[541] RAP1GAP Is a Novel Marker of Malignancy for Thyroid Tumors

H Zuo, M Gandhi, MN Nikiforova, P Zhang, YE Nikiforov. University of Pittsburgh, Pittsburgh, PA

Background: RAP1 GTPase activating protein, also known as RAP1GAP, functions by switching off RAP1, the RAS-like protein that has been implicated in cell adhesion and migration. Recent findings suggest that RAP1GAP is frequently inactivated in several tumor types and may function as a tumor suppressor. The aim of this study was to investigate the alterations of the RAP1GAP gene and its expression in thyroid tumors and its potential value as a diagnostic marker for thyroid cancer.
Design: We studied 197 thyroid nodular lesions, including 40 hyperplastic nodules (HNs), 48 follicular adenomas (FAs), 28 follicular carcinomas (FCs), 78 papillary carcinomas (PCs), and 3 anaplastic carcinomas (ACs). Loss of heterozygosity (LOH) in the RAP1GAP region was studied using three microsatellite loci located on 1p36.1-p35 within or close to the location of the RAP1GAP gene. RAP1GAP mRNA expression levels were detected by qRT-PCR and protein levels by western blot. Immunohistochemistry was performed using RAP1GAP antibody (Santa Cruz, dilution 1:100).
Results: LOH at the RAP1GAP region was found in 21% of FCs, 40% of PCs, and 67% of ACs. RAP1GAP mRNA levels were significantly decreased in malignant thyroid tumors, down to 0.45 from the expression level in normal thyroid cells in FCs, 0.25 in PCs, and 0.04 in ACs. The presence of LOH correlated with the decreased expression levels of mRNA (p=0.039). Western blot analysis confirmed the decrease or complete loss of RAP1GAP protein in 3 PCs as compared to adjacent normal thyroid tissue. Using immunohistochemistry, significant decrease or loss of RAP1GAP expression was observed in 0/28 HNs, 2/32 (6%) FAs, 6/16 (38%) FCs and 21/29 (72%) of PCs. Among PCs, the decreased or loss of RAP1GAP immunostaining was observed in 20/21 (95%) of invasive PCs but only in 1/9 (11%) encapsulated PCs. Among FCs, the decrease or loss of immunostaining was found in 5/5 (100%) widely invasive FCs and 1/11 (9%) minimally invasive FCs.
Conclusions: RAP1GAP is likely to serve as a tumor suppressor gene which is frequently affected by LOH and lose expression in a significant proportion of thyroid cancers, especially in those with invasive growth. The loss of the RAP1GAP protein can be detected by immunohistochemistry and may serve as a diagnostic marker of malignancy in thyroid nodules.
Category: Endocrine

Tuesday, March 10, 2009 1:00 PM

Poster Session IV # 84, Tuesday Afternoon