Molecular characterisation and liquid biomarkers in Carcinoma of Unknown Primary (CUP): taking the 'U' out of 'CUP'

Abstract

Cancers of Unknown Primary (CUP) comprise a heterogeneous clinical entity of confirmed metastatic cancer where the primary site of origin is undetectable. It has a poor prognosis with limited treatment options. CUP is historically under-researched; however, understanding its biology has the potential to not only improve treatment and survival by implementation of biomarkers for patient management, but also to greatly contribute to our understanding of carcinogenesis and metastasis across all cancer types. Here we review the current advances in CUP research and explore the debated hypotheses underlying its biology. The evolution of molecular profiling and tissue-of-origin classifiers have the potential to transform the diagnosis, classification and therapeutic management of patients with CUP but robust evidence to support widespread use is lacking. Precision medicine has transformed treatment strategy in known tumour types; in CUP, however, there remains a clinical need for a better understanding of molecular characteristics to establish the potential role of novel or existing therapeutics. The emergence of liquid biopsies as a source of predictive and prognostic biomarkers within known tumour types is gaining rapid ground and this review explores the potential utility of liquid biopsies in CUP.

Fig. 1

Current management and treatment options for favourable and unfavourable subtypes of CUP. Following tissue biopsy diagnosis of confirmed CUP, patients with favourable clinical features are treated as the analogous tumour type. Those patients within the unfavourable clinical subtype can be offered palliative chemotherapy if they have good performance status. There is no standard second line therapy. IHC immunohistochemistry, PS performance status, CK cytokeratin, LDH lactate dehydrogenase, SCC squamous cell carcinoma

Fig. 2

Potential clinical and research applications of liquid biopsies for the management of CUP. Clinical and research applications of liquid biopsies are wide-reaching, from early detection and diagnosis to monitoring response to therapy and earlier detection of disease relapse. Liquid biopsies contain genetic information from the tumour in the form of circulating tumour cells (CTCs), tumour-educated platelets (TEPs), mircoRNAs contained within exomes and circulating free tumour DNA (ctDNA). ctDNA is a component of circulating free DNA (cfDNA); fragments of DNA either passively released by cells as a consequence of apoptosis and cell death, or actively released by cells as a potential messaging signal. Patients with cancer have a much higher proportion of cfDNA in the blood compared with healthy normal volunteers (HNV); a greater proportion is tumour-derived ctDNA that is shed from the highly proliferating tumour cells and/or tumour cell death.^, CTCs are released into the bloodstream by a passive process of tumour shedding or through active intravasation, including processes such as epithelial-to-mesenchymal transition (EMT), cell-to-cell cooperation and vasculogenic mimicry.^, Epithelial cells undergoing EMT lose their characteristic cell-to-cell interactions, become mobile and gain invasive properties. CTCs that underwent EMT may reverse this process during the process of metastasis formation; however, only a very small proportion of CTCs subsequently propagate a distant metastasis. Molecular analyses that can be performed on genetic material include copy number alterations, actionable mutation detection, amplifications, and deletions, as well as epigenetic and transcriptome analysis. CTCs can be implanted into immune-compromised mice as CTC explants (CDX) or cultured directly as CTC organoids for drug testing

Fig. 3

Proposed role of liquid biopsies in CUP research and future diagnosis, management stratification and monitoring of CUP patients. A single blood test at presentation of CUP could determine tissue of origin, evaluate prognostic and predictive biomarkers and stratify patients to appropriate treatment. Serial blood samples could monitor early response to therapy or resistance, enabling timely switch or halting of futile treatments. EGFR epidermal growth factor receptor, IHC immunohistochemistry, MSI microsatellite instability, PS performance status.