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WHIM Syndrome (WS) is a rare immunodeficiency caused by gain-of-function CXCR4 mutations. Here we report for the first time a substantial decrease in bone mineral density in 25% of WS patients and bone defects leading to osteoporosis in a WS mouse model. Reduction in bone content involved impaired CXCR4 desensitization that disrupts cell cycle progression and osteogenic specification of mouse bone marrow (BM)-residing skeletal stromal/stem cells (SSCs). This was also evidenced in BM stromal cells from WS patients. Consistent with this, chronic treatment with the CXCR4 antagonist AMD3100 normalized in vitro osteogenic fate of mutant SSCs and reversed in vivo loss in skeletal cells, thus demonstrating that proper CXCR4 desensitization is required for the osteogenic specification of BM SSCs. Our study provides novel mechanistic insights into how CXCR4 signaling regulates the osteogenic fate of BM SSCs.

Summary Patients with acute myeloid leukemia (AML) carrying high-risk genetic lesions or high residual disease levels after therapy are particularly exposed to the risk of relapse. Here, we identified the long non-coding RNA CDK6-AS1 able to cluster an AML subgroup with peculiar gene signatures linked to hematopoietic cell differentiation and mitochondrial dynamics. CDK6-AS1 silencing triggered hematopoietic commitment in healthy CD34+ cells, whereas in AML cells the pathological undifferentiated state was rescued. This latter phenomenon derived from RUNX1 transcriptional control, responsible for the stemness of hematopoietic precursors and for the block of differentiation in AML. By CDK6-AS1 silencing in vitro, AML mitochondrial mass decreased with augmented pharmacological sensitivity to mitochondria-targeting drugs. In vivo, the combination of tigecycline and cytarabine reduced leukemia progression in the AML-PDX model with high CDK6-AS1 levels, supporting the concept of a mitochondrial vulnerability. Together, these findings uncover CDK6-AS1 as crucial in myeloid differentiation and mitochondrial mass regulation., Graphical abstract, Highlights • CDK6-AS1 acts in concert with CDK6 • High CDK6-AS1 levels trigger RUNX1 early differentiation arrest in myeloid cells • CDK6-AS1 controls mitochondrial mass of AML blasts • CDK6-AS1 levels impact on mitochondrial-targeted agents sensitivity, Molecular biology; Cell biology; Cancer

Context: Systemic inflammatory and auto-immune diseases (SIAD) are observed in 25% of patients with MDS or CMML, who harbor a high frequency of mutations in TET2, IDH1/2 and SRSF2 as we previously reported (Zhao et al, Leukemia 2021). Recently, the identification of mutations in UBA1 in patients with autoinflammatory disorders, some of whom have dysplastic bone marrow, defined novel disease called VEXAS syndrome (Beck et al, NEJM 2020). Objective: To explore the prevalence of UBA1 mutations in our initial cohort of MDS/CMML patients with SIAD (n=85) (Zhao et al, Leukemia 2021). Design and Patients: All male patients for whom material was available (n=33) were analyzed for the presence of UBA1 mutations. Primary Outcome: UBA1 mutations screening with Sanger sequencing. Results: Four out of 33 (12%) patients had UBA1 mutations. All of them had MDS without excess of blasts. Cytomorphologic review of bone marrow smears showed numerous vacuoles in precursors in all 4 cases. Karyotype was normal in 3/4 patients, and one patient had deletion 9q. One patient also had somatic mutations in DNMT3A (VAF 11%) and TP53 (VAF 2%), and another patient had a mutation of TET2 (VAF 18%). Three patients in our MDS/CMML SIAD cohort had relapsing polychondritis, and 2 of them had UBA1 mutations, while 6 patients from our initial SIAD cohort had a diagnosis of Sweet syndrome, including 2 with UBA1 mutations. Three out of 4mut patients had steroid refractory SIAD and were heavily treated with other immunosuppressive therapies including methotrexate, azathioprine or cytokine targeting agents. At last follow-up, none of the 4 patients with UBA1 mutations had MDS progression, and one of them had died from a stroke, 2.4 years after MDS diagnosis. Conclusions: Our retrospective study represents the first assessment of VEXAS syndrome among patients with myeloid malignancies associated with SIAD. The low prevalence of UBA1 mutations in our cohort suggests that other pathological mechanisms may drive inflammation in the large majority of MDS/CMML patients with associated SIAD, likely in relation with mutations in epigenetic regulators TET2/IDH and SRSF2 as we previously proposed.