Robert C. Olney

Expression of the components of the insulin-like growth factor axis across the growth-plate.

Linear bone growth occurs as the result of proliferation and differentiation of growth-plate chondrocytes. These two phases of chondrocyte growth are regulated separately, with insulin-like growth factor I (IGF-I) being the primary stimulator of proliferation. We studied the expression of the components of the growth hormone GH/IGF system to learn if this proliferative signal is altered as chondrocytes undergo differentiation. Growth-plate chondrocytes were isolated from fetal cows and fractionated on discontinuous Percoll gradients. Five populations were recovered, ranging from high density cells (proliferative chondrocytes) to low density cells (hypertrophic chondrocytes). Messenger RNAs (mRNAs) were analyzed by a reverse transcriptase/quantitative polymerase chain reaction (RT/qPCR) technique. Results showed that mRNA of IGF-I and IGF-II in proliferative chondrocytes was 32 and five fold more abundant, respectively, than in hypertrophic chondrocytes. Of the four major IGF-I mRNA transcripts, the class 1-Ea transcript was predominant. Messenger RNA levels for IGFBP-3, -4, and -5 were also reduced in hypertrophic chondrocytes. Levels of GH receptor, the type 1 IGF receptor, and IGF binding protein-2 (IGFBP-2) mRNAs were unchanged across the growth-plate. Since IGF-I and -II are potent stimulators of proliferation, the down-regulation of these genes may be necessary in order for hypertrophy to proceed.

Mechanisms of Impaired Growth: Effect of Steroids on Bone and Cartilage

Background: Long-term treatment with high-dose glucocorticoids (GCs) has profound effects on bone metabolism and linear growth. Bone metabolism is a balance between bone resorption by osteoclasts and new bone formation by osteoblasts. Systemically, GC treatment reduces circulating levels of estrogen and modestly increases parathyroid hormone levels. At the local level, GCs decrease insulin-like growth factor I (IGF-I) production, induce IGF-I resistance and increase nuclear factor κB ligand production by osteoblasts. These alterations inhibit new bone formation and stimulate bone resorption, with a net loss of bone over time. Clinically, this results in decreased bone mineral density, osteoporosis and increased risk for fracture. Local effects of GCs at the growth plate include reduction of IGF-I production, inducing IGF-I resistance and reducing production of C-type natriuretic peptide, which results in a reduction of chondrocyte proliferation, matrix synthesis and hypertrophy. These reductions in chondrocyte function result in decreased linear growth. Conclusions: The effects of GCs on bone metabolism and linear growth are sensitive and specific and represent an evolutionary adaptation to redirect resources during times of physiologic stress. Since many of these effects result from alterations in IGF-I production, growth hormone therapy is a potential approach to ameliorate these problems.

Heterozygous mutations in natriuretic peptide receptor-B (NPR2) gene as a cause of short stature

Based on the observation of reduced stature in relatives of patients with acromesomelic dysplasia, Maroteaux type (AMDM), caused by homozygous or compound heterozygous mutations in natriuretic peptide receptor‐B gene (NPR2), it has been suggested that heterozygous mutations in this gene could be responsible for the growth impairment observed in some cases of idiopathic short stature (ISS). We enrolled 192 unrelated patients with short stature and 192 controls of normal height and identified seven heterozygous NPR2 missense or splice site mutations all in the short stature patients, including one de novo splice site variant. Three of the six inherited variants segregated with short stature in the family. Nine additional rare nonsynonymous NPR2 variants were found in three additional cohorts. Functional studies identified eight loss‐of‐function mutations in short individuals and one gain‐of‐function mutation in tall individuals. With these data, we were able to rigorously verify that NPR2 functional haploinsufficiency contributes to short stature. We estimate a prevalence of NPR2 haploinsufficiency of between 0 and 1/26 in people with ISS. We suggest that NPR2 gain of function may be a more common cause of tall stature than previously recognized.

Impact of age, phenotype and cardio‐renal function on plasma C‐type and B‐type natriuretic peptide forms in an adult population

In contrast to the cardiac hormones, atrial natriuretic peptide (ANP) and B‐type natriuretic peptide (BNP), variations in plasma concentrations of C‐type natriuretic peptide (CNP) in healthy adults are ill‐defined, limiting their clinical application.

Amino-terminal propeptide of C-type natriuretic peptide (NTproCNP) predicts height velocity in healthy children

C‐type natriuretic peptide (CNP) is a paracrine regulatory factor of the growth plate and plays a key role in endochondral growth. Its amino‐terminal propeptide (NTproCNP) is an equimolar product of CNP biosynthesis and is easily measured in plasma. Preliminary studies suggest that NTproCNP levels correlate with height velocity in sheep and children. The objectives of the study were to correlate NTproCNP levels with height velocity and to define the reference range for plasma CNP and NTproCNP across childhood.

Healthy Children With Frequent Fractures: How Much Evaluation Is Needed?

OBJECTIVE. We performed a case-control study to determine whether occult bone disease is associated with a history of frequent fractures in children. METHODS. Healthy children with ≥2 incidences of low-energy fractures were recruited (n = 68). Children with no history of fractures served as control subjects (n = 57). Food logs, activity surveys, physical examinations, laboratory tests, and dual-energy radiographic absorptiometry were used. RESULTS. Bone mineral density z scores were significantly reduced in case subjects, compared with control subjects. Three case subjects (4.3%) and 1 control subject (1.8%) had bone mineral density z scores below the expected range. Of those 4 subjects, 2 had dairy avoidance and 2 had delayed puberty. An additional case subject had evidence of vitamin D deficiency. A significant number of subjects (20% of case subjects and 23% of control subjects) had idiopathic hypercalcuria, based on 24-hour urine collections. Among the case subjects, bone mineral density z scores were significantly lower for those with idiopathic hypercalcuria. Among the control subjects, the presence of idiopathic hypercalcuria did not affect bone mineral density. The case subjects with idiopathic hypercalcuria accounted for virtually all of the differences in bone mineral density between the case and control groups. Analysis of parathyroid hormone and 1,25-dihydroxy-vitamin D levels showed that children with frequent fractures and hypercalcuria had renal hypercalcuria, whereas children with no fractures and hypercalcuria had absorptive hypercalcuria. CONCLUSIONS. We identified a significant association between a history of frequent fractures and hypercalcuria in children. We propose that the appropriate screening evaluation for children who present with a history of frequent fractures consists of a dietary history targeted at calcium and vitamin D intakes, a physical examination to assess for pubertal delay, and urinary calcium concentration/creatinine ratio determination to assess for hypercalcuria. Children with abnormalities in this screening should undergo dual-energy radiographic absorptiometry and appropriate evaluation.

C-type natriuretic peptide plasma levels are elevated in subjects with achondroplasia, hypochondroplasia, and thanatophoric dysplasia

CONTEXT C-type natriuretic peptide (CNP) is a crucial regulator of endochondral bone growth. In a previous report of a child with acromesomelic dysplasia, Maroteaux type (AMDM), caused by loss-of-function of the CNP receptor (natriuretic peptide receptor-B [NPR-B]), plasma levels of CNP were elevated. In vitro studies have shown that activation of the MAPK kinase (MEK)/ERK MAPK pathway causes functional inhibition of NPR-B. Achondroplasia, hypochondroplasia, and thanatophoric dysplasia are syndromes of short-limbed dwarfism caused by activating mutations of fibroblast growth factor receptor-3, which result in overactivation of the MEK/ERK MAPK pathway. OBJECTIVE The purpose of this study was to determine whether these syndromes exhibit evidence of CNP resistance as reflected by increases in plasma CNP and its amino-terminal propeptide (NTproCNP). DESIGN This was a prospective, observational study. SUBJECTS Participants were 63 children and 20 adults with achondroplasia, 6 children with hypochondroplasia, 2 children with thanatophoric dysplasia, and 4 children and 1 adult with AMDM. RESULTS Plasma levels of CNP and NTproCNP were higher in children with achondroplasia with CNP SD scores (SDSs) of 1.0 (0.3-1.4) (median [interquartile range]) and NTproCNP SDSs of 1.4 (0.4-1.8; P < .0005). NTproCNP levels correlated with height velocity. Levels were also elevated in adults with achondroplasia (CNP SDSs of 1.5 [0.7-2.1] and NTproCNP SDSs of 0.5 [0.1-1.0], P < .005). In children with hypochondroplasia, CNP SDSs were 1.3 (0.7-1.5) (P = .08) and NTproCNP SDSs were 1.9 (1.8-2.3) (P < .05). In children with AMDM, CNP SDSs were 1.6 (1.4-3.3) and NTproCNP SDSs were 4.2 (2.7-6.2) (P < .01). CONCLUSIONS In these skeletal dysplasias, elevated plasma levels of proCNP products suggest the presence of tissue resistance to CNP.

C-type natriuretic peptide forms in adult hyperthyroidism: correlation with thyroid hormones and markers of bone turnover.

Context  Plasma C‐type natriuretic peptide (CNP) forms correlate with linear growth velocity in juveniles. In hyperthyroid children, plasma CNP products fall in parallel with height velocity and thyroid hormones (TH) as euthyroidism is restored. The effect of TH on CNP forms after completion of endochondral growth is unknown.

Acute inflammation in young children inhibits C-type natriuretic peptide

Background:C-type natriuretic peptide (CNP) is a paracrine growth factor critical in endochondral bone growth. Amino-terminal CNP (NTproCNP), measurable in plasma, correlates with growth-plate activity and can be used as a biomarker of growth velocity in children. Because severe inflammation in adults increases CNP, we studied CNP peptides and inflammatory markers in children with acute illness.Methods:Forty-two children aged 2 mo to 5 y with acute illness warranting admission to an acute assessment unit were studied. Fifteen age-matched healthy children attending an outpatient clinic served as controls. Venous CNP concentrations were measured at admission, along with markers of acute inflammation (body temperature, C-reactive protein (CRP), and white blood cell count) in children with acute illness.Results:NTproCNP and CNP SD scores (SDSs) in the acutely ill group were significantly suppressed (P < 0.001) as compared with those of healthy children or healthy population norms. NTproCNP SDS was significantly inversely related to body temperature (r = −0.42, P < 0.01) and CRP (r = −0.56, P < 0.001).Conclusion:Acute inflammation in young children potently reduces CNP production, which needs to be considered when screening for growth disorders. Our data raise the possibility that the adverse effects of inflammatory cytokines on skeletal growth may be mediated in part by reduced CNP.

Plasma C‐type natriuretic peptide forms and thyroid status in prepubertal children with acquired thyroid disease

Objective  C‐type natriuretic peptide (CNP) and thyroid hormone (TH) are essential for normal skeletal growth. Plasma CNP peptides correlate with growth velocity, but the relationship between thyroid status and CNP production is unknown. This study examined the impact of restoring normal TH levels on CNP and height velocity (HV) in children with acquired hypo‐ and hyperthyroidism.