Rare, difficult to diagnose, without specific therapies: cancers of unknown primary origin or CUP, accounting for 1-2 percent of all malignancies, with a life expectancy of less than one year and in the absence effective treatments are a real challenge for research and medicine. Hence a recent Italian multicenter study, published in Nature Communications (https://www.nature.com/articles/s41467-021-22643-w), is finally good news: researchers have managed to isolate CUP stem cells in vitro, propagate them in vivo and observe that they display high tumorigenicity ricapitulating the same CUP phenotype in experimental animals, thus obtaining a valid study model and paving the way for further characterizations of the CUPs. The model is the mainstay of an innovative research project supported by AIRC and directed by professor Paolo Comoglio, which will be implemented in collaboration with Gianni Bonadonna Foundation.
So far, CUPs have been addressed by trying to ‘deduce’ a putative ‘tissue of origin’, and treat each CUP as a high-grade metastatic tumor of that tissue or organ, or by trying to identify mutated cancer genes and inform personalized targeted therapies. The approach of the multidisciplinary team lead by Carla Boccaccio of the Cancer Stem Cell Research Laboratory at the IRCCS Candiolo Cancer Institute was different: CUPs were investigated as a group of tumors sharing the common ability to disseminate in a way that is early and rapidly progressing, unrestrained by those barriers in the tissue of origin that facilitate the growth of a detectable primary mass, and associated with an early cell differentiation block, which precludes formation of recognizable tissues. The first step was therefore to isolate CUP stem cells; as Boccaccio explains, «Stem cells responsible for tissue repair and regeneration can be found in every tissue and the same is true for tumors, where stem cells maintain the population of cancer cells over time. They are insidious cells, more resistant to therapies than cancer cells that undergo pseudo-differentiation processes: a sort of ‘cancer root’, more difficult to eradicate. We were able to isolate them in vitro from CUPs of eight patients and then we transferred them to immunocompromised mice».
From CUP stem cells it was possible to grow agnospheres, or spheroids derived from tumors of unknown origin enriched in stem cells; agnospheres were proven to be capable of long-term propagating in the complete absence of any exogenous growth factor, usually required for in vitro isolation and maintenance of the stem phenotype in cancer stem cells from highly aggressive tumors. Furthermore agnospheres retained CUP genetic and molecular characteristics and were able to reproduce a faithful disease model that features early cell dissemination and formation of established multi-organ metastases, identical to that of the patient from which they were derived. «One case was particularly interesting», says Boccaccio. «Usually the derivation process leading from cell isolation to implantation takes months, but with CUP stem cells it was very quick, due to the high tumor aggressiveness: we isolated tumor stem cells from circulating tumor cells and with the patient alive we obtained a mouse with metastases in the same organs, even in muscles, a rare site of metastasis. It was therefore possible to have a sort of patient’s ‘avatar’, a living organism-model to be used for testing therapies: it’s not usual to obtain such an avatar, but it’s another proof that the CUPs study model of agnospheres can help shed light on the enigma of these tumors», Boccaccio says.
