Tissue-specific carcinogenesis in transgenic mice expressing the RET proto-oncogene with a multiple endocrine neoplasia type 2A mutation. 
Germ line mutations of the RET proto-oncogene are responsible for the development of multiple endocrine neoplasia type 2A (MEN 2A), an inherited cancer syndrome characterized by medullary thyroid carcinoma, pheochromocytoma, and parathyroid hyperplasia.  To study the mechanism of tissue-specific tumor development by RET with a MEN2A (cysteine 634-->arginine) mutation, we generated transgenic mice by introducing the RET-MEN2A gene fused to Moloney murine leukemia virus long terminal repeat.  Expression of the transgene and its product was detected at variable levels in a variety of tissues including thyroid, heart, liver, colon, parotid gland, and brain.  All of 29 mice analyzed developed thyroid C-cell hyperplasia or medullary carcinoma, accompanying high levels of serum calcitonin.  In addition, development of mammary or parotid gland adenocarcinoma was observed in one-half of the transgenic mice.  RET dimerization and its complex formation with Shc and Grb2 adaptor proteins were detected in tumor tissues.  Unexpectedly, no tumor formation was found in other tissues despite RET-MEN2A expression where RET dimerization was undetectable.  Because these tissues but not tumors expressed glial cell line-derived neurotrophic factor family receptor alpha (GFR alpha) at high levels, this suggested that GFR alpha expression may interfere in the dimerization of the RET-MEN2A mutant proteins, leading to tissue-specific tumor development in vivo.