hormone-independent, cell-autonomous activation of adenylyl cyclase. Mutation-bearing cells are distributed in a mosaic pattern that in the skin can be seen to follow the developmental lines of Blashko (e.g., the unusual pattern of CALM lesions) and lead to phenotypic consequences only in cells that are affected by excess cyclic AMP production. Further variability in the phenotype arises as a consequence of the parental origin of the GNAS allele bearing the post-zygotic mutation, as paternal imprinting of Gα_s leads to the milder effect of mutations that occur on the paternal allele. The proportion of cells with an activating variant of GNAS within a tissue will determine the extent of the phenotype. For example, the magnitude of fibrous dysplasia may range from a single, asymptomatic lesion to multiple, diffuse, polyostotic lesions and substantial impairment of skeletal shape and strength. Patients with extensive fibrous dysplasia often have increased serum concentrations of FGF23 which lead to hypophosphatemic rickets and osteomalacia. Endocrinopathies associated with the McCune–Albright syndrome include: gonadotropin-independent isosexual precocious puberty and recurrent ovarian cysts in affected females; Leydig and Sertoli cell hyperplasia with macro-orchidism in males, hyperthyroidism associated with thyroid nodularity, neonatal hypercortisolism, hypersomatotropism, and hyperprolactinemia. Many of these endocrinopathies occur as isolated disorders due to identical somatic Arg201 GNAS mutations that occur much later in development (and even postnatally) and are present only in a single tissue.

Abnormalities of vitamin D metabolism or function

Vitamin D-dependent rickets (VDDR) is distinct from nutritional vitamin D deficiency and is the consequence of abnormalities in the metabolism of this vitamin or in the tissue responsiveness to its active metabolite – 1,25-dihydroxyvitamin D/calcitriol. Cholecalciferol is first 25-hydroxylated by the hepatic 25-hydroxylase enzyme (CYP2R1, OMIM 608717) to form calcifidiol (25-hydroxyvitamin D) and then by renal 25-hydroxyvitamin D3-1α-hydroxylase (CYP27B1, OMIM 609506) to form calcitriol, the active metabolite of vitamin D that binds to the vitamin D receptor (VDR, OMIM 601769) and modulates target gene expression (Figure 2). There are five forms of VDDR [28, 29]. VDDR1A (OMIM 264700) is the result of inactivating variants of CYP27B1 – thus impairing conversion of calcifidiol to calcitriol. VDDR1B (OMIM 600081) is a consequence of biallelic mutations in CYP2R1 – thereby depressing synthesis of calcidiol. VDDR2A (OMIM 277440) is due to a loss-of-function variant of VDR resulting in a functionally abnormal vitamin D receptor. VDDR2B (OMIM 600785) is associated with normal or increased synthesis of calcidiol and calcitriol and a normal vitamin D receptor whose pathogenesis is as yet unidentified, but may be related to an abnormality of the interaction between the vitamin D receptor and its target hormone response element. VDDR1 and VDDR2 are autosomal recessive in transmission while VDDR3 (OMIM 619073) is transmitted as an autosomal dominant disorder; VDDR3 is due to a recurrent, gain-of-function variant of CYP3A4 (OMIM 124010) that leads to rapid inactivation of vitamin D metabolites [30]. In contrast to genetic disorders that impair vitamin D action, hypercalcemia of infancy, type 1 (OMIM 143880) is associated with excessive vitamin D action as a consequence of biallelic inactivating variants of CYP24A1 (OMIM 126065), encoding the enzyme – 25-hydroxyvitamin D-24-hydroxylase – that inactivates calcifediol and calcitriol. Patients with this disorder have hypercalcemia and hypercalciuria with markedly elevated serum concentrations of calcitriol and very low levels of PTH. Hypercalcemia of infancy, type 2 (OMIM 616963) results from inactivating variants of SLC34A1 (OMIM 182309) encoding the renal sodium-phosphate co-transporter NPT2a, the functional loss of which results in hypophosphatemia, suppressed levels of FGF23, and consequent increased production of calcitriol [31].

Figure 2:

Metabolism of vitamin D – causes of non-nutritional vitamin D deficiency: cholecalciferol (vitamin D3) or ergocalciferol (vitamin D2) is initially hydroxylated at carbon 25 by 25-hydroxylase encoded by CYP2R1 (Table 1) to form calcidiol and then by 25-hydroxyvitamin D3–1-alpha hydroxylase (CYP27B1) to form calcitriol. Hydroxylation at carbon 24 by 25-hydroxyvitamin D3–24-hydroxylase (CYP24A1) leads to water soluble calcitroic acid (1,24,25-trihydroxycholecalciferol) and its excretion in urine or bile. Hydroxylation at carbon 23 by the enzyme encoded by CYP3A4 results in water soluble 1,23,25-trihydroxycholecalciferol. Loss of function variants of CYP2R1 and CYP27B1 interfere with synthesis of calcitriol, while gain–of-function variants of CYP3A4 increase its rate of excretion (see text). [Reproduced with permission from [29] Stoffers AJ, Weber DR, Levine MA. An update on vitamin D deficiency in the twenty-first century: nature and nurture. Curr Opin Endocrinol Diabetes Obes 2022;29:36–43. DOI: 10.1097/MED.0000000000000691].

Hypophosphatasia

Hypophosphatasia (OMIM 241500) is a disorder of bone mineralization due to loss-of-function variants of TNSALP encoding tissue nonspecific (liver, bone and kidney) alkaline phosphatase (OMIM 171760) resulting in clinical manifestations of rickets. Alkaline phosphatase is a phosphohydrolase ectoenzyme linked to the outer membrane of osteoblasts. Depending on the mutant site within TNSALP and whether the mutant site is monoallelic or biallelic, variable degrees of loss of bone alkaline phosphatase activity result in clinical manifestations of rickets of increasing severity [32, 33]. Thus, there are perinatal, infantile, childhood, and adult forms of hypophosphatasia as well as types designated odontohypo-phosphatasia (clinical manifestations related to abnormal dentinogenesis and premature shedding of teeth) and pseudohypophosphatasia (a disorder in which alkaline phosphatase activity toward artificial substrates is intact, but biologic alkaline phosphatase activity toward endogenous substrates is compromised). In patients with hypophosphatasia, inability to free protein-bound phosphate leads to extracellular accumulation of inorganic pyrophosphate that impairs normal growth of hydroxyapatite crystals and hence results in subnormal skeletal mineralization. With the development of a recombinant bone-targeted human TNSALP termed asfotase alpha, management of neonates, infants, and children with hypophosphatasia has changed dramatically; in the perinatal and infantile forms of hypophosphatasia administration of asfotase alpha increases skeletal mineralization leading to improvement in respiratory function and improved survival rate [34].

Hyperparathyroidism – familial hypocalciuric hypercalcemia

A single autonomously functioning parathyroid adenoma is present in most patients with sporadic primary hyperparathyroidism. By contrast, familial forms of primary hyperparathyroidism are associated with enlargement and excess function of all four parathyroid glands. Familial hyperparathyroidism is usually inherited as an autosomal dominant disorder and most often occurs as a component of complex syndromes that involve multiple tissues: multiple endocrine neoplasia types: 1 – due to deleterious variants of MEN1 (OMIM 613733) encoding the tumor suppressor menin; 2A – related to inactivating mutations in RET (OMIM 164761) specifying the RET protooncogene, a receptor tyrosine kinase; and 4 – attributable to variants of CDKN1B (OMIM 600778) encoding cyclin-dependent kinase inhibitor 1B, a molecule that impairs the normal transfer of a phosphate group from cyclic AMP to a recipient protein and hence interrupts progression of the normal cell cycle [35]. Familial isolated hyperparathyroidism type 1 (OMIM 145000) has been ascribed to mutations in CDC73 (OMIM 607393), variants of which are also associated with the hyperparathyroidism-ossifying jaw tumor syndrome (OMIM 145001). CDC73 (OMIM 607393) encodes parafibromin, a tumor suppressor that is involved in transcriptional and post-transcriptional pathways that control cell cycling. Patients with CDC73 mutations usually have single-gland parathyroid disease and have a high risk of developing parathyroid carcinoma. Familial isolated hyperparathyroidism type 2 (OMIM 618883) has been related to activating variants of the transcription factor glial cells missing-2 (GCM2, OMIM 603716) [36]. Variants of hyperparathyroidism include three forms of familial hypocalciuric hypercalcemia (FHH) which differ from other forms of familial primary hyperparathyroidism due to the very low fractional excretion of calcium characteristic of these conditions, onset of mild hypercalcemia within the first days of life, and their generally benign natural history. FHH1 is the consequence of heterozygous loss-of-function mutations in CASR (OMIM 601199) encoding the plasma membrane CASR; FHH2 is due to loss-of-function mutations in GNA11 (OMIM 139313) encoding the α subunit of G11 that interacts with the CASR, and FHH3 is due to loss-of-function mutations in AP2S1 (OMIM 602242) encoding the adaptor-related protein complex 2, sigma 1 subunit, a component of a large protein complex required for proper insertion of the CASR into the plasma membrane [37]. Conversely, activating variants of CASR result in autosomal dominant hypocalcemia type 1 (OMIM 601198), while activating mutations in GNA11 are responsible for autosomal dominant hypocalcemia type 2 (OMIM 615361).

Abnormalities of bone strength: osteogenesis imperfecta and osteopetrosis

Cartilage is formed by chondroblasts that are derived from mesenchymal stem cells in response to the product of SOX9 (OMIM 608160) [38, 39]. Bone is formed by osteoblasts that have differentiated from mesenchymal cells – either directly as membranous bone (e.g., skull, clavicles) or after antecedent cartilage formation (e.g., long bones, vertebrae) under the direction of bone morphogenetic protein (BMP1, OMIM 112264) or follow the WNT-βCatenin pathway involving the low density lipoprotein receptor-related proteins 5 and 6 and epigenetic factors including a number of miRNAs, lncRNAs, and circRNAs [40]. Osteoclasts arise from bone marrow hematopoietic monocyte/macrophage precursor cells that undergo differentiation – initially into mononuclear osteoclasts – in response to the osteoblast – derived ligand for receptor activator of nuclear factor kβ (NFkβ) (RANK-ligand) (TNFSF11, OMIM 602642), macrophage colony-stimulating factor (CSF1; 120420), and immunoreceptor tyrosine-based activation motif (ITAM)-mediated co-stimulatory signaling (NFAM1, OMIM 608740) [40]. RANK-ligand links to RANK expressed on the cell membrane of the osteoclast precursor cell.

Osteogenesis imperfecta is characterized by bone fragility and increased risk/frequency of fracture with minimal or no apparent injury. It is due to variants of genes whose malfunction impairs bone strength by interfering with formation or modeling of bone collagen or its proper mineralization (Table 2) [41–43]. It may be due to deleterious variants of genes required for osteoblast differentiation and maturation or impairing osteoblast function; e.g., those encoding osteoblast synthesis of procollagens types I-IV or those necessary for the normal processing, structure, modifications, and calcification/mineralization of bone collagen [38–43]. As of October 2022, 22 “types” of osteogenesis imperfecta have been formally categorized by OMIM. Osteogenesis imperfecta type 22 is due to deleterious variants of CCDC134 (OMIM 618788) encoding a secreted protein that regulates intracellular signal transduction through the mitogen-activated protein kinase pathway. Inactivating variants of CCDC134 have been identified in patients with extreme bone fragility and multiple bone fractures – at times beginning in utero, pseudoarthroses, scoliosis, and impaired growth but normal dentinogenesis; in these patients the expression of COL1A1 is decreased [44–46]. Additionally, there are other gene variants listed in Table 2 that are associated with decreased bone strength. For example, PLOD2 encodes lysyl hydroxylase 2, an enzyme required for cross-linking of collagen fibers that is mutated in patients with Bruck syndrome 2 (OMIM 609220), manifested by the association of congenital contractures, pterygia, and extreme bone fragility [47]. P4HB encodes the beta subunit of prolyl-4-hydroxylase that is mutated in patients with one form of the Cole-Carpenter syndrome 1 (OMIM 112240) of craniosynostosis and bone fragility. Zoledronic acid, a modified bisphosphonate, has proven effective in the treatment of patients with osteogenesis imperfecta; by decreasing bone resorption, zoledronic acid increases bone mineral content [48, 49].

Table 2:

Osteogenesis imperfecta.

Type	OMIM phenotype	Defective gene	Protein	OMIM	Locus gene	Inheritance, associated features
Defects in procollagen/collagen synthesis and processing
I	166200	COL1A1	Collagen, type I, alpha −1	120150	17q21.33	AD, variant structure of the collagen propeptide, modifications, helical formation; multiple bone fractures with minimal trauma, blue sclerae, normal teeth, near normal height
II	166219	COL1A1, COL1A2	Collagen, type I, alpha-2	120160	7q21.3	AD, in utero and perinatal fractures, death often in perinatal period
III	259420	COL1A1, COL1A2				AD, fading blue sclerae, progressive deformities in late childhood, dentinogenesis imperfecta of primary teeth
IV	166220	COL1A1, COL1A2				AD, Prepubertal bone fragility nearly white sclerae
XIII	614856	BMP1	Bone morphogenetic protein	112264	8p21.3	AR, deficiency of C-propeptidase, 1 severe skeletal deformity
XXII	618788	CCDC134	Coiled-coil domain-containing protein 134	618788	22q13.2	AR, impacts transcription, intracellular signal transduction, decreases expression of COL1A1
Ehlers-Danlos syndrome/Osteogenesis imperfecta
	619115	COL1A1	Collagen, type I, alpha 1	120150	17q21.33	AD, Bone fragility with joint hyperextensibility
Defects in collagen modification
VII	610682	CRTAP	Cartilage-associated protein	605497	3p22.3	AR, scafolding protein forming complex with prolyl 3-hydroxylase; severe to lethal
VIII	610915	P3H1	Prolyl 3-hydroxylase 1	610339	1p34.2	AR, Hydroxylates carbon 3 of proline at position 986 of collagen type 1 alpha 1- essential for collagen cross-linking, severe to lethal
IX	259440	PPIB	Peptidyl-prolyl isomerase B	12384	15q22.31	AR, catalyzes isomerization of proline, an amino acid essential for synthesis of bone collagen, gray sclerae, bowing of femur, tibia
XIV	615066	TMEM38B	Transmembrane protein 38B	611236	9q31.2	AR, monovalent cation channel in endoplasmic reticulum important for intracellular collagen movement, normal to blue sclerae, severe
Abnormalities of collagen folding and cross-linking
X	613848	SERPINH1	Serpin peptidase inhibitor	600943	11q13.5	AR, collagen binding protein, also termed heat shock protein 47; monitors rate of movement of type I procollagen from endoplasmic reticulum to the Golgi apparatus, severe, dentinogenesis imperfecta
XI	610968	FKBP10	FK506-binding protein	607063	17q21.2	AR, collagen chaperone assisting in collagen folding; interacts with SERPINH1, joint contractures, Bruck syndrome #1
	259450	FKBP10				AR, bone fragility, congenital contractures, pterygia, Bruck syndrome #2
NAa	609220	PLOD2	Procollagen-lysine, 2-oxoglutarate, 5-dioxygenase 2	601865	3q24	AR, lysyl hydroxylase 2 – enzyme that hydroxylates lysine residues in telopeptides of domains enabling cross-linking of collagen strands; associated with bone fragility, joint contractures, pterygia Bruck syndrome #2/osteogenesis imperfecta
XXI	619131	KDELR2	KDEL endoplasmic reticulum protein retention receptor 2	609024	7p22.1	AR, receptor that cycles between the endoplasmic reticulum and Golgi apparatus transferring proteins with a carboxyl-terminal sequence of Lys-Asp-Glu-Leu (KDEL)
Abnormalities of osteoblast differentiation and function
XII	613849	SP7	Transcription factor SP7	606633	12q13.13	AR, controls osteoblastic bone formation, moderately severe
XV	615220	WNT1	Wingless-type MMTV integrative site family, member 1	164820	12q13.12	AR/AD, stimulates osteoblast differentiation, severe skeletal deformities, neurological defects
XVI	616229	CREB3L1	cAMP Response element binding protein 3-like 1	616215	11 p11.2	AR, affects the unfolded protein response resulting in alteration of protein processing in the osteoblast, developmental delay
XVII	616507	SPARC	Secreted protein, acidic, cysteine-rich	182120	5q33.1	AR, glycoprotein that binds to type 1 collagen in the extracellular matrix, pectus deformity
XVIII	617952	TENT5A	Terminal nucleotidyl-transferase 5A	611357	6q14.1	AR, increases expression of COL1A1 and COL1A2, joint contractures
XIX	301014	MBTPS2	Membrane-bound transcription factor protease, site 2	300294	Xp22.12	XLR, acts within Golgi apparatus to release membrane-bound proteins
XX	618644	MSED	Mesoderm development LRP chaperone LRP 5 and 6	607783	15q25.1	AR, essential for osteoblast differentiation as a chaperone for defects in mineralization of bone
V	610967	IFITM5	Interferon-induced transmembrane protein 5	614757	11 p15.5	AD, ossification of forearm interosseous membrane, hyperplastic callus formation
VIa		IFITM5				Fish-scale pattern in lamellar bone, increased osteoid
VI	613982	SERPINF1	Serpin peptidase inhibitor, Clade F, Member 1	172860	17p13.3	AR, fish-scale pattern in lamellar bone, increased osteoid
NAa - Cole-Carpenter syndrome type 1	112240	P4HB	Prolyl 4-hydroxylase, beta subunit	176790	17q25.3	AD, bone fragility, craniosynostosis; hydroxylates prolyl residues in preprocollagen
NAa - Cole-Carpenter syndrome type 2	616294	SEC24D	SEC24-related gene family, member D	607186	17q25.3	AR, bone fragility, craniosynostosis; mediates protein transport from endoplasmic reticulum

^aNA, not assigned; AD, autosomal dominant; AR, autosomal recessive; XLR, X-linked recessive. [Modified from 41. Jovanovic M, Guterman-Ram G, Marini JC. Osteogenesis imperfecta: Mechanisms and signaling pathways connecting classical and rare OI types. Endocrine Rev 2022;43:61–90. doi:10.1210/endrev/bnab017. 43. Etich J, Leβmeier L, Rehberg M, et al. Osteogenesis imperfecta – pathophysiology and therapeutic options. Mol Cell Pediatr 2020;7:9. https://doi.org/10.1186/s40348-020-00101-9].

Osteopetrosis is a disorder of bone formation characterized by increased bone mass/density due to abnormalities in osteoclast differentiation or function and hence decreased bone mineral reabsorption [50]. Paradoxically, however, osteopetrosis is associated with increased “brittleness” of bone and predisposition to fracture [50–52]. The forms of osteopetrosis may be classified according to the site of abnormality of osteoclast differentiation (osteoclastogenesis), osteoclast function, or interaction with matrix (Table 3). Thus, FERMT3 (OMIM 607901) and ITGB3 (OMIM 173470) encode integrins – adhesion factors that facilitate cell-to-cell-to-matrix communication [53].

Table 3:

Osteopetrosis.

Type	OMIM	Defective gene	Protein	OMIM	Locus	Inheritance, associated features
Osteoclast differentiation
AR2	259710	TNFSF11	Tumor necrosis factor ligand superfamily, member 11	602642	13q14.11	AR, receptor activator of NFkB – ligand (RANKL)
AR7	612301	TNFRSF11A	Tumor necrosis factor receptor superfamily, member 11A	603499	18q21.33	AR, receptor activator of NFkB – (RANK)
AD1	607634	LRP5	Low density lipoprotein-receptor-related protein 5	603506	11 q13.2	AD, transduces WNT signaling enabling osteoclastogenesis
NA	300248	IKBKG	IKBKG Inhibitor of nuclear factor kappa-B kinase, Regulatory subunit gamma	300248	Xq28	XLR, enables NFkB- mediated osteoclast differentiation, associated with ectodermal dysplasia
NA		IGSF23	Immunoglobulin superfamily member 23	NA	19q13.31	AR, receptor promoting M-CSF and RANK ligand – mediated differentiation of osteoclasts by activating MAPK signaling
NA	617306	MITF	Microphthalmia -associated transcription factor	156845	3p13	AR, developmental protein, variant associated with syndrome of coloboma, osteopetrosis, microphthalmia, albinism
Osteoclast function
NA		FERMT3	Fermitin family, member 3	607901	11q13.1	AR. transmembrane adhesion receptor facilitating cell-to-cell-to-extracellular matrix protein interaction
NA		ITGB3	Integrin, beta 3	173470	17q21.32	AR, transmembrane adhesion receptor that facilitates cell-to-cell-to-extracellular matrix protein interaction
AR22		CCDC134	Coil-coiled domain-containing protein 134	618788	22q13	AR, regulator of transcription, signal transduction
Synthesis of hydrochloric acid
AR3	259730	CA2	Carbonic anhydrase II	611492	8q21.2	AR, catalyzes dissociation of carbonic acid, associated with renal tubular acidosis
Acid transport/secretion/mineral dissolution
AR8	615085	SNX10	Sorting nexin 10	614780	7p15.2	AR, necessary for vacuole formation and trafficking severe neonatal form
AR6	611497	PLEKHM1	Pleckstrin homology domain containing protein,family M, member 1	611466	17q21.31	AR/AD, enables vesicular transport in osteoclasts, intermediate severity
AD3	618107	PLEKHM1
AR4	611490	CLCN7	Chloride channel 7	602727	16p13.3	AR/AD, chloride channel that interacts with OSTM1, variable severity
AD2	166600	CLCN7
AR5	259720	OSTM1	Osteopetrosis-associated transmembrane protein1	607649	6q21	AR, forms stable molecular complex with CLCN7, infantile form with involvement of nervous system
AR1	259700	TCIRG1	T Cell immune regulator 1	604592	11 q13.2	AR, osteoclast proton pump, severe neonatal form
Other
Pycknodysostosis	265800	CTSK	Cathepsin K	601105	1q21.3	AR, cysteine proteinase that degrades bone collagen matrix; bone fragility despite osteosclerosis
Sclerosteosis 1	269500	SOST	Sclerostin	605740	17q21.31	AR, bone morphogenic protein antagonist secreted by osteocytes; interacts with LRP5, LRP6

NA, not assigned; AD, autosomal dominant; AR, autosomal recessive; XLR, X-linked recessive.

Conclusions

In this brief overview we have presented some highlights and discoveries that have been made over the past fifty years in pediatric bone and mineral metabolism. These remarkable achievements have led to the generation of new diagnostic algorithms and stimulated the development of novel treatments that address the fundamental biological defect that is responsible for the clinical disorder. It is clear that the pace of discovery has accelerated with the resolution of the genome at the single base level, and we anticipate that many new and important advances in understanding physiology and disease will soon be disclosed through enhanced understanding of the mechanisms that regulate gene transcription and by further definition of the physical interactions of proteins with genes and chromatin. Further, new knowledge from the emerging field of epigenetics also promises to extend our understanding of human disease. We predict that over the next ten years we will see the harnessing of these discoveries to develop even more bold approaches to therapeutics, including gene therapy.