We previously found that when transplanted with 2.5 × 105
bone marrow cells (CD45.2+
) isolated from Nras G12D/G12D mice along with an equal number of wild-type competitor cells (CD45.1+
), 0 out of 64 recipient mice developed a sustained MPD.1
Moreover, we noticed that the transient MPD phenotypes always diminished after Nras G12D/G12D HSC activity disappeared as demonstrated by the disappearance of donor-derived CD45.2
cells in peripheral blood over time (data not shown). Thus, we hypothesized that MPD phenotypes are maintained by genetically altered HSCs and Nras G12D/G12D signaling might substantially alter HSC behaviors (e.g., induce their proliferative exhaustion) so that these HSCs no longer sustain MPD phenotypes to a lethal stage in recipient mice. To test this hypothesis, we increased donor cell number to 1 × 106
Nras G12D/G12D bone marrow cells with an equal number of wild-type competitor cells. If our hypothesis is correct, the increased number of Nras G12D/G12D HSCs may be able to sustain the development of a lethal MPD in recipient mice.
At this higher dose of HSCs, all the recipient mice died of lethal hematopoietic malignancies (n = 18) 4–8.5 mo after transplant (). This disease latency is not significantly different from that reported at a lower cell dose.1
We analyzed hematopoietic phenotypes in diseased animals. Fourteen mice developed MPD (), similar to the chronic MPD developed in recipient mice transplanted with Nras G12D/+ bone marrow cells (). Clearly, at its endogenous level, Nras G12D/G12D does not promote the transformation of chronic MPD to acute myeloid leukemia. It does, however, significantly shorten the disease latency compared with Nras G12D/+ (6–24 mo after transplant).1
Four recipient mice developed a thymic T-cell disease closely resembling TALL (). Interestingly, unlike the TALL developed in recipient mice with a lower cell dose, the TALL developed at a higher cell dose were CD44 negative (). Three recipient mice developed a B-cell disease in thymus and lymph nodes, closely resembling acute B-cell lymphoblastic leukemia/lymphoma (BALL) ( and E
). This disease has not been found in recipient mice transplanted with either Nras G12D/+ cells4
or Nras G12D/G12D cells at a lower cell dose,1
suggesting that BALL requires both Nras G12D/G12D signaling and sustained HSC activity. We examined the immunophenotypes of the malignant B cells and found that they were CD43+/−
(data not shown). The development of BALL is not a total surprise as we previously observed abnormal B-cell development in 1 out of 10 recipient mice transplanted with Kras G12D cells.5
However, this mouse died of a lethal MPD closely resembling juvenile myelonomocytic leukemia and TALL ~3.5 mo after transplant and did not develop a lethal B-cell disease in a timely manner. In contrast, recipient mice transplanted with Nras G12D/G12D cells developed a lethal MPD and/or TALL significantly more slowly, which allowed BALL development. It is notable that some of the MPD animals simultaneously developed TALL or BALL.
Figure 2 Recipient mice transplanted with a higher dose of Nras G12D/G12D cells develop lethal malignancies in several lineages of hematopoietic cells. Lethally irradiated mice (CD45.1+) were transplanted with 1 × 106 total bone marrow cells of Nras G12D/G12D (more ...)
The development of highly penetrant MPD in recipient mice transplanted with a higher dose of Nras G12D/G12D cells supports our second hypothesis. Currently, we are actively investigating how Nras G12D/G12D signaling affects HSC function.