An intronic splicing mutation of the MEN1 gene.

Abstract

Identification of a pathogenic germline mutation of the responsible gene is essential for DNA diagnosis of certain familial cancer syndromes. However, such a mutation, if detected as a nucleotide change, is difficult to interpret unless its biological significance is apparent. MEN1 is a tumor-suppressor gene responsible for multiple endocrine neoplasia type 1 (MEN1) (Chandrasekharappa et al., 1997), a familial cancer syndrome characterized by combined occurrence of parathyroid, pituitary, and enteropancreatic endocrine tumors, among which parathyroid tumors are most prevalent with a frequency of 95% (Trump et al., 1996). Adrenal cortical tumors, carcinoids, and lipomas are also observed in association with MEN1. Görtz et al. (1999) identified an intronic base substitution 9 nucleotides upstream of a splicing junction of the MEN1 gene in a patient with adrenocortical adenoma and lung carcinoid and concluded that this nucleotide change was the germline mutation causing these tumors. In contrast, the same nucleotide substitution in the MEN1 gene had previously been reported as a polymorphism of heterozygous frequency of 3% (Bassett et al., 1998). We identified the same germline mutation (894-9 G→A) in 2 unrelated Japanese pedigrees affected with MEN1 (Kishi et al., 1999). Case 1 had multiple parathyroid tumors causing hyperparathyroidism, pancreatic insulinomas, and s.c. lipomas. This patient had no definite family history of MEN1 and was an apparently sporadic case. In the case 2 family, there were at least 4 patients with parathyroid tumors, 1 of whom also had pancreatic insulinomas. None of the patients in either pedigree had pituitary tumors. The wild-type MEN1 sequence was lost in the pancreatic islet tumor resected from case 1. Abnormal mRNA was identified in the tumor, which retained an intronic 7-nucleotide sequence, indicating aberrant mRNA splicing at a new splicing acceptor site created by the mutation, as suggested by Görtz et al. (1999). These findings indicate that this nucleotide substitution is not a benign polymorphism but a pathogenic mutation. Although the patient reported by Görtz et al. (1999) showed an atypical MEN1 phenotype, having only adrenal adenoma and lung carcinoid without parathyroid involvement, our patients had typical MEN1-related tumors. Thus, this mutation does not appear to be related to a special variant MEN1 phenotype. However, it is possible that the mutant MEN1 gene may produce a small amount of normal mRNA generated by correct splicing at the authentic acceptor site and may cause a milder form of MEN1 as suggested by analogy of other genes, such as β-globin (Treisman et al., 1983) and cystic fibrosis (Highsmith et al., 1997), in which similar splicing mutations caused a milder phenotype. Further investigation is necessary to determine whether this MEN1 splicing mutation is really associated with a mild form of MEN1. Yours sincerely, Toshihiko Tsukada [email protected], Mari Kishi, Takao Obara, Ken Yamaguchi