The Glu632-Leu633 deletion in cysteine rich domain of Ret induces constitutive dimerization and alters the processing of the receptor protein

Mutations of the RET gene, encoding a receptor tyrosine kinase, have been associated with the inherited cancer syndromes MEN 2A and MEN 2B. They have also further been associated with both familial and sporadic medullary thyroid carcinomas. Missense mutations affecting cysteine residues within the extracellular domain of the receptor causes constitutive tyrosine kinase activation through the formation of disulfide-bonded homodimers. We have recently reported that a somatic 6 bp in-frame deletion, originally coding for Glu632-Leu633, potently activates the RET gene. This activation is increased with respect to the frequent MEN 2A-associated missense mutation Cys634Arg. This finding specifically correlated to the clinic behavior of the corresponding tumor, which was characterized by an unusually aggressive progression with both multiple and recurrent metastases. By examining the possibility that this deletion acts in a manner similar to cysteine substitution, we have analysed the molecular mechanism by which this oncogenic activation occurs. Phosphorylated dimers of the deleted Ret receptor were detected in immunoprecipitates separated under non-reducing conditions. Like other Cys point mutations, this 6 bp deletion affecting two amino acid residues between two adjacent Cys, is capable of activating the transforming ability of Ret by promoting receptor dimerization. These results suggest that alteration to cysteine residue position or pairing is capable of inducing ligand independent dimerization. Furthermore, we present data demonstrating that the processing and sorting of the Ret membrane receptor to the cell surface is affected by mutation type.

Full activation of MEN2B mutant RET by an additional MEN2A mutation or by ligand GDNF stimulation

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).

Molecular and biochemical analysis of RET/PTC4, a novel oncogenic rearrangement between RET and ELE1 genes, in a post-Chernobyl papillary thyroid cancer

A post-Chernobyl papillary thyroid cancer, displaying a novel ELE1/RET oncogenic rearrangement with an anomalous fusion transcript, was molecularly characterized. In spite of the presence of a normal breakpoint in exon 5 of the activating ELE1 gene, the sequence of the rearranged genomic DNA showed a previously unreported intra-exonic breakpoint in the RET protooncogene. As a consequence, a cDNA sequence 93 nucleotides larger than the regular one, and with the exon 5 of ELE1 joined to exon 11 instead of exon 12 of RET, is formed. To characterize the product of this new oncogenic ELE1/RET rearrangement, here designated as RET/PTC4, we performed an immunoprecipitation and Western blot analysis on cell extracts from NIH3T3 transfectants. The results showed the presence of two isoforms of the chimeric protein, displaying a constitutive tyrosine phosphorylation. As expected, the molecular weight of this protein was higher than that of RET/ PTC3 protein (p80 and p85, instead of p76 and p81). Previous reports, from our and other laboratories, showed that post-Chernobyl papillary thyroid carcinomas are characterized by a high frequency (about 60%) of RET oncogenic rearrangements (Fugazzola et al., 1995; Klugbauer et al., 1995; Ito et al., 1994). These events predominantly involve ELE1 activating sequence, thus producing RET/PTC3 oncogene (Fugazzola et al., 1995; Klugbauer et al., 1995). Hence, this elevated frequency of RET rearrangements could increase the probability of selecting unusual events as that here described. Alternatively, targeted radiation effects could be responsible for the atypical RET rearrangement producing RET/PTC4 oncogene.

Cardiologic abnormalities in patients with long-term lupus nephritis

To assess the cardiological status of patients with long-term lupus nephritis we evaluated 30 patients (mean age 43 +/- 11 years) with lupus nephritis lasting from at least 10 years (mean 15 +/- 5 years). At the time of cardiological evaluation the mean plasma creatinine was 132.6 +/- 11.1 mumol/l and in 28 patients lupus had been quiescent for at least 3 years. Fourteen patients (46.6%) showed one or more cardiac abnormalities: 10 had valvular lesions (1 verrucous endocarditis, 9 thickening and stiffness of one or more valves)--4 patients had regional myocardial akinesis as a consequence of a previous cardiac infarct (one had valvular abnormalities too). One patient had pulmonary hypertension probably secondary to pulmonary vasculitis. No patient had pericarditis. These cardiac abnormalities proved to be statistically correlated with the number of ARA criteria (p = 0.045), the number of lupus flares (p = 0.004), the serum levels of cholesterol (p = 0.04) and of triglycerides (p = 0.025) as well as the duration of hypercholesterolemia (p = 0.005) and of hypertriglyceridemia (p = 0.007). In conclusion, in patients with long-term lupus nephritis cardiac lesions are frequent. The main lesions are non-verrucous valvulopathy (probably a consequence of healing verrucous endocarditis) and cardiac infarct (caused by an accelerated atherosclerosis). On the contrary cardiac lesions caused by active lupus as pericarditis, myocarditis and verrucous endocarditis are rare.