Screening of variants of unkown significance (VUS) in the RET proto-oncogene in patients with multiple endocrine neoplasia type 2 and healthy individual controls

Abstract

The multiple endocrine neoplasia type 2 (MEN2) is a tumor syndrome transmitted by autosomal dominant inheritance. The glands involved are thyroid, parathyroid and medullar of adrenals and their main clinical manifestations are the medullary thyroid carcinoma (MTC), pheochromocytoma (PHEO) and primary hyperparathyroidism (HPT). The MEN2 syndrome is subdivided in MEN2A, MEN 2B and familial CMT, wherein each sub-form displays a characteristic phenotype.This genetic syndrome is caused by germline mutations in the RET proto-oncogene, which account for> 95% of cases of MEN2. The RET activating mutations leads to constitutive structural alterations in the encoded protein, RET, regardless of the ligand and this event triggers the transformation of a normal cell in a neoplastic cell. According to the International 2001-consensus on NEMs and the 2009-consensus on MTC, to RET germline mutation carriers it should be recommended preventive total thyroidectomy (PTTx) at appropriate and specific ages, attempting to achieve clinical and biochemical cure (calcitonin <2pg/ml) of CMT. Currently, with the use of new technologies an increasing number of RET genetic variants has been reported. However, the pathogenicity of several out of these variants remains unknown, as those of variants of unknown significance (VUS). This event may have occurred due to several reasons. One of them is that many VUS have been described in sporadic cases of MTC (no family history), thereby precluding the analysis of co-segregation and genotype-phenotype correlation.The identification of a RET variant with a previously known pathogenicity (disease-causing mutation) consists in a consensual indication for (of) PTTx, either in affected MTC cases or in young asymptomatic carriers. In the latter cases, this procedure will greatly increase the chance of prevention and cure of MTC. Instead, the identification of RET common polymorphisms rules out the recommendation of PTTx and individuals harboring these polymorphisms should be excluded from clinical follow-up.The presence of a VUS generates a more complex situation. Thus, this information does not allow us to decide in favor or against, although it also does not exclude the need for performing the periodic clinical screening. Therefore, patients with RET VUS should be followed annually by imaging studies and serum calcitonin, the tumor marker of MTC, without knowing whether these cases actually display or not a genetic predisposition for MTC/MEN2. Of note, many MTC cases are frequently recognized at advanced stages, when MTC metastasis are already present and the chances for clinical/biochemical "cure" are low. Therefore, in the present study we aim to study the allelic frequencies of 48 VUS already reported in RET hotspot exons in 1,000 healthy control subjects of the population, and these results will be compared with those from 50 patients with MTC/MEN2. VUS genotyping will be performed by Real Time PCR with TaqMan ® probes using the platform OpenArray ® Real-Time PCR. Further, we will approach these variants using, in silico, seven predictive software programs to evaluate the potential pathogenicity of each VUS. Data from the present study may potentially contribute with the literature, aiming to a) improve the molecular diagnosis of MEN2-related MTC cases and their at-risk relatives and, b) collaborate to a better clinical and surgical management of these patients and at-risk individuals.