Intratumoral heterogeneity as a therapy resistance mechanism: role of melanoma subpopulations

Abstract

Malignant melanoma is an aggressive form of skin cancer whose incidence continues to increase worldwide. Increased exposure to sun, ultraviolet radiation, and the use of tanning beds can increase the risk of melanoma. Early detection of melanomas is the key to successful treatment mainly through surgical excision of the primary tumor lesion. But in advanced stage melanomas, once the disease has spread beyond the primary site to distant organs, the tumors are difficult to treat and quickly develop resistance to most available forms of therapy. The advent of molecular and cellular techniques has led to a better characterization of tumor cells revealing the presence of heterogeneous melanoma subpopulations. The discovery of gene mutations and alterations of cell-signaling pathways in melanomas has led to the development of new targeted drugs that show dramatic response rates in patients. Single-agent therapies generally target one subpopulation of tumor cells while leaving others unharmed. The surviving subpopulations will have the ability to repopulate the original tumors that can continue to progress. Thus, a rational approach to target multiple subpopulations of tumor cells with a combination of drugs instead of single-agent therapy will be necessary for long-lasting inhibition of melanoma lesions. In this context, the recent development of immune checkpoint reagents provides an additional armor that can be used in combination with targeted drugs to expand the presence of melanoma reactive T cells in circulation to prevent tumor recurrence.

Figure 1. Molecular heterogeneity of melanomas

Precursor melanocytic lesions frequently harbor single gene mutations (*) such as BRAF, NRAS, C-KIT or GNAQ/GNA11 with a potential for neoplastic transformation. Additional oncogenic events (ϕ) such as deletions, mutations or loss of tumor suppressor genes (PTEN, p16^INK4A/p14^ARF, p53), alterations in genes associated with cell-cycle regulation (CCND1/CDK4, MITF [dashed circle]) or activation (black arrow) of signaling pathways (PI3K/AKT [dotted oval]; sometimes PI3K/AKT mutations can also be found in low frequency) are needed for malignant transformation of benign nevi to primary tumor and then to progressive metastatic melanoma. The most frequent genetic alterations are depicted for simplicity. Mutations of tumor suppressor genes (p16 ^INK4A, p14 ^ARF and p53) may happen very early in the process of malignant transformation but there is no concrete evidence of their exact occurrence. Genomic instability further contributes to genetic heterogeneity.

Figure 2. Induction of melanoma subpopulations: the role of TME, chemo- or targeted-therapy and immune related stress

TME niche and therapy-induced infiltration of leukocytes supports and promotes the induction of tumor subpopulations which express increased levels of drug efflux proteins, DNA repair enzymes and anti-apoptotic proteins resulting in activation of pro-tumor survival mechanisms.

Figure 3. Potential new therapeutic approaches to target melanoma

A heterogeneous tumor such as melanoma will require multi-targeted inhibition of signaling pathways (e.g. BRAF) or cell-cycle regulatory proteins (e.g. CDK inhibitors) (1–8) and depletion of minor subpopulations (e.g. CD20) that sustain the tumor using a combination of antibodies or inhibitors (9–13). This strategy will help prevent tumor recurrence and thus obtain long-lasting responses.