Abstract #1184452: Thyroid Fine Needle Aspirates Containing both BRAF V600E and TERT Promoter Mutations Show Unique Differential microRNA Expression Patterns

Abstract

Thyroid nodules, testing positive for both BRAF V600E and TERT promoter mutations via next-generation sequencing (NGS), show aggressive biological behavior and may serve as candidates for molecular-driven targeted therapies. MicroRNA (miRNA) expression profiling reflects epigenetic growth control which is unique to each neoplasm and driven by dominant genomic alterations. We analyzed differential miRNA expression patterns in a large series of thyroid nodules containing both BRAF V600E and TERT mutations searching for molecular heterogeneity that could impact treatment. A cohort of 54 thyroid FNAs, containing both a BRAF V600E and TERT mutation, underwent NGS testing confirming mutation genotype and variant allele frequency percentage (VAF%). Samples consisted of needle aspirates in an RNA buffer and microdissection of stained cytology slides. The differential miRNA expression of 6 growth promoting and 5 growth suppressing miRNAs were measured by quantitative PCR and evaluated for 55 pairwise relationships. Separate cohorts of 30 BRAF V600E only and 30 TERT only samples were used as the quantitative standard for defining the miRNA expression profile for each mutational genotype. The degree of homology to each specific mutation genotype was determined. Mutation dominant was defined by having 50 or greater pairwise expression values fall within 2 standard deviations of the average for each mutation type. Cases not meeting the thresholds were designated as having a mixed miRNA expression profile. Most nodules bearing both BRAF V600E and TERT mutations closely matched the pure BRAF V600E miRNA expression profile (29/54, 53.7%). A mixed expression pattern (18/54, 33.3%) was next common. TERT dominance was least common (7/54, 13.0%). Mutation VAF% ranged from 3.5% to 48% between cases, however, VAF% within individual cases were similar in the majority (49/54, 90.7%) being slightly higher for TERT in keeping with tightly linked temporal mutational acquisition with TERT mutation acquired first. Combined BRAF V600E and TERT mutated thyroid FNA samples can be expected to differentially express each of these strong driver genomic alterations. Similar VAF levels supports coordinated mutational acquisition, likely within one neoplastic clone. However, differential miRNA expression patterns were seen with BRAF V600E dominance in most cases, followed by a mixed pattern involving both oncogenes. Most importantly, miRNA expression profiling of nodules containing both BRAF V600E and TERT mutations may be helpful in selecting optimal treatment by matching therapeutics to the miRNA expression pattern of dominant oncogenes.