Background: Multiple Endocrine Neoplasia (MEN) 2B is a rare hereditary disorder characterized by medullary thyroid cancer (MTC) in early childhood, pheochromocytoma, and mucosal neuromas. Patients with advanced MTC are treated with rearranged during transfection (RET) targeting tyrosine kinase inhibitors (TKIs) such as vandetanib. Despite initial responses, many patients progress on TKI therapy and mechanisms of resistance are yet to be elucidated. We analyzed tumor samples from seven children with MEN2B and MTC enrolled in a natural history study (NCT01660984).

Methods: DNA samples from tumor and adjacent normal tissue from paraffin embedded blocks or unstained slides were analyzed by a custom capture next-generation sequencing panel. Bioinformatics analyses identified point mutations, insertions, deletions and copy number variations.

Results: Seven patients (median age at study enrollment 14 years, range 11-17 years) with the RET p.Met918Thr germline mutation were included in analysis. Tumor samples were available for three patients pre-TKI (four samples), two patients at progression on TKI (three samples), and two patients both, pre-TKI and at progression (five samples). Pre-TKI samples exhibited few tumor specific mutations or copy number variations and 4/5 patients had loss of chromosome 1p. Progression for one patient was associated with acquisition of a previously unidentified p.Leu790Phe mutation within the kinase domain of the RET gene. Loss of heterozygosity and increase in copy number variations were noted in 4/5 samples at tumor progression. Two patients had copy number loss of chromosome 14 and three had copy number gain of chromosome 1q. Recurrent, somatic, non-synonymous mutations were not identified in the sample set.

Conclusion: In children with MEN2B and MTC, we identified increase in copy number variations and a somatic mutation within the RET gene as potential mechanisms of drug resistance. Our data imply that a common genetic mechanism for progression on TKI may not exist within this small sample set and highlight the need for serial collection of tumor tissue. Further, whole genome aneuploidy may provide rationale for the evaluation of cytotoxic chemotherapy in patients who experience progressive disease on TKI therapy. Analysis of additional samples and whole exome DNA and RNA sequencing are ongoing.

Citation Format: Ira L. Kraft, Srivandana Akshintala, Keith J. Killian, Robert B. Hufnagel, John W. Glod, Claudia Derse-Anthony, Yuelin Zhu, Holly S. Stevenson, Diana Bradford, Maria J. Merino, Frank M. Balis, Elizabeth Fox, Brigitte C. Widemann, Jack F. Shern, Paul S. Meltzer. Genomic mechanisms of disease progression in pediatric medullary thyroid cancer (MTC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4882. doi:10.1158/1538-7445.AM2017-4882