ESR2 Gene and Medullary Thyroid Carcinoma

Dear Editor:

Approximately 25% of medullary thyroid carcinoma (MTC) cases occur within Multiple Endocrine Neoplasia type 2 (MEN 2) syndrome, an autosomal dominant inherited disorder that includes three different clinical phenotypes: familial MTC (FMTC), MEN 2A, and MEN 2B (1). In >95% of the cases, the three forms of MEN 2 are caused by specific gain-of-function germline mutations of the RET proto-oncogene, which encodes a receptor tyrosine-kinase expressed in derivatives and tumors of neural crest origin (2). However, some families with apparent MTC predisposition do not have a detectable germline RET mutation, suggesting that further predisposing genetic alterations remain to be identified.

Recently, the novel constitutional frameshift c.948delT mutation (p.G318Afs*22) in ESR2 was found to segregate with the disease in a kindred with apparent MTC/C cell hyperplasia (CCH) predisposition and no identifiable RET mutation (3). Specifically, this mutation was detected in two patients using exome sequencing and in another two patients by direct Sanger sequencing of the specific region. Of the four patients who were positive for the mutation, only one had histologic confirmation of MTC, while the other three just presented CCH. The three family members with normal thyroid histology did not carry the mutation. ESR2 encodes estrogen receptor β, ERβ, which either forms homodimers or, preferentially, heterodimers with ERα that bind to the DNA at specific estrogen-responsive elements (EREs) within the promoters of target genes to regulate their transcription. ERα is the most potent activator, whereas ERβ can repress ERα, and transcriptional activity is determined by the relative ERα/ERβ proportion. To investigate the role of ESR2 in MTC tumorigenesis, immunohistochemical studies showed that this mutation was associated with loss of ERβ expression in tumor samples from affected family members, with retained staining in the corresponding normal thyroid tissue. In addition, loss of ERβ was demonstrated to cause ERα-mediated ERE activation and increased RET expression, since three EREs are present in the RET promoter. Interestingly, these findings provide a novel mechanism causing increased RET expression in tumorigenesis through ESR2 mutations. However, no additional ESR2 mutations were detected when sequencing the coding region of the gene in another three MTC families and 19 individuals with apparently sporadic MTC and no RET mutation.

In order to contribute to evaluating the role of ESR2 in familial forms of MTC, using Sanger sequencing, we have analyzed the complete coding sequence of this gene in six Spanish families with FMTC, including 12 affected individuals and two individuals with prophylactic thyroidectomy because of abnormal calcitonin levels after pentagastrin stimulation. Those families were defined by the occurrence of MTC in two or more first- or second-degree relatives. The complete coding region of the RET gene had been previously sequenced, and no germline variants were detected other than known polymorphisms. Written informed consent was obtained from all participants for clinical and molecular genetic studies. Our study complies with the tenets of the Declaration of Helsinki.

No mutations were detected in ESR2 other than known polymorphisms when we analyzed these FMTC cases. Therefore, currently, the association of a mutation in ESR2 with familial forms of MTC derives from just one kindred. Further replicative studies should be carried out, not only in MTC families but also in sporadic patients and control groups. The putative role of this gene in familial forms of MTC should be carefully examined, and additional data are required before using them in risk assessment.