Immunoproteasome function maintains oncogenic gene expression in KMT2A-complex driven leukemia.

Pharmacologic targeting of chromatin-associated protein complexes has shown significant responses in KMT2A-rearranged (KMT2A-r) acute myeloid leukemia (AML) but resistance frequently develops to single agents. This points to a need for therapeutic combinations that target multiple mechanisms. To enhance our understanding of functional dependencies in KMT2A-r AML, we have used a proteomic approach to identify the catalytic immunoproteasome subunit PSMB8 as a specific vulnerability. Genetic and pharmacologic inactivation of PSMB8 results in impaired proliferation of murine and human leukemic cells while normal hematopoietic cells remain unaffected. Disruption of immunoproteasome function drives an increase in transcription factor BASP1 which in turn represses KMT2A-fusion protein target genes. Pharmacologic targeting of PSMB8 improves efficacy of Menin-inhibitors, synergistically reduces leukemia in human xenografts and shows preserved activity against Menin-inhibitor resistance mutations. This identifies and validates a cell-intrinsic mechanism whereby selective disruption of proteostasis results in altered transcription factor abundance and repression of oncogene-specific transcriptional networks. These data demonstrate that the immunoproteasome is a relevant therapeutic target in AML and that targeting the immunoproteasome in combination with Menin-inhibition could be a novel approach for treatment of KMT2A-r AML.

Histone demethylase KDM4C is a functional dependency in JAK2-mutated neoplasms.

Mutations of the JAK2 gene are frequent aberrations in the aging hematopoietic system and in myeloid neoplasms. While JAK-inhibitors efficiently reduce hyperinflammation induced by the constitutively active mutated JAK2 kinase, the malignant clone and abundance of mutated cells remains rather unaffected. Here, we sought to assess for genetic vulnerabilities of JAK2-mutated clones. We identified lysine-specific demethylase KDM4C as a selective genetic dependency that persists upon JAK-inhibitor treatment. Genetic inactivation of KDM4C in human and murine JAK2-mutated cells resulted in loss of cell competition and reduced proliferation. These findings led to reduced disease penetrance and improved survival in xenograft models of human JAK2-mutated cells. KDM4C deleted cells showed alterations in target histone residue methylation and target gene expression, resulting in induction of cellular senescence. In summary, these data establish KDM4C as a specific dependency and therapeutic target in JAK2-mutated cells that is essential for oncogenic signaling and prevents induction of senescence.

Immunoproteasome Function in Normal and Malignant Hematopoiesis.

The ubiquitin-proteasome system (UPS) is a central part of protein homeostasis, degrading not only misfolded or oxidized proteins but also proteins with essential functions. The fact that a healthy hematopoietic system relies on the regulation of protein homeostasis and that alterations in the UPS can lead to malignant transformation makes the UPS an attractive therapeutic target for the treatment of hematologic malignancies. Herein, inhibitors of the proteasome, the last and most important component of the UPS enzymatic cascade, have been approved for the treatment of these malignancies. However, their use has been associated with side effects, drug resistance, and relapse. Inhibitors of the immunoproteasome, a proteasomal variant constitutively expressed in the cells of hematopoietic origin, could potentially overcome the encountered problems of non-selective proteasome inhibition. Immunoproteasome inhibitors have demonstrated their efficacy and safety against inflammatory and autoimmune diseases, even though their development for the treatment of hematologic malignancies is still in the early phases. Various immunoproteasome inhibitors have shown promising preliminary results in pre-clinical studies, and one inhibitor is currently being investigated in clinical trials for the treatment of multiple myeloma. Here, we will review data on immunoproteasome function and inhibition in hematopoietic cells and hematologic cancers.

The cell fate determinant Scribble is required for maintenance of hematopoietic stem cell function.

Cell fate determinants influence self-renewal potential of hematopoietic stem cells. Scribble and Llgl1 belong to the Scribble polarity complex and reveal tumor-suppressor function in drosophila. In hematopoietic cells, genetic inactivation of Llgl1 leads to expansion of the stem cell pool and increases self-renewal capacity without conferring malignant transformation. Here we show that genetic inactivation of its putative complex partner Scribble results in functional impairment of hematopoietic stem cells (HSC) over serial transplantation and during stress. Although loss of Scribble deregulates transcriptional downstream effectors involved in stem cell proliferation, cell signaling, and cell motility, these effectors do not overlap with transcriptional targets of Llgl1. Binding partner analysis of Scribble in hematopoietic cells using affinity purification followed by mass spectometry confirms its role in cell signaling and motility but not for binding to polarity modules described in drosophila. Finally, requirement of Scribble for self-renewal capacity also affects leukemia stem cell function. Thus, Scribble is a regulator of adult HSCs, essential for maintenance of HSCs during phases of cell stress.