Abstract 3869: Allelic loss of a specific 3p segment: A key target loci involved in the development of highly malignant thyroid cancer histotypes

Abstract

Abstract There is compelling genetic evidence that constitutive activation of effectors along the PTEN-RAS-PI3K-AKT pathway is pivotal in thyroid tumour progression and dedifferentiation. We and others have shown that oncogenic PIK3CA mutations and gains in PIK3CA copy number are primarily associated with anaplastic thyroid carcinomas (ATC) and poorly differentiated thyroid carcinomas (PDC), which are genetically unstable thyroid cancer histotypes that show DNA aneuploidy and chromosomal aberrations (trisomy, tetrasomy or high polysomy). In this study we sought to determine whether gains in PIK3CA copy number, in ATC and PDC, represent the amplification of the gene itself or aneuploidy of chromosome 3, with other candidate genes at 3p or 3q involved in the amplified region. Herein, 20 ATC, 9 PDC and 8 ATC cell lines, which were previously screened for changes in PIK3CA gene dosage, were investigated for gains/losses at different loci of chromosome 3. A targeted 12 probe mixture spanning chromosome 3 (3p25.3 to 3q29) was designed and included on a multiplex PCR reaction. Tumor gene dosages were compared to mean ratios in non tumoral DNA (normal thyroids and/or blood from healthy blood donors). Cell lines were studied in parallel by means of CGH-microarray. PIK3CA gains in ATC and PDC were found to represent amplification of the gene itself and not a chromosome 3 polysomy. None of the ATC and PDC analyzed demonstrated gains in the exonic segments screened. Conversely, intragenic homozygous and heterozygous deletions on the short arm of chromosome 3 emerged as associated with ATC and PDC supporting the idea that genetic material in that locus is crucial in the development of aggressive thyroid carcinomas. Sixty percent of the ATC (12/20), 66.6% of the PDC (6/9) and 37.5% of the ATC cell lines (3/8) exhibited homozygous or heterozygous deletions of a specific 3p segment. Presumably, the truncated transcripts are non-functional because the encoded gene protein is highly evolutionary conserved and all the exonic rearrangements involve the translation start site. Among the 12 ATC showing loss of at least 1 allele at the 3p locus and gains at PIK3CA, 7 (58%) were mutated at RAS, 5 (42%) at PIK3CA and 2 (17%) at BRAF. Similarly, 2 of the 3 (67%) ATC cell lines bearing intragenic homozygous deletions were mutated at RAS. Among the 6 PDC with losses at 3p, 4 (67%) were mutated at RAS, 1 (17%) at PIK3CA and 2 (33%) at BRAF. Given that allelic losses at 3p were not seen in 5 PTC included in the study it is likely that inactivation of the 3p locus might be a late event in the multistep process of thyroid carcinogenesis. This finding may significantly impact on the rationale of future, tailored gene targeted therapies applied to patients with ATC and PDC. Our data suggest that inactivation of the 3p locus precedes or is coincident with RAS and PI3K activation and contributes to thyroid tumor progression and dedifferentiation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3869. doi:10.1158/1538-7445.AM2011-3869