Rates, Predictors, and Outcomes of Ustekinumab Dose Escalation in Inflammatory Bowel Disease.

OBJECTIVE: Ustekinumab (UST) is effective for the induction and maintenance of remission in inflammatory bowel disease (IBD). However, a significant proportion of patients will require UST dose escalation. We sought to determine the rates, predictors, and outcomes of UST dose escalation in patients with IBD. PATIENTS AND METHODS: This was a multicenter, retrospective study of all patients with IBD who received UST from January 1, 2014 to March 1, 2022. Primary outcomes were the rates and predictors of UST dose escalation. Secondary outcomes included steroid-free clinical remission, endoscopic healing, and normalization of serum c-reactive protein in patients who underwent UST dose escalation. RESULTS: A total of 198 patients were included (58% females and 76.7% with Crohn's disease). UST dose was escalated by 55.5% (n = 110). Mean baseline albumin was lower in the UST dose escalation group at 3.86 ± 0.47 versus 4.03 ± 0.45 g/dL (P = 0.044). The mean hemoglobin was lower in the UST dose escalation group at 12.1 ± 1.83 versus 12.7 ± 1.42 (P = 0.049). On multivariate analysis, male sex alone was associated with the need for dose escalation (odds ratio: 4.08, 95% CI: 1.20 - 13.90; P = 0.025). In the UST dose escalation group, 66.1% achieved steroid-free clinical remission, 55.8% had normalization of c-reactive protein, and 35.8% achieved endoscopic healing. CONCLUSIONS: UST dose escalation was needed in more than half of patients with IBD in this real-world cohort. UST dose escalation resulted in clinical remission in more than half of the cohort and endoscopic healing in one-third of patients.

Ustekinumab Drug Levels and Outcomes in Inflammatory Bowel Disease.

BACKGROUND: Data regarding the utility of therapeutic drug monitoring with ustekinumab (UST) are sparse. Our aim was to determine the correlation of UST levels with outcomes in a cohort of patients with inflammatory bowel disease (IBD). METHODS: This was a multicenter, retrospective study of all patients with IBD who received UST from January 1, 2014 to March 1, 2022. The primary outcomes were the correlation of UST level with clinical remission (per physician global assessment), endoscopic healing [the absence of ulcers/erosions in Crohn's disease (CD) and Mayo endoscopic score ≤1 for ulcerative colitis (UC)], and normal serum C-reactive protein (CRP) (≤5 mg/L). Secondary outcomes included defining optimal UST trough levels associated with favorable outcomes. RESULTS: A total of 71 patients (74.6% with CD; 57.7% female) were included. The median age was 39.5 years [interquartile range (IQR): 26 to 52] and 12.6% were on combination therapy with immunomodulators. Median UST trough levels were significantly higher in patients who achieved endoscopic healing at 5.4 µg/mL versus 3.5 µg/mL (P=0.035) and normal CRP at 5.5 µg/mL versus. 3.1 µg/mL (P=0.002). A cutoff UST level of 4.8 µg/mL yielded the highest area under the curve (AUC) of 0.73 (95% CI: 0.61-0.80) to predict a normal CRP followed by a cutoff of 3.5 µg/mL which yielded an AUC of 0.66 (95% CI: 0.52-0.81) to predict endoscopic healing. CONCLUSIONS: UST trough levels were significantly higher in patients who achieved a normal CRP and endoscopic healing. A cutoff UST level of 4.8 µg/mL reliably predicted CRP normalization.

Comparative Safety of Biologic Agents in Patients With Inflammatory Bowel Disease With Active or Recent Malignancy: A Multi-Center Cohort Study.

BACKGROUND & AIMS: Safety of biologic agents is a key consideration in patients with inflammatory bowel disease (IBD) and active or recent cancer. We compared the safety of tumor necrosis factor (TNF)-α antagonists vs non-TNF biologics in patients with IBD with active or recent cancer. METHODS: We conducted a multicenter retrospective cohort study of patients with IBD and either active cancer (cohort A) or recent prior cancer (within ≤5 years; cohort B) who were treated with TNFα antagonists or non-TNF biologics after their cancer diagnosis. Primary outcomes were progression-free survival (cohort A) or recurrence-free survival (cohort B). Safety was compared using inverse probability of treatment weighting with propensity scores. RESULTS: In cohort A, of 125 patients (483.8 person-years of follow-up evaluation) with active cancer (age, 54 ± 15 y, 75% solid-organ malignancy), 10 of 55 (incidence rate [IR] per 100 py, 4.4) and 9 of 40 (IR, 10.4) patients treated with TNFα antagonists and non-TNF biologics had cancer progression, respectively. There was no difference in the risk of progression-free survival between TNFα antagonists vs non-TNF biologics (hazard ratio, 0.76; 95% CI, 0.25-2.30). In cohort B, of 170 patients (513 person-years of follow-up evaluation) with recent prior cancer (age, 53 ± 15 y, 84% solid-organ malignancy; duration of remission, 19 ± 19 mo), 8 of 78 (IR, 3.4) and 5 of 66 (IR 3.7) patients treated with TNFα antagonists and non-TNF biologics had cancer recurrence, respectively. The risk of recurrence-free survival was similar between both groups (hazard ratio, 0.94; 95% CI, 0.24-3.77). CONCLUSIONS: In patients with IBD with active or recent cancer, TNFα antagonists and non-TNF biologics have comparable safety. The choice of biologic should be dictated by IBD disease severity in collaboration with an oncologist.

A role for the histone H2A deubiquitinase MYSM1 in maintenance of CD8+ T cells.

Several previous studies outlined the importance of the histone H2A deubiquitinase MYSM1 in the regulation of stem cell quiescence and haematopoiesis. In this study we investigated the role of MYSM1 in T-cell development. Using mouse models that allow conditional Mysm1 ablation at late stages of thymic development, we found that MYSM1 is intricately involved in the maintenance, activation and survival of CD8+ T cells. Mysm1 ablation resulted in a twofold reduction in CD8+ T-cell numbers, and also led to a hyperactivated CD8+ T-cell state accompanied by impaired proliferation and increased pro-inflammatory cytokine production after ex vivo stimulation. These phenotypes coincided with an increased apoptosis and preferential up-regulation of p53 tumour suppressor protein in CD8+ T cells. Lastly, we examined a model of experimental cerebral malaria, in which pathology is critically dependent on CD8+ T cells. In the mice conditionally deleted for Mysm1 in the T-cell compartment, CD8+ T-cell numbers remained reduced following infection, both in the periphery and in the brain, and the mice displayed improved survival after parasite challenge. Collectively, our data identify MYSM1 as a novel factor for CD8+ T cells in the immune system, increasing our understanding of the role of histone H2A deubiquitinases in cytotoxic T-cell biology.

p53 mediates loss of hematopoietic stem cell function and lymphopenia in Mysm1 deficiency.

MYSM1 is a chromatin-binding transcriptional cofactor that deubiquitinates histone H2A. Studies of Mysm1-deficient mice have shown that it is essential for hematopoietic stem cell (HSC) function and lymphopoiesis. Human carriers of a rare MYSM1-inactivating mutation display similar lymphopoietic deficiencies. However, the mechanism by which MYSM1 regulates hematopoietic homeostasis remains unclear. Here, we show that Mysm1-deficiency results in p53 protein elevation in many hematopoietic cell types. p53 is a central regulator of cellular stress responses and HSC homeostasis. We thus generated double-knockout mice to assess a potential genetic interaction between Mysm1 and p53 in hematopoiesis. Mysm1(-/-)p53(-/-) mouse characterization showed a full rescue of Mysm1(-/-) developmental and hematopoietic defects. This included restoration of lymphopoiesis, and HSC numbers and functions. These results establish p53 activation as the driving mechanism for hematopoietic abnormalities in Mysm1 deficiency. Our findings may advance the understanding of p53 regulation in hematopoiesis and implicate MYSM1 as a potential p53 cofactor.

Use of glucagon-like peptide-1 receptor agonists for type 2 diabetes mellitus and outcomes of inflammatory bowel disease.

BACKGROUND: Glucagon-like peptide-1 receptor agonists (GLP-1RA) show anti-inflammatory properties. AIM: To evaluate their clinical impact on inflammatory bowel disease (IBD) outcomes. METHODS: Retrospective cohort study utilising the TriNetX database comparing IBD-specific outcomes in patients with ulcerative colitis (UC) or Crohn's disease (CD) and type 2 diabetes mellitus (T2DM) on GLP-1RA compared to oral hypoglycaemic agents. The primary outcome was hospitalisation requiring intravenous steroids and IBD-related surgery within 3 years. We performed 1:1 propensity score matching (PSM) for demographics, co-morbid conditions, BMI, laboratory values, HbA1c, and IBD medications including steroids. RESULTS: We identified 1130 patients in the UC GLP-1RA cohort (mean age: 58.9 ± 11.6 years, 56.3% female, 70.2% White, 57.2% with obesity) and 1140 patients in the CD GLP-1RA cohort (mean age: 56.7 ± 11.5, 61.9% female, 73.6% White, 56.2% with obesity). After PSM, there was no difference in the risk of intravenous steroid use (aHR: 1.21, 95% CI: 0.92-1.59) but a lower risk of colectomy (aHR: 0.37, 95% CI: 0.14-0.97) between the UC GLP-1RA and control cohort. There was no difference in the risk of intravenous steroid use (aHR: 1.04, 95% CI: 0.80-1.34) but a lower risk of surgery (aHR: 0.55, 95% CI: 0.36-0.84) between the CD GLP-1RA and CD control cohort. There was no difference in the risk of oral steroid use or advanced therapy initiation in the UC and CD GLP-1RA than control cohorts. CONCLUSIONS: We found an association between lower risk of IBD-related surgery and GLP-1RA use for T2DM in patients with UC or CD.

Metformin Is Associated With Improved Inflammatory Bowel Disease Outcomes in Patients With Type 2 Diabetes Mellitus: A Propensity-Matched Cohort Study.

BACKGROUND: Metformin exerts anti-inflammatory properties through a positive effect on oxidative stress, gut barrier integrity, and the gut microbiota. Our aim was to evaluate the influence of metformin on inflammatory bowel disease (IBD) outcomes in patients with type 2 diabetes mellitus (T2DM). METHODS: We conducted a retrospective cohort study using the TriNetX database in patients with IBD and T2DM who initiated metformin vs oral hypoglycemics or insulin (control cohort) between August 31, 2002, and August 31, 2022. One-to-one propensity score matching was performed. Primary outcomes were need for intravenous (IV) steroid use or IBD-related surgery within 1, 2, and 3 years after metformin initiation. RESULTS: Our cohorts included 1323 patients with ulcerative colitis (UC) (mean age 58.7 ±â€…12.2 years, 50.1% female, 77.3% White) and 1278 patients with Crohn's disease (CD) (mean age 56.3 ±â€…12.6 years, 58.2% female, 76.5% White). At 1 year, patients with UC and CD were less likely to require IV steroids (UC: adjusted odds ratio [aOR], 0.45; 95% confidence interval [CI], 0.34-0.59; P < .01; CD: aOR, 0.67; 95% CI, 0.53-0.85; P < .01). The decreased need for IV steroids persisted in all metformin groups at 2 and 3 years. Patients with CD were at a lower risk for IBD-related surgery at year 1 (aOR, 0.5; 95% CI, 0.31-0.81; P < .01), and this finding persisted at 3 years (aOR, 0.62; 95% CI, 0.43-0.89; P < .01). Metformin did not affect risk for surgery in patients with UC. CONCLUSIONS: Patients with IBD and T2DM on metformin had a decreased likelihood of worse IBD outcomes.

Our study shows that metformin is associated with decreased risk of corticosteroids in patients with ulcerative colitis and Crohn's disease and decreased risk of surgery in patients with Crohn's disease.

Metagenomic and bile acid metabolomic analysis of fecal microbiota transplantation for recurrent Clostridiodes difficile and/or inflammatory bowel diseases.

Background: Fecal microbiota transplantation (FMT) is an effective treatment of recurrent Clostridioides difficile infections (rCDI), but has more limited efficacy in treating either ulcerative colitis (UC) or Crohn's disease (CD), two major forms of inflammatory bowel diseases (IBD). We hypothesize that FMT recipients with rCDI and/or IBD have baseline fecal bile acid (BA) compositions that differ significantly from that of their healthy donors and that FMT will normalize the BA compositions. Aim: To study the effect of single colonoscopic FMT on microbial composition and function in four recipient groups: 1.) rCDI patients without IBD (rCDI-IBD); 2.) rCDI with IBD (rCDI+IBD); 3.) UC patients without rCDI (UC-rCDI); 4.) CD patients without rCDI (CD-rCDI). Methods: We performed 16S rRNA gene sequence, shotgun DNA sequence and quantitative bile acid metabolomic analyses on stools collected from 55 pairs of subjects and donors enrolled in two prospective single arm FMT clinical trials (Clinical Trials.gov ID NCT03268213, 479696, UC no rCDI ≥ 2x IND 1564 and NCT03267238, IND 16795). Fitted linear mixed models were used to examine the effects of four recipient groups, FMT status (Donor, pre-FMT, 1-week post-FMT, 3-months post-FMT) and first order Group*FMT interactions on microbial diversity and composition, bile acid metabolites and bile acid metabolizing enzyme gene abundance. Results: The pre-FMT stools collected from rCDI ± IBD recipients had reduced α-diversity compared to the healthy donor stools and was restored post-FMT. The α-diversity in the pre-FMT stools collected from UC-rCDI or CD-rCDI recipients did not differ significantly from donor stools. FMT normalized some recipient/donor ratios of genus level taxa abundance in the four groups. Fecal secondary BA levels, including some of the secondary BA epimers that exhibit in vitro immunomodulatory activities, were lower in rCDI±IBD and CD-rCDI but not UC-rCDI recipients compared to donors. FMT restored secondary BA levels. Metagenomic baiE gene and some of the eight bile salt hydrolase (BSH) phylotype abundances were significantly correlated with fecal BA levels. Conclusion: Restoration of multiple secondary BA levels, including BA epimers implicated in immunoregulation, are associated with restoration of fecal baiE gene counts, suggesting that the 7-α-dehydroxylation step is rate-limiting.

Gallbladder Disorders: A Comprehensive Review.

Gallbladder disorders encompass a wide breadth of diseases that vary in severity. We present a comprehensive review of literature for the clinical presentation, pathophysiology, diagnostic evaluation, and management of cholelithiasis-related disease, acute acalculous cholecystitis, functional gallbladder disorder, gallbladder polyps, gallbladder hydrops, porcelain gallbladder, and gallbladder cancer.

p53-dependent induction of P2X7 on hematopoietic stem and progenitor cells regulates hematopoietic response to genotoxic stress.

Stem and progenitor cells are the main mediators of tissue renewal and repair, both under homeostatic conditions and in response to physiological stress and injury. Hematopoietic system is responsible for the regeneration of blood and immune cells and is maintained by bone marrow-resident hematopoietic stem and progenitor cells (HSPCs). Hematopoietic system is particularly susceptible to injury in response to genotoxic stress, resulting in the risk of bone marrow failure and secondary malignancies in cancer patients undergoing radiotherapy. Here we analyze the in vivo transcriptional response of HSPCs to genotoxic stress in a mouse whole-body irradiation model and, together with p53 ChIP-Seq and studies in p53-knockout (p53KO) mice, characterize the p53-dependent and p53-independent branches of this transcriptional response. Our work demonstrates the p53-independent induction of inflammatory transcriptional signatures in HSPCs in response to genotoxic stress and identifies multiple novel p53-target genes induced in HSPCs in response to whole-body irradiation. In particular, we establish the direct p53-mediated induction of P2X7 expression on HSCs and HSPCs in response to genotoxic stress. We further demonstrate the role of P2X7 in hematopoietic response to acute genotoxic stress, with P2X7 deficiency significantly extending mouse survival in irradiation-induced hematopoietic failure. We also demonstrate the role of P2X7 in the context of long-term HSC regenerative fitness following sublethal irradiation. Overall our studies provide important insights into the mechanisms of HSC response to genotoxic stress and further suggest P2X7 as a target for pharmacological modulation of HSC fitness and hematopoietic response to genotoxic injury.

MYSM1 maintains ribosomal protein gene expression in hematopoietic stem cells to prevent hematopoietic dysfunction.

Ribosomopathies are congenital disorders caused by mutations in the genes encoding ribosomal and other functionally related proteins. They are characterized by anemia, other hematopoietic and developmental abnormalities, and p53 activation. Ribosome assembly requires coordinated expression of many ribosomal protein (RP) genes; however, the regulation of RP gene expression, especially in hematopoietic stem cells (HSCs), remains poorly understood. MYSM1 is a transcriptional regulator essential for HSC function and hematopoiesis. We established that HSC dysfunction in Mysm1 deficiency is driven by p53; however, the mechanisms of p53 activation remained unclear. Here, we describe the transcriptome of Mysm1-deficient mouse HSCs and identify MYSM1 genome-wide DNA binding sites. We establish a direct role for MYSM1 in RP gene expression and show a reduction in protein synthesis in Mysm1-/- HSCs. Loss of p53 in mice fully rescues Mysm1-/- anemia phenotype but not RP gene expression, indicating that RP gene dysregulation is a direct outcome of Mysm1 deficiency and an upstream mediator of Mysm1-/- phenotypes through p53 activation. We characterize a patient with a homozygous nonsense MYSM1 gene variant, and we demonstrate reduced protein synthesis and increased p53 levels in patient hematopoietic cells. Our work provides insights into the specialized mechanisms regulating RP gene expression in HSCs and establishes a common etiology of MYSM1 deficiency and ribosomopathy syndromes.

Targeting Two Antigens Associated with B-ALL with CD19-CD123 Compound Car T Cell Therapy.

The recent FDA approval of the first CAR immunotherapy marks a watershed moment in the advancement toward a cure for cancer. CD19 CAR treatment for B cell acute lymphocytic leukemia has achieved unprecedented remission rates. However, despite success in treating previously relapsed and refractory patients, CD19 CAR faces similar challenges as traditional chemotherapy, in that malignancy can adapt and overcome treatment. The emergence of both antigen positive and negative blasts after CAR treatment represents a need to bolster current CAR approaches. Here, we report on the anti-tumor activity of a CAR T cell possessing 2 discrete scFv domains against the leukemic antigens CD19 and CD123. We determined that the resulting compound CAR (cCAR) T cell possesses consistent, potent, and directed cytotoxicity against each target antigen population both in vitro and in vivo. Our findings indicate that targeting CD19 and CD123 on B-ALL cells may be an effective strategy for augmenting the response against leukemic blasts and reducing rates of relapse.

Preclinical Targeting of Human Acute Myeloid Leukemia Using CD4-specific Chimeric Antigen Receptor (CAR) T Cells and NK Cells.

Acute myeloid leukemia (AML) is an aggressive malignancy lacking targeted therapy due to shared molecular and transcriptional circuits as well as phenotypic markers with normal hematopoietic stem cells (HSCs). Identifying leukemia specific markers expressed on AML or AML subtypes for therapeutic targeting is of exquisite clinical value. Here we show that CD4, a T lymphocytes membrane glycoprotein that interacts with major histocompatibility complex class II antigens and is also expressed in certain AML subsets but not on HSCs is a proper target for genetically engineered chimeric antigen receptor T cells (CAR-T cells). Treatment with CD4 redirected CAR-T cell (CD4CAR) specifically eliminated CD4-expressing AML cell lines in vitro and exhibited a potent anti-leukemic effect in a systemic AML murine model in vivo. We also utilized natural killers as another vehicle for CAR engineered cells and this strategy similarly and robustly eliminated CD4- expressing AML cells in vitro and had a potent in vivo anti-leukemic effect and was noted to have shorter in vivo persistence. Our data offer a proof of concept for immunotherapeutic targeting of CD4 as a strategy to treat CD4 expressing refractory AML as a bridge to stem cell transplant (SCT) in a first in human clinical trial.

Compound CAR T-cells as a double-pronged approach for treating acute myeloid leukemia.

Acute myeloid leukemia (AML) bears heterogeneous cells that can consequently offset killing by single-CAR-based therapy, which results in disease relapse. Leukemic stem cells (LSCs) associated with CD123 expression comprise a rare population that also plays an important role in disease progression and relapse. Here, we report on the robust anti-tumor activity of a compound CAR (cCAR) T-cell possessing discrete scFv domains targeting two different AML antigens, CD123, and CD33, simultaneously. We determined that the resulting cCAR T-cells possessed consistent, potent, and directed cytotoxicity against each target antigen population. Using four leukemia mouse models, we found superior in vivo survival after cCAR treatment. We also designed an alemtuzumab safety-switch that allowed for rapid cCAR therapy termination in vivo. These findings indicate that targeting both CD123 and CD33 on AML cells may be an effective strategy for eliminating both AML bulk disease and LSCs, and potentially prevent relapse due to antigen escape or LSC persistence.

Mysm1 expression in the bone marrow niche is not essential for hematopoietic maintenance.

Myb-Like SWIRM and MPN domains 1 (MYSM1) is a chromatin-binding protein, essential for hematopoietic stem cell (HSC) maintenance and differentiation in humans and mouse models. HSCs in mammalian bone marrow exist in close interactions with many non-hematopoietic cell types in their microenvironment, collectively known as the bone marrow niche. Although cell-intrinsic activities of MYSM1 within the hematopoietic cells are known to play an important role in hematopoietic homeostasis, Mysm1 expression is also widely observed in non-hematopoietic cells, and MYSM1 is implicated as an important regulator of mesenchymal stem cell differentiation, osteoblast function, and adipogenesis within the bone marrow. In this work we for the first time test the roles of Mysm1 within the non-hematopoietic cells of the bone marrow niche in the maintenance of HSCs and hematopoietic homeostasis. For this purpose, wild-type mouse bone marrow was transplanted into Mysm1fl/flTg.CreERT2 recipient mice, followed by tamoxifen-induced ablation of Mysm1 specifically within the non-hematopoietic cells of the recipient. HSC functions and hematopoiesis in these chimeras, with wild-type HSCs and a Mysm1-deficient bone marrow environment, are characterized in the current work. We report that the selective deletion of Mysm1 in non-hematopoietic cells did not affect HSC maintenance and differentiation, with the mice maintaining the normal number and viability of HSCs, diverse hematopoietic progenitors, and mature hematopoietic and immune cell types. Overall we conclude that Mysm1 expression within the niche is not essential for the maintenance of hematopoietic homeostasis.

MYSM1-dependent checkpoints in B cell lineage differentiation and B cell-mediated immune response.

MYSM1 is a chromatin-binding histone deubiquitinase. MYSM1 mutations in humans result in lymphopenia whereas loss of Mysm1 in mice causes severe hematopoietic abnormalities, including an early arrest in B cell development. However, it remains unknown whether MYSM1 is required at later checkpoints in B cell development or for B cell-mediated immune responses. We analyzed conditional mouse models Mysm1fl/flTg.mb1-cre, Mysm1fl/flTg.CD19-cre, and Mysm1fl/flTg.CD21-cre with inactivation of Mysm1 at prepro-B, pre-B, and follicular B cell stages of development. We show that loss of Mysm1 at the prepro-B cell stage in Mysm1fl/flTg.mb1-cre mice results in impaired B cell differentiation, with an ∼2-fold reduction in B cell numbers in the lymphoid organs. Mysm1fl/flTg.mb1-cre B cells also showed increased expression of activation markers and impaired survival and proliferation. In contrast, Mysm1 was largely dispensable from the pre-B cell stage onward, with Mysm1fl/flTg.CD19-cre and Mysm1fl/flTg.CD21-cre mice showing no alterations in B cell numbers and largely normal responses to stimulation. MYSM1, therefore, has an essential role in B cell lineage specification but is dispensable at later stages of development. Importantly, MYSM1 activity at the prepro-B cell stage of development is important for the normal programming of B cell responses to stimulation once they complete their maturation process.

Deubiquitinase MYSM1 Is Essential for Normal Fetal Liver Hematopoiesis and for the Maintenance of Hematopoietic Stem Cells in Adult Bone Marrow.

MYSM1 is a chromatin-interacting deubiquitinase recently shown to be essential for hematopoietic stem cell (HSC) function and normal progression of hematopoiesis in both mice and humans. However, it remains unknown whether the loss of function in Mysm1-deficient HSCs is due to the essential role of MYSM1 in establishing the HSC pool during development or due to a continuous requirement for MYSM1 in adult HSCs. In this study we, for the first time, address these questions first, by performing a detailed analysis of hematopoiesis in the fetal livers of Mysm1-knockout mice, and second, by assessing the effects of an inducible Mysm1 ablation on adult HSC functions. Our data indicate that MYSM1 is essential for normal HSC function and progression of hematopoiesis in the fetal liver. Furthermore, the inducible knockout model demonstrates a continuous requirement for MYSM1 to maintain HSC functions and antagonize p53 activation in adult bone marrow. These studies advance our understanding of the role of MYSM1 in HSC biology, and provide new insights into the human hematopoietic failure syndrome resulting from MYSM1 deficiency.