Mutational and Copy Number Analysis of Candidate Tumor Suppressor Gene FBXO4 in Sporadic Parathyroid Adenoma

Presentation Number: SAT 342
Date of Presentation: April 1st, 2017

Kelly Brewer*1, Isabel Nip1, Jessica Costa-Guda2 and Andrew Arnold1
1University of Connecticut School of Medicine, Farmington, CT, 2University of Connecticut School of Dental Medicine, Farmington, CT

Abstract

The molecular mechanisms underlying the tumorigenesis of sporadic parathyroid adenomas are incompletely understood. Overexpression of the oncoprotein cyclin D1, encoded by CCND1/PRAD1, has been documented in up to 40% of sporadic parathyroid adenomas; however, known aberrations in the gene encoding cyclin D1 account for less than half of these cases, and thus far no other potential mechanisms of cyclin D1 overexpression have been identified in this tumor type. Degradation of cyclin D1 is tightly regulated and genetic alterations of FBXO4, which encodes one essential member of the cyclin D1-degrading complex, have been identified in a number of human tumors (1). Genetic aberrations in FBXO4 and CCND1 appear to be mutually exclusive, strongly suggesting a common pathway. We postulated that genetic inactivation of FBXO4 and the consequential increase in stability of cyclin D1 may contribute to the development of sporadic parathyroid adenoma. We therefore sought to identify FBXO4 coding region mutations by direct Sanger sequencing of genomic DNA samples isolated from sporadic parathyroid adenomas, as well as identify FBXO4 copy number changes by loss of heterozygosity. The entire coding region and intron-exon boundaries of FBXO4 were examined in a cohort of seventy-six typically presenting, sporadic parathyroid adenomas. In addition, allelic imbalance was assessed by analysis of two microsatellite markers flanking the FBXO4 genomic region in seventy-one of the seventy-six samples (those for which matched germline DNA was available). No clearly inactivating or cancer-associated genetic alterations (e.g. early stop codons, frameshifts, indels, etc.) were found. One known polymorphism, rs144096644, was identified in one sample; however, this SNP does not effect any changes in amino acid sequence and is not associated with cancer. A second sample exhibited loss of one copy of the proximal microsatellite yet retained two copies of the distal microsatellite, suggesting the possibility of partial or whole deletion of FBXO4, but not contradicting the possibility that the gene may have remained intact. The absence of intragenic mutations in FBXO4, as well as the absence or exceedingly low frequency of allelic loss in the surrounding genomic region, suggests that such aberrations do not commonly contribute to the development of sporadic parathyroid adenoma. However, because such intragenic mutations could be rare, a significant role for cyclin D1 accumulation due inactivation of FBXO4 in parathyroid tumor formation cannot be ruled out. Further research must be done to elucidate the mechanism(s) of cyclin D1 accumulation in parathyroid adenomas in which CCND1 aberrations are not present.

 

Nothing to Disclose: KB, IN, JC, AA