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C. Côme et al.
tions). Moreover, to our knowledge, AML PDX models based on this approach have only been described by one laboratory so far.39,42 Therefore, our second proposed model is hBMLS based on gelatin scaffolds. This tech- nique is quite simple and, as for ossicles, up to four scaf- folds can be inserted per animal. Moreover, this strategy does not involve a long period of in vivo incubation in order to generate ossicles and, importantly, does not require pre-conditioning with irradiation.41 Using this technique, we have succeeded in generating MDS PDX models covering several MDS subtypes in both our labo- ratories. A limitation of this approach, as for other hBMLS models, is the use of BM-derived MSC because these MSC have various alterations compared to those derived from healthy donors, such as DNA methylation status47,48 and in vitro proliferation/differentiation capacity.47 There is therefore a risk that the use of healthy allogeneic MSC may affect the behavior of the MDS clone(s) in vivo. Encouragingly, the few studies that have compared the use of healthy and patient-specific MSC have not suggest- ed a major impact of the MSC origin on the engraftment levels of MDS in immunocompromised mice receiving intra-femoral injections.26,29 Nevertheless, MDS-derived BM MSC do have an impact on the survival and differen- tiation capacities of CD34+ hematopoietic stem and pro- genitor cells in vitro and in vivo,47,49 and they can also respond favorably to the hypomethylating agent azacyti- dine, the current treatment regimen for high-risk MDS.49 Consequently, investigation are needed to determine whether autologous MDS-BM MSC would be better at recapitulating the complexity of the disease in this model rather than BM MSC from healthy donors.
A major unresolved issue for the hBMLS approaches is that MSC display significant donor-to-donor variations and it would therefore be extremely useful to have a standard- ized source of MSC, i.e. in the form of BM MSC lines. Importantly, such cell lines have been generated recently and it would be of paramount importance to determine whether they retain their capacity to generate hBMLS in vivo50 and whether MDS material could engraft and expand in these structures. As MDS MSC have been shown to have a strong impact on the in vivo potential of CD34+ hematopoietic stem and progenitor cells, notably by showing altered extracellu- lar signaling such as reduced CXCL12 expression,48,49 such a cell line should either retain the features of MDS MSC or be receptive to “education” by MDS cells. However, if a MSC cell line that robustly retains these features could be obtained, this would provide an experimental platform for genetic manipulation of niche-derived cells, thereby facilitat- ing studies into niche-MDS cell interactions.
Conclusions and perspectives
MDS is a very heterogeneous group of blood disorders, associated with lesions in dozens of driver genes.2,3 Genetically engineered mouse models harboring muta- tions in the most common MDS driver genes display sev- eral characteristics of MDS11-13 but remain imperfect as an experimental tool since they generally only recapitulate a subset of the phenotypes associated with human MDS. During the past few decades, in particular during the past 5 years, we have seen several improvements in the tool- box available for the generation of MDS PDX.18-20,22-27,29,31,34 Moreover, various alternative methods, especially hBMLS models, appear to be extremely promising in terms of facilitating a more robust generation of MDS PDX.39-41 This is important since an increase in the number of MDS PDX models will allow us to cover the broad genetic and phe- notypic spectra of human MDS more comprehensively and provide tools to address key aspects of MDS biology.
Despite the recent developments in MDS PDX, these models may be further improved by incorporating addi- tional human niche cells, such as endothelial cells. Indeed, these cells are functional in hBMLS settings51,52 and endothelial cells from low-risk MDS patients influence hematopoietic stem cell behavior in vitro.53 However, the recent developments of hBMLS models already provide an excellent opportunity to characterize the interaction between MDS tumor cells and their microenvironment better. As indicated above, the tumor microenvironment plays a key role in the pathogenesis of MDS and if we could manipulate MSC in the hBMLS models, we would have a precise tool to discern the biological importance of the niche. Finally, the increasing armory of MDS PDX also holds great promise as preclinical translational models for the development and validation of novel therapies as well as for personalized medicine along the lines already occur- ring in solid cancers.
Acknowledgement
Work in the Porse laboratory was supported through a center grant from the Novo Nordisk Foundation (Novo Nordisk Foundation Center for Stem Cell Biology, DanStem; Grant Number NNF17CC0027852). The present work is also part of the Danish Research Center for Precision Medicine in Blood Cancers funded by the Danish Cancer Society (grant n. R223- A13071) and Greater Copenhagen Health Science Partners. Work in the Bonnet laboratory was supported by the Francis Crick Institute, which receives its core funding from Cancer Research UK (FC001045), The UK Medical Research Council (FC001045) and the Wellcome Trust (FC001045).
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