Drastic genetic instability of tumors and normal tissues in Turcot syndrome

Turcot syndrome is characterized by an association ofmalignant brain tumors and colon cancer developing inthe patient’s teens. Since the mechanism of carcinogen-esis in Turcot syndrome is still unclear, we analysedgenetic changes in tumors from a Turcot patient with nofamily history of the condition. All tumors, including oneastrocytoma, three colon carcinomas, and two colonadenomas, exhibited severe replication error (RER), andall colon tumors showed somatic muations at repeatedregions of TGFbRII, E2F-4, hMSH3, and/or hMSH6genes. Somatic APC mutations were detected in three ofthree colon carcinomas, and somatic p53 mutations weredetected in the astrocytoma and two of three coloncarcinomas, both of which showed two mutations withoutallele loss. We also found that normal colon mucosa,normal skin fibroblasts and normal brain tissue from thispatient showed respective high frequencies of RER, incontrast to usual HNPCC patients in which RER wasvery rare in normal tissues. These results suggest thatextreme DNA instability in normal tissues causes theearly development of multiple cancer in Turcot syn-drome. A missense mutation (GAG to AAG) at codon705 of hPMS2 gene was detected in one allele of thispatient, which was inherited from his mother withouttumors. Additional unknown germline mutation maycontribute to the genetic instability in normal tissues.Keywords: Turcot syndrome; genetic instability; colontumor; brain tumor; RER in normal tissueTurcot syndrome is characterized by an association ofmalignant tumors of central nervous system and colontumors. It has been suggested that there are three typesof Turcot syndrome (Turcot et al., 1959; Itoh andOhsato, 1985): an autosomal dominant type associatedwith familial adenomatous polyposis (FAP), anautosomal dominant type associated with hereditarynonpolyposis colorectal cancer (HNPCC), and arecessively inherited type. Although about 10 dom-inantly inherited cases with germline mutation of APCgene (Mori et al., 1994; Hamilton et al., 1995) and twodominant cases with germline mutation of DNAmismatch repair genes (Hamilton et al., 1995), havebeen reported as Turcot, the causative gene of recessivecases has not been identified, and the mechanism ofcarcinogenesis in every type of Turcot syndrome is stillunclear.The present case of Turcot had no family history oftumors of any kind in his parents. This patientdeveloped a grade IV astrocytoma at the age of 7,which recurred at 9, a malignant lymphoma at 15, afibroma at 15, three independent colon adenocarcino-mas and nearly 10 colonic polyps in the period betweenthe ages of 13 and 16. The patient, who had su•eredfrom slight mental retardation, died at 16. We analysedgenetic changes in six tumors, including one astro-cytoma (As, at age 9), two carcinomas of the sigmoidcolon (CoCa1, which occurred at age 13, and CoCa2,at 15), one carcinoma of the ascending colon (CoCa3,at 16), and two tubular adenomas of the colon(CoAd1, at 13, and CoAd3, at 15).To examine whether the patient was FAP with newAPC germline mutation, DNA samples from tumorsand normal tissue of the same patient were thoroughlyanalysed for mutation in the entire coding sequence ofthe APC gene; however germline mutation was notdetected. We then examined the possibility that thispatient may have been a kind of HNPCC. Micro-satellite alteration, which is the characteristic ofHNPCC tumors (Aaltonen et al., 1993), was checkedfor in six tumors at five regions. All six tumorsexhibited obvious alteration at three or more regions,as shown in Figure 1a and Table 1. Moreover, all ofthe colon adenomas and carcinomas also showedinsertion or deletion mutations at (A)