Thyroid cancer is a common endocrine malignancy that encompasses well-differentiated as well as dedifferentiated cancer types. The latter tumors have high mortality and lack effective therapies. Using a paired-end RNA-sequencing approach, we report the discovery of rearrangements involving the anaplastic lymphoma kinase (ALK) gene in thyroid cancer. The most common of these involves a fusion between ALK and the striatin (STRN ) gene, which is the result of a complex rearrangement involving the short arm of chromosome 2. STRN-ALK leads to constitutive activation of ALK kinase via dimerization mediated by the coiled-coil domain of STRN and to a kinase-dependent, thyroid-stimulating hormoneindependent proliferation of thyroid cells. Moreover, expression of STRN-ALK transforms cells in vitro and induces tumor formation in nude mice. The kinase activity of STRN-ALK and the ALKinduced cell growth can be blocked by the ALK inhibitors crizotinib and TAE684. In addition to well-differentiated papillary cancer, STRN-ALK was found with a higher prevalence in poorly differentiated and anaplastic thyroid cancers, and it did not overlap with other known driver mutations in these tumors. Our data demonstrate that STRN-ALK fusion occurs in a subset of patients with highly aggressive types of thyroid cancer and provide initial evidence suggesting that it may represent a therapeutic target for these patients.

It has been widely reported that the small GTP-binding protein Rap1 has an anti-Ras and antimitogenic activity. Thus, it is generally accepted that a normal physiological role of Rap1 proteins is to antagonize Ras mitogenic signals, presumably by forming nonproductive complexes with proteins that are typically effectors or modulators of Ras. Rap1 is activated by signals that raise intracellular levels of cAMP, a molecule that has long been known to exert both inhibitory and stimulatory effects on cell growth. We have now tested the intriguing hypothesis that Rap1 could have mitogenic effects in systems in which cAMP stimulates cell proliferation. The result of experiments addressing this possibility revealed that Rap1 has full oncogenic potential. Expression of Rap1 in these cells results in a decreased doubling time, an increased saturation density, and an unusual anchorage-dependent morphological transformation. Most significantly, however, Rap1-expressing cells formed tumors when injected into nude mice. Thus, we propose that the view that holds Rap1 as an antimitogenic protein should be restricted and conclude that Rap1 is a conditional oncoprotein.

Rap1 proteins belong to the Ras superfamily of small molecular weight GTP-binding proteins. Although Rap1 and Ras share approximately 50% overall amino acid sequence identity, the effector domains of the two proteins are identical, suggesting either similar or antagonistic signaling roles. Several pathways leading to Ras activation have been defined, including those initiated by agonist binding to tyrosine kinase or Gi-coupled receptors. Nothing is known about such events for Rap1 proteins. The cAMP-mediated inhibition of Ras-dependent MAP kinase activation is well documented and resembles that caused by expression of GTPase-deficient Rap1. We have developed a system whereby signals leading to Rap1b activation, i.e. an increase in Rap1b-bound GTP/GDP ratio, can be measured. We report here that treatment of cells with agents that elevate intracellular cAMP levels result in Rap1b activation. These results demonstrate for the first time agonist-dependent activation of Rap1 proteins.