Medullary thyroid carcinoma (MTC) is a neuro-endocrine tumor arising from the calcitonin (CT) secreting C-cells in the thyroid gland, for which no curative therapy is available. Specific mutations in the RET tyrosine kinase gene cause different clinical subtypes of the inherited cancer syndrome, multiple endocrine neoplasia type 2 (MEN 2A, -2B, FMTC). MEN 2A accounts for approximately 90% of all inherited MTC. The most common mutation associated with MEN 2A affects codon 634, with the C634R substitution found in ca. 50% of all MEN 2A patients. A model for the MEN 2A syndrome is represented by the transgenic mouse line CT2A-3 that expresses the 3' long isoform of the human RET cDNA carrying the C634R activating mutation (Cranston & Ponder, Cancer Res., 2003). This transgenic line produces thyroid tumors resembling human MTC. Depending on the strain background, the incidence of thyroid tumours developing in these transgenic mice at 43 weeks of age ranged from 0% (FVB/N) to 98% (CBA/Ca), indicating that tumor penetrance is modulated by the genetic background. We found comparably low levels of RET transgene expression in normal thyroids of both susceptible and resistant CT2A-3 transgenic lines, and comparable amounts of Ret protein in mouse MTC and TT and MZ-CRC-1 cell lines. These findings strengthen the importance of genetic background on MTC phenotype, and suggest that the transgene is not expressed at aberrant levels, making this transgenic model suitable for studies on RET and MTC modifier genes. To set the conditions for genetic linkage analysis, we validated the use of plasma CT level as a quantitative marker in MTC progression. A high correlation between plasma CT levels and overall MTC volume in CT2A-CBA/Ca (r = 0.92, P<0.001, Spearman rank correlation test) was found. A time-course study in the susceptible (CT2A-CBA/Ca), in the resistant (CT2A-FVB/N), and in hybrid (FVB/N x CT2A-CBA/Ca) F1 transgenic mice was carried out using plasma CT levels. The FVB/N parental strain was found to carry dominant allele/s that produce a 10-15 fold reduction in the plasma CT level of hybrid mice with respect to the susceptible CT2A-CBA/Ca line. A whole genome-wide scan using 108 polymorphic markers in 305 CBA/Ca (FVB/N x CT2A-CBA/Ca) backcross mice showed co-segregation of the phenotype (CT levels measured at 23, 33 and 43 weeks of age) with genetic markers. This led to the identification of two different quantitative trait loci (QTL), on chromosomes 8 and 19. The inhibitory effects on MTC progression can be explained by the inherited parental FVB/N alleles. The proportion of variance explained by a two-QTL model is 24%, with a LOD score of 10, with a major effect played by the locus on chromosome 19.
Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 4923.
100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO