Luuk Heitink

The mitochondrial E3 ligase MARCH5 has consistently emerged as a dependency in unbiased screens in acute myeloid leukemia and myeloma, yet the underpinning mechanism remains ill-defined. Here, we show that MARCH5 cooperates with UBE2J2 and MFN2, forming a stress-sensing complex at mitochondria-ER contact sites (MERCS) that restrains apoptosis in response to diverse organellar damage signals. Loss of MARCH5 potently sensitizes diverse blood cancer cell lines to BCL-2 and BCL-XL inhibition and compromises stress tolerance. By contrast, non-hematopoietic cell lines exhibit a phenotype largely restricted to BCL-XL dependence, permitting tissue-selective therapeutic synergy with venetoclax and other agents. Mechanistically, spatial organization underpins this specificity. The complex assembles at MERCS, where it co-localizes with BCL-2 and BCL-XL but not MCL-1. Upon organellar damage, it dissociates prior to BAX/BAK activation, lowering the apoptotic threshold and enforcing reliance on neighboring BCL-2 and BCL-XL. Consistent with its distribution, MARCH5 loss minimally alters MCL-1 dependence, revealing a spatially encoded mechanism integrating diverse stress signals into cell-death decisions. To guide future therapeutics, we demonstrate that disrupting key protein-protein interactions within this complex is sufficient to sensitize blood cancer cell lines, restoring venetoclax responsiveness and prolonging survival in a murine model of refractory lymphoma. Genetic deletion of MARCH5 or UBE2J2 restored BH3-mimetic sensitivity to primary chronic lymphocytic leukemia cells rendered resistant by cytokine stimulation. These findings establish the MERCS-resident MARCH5 complex as a central regulator of malignant cell stress tolerance and highlight tractable protein interfaces for therapeutic targeting.

Genetic alterations activating NF‑κB have been described in mature B‑cell malignancies. These alterations include loss of the negative regulator TRAF3, allowing NIK (NF‑κB-inducing kinase) to drive non-canonical NF‑κB (ncNF‑κB) signaling. In this study, we characterized two recurrent synonymous variants of MAP3K14, the gene encoding NIK, across mature B‑cell malignancies including chronic lymphocytic leukemia, lymphoplasmacytic lymphoma/Waldenström's macroglobulinemia, multiple myeloma and mantle cell lymphoma. These variants were shown to introduce splice donor sites that result in removal of the critical TRAF3 binding motif. Cell line models prime edited to contain the synonymous variants demonstrated NF‑κB activation and transcriptional reprogramming with enhanced cell proliferation and survival, as well as increased secretion of several cytokines/chemokines including IP10/CXCL10. We also characterized distinctive cell surface proteomic changes with NF‑κB activation, identifying candidate markers (e.g.,CD132/IL‑2Rγ) for assessing NF‑κB activation at a single cell level using flow cytometry. This study expands the spectrum of genetic lesions that result in activation of NF‑κB in mature B‑cell malignancies and highlights our incomplete understanding of NF‑κB drivers in mature B‑cell malignancies.

Inheritance of a BRCA2 pathogenic variant conveys a substantial life-time risk of breast cancer. Identification of the cell(s)-of-origin of BRCA2-mutant breast cancer and targetable perturbations that contribute to transformation remains an unmet need for these individuals who frequently undergo prophylactic mastectomy. Using preneoplastic specimens from age-matched, premenopausal females, here we show broad dysregulation across the luminal compartment in BRCA2^mut/+ tissue, including expansion of aberrant ERBB3^lo luminal progenitor and mature cells, and the presence of atypical oestrogen receptor (ER)-positive lesions. Transcriptional profiling and functional assays revealed perturbed proteostasis and translation in ERBB3^lo progenitors in BRCA2^mut/+ breast tissue, independent of ageing. Similar molecular perturbations marked tumours bearing BRCA2-truncating mutations. ERBB3^lo progenitors could generate both ER⁺ and ER⁻ cells, potentially serving as cells-of-origin for ER-positive or triple-negative cancers. Short-term treatment with an mTORC1 inhibitor substantially curtailed tumorigenesis in a preclinical model of BRCA2-deficient breast cancer, thus uncovering a potential prevention strategy for BRCA2 mutation carriers.