Whole-Exome Sequencing in Papillary Microcarcinoma: Potential Early Biomarkers of Lateral Lymph Node Metastasis

Early identification of patients with high-risk papillary thyroid microcarcinoma (PTMC) that is likely to progress has become a critical challenge. We aimed to identify somatic mutations associated with lateral neck lymph node (LN) metastasis (N1b) in patients with PTMC. Whole-exome sequencing (WES) of 14 PTMCs with no LN metastasis (N0) and 13 N1b PTMCs was performed using primary tumors and matched normal thyroid tissues. The mutational burden was comparable in N0 and N1b tumors, as the median number of mutations was 23 (range, 12 to 46) in N0 and 24 (range, 12 to 50) in N1b PTMC (P=0.918). The most frequent mutations were detected in PGS1, SLC4A8, DAAM2, and HELZ in N1b PTMCs alone, and the K158Q mutation in PGS1 (four patients, Fisher's exact test P=0.041) was significantly enriched in N1b PTMCs. Based on pathway analysis, somatic mutations belonging to the receptor tyrosine kinase-RAS and NOTCH pathways were most frequently affected in N1b PTMCs. We identified four mutations that are predicted to be pathogenic in four genes based on Clinvar and Combined Annotation-Dependent Depletion score: BRAF, USH2A, CFTR, and PHIP. A missense mutation in CFTR and a nonsense mutation in PHIP were detected in N1b PTMCs only, although in one case each. BRAF mutation was detected in both N0 and N1b PTMCs. This first comprehensive WES analysis of the mutational landscape of N0 and N1b PTMCs identified pathogenic genes that affect biological functions associated with the aggressive phenotype of PTMC.