Children grow taller because their bones grow longer. This bone elongation occurs at the growth plate, a thin layer of cartilage found near the ends of juvenile bones. Consequently, mutations in genes that regulate growth plate chondrogenesis cause abnormal bone growth in children. Depending on the severity and nature of the genetic abnormality, the clinical phenotype can range from chondrodysplasias with short, malformed bones, to severe, often disproportionate, short stature, to mild proportionate short stature. If the genetic defect affects tissues other than the growth plate cartilage, the child may present with a more complex syndrome that includes other clinical abnormalities. For many children who are brought to medical attention for linear growth disorders, clinical evaluation and laboratory evaluation fail to identify the underlying etiology. To discover new genetic causes of childhood growth disorders, we are using powerful genetic approaches including SNP arrays to detect large deletions, duplications, mosaicism, and uniparental disomy combined with exome sequencing to detect single nucleotide variants and small insertions/deletions in coding regions and splice sites. This analysis has led to our identification of heterozygous mutations in ACAN causing autosomal dominant short stature with advanced bone age and premature osteoarthritis. We also identified compound heterozygous mutations in BRF1 in a family with autosomal recessive growth failure, markedly delayed bone age, and central nervous system anomalies, confirming a previous report that BRF1 mutations cause cerebellar-facial-dental syndrome and further elucidating the phenotype. We also studied a subject with markedly accelerated skeletal and dental development, retinal scarring, and autism-spectrum disease and found elevated retinoic acid levels and a microdeletion on chromosome 18 10q23.2-23.33 which included CYP26A1 and C1, both major retinoic acid metabolizing enzymes. Because retinoic acid accelerates skeletal development and is involved in development of the eye and central nervous system, it is likely that these gene deletions contributed to the patients disorder. Many of the mechanisms that regulate mammalian body growth, when disrupted, can contribute to the development of malignancies. We have studied the role of two heparin-binding growth factors, pleiotrophin and midkine, in the regulation both of normal body growth and in the unregulated growth of malignancies. To delineate the role of midkine and pleiotrophin in human development, we developed high sensitivity assays to measure their concentrations in amniotic fluid at various gestational ages in both healthy and complicated pregnancies. We found that both of these growth factors could be readily measured in amniotic fluid and that the concentrations were higher than most cytokines previously reported in amniotic fluid. The concentration of midkine but not that of pleiotrophin declined with gestational age. Both midkine and pleiotrophin concentrations were found to be lower in pregnancies that were complicated by chorioamnionitis at term, raising the possibility that these growth factors might be useful as markers for infection. Previously, we measured midkine concentrations in fine-needle aspirate samples from benign and malignant thyroid nodules to explore the possibility that midkine measurement might aid in the evaluation of thyroid nodules and found that, in FNA samples, the midkine/thyroglobulin ratio in papillary thyroid cancer was greater than in benign thyroid nodules. More recently, we performed an analogous study of pleiotrophin and found that the pleiotrophin to thyroglobulin ratio was also higher in papillary thyroid cancer samples than in benign thyroid nodules. The findings raise the possibility that measurement of midkine and pleiotrophin concentrations in fine-needle aspirate samples may provide useful diagnostic and/or prognostic information in the evaluation of thyroid nodules. Currently, treatment approaches for linear growth disorders are limited. Recombinant human growth hormone treatment has limited efficacy for severe disease, including many skeletal dysplasias, and has significant known and potential adverse effects. Therefore, better treatments for severe growth disorders are needed. Recent studies have identified many paracrine factors that positively regulate growth plate chondrogenesis and therefore might be used therapeutically. However, the development of these molecules into effective treatment has been hampered by their mechanism of action; these growth factors are produced locally and act locally in the growth plate, and thus do not lend themselves to systemic therapeutic approaches. We envisioned that these locally-acting molecules could be targeted to the growth plate by linking them to cartilage-binding proteins, such as antibody fragments. When administered systemically, these hybrid molecules would be preferentially taken up by growth plate cartilage, and thus might augment the therapeutic effect on the target organ while diminishing adverse effects due to actions on other tissues. Similarly, growth-promoting endocrine factors, such as growth hormone and insulin-like growth factor-I might be targeted to the growth plate to enhance the therapeutic effects on chondrogenesis and reduce adverse effects on non-target tissues. To develop cartilage-targeting therapy, yeast display was previously used to identify antibody fragments that bound with high affinity to matrilin-3, an extracellular matrix protein expressed with high tissue specificity in cartilage. In vivo, these antibody fragments homed specifically to cartilage tissue in mice. Coupling these antibody fragments to endocrine and paracrine factors that stimulate chondrogenesis could be used to direct these potent molecules specifically to cartilage tissue and thus has the potential to open up new pharmacological approaches to treat childhood skeletal growth disorders. These approaches are currently under study in the laboratory.
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