CUPs (Cancers of unknown Primary) are generally defined as a mixed cluster of metastatic tumors disseminated from unknown sites. It is currently challenging to ascertain the site of origin (tissue) by means of a standardized diagnostic clinical approach. About 3%–5% of all malignancies are considered CUP. Because the origin of malignancy is not known, it becomes challenging to devise a clinically appropriate and successful treatment regimen.
Classification and Sub-classification
About 15% of CUP can be diagnosed into sub-classes like adenocarcinoma in axillary lymph nodes or squamous carcinoma in cervical lymph nodes, peritoneal papillary serous carcinoma, and poorly differentiated neuroendrocrine CUP. The remaining 85% of patients do not fit into these specific sub-classes. Amongst the classified ones, about 50-70% of CUP cases is well-differentiated or moderately differentiated adenocarcinomas, while poorly differentiated carcinomas/adenocarcinomas make up 20-30% of them. The remaining cases are divided into squamous cell carcinomas (5-8%) and poorly differentiated or undifferentiated malignant neoplasms (2-3%).
Incidence and prevalence
Epidemiological reports of CUP cases are rare and challenging to outline mainly due to the difficulties in diagnosis and classification. There are reports mainly from northern European countries (Netherlands, Sweden, Scotland and Norway), Australia and USA (1960–2010) that outline the characteristic trends of this disease. They reveal an increasing incidence of CUP over the period from 1960 to 1980, to a level of 16 per 100,000 inhabitants (1).
Currently the exact cause of CUP is unknown (2). Chromosomal instability has been a characteristic feature of the CUPs that could potentially distinguish them from metastases of known origin (3).
The initiation and progression of metastasis can occur in one of two broad ways: 1) growth within the primary tissue before dissemination of fully malignant cells to secondary sites; and 2) cells from the tumor disseminate from the primary tissue, lodges and grows in a secondary site independently. However a third possibility is that metastatic dissemination can occur via inherent metastatic aggressiveness of cancer cells without the growth of any kind of tissue-specific tumor. This kind of parallel and independent metastatic behavior might be a recurring feature of CUPs.
The most commonly used diagnostic approach is IHC-based pathology from a good quality tissue. There are 2 classes of antibody markers that are usually implemented in CUP characterization: antibodies to cytokeratins followed by antibodies to organ-restricted markers. Specific cytokeratins used are CK7 and CK20 which provide general indications of a possible primary site. This is usually followed by tissue-specific markers - such as chromogranin A and synaptophysin for ascertaining neuroendocrine differentiation, K7+, WT-1+, PAX8+, CK20− for ovarian cancer and RCC+, PAX8+ for renal cancer. Other such tissue-specific biomarkers: thyroid-specific thyroid transcription factor-1 (TTF1), breast-specific mammoglobin, intestine-specific transcription factor (CDX2) and hepatocyte-specific antigen 1 (HepPar-1). Readers are referred to an excellent representation of CUP screening wherein both primary staining pathology followed by tissue-specific biomarker approach is discussed (4). One caveat in IHC-based biomarkers is lack of a standardized panel of IHC markers used to address this challenge. Another concern is the subjective bias in IHC observations that can lead to errors in reproducibility.
Both computer tomography (CT) and conventional magnetic resonance imaging (MRI) scans have been predominantly used in conjunction with the clinico-pathological assays for diagnosis and prognosis. Due to the small size, soft tissue contrast, quantitative limitation and regression of the primary tumor in CUP cases, imaging presents its own difficulties. This has been circumvented by 18F-fluoro-2-deoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) scans (5).
Gene (mRNA) based profiling
Studies predicting the tissue of origin in CUP cases by a comparative approach of its gene (mRNA) expression profile (GEP)with that of a known set of primary/metastatic tumors have enabled the correct identification of 83-89% of cases. A comprehensive approach to gene expression profiling as well as a comparative analysis of IHC versus GEP is outlined in (6).
Epigenomics - DNA hyper/hypomethylation
Cancer exhibits three types of aberrations in DNA methylation - hypermethylation of tumor suppressor genes, aberrant expression of methylating enzymes like DNMT1, and hypomethylation of certain genes. All these types can be harnessed via current whole-genome approaches for cancer diagnostic and prognostic applications (1,7). Many of these studies where CUP patients were analyzed and classified with a methylation-based classifier algorithm approach have been successfully confirmed with pathological data.
Epigenomics - miRNA based profiling
Traditional gene expression microarrays (mRNA profiling) can be challenging to perform on FPPE tissue samples as mRNA is not amenable to processing for this kind of assay. Assaying for miRNA is particularly advantageous due to its stability and robustness to experimental processes. miRNA microarray profiles could be faster to generate—but equally accurate—than a panel of RT-qPCR-based genes. Recently a group of researchers investigated miRNA expression profiling as a tool for identifying the tissue of origin of metastases samples from primary cancers and metastasis samples by using a microarray platform. A miRNA specific signature was successfully applied to predict CUPs (8).
Perspectives - Integrative Disease management
The current molecular assays ascertaining the primary site of CUP are so designed so as to identify similarities (and not differences) between CUP and metastatic solid tumors. This approach restricts the metastatic tissue properties to that of primary tissues and overlooks the novel characteristic features of newly emerging metastatic cells. A pressing future necessity is the creation of a clinical diagnostic algorithm that integrates both pathological data and molecular profiling tests – this integrative approach along with critical clinical evaluation will result in better CUP diagnosis and management.
- Moran S, Martinez-Cardús A, Boussios S, Esteller M. Precision Medicine Based in Epigenomics: The Paradigm of Carcinoma of Unknown Primary. Nat Rev Clin Oncol. 2017; 14(11): 682-694
- Massard C, Loriot Y, Fizazi K. Carcinomas of an unknown primary origin—diagnosis and treatment. Nat Rev Clin Oncol 2011; 8: 701–710.
- Vikeså J, Møller AK, Kaczkowski B et al. Cancers of unknown primary origin (CUP) are characterized by chromosomal instability (CIN) compared to metastasis of know origin. BMC Cancer 2015; 15: 151.
- Tomuleasa C, Zaharie F, Muresan MS, Pop L, Fekete Z, Dima D, Frinc I, Trifa A, Berce C, Jurj A, Berindan-Neagoe I, Zdrenghea M. How to Diagnose and Treat a Cancer of Unknown Primary Site. Gastrointestin Liver Dis. 2017; 26(1):69-79.
- Kwee, T. C. & Kwee, R. M. Combined FDG-PET/CT for the detection of unknown primary tumors: Systematic review and meta-analysis. Eur. Radiol. 2009; 19, 731–744
- Hainsworth J and Greco A. Gene expression profiling in patients with carcinoma of unknown primary site: from translational research to standard of care. Virchows Arch 2014; 464:393–402
- Tang W, Wan S, Yang Z, Teschendorff AE, Zou Q. Tumor origin detection with tissue-specific miRNA and DNA methylation markers. Bioinformatics. 2018; 34(3): 398-406.
- Ferracin M, Pedriali M, Veronese A, Zagatti B, Gafà R, Magri E, Lunardi M, Munerato G, Querzoli G, Maestri I, Ulazzi L, Nenci I, Croce CM, Lanza G, Querzoli P, Negrini M. MicroRNA profiling for the identification of cancers with unknown primary tissue-of-origin. J Pathol. 2011;225(1):43-53.