Real-time quantitative PCR (qPCR) is a well-established tool for minimal residual disease (MRD) detection in mature lymphoid malignancies. Despite remarkable sensitivity and specificity, qPCR has some limitations, particularly in the need for a reference standard curve, based on target serial dilutions. In this study, we established droplet digital PCR (ddPCR) for MRD monitoring in multiple myeloma, mantle cell lymphoma, and follicular lymphoma and compared it head-to-head with qPCR. We observed that ddPCR has sensitivity, accuracy, and reproducibility comparable with qPCR. We then compared the two approaches in 69 patients with a documented molecular marker at diagnosis (18 multiple myelomas, 21 mantle cell lymphomas assessed with the immunoglobulin gene rearrangement, and 30 follicular lymphomas with the use of the BCL2/immunoglobulin gene major breakpoint region rearrangement). ddPCR was successful in 100% of cases, whereas qPCR failed to provide a reliable standard curve in three patients. Overall, 222 of 225 samples were evaluable by both methods. The comparison highlighted a good concordance (r = 0.94, P < 0.0001) with 189 of 222 samples (85.1%; 95% CI, 80.4%-89.8%) being fully concordant. We found that ddPCR is a reliable tool for MRD detection with greater applicability and reduced labor intensiveness than qPCR. It will be necessary to authorize ddPCR as an outcome predictor tool in controlled clinical settings and multilaboratory standardization programs.

Mono-ADP-ribosylation is a reversible posttranslational modification that modulates the function of target proteins. The enzymes that catalyze this reaction in mammalian cells are either bacterial pathogenic toxins or endogenous cellular ADP-ribosyltransferases. For the latter, both the enzymes and their targets have largely remained elusive, mainly due to the lack of specific techniques to study this reaction. The recent discovery of the macro domain, a protein module that interacts selectively with ADP-ribose, prompted us to investigate whether this interaction can be extended to the identification of ADP-ribosylated proteins. Here, we report that macro domains can indeed be used as selective baits for high-affinity purification of mono-ADP-ribosylated proteins, which can then be identified by mass spectrometry. Using this approach, we have identified a series of cellular targets of ADP-ribosylation reactions catalyzed by cellular ADP-ribosyltransferases and toxins. These proteins include most of the known targets of ADP-ribosylation, indicating the validity of this method, and a large number of other proteins, which now need to be individually validated. This represents an important step toward the discovery of new ADP-ribosyltransferase targets and an understanding of the physiological role and the pharmacological potential of this protein modification.ADP-ribosylation ͉ ADP-ribosyltransferase ͉ NAD ͉ toxin ͉ posttranslational modification

The mono(ADP‐ribosyl)ation reaction is a post‐translational modification that is catalysed by both bacterial toxins and eukaryotic enzymes, and that results in the transfer of ADP‐ribose from βNAD+ to various acceptor proteins. In mammals, both intracellular and extracellular reactions have been described; the latter are due to glycosylphosphatidylinositol‐anchored or secreted enzymes that are able to modify their targets, which include the purinergic receptor P2X7, the defensins and the integrins. Intracellular mono(ADP‐ribosyl)ation modifies proteins that have roles in cell signalling and metabolism, such as the chaperone GRP78/BiP, the β‐subunit of heterotrimeric G‐proteins and glutamate dehydrogenase. The molecular identification of the intracellular enzymes, however, is still missing. A better molecular understanding of this reaction will help in the full definition of its role in cell physiology and pathology.

We have recently demonstrated that the beta subunit of the heterotrimeric G-proteins is endogenously mono-ADP-ribosylated in intact cells. The modified betagamma heterodimer loses its ability to inhibit calmodulin-stimulated type 1 adenylate cyclase and, remarkably, is de-ADP-ribosylated by a cytosolic hydrolase that completes an ADP-/de-ADP-ribosylation cycle of potential physiological relevance. In the present study, we show that this ADP-ribosylation might indeed be a general mechanism for termination of betagamma signalling, since the ADP-ribosylated betagamma subunit is also unable to activate both phosphoinositide 3-kinase-gamma and phospholipase C-beta2. Moreover, we show that beta subunit ADP-ribosylation is induced by G-protein-coupled receptor activation, since hormone stimulation of Chinese-hamster ovary plasma membranes leads to increases in beta subunit labelling. This occurs when betagamma is in its active heterodimeric conformation, since full inhibition of this modification can be achieved by binding of GDP-alphai3 to the betagamma heterodimer. Taken together, these findings delineate a pathway that arises from the activation of a G-protein-coupled receptor and leads to the inhibition of betagamma activity through its reversible mono-ADP-ribosylation.

The MET oncogene is aberrantly overexpressed in human osteosarcomas. We have previously converted primary cultures of human bone-derived cells into osteosarcoma cells by overexpressing MET. To determine whether MET transforms mesenchymal stem cells or committed progenitor cells, here we characterize distinct MET overexpressing osteosarcoma (MET-OS) clones using genome-wide expression profiling, cytometric analysis, and functional assays. All the MET-OS clones consistently display mesenchymal and stemness markers, but not most of the mesenchymal-stem cell-specific markers. Conversely, the MET-OS clones express genes characteristic of early osteoblastic differentiation phases, but not those of late phases. Profiling of mesenchymal stem cells induced to differentiate along osteoblast, adipocyte, and chondrocyte lineages confirms that MET-OS cells are similar to cells at an initial phase of osteoblastic differentiation. Accordingly, MET-OS cells cannot differentiate into adipocytes or chondrocytes, but can partially differentiate into osteogenic-matrix-producing cells. Moreover, in vitro MET-OS cells form self-renewing spheres enriched in cells that can initiate tumors in vivo. MET kinase inhibition abrogates the self-renewal capacity of MET-OS cells and allows them to progress toward osteoblastic differentiation. These data show that MET initiates the transformation of a cell population that has features of osteo-progenitors and suggest that MET regulates self-renewal and lineage differentiation of osteosarcoma cells. ß