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

Abstract

Using conditional knock-in mouse models, we and others have shown that despite the very high sequence identity between Nras and Kras proteins, oncogenic Kras displays a much stronger leukemogenic activity than oncogenic Nras in vivo. In this manuscript, we will summarize our recent work of characterizing wild-type Kras function in adult hematopoiesis and in oncogenic Kras-induced leukemogenesis. We attribute the strong leukemogenic activity of oncogenic Kras to 2 unique aspects of Kras signaling. First, Kras is required in mediating cell type-and cytokine-specific ERK1/2 signaling. Second, oncogenic Kras, but not oncogenic Nras, induces hyperactivation of wild-type Ras, which significantly enhances Ras signaling in vivo. We will also discuss a possible mechanism that mediates oncogenic Krasevoked hyperactivation of wild-type Ras and a potential approach to down-regulate oncogenic Kras signaling. In mammals, there are 3 Ras genes (Hras, Nras, and Kras) encoding 4 homologous 21kD proteins: Hras, Nras, Kras4A and Kras4B. 1 The first 85 amino-terminal residues are identical and the middle 78 amino acids share an 85% -90% identity among all Ras isoforms, while the last 25 amino acids at the carboxyl-terminus are highly variable. 2 In haematopoietic neoplasms, oncogenic mutations in the NRAS and KRAS genes are common, but rare in the HRAS gene. 2 Despite the high similarity in protein sequences and largely overlapping expression patterns, accumulating evidence suggest that Kras G12D/C expressed from its endogenous locus demonstrates much stronger leukemogenic activity than oncogenic Nras alleles (including Nras G12D alleles) (Fig. 1). Using conditional knock-in mouse models, we and others characterized multiple oncogenic Ras alleles in a pure C57BL/6 background, including the weak Nras G12D/ C , the intermediate Nras Q61R/C and Nras G12D/G12D , and the strong Kras G12D/C . 3-8 All of these models involve a conditional knock-in oncogenic Ras allele and the interferon-inducible Mx1-Cre. Upon polyinosinic-polycytidylic acid (pI-pC) injections to induce the expression of Cre and subsequently the oncogenes, Kras G12D/C mice die rapidly, while Nras G12D/G12D and Nras G12D/C mice show incrementally prolonged survival (Fig. 1A). To eliminate the impact of acute interferon signaling on animal survival, we also took advantage of the leaky expression of Mx1-Cre over the time. Again, the non-pIpC treated Kras G12D/C mice display a significantly shorter survival than the corresponding Nras G12D/G12D mice (Fig. 1B). These results clearly demonstrate that oncogenic Kras is a much more potent oncogene than oncogenic Nras in leukemogenesis.To decipher the unique function of oncogenic Kras signaling in leukemogenesis, we first investigated how loss of wild-type (WT) Kras impacts on adult hematopoiesis. 9 We found that loss of Kras leads to greatly reduced TPO signaling in haematopoietic stem cells (HSCs), while SCF-evoked ERK1/2 activation is not affected. The compromised TPO signaling is associated with reduced long term-and i...

Expression of Nras Q61R and MYC transgene in germinal center B cells induces a highly malignant multiple myeloma in mice

“…Despite the high degree of sequence identity between Kras and Nras proteins (.80%), we and others observed that oncogenic Kras is a much more potent oncogene than oncogenic Nras in myeloid diseases, demonstrated by earlier disease onset, more severe phenotypes, and shorter survival in Kras G12D/1 mice compared with Nras G12D/1 , Nras G12D/G12D , and Nras Q61R/1 mice (reviewed in Chang et al 48 ). Interestingly, when activated throughout the hematopoietic system, Kras G12D/1 mice never developed B-cell malignancies whereas a fraction of oncogenic Nras mice did.…”

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

“…Although typically secondary events late in leukemogenesis, 51 mutations in the RAS pathway are found in AML in general and at a high frequency in MLLr leukemia (;10% NRAS and KRAS mutations), 52 and they play a role in disease pathogenesis. 53,54 In a negative-selection screen of epigenetic regulators in mouse MLL-AF9 leukemia cells overexpressing an activating form of Ras, NRas G12D , Shi et al 26 did not observe depletion of JmjC sgRNAs e20.1, e21.1, e22.1, e23.1, and e23.2. RAS mutations are also found in common AML cell lines, including NOMO1 (KRAS G13D ) and THP1 (NRAS G12D ; Broad Institute Cancer Cell Line Encyclopedia).…”

Unique dependence on Sos1 in KrasG12D-induced leukemogenesis

“…Despite their similar expression patterns and high protein sequence similarities, we found that Kras G12D is a much more potent oncogene than oncogenic Nras; expression of Kras G12D , through either pI-pC induction or leaky expression of Mx1-Cre, drives an acute myeloproliferative neoplasm (MPN) and leads to much earlier lethality than oncogenic Nras in mice (reviewed in Chang et al 5 ). Consistently, juvenile myelomonocytic leukemia with heterozygous somatic KRAS mutations are particularly aggressive and generally require an urgent bone marrow transplantation therapy.…”