1,5 p53 loss of heterozygosity (p53LOH) is frequently observed in Li-Fraumeni syndrome (LFS) patients who carry a mutant (Mut) p53 germ-line mutation. Here, we focused on elucidating the link between p53LOH and tumor development in stem cells (SCs). Although adult mesenchymal stem cells (MSCs) robustly underwent p53LOH, p53LOH in induced embryonic pluripotent stem cells (iPSCs) was significantly attenuated. Only SCs that underwent p53LOH induced malignant tumors in mice. These results may explain why LFS patients develop normally, yet acquire tumors in adulthood. Surprisingly, an analysis of single-cell sub-clones of iPSCs, MSCs and ex vivo bone marrow (BM) progenitors revealed that p53LOH is a bi-directional process, which may result in either the loss of wild-type (WT) or Mut p53 allele. Interestingly, most BM progenitors underwent Mutp53LOH. Our results suggest that the bi-directional p53LOH process may function as a cell-fate checkpoint. The loss of Mutp53 may be regarded as a DNA repair event leading to genome stability. Indeed, gene expression analysis of the p53LOH process revealed upregulation of a specific chromatin remodeler and a burst of DNA repair genes. However, in the case of loss of WTp53, cells are endowed with uncontrolled growth that promotes cancer.

Emerging evidence suggests that immunological mechanisms underlie metabolic control of adipose tissue. Here, we have shown the regulatory impact of a rare subpopulation of dendritic cells, rich in perforin-containing granules (perf-DCs). Using bone marrow transplantation to generate animals selectively lacking perf-DCs, we found that these chimeras progressively gained weight and exhibited features of metabolic syndrome. This phenotype was associated with an altered repertoire of T cells residing in adipose tissue and could be completely prevented by T cell depletion in vivo. A similar impact of perf-DCs on inflammatory T cells was also found in a well-defined model of multiple sclerosis, experimental autoimmune encephlalomyelitis (EAE). Thus, perf-DCs probably represent a regulatory cell subpopulation critical for protection from metabolic syndrome and autoimmunity.

Mesenchymal stromal cells (MSC) are used extensively in clinical trials; however, the possibility that MSCs have a potential for malignant transformation was raised. We examined the genomic stability versus the tumorforming capacity of multiple mouse MSCs. Murine MSCs have been shown to be less stable and more prone to malignant transformation than their human counterparts. A large series of independently isolated MSC populations exhibited low tumorigenic potential under syngeneic conditions, which increased in immunocompromised animals. Unexpectedly, higher ploidy correlated with reduced tumor-forming capacity. Furthermore, in both cultured MSCs and primary hepatocytes, polyploidization was associated with a dramatic decrease in the expression of the long noncoding RNA H19. Direct knockdown of H19 expression in diploid cells resulted in acquisition of polyploid cell traits. Moreover, artificial tetraploidization of diploid cancer cells led to a reduction of H19 levels, as well as to an attenuation of the tumorigenic potential. Polyploidy might therefore serve as a protective mechanism aimed at reducing malignant transformation through the involvement of the H19 regulatory long noncoding RNA. Cancer Res; 72(24); 6403-13. Ó2012 AACR.

Umbilical cord blood (UCB) is a good source of hematopoietic progenitors with increasing implementation in the clinical transplant setting. This study evaluates the molecular mechanisms of progenitor resistance to apoptosis triggered by Fas cross-linking. CD34(+) and lineage-negative progenitors survive short-term ex vivo incubation and are not induced into apoptosis by Fas cross-linking. Furthermore, brief exposure of UCB cells to Fas-ligand for 24-48 h does not impair quantitative severe combine immune deficiency (SCID) reconstitution activity and appears to foster myelomonocyte reconstitution. The transcriptome of Fas receptor-positive CD34(+) cells that survived an apoptotic challenge showed significant transcriptional upregulation of caspase-8, mucosa-associated lymphoid tissue lymphoma translocation gene-1 (MALT1), HtrA2, and GSK3β in addition to higher levels of c-FLICE inhibitory protein (FLIP), Bcl-2, and cytosolic inhibitor of apoptosis protein (cIAP) in all Fas-positive cells. Most prominent is the transcriptional upregulation of several key components the NFκB1 pathway including the membrane receptors TGF-β, interleukin-1 (IL-1), and TCR, the associated factor TNF receptor-associated factor-6 (TRAF6), and the converting enzymes TGF-β-activated kinase-1 (TAK1), double-stranded RNA-activated protein kinase (PKR), and α-catalytic subunit of IκB kinase (IKKα), that promote activation and nuclear translocation of this transcription factor. These data indicate that hematopoietic progenitors are not insensitive to apoptosis but are actively shielded from the extrinsic and intrinsic apoptotic pathways. This may occur through inherent transcriptional upregulation of the entire NFκB pathway in the presence of competent apoptotic signaling.