A novel activating mutation in the RET tyrosine kinase domain mediates neoplastic transformation

Identifying novel oncogenic RET mutations and characterising their sensitivity to RET-specific inhibitors

BACKGROUND

Rearranged during transfection (RET) is a well-known proto-oncogene. Multiple RET oncogenic alterations have been identified, including fusions and mutations. Although RET fusions have been reported in multiple cancers, RET mutations were mainly found in multiple endocrine neoplasia type 2 and medullary thyroid carcinoma. RET mutations in other cancers were underinvestigated and their functional annotation was less well studied.

METHODS

We retrospectively reviewed next-generation sequencing data from 37 056 patients with cancer to search for RET mutations. We excluded patients with other co-occurring known driver mutations to enrich potential activating RET mutations for further analysis. Moreover, we performed in vitro functional validation of the oncogenic property of several high frequent and novel RET mutants and their sensitivity to RET-specific inhibitors LOXO-292 and BLU-667.

RESULTS

Within 560 (1.5%) patients with cancer who harbour RET mutations, we identified 380 distinct RET mutation sites, including 252 sites without co-occurring driver mutations. RET mutations were more frequently found in thyroid cancer, mediastinal tumour and several other cancers. The mutation sites spread out through the whole protein with a few hotspots within the kinase domain. In addition, we functionally validated that 898-901del, T930P and T930K were novel RET-activating mutations and they were all sensitive to RET inhibitors.

CONCLUSION

Our results demonstrated the frequency of RET mutations across different cancers. We reported and/or validated several previously uncharacterised RET oncogenic mutations and demonstrated their sensitivity to RET-specific inhibitors. Our results help to stratify patients with cancer based on their RET mutation status and potentially provide more targeted treatment options.

Pheochromocytoma as the first manifestation of MEN2A with RET mutation S891A: report of a case

We report a rare case with pheochromocytoma as the first manifestation of multiple endocrine neoplasia type 2A with RET mutation S891A. Bilateral pheochromocytomas were identified in a 54-year-old woman. Screening for RET revealed a rare S891A mutation located in the intracellular tyrosine kinase domain. This mutation was previously recognized as one of the mutations only in cases manifesting solely medullary thyroid carcinomas (MTCs). Since calcitonin stimulation test indicated positive result, total thyroidectomy was performed 1 year after the bilateral adrenalectomy, and C-cell hyperplasia was diagnosed by histopathological examination. Our report suggests that cases with S891A mutation, akin to those with other RET mutations, require screening for pheochromocytoma. In addition, it is indicated that calcitonin stimulation test should be performed even in the unaffected elder cases with S891A mutation although the mutation is classified as lowest risk group on MTC in guidelines.

Targeting RET to induce medullary thyroid cancer cell apoptosis: an antagonistic interplay between PI3K/Akt and p38MAPK/caspase-8 pathways

Mutations in REarranged during Transfection (RET) receptor tyrosine, followed by the oncogenic activation of RET kinase is responsible for the development of medullary thyroid carcinoma (MTC) that responds poorly to conventional chemotherapy. Targeting RET, therefore, might be useful in tailoring surveillance of MTC patients. Here we showed that theaflavins, the bioactive components of black tea, successfully induced apoptosis in human MTC cell line, TT, by inversely modulating two molecular pathways: (i) stalling PI3K/Akt/Bad pathway that resulted in mitochondrial transmembrane potential (MTP) loss, cytochrome-c release and activation of the executioner caspases-9 and -3, and (ii) upholding p38MAPK/caspase-8/caspase-3 pathway via inhibition of Ras/Raf/ERK. Over-expression of either constitutively active myristoylated-Akt-cDNA (Myr-Akt-cDNA) or dominant-negative-caspase-8-cDNA (Dn-caspase-8-cDNA) partially blocked theaflavin-induced apoptosis, while co-transfection of Myr-Akt-cDNA and Dn-caspase-8-cDNA completely eradicated the effect of theaflavins thereby negating the possibility of existence of other pathways. A search for the upstream signaling revealed that theaflavin-induced disruption of lipid raft caused interference in anchorage of RET in lipid raft that in turn stalled phosphorylation of Ras and PI3Kinase. In such anti-survival cellular micro-environment, pro-apoptotic signals were triggered to culminate into programmed death of MTC cell. These findings not only unveil a hitherto unexplained mechanism underlying theaflavin-induced MTC death, but also validate RET as a promising and potential target for MTC therapy.

Four novel RET germline variants in exons 8 and 11 display an oncogenic potential in vitro

CONTEXT

Most germline-activating mutations of the RET proto-oncogene associated with inherited medullary thyroid cancer (MTC) are localized in exons 10, 11 and 13-15. Four novel RET variants, located in the extracellular domain (p.A510V, p.E511K and p.C531R) coded by exon 8 and in the intracellular juxtamembrane region (p.K666N) coded by exon 11, were identified on the leukocyte DNA from apparently sporadic cases.

METHODS

Plasmids carrying Ret9-wild-type (Ret9-WT), Ret9-C634R and all Ret9 variants were transfected, and the phosphorylation levels of RET and ERK were evaluated by western blot analyses. The transforming potentials were assessed by the focus formation assay.

RESULTS

The p.A510V, p.E511K and p.C531R variants were found to generate RET and ERK phosphorylation levels and to have a transforming activity higher than that of Ret9-WT variant, but lower than that of Ret9-C634R variant. Differently, the p.K666N variant, located immediately downstream of the transmembrane domain, and involving a conserved residue, displayed high kinase and transforming activities. Computational analysis predicted non-conservative alterations in the mutant proteins consistent with putative modifications of the receptor conformation.

CONCLUSIONS

The molecular analyses revealed an oncogenic potential for all the novel germline RET variants. Therefore, the prevalence of exon 8 genomic variations with an oncogenic potential may be higher than previously thought, and the analysis of this exon should be considered after the exclusion of mutations in the classical hotspots. In addition, on the basis of these functional data, it is advisable to extend the genetic screening to all the first-degree relatives of the MTC patients, and to perform a strict follow-up of familial carriers.

Medullary thyroid cancer: management guidelines of the American Thyroid Association

BACKGROUND

Inherited and sporadic medullary thyroid cancer (MTC) is an uncommon and challenging malignancy. The American Thyroid association (ATA) chose to create specific MTC Clinical Guidelines that would bring together and update the diverse MTC literature and combine it with evidence-based medicine and the knowledge and experience of a panel of expert clinicians.

METHODS

Relevant articles were identified using a systematic PubMed search and supplemented with additional published materials. Evidence-based recommendations were created and then categorized using criteria adapted from the United States Preventive Services Task Force, Agency for Healthcare Research and Quality.

RESULTS

Clinical topics addressed in this scholarly dialog included: initial diagnosis and therapy of preclinical disease (including RET oncogene testing and the timing of prophylactic thyroidectomy), initial diagnosis and therapy of clinically apparent disease (including preoperative testing and imaging, extent of surgery, and handling of devascularized parathyroid glands), initial evaluation and treatment of postoperative patients (including the role of completion thyroidectomy), management of persistent or recurrent MTC (including the role of tumor marker doubling times, and treatment of patients with distant metastases and hormonally active metastases), long-term follow-up and management (including the frequency of follow-up and imaging), and directions for future research.

CONCLUSIONS

One hundred twenty-two evidence-based recommendations were created to assist in the clinical care of MTC patients and to share what we believe is current, rational, and optimal medical practice.

Nuclear factor-kappaB activation is associated with somatic and germ line RET mutations in medullary thyroid carcinoma

The nuclear factor-kappaB (NF-kappaB) family of transcription factors regulates a wide variety of cellular processes including cell growth, differentiation, and apoptosis. NF-kappaB has been shown to be activated through several signaling pathways that involve growth factor receptors. The aim of the study was to assess the immunohistochemical expression of members of the NF-kappaB family and the putative targets of NF-kappaB in a series of medullary thyroid carcinomas (MTCs), in correlation with RET mutational status. A tissue microarray was constructed from paraffin-embedded blocks of 48 MTCs (13 familial, 35 sporadic) previously evaluated for germ line and somatic RET mutations. Immunohistochemical evaluation included members of the NF-kappaB (p50, p65, p52, c-Rel, RelB) family, as well as putative targets of NF-kappaB such as Flip, Bcl-xL, and cyclin D1. Nuclear immunostaining for members of NF-kappaB was frequent in MTCs (p50, 19%; p65, 68%; p52, 86.6%; c-Rel, 75%; RelB, 36%). MTCs with germ line or somatic RET mutations (29 cases) showed NF-kappaB nuclear translocation (particularly of p65, P = .035) more frequently than MTCs without RET mutations (19 cases). Immunostaining for putative targets of NF-kappaB showed a significant statistical association between p65 and Bcl-xL (P = .024). In addition, Bcl-xL expression was statistically higher in the tumors with exon 16 RET mutation in comparison with those with exon 10 and 11 RET mutations or wild-type RET (P = .002). Moreover, the significance of RETsignaling in NF-kappaB activation was evaluated in the RET-mutated TT cell line. TT cells were infected with lentiviruses carrying short hairpin RNA to knock down RET expression, and NF-kappaB activity was assessed by luciferase reporter assays. Silencing of RET in the TT cell line produced a significant decrease in NF-kappaB activation and reduction in ERK1/2. The results suggest that the NF-kappaB is frequently activated in MTCs. The results also support the hypothesis that RET activation by somatic or germ line mutations may be responsible for NF-kappaB activation in MTCs.