Thorotrast, a colloidal suspension of radioactive 232 ThO 2 that emits ␣ particles, was used as a radiographic contrast during World War II. It is known to induce liver cancers, most frequently ICC, decades after injection. Since radiation induces genomic instability, we analyzed MSI in Thorotrastinduced ICC. The frequency of MSI ؉ cases was 62.5% in Thorotrast ICC, whereas it was 22.7% in non-Thorotrast ICC. However, frameshift mutations of mononucleotide repeats were not observed in Thorotrast ICC. In addition, the MSI ؉ phenotype was not associated with the quantity of Thorotrast deposited or the latency period of ICC induction. Promoter regions of both the hMLH1 and the hMSH2 MMR genes tended to be hypermethylated in the tumor part compared to the adjacent nontumor part in Thorotrast ICC. Methylation of the hMLH1 promoter was associated with the MSI ؉ phenotype in Thorotrast ICC. In contrast, methylation status of these promoter regions was not related to MSI in nonThorotrast ICC cases. These findings suggest that MSI induced by exposure to Thorotrast mainly reflects clonal expansion of cancer cells and is partly due to inactivation of hMLH1 by hypermethylation. © 2002 Wiley-Liss, Inc.
Key words: Thorotrast; microsatellite instability; cholangiocellular carcinomaIdentification of genetic changes in radiation-induced cancer can contribute to risk assessment and prevention. Thorotrast is the trade name for a 25% colloidal suspension of radioactive 232 ThO 2 that naturally emits ␣ particles (90%),  particles and ␥-rays (10%). It was used as a radiographic contrast agent from the 1930s to the 1950s. 1 Intravascularly injected, Thorotrast remains in the reticuloendothelial system for life, and those organs are chronically irradiated by ␣-particles. In particular, Ͼ60% of total Thorotrast is located in the liver. 2 Several decades after injection, Thorotrast has been known to induce liver cancers, of which ICC is the most frequent type. The development of cancer is thought to be a consequence of multiple carcinogenic steps, including genetic changes such as activation of proto-oncogenes and inactivation of tumor-suppressor genes. 3 It has become evident that cancer cells exhibit a mutator phenotype. Mutator phenotype or genomic instability results from aberrations of genes that normally function in the maintenance of genetic stability.There is now considerable evidence that cells that have survived irradiation may produce descendants that manifest various biologic consequences induced by genomic instability other than irreversible damage at the time of irradiation. 4,5 A higher frequency of nonclonal chromosomal aberrations appears in clonal descendants of mouse hematopoietic stem cells more than 12 generations after ␥-irradiation. 6 A persistently increased rate of gene mutation is observed in cell populations many generations after exposure to X-rays and ␣ particles. 7 To search for genetic changes characteristic of radiation exposure, we previously analyzed p53 gene mutations in Thorotrast-laden livers. Multip...