MAPK pathway modulation and its implications in KRAS mutant lung adenocarcinoma
Sergio de Hita Román
Centro de Investigación del Cáncer (Unviersidad de Salamanca-CSIC)
In lung adenocarcinoma (LUAD), virtually all genetic alterations driving tumor progression are directly linked to an activation of the RAS-MAPK pathway. However, excessive MAPK signaling is detrimental. Through uncertain mechanisms, the tumor must undergo a process of selection for a set of both positive and negative regulators to keep balance, and this regulatory system has an unknown clinical impact.In order to elucidate how MAPK output shapes tumor progression, we refined a specific transcriptional signature to assess MAPK activity levels in KRAS mutant human LUAD. Strikingly, high MAPK tumors had a better prognosis compared to those with lower pathway intensity, implying moderate MAPK activation may confer greater aggressiveness, while highly active lesions are associated with stress phenotypes.Paradoxically, DUSP4, a dual specificity phosphatase that forms a transcriptional negative feedback loop with ERK, exhibited anti-correlation with pathway activation in KRAS-mutant patients. We then discovered that DUSP4 was submitted to recurrent copy number variations (CNVs). Low MAPK patients showed increased copy number gains of DUSP4, while high MAPK patients exhibited more frequent copy number losses since early stages of the disease.
Using in vivo models, we found that genetic ablation of DUSP4 grants a proliferative advantage to early KRAS driven neoplastic lesions. However, in later stages of this negative regulator increases MAPK activity and results in detrimental effects. DUSP4 knockout induces stress phenotypes including genomic, oxidative damage, and
concomitant apoptosis.
All these results suggest that mechanisms that govern MAPK modulation and limit excessive hyper-signaling are crucial for tumor promotion. We sought to identify additional modulators of the pathway by performing an in vitro CRISPR screening. For this purpose, we engineered a cellular model capable of generating inducible mild MAPK-
dependent toxicities to be used as a selective pressure to carry out the screening. By these means, we identified several candidates, including phosphatases, to be potential novel pathway regulators. The characterization of these novel candidates will continue to provide molecular insights of the fine tuning of the MAPK signaling cascade,
which, down the road, will contribute to the development of innovative therapeutic strategies.