Immunoradiometric assay of parathyrin with polyclonal and monoclonal region-specific antibodies

Hypocalcemia and hyperphosphatemia are encountered in idiopathic hypoparathyroidism (IHP) and pseudohypoparathyroidism type Ib (PHP1B). In contrast to PHP1B, which is caused by resistance toward parathyroid hormone (PTH), the genetic defects leading to IHP impair production of this important regulator of mineral ion homeostasis. So far, only five PTH mutations were shown to cause IHP, each of which is located in the hormone’s pre-pro leader segment and thus impair hormone secretion. In three siblings affected by IHP, we now identified a homozygous arginine-to-cysteine mutation at position 25 (R25C) of the mature PTH(1-84) polypeptide; heterozygous family members are healthy. Depending on the assay used for evaluating these patients, plasma PTH levels were either low or profoundly elevated, thus leading to ambiguities regarding the underlying diagnosis, namely IHP or PHP1B. Consistent with increased PTH levels, recombinant [Cys25]PTH(1-84) and wild-type PTH(1-84) were secreted equally well by transfected COS-7 cells. However, synthetic [Cys25]PTH(1-34) was found to have a lower binding affinity for the PTH receptor type-1 (PTH1R) than PTH(1-34) and consequently a lower efficiency for stimulating cAMP formation in cells expressing this receptor. Consistent with these in vitro findings, long-term infusion of [Cys25]PTH(1-34) resulted only in minimal calcemic and phosphaturic responses, despite readily detectable levels of [Cys25]PTH(1-34) in plasma. The mineral ion abnormalities observed in the three IHP patients are thus most likely caused by the inherited homozygous missense PTH mutation, which reduces bioactivity of the secreted hormone. Based on these findings, screening for PTH(1-84) mutations should be considered when clinical and laboratory findings are consistent with PHP1B, but GNAS methylation changes have been excluded. Differentiating between IHP and PHP1B has considerable implications for genetic counseling, therapy, and long-term outcome because treatment of IHP patients with inappropriately high doses of active vitamin D and calcium can contribute to development of nephrocalcinosis and chronic kidney disease.

Section: Discussionsupporting

confidence: 79%

A Homozygous [Cys25]PTH(1-84) Mutation That Impairs PTH/PTHrP Receptor Activation Defines a Novel Form of Hypoparathyroidism

Lee

Mannstadt

Guo

et al. 2015

“…Both the free 25(OH)D index (based on the molar ratio of 25(OH)D to DBP) and the free 1,25(OH) 2 D index (based on the molar ratio of 1,25(OH) 2 D to DBP) were calculated. These molar ratios have been documented to be adequate estimations of the true free concentrations 29. Serum intact parathyroid hormone (PTH) was measured by a two‐step immunochemical method, involving an amino‐terminal capture and a midregional detecting antibody, as described previously 29.…”

Section: Methodsmentioning

confidence: 99%

Age-Related (Type II) Femoral Neck Osteoporosis in Men: Biochemical Evidence for Both Hypovitaminosis D– and Androgen Deficiency–Induced Bone Resorption

Boonen

Vanderschueren

Cheng

et al. 1997

120

The problem of osteoporosis in men has recently been recognized as an important public health issue. To test the hypothesis that endocrine deficiency-mediated alterations in bone metabolism might contribute to osteoporotic fracture risk in elderly men, serum levels of 25-hydroxycholecalciferol (25(OH)D), 1,25-dihydroxycholecalciferol (1,25(OH) 2 D), intact parathyroid hormone (PTH), testosterone, and estradiol were measured in 40 males (mean age 73 years) who were consecutively recruited within 18 h following a fracture of the proximal femur, and in an equal number of community-living older men (mean age 72 years) who served as controls. In addition, circulating osteocalcin and urinary excretion of (deoxy)pyridinoline were determined as markers of bone formation and resorption, respectively. No differences were observed between the mean serum concentrations of osteocalcin and estradiol. Serum levels of 25(OH)D, 1,25(OH) 2 D, and testosterone, however, were decreased in hip fracture patients. When correcting for differences in vitamin D binding protein, differences in 1,25(OH) 2 D did not persist, whereas serum 25(OH)D was still significantly lower in patients than in controls (6.1 ؎ 4.3 vs. 7.6 ؎ 2.8, p ‫؍‬ 0.01). Similarly, a highly significant deficit was observed in the free testosterone index, calculated from total testosterone and the level of sex hormone binding globulin (2.6 ؎ 1.3 vs. 8.2 ؎ 2.9, p < 0.001). Serum PTH and urinary pyridinium cross-links, however, were markedly increased in the fracture group. Moreover, in fracture patients, free 25(OH)D and free testosterone were both significant and mutually independent negative predictors of (deoxy)pyridinoline excretion. Although limited by its cross-sectional design, the present study suggests that both hypovitaminosis D and androgen deficiency may predispose to bone resorption in elderly men and in turn to remodeling imbalance and fracture

“…(33) Serum intact PTH (reference range 3-40 pg/ml) was measured by a twostep immunochemical method, involving an amino-terminal capture and a midregional detecting antibody, as described previously. (34) Human osteocalcin (OC) was determined by a previously reported radioimmunoassay (RIA). (35) Serum levels of skeletal alkaline phosphatase were determined by the heat inactivation method.…”

Section: Biochemical Measurementsmentioning

confidence: 99%

“…Clinical and biochemical data from the elderly controls and the hip fracture patients are indicated in Table 1. According to the 95% reference interval of a healthy young reference group, (34) 40% of the fracture patients had elevated levels of PTH (serum PTH >40 pg/ml) compared with none of the elderly controls. Vitamin D deficiency, as evidenced by serum 25(OH)D 3 levels below 12 ng/ml, (42) was observed in 64% of the osteoporotic patients compared with 8% of the control subjects.…”

Section: Characteristics Of the Populationmentioning

confidence: 99%

Down-Regulation of the Serum Stimulatory Components of the Insulin-like Growth Factor (IGF) System (IGF-I, IGF-II, IGF Binding Protein [BP]-3, and IGFBP-5) in Age-Related (Type II) Femoral Neck Osteoporosis

Boonen

Mohan

Dequeker

et al. 1999

102

Both a decrease in bone formation and an increase in bone resorption have been implicated in the pathogenesis of age-related (type II) femoral neck osteoporosis. While the increase in the bone resorption rate has been shown to be partially related to secondary hyperparathyroidism, the mechanisms underlying the decline in bone formation have not yet been identified. The aim of the present study was to test the hypothesis that the bone formation deficit associated with type II osteoporosis might be due to secondary hyperparathyroidism and/or to a deficiency of the insulin-like growth factor (IGF) system. Circulating concentrations of IGF-I, IGF-II, IGF binding protein (IGFBP)-3, IGFBP-4, IGFBP-5, 25-hydroxycholecalciferol (25(OH)D 3 ), and intact parathyroid hormone (PTH) were measured in 50 elderly women after sustaining a hip fracture and in 50 healthy age-matched controls. In addition, serum levels of osteocalcin (OC), skeletal alkaline phosphatase, and N-terminal procollagen peptide and urinary pyridinium cross-links were determined as markers of bone remodeling, and bone mineral density (BMD) was assessed at the proximal femur. In the patient group, serum was drawn within 18 h of the fracture and prior to surgery. Circulating protein concentrations did not change over this time frame. No difference was found between mean IGFBP-4 serum levels in the two groups studied, while mean levels of IGF-I, IGF-II, IGFBP-3, IGFBP-5, 25(OH)D 3 , and markers of bone formation were significantly lower (p < 0.006) in patients as compared with healthy subjects. Serum PTH and urinary pyridinium cross-links, however, were markedly increased (p < 0.001) in the osteoporotic group. In pooled data from the normal and osteoporotic populations, age-adjusted multiple regression models based on IGF-I, IGF-II, IGFBP-3, and IGFBP-5 were found to be highly predictive of serum OC (R 2 = 19%, p < 0.001) and BMD of femoral neck (R 2 = 49%, p < 0.0001), consistent with an effect of the anabolic IGF components on overall bone formation rate. Similar models based on 25(OH)D 3 and PTH, however, were statistically unrelated to OC. To address further the potential impact of trauma on circulating IGF system components, we measured IGF system component levels in 10 male patients within 18 h following tibial fracture and in 10 age-matched normal male subjects. There was no significant difference in serum level of any of the IGF system components between the two groups. Although limited by its cross-sectional design, the present study