MON-335 Phenocopy of Multiple Endocrine Neoplasia Type 1 (MEN1) Due to a Germline Cell Division Cycle 73 (CDC73) Variant

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant disorder characterized by the combined occurrence of parathyroid tumors, and neuroendocrine tumors (NETs) of the pituitary and pancreas. MEN1 is caused by germline mutations of the tumor suppressor gene MEN1, which are found in >75% of MEN1 patients. The remaining patients may have mutations involving: as yet unidentified genes; or other genes with known involvement in endocrine tumors, e.g. cyclin dependent kinase inhibitor 1B (CDKN1B) which is associated with MEN4. Here, we report a patient who had primary hyperparathyroidism, acromegaly, and a pancreatic NET that showed immuno-staining for glucagon and chromogranin A, which are consistent with MEN1. However, mutational analysis did not identify a MEN1 mutation, and analysis of other genes was undertaken, after informed consent was obtained from the patient. These genes include: CDKN1B; CDKN1A; CDKN2B; CDKN2C;cell cycle division 73 (CDC73), causative for hyperparathyroidism-jaw tumour (HPT-JT) syndrome, an autosomal dominant disorder characterised by occurrence of parathyroid tumors, ossifying fibromas of the jaw, renal tumours and uterine tumors; calcium sensing receptor (CASR), causative for familial hypocalciuric hypercalcemia type 1 (FHH1); and aryl-hydrocarbon receptor-interacting protein (AIP), causative for familial pituitary tumors. This revealed a heterozygous c.1138C>T (p.Leu380Phe) missense mutation of CDC73. This is likely a disease-causing mutation as: 1) the p.Leu380Phe substitution is not present in >120,000 alleles of The Exome Aggregation Consortium (ExAc) database; 2) involves an evolutionary conserved Leu380 residue; and 3) is situated within the parafibromin domain predicted to interact with the polymerase associated factor 1 (PAF1) and RNA polymerase II, (POLR2A) complex. In addition, RNA-Scope analysis, to detect specific CDC73 mRNA transcripts in paraffin embedded sections, of the patient’s pancreatic NET revealed that CDC73 mRNA expression was significantly decreased by >13% (p<0.005) in the tumor cells, compared to normal adjacent islets. Thus, our results suggest the spectrum of tumours associated with CDC73 mutations also includes pancreatic NETs, and demonstrate that CDC73 mutations may result in a phenocopy of MEN1.