Noonan syndrome [NS; Mendelian Inheritance in Men (MIM) #163950] and related syndromes [Noonan syndrome with multiple lentigines (formerly called LEOPARD syndrome; MIM #151100), Noonan-like syndrome with loose anagen hair (MIM #607721), Costello syndrome (MIM #218040), cardio-facio-cutaneous syndrome (MIM #115150), type I neurofibromatosis (MIM #162200), and Legius syndrome (MIM #611431)] are a group of related genetic disorders associated with distinctive facial features, cardiopathies, growth and skeletal abnormalities, developmental delay/mental retardation, and tumor predisposition. NS was clinically described more than 50 years ago, and disease genes have been identified throughout the last 3 decades, providing a molecular basis to better understand their physiopathology and identify targets for therapeutic strategies. Most of these genes encode proteins belonging to or regulating the so-called RAS/MAPK signaling pathway, so these syndromes have been gathered under the name RASopathies. In this review, we provide a clinical overview of RASopathies and an update on their genetics. We then focus on the functional and pathophysiological effects of RASopathy-causing mutations and discuss therapeutic perspectives and future directions.

SHP2 was recently found to down-regulate PI3K activation by dephosphorylating Gab1 but the mechanisms explaining the positive role of the Gab1/SHP2 pathway in EGF-induced Ras activation remain ill defined. Substrate trapping experiments now suggest that SHP2 dephosphorylates other Gab1 phosphotyrosines located within a central region displaying four YXXP motifs. Because these sites are potential docking motifs for Ras-GAP, we tested whether SHP2 dephosphorylates them to facilitate Ras activation. We observed that a Gab1 construct preventing SHP2 recruitment promoted membrane relocation of RasGAP. Moreover, a RasGAP-inactive mutant restored the activation of Ras in cells transfected with SHP2-inactivating Gab1 mutant or in SHP2-deficient fibroblasts, supporting the hypothesis that RasGAP is a downstream target of SHP2. To determine whether Gab1 is a RasGAP-binding partner, a Gab1 mutant deleted of four YXXP motifs was produced. The deletion suppressed RasGAP redistribution and restored the defective Ras activation caused by SHP2-inactivating mutations. Moreover, Gab1 was found to interact with RasGAP SH2 domains, only under conditions where SHP2 is not activated. To identify Ras-GAP-binding sites, Tyr to Phe mutants of Gab1 YXXP motifs were produced. Gab1 constructs mutated on Tyr³¹⁷ were severely affected in RasGAP binding and were the most active in compensating for Ras-defective activation and blocking RasGAP redistribution induced by SHP2 inactivation. We have thus localized on Gab1 a Ras-negative regulatory tyrosine phosphorylation site involved in RasGAP binding and showed that an important SHP2 function is to down-regulate its phosphorylation to disengage RasGAP and sustain Ras activation.

Inactivation of the HRPT2 tumor suppressor gene is associated with the pathogenesis of the hereditary hyperparathyroidism-jaw tumor syndrome and malignancy in sporadic parathyroid tumors. The cellular function of the HPRT2 gene product, parafibromin, has not been defined yet. Here we show that parafibromin physically interacts with human orthologs of yeast Paf1 complex components, including PAF1, LEO1, and CTR9, that are involved in transcription elongation and 3′ end processing. It also associates with modified forms of the large subunit of RNA polymerase II, in particular those phosphorylated on serine 5 or 2 within the carboxy-terminal domain, that are important for the coordinate recruitment of transcription elongation and RNA processing machineries during the transcription cycle. These interactions depend on a C-terminal domain of parafibromin, which is deleted in ca. 80% of clinically relevant mutations. Finally, RNAi-induced downregulation of parafibromin promotes entry into S phase, implying a role for parafibromin as an inhibitor of cell cycle progression. Taken together, these findings link the tumor suppressor parafibromin to the transcription elongation and RNA processing pathway as a PAF1 complex- and RNA polymerase II-bound protein. Dysfunction of this pathway may be a general phenomenon in the majority of cases of hereditary parathyroid cancer.

Although the mechanisms involved in the activation of mitogen-activated protein kinases (MAPK) by receptor tyrosine kinases do not display an obvious role for phosphoinositide 3-kinases (PI3Ks), we have observed in the nontransformed cell line Vero stimulated with epidermal growth factor (EGF) that wortmannin and LY294002 nearly abolished MAPK activation. The effect was observed under strong stimulation and was independent of EGF concentration. In addition, three mutants of class Ia PI3Ks were found to inhibit MAPK activation to an extent similar to their effect on Akt/protein kinase B activation. To determine the importance of PI3K lipid kinase activity in MAPK activation, we have used the phosphatase PTEN and the pleckstrin homology domain of Tec kinase. Overexpression of these proteins, but not control mutants, was found to inhibit MAPK activation, suggesting that the lipid products of class Ia PI3K are necessary for MAPK signaling. We next investigated the location of PI3K in the MAPK cascade. Pharmacological inhibitors and dominant negative forms of PI3K were found to block the activation of Ras induced by EGF. Upstream from Ras, although association of Grb2 with its conventional effectors was independent of PI3K, we have observed that the recruitment of the tyrosine phosphatase SHP2 required PI3K. Because SHP2 was also essential for Ras activation, this suggested the existence of a PI3K/SHP2 pathway leading to the activation of Ras. In addition, we have observed that the docking protein Gab1, which is involved in PI3K activation during EGF stimulation, is also implicated in this pathway downstream of PI3K. Indeed, the association of Gab1 with SHP2 was blocked by PI3K inhibitors, and expression of Gab1 mutant deficient for binding to SHP2 was found to inhibit Ras stimulation without interfering with PI3K activation. These results show that, in addition to Shc and Grb2, a PI3K-dependent pathway involving Gab1 and SHP2 is essential for Ras activation under EGF stimulation.