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Capillary malformation–arteriovenous malformation (CM-AVM) is an autosomal dominant blood vascular (BV) disorder characterized by CM and fast flow BV lesions. Inactivating mutations of the RASA1 gene are the cause of CM-AVM in most cases. RASA1 is a GTPase-activating protein that acts as a negative regulator of the Ras small GTP-binding protein. In addition, RASA1 performs Ras-independent functions in intracellular signal transduction. Whether CM-AVM results from loss of an ability of RASA1 to regulate Ras or loss of a Ras-independent function of RASA1 is unknown. To address this, we generated Rasa1 knockin mice with an R780Q point mutation that abrogates RASA1 catalytic activity specifically. Homozygous Rasa1 R780Q/R780Q mice showed the same severe BV abnormalities as Rasa1 -null mice and died midgestation. This finding indicates that BV abnormalities in CM-AVM develop as a result of loss of an ability of RASA1 to control Ras activation and not loss of a Ras-independent function of this molecule. More important, findings indicate that inhibition of Ras signaling is likely to represent an effective means of therapy for this disease.

A critical problem within transcription factor families is how diverse regulatory programs are directed by highly related members. Androgen and glucocorticoid receptors (AR, GR) recognize a consensus DNA hormone response element (HRE), but they activate target genes with precise specificity, largely dependent on the promoter and cell context. We have assessed the role of different receptor domains in hormone-specific response by testing chimeras of AR and GR for their ability to activate the androgen-specific enhancer of the mouse sex-limited protein (Slp) gene. Although all of the mutant receptors activated simple HREs, only a few activated the androgen-specific element. One component shared by receptors functional on the AR-specific target was the AR DNA binding domain. Activation was not due to differential DNA affinity but rather to the AR DNA binding domain escaping suppression directed at the GR DNA binding domain in this enhancer context. A further mechanism increasing specific activation was cooperation of receptors at multiple and weak HREs, which was accentuated in the presence of both the AR N terminus and ligand binding domain. These domains together increased recognition of weak HREs, as demonstrated by in vitro DNase I footprinting and transactivation of mutant enhancers. Further, AR N-terminal subdomains reported to interact directly with the ligand binding domain relieved an inhibitory effect imposed by that domain. Therefore, functions intrinsic to AR augment steroid-specific gene activation, by evading negative regulation operating on the domains of other receptors and by enhancing cooperativity through intra- and inter-receptor domain interactions. These subtle distinctions in AR and GR behavior enforce transcriptional specificity established by the context of nonreceptor factors.