Patients with local–regionally advanced or metastatic thyroid carcinoma are a growing concern for head and neck surgeons, endocrinologists and medical oncologists. With the increasing incidence of thyroid carcinoma, now representing 5% of cancers diagnosed in women, this has resulted in over 650,000 patients living with a thyroid cancer diagnosis in the United States. Up to 30% of these patients will develop local regional recurrence and up to 15% will develop distant metastatic disease. Although surgery and radioactive iodine provide initial therapies for follicular derived carcinomas, tumors often become non-avid for radioactive iodine limiting this treatment modality. Despite the increasing number of patients with advanced thyroid carcinoma, there remains a limited set of drugs approved for advanced follicular derived thyroid carcinomas (sorafenib and lenvatinib). Ideally, molecular assessment of the tumor tissue will allow for directed therapy selection through the identification of altered biologic pathways. However, which testing methodology should be used in this patient population is debated based on time, cost, and information gained.
To better inform treatment decisions in advanced thyroid carcinoma patients, the landscape of mutational alterations particularly mutations associated with potential progression and metastasis is needed. Similarly, recognizing the distinct characteristics associated with oncogenes versus tumor suppressor genes in thyroid tumorigenesis and potential targetability is required.
BRAF V600E, a point mutation at codon 600 leading to a valine substitution to glutamic acid, represents the most common oncogene mutated in thyroid cancer occurring in 40% to 60% of papillary thyroid carcinomas, 33% of poorly differentiated thyroid carcinomas, and 20% to 45% of anaplastic thyroid carcinoma.
Another oncogene, RAS, shows mutations in up to 30% of follicular carcinomas, 28% of poorly differentiated thyroid carcinoma, 24% of anaplastic thyroid carcinoma, and 10% to 20% of medullary thyroid carcinoma.
Mutations in RET proto-oncogene define hereditary medullary thyroid carcinoma and also occur in 40% to 50% of somatic cases.
Mutations in oncogenes often lead to currently “targetable therapies” either as specific gene inhibitors or often broader pathway inhibitors. Drugs targeting overactive pathways and oncogenes have expanded over the past decade. This includes a host of tyrosine kinase inhibitors and gene-specific inhibitors including BRAF V600E inhibitors, which has led to increased clinical trials in thyroid cancer.
These oncogenes are key drivers of the activated RAS–RAF–MEK–ERK signaling pathway, which contributes to thyroid carcinogenesis.
Concurrently, the recent development of a BRAF V600E specific antibody for immunohistochemical assessment has facilitated a more rapid and readily available technique for determining BRAF V600E status compared to molecular assays. This single gene testing approach fails to account for potentially altered secondary pathways, which may confer escape mechanisms. Moreover, about one third of papillary thyroid carcinomas will be BRAF V600E negative, thus limited testing leaves an unknown if other molecular alterations may exist or be of aid for therapeutic consideration (17).
Although oncogene alterations frequently occur at hotspots and are relatively easy to test, only with modern next-generation sequencing (NGS) can analysis be performed to gain insight into alterations within tumor suppressor genes. Dysregulation of TP53, a prototypic tumor suppressor gene, contributes to a vast array of tumors and is currently viewed as “nontargetable” based on current therapeutic considerations. TP53 gene mutations occur late in the progression of thyroid carcinoma with increasing incidence from papillary to poorly differentiated and finally anaplastic thyroid carcinomas. Although not currently targetable, knowledge of the presence of a TP53 mutation may be informative as a mechanism of resistance and possible biomarker portending a more aggressive clinical course.