OBJECTIVE: Mutations in the RET proto-oncogene are found in about one third of 
sporadic medullary thyroid carcinomas (MTCs), mostly affecting codon 918. Glial 
cell line derived neurotropic factor (GDNF) and its membrane-bound GDNF family 
receptor alpha (GFRalpha-1), as well as neurturin (NTN) and its membrane-bound 
receptor GFRalpha-2 form a complex with the RET product, a receptor tyrosine 
kinase, resulting in downstream signaling to the nucleus.
DESIGN: To elucidate the role of these RET ligands in MTC tumorigenesis, their 
expression was determined in 15 MTC samples, one papillary thyroid carcinoma 
(PTC) and three normal thyroid tissue specimens.
METHODS: The mRNA expression of RET, GDNF, GFRalpha-1, NTN and GFRalpha-2 was 
investigated by mRNA in situ hybridization, and confirmed by reverse 
transcription-PCR analysis.
RESULTS: None of the five genes was expressed in the normal thyroids or in the 
PTC. All MTCs showed expression of RET, 13 expressed GDNF, 12 expressed 
GFRalpha-1 and 9 expressed NTN and GFRalpha-2. In 7 of the tumors RET, GDNF and 
GFRalpha-1 were expressed at high levels, and in five of these seven tumors NTN 
and GFRalpha-2 genes were also expressed at high levels. The high level of 
expression was preferentially seen in tumor cells adjacent to stroma and 
connective tissue. All MTCs without expression of the RET ligands harbored the 
RET codon 918 mutation.
CONCLUSIONS: The results suggest that this signaling pathway is important for 
MTC development, and that it may be activated by expression of the RET ligand 
complexes by the tumor cells themselves.