Ppp6c haploinsufficiency accelerates UV-induced BRAF(V600E)-initiated melanomagenesis

Molecular Modeling Unveils the Effective Interaction of B-RAF Inhibitors with Rare B-RAF Insertion Variants

The Food and Drug Administration (FDA) has approved MAPK inhibitors as a treatment for melanoma patients carrying a mutation in codon V600 of the BRAF gene exclusively. However, BRAF mutations outside the V600 codon may occur in a small percentage of melanomas. Although these rare variants may cause B-RAF activation, their predictive response to B-RAF inhibitor treatments is still poorly understood. We exploited an integrated approach for mutation detection, tumor evolution tracking, and assessment of response to treatment in a metastatic melanoma patient carrying the rare p.T599dup B-RAF mutation. He was addressed to Dabrafenib/Trametinib targeted therapy, showing an initial dramatic response. In parallel, in-silico ligand-based homology modeling was set up and performed on this and an additional B-RAF rare variant (p.A598_T599insV) to unveil and justify the success of the B-RAF inhibitory activity of Dabrafenib, showing that it could adeptly bind both these variants in a similar manner to how it binds and inhibits the V600E mutant. These findings open up the possibility of broadening the spectrum of BRAF inhibitor-sensitive mutations beyond mutations at codon V600, suggesting that B-RAF V600 WT melanomas should undergo more specific investigations before ruling out the possibility of targeted therapy.

PP6 deficiency in mice with KRAS mutation and Trp53 loss promotes early death by PDAC with cachexia-like features

To examine effects of PP6 gene (Ppp6c) deficiency on pancreatic tumor development, we developed pancreas-specific, tamoxifen-inducible Cre-mediated KP (KRAS(G12D) plus Trp53-deficient) mice (cKP mice) and crossed them with Ppp6cflox / flox mice. cKP mice with the homozygous Ppp6c deletion developed pancreatic tumors, became emaciated and required euthanasia within 150 days of mutation induction, phenotypes that were not seen in heterozygous or wild-type (WT) mice. At 30 days, a comparative analysis of genes commonly altered in homozygous versus WT Ppp6c cKP mice revealed enhanced activation of Erk and NFκB pathways in homozygotes. By 80 days, the number and size of tumors and number of precancerous lesions had significantly increased in the pancreas of Ppp6c homozygous relative to heterozygous or WT cKP mice. Ppp6c-/- tumors were pathologically diagnosed as pancreatic ductal adenocarcinoma (PDAC) undergoing the epithelial-mesenchymal transition (EMT), and cancer cells had invaded surrounding tissues in three out of six cases. Transcriptome and metabolome analyses indicated an enhanced cancer-specific glycolytic metabolism in Ppp6c-deficient cKP mice and the increased expression of inflammatory cytokines. Individual Ppp6c-/- cKP mice showed weight loss, decreased skeletal muscle and adipose tissue, and increased circulating tumor necrosis factor (TNF)-α and IL-6 levels, suggestive of systemic inflammation. Overall, Ppp6c deficiency in the presence of K-ras mutations and Trp53 gene deficiency promoted pancreatic tumorigenesis with generalized cachexia and early death. This study provided the first evidence that Ppp6c suppresses mouse pancreatic carcinogenesis and supports the use of Ppp6c-deficient cKP mice as a model for developing treatments for cachexia associated with pancreatic cancer.

Ppp6c haploinsufficiency accelerates UV-induced BRAF(V600E)-initiated melanomagenesis

According to TCGA database, mutations in PPP6C (encoding phosphatase PP6) are found in c. 10% of tumors from melanoma patients, in which they coexist with BRAF and NRAS mutations. To assess PP6 function in melanoma carcinogenesis, we generated mice in which we could specifically induce BRAF(V600E) expression and delete Ppp6c in melanocytes. In these mice, melanoma susceptibility following UVB irradiation exhibited the following pattern: Ppp6c semi‐deficient (heterozygous) > Ppp6c wild‐type > Ppp6c‐deficient (homozygous) tumor types. Next‐generation sequencing of Ppp6c heterozygous and wild‐type melanoma tumors revealed that all harbored Trp53 mutations. However, Ppp6c heterozygous tumors showed a higher Signature 1 (mitotic/mitotic clock) mutation index compared with Ppp6c wild‐type tumors, suggesting increased cell division. Analysis of cell lines derived from either Ppp6c heterozygous or wild‐type melanoma tissues showed that both formed tumors in nude mice, but Ppp6c heterozygous tumors grew faster compared with those from the wild‐type line. Ppp6c knockdown via siRNA in the Ppp6c heterozygous line promoted the accumulation of genomic damage and enhanced apoptosis relative to siRNA controls. We conclude that in the presence of BRAF(V600E) expression and UV‐induced Trp53 mutation, Ppp6c haploinsufficiency promotes tumorigenesis.