Choline imaging in oncology using PET and 1H-MRS : comparison, assessment as monitoring tool and insights into metabolism in an experimental model

Purpose: To primarily investigate at gene expression level which from transmembrane choline transporters and intracellular choline kinases had the prominent role in the elevation of the total Choline (tCho) peak on proton MR spectra in a rodent rabdomyosarcoma model Materials and methods: Twenty-two rats bearing grafted syngenic rhabdomyosarcoma in both thighs were examined on a 3T MR system. Total choline concentration was measured on proton MR spectra using centimetric cubic volumes of interest (VOIs) placed contiguously in the greater axis of the tumour. After euthanasia, centimetric sub-regions of the tumours corresponding to spectroscopic VOIs were excised and frozen in liquid nitrogen. From 44 tumours (89 H-MRS voxels), only 39 H-MRS voxels were included into final statistical analysis because the 50 remainders corresponded to necrotic tissue at excision. RNA was extracted from all the 39 analyzable samples and was then reverse-transcribed into cDNA by PCR. Choline kinase α and β genes were studied as well as genes of transmembrane transporters OCT1, OCT2, OCT3, CTL1, CTL3, CTL4 and CHT1. The expression level of each gene (‘ΔCt’), was referenced to that of the RPL19 gene. Spearman rank correlation coefficient was used for analysis between variables. Results: High quality H-MR spectra were obtained for each H-MRS VOI. OCT1 was the more expressed gene but was not significantly correlated with the tCho peak at H-MRS (p=0.53). In turn, the less expressed CTL1 transporter gene was significantly correlated with the tCho peak (p=0.02; r=0.38). A moderate but more significant correlation between tCho concentration at H-MRS and the choline kinase α sub-type expression (p=0.002; r=0.51) was demonstrated. Conclusion: Choline transporters seemed to play a prominent role in the increase in choline concentration at gene expression level.