Dr. med. Dr. rer. nat. Alpaslan Tasdogan

Dr. med. Dr. rer. nat. Alpaslan Tasdogan
Institut für Immunologie

»DNA Damage-Induced HSPC Malfunction Depends on ROS Accumulation Downstream of IFN-1 Signaling and Bid Mobilization«

Cytogenetic abnormalities are highly characteristic of cancer cells. Also leukemic cells in many hematopoietic malignancies hold various chromosomal aberrations, including deletions, duplications and translocations. These most often lead to dysregulated gene expression profiles. There is ample experimental evidence documenting that these genetic changes contribute to disease initiation, progression and malignant transformation.

Deletions on the long arm of human chromosome 7 or loss of the entire chromosome (7/7qdeletions) are common cytogenetic alterations in myeloid malignancies, often indicating poor prognosis. In 2002, researchers at the University Ulm (Prof. Konstanze Döhner, Innere Medizin III) and UCSF (USA) identified a hitherto unknown gene located within a consensus deletion region. Based on sequence homology, the novel gene was classified as a new member of the “Mixed-Lineage-Leukemia” (MLL)/Trithorax family and designated MLL5. Importantly, mutations and translocations involving any of the previously identified members of this family are well-established contributors to human cancer. The homology to MLL-proteins and the intriguing association of 7/7q-deletions with myeloid malignancy strongly suggest a role for MLL5 in leukemogenesis. However, molecular studies of MLL5 function are scarce and no direct experimental evidence for a pathogenic role of MLL5 has been reported yet.

In my thesis I uncovered a previously unknown phenotype, namely that Mll5−/− mice accumulate DNA breaks and exhibit supra-physiological levels of intracellular reactive oxygen species (ROS). Strikingly, oral administration of the antioxidant ROS-scavenger N-Acetyl-L-Cysteine (NAC) not only restored critical parameters of hematopoietic stem and progenitor cell (HSPC) function but also revealed a marked 50% reduction of perinatal lethality among Mll5−/− pups, highlighting oxidative stress as a major cause of newborn death among offspring lacking Mll5. Global gene expression analysis (Affymetrix) revealed a striking up-regulation of select target genes of type I interferon (IFN-1) signaling in hematopoietic stem cells. Importantly, Mll5−/−Ifnar−/− compound mouse mutants exhibited a significant reduction of ROS levels in HSPCs and a substantial rescue of HSPC defects. In addition, we firmly demonstrated that IFN-1 signaling in HSPCs induces elevated ROS levels in wild-type mice. Our work thus highlights a novel pathogenic pathway linking DNA damage, IFN-1 signaling, ROS induction and HSPC attrition in mice. In another line of investigation, we discovered a striking correlation between elevated ROS and accumulation of the Bcl-2 family member Bid in mitochondria of Mll5−/− HSPCs. Subsequent studies with Bid-deficient mice provided unequivocal genetic evidence for a previously unknown role of Bid in toxic ROS induction. These new findings provide an attractive working hypothesis for molecular studies to understand hematopoietic stem cell defects in several other mouse mutants with elevated levels of ROS. With respect to leukemogenesis, our novel findings are thus not only of high scientific, but also of potentially high clinical relevance.

Figure 1: Graphical abstract showing the main findings of the thesis. 
HSPCs in Mll5-deficient mice accumulate DNA damage and ROS. Further experiments showed that IFN-1-dependent Bid mobilization is key for toxic ROS accumulation. Interestingly, the Ink4a/Arf locus also contributes to ROS production and hematopoietic defects.

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