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Mutations in one allele of the TP53 gene in cancer early stages are frequently followed by the loss of the remaining wild-type allele (LOH) during tumor progression. However, the clinical impact of TP53 mutations and p53LOH, especially in the context of genotoxic modalities, remains unclear. Using MMTV;ErbB2 model carrying a heterozygous R172H p53 mutation, we report a previously unidentified oncogenic activity of mutant p53 (mutp53): the exacerbation of p53LOH after irradiation. We show that wild-type p53 allele is partially transcriptionally competent and enables the maintenance of the genomic integrity under normal conditions in mutp53 heterozygous cells. In heterozygous cells γ-irradiation promotes mutp53 stabilization, which suppresses DNA repair and the cell cycle checkpoint allowing cell cycle progression in the presence of inefficiently repaired DNA, consequently increases genomic instability leading to p53LOH. Hence, in mutp53 heterozygous cells, irradiation facilitates the selective pressure for p53LOH that enhances cancer cell fitness and provides the genetic plasticity for acquiring metastatic properties., Amr Ghaleb et al. find that, in an ErbB2 mouse model, irradiation of premalignant mammary lesions enhances tumorigenesis in tumours that are heterozygous for p53 containing a p53 R172H mutation. This is associated with loss of the wild-type p53 allele, stabilisation of mutant p53 protein and activation of mTOR signaling.

Neurofibromatosis type I (NF1) is characterized by prominent skeletal manifestations caused by NF1 loss. While inhibitors of the ERK activating kinases MEK1/2 are promising as a means to treat NF1, the broad blockade of the ERK pathway produced by this strategy is potentially associated with therapy limiting toxicities. Here, we have sought targets offering a more narrow inhibition of ERK activation downstream of NF1 loss in the skeleton, finding that MEKK2 is a novel component of a noncanonical ERK pathway in osteoblasts that mediates aberrant ERK activation after NF1 loss. Accordingly, despite mice with conditional deletion of Nf1 in mature osteoblasts (Nf1fl/fl;Dmp1-Cre) and Mekk2−/− each displaying skeletal defects, Nf1fl/fl;Mekk2−/−;Dmp1-Cre mice show an amelioration of NF1-associated phenotypes. We also provide proof-of-principle that FDA-approved inhibitors with activity against MEKK2 can ameliorate NF1 skeletal pathology. Thus, MEKK2 functions as a MAP3K in the ERK pathway in osteoblasts, offering a potential new therapeutic strategy for the treatment of NF1., Neurofibromatosis type I (NF1) is characterized by prominent skeletal abnormalities mediated in part by aberrant ERK pathway activation due to NF1 loss-of-function. Here, the authors report the MEKK2 is a key mediator of this aberrant ERK activation and that MEKK2 inhibitors, including ponatinib, ameliorate skeletal defects in a mouse model of NF1.

Despite success of ERBB2-targeted therapies such as lapatinib, resistance remains a major clinical concern. Multiple compensatory receptor tyrosine kinase (RTK) pathways are known to contribute to lapatinib resistance. The heterogeneity of these adaptive responses is a significant hurdle for finding most effective combinatorial treatments. The goal of this study was to identify a unifying molecular mechanism whose targeting could help prevent and/or overcome lapatinib resistance. Using the MMTV-ERBB2;mutant p53 (R175H) in vivo mouse model of ERBB2-positive breast cancer, together with mouse and human cell lines, we compared lapatinib-resistant vs. lapatinib-sensitive tumor cells biochemically and by kinome arrays and evaluated their viability in response to a variety of compounds affecting heat shock response. We found that multiple adaptive RTKs are activated in lapatinib-resistant cells in vivo, some of which have been previously described (Axl, MET) and some were novel (PDGFRα, PDGFRβ, VEGFR1, MUSK, NFGR). Strikingly, all lapatinib-resistant cells show chronically activated HSF1 and its transcriptional targets, heat shock proteins (HSPs), and, as a result, superior tolerance to proteotoxic stress. Importantly, lapatinib-resistant tumors and cells retained sensitivity to Hsp90 and HSF1 inhibitors, both in vitro and in vivo, thus providing a unifying and actionable therapeutic node. Indeed, HSF1 inhibition simultaneously downregulated ERBB2, adaptive RTKs and mutant p53, and its combination with lapatinib prevented development of lapatinib resistance in vitro. Thus, the kinome adaptation in lapatinib-resistant ERBB2-positive breast cancer cells is governed, at least in part, by HSF1-mediated heat shock pathway, providing a novel potential intervention strategy to combat resistance.

Hox11 genes are essential for zeugopod skeletal element development but their roles in synovial joint formation remain largely unknown. Here, we show that the elbow and knee joints of mouse embryos lacking all Hox11 paralogous genes are specifically remodeled and reorganized. The proximal ends of developing mutant ulna and radius elements became morphologically similar and formed an anatomically distinct elbow joint. The mutant ulna lacked the olecranon that normally attaches to the triceps brachii muscle tendon and connects the humerus to the ulna. In its place, an ulnar patella-like element developed that expressed lubricin on its ventral side facing the joint and was connected to the triceps muscle tendon. In mutant knees, both tibia and fibula fully articulated with an enlarged femoral epiphyseal end that accommodated both elements, and the neo-tripartite knee joint was enclosed in a single synovial cavity and displayed an additional anterior ligament. The mutant joints also exhibited a different organization of the superficial zone of articular cartilage that normally exerts an anti-friction function. In conclusion, Hox11 genes co-regulate and coordinate the development of zeugopod skeletal elements and adjacent elbow and knee joints, and dictate joint identity, morphogenesis and anatomical and functional organization. Notably, the ulnar patella and tripartite knee joints in the mouse mutants actually characterize several lower vertebrates, including certain reptiles and amphibians. The re-emergence of such anatomical structures suggests that their genetic blueprint is still present in the mouse genome but is normally modified to the needs of the mammalian joint-formation program by distinct Hox11 function.