Evasion of the p53 tumour surveillance network by tumour-derived MYC mutants

Abstract

The c-Myc oncoprotein promotes proliferation and apoptosis, such that mutations that disable apoptotic programmes often cooperate with MYC during tumorigenesis. Here we report that two common mutant MYC alleles derived from human Burkitt's lymphoma uncouple proliferation from apoptosis and, as a result, are more effective than wild-type MYC at promoting B cell lymphomagenesis in mice. Mutant MYC proteins retain their ability to stimulate proliferation and activate p53, but are defective at promoting apoptosis due to a failure to induce the BH3-only protein Bim (a member of the B cell lymphoma 2 (Bcl2) family) and effectively inhibit Bcl2. Disruption of apoptosis through enforced expression of Bcl2, or loss of either Bim or p53 function, enables wild-type MYC to produce lymphomas as efficiently as mutant MYC. These data show how parallel apoptotic pathways act together to suppress MYC-induced transformation, and how mutant MYC proteins, by selectively disabling a p53-independent pathway, enable tumour cells to evade p53 action during lymphomagenesis.

Figure 1. Tumour-derived MYC mutants show enhanced oncogenicity in vivo

a, Kaplan–Meier curve showing survival at various times after adoptive transfer. b, In vivo GFP imaging shows disseminated lymphomas in a mouse 60 days after reconstitution with HSCs transduced with P57S, but not wild-type MYC. c, Haematoxylin/eosin staining and immunohistochemical staining for B220 (a B cell lineage marker) of lymph node and liver sections of animals harbouring wild-type MYC and P57S lymphomas, showing an aggressive B cell disease with perivascular infiltration of B220-positive tumour cells into the liver.

Figure 2. Wild-type and mutantMYC show apoptotic, but not proliferative, differences in vivo

a, Flow cytometry showing BrdU incorporation in GFP-positive bone marrow cells 24 days after reconstitution. Data are representative of three independent experiments. b, Kaplan–Meier curve showing survival following adoptive transfer with HSCs co-infected with the indicated MYC-expressing retroviruses and a Bcl2-expressing retrovirus. c, In vivo GFP imaging showing disseminated lymphomas in mice 35 days after adoptive transfer with HSCs co-transduced with wild-type or mutant MYC and Bcl2. d, Representative histograms of DNA content of wild-type MYC/Bcl2 and mutant MYC/Bcl2 lymphomas.

Figure 3. Impaired Bim induction contributes to the increased oncogenicity of mutant MYC alleles

a, b, Western blot analysis showing ARF and p53 levels (a) or Bim levels (b) in MEFs stably infected with control (labelled C), wild-type MYC or mutant MYC vectors. The long (BimL) and extra long (BimEL) isoforms of Bim are shown. c,Western blot analysis of HSCs using antibodies against the indicated proteins. d, Kaplan–Meier curve showing free–free survival following adoptive transfer with Bim^−/− HSCs infected with the indicated MYC-expressing retroviruses. e, In vivo GFP imaging showing disseminated lymphomas in mice 34 days after adoptive transfer with Bim^−/− HSCs transduced with mutant or wild-type MYC. f,Wild type (Bcl2^+/+) and Bcl2^−/− MEFs transduced with the indicated retroviruses were incubated in low serum and viability was assessed by trypan blue exclusion. Data represent the mean ± standard deviation of three independent experiments.

Figure 4. Impact of mutations in MYC on p53 tumour suppressor action and Bim induction in human Burkitt’s lymphomas

a, Kaplan–Meier curve showing mouse survival after adoptive transfer with p53^+/− HSCs infected with the indicated MYC retroviral constructs. Inset, Representative allele-specific PCR assay showing retention of the wild-type p53 allele in lymphomas produced by mutant but not wild-type MYC. b, Kaplan–Meier curve showing mouse survival after adoptive transfer with p53^−/− HSCs infected as in a. Inset, western blot analysis of representative lymphomas harbouring the indicated MYC allele. c, Bim expression in DLBCL and wild-type MYC-expressing and mutant MYC-expressing Burkitt’s lymphomas as assessed by immunohistochemisty using an anti-Bim antibody. The frequency of Bim-positive tumours in each category is indicated.