Haploinsufficiency of Sox9 results in defective cartilage primordia and premature skeletal mineralization

Abstract

In humans, SOX9 heterozygous mutations cause the severe skeletal dysmorphology syndrome campomelic dysplasia. Except for clinical descriptions, little is known about the pathogenesis of this disease. We have generated heterozygous Sox9 mutant mice that phenocopy most of the skeletal abnormalities of this syndrome. The Sox9 ؉/؊ mice died perinatally with cleft palate, as well as hypoplasia and bending of many skeletal structures derived from cartilage precursors. In embryonic day (E)14.5 heterozygous embryos, bending of radius, ulna, and tibia cartilages was already prominent. In E12.5 heterozygotes, all skeletal elements visualized by using Alcian blue were smaller. In addition, the overall levels of Col2a1 RNA at E10.5 and E12.5 were lower than in wild-type embryos. We propose that the skeletal abnormalities observed at later embryonic stages were caused by delayed or defective precartilaginous condensations. Furthermore, in E18.5 embryos and in newborn heterozygotes, premature mineralization occurred in many bones, including vertebrae and some craniofacial bones. Because Sox9 is not expressed in the mineralized portion of the growth plate, this premature mineralization is very likely the consequence of allele insufficiency existing in cells of the growth plate that express Sox9. Because the hypertrophic zone of the heterozygous Sox9 mutants was larger than that of wild-type mice, we propose that Sox9 also has a role in regulating the transition to hypertrophic chondrocytes in the growth plate. Despite the severe hypoplasia of cartilages, the overall organization and cellular composition of the growth plate were otherwise normal. Our results suggest the hypothesis that two critical steps of the chondrocyte differentiation pathway are sensitive to Sox9 dosage. First, an early step presumably at the stage of mesenchymal condensation of cartilage primordia, and second, a later step preceding the transition of chondrocytes into hypertrophic chondrocytes.T he differentiation of chondrocytes from mesenchymal cells occurs along a multistep pathway, during which committed mesenchymal cells first aggregate together to form precartilaginous condensations that prefigure the overall shape of future bones. Then expression of cartilage-specific proteins is initiated, and the cells become surrounded by abundant extracellular matrix (1). In the growth plate of endochondral bones, chondrocytes become flattened and undergo a unidirectional proliferation that is primarily responsible for the longitudinal growth of bones. After these cells stop proliferating, they change their genetic program and become hypertrophic. The extracellular matrix surrounding the hypertrophic chondrocytes that are closest to the metaphyses become mineralized and the cells undergo apoptosis, leaving behind a calcified cartilaginous matrix that is degraded subsequently and replaced by bone matrix (2).Recent studies have identified SOX9 as an essential transcription factor in chondrogenesis (3). In the absence of Sox9, there is a complete block in chondroc...

Acetylation reduces SOX 9 nuclear entry and ACAN gene transactivation in human chondrocytes

“…Most of the work regarding SOX9 establishes its role in skeletal development until postnatal stages (Bi et al ., 2001; Dy et al ., 2012). However, several studies in rat OA models and human healthy and OA samples conclude that SOX9 mRNA levels are extremely low during OA pathogenesis (Haag et al ., 2008; Kim & Im, 2011; Kim et al ., 2013), which coincides with our observations in Fig.…”

Conditional Sox9 ablation reduces chondroitin sulfate proteoglycan levels and improves motor function following spinal cord injury

“…Conventional Sox9 knock-out mice have been generated but are unsuitable for studies of SCI as both Sox9 knock-out (Sox9 −/− ) and heterozygote (Sox9 +/− ) embryos do not survive to birth (Bi et al, 2001). To evaluate SOX9 loss-of-function after SCI, in a nervous system that developed with normal levels of SOX9 activity, a tamoxifen-inducible conditional Sox9 knock-out strategy was used.…”

Frontal nasal prominence expression driven by Tcfap2a relies on a conserved binding site for STAT proteins

“…Several Sox factors are known to play critical roles in craniofacial and limb skeletogenesis and are good candidates to regulate Tcfap2a (Bi et al, 1999(Bi et al, , 2001Smits et al, 2001;Sock et al, 2004). The Group E factor, Sox9, is an enticing candidate, given both its role in chondrogenesis and its responsiveness to retinoic acid (Bi et al, 1999(Bi et al, , 2001Sekiya et al, 2000;Akiyama et al, 2002Akiyama et al, , 2004MoriAkiyama et al, 2003;Sahar et al, 2005), an agent known to alter AP-2␣ expression in vitro and in vivo (Williams et al, 1988;Lü scher et al, 1989;Philipp et al, 1994;Shen et al, 1997). Another Group E candidate, Sox8, is expressed coincident with Tcfap2a in the face and limbs (Wegner, 1999;Schepers et al, 2000).…”