The molecular mechanisms driving malignant transformation of neuroblastoma are still incompletely understood.

Here, we aim to determine the role of MAP2K7 inactivation in neuroblastoma tumorigenesis, a gene that has not been recognized in the context of this malignancy before.

Our study will likely expand our knowledge on the complex genetic etiology and the molecular pathogenesis of high-risk neuroblastoma, which is prerequisite for developing novel therapeutic strategies against this deadly disease.

Neuroblastoma is a malignant pediatric tumor of the sympathetic nervous system with a broad spectrum of clinical courses, ranging from spontaneous differentiation or regression to fatal progression. We previously discovered that high-risk neuroblastoma is characterized by activation of telomere maintenance mechanisms, which are associated with genomic alterations of MYCN, TERT, or ATRX. In a substantial fraction of high-risk tumors, however, the genes driving malignant transformation have remained unknown.

We recently identified inactivating mutations in MAP2K7, encoding for a kinase of the JNK pathway, in a subset of primary neuroblastomas. Here, we aim to examine the mechanistic relevance of MAP2K7 inactivation in malignant transformation of neuroblastoma. To this end, we will determine whether genomic inactivation of MAP2K7 drives neuroblastoma tumorigenesis in genetically engineered mouse models. We will also generate MAP2K7 knock-out neuroblastoma cell lines to assess the phenotypic consequences of its inactivation in established neuroblastoma models. Finally, we will use both MAP2K7-mutated model systems and primary tumors to gain insights into the pathways by which MAP2K7 inactivation may promote neuroblastoma development.

1. Does MAP2K7 inactivation drive malignant transformation of neuroblastic tumors
2. Which are the phenotypic consequences of MAP2K7 inactivation in neuroblastoma cell line models?
3. Which downstream pathways mediate the transforming effects upon MAP2K7 inactivation?

In our previous work, we have extensively characterized genomic and transcriptomic characteristics of the distinct neuroblastoma subtypes to gain insights into the molecular pathogenesis of this malignancy, to establish biomarkers for risk assessment, and to identify novel therapeutic targets. We discovered that high-risk neuroblastomas are recurrently affected by genomic rearrangements of the TERT (telomerase reverse transcriptase) locus (Peifer et al., Nature 2015).

The rearrangements invariably led to massive induction of TERT expression and telomerase activity, and occurred in mutually exclusive fashion with MYCN amplification and ATRX mutations. We also found that telomerase activity was strongly up-regulated in MYCN-amplified tumors, while the ALT pathway was activated in neuroblastomas bearing ATRX mutations. In contrast to high-risk tumors, telomere maintenance mechanisms were completely absent in low-risk neuroblastomas. Based on these observations, we hypothesized that activation of telomere maintenance mechanisms is a key feature of high-risk neuroblastoma, while low-risk tumors are defined by the lack of such alterations.

Telomere maintenance is essential for cancer cells to overcome replicative senescence by maintaining telomere length above a critical threshold, thus providing the capacity to divide infinitely. Our data suggest that low-risk neuroblastoma presumably lack immortal proliferation capacity, which may account for their propensity to undergo spontaneous regression or terminal differentiation.

To test our hypothesis, we determined alterations associated with telomerase activation and ALT in a cohort of 208 primary neuroblastomas (Ackermann et al., Science 2018).

We also wondered how mutations in genes of the RAS or p53 pathway, which occurred in 18% of the cases, affect the clinical phenotype. We observed that telomere maintenance mechanisms were strongly associated with poor survival, whereas patients whose tumors lacked such mechanisms had excellent outcome.

In addition, we noticed that the prognostic significance of RAS and p53 pathway mutations was strictly dependent on the presence of telomere maintenance mechanisms. Patients whose tumors were telomere maintenance-positive had dramatically inferior outcome when RAS/p53 pathway mutations were present, as compared to patients without such alterations. By contrast, patients whose tumors lacked telomere maintenance mechanisms had excellent outcome, and spontaneous regression or differentiation occurred both in the presence and absence of RAS/p53 pathway mutations.

Together, our data suggest a clear pathogenetic hierarchy of genetic alterations in neuroblastoma development, with telomere maintenance being the key characteristic of high-risk disease. Our studies thus provide a novel, clinically relevant classification of neuroblastoma based on a mechanistic understanding of the underlying tumor biology (Figure 1).