IT has long been known that some cellular proteins bind guanine nucleotides such as guanosine tri-phosphate (GTP) with high affinity. These binding proteins are called G proteins and have been linked to such critical intracellular functions as protein synthesis.1 In the past decade three new G proteins have been discovered that reside in the plasma membrane and transmit information from the outside to the inside of the cell. Two of these new G proteins form part of the adenylate cyclase system, a membrane-bound enzyme complex found in virtually every cell, and the third G protein is found in the disk . . .
During normal childhood and adolescence, the skeleton undergoes tremendous change. Utilizing the processes of modeling and remodeling, the skeleton acquires its adult configuration and ultimately achieves peak bone mass. Optimization of peak bone mass requires the proper interaction of environmental, dietary, hormonal, and genetic influences. A variety of acute and chronic conditions, as well as genetic polymorphisms, are associated with reduced bone density, which can lead to an increased risk of fracture both in childhood and later during adulthood. Bone densitometry has an established role in the evaluation of adults with bone disorders, and the development of suitable reference ranges for children now permits the application of this technology to younger individuals. We present a brief overview of the factors that determine bone density and the emerging role of bone densitometry in the assessment of bone mass in growing children and adolescents.
Genetic forms of vitamin D-dependent rickets (VDDRs) are due to mutations impairing activation of vitamin D or decreasing vitamin D receptor responsiveness. Here we describe two unrelated patients with early-onset rickets, reduced serum levels of the vitamin D metabolites 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, and deficient responsiveness to parent and activated forms of vitamin D. Neither patient had a mutation in any genes known to cause VDDR; however, using whole exome sequencing analysis, we identified a recurrent de novo missense mutation, c.902T>C (p.I301T), in CYP3A4 in both subjects that alters the conformation of substrate recognition site 4 (SRS-4). In vitro, the mutant CYP3A4 oxidized 1,25-dihydroxyvitamin D with 10-fold greater activity than WT CYP3A4 and 2-fold greater activity than CYP24A1, the principal inactivator of vitamin D metabolites. As CYP3A4 mutations have not previously been linked to rickets, these findings provide insight into vitamin D metabolism and demonstrate that accelerated inactivation of vitamin D metabolites represents a mechanism for vitamin D deficiency.
We hypothesized that a decrease in Gsα expression occurs with osteogenic differentiation and that when Gsα expression was decreased by antisense oligonucleotides or direct inhibition of protein kinase A there was a concomitant increase in Runx2/Cbfa1. We also investigated the mechanism involved in the change in Runx2/Cbfa1 levels and whether the expression of other genes known to be involved in bone formation was altered. There was a decrease in Gsα expression with osteogenic differentiation and antisense oligonucleotides, and protein kinase A inhibition led to increased expression and DNA binding of the osteoblast-specific Runx2/Cbfa1. Additionally, with decreased Gsα expression or protein kinase A inhibition, Runx2/Cbfa1 protein was serine phosphorylated and ubiquitinated less. Microarray analysis, after the addition of antisense Gsα, showed a more than 10-fold increase in collagen Type I Alpha 2 mRNA (a target of Runx2/Cbfa1). These data show that reduced expression of Gsα can induce an osteoblast-like phenotype. The results also indicate a potential pathophysiologic role in patients with heterozygous inactivating mutations in GNAS1, the gene for the alpha chain (Gsα) of the heterotrimeric G protein, present in three disorders with ectopic intramembranous bone: Albright's hereditary osteodystrophy, progressive osseous heteroplasia, and osteoma cutis.
The landmark guide to pediatric medicine - updated and streamlined for today's clinicians and studentsHighlighting the numerous advances and developements in pediatrics, the new second edition provides a complete update of this premier clinical reference including signs and symptoms, behavioral health, care of healthy and high-risk infants, adolescent health, critical situations, practice management, ethical and legal concerns, and much more.
Activation of CRH receptors type 1 (CRH-R1) by CRH or urocortin (UCN) leads to stimulation of multiple G proteins with consequent effects on diverse signaling cascades in a tissue-specific manner. In human myometrium and human embryonic kidney (HEK)293 cells, binding of UCN to CRH-R1α receptors activates both the Gs and Gq, leading to activation of the adenylyl cyclase/protein kinase A (PKA) and the phospholipase C/protein kinase C and ERK1/2 signaling pathways, respectively. The overall result of these signals is often unpredictable, as these two signaling pathways can interact in many cellular systems, with either potentiation or inhibition of ERK1/2 activity. In the present studies we investigated potential signaling interactions after stimulation of CRH-R1α receptors in human cultured pregnant myometrial cells or HEK293 cells overexpressing recombinant CRH-R1α receptors. We found that the adenylyl cyclase/PKA pathway has the capacity to markedly decrease UCN-induced ERK1/2 activation, and that these effects were due in part to the ability of PKA to phosphorylate the CRH-R1α at position Ser301 in the third intracellular loop. Mutant CRH-R1α receptors with substitutions at position Ser301, which is the only potential PKA phosphorylation site, were resistant to PKA-dependent phosphorylation and showed altered signaling characteristics, which were dependent upon the amino acid substitution at this position.
Hypoparathyroidism is a disorder characterized by hypocalcemia due to insufficient secretion of PTH. Pseudohypoparathyroidism is a less common disorder due to target organ resistance to PTH. This report summarizes the results of the findings and recommendations of the Working Group on Epidemiology and Diagnosis of Hypoparathyroidism. Each contributing author reviewed the recent published literature regarding epidemiology and diagnosis of hypoparathyroidism using PubMed and other medical literature search engines. The prevalence of hypoparathyroidism is an estimated 37 per 100 000 person-years in the United States and 22 per 100 000 person-years in Denmark. The incidence in Denmark is approximately 0.8 per 100 000 person-years. Estimates of prevalence and incidence of hypoparathyroidism are currently lacking in most other countries. Hypoparathyroidism increases the risk of renal insufficiency, kidney stones, posterior subcapsular cataracts, and intracerebral calcifications, but it does not appear to increase overall mortality, cardiovascular disease, fractures, or malignancy. The diagnosis depends upon accurate measurement of PTH by second- and third-generation assays. The most common etiology is postsurgical hypoparathyroidism, followed by autoimmune disorders and rarely genetic disorders. Even more rare are etiologies including parathyroid gland infiltration, external radiation treatment, and radioactive iodine therapy for thyroid disease. Differentiation between these different etiologies is aided by the clinical presentation, serum biochemistries, and in some cases, genetic testing. Hypoparathyroidism is often associated with complications and comorbidities. It is important for endocrinologists and other physicians who care for these patients to be aware of recent advances in the epidemiology, diagnosis, and genetics of this disorder.
ENPP1 encodes ENPP1, an ectonucleotidase catalyzing hydrolysis of ATP to AMP and inorganic pyrophosphate (PPi), and an endogenous plasma protein physiologically preventing ectopic calcification of connective tissues. Mutations in ENPP1 have been reported in association with a range of human genetic diseases. In this mutation update, we provide a comprehensive review of all the pathogenic variants, likely pathogenic variants, and variants of unknown significance in ENPP1 associated with three autosomal recessive disorders-generalized arterial calcification of infancy (GACI), autosomal recessive hypophosphatemic rickets type 2 (ARHR2), and pseudoxanthoma elasticum (PXE), as well as with a predominantly autosomal dominant disorder-Cole disease. The classification of all variants is determined using the latest ACMG guidelines. A total of 140 ENPP1 variants were curated consisting of 133 previously reported variants and seven novel variants, with missense variants being the most prevalent (70.0%, 98/140). While the pathogenic variants are widely distributed in the ENPP1 gene of patientsgen without apparent genotype-phenotype correlation, eight out of nine variants associated with Cole disease are confined to the somatomedin-B-like (SMB) domains critical for homo-dimerization of the ENPP1 protein.
Osteopetrosis encompasses a group of rare metabolic bone diseases characterized by impaired osteoclast activity or development, resulting in high bone mineral density. Existing guidelines focus on treatment of the severe infantile forms with hematopoietic cell transplantation (HCT) but do not address the management of patients with less severe forms for whom HCT is not the standard of care. Therefore, our objective was to develop expert consensus guidelines for the management of these patients. A modified Delphi method was used to build consensus among participants of the Osteopetrosis Working Group, with responses to an anonymous online survey used to identify areas of agreement and conflict and develop a follow-up survey. The strength of recommendations and quality of evidence was graded using the Grading of Recommendations Assessment, Development and Evaluation system. Consensus was found in the areas of diagnosis, monitoring, and treatment. We recommend relying on characteristic radiographic findings to make the diagnosis and found that genetic testing adds important information by identifying mutations associated with unique disease complications. We recommend ongoing monitoring for changes in mineral metabolism and other complications, including cranial nerve impingement, anemia, leukopenia, and dental disease. We suggest that calcitriol should not be used in high doses and instead recommend symptom-based supportive therapy for disease complications because noninfantile osteopetrosis has no effective treatment. Scarcity of published studies on osteopetrosis reduce the ability to develop evidence-based guidelines for the management of these patients. Expert opinion-based guidelines for this rare condition are nevertheless important to enable improved care.
