High incidence, early onset of histiocytic sarcomas in mice with Hertwig's anemia.

OBJECTIVE: Histiocytic sarcoma (HS) is a rare, rapidly disseminated, usually lethal tumor in humans. Treatment specific for HS has not been developed primarily due to deficiencies of appropriate animal models with high incidence/early onset. Mice with Hertwig's anemia (an/an) provide a potential model. METHODS: Here, we compare HS susceptibility in an/an and unaffected control mice maintained on three genetic backgrounds. As a potential therapeutic measure, genetically marked bone marrow is transplanted between high and low susceptibility animals. RESULTS: HS is detected earlier and the overall incidence is 15-fold higher in WBB6F1(F1)-an/an than in F1-+/?, B6-an/an and -+/? mice. Neither WB-an/an nor their normal WB-+/? littermates present with HS. Liver myelopoiesis and aneuploidy coexist with HS but the former is also rampant (33.7% incidence) in HS-free +/? and an/an mice. Marrow transplantation experiments provide evidence that (1) myelopoiesis is associated with HS and (2) early-onset/high-incidence HS is blocked by using late-onset F1-+/+ mice, as either donor or recipient. CONCLUSIONS: Homozygosity for an on an F1 genetic background is essential for high-incidence/early-onset HS; myelopoiesis and HS coexist; and therapeutic transplantation may be feasible.

Accelerated leukemogenesis by truncated CBF beta-SMMHC defective in high-affinity binding with RUNX1.

Dominant RUNX1 inhibition has been proposed as a common pathway for CBF leukemia. CBF beta-SMMHC, a fusion protein in human acute myeloid leukemia (AML), dominantly inhibits RUNX1 largely through its RUNX1 high-affinity binding domain (HABD). However, the type I CBF beta-SMMHC fusion in AML patients lacks HABD. Here, we report that the type I CBF beta-SMMHC protein binds RUNX1 inefficiently. Knockin mice expressing CBF beta-SMMHC with a HABD deletion developed leukemia quickly, even though hematopoietic defects associated with Runx1-inhibition were partially rescued. A larger pool of leukemia-initiating cells, increased MN1 expression, and retention of RUNX1 phosphorylation are potential mechanisms for accelerated leukemia development in these mice. Our data suggest that RUNX1 dominant inhibition may not be a critical step for leukemogenesis by CBF beta-SMMHC.

In vitro hematopoietic differentiation of mouse embryonic stem cells requires the tumor suppressor menin and is mediated by Hoxa9.

Inactivating mutations in the tumor suppressor gene MEN1 cause the inherited cancer syndrome multiple endocrine neoplasia type 1 (MEN1). The ubiquitously expressed MEN1 encoded protein, menin, interacts with MLL (mixed-lineage leukemia protein), and together they are essential components of a multiprotein complex with histone methyl transferase activity. MLL is also essential for hematopoiesis, and plays a critical role in leukemogenesis via epigenetic regulation of Hoxa9 expression that also requires menin. Therefore we chose to explore the role of menin in hematopoiesis. We generated Men1(-/-) embryonic stem (ES) cell lines, and induced them to differentiate in vitro. While these cells were able to form embryoid bodies (EBs) expressing the early markers Flk-1 and c-Kit, their ability to further differentiate into hematopoietic colonies was compromised. The Men1(-/-) ES cells show reduced expression of Hoxa9 that can be recovered by reexpression of Menin. We demonstrate that the block in differentiation of Men1(-/-) ES cell lines can be rescued not only by the expression of menin but also that of Hoxa9. These results suggest that, similar to MLL, menin is required for hematopoiesis, and this requirement may be mediated through regulation of Hoxa9 expression.

Runx1 deficiency predisposes mice to T-lymphoblastic lymphoma.

Chromosomal rearrangements affecting RUNX1 and CBFB are common in acute leukemias. These mutations result in the expression of fusion proteins that act dominant-negatively to suppress the normal function of the Runt-related transcription factor 1 (RUNX)/core binding factor beta (CBFbeta) complexes. In addition, loss-of-function mutations in Runt-related transcription factor 1 (RUNX1) have been identified in sporadic cases of acute myeloid leukemia (AML) and in association with the familial platelet disorder with propensity to develop AML (FPD/AML). In order to examine the hypothesis that decreased gene dosage of RUNX1 may be a critical event in the development of leukemia, we treated chimeric mice generated from Runx1(lacZ/lacZ) embryonic stem (ES) cells that have homozygous disruption of the Runx1 gene with N-ethyl-N-nitrosourea (ENU). We observed an increased incidence of T-lymphoblastic lymphoma in Runx1(lacZ/lacZ) compared with wild-type chimeras and confirmed that the tumors were of ES-cell origin. Our results therefore suggest that deficiency of Runx1 can indeed predispose mice to hematopoietic malignancies.