The RET Functional Variant c*587T>C is Not Associated with Susceptibility to Sporadic Medullary Thyroid Cancer

Dear Editor:

Medullary thyroid carcinoma (MTC) originates from the C cells of the thyroid and represents approximately 5–10% of all thyroid malignancies (1,2). About 75% of all MTC cases are believed to be sporadic (sMTC), whereas the remaining 25% correspond to inherited cancer syndromes known as multiple endocrine neoplasia type 2 (MEN 2, OMIM 171400; Online Mendelian Inheritance in Man^®, http://www.ncbi.nlm.nih.gov/omim/). While germline gain-of-function mutations in the RET (rearranged during transfection) proto-oncogene (OMIM 164761) were shown to be responsible for approximately 92% of MEN 2 cases (2,3), very little is known about the mechanism causing the majority of sporadic tumors. For this reason, the search for genetic factors involved or conferring susceptibility to this phenotype is currently of great interest, especially within the RET proto-oncogene. Susceptibility to a disease is a polygenic discretely variable entity, whose extent depends on random combination of favorable or unfavorable gene variants. Therefore, phenotypic expression relies on both the effects of susceptibility and protective alleles. In this sense, data from different series of sMTC patients are highly discrepant. Conflicting results have been reported for the RET variants G691S (c.2071C>A, rs1799939), S904S (c.2712C>G, rs1800863), S836S (c.2508C>T, rs1800862), or IVS1-126G>T (rs2565206) (4 –10). Based on those findings, since the direct effect of the RET variants considered MTC-risk factors has not been proven (4,10,11), we hypothesized about the existence of at least two still unidentified sMTC loci, in linkage disequilibrium with S836S-IVS1-126G>T or with G691S-S904S (9). Interestingly several genetic observations have been recently collected in favor of a variant located in the 3′UTR extreme of RET, namely c*587T>C (g.128496T>C, rs3026785), that could putatively support a pathogenic mechanism leading to the disease. First, strong linkage disequilibrium is reported between the c*587C allele and c.2508T (836S) (12), which in turn is linked to IVS1-126T, both of them being repeatedly considered as MTC-predisposing alleles (4 –6,9 –11). Second, and even more interesting, a recent report strongly suggests a functional effect of c*587T>C on mRNA stability (13). Those authors demonstrated that c*587C lowers RET mRNA degradation in human neuroblastoma cells, ultimately leading to an increase of transcription product and probably an increase in the amount of total RET protein at the cell membrane (13). Therefore all this prompted us to evaluate this variant as the susceptibility factor for the development of sMTC linked to S836S in our series.

A typical case–control study was designed which included a series of 83 sporadic MTC (sMTC) patients from southern Spain and a group of 190 healthy controls comprising unrelated race-, age-, and sex-matched individuals. In addition, we also analyzed a group of 183 Spanish patients affected with Hirschsprung disease (HSCR, OMIM 142623), a developmental disorder characterized by the congenital absence of intrinsic ganglion cells in the myenteric and submucosal plexuses of the gastrointestinal tract, which is also related to RET mutations but by opposing mechanisms to those leading to MTC.

Informed consent was obtained from all the participants for clinical and molecular genetic studies, which conformed to the tenets of the declaration of Helsinki.

Large-scale genotyping using the Taqman technique was performed under conditions previously described (Applied Biosystems, Foster City, CA) (8). Statistical analysis of the data was done employing the SPSS Version 14.0 for Windows (SPSS Inc., Chicago, IL), and calculated using chi squared analysis, with statistical significance set at p < 0.05.

We observed that allelic frequencies for the c*.587C allele were similar in the two groups of patients and in controls, and that the distribution was not statistically different for any of the comparisons (sMTC versus controls: χ² = 0.03, p = 0.87; HSCR versus controls: χ² = 1.81, p = 0.18). Since genetic inheritance operates through the transmission of chromosomal segments, haplotype information is considered essential for identifying causative/protective variations. Therefore, with the available data from previous studies (4,6,9) we analyzed the distribution of the c*587T>C variant in the context of RET haplotypes. More specifically, and because it seemed to be especially interesting, we tried to analyze in our series of patients and controls, the distribution of c*587T>C, according to the presence/absence of c.2508C>T (S836S) and IVS1-126G>T variants. We found that the c*587C allele was in linkage disequilibrium with c.2508T as previously reported (12,13) and with IVS1-126T. Whenever the C allele appeared at the c*587 position, the T variant was also present at c.2508 and IVS1-126, although this phenomenon did not occur in the contrary. This observation would suggest that the variant c*587T>C, at the 3′UTR region, is a more recent genetic event that occurred on a RET allele carrying both c.2508T and IVS1-126T. In any case, our findings seem to discard the involvement of the RET 3′UTR variant in the pathogenesis of sMTC, despite the functional role proposed for it after extensive in silico and in vitro analyses (13).

The mechanism causing the majority of sporadic medullary thyroid carcinomas remains elusive. Although the initial hypothesis about the involvement of the RET 3′UTR locus in the pathogenesis of sMTC was promising, our data revealed no kind of association of the studied variant with the phenotype. In fact, previous in vitro analysis (13) evaluating c*587T>C in MTC-TT cells showed no further variation when comparing the wild-type and the variant alleles, which not only suggests that the activity of the variant might depend on cell type, but would also corroborate our findings supporting the exclusion of the variant as a modifier locus for sMTC.

On the other hand, in contrast with what has been previously reported (13), our results would also clearly discard any kind of association of the 3′UTR variant in the context of HSCR. Therefore, the role in the posttranscriptional control of a subset of RET transcripts proposed for c*587T>C (13) would not be supported by the similar allelic distribution observed in our HSCR patients as compared to the normal population.

In summary and conclusion, an understanding of the genetic events that occur in sMTC is a prerequisite for identifying potential molecular therapeutic targets in this disease. Therefore further analyses similar to this, aimed at identifying the RET loci responsible for the sporadic forms of the disease, will be an important step contributing to improve the efficacy of RET-targeted therapies.