SOHO 2019 | XPO7 as a novel therapeutic target for TP53-AML

During the 7th Society of Hematologic Oncology (SOHO) annual meeting, in Houston, TX, US, a session on acute myeloid leukemia (AML) was held. Yuichiro Semba, Kyushu University Graduate School of Medical Sciences, Fukuoka, JP, presented a talk entitled “CRISPR-Cas9 Screen Identifies XPO7 as a Novel Therapeutic Target for TP53-Mutated AML”.1

AML is a heterogeneous disease characterized by various genetic aberrations, some of which can be prognostic. It has already been established that patients with complex karyotype have poor outcomes.2 Mutations in TP53 are commonly associated with a complex karyotype, with an incidence of 69-78%, are a strong AML risk-factor.1,2 In this pre-clinical genomic study, the investigators sought to identify specific gene targets and pathways that are critical for the survival of TP53-mutated AML cells with the hope of findings novel drug targets. For this, they performed a genome-wide CRISPR-Cas9 screen in AML cell lines in vitro.

AML cell lines
  • All cell lines were established from mice models that were transplanted with bone marrow stem cells harboring the MLL/AF9 leukemia oncogene (AML mice)
  • Leukemic cells were harvested from the AML mice and were established as cell lines with a normal wild-type Trp53 (the mouse homolog of TP53) and normal karyotype
  • Trp53-mutated AML cell lines were established by single-guide RNA targeting of Trp53 in the wild-type lines
Genome-wide CRISPR-Cas9 screen 
  • A CRISPR-Cas9 library screen was used to identify Trp53-dependent genes whose loss was lethal in Trp53-mutated cells but not in wildtype Trp53 AML cells
  • The authors identified XPO7, a nuclear transport receptor gene expressed in leukemias as one of the genes that were dependent on Trp53
  • To further validate this the authors compared the proliferation rate and cell cycle transition of wild-type or Trp53-knockout AML cell lines following XPO7 downregulation in vitro
    • Loss of XPO7 led to a significant decrease in proliferation and cell cycle transition only in the TrpP53-knockout AML cell lines and not the wild-type ones
  • By Western blotting and qPCR, the investigators uncovered that XPO7 retains Trp53 in the nucleus of the Trp53-mutated AML cells
    • This mechanism shows that XPO7 acts as a TP53-dependent tumor suppressor in wild-type AML cells
  • This result was further verified in human TP53-mutated AML cell lines in vitro, with XPO7 knockout being toxic and delaying cell cycle progression
Conclusions 

The investigators identified a synthetic lethal association between expression of XPO7 and TP53 with XPO7 loss leading to the suppression of TP53-mutated AML cell proliferation. Based on these results and the fact that XPO7 mRNA is significantly upregulated in AML patients (The Cancer Genome Atlas database)3 it is likely that the nuclear transport protein XPO7 may serve as a novel therapeutic target for TP53-mutated AML.

References
  1. Semba Y. CRISPR-Cas9 Screen Identifies XPO7 as a Novel Therapeutic Target for TP53-Mutated AML. Oral Abstract AML-231. 2019 Sep 12. SOHO Annual Meeting, Houston, TX, US
  2. Ferrari, A. et al. (2014). Leukemia Associated TP53 Mutations in AML Patients ARE Strongly Associated with Complex Karyotype and Poor Outcome. Blood, 124(21)
  3. The Cancer Genome Atlas (TCGA). https://portal.gdc.cancer.gov [Accessed 08 Oct 2019]
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