The mystery of oncogenic KRAS: Lessons from studying its wild-type counter part

Nras Q61R/+ and Kras-/- cooperate to downregulate Rasgrp1 and promote lympho-myeloid leukemia in early T-cell precursors

Kras^−/−; Nras^Q61R/+ mice develop early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL.

We identify Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is an aggressive subtype of T-cell ALL. Although genetic mutations hyperactivating cytokine receptor/Ras signaling are prevalent in ETP-ALL, it remains unknown how activated Ras signaling contributes to ETP-ALL. Here, we find that in addition to the frequent oncogenic RAS mutations, wild-type (WT) KRAS transcript level was significantly downregulated in human ETP-ALL cells. Similarly, loss of WT Kras in Nras^Q61R/+ mice promoted hyperactivation of extracellular signal-regulated kinase (ERK) signaling, thymocyte hyperproliferation, and expansion of the ETP compartment. Kras^−/−; Nras^Q61R/+ mice developed early onset of T-cell malignancy that recapitulates many biological and molecular features of human ETP-ALL. Mechanistically, RNA-sequencing analysis and quantitative proteomics study identified that Rasgrp1, a Ras guanine nucleotide exchange factor, was greatly downregulated in mouse and human ETP-ALL. Unexpectedly, hyperactivated Nras/ERK signaling suppressed Rasgrp1 expression and reduced Rasgrp1 level led to increased ERK signaling, thereby establishing a positive feedback loop to augment Nras/ERK signaling and promote cell proliferation. Corroborating our cell line data, Rasgrp1 haploinsufficiency induced Rasgrp1 downregulation and increased phosphorylated ERK level and ETP expansion in Nras^Q61R/+ mice. Our study identifies Rasgrp1 as a negative regulator of Ras/ERK signaling in oncogenic Nras-driven ETP-like leukemia.

Expression of NrasQ61R and MYC transgene in germinal center B cells induces a highly malignant multiple myeloma in mice

NRAS Q61 mutations are prevalent in advanced/relapsed multiple myeloma (MM) and correlate with poor patient outcomes. Thus, we generated a novel MM model by conditionally activating expression of endogenous NrasQ61R and an MYC transgene in germinal center (GC) B cells (VQ mice). VQ mice developed a highly malignant MM characterized by a high proliferation index, hyperactivation of extracellular signal-regulated kinase and AKT signaling, impaired hematopoiesis, widespread extramedullary disease, bone lesions, kidney abnormalities, preserved programmed cell death protein 1 and T-cell immunoreceptor with immunoglobulin and immunoreceptor tyrosine-based inhibition motif domain immune-checkpoint pathways, and expression of human high-risk MM gene signatures. VQ MM mice recapitulate most of the biological and clinical features of human advanced/high-risk MM. These MM phenotypes are serially transplantable in syngeneic recipients. Two MM cell lines were also derived to facilitate future genetic manipulations. Combination therapies based on MEK inhibition significantly prolonged the survival of VQ mice with advanced-stage MM. Our study provides a strong rationale to develop MEK inhibition-based therapies for treating advanced/relapsed MM.

Critical role of Jumonji domain of JMJD1C in MLL-rearranged leukemia

JMJD1C, a member of the lysine demethylase 3 family, is aberrantly expressed in mixed lineage leukemia (MLL) gene-rearranged (MLLr) leukemias. We have shown previously that JMJD1C is required for self-renewal of acute myeloid leukemia (AML) leukemia stem cells (LSCs) but not normal hematopoietic stem cells. However, the domains within JMJD1C that promote LSC self-renewal are unknown. Here, we used clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein-9 nuclease (Cas9) negative-selection screening and identified a requirement for the catalytic Jumonji (JmjC) domain and zinc finger domain for leukemia cell survival in vitro and in vivo. In addition, we found that histone H3 lysine 36 methylation (H3K36me) is a marker for JMJD1C activity at gene loci. Moreover, we performed single cell transcriptome analysis of mouse leukemia cells harboring a single guide RNA (sgRNA) against the JmjC domain and identified increased activation of RAS/MAPK and the JAK-STAT pathway in cells harboring the JmjC sgRNA. We discovered that upregulation of interleukin 3 (IL-3) receptor genes mediates increased activation of IL-3 signaling upon JMJD1C loss or mutation. Along these lines, we observed resistance to JMJD1C loss in MLLr AML bearing activating RAS mutations, suggesting that RAS pathway activation confers resistance to JMJD1C loss. Overall, we discovered the functional importance of the JMJD1C JmjC domain in AML leukemogenesis and a novel interplay between JMJD1C and the IL-3 signaling pathway as a potential resistance mechanism to targeting JMJD1C catalytic activity.

Unique dependence on Sos1 in Kras G12D -induced leukemogenesis

We and others have previously shown that Kras ^G12D is a much more potent oncogene than oncogenic Nras in hematological malignancies. We attributed the strong leukemogenic activity of Kras^G12D at least partially to its unique capability to hyperactivate wild-type (WT) Nras and Hras. Here, we report that Sos1, a guanine nucleotide exchange factor, is required to mediate this process. Sos1 is overexpressed in Kras ^G12D/+ cells, but not in Nras ^Q61R/+ and Nras ^G12D/+ cells. Kras^G12D proteins form a complex with Sos1 in vivo. Sos1 deficiency attenuates hyperactivation of WT Nras, Hras, and the downstream ERK signaling in Kras ^G12D/+ cells. Thus, Sos1 deletion ameliorates oncogenic Kras-induced myeloproliferative neoplasm (MPN) phenotypes and prolongs the survival of Kras ^G12D/+ mice. In contrast, Sos1 is dispensable for hyperactivated granulocyte-macrophage colony-stimulating factor signaling in Nras ^Q61R/+ cells, and Sos1 ^-/- does not affect MPN phenotypes in Nras ^Q61R/+ mice. Moreover, the survival of Kras ^G12D/+ ; Sos1 ^-/- recipients is comparable to that of Kras ^G12D/+ recipients treated with combined MEK and JAK inhibitors. Our study suggests that targeting Sos1-oncogenic Kras interaction may improve the survival of cancer patients with KRAS mutations.