Dnmt3a deletion cooperates with the Flt3/ITD mutation to drive leukemogenesis in a murine model.

Internal tandem duplications of the juxtamembrane domain of FLT3 (FLT3/ITD) are among the most common mutations in Acute Myeloid Leukemia (AML). Resulting in constitutive activation of the kinase, FLT3/ITD portends a particularly poor prognosis, with reduced overall survival and increased rates of relapse. We previously generated a knock-in mouse, harboring an internal tandem duplication at the endogenous Flt3 locus, which develops a fatal myeloproliferative neoplasm (MPN), but fails to develop acute leukemia, suggesting additional mutations are necessary for transformation. To investigate the potential cooperativity of FLT3/ITD and mutant DNMT3A, we bred a conditional Dnmt3a knockout to a substrain of our Flt3/ITD knock-in mice, and found deletion of Dnmt3a significantly reduced median survival of Flt3ITD/+ mice in a dose dependent manner. As expected, pIpC treated Flt3ITD/+ mice solely developed MPN, while Flt3ITD/+;Dnmt3af/f and Flt3ITD/+;Dnmt3af/+ developed a spectrum of neoplasms, including MPN, T-ALL, and AML. Functionally, FLT3/ITD and DNMT3A deletion cooperate to expand LT-HSCs, which exhibit enhanced self-renewal in serial re-plating assays. These results illustrate that DNMT3A loss cooperates with FLT3/ITD to generate hematopoietic neoplasms, including AML. In combination with FLT3/ITD, homozygous Dnmt3a knock-out results in reduced time to disease onset, LT-HSC expansion, and a higher incidence of T-ALL compared with loss of just one allele. The co-occurrence of FLT3 and DNMT3A mutations in AML, as well as subsets of T-ALL, suggests the Flt3ITD/+;Dnmt3af/f model may serve as a valuable resource for delineating effective therapeutic strategies in two clinically relevant contexts.

Increased Skin Tumor Incidence and Keratinocyte Hyper-Proliferation in a Mouse Model of Down Syndrome.

Down syndrome (DS) is a genetic disorder caused by the presence of an extra copy of human chromosome 21 (Hsa21). People with DS display multiple clinical traits as a result of the dosage imbalance of several hundred genes. While many outcomes of trisomy are deleterious, epidemiological studies have shown a significant risk reduction for most solid tumors in DS. Reduced tumor incidence has also been demonstrated in functional studies using trisomic DS mouse models. Therefore, it was interesting to find that Ts1Rhr trisomic mice developed more papillomas than did their euploid littermates in a DMBA-TPA chemical carcinogenesis paradigm. Papillomas in Ts1Rhr mice also proliferated faster. The increased proliferation was likely caused by a stronger response of trisomy to TPA induction. Treatment with TPA caused hyperkeratosis to a greater degree in Ts1Rhr mice than in euploid, reminiscent of hyperkeratosis seen in people with DS. Cultured trisomic keratinocytes also showed increased TPA-induced proliferation compared to euploid controls. These outcomes suggest that altered gene expression in trisomy could elevate a proliferation signalling pathway. Gene expression analysis of cultured keratinocytes revealed upregulation of several trisomic and disomic genes may contribute to this hyperproliferation. The contributions of these genes to hyper-proliferation were further validated in a siRNA knockdown experiment. The unexpected findings reported here add a new aspect to our understanding of tumorigenesis with clinical implications for DS and demonstrates the complexity of the tumor repression phenotype in this frequent condition.

Genomic alterations in myeloid neoplasms with novel, apparently balanced translocations.

Characterization of gross chromosomal rearrangements, particularly translocations in neoplasms, has proven to be valuable in patient management by aiding in diagnosis, defining prognosis, and leading to new therapeutic interventions. In this report, we investigate two apparently balanced translocations, t(6;17)(q23.3;p13.3) and t(2;13)(p21;q14.11), in patients with myeloid neoplasms and uncover concomitant microdeletions associated with the breakpoints. Breakpoint mapping by fluorescence in situ hybridization (FISH) detected deletions at or adjacent to all breakpoints. Subsequently, array comparative genomic hybridization on the 244 K Agilent platform refined the deletion boundaries, revealing a 1.7 Mb deletion directly adjacent to the 6q23.3 breakpoint, and a 562 kb deletion at 17p13.3 in the first case. The second case was found to harbor a 195 kb deletion at 2p21 and a 1.4 Mb deletion distal to the 13q breakpoint at 13q14.3. Additionally, a 133 kb deletion within the breakpoint region at 13q14.11 and a 265 kb deletion proximal to the breakpoint were discovered, neither of which was detected by FISH. Although a gene fusion resulting from either novel rearrangement cannot be determined from these data, formation of a fusion transcript cannot be excluded because the resolution of the techniques used does not allow definite delineation of the breakpoint locations. Although the incidence and clinical relevance of these focal imbalances remains to be evaluated, the cases presented here support high resolution evaluation of presumably balanced rearrangements in neoplasms. Such imbalances may portend important hitherto unrecognized prognostic and diagnostic categories.

Novel SSBP2-JAK2 fusion gene resulting from a t(5;9)(q14.1;p24.1) in pre-B acute lymphocytic leukemia.

Activating mutations in JAK2 are found in virtually all patients with polycythemia vera, and about half of those with essential thrombocythemia and primary myelofibrosis. In addition, less common aberrations (particularly gene fusions) involving JAK2 have been described in acute leukemias. With the advent of JAK2 inhibitor trials in myeloproliferative disorders, tumors with JAK2 mutations or rearrangements have become candidates for targeted therapy. In this report, we identify SSBP2 as a new JAK2 fusion partner in a patient with pre-B cell acute lymphocytic leukemia. This finding adds to the expanding compendium of JAK2 aberrations found in various hematopoietic malignancies, as well as the potential need for a diagnostic FISH analysis in the appropriate clinical setting.