Regulation Of Childhood Growth

孩子长得高是因为他们的骨头长得长。这种骨骼的延长发生在生长板，一层在幼年骨骼末端附近发现的软骨薄层。因此，调节生长板软骨形成的基因突变会导致儿童骨骼生长异常。根据遗传异常的严重程度和性质，临床表型的范围可以从软骨发育不良伴短、畸形骨到严重、通常不成比例的身材矮小，再到轻度成比例的身材矮小。如果遗传缺陷影响生长板软骨以外的组织，儿童可能会出现更复杂的综合征，包括其他临床异常。对于许多因线性生长障碍而就医的儿童，临床评价和实验室评价未能确定潜在的病因。 为了发现儿童生长障碍的新遗传原因，我们正在使用强大的遗传学方法，包括SNP阵列来检测大缺失，重复，镶嵌和单亲二体性，并结合外显子组测序来检测编码区和剪接位点中的单核苷酸变异和小插入/缺失。这项分析使我们确定了ACAN杂合突变，导致常染色体显性遗传性身材矮小伴骨龄提前和过早骨关节炎。最近，我们参与了一项国际合作，以识别更多患有这种疾病的患者，从而阐明该疾病的表型谱 我们还探讨了生长板上骨骼生长的表观遗传调控。组蛋白甲基转移酶EZH 1和EZH 2催化组蛋白H3在赖氨酸27（H3 K27）处的三甲基化，其充当染色质凝聚和转录抑制的表观遗传信号。在人类中，EZH 2的杂合突变导致韦弗综合征，包括明显的骨骼过度生长。此外，通过全基因组关联研究，EZH 2基因位于与人类身高变异相关的位点，进一步证明EZH 2在调节骨骼生长中发挥重要作用。由于纵向骨生长的结果从软骨形成在生长板，我们探讨了EZH 1和2在这个过程中的作用。在小鼠中，软骨特异性敲除Ezh 2和Ezh 1都不会影响骨骼生长，但软骨中两种组蛋白甲基转移酶的联合丢失会减少H3 K27的三甲基化，严重损害骨骼生长。生长板软骨形成的两个主要过程，软骨细胞增殖和肥大，都受到损害。软骨细胞增殖的减少部分是由于细胞周期蛋白依赖性激酶抑制剂Ink 4a/B的去抑制，而无效的软骨细胞肥大是由于通过增加IGF结合蛋白的表达抑制IGF信号传导。总的来说，我们的研究结果揭示了H3 K27甲基化在生长板中软骨细胞增殖和肥大的调节中的关键作用，这是骨骼生长的中心决定因素。我们目前正在研究导致韦弗综合征的EZH 2错义突变，以了解该疾病的分子病理生理学。 我们的研究还集中在骨形态发生蛋白在生长板和关节软骨调节中的作用。关节软骨和生长板软骨都是由间充质细胞凝聚而成，但最终在组织学和功能上有重要的差异。每一层都由三层组成，即关节软骨的浅、中、深区和生长板软骨的静止、增殖和肥大区。在整个生长板软骨中观察到BMP相关基因表达的梯度，可能在带状分化中发挥作用。为了研究关节软骨中类似表达梯度的存在，我们使用激光捕获显微切割（LCM）将小鼠胫骨近端的生长板和关节软骨分离成6个组成区域，并使用颜色编码探针的溶液杂交测定来定量每个区域中30种不同BMP相关基因的mRNA。原位杂交和免疫组织化学，然后用于确认空间表达模式。我们发现的证据表明，BMP信号梯度存在于生长板和关节软骨，这些梯度有助于出生后软骨内骨骼的软骨细胞的空间分化。

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. Recently, we have participated in an international collaboration to identify more patients with this condition and thereby elucidate the phenotypic spectrum of the disorder We have also explored the epigenetic regulation of skeletal growth at the growth plate. Histone methyltransferases EZH1 and EZH2 catalyze the trimethylation of histone H3 at lysine 27 (H3K27), which serves as an epigenetic signal for chromatin condensation and transcriptional repression. In humans, heterozygous mutations in EZH2 cause Weaver syndrome, which includes marked skeletal overgrowth. In addition, the EZH2 gene lies in a locus associated with human height variation by genome-wide association studies, providing further evidence that EZH2 plays an important role in regulating skeletal growth. Because longitudinal bone growth results from chondrogenesis at the growth plate, we explored the role of Ezh1 and 2 in this process. In mice, neither cartilage-specific knockout of Ezh2 nor generalized knockout of Ezh1 affected skeletal growth, but the combined losses of both histone methyltransferases in cartilage diminished H3K27 trimethylation and severely impaired skeletal growth. Both of the principal process underlying growth plate chondrogenesis, chondrocyte proliferation and hypertrophy, were compromised. The decrease in chondrocyte proliferation is due in part to derepression of cyclin dependent kinase inhibitors Ink4a/b, while the ineffective chondrocyte hypertrophy is due to suppression of IGF signaling by increased expression of IGF binding proteins. Collectively, our findings reveal a critical role for H3K27 methylation in the regulation of chondrocyte proliferation and hypertrophy in the growth plate, which are the central determinants of skeletal growth. We are currently studying the missense mutations in EZH2 that cause Weaver syndrome to understand the molecular pathophysiology of the disorder. Our research has also focused on the role of bone morphogenetic proteins in the regulation of the growth plate and articular cartilage. Articular and growth plate cartilage both arise from condensations of mesenchymal cells, but ultimately develop important histological and functional differences. Each is composed of three layers the superficial, mid and deep zones of articular cartilage and the resting, proliferative and hypertrophic zones of growth plate cartilage. A gradient in expression of BMP-related genes has been observed across growth plate cartilage, likely playing a role in zonal differentiation. To investigate the presence of a similar expression gradient in articular cartilage, we used laser capture microdissection (LCM) to separate murine growth plate and articular cartilage from the proximal tibia into their six constituent zones, and used a solution hybridization assay with color-coded probes to quantify mRNAs for 30 different BMP-related genes in each zone. In situ hybridization and immunohistochemistry were then used to confirm spatial expression patterns. We found evidence that BMP signaling gradients exist across both growth plate and articular cartilage and that these gradients contribute to the spatial differentiation of chondrocytes in the postnatal endochondral skeleton.