Bongarzone I, Vigano E, Alberti L, Borrello M G, Pasini B, Greco A, Mondellini P, Smith D P, Ponder B A, Romeo G, Pierotti M A
Division of Experimental Oncology A, Istituto Nazionale Tumori, Milan, Italy.
Oncogene. 1998 May 7;16(18):2295-301. doi: 10.1038/sj.onc.1201759.
Germline mutations of RET gene, encoding a receptor tyrosine kinase, have been associated with the MEN2A and MEN2B inherited cancer syndromes. In MEN2A mutations affecting cysteine residues in the extracellular domain of the receptor cause constitutive activation of the tyrosine kinase by the formation of disulfide-bonded homodimers. In MEN2B a single mutation in the tyrosine kinase domain (Met918Thr) has been identified. This mutation does not lead to dimer formation, but has been shown (both biologically and biochemically) to cause ligand-independent activation of the Ret protein, but to a lesser extent than MEN2A mutations. Intramolecular activation by cis-autophosphorylation of RetMEN2B monomers has been proposed as a model for activation, although alternative mechanisms can be envisaged. Here we show that the activity of RetMEN2B can be increased by stable dimerization of the receptor. Dimerization was achieved experimentally by constructing a double mutant receptor with a MEN2A mutation (Cys634Arg) in addition to the MEN2B mutation, and by chronic exposure of RetMEN2B-expressing cells to the Ret ligand GDNF. In both cases full activation of RetMEN2B, measured by 'in vitro' transfection assays and biochemical parameters, was seen. These results indicate that the MEN2B phenotype could be influenced by the tissue distribution or concentration of Ret ligand(s).
编码受体酪氨酸激酶的RET基因的种系突变与MEN2A和MEN2B遗传性癌症综合征相关。在MEN2A中，影响受体胞外结构域中半胱氨酸残基的突变通过形成二硫键连接的同型二聚体导致酪氨酸激酶的组成性激活。在MEN2B中，已鉴定出酪氨酸激酶结构域中的单个突变（Met918Thr）。该突变不会导致二聚体形成，但已显示（在生物学和生物化学方面）会导致Ret蛋白的配体非依赖性激活，但程度低于MEN2A突变。尽管可以设想其他机制，但已提出RetMEN2B单体通过顺式自磷酸化进行分子内激活作为激活模型。在这里，我们表明RetMEN2B的活性可以通过受体的稳定二聚化而增加。通过构建除MEN2B突变外还具有MEN2A突变（Cys634Arg）的双突变受体，以及通过将表达RetMEN2B的细胞长期暴露于Ret配体GDNF，实验性地实现了二聚化。在这两种情况下，通过“体外”转染试验和生化参数测量，均可见RetMEN2B的完全激活。这些结果表明，MEN2B表型可能受Ret配体的组织分布或浓度影响。
