RET mutated C-cells proliferate more rapidly than non-mutated neoplastic cells

Abstract

A statistically significant higher prevalence of the RET p.Met918Thr somatic mutation, identified by direct sequencing, was previously reported in MTC > 2 cm than in smaller tumors. Aim of this study was to correlate the full RET and RAS mutation profile, identified by a Next Generation Sequencing approach, with the growth rate, proliferation and tumor size of MTC. Data of 149 sporadic MTC patients were correlated with RET mutations and Ki67 positivity. Eighty-one cases had a somatic RET mutation, 40 had a RAS mutation and 28 were negative. A statistically significant higher prevalence of RET mutations was found in MTC > 2 cm. A higher prevalence of RET more aggressive mutations, higher allelic frequencies and, higher percentage of Ki67 positive cells were found in larger tumors which had also a worse outcome. Our study highlights the predominant role of RET somatic mutations in MTC tumorigenesis. We demonstrate that RET mutation prevalence and allelic frequency (AF) are significantly higher in larger tumors. Based on these results, we can conclude that RET mutated C-cells's growth and proliferation are more rapid than those of non-mutated cells and give origin to bigger and more aggressive MTC.

Keywords: Ki67; RAS; RET; allelic frequency; cells’ growth; medullary thyroid cancer.

Figure 1

Prevalence of somatic RET and RAS mutations according to tumor size. A statistically significant difference in the mutations profile was observed both when 4 different groups were considered (group 1, ≤1 cm; group 2, >1 and ≤2 cm; group 3, >2 and ≤3 cm; and group 4, >3 cm (P = 0.02)) (panel A) and when grouped into 2 bigger groups (1 + 2, ≤2 cm and 3 + 4, >2 cm) (P = 0.01) (panel B).

Figure 2

Prevalence of somatic highest, high and moderate RET mutations (17) according to tumor size. Four different groups were considered (group 1, ≤1 cm; group 2, >1 and ≤ 2 cm; group 3, >2 and ≤3 cm; and group 4, >3 cm) (panel A); groups A and B vs groups C and D have been considered (panel B). A statistically higher prevalence of the most aggressive RET mutations was observed (P= 0.0027 and P = 0.0004, respectively).

Figure 3

Correlation between the allelic frequency (AF) of the driver mutations and tumor size. RET AF is higher in larger tumors. Four different groups were considered (group 1, ≤1 cm; group 2, >1 and ≤2 cm; group 3, >2 and ≤ 3 cm; and group 4, >3 cm) (panel A); groups 1 and 2 vs groups 3 and 4 have been considered. (P < 0.001) (panel B). RAS AF was not correlated to tumor size when both the four groups (panel C) and when considered groups 1 and 2 vs groups 3 and 4 (panel D) were considered.

Figure 4

A statistically significant correlation was found when Ki67 low, intermediate and high positivity was correlated to the tumor size (panel A); although not statistically significant, a trend of correlation was observed when Ki67 positivity was analyzed according to the type of mutation (panel B). The correlation of Ki67 positivity was statistically significant when the analysis was performed with the AF of RET mutation in RET positive cases (panel C).