MOLECULAR-GENETICS OF MALIGNANT INSULINOMA

Malignant insulinoma is an rare form of cancer with poor prognosis and a reported 5-year survival of 35%. Relatively little is known about t he etiology of this disease or of the oncogenes and tumor suppressor g enes that participate in its genesis and progression. To address this issue, several protooncogenes, including K-ras, N-ras, erb-2, erb-3, c -myc, c-fos, c-jun were examined. Also analyzed was the expression of the growth factors TGF-alpha, EGF, and insulin as well as the EGF rece ptor (EGF-R), p53 and the putative anti-metastasis gene nm23-H1. These were examined in malignant insulinomas, benign insulinomas, pancreati c B cell hyperplasias and in normal endocrine pancreas. Normal endocri ne pancreas showed moderate immunoreaction for c-myc and a strong reac tion for insulin. All other parameters were negative. Benign pancreati c B cell hyperplasias were slightly or moderately positive for N-ras a nd TGF-alpha, and were weakly positive for EGF-R. They were strongly p ositive for c-myc and insulin. In malignant insulinomas there was stro ng immunoreaction for c-myc, TGF-alpha, N-ras, K-ras and p53. Insulin reaction was moderate or strong. Molecular genetic studies have been p erformed for the presence of activating point mutations in codon 12 of the c-K-ras oncogene. Mutations were detected using primer-mediated, mutant-enriched, polymerase chain reaction - restriction fragment leng th polymorphism analysis and were further characterized by allele-spec ific oligonucleotide hybridization. Four out of six patients with mali gnant insulinoma and two out of eight patients with benign insulinoma harbored K-ras point mutations at codon 12. All patients with mutated K-ras oncogene also had elevated levels of p53 protein as well as c-my c and TGF-alpha. In one extremely malignant case we found concomitant mutation at condon 12 of K-ras and codon 61 of the N-ras gene. Our dat a were consistent with the idea that malignant progression is accompan ied by the progressive accumulation of multiple genetic lesions and su ggest that activation of myc, TGF-alpha and ras genes may be early eve nts in the development of insulinoma.