Inducible Expression of Leukemia-Associated Shp2 (Ptpn11) Affects Multiple Stages of Hematopoiesis and Causes a Fatal Myeloproliferative Disorder (MPD) in Mice

Abstract

Germ-line mutations in ptpn11, which encodes the protein tyrosine phosphatase Shp2, cause ∼50% of Noonan Syndrome (NS), which is associated with an increased risk of juvenile myelomonocytic leukemia (JMML). Somatic Shp2 mutations are found in ∼35% of sporadic JMML; nearly all other cases have either activating Ras mutations or homozygous Nf1 deficiency. Shp2 mutations are also found at lower incidence in acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), and B-cell acute lymphoblastic leukemia (B-ALL). NS and leukemia-associated Shp2 mutations can affect the same residue, but result in different substitutions. We showed previously that the leukemia-associated mutants E76K or D61Y, but not wild type (WT), Shp2 transform bone marrow (BM) or fetal liver cells. Transplantation of E76K- or D61Y-transduced BM evoked invasive MPD in ∼60% of recipients, with most dying at 6–7 months. In this model, mutant Shp2 is expressed under retroviral promoter control and the phenotype is variable and incompletely penetrant. To assess the effect of expressing a leukemogenic Shp2 allele under endogenous promoter control, we generated knock-in mice that can conditionally express Shp2D61Y (LSL-Shp2D61Y). Global expression of the D61Y heterozygous allele was embryonic lethal. Post-natal expression of D61Y, induced by treating Mx-1 Cre; LSL-Shp2D61Y mice with pI-pC, evoked fatal MPD with ∼50% mice dying within 5–7 months. Mutant mice showed a marked increase in WBC, expansion of the Gr-1+/Mac-1+population in BM and spleen, and histopathological evidence of infiltrating MPD. Unlike in our retroviral transduction/transplant model (and similar to JMML patients), most induced LSL-Shp2D61Y mice were also anemic. Mutant mice exhibited a marked depletion of quiescent LSK (Lin−Sca1+cKit+) cells in the BM, with a concomitant increase in LSK cells in the spleen. Interestingly, Shp2 mutant-expressing BM failed to promote long term reconstitution in BM transplant assays. Cells from mutant spleens did have some long term multi-lineage reconstitution activity, but the level was substantially less than predicted by their LSK content. Moreover, no transplant recipients develop MPD. In contrast to these effects on the stem cell compartment, D61Y directly allowed cytokine-independent differentiation of CMP and GMP in vitro. Macrophages from mutant mice showed enhanced GM-CSF-mediated proliferation and ERK activation. Our results showed that expression of leukemia-associated Shp2 at endogenous levels is sufficient to evoke MPD and has cell type-specific effect on different stages of hematopoiesis.