IMPE Abstracts

Thyroid cancer is rare in childhood accounting for 0–3% of all pediatric malignancies. Children with thyroid cancer demonstrate higher rates of cervical lymph node involvement and pulmonary metastases, the long-term outcome is rather favorable and mortality rate is low compared with adults. An increasing incidence of the disease has been observed in the last years. There is not enough evidence about the molecular mechanisms implicated and the correlation with prognosis and clinical management particularly in pediatric patients. Furthermore pediatric cases have distinct clinicopathological features and actually the genetic profile differs substantially from adult cases. Therefore, establishing the molecular causes remains a real challenge. The aim of this study was to examine the diagnostic yield of next-generation sequencing (NGS) in pediatric Papillary thyroid carcinoma (PTC) mainly to reinforce knowledge on gene variants and fusions implicated. We have developed a DNA-based NGS panel, targeting 54 thyroid cancer-related genes, with a strategy to detect potential gene fusion in 9 tyrosine kinase genes with ability to reveal both previously described rearrangements as well as fusions with new partners. Mean coverage was 400x to increase sensitivity to identify tumor minority variants. The cohort consisted of 65 pediatric PTC patients followed in a single tertiary center. DNA was extracted from fresh frozen tissue samples, cryobar freezing wells and/or formalin-fixed paraffin-embedded blocks. Variants found were classified and prioritized according to ACMG guides, pathological molecular alterations were validated by DNA sanger sequencing in tissue DNA. To provide evidence of a germline mutation, peripheral blood DNA samples were analyzed when it was available. Fusions were validated by PCR amplification and sequencing analysis of hybrid genes, CGH array or inmunohistochemistry techniques. At least one pathogenic variant or gene fusion was detected in 43/65 samples (66%). BRAF p.Val600Glu 8/65 (12%), RET-fusion 13/65 (20%) and ALK-fusions 9/65 (14%) were the most frequently alterations observed. It is important to highlight that pathological germline variants were detected in genes implicated in hereditary cancer predisposition (DICER1, PTEN) in 4/65 patients (6%). Moreover, accordingly to American Thyroid Association relative risk classification, fusions were significantly higher (78%) in high risk cases compared to intermediate and low risk cases (27%) P<0,0014. Finally DNA NGS might be an efficient molecular diagnosis tool. More studies are needed to confirm the clinical utility of oncogene testing in order to guide management recommendations for surgery, adjuvant radioactive iodine, and the individualized selection of systemic therapy for patients with refractory disease.