The SMAD-binding domain of SKI: A hotspot for de novo mutations causing Shprintzen-Goldberg syndrome

Shprintzen-Goldberg syndrome (SGS) is a rare, systemic connective tissue disorder characterized by craniofacial, skeletal, and cardiovascular manifestations that show a significant overlap with the features observed in the Marfan (MFS) and Loeys-Dietz syndrome (LDS). A distinguishing observation in SGS patients is the presence of intellectual disability, although not all patients in this series present this finding. Recently, SGS was shown to be due to mutations in the SKI gene, encoding the oncoprotein SKI, a repressor of TGFβ activity. Here, we report eight recurrent and three novel SKI mutations in eleven SGS patients. All were heterozygous missense mutations located in the R-SMAD binding domain, except for one novel in-frame deletion affecting the DHD domain. Adding our new findings to the existing data clearly reveals a mutational hotspot, with 73% (24 out of 33) of the hitherto described unrelated patients having mutations in a stretch of five SKI residues (from p.(Ser31) to p.(Pro35)). This implicates that the initial molecular testing could be focused on mutation analysis of the first half of exon 1 of SKI. As the majority of the known mutations are located in the R-SMAD binding domain of SKI, our study further emphasizes the importance of TGFβ signaling in the pathogenesis of SGS. © 2015 Macmillan Publishers Limited All rights reserved

Glucagon decreases cytokine induction of nitric oxide synthase and action on insulin secretion in RIN5F cells and rat and human islets of Langerhans

Nitric oxide synthase, induced by cytokines in insulin-containing cells, produces nitric oxide which inhibits function and may promote cell killing. Since glucagon was shown to prevent inducible nitric oxide synthase (iNOS) expression in rat hepatocytes it was of interest to examine the action of glucagon (and cyclic AMP) on iNOS induction in insulin-producing cells. Cultured RIN5F cells and primary rat and human islets of Langerhans were treated with interleukin 1ß (IL-1ß) or a combination of cytokines, and were co-treated or pre-treated with glucagon. In RIN5F cells, the activity of iNOS induced by IL-1ß (10 pM, 24 h), was significantly reduced by glucagon (1000 nM), which raises cyclic AMP, and by forskolin (1-10 µM), a non specific activator of adenylate cyclase. Glucagon and forskolin also decreased iNOS expression in RIN5F cells, and rat and human islets, as shown by Western blotting. The inhibitory action of IL-1ß (100 pM, 24 h) on rat islet insulin secretion was partially reversed by 1-h pre-treatment with glucagon (10-1000 nM), while the contrasting stimulatory effect of 48-h treatment with cytokines on insulin secretion from human islets was similarly prevented by glucagon (1000 nM) pre-treatment. These results suggest that glucagon inhibits iNOS expression in insulin-containing cells and imply that glucagon could modulate the inhibitory effects of cytokines