Mutations in the TGF-β repressor SKI cause Shprintzen-Goldberg syndrome with aortic aneurysm

Abstract

Elevated transforming growth factor (TGF)-β signaling has been implicated in the pathogenesis of syndromic presentations of aortic aneurysm, including Marfan syndrome (MFS) and Loeys-Dietz syndrome (LDS). However, the location and character of many of the causal mutations in LDS intuitively imply diminished TGF-β signaling. Taken together, these data have engendered controversy regarding the specific role of TGF-β in disease pathogenesis. Shprintzen-Goldberg syndrome (SGS) has considerable phenotypic overlap with MFS and LDS, including aortic aneurysm. We identified causative variation in ten individuals with SGS in the proto-oncogene SKI, a known repressor of TGF-β activity. Cultured dermal fibroblasts from affected individuals showed enhanced activation of TGF-β signaling cascades and higher expression of TGF-β-responsive genes relative to control cells. Morpholino-induced silencing of SKI paralogs in zebrafish recapitulated abnormalities seen in humans with SGS. These data support the conclusions that increased TGF-β signaling is the mechanism underlying SGS and that high signaling contributes to multiple syndromic presentations of aortic aneurysm.

Figure 1

SKI mutations in patients with Shprintzen-Goldberg syndrome (SGS). A) Clinical features and mutations seen in SGS patients. Illustrated features involve the craniofacial (abnormal head shape due to craniosynostosis, widely-spaced eyes, small and receding chin and high-arched palate), skeletal (long fingers, joint contractures, chest wall deformity, spine curvature, foot deformity) and cardiovascular (aortic root aneurysm indicated by black arrowheads and mitral valve prolapse indicated by white arrowhead) systems. Pedigrees indicate that all cases were sporadic (affected, black symbols; unaffected, open symbols). Mutation status is indicated below each individual (−/−, mutation negative; +/−, heterozygous; blank, not available for testing). Permission to publish photographs was obtained from the affected individuals or their parents. B) Location of mutations in relation to known binding sites (highlighted in yellow) of SKI binding partners. The position of the Dachshund homology domain is indicated.

Figure 2

Assessment of TGF-β signaling in dermal fibroblasts. A) Western blot analysis of pSMAD2, pSMAD3, pERK1/2, pJNK1/2, pp38 and β-Actin (loading control) at steady state (baseline) and in response to TGF-β2. Representative Shprintzen-Goldberg syndrome (SGS) and control blots are shown (for a single SGS patient and single control with 3 biological replicates for each). Graphs display Western blot quantification of 3 biological replicates in each of 2 SGS patients and 2 controls. All data are normalized to β-Actin. The control baseline value for each graph was set to 1.0; all other values represent fold change in comparison to this sample. B) Relative mRNA expression for TGF-β target genes (normalized to β-Actin), as assessed by quantitative polymerase chain reaction analysis. Quantification was performed on 3 biological replicates in each of 2 SGS patients and 2 controls. The upper and lower margins of the box define the 75^th and 25^th percentiles respectively; the internal line defines the median and the whiskers define the range. NS, not significant; * p<0.05; ** p<0.01; † p<0.001; †† p<0.0001.

Figure 3

Assessment of SKI in model systems. A) Murine SKI expression at embryonic day 13.5 (E13.5) is high in the mitral valve (MV), tricupsid valve (TV), pulmonary trunk (Pu), pulmonary valve (PuV), bronchus (Br) and proximal aorta (Ao; both nucleus and cytoplasm). Lower expression is seen in the descending aorta (dAo), esophagus (E), right and left ventricles (RV and LV) and left atrium (LA). Red, SKI; green, myosin heavy chain; blue, nuclei. At birth (P0), SKI is expressed in the aortic root (AoR), distal ascending aorta (ascAo) and endothelial surface of the mitral and aortic (AoV) valves, with less nuclear expression than at E13.5. In adulthood, SKI is expressed exclusively in the cytoplasm of the AoV and the media of the AoR; in the ascAo SKI is expressed in the intima (I) and adventitia (A), but excluded from the central media (M). B) Disruption of skia or skib expression in zebrafish. Top panels, assessment of cardiovascular anatomy at 3 days post-fertilization (dpf). Uninjected embryos display proper cardiac looping with normal relation between the atrium (A, dotted outline) and ventricle (V), and a distinct bulbous arteriosus (arrow) defining the outflow tract (OFT). skia morphants show incomplete looping with an irregular OFT, while skib morphants show failure of looping and an ill-defined OFT. Asterisk, atrioventricular cushion. Lower panels, alcian blue staining of cartilage at 6 dpf. skia and skib morphants display craniofacial cartilage deficits and prominent mandibular malformation. Meckel's cartilage, m; palatoquadrates, pq; ceratohyales, ch; ceratobranchial arches, cb. Scale bars, μm.