Medullary Thyroid Cancer: Molecular Drivers and Immune Cellular Milieu of the Tumour Microenvironment—Implications for Systemic Treatment.

Simple Summary: Medullary thyroid carcinoma (MTC) is driven by a small number of pathogenic genetic variants and tumours usually exhibit a correspondingly low tumour mutational burden. This reduces tumour visibility to the immune system and impacts the immune cell profile of the tumour microenvironment. In the last decade targeted pathway inhibitors have revolutionized the therapeutic landscape for patients with advanced disease, with increasing options for systemic therapy tailored to the molecular signature of the tumour. Therefore, understanding the molecular basis of disease, pathogenesis of immune evasion and mechanisms of escape of pathway inhibition is of paramount importance. Here, we summarize genetic and molecular drivers of MTC and their relevance to tumour immunogenicity, the cellular milieu of the tumour microenvironment, and response to targeted therapy. In this review, we explore the underlying molecular biology of medullary thyroid carcinoma (MTC) and its interplay with the host immune system. MTC is consistently driven by a small number of specific pathogenic variants, beyond which few additional genetic events are required for tumorigenesis. This explains the exceedingly low tumour mutational burden seen in most MTC, in contrast to other cancers. However, because of the low tumour mutational burden (TMB), there is a correspondingly low level of tumour-associated neoantigens that are presented to the host immune system. This reduces tumour visibility and vigour of the anti-tumour immune response and suggests the efficacy of immunotherapy in MTC is likely to be poor, acknowledging this inference is largely based on the extrapolation of data from other tumour types. The dominance of specific RET (REarranged during Transfection) pathogenic variants in MTC tumorigenesis rationalizes the observed efficacy of the targeted RET-specific tyrosine kinase inhibitors (TKIs) in comparison to multi-kinase inhibitors (MKIs). Therapeutic durability of pathway inhibitors is an ongoing research focus. It may be limited by the selection pressure TKI treatment creates, promoting survival of resistant tumour cell clones that can escape pathway inhibition through binding-site mutations, activation of alternate pathways, and modulation of the cellular and cytokine milieu of the tumour microenvironment (TME). [ABSTRACT FROM AUTHOR]