Skeletal Muscle-Specific Methyltransferase METTL21C Trimethylates p97 and Regulates Autophagy-Associated Protein Breakdown

Summary: Protein aggregates and cytoplasmic vacuolization are major hallmarks of multisystem proteinopathies (MSPs) that lead to muscle weakness. Here, we identify METTL21C as a skeletal muscle-specific lysine methyltransferase. Insertion of a β-galactosidase cassette into the Mettl21c mouse locus revealed that METTL21C is specifically expressed in MYH7-positive skeletal muscle fibers. Ablation of the Mettl21c gene reduced endurance capacity and led to age-dependent accumulation of autophagic vacuoles in skeletal muscle. Denervation-induced muscle atrophy highlighted further impairments of autophagy-related proteins, including LC3, p62, and cathepsins, in Mettl21c−/− muscles. In addition, we demonstrate that METTL21C interacts with the ATPase p97 (VCP), which is mutated in various human MSP conditions. We reveal that METTL21C trimethylates p97 on the Lys315 residue and found that loss of this modification reduced p97 hexamer formation and ATPase activity in vivo. We conclude that the methyltransferase METTL21C is an important modulator of protein degradation in skeletal muscle under both normal and enhanced protein breakdown conditions. : Wiederstein et al. describe the skeletal muscle methyltransferase Mettl21c. They found that ablation of Mettl21c in mice results in muscle weakness and disturbance of the protein degradation machinery. Those changes are hallmarks of multisystem proteinopathies. They demonstrate that Mettl21c modulates p97 activity, which is frequently mutated in human patients with muscle weakness. Keywords: methyltransferases, skeletal muscle, p97, atrophy, autophag

ATM activity in T cells is critical for immune surveillance of lymphoma in vivo

The proximal DNA damage response kinase ATM is frequently inactivated in human malignancies. Germline mutations in the ATM gene cause Ataxia-telangiectasia (A-T), characterized by cerebellar ataxia and cancer predisposition. Whether ATM deficiency impacts on tumor initiation or also on the maintenance of the malignant state is unclear. Here, we show that Atm reactivation in initially Atm-deficient B- and T cell lymphomas induces tumor regression. We further find a reduced T cell abundance in B cell lymphomas from Atm-defective mice and A-T patients. Using T cell-specific Atm-knockout models, as well as allogeneic transplantation experiments, we pinpoint impaired immune surveillance as a contributor to cancer predisposition and development. Moreover, we demonstrate that Atm-deficient T cells display impaired proliferation capacity upon stimulation, due to replication stress. Altogether, our data indicate that T cell-specific restoration of ATM activity or allogeneic hematopoietic stem cell transplantation may prevent lymphomagenesis in A-T patients

UBQLN4 Represses Homologous Recombination and Is Overexpressed in Aggressive Tumors

Genomic instability can be a hallmark of both human genetic disease and cancer. We identify a deleterious UBQLN4 mutation in families with an autosomal recessive syndrome reminiscent of genome instability disorders. UBQLN4 deficiency leads to increased sensitivity to genotoxic stress and delayed DNA double-strand break (DSB) repair. The proteasomal shuttle factor UBQLN4 is phosphorylated by ATM and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DSB repair (HRR). Loss of UBQLN4 leads to chromatin retention of MRE11, promoting non-physiological HRR activity in vitro and in vivo. Conversely, UBQLN4 overexpression represses HRR and favors non-homologous end joining. Moreover, we find UBQLN4 overexpressed in aggressive tumors. In line with an HRR defect in these tumors, UBQLN4 overexpression is associated with PARP1 inhibitor sensitivity. UBQLN4 therefore curtails HRR activity through removal of MRE11 from damaged chromatin and thus offers a therapeutic window for PARP1 inhibitor treatment in UBQLN4-overexpressing tumors.Control of MRE11 association with chromatin by UBQLN4 during double-strand break repair influences repair pathway choice and can be dysregulated in tumorigenesis.This work was funded through the Dr. M. and S.G. Adelson Medical Research Foundation, The Israel Science Foundation joint ISF-NSFC Research Program and The Israel Cancer Research Fund (to Y.S.), the German-Israeli Foundation for Research and Development (I-65-412.20-2016 to Y.S. and H.C.R.), the Deutsche Forschungsgemeinschaft (KFO-286-RP2 to H.C.R., KFO-286-CP2 to M.P., JA2439/1-1 to R.D.J., DI1731/2-1 to F.D.), the Else Kröner-Fresenius Stiftung (2014-A06 to H.C.R., 2016_Kolleg.19 to R.D.J.), the Deutsche Krebshilfe (1117240 to H.C.R. and the Mildred-Scheel Professorship to M.P.), the German Ministry of Education and Research (BMBF 01GM1211B to D.W., BMBF e:Med 01ZX1303A and 01ZX1406 to M.P., BMBF e:Med 01ZX1303 and 01ZX1307 to M.F.), and R+D+I grants from the Spanish Ministry of Economy and Competitivity (SAF2013-43255-P and SAF2016-74855-P to P.H.). Y.S. is a Research Professor of the Israel Cancer Research Fund

-Active AKT signaling triggers CLL towards Richter's transformation via over-activation of Notch1

Richter's transformation (RT) is an aggressive lymphoma which occurs upon progression from chronic lymphocytic leukemia (CLL). Transformation has been associated with genetic aberrations in the CLL-phase involving TP53, CDKN2A, MYC, and NOTCH1, however a significant proportion of RT cases lack CLL-phase associated events. Here, we report that high levels of AKT phosphorylation occurs both in high-risk CLL patients harboring TP53 and NOTCH1 mutations as well as in RT patients. Genetic over-activation of Akt in the murine E-TCL1 CLL mouse model resulted in CLL to RT with significantly reduced survival and an aggressive lymphoma phenotype. In the absence of recurrent mutations, we identified a profile of genomic aberrations intermediate between CLL and DLBCL. Multi-omics assessment by phosphoproteomic/proteomic and single-cell transcriptomic profiles of this Akt-induced murine RT revealed a S100-protein-defined subcluster of highly aggressive lymphoma cells, which developed from CLL cells, through activation of Notch via Notch ligand expressed by T cells. Constitutively active Notch1 similarly induced RT of murine CLL. We identify Akt activation as an initiator of CLL transformation towards aggressive lymphoma by inducing Notch signaling between RT cells and microenvironmental T cells