Clonal hematopoiesis in sickle cell disease

How I treat sickle cell disease with gene therapy

ABSTRACT

In 2023, 2 different gene therapies were approved for individuals with severe sickle cell disease (SCD). The small number of patients treated on the pivotal clinical trials that led to these approvals have experienced dramatic short-term reductions in the occurrence of painful vaso-occlusive crises, but the long-term safety and efficacy of these genetic therapies are yet to be ascertained. Several challenges and treatment-related concerns have emerged in regard to administering these therapies in clinical practice. This article discusses the selection and preparation of individuals with SCD who wish to receive autologous gene therapy, as well as the salient features of the care needed to support them through a long and arduous treatment process. I specifically focus on postinfusion care, as it relates to immune monitoring and infection prevention. Compared with allogeneic hematopoietic cell transplantation, delivering autologous gene therapy to an individual with SCD has distinct nuances that require awareness and special interventions. Using clinical vignettes derived from real-life patients, I provide perspectives on the complex decision-making process for gene therapy for SCD based on currently available data and make recommendations for evaluating and supporting these patients.

Late effects of hemopoietic stem cell transplant for sickle cell disease: monitoring and management

INTRODUCTION

Allogeneic hemopoietic stem cell transplantation (HSCT) is a curative therapy for sickle cell disease (SCD). Exposure to both SCD and HSCT conditioning regimens is associated with late health effects.

AREAS COVERED

This review addresses post-HSCT outcomes and late health effects among individuals with SCD exposed to allogeneic HSCT regimens, summarizes recommendations for long-term care, and identifies future survivorship research needs.

EXPERT OPINION

Individuals with SCD exposed to HSCT and gene therapy require multidisciplinary care to monitor late health effects. To optimize care, multi-disciplinary clinics that include experts in late effects of HSCT exposure, SCD, complex chronic pain, mental health, and social work are needed. Research defining the late effects of exposure is needed to inform patient management and build clinical care infrastructure.

Secondary haematological dysplasia after CAR-T-cell therapy for acute lymphoblastic leukaemia in children

The use of CAR-T is becoming more widespread in the treatment of haematological malignancies. In adults, secondary myelodysplastic syndromes (MDS) after CAR-T have been described. However, there are currently no data on the risk of MDS following CAR-T in children treated for acute lymphoblastic leukaemia (ALL). We studied all children treated with CAR-T cells at Hospital Sant Joan de Déu in Barcelona and those with persistent cytopenias were evaluated at the cytological, cytogenetic, and molecular levels to look for MDS. A total of 106 patients received CAR-T for ALL. Among 40 patients without early relapse or subsequent therapy after CAR-T, four fulfilled the WHO criteria for myelodysplasia. These four patients had received a haematopoietic stem cell transplantation (HSCT) prior to CAR-T and presented cytopenias with severe dysplastic changes in bone marrow after CAR-T. One patient had clonal MDS with high-risk cytogenetics arising from the host cells requiring a HSCT. Three patients had non-progressive dysplasia arising from the donor cells. Two are alive in complete remission with stable cytopenias and one succumbed to ALL relapse. This is the first description of post-CAR-T MDS and haematological dysplasia in children and highlights the need to monitor children with persistent post-CAR-T cytopenias.

A simplified G-CSF-free procedure allows for in vivo HSC gene therapy of sickle cell disease in a mouse model

ABSTRACT

We have reported the direct repair of the sickle cell mutation in vivo in a disease model using vectorized prime editors after hematopoietic stem cell (HSC) mobilization with granulocyte colony-stimulating factor (G-CSF)/AMD3100. The use of G-CSF for HSC mobilization is a hurdle for the clinical translation of this approach. Here, we tested a G-CSF-free mobilization regimen using WU-106, an inhibitor of integrin α4β1, plus AMD3100 for in vivo HSC prime editing in sickle cell disease (SCD) mice. Mobilization with WU-106 + AMD3100 in SCD mice was rapid and efficient. In contrast to the G-CSF/AMD3100 approach, mobilization of activated granulocytes and elevation of the key proinflammatory cytokine interleukin-6 in the serum were minimal. The combination of WU-106 + AMD3100 mobilization and IV injection of the prime editing vector together with in vivo selection resulted in ∼23% correction of the SCD mutation in the bone marrow and peripheral blood cells of SCD mice. The treated mice demonstrated phenotypic correction, as reflected by normalized blood parameters and spleen size. Editing frequencies were significantly increased (29%) in secondary recipients, indicating the preferential mobilization/transduction of long-term repopulating HSCs. Using this approach, we found <1% undesired insertions/deletions and no detectable off-target editing at the top-scored potential sites. Our study shows that in vivo transduction to treat SCD can now be done within 2 hours involving only simple IV injections with a good safety profile. The same-day mobilization regimen makes in vivo HSC gene therapy more attractive for resource-poor settings, where SCD does the most damage.

Azathioprine/hydroxyurea preconditioning prior to nonmyeloablative matched sibling donor hematopoietic stem cell transplantation in adults with sickle cell disease: A prospective observational cohort study

Nonmyeloablative, matched sibling donor hematopoietic stem cell transplantation with alemtuzumab/total body irradiation (TBI) conditioning is a curative therapy with low toxicity for adults with sickle cell disease (SCD). However, relatively low donor chimerism levels and graft rejection remain important challenges. We hypothesized that adding azathioprine/hydroxyurea preconditioning will improve donor chimerism levels and reduce graft failure rate. In this prospective cohort study, we enrolled consecutive adult patients with SCD undergoing matched sibling donor transplantation at the Amsterdam UMC. Patients received azathioprine 150 mg/day and hydroxyurea 25 mg/kg/day for 3 months prior to alemtuzumab 1 mg/kg and 300 cGy TBI conditioning. Twenty patients with SCD (median age 26 years [range 19-49], 13 females) were transplanted. Median follow-up was 46.0 months (IQR 21.8-57.9). One-year overall survival and event-free survival (graft failure or death) were both 95% (95% confidence interval 86-100). Mean donor myeloid and T-cell chimerism 1-year post-transplant were 95.2% (SD ±10.6) and 67.3% (±15.3), respectively. One patient (5%) experienced graft failure without autologous regeneration, resulting in infections and death. All other patients had a corrected SCD phenotype and were able to discontinue sirolimus. Three patients were successfully treated with alemtuzumab (1 mg/kg) after the transplant because of declining donor chimerism and cytopenias to revert impending graft rejection. Toxicity was mostly related to sirolimus and alemtuzumab. One patient developed steroid-responsive grade II intestinal acute graft-versus-host disease. Collectively, preconditioning with azathioprine/hydroxyurea prior to nonmyeloablative matched sibling donor transplantation resulted in excellent event-free survival and robust donor T-cell chimerism, enabling the successful withdrawal of sirolimus. ClinicalTrials.gov: NCT05249452.

Is There any Relationship Between the Repeated Complications of Sickle Cell Disease and the Potential Development of Acute Leukemia?

Sickle cell disease (SCD) is a severe monogenic hereditary hemoglobinopathy that is characterized by repeated clinical and biological manifestations able to generate stress erythopoiesis. A clonal hematopoiesis involving mainly variants of TP53, DNMT3A, ASXL1, and/or TET2 may be more prevalent in patients with SCD, suggesting that mutations in these genes may lead to an increased risk of leukemia. An increased prevalence of leukemia in patients with SCD has been confirmed by an increasing number of acute myeloid leukemia cases with myelodysplastic features reported in this patient population even in the absence of disease-modifying treatments. This leads to the hypothesis of a mechanism involving multifactorial causes through the pathophysiologic manifestations of SCD, in which cells are undergoing constant hematopoietic hyperplasia, inducing genomic damage and somatic mutations.

Clonal hematopoiesis in children with predisposing conditions

Clonal hematopoiesis in children and young adults differs from that occuring in the older adult population. A variety of stressors drive this phenomenon, sometimes independent of age-related processes. For the purposes of this review, we adopt the term clonal hematopoiesis in predisposed individuals (CHIPI) to differentiate it from classical, age-related clonal hematopoiesis of indeterminate potential (CHIP). Stress-induced CHIPI selection can be extrinsic, such as following immunologic, infectious, pharmacologic, or genotoxic exposures, or intrinsic, involving germline predisposition from inherited bone marrow failure syndromes. In these conditions, clonal advantage relates to adaptations allowing improved cell fitness despite intrinsic defects affecting proliferation and differentiation. In certain contexts, CHIPI can improve competitive fitness by compensating for germline defects; however, the downstream effects of clonal expansion are often unpredictable - they may either counteract the underlying pathology or worsen disease outcomes. A more complete understanding of how CHIPI arises in young people can lead to the definition of preleukemic states and strategies to assess risk, surveillance, and prevention to leukemic transformation. Our review summarizes current research on stress-induced clonal dynamics in individuals with germline predisposition syndromes.

How I treat anemia in older adults

ABSTRACT

With the global growing older adult population, clinicians face the common, yet complex challenge of how to evaluate and manage anemia in this population. Older age predisposes to common causes of anemia such as nutritional deficiencies, inflammatory disorders, chronic kidney disease, and hematologic malignancies. Failure to diagnose and appropriately manage anemia may result in decreased quality of life, impaired cognition, impaired mobility, and increased mortality. Anemia diagnosis in older adults presents a diagnostic conundrum because anemia may have a single cause, may be multifactorial, or may have no apparent cause even after an extensive evaluation. We believe a systematic approach to diagnosis ensures appropriate testing and avoids the pitfall of undertreatment and overtreatment. In this article we present our recommended approach through common scenarios for the management of anemia in the older adult.

Haematopoietic stem cell health in sickle cell disease and its implications for stem cell therapies and secondary haematological disorders

Gene modification of haematopoietic stem cells (HSCs) is a potentially curative approach to sickle cell disease (SCD) and offers hope for patients who are not eligible for allogeneic HSC transplantation. Current approaches require in vitro manipulation of healthy autologous HSC prior to their transplantation. However, the health and integrity of HSCs may be compromised by a variety of disease processes in SCD, and challenges have emerged in the clinical trials of gene therapy. There is also concern about increased susceptibility to haematological malignancies during long-term follow up of patients, and this raises questions about genomic stability in the stem cell compartment. In this review, we evaluate the evidence for HSC deficits in SCD and then discuss their potential causation. Finally, we suggest several questions which need to be addressed in order to progress with successful HSC manipulation for gene therapy in SCD.

Gene therapy for sickle cell disease

Sickle cell disease (SCD) is potentially curable after allogeneic hematopoietic stem cell transplantation (HSCT) or autologous HSCT after ex vivo genetic modification. Autologous HSCT with gene therapy has the potential to overcome many of the limitations of allogeneic HSCT that include the lack of suitable donors, graft-versus-host disease, the need for immune suppression, and the potential for graft rejection. Significant progress in gene therapy for SCD has been made over the past several decades, now with a growing number of clinical trials investigating various gene addition and gene editing strategies. Available results from a small number of patients, some with relatively short follow-up, are promising as a potentially curative strategy, with current efforts focused on continuing to improve the efficacy, durability, and safety of gene therapies for the cure of SCD.

Clonal hematopoiesis in frequent whole blood donors

Healthy volunteer donors are committed to contributing key medical resources. Repeated, regular donation of whole blood represents a specific trigger of hematopoietic stress. Hematopoietic stem cells (HSCs) are known to respond to environmental triggers by altering their differentiation and/or proliferative behavior. This can manifest in long-term changes in the clonal dynamics of HSCs, such as the age-associated expansion of HSCs carrying somatic mutations in genes associated with hematologic cancers-that is, clonal hematopoiesis (CH). A recent study revealed a higher prevalence of CH in frequent donors driven by low-risk mutations in genes encoding for epigenetic modifiers, with DNMT3A and TET2 being the most common. No difference in the prevalence of known preleukemic driver mutations was detected between the cohorts, underscoring the safety of repetitive blood donations. Functional analyses suggest a link between the presence of selected DNMT3A mutations found in the frequent donor group and the responsiveness of the cells to the molecular mediator of bleeding stress, erythropoietin (EPO), but not inflammation. These findings define EPO as one of the environmental factors that provide a fitness advantage to specific mutant HSCs. Analyzing CH prevalence and characteristics in other donor cohorts will be important to comprehensively assess the health risks associated with the different types of donation.

Clonal selection of hematopoietic stem cells after gene therapy for sickle cell disease

Gene therapy (GT) provides a potentially curative treatment option for patients with sickle cell disease (SCD); however, the occurrence of myeloid malignancies in GT clinical trials has prompted concern, with several postulated mechanisms. Here, we used whole-genome sequencing to track hematopoietic stem cells (HSCs) from six patients with SCD at pre- and post-GT time points to map the somatic mutation and clonal landscape of gene-modified and unmodified HSCs. Pre-GT, phylogenetic trees were highly polyclonal and mutation burdens per cell were elevated in some, but not all, patients. Post-GT, no clonal expansions were identified among gene-modified or unmodified cells; however, an increased frequency of potential driver mutations associated with myeloid neoplasms or clonal hematopoiesis (DNMT3A- and EZH2-mutated clones in particular) was observed in both genetically modified and unmodified cells, suggesting positive selection of mutant clones during GT. This work sheds light on HSC clonal dynamics and the mutational landscape after GT in SCD, highlighting the enhanced fitness of some HSCs harboring pre-existing driver mutations. Future studies should define the long-term fate of mutant clones, including any contribution to expansions associated with myeloid neoplasms.

Clonal hematopoiesis and inflammation: A review of mechanisms and clinical implications

Clonal hematopoiesis (CH) is defined by the presence of somatic mutations in hematopoietic stem and progenitor cells (HSPC). CH is associated primarily with advancing age and confers an elevated risk of progression to overt hematologic malignancy and cardiovascular disease. Increasingly, CH is associated with a wide range of diseases driven by, and sequelae of, inflammation. Accordingly, there is great interest in better understanding the pathophysiologic and clinical relationship between CH, aging, and disease. Both observational and experimental findings support the concept that CH is a potential common denominator in the inflammatory outcomes of aging. However, there is also evidence that local and systemic inflammatory states promote the growth and select for CH clones. In this review, we aim to provide an up-to-date summary of the nature of the relationship between inflammation and CH, which is central to unlocking potential therapeutic opportunities to prevent progression to myeloid malignancy.

Gene Therapy for Hemoglobinopathies

β-Thalassemia and sickle cell disease are autosomal recessive disorders of red blood cells due to mutations in the adult β-globin gene, with a worldwide diffusion. The severe forms of hemoglobinopathies are fatal if untreated, and allogeneic bone marrow transplantation can be offered to a limited proportion of patients. The unmet clinical need and the disease incidence have promoted the development of new genetic therapies based on the engineering of autologous hematopoietic stem cells. Here, the steps of ex vivo gene therapy development are reviewed along with results from clinical trials and recent new approaches employing cutting edge gene editing tools.

Clonal hematopoiesis and inflammation - the perpetual cycle

Acquired genetic or cytogenetic alterations in a blood stem cell that confer clonal fitness promote its relative expansion leading to clonal hematopoiesis (CH). Despite a largely intact hematopoietic output, CH is associated with a heightened risk of progression to hematologic malignancies and with non-hematologic health manifestations, including cardiovascular disease and overall mortality. We focus on the evidence for the role of inflammation in establishing, maintaining and reciprocally being affected by CH. We describe the known pro-inflammatory signals associated with CH and preclinical studies that elucidated the cellular mechanisms involved. We review the evolving literature on early-onset CH in germline predisposition conditions and the possible role of immune dysregulation in this context.

Sickle cell disease and acute leukemia: one case report and an extensive review

Population-based studies and case reports suggest that there may be an increased risk of acute leukemia associated with sickle cell disease (SCD). Following the description of a new case report, an extensive review of the literature identified 51 previously described cases. Most cases study showed myelodysplastic features confirmed, when available, by genetic markers such as chromosome 5 and/or chromosome 7 abnormalities and TP53 gene mutations. The increased risk of leukemogenesis is certainly multifactorial and related to the pathophysiologic mechanisms of the clinical manifestations of SCD. Chronic hemolysis and secondary hemochromatosis may cause increased chronic inflammation, resulting in persistent marrow stress, which could potentially compromise the genomic stability of the hematopoietic stem cells generating genomic damage and somatic mutations over the course of SCD and its treatment, resulting in a clone that led to acute myeloid leukemia.

In vivo HSC prime editing rescues sickle cell disease in a mouse model

Sickle cell disease (SCD) is a monogenic disease caused by a nucleotide mutation in the β-globin gene. Current gene therapy studies are mainly focused on lentiviral vector-mediated gene addition or CRISPR/Cas9-mediated fetal globin reactivation, leaving the root cause unfixed. We developed a vectorized prime editing system that can directly repair the SCD mutation in hematopoietic stem cells (HSCs) in vivo in a SCD mouse model (CD46/Townes mice). Our approach involved a single intravenous injection of a nonintegrating, prime editor-expressing viral vector into mobilized CD46/Townes mice and low-dose drug selection in vivo. This procedure resulted in the correction of ∼40% of βS alleles in HSCs. On average, 43% of sickle hemoglobin was replaced by adult hemoglobin, thereby greatly mitigating the SCD phenotypes. Transplantation in secondary recipients demonstrated that long-term repopulating HSCs were edited. Highly efficient target site editing was achieved with minimal generation of insertions and deletions and no detectable off-target editing. Because of its simplicity and portability, our in vivo prime editing approach has the potential for application in resource-poor countries where SCD is prevalent.

Secondary Neoplasms After Hematopoietic Cell Transplant for Sickle Cell Disease

PURPOSE

To report the incidence and risk factors for secondary neoplasm after transplantation for sickle cell disease.

METHODS

Included are 1,096 transplants for sickle cell disease between 1991 and 2016. There were 22 secondary neoplasms. Types included leukemia/myelodysplastic syndrome (MDS; n = 15) and solid tumor (n = 7). Fine-Gray regression models examined for risk factors for leukemia/MDS and any secondary neoplasm.

RESULTS

The 10-year incidence of leukemia/MDS was 1.7% (95% CI, 0.90 to 2.9) and of any secondary neoplasm was 2.4% (95% CI, 1.4 to 3.8). After adjusting for other risk factors, risks for leukemia/MDS (hazard ratio, 22.69; 95% CI, 4.34 to 118.66; P = .0002) or any secondary neoplasm (hazard ratio, 7.78; 95% CI, 2.20 to 27.53; P = .0015) were higher with low-intensity (nonmyeloablative) regimens compared with more intense regimens. All low-intensity regimens included total-body irradiation (TBI 300 or 400 cGy with alemtuzumab, TBI 300 or 400 cGy with cyclophosphamide, TBI 200, 300, or 400 cGy with cyclophosphamide and fludarabine, or TBI 200 cGy with fludarabine). None of the patients receiving myeloablative and only 23% of those receiving reduced-intensity regimens received TBI.

CONCLUSION

Low-intensity regimens rely on tolerance induction and establishment of mixed-donor chimerism. Persistence of host cells exposed to low-dose radiation triggering myeloid malignancy is one plausible etiology. Pre-existing myeloid mutations and prior inflammation may also contribute but could not be studied using our data source. Choosing conditioning regimens likely to result in full-donor chimerism may in part mitigate the higher risk for leukemia/MDS.

Management of Older Adults with Sickle Cell Disease: Considerations for Current and Emerging Therapies

People with sickle cell disease (SCD) are living longer than ever before, with the median survival increasing from age 14 years in 1973, beyond age 40 years in the 1990s, and as high as 61 years in recent cohorts from academic centers. Improvements in survival have been attributed to initiatives, such as newborn screening, penicillin prophylaxis, vaccination against encapsulated organisms, better detection and treatment of splenic sequestration, and improved transfusion support. There are an estimated 100,000 people living with SCD in the United States and millions of people with SCD globally. Given that the number of older adults with SCD will likely continue to increase as survival improves, better evidence on how to manage this population is needed. When managing older adults with SCD (defined herein as age ≥ 40 years), healthcare providers should consider the potential pitfalls of extrapolating evidence from existing studies on current and emerging therapies that have typically been conducted with participants at mean ages far below 40 years. Older adults with SCD have historically had little to no representation in clinical trials; therefore, more guidance is needed on how to use current and emerging therapies in this population. This article summarizes the available evidence for managing older adults with SCD and discusses potential challenges to using approved and emerging drugs in this population.

Variation and impact of polygenic hematologic traits in monogenic sickle cell disease

Several of the complications observed in sickle cell disease (SCD) are influenced by variation in hematologic traits (HT), such as fetal hemoglobin (HbF) level and neutrophil count. Previous large-scale genome-wide association studies carried out in largely healthy individuals have identified thousands of variants associated with HT, which have then been used to develop multi-ancestry polygenic trait scores (PTS). Here, we tested whether these PTS associate with HT in SCD patients and if they can improve statistical models associated with SCD-related complications. In 2,056 SCD patients, we found that the PTS predicted less HT variance than in non-SCD individuals of African ancestry. This was particularly striking at the Duffy/DARC locus, where we observed an epistatic interaction between the SCD genotype and the Duffy null variant (rs2814778) that led to a two-fold weaker effect on neutrophil count. PTS for these HT which are measured as part of routine practice were not associated with complications in SCD. In contrast, we found that a simple PTS for HbF that includes only six variants explained a large fraction of the phenotypic variation (20.5-27.1%), associated with acute chest syndrome and stroke risk, and improved the statistical modeling of the vaso-occlusive crisis rate. Using Mendelian randomization, we found that increasing HbF by 4.8% reduces stroke risk by 39% (P=0.0006). Taken together, our results highlight the importance of validating PTS in large diseased populations before proposing their implementation in the context of precision medicine initiatives.

Knowledge to date on secondary malignancy following hematopoietic cell transplantation for sickle cell disease

Allogeneic hematopoietic cell transplantation, gene therapy, and gene editing offer a potential cure for sickle cell disease (SCD). Unfortunately, myelodysplastic syndrome and acute myeloid leukemia development have been higher than expected after graft rejection following nonmyeloablative conditioning and lentivirus-based gene therapy employing myeloablative busulfan for SCD. Somatic mutations discovered in 2 of 76 patients who rejected their grafts were identified at baseline at much lower levels. While a whole-genome sequencing analysis reported no difference between patients with SCD and controls, a study including whole-exome sequencing revealed a higher prevalence of clonal hematopoiesis in individuals with SCD compared with controls. Genetic risk factors for myeloid malignancy development after curative therapy for SCD are currently being explored. Once discovered, decisions could be made about whether gene therapy may be feasible vs allogeneic hematopoietic cell transplant, which results in full donor chimerism. In the meantime, care should be taken to perform a benefit/risk assessment to help patients identify the best curative approach for them. Long-term follow-up is necessary to monitor for myeloid malignancies and other adverse effects of curative therapies for SCD.

Effective therapies for sickle cell disease: are we there yet?

Sickle cell disease (SCD) is a common genetic blood disorder associated with acute and chronic pain, progressive multiorgan damage, and early mortality. Recent advances in technologies to manipulate the human genome, a century of research and the development of techniques enabling the isolation, efficient genetic modification, and reimplantation of autologous patient hematopoietic stem cells (HSCs), mean that curing most patients with SCD could soon be a reality in wealthy countries. In parallel, ongoing research is pursuing more facile treatments, such as in-vivo-delivered genetic therapies and new drugs that can eventually be administered in low- and middle-income countries where most SCD patients reside.

Clonal hematopoiesis: Mutation-specific adaptation to environmental change

Clonal hematopoiesis of indeterminate potential (CHIP) describes a widespread expansion of genetically variant hematopoietic cells that increases exponentially with age and is associated with increased risks of cancers, cardiovascular disease, and other maladies. Here, we discuss how environmental contexts associated with CHIP, such as old age, infections, chemotherapy, or cigarette smoking, alter tissue microenvironments to facilitate the selection and expansion of specific CHIP mutant clones. Further, we consider major remaining gaps in knowledge, including intrinsic effects, clone size thresholds, and factors affecting clonal competition, that will determine future application of this field in transplant and preventive medicine.

Clonal Hematopoiesis and the Risk of Hematologic Malignancies after Curative Therapies for Sickle Cell Disease

Sickle cell disease (SCD) is associated with severe morbidity and early mortality. Two large population studies found an increased risk for leukemia in individuals with SCD. Notably, while the relative risk of leukemia development is high, the absolute risk is low in individuals with SCD who do not receive cell-based therapies. However, the risk of leukemia in SCD is high after graft rejection and with gene therapy. Clonal hematopoiesis (CH) is a well-recognized premalignant condition in the general population and in patients after high-dose myelotoxic therapies. Recent studies suggest that CH may be more common in SCD than in the general population, outside the cell-based therapy setting. Here, we review risk factors for CH and progression to leukemia in SCD. We surmise why patients with SCD are at an increased risk for CH and why leukemia incidence is unexpectedly high after graft rejection and gene therapy for SCD. Currently, we are unable to reliably assess genetic risk factors for leukemia development after curative therapies for SCD. Given our current knowledge, we recommend counseling patients about leukemia risk and discussing the importance of an individualized benefit/risk assessment that incorporates leukemia risk in patients undergoing curative therapies for SCD.

The hematopoietic saga of clonality in sickle cell disease

Sickle cell disease (SCD) is associated with an increased risk of vascular-occlusive events and of leukemia. Clonal hematopoiesis (CH) may increase both risks. In turn, physiologic abnormalities in SCD may modify the incidence and/or distribution of genetic alterations in CH. In a recent issue of the JCI, Liggett et al. found no difference in CH rate between individuals with versus without SCD. Here we contextualize this report and discuss the complex interplay between CH and SCD with particular attention to consequences for emerging gene therapies. We further consider the limitations in our current understanding of these topics that must be addressed in order to optimize therapeutic strategies for SCD.

Clonal hematopoiesis in sickle cell disease

BACKGROUND

Curative gene therapies for sickle cell disease (SCD) are currently undergoing clinical evaluation. The occurrence of myeloid malignancies in these trials has prompted safety concerns. Individuals with SCD are predisposed to myeloid malignancies, but the underlying causes remain undefined. Clonal hematopoiesis (CH) is a pre-malignant condition that also confers significant predisposition to myeloid cancers. While it has been speculated that CH may play a role in SCD-associated cancer predisposition, limited data addressing this issue have been reported.

METHODS

Here, we leveraged 74,190 whole genome sequences to robustly study CH in SCD. Somatic mutation calling methods were used to assess CH in all samples and comparisons between individuals with and without SCD were performed.

RESULTS

While we had sufficient power to detect a greater than 2-fold increased rate of CH, we found no detectable variation in rate or clone properties between individuals affected by SCD and controls. The rate of CH in individuals with SCD was unaltered by hydroxyurea use.

CONCLUSIONS

We did not observe an increased risk for acquiring detectable CH in SCD, at least as measured by whole genome sequencing. These results should help guide ongoing efforts and further studies that seek to better define the risk factors underlying myeloid malignancy predisposition in SCD and help ensure that curative therapies can be more safely applied.

FUNDING

Funding was provided by the New York Stem Cell Foundation and National Institutes of Health. The funders had no role in study design or reporting.

Editing outside the body: Ex vivo gene-modification for β-hemoglobinopathy cellular therapy

Genome editing produces genetic modifications in somatic cells, offering novel curative possibilities for sickle cell disease and β-thalassemia. These opportunities leverage clinical knowledge of hematopoietic stem cell transplant and gene transfer. Advantages to this mode of ex vivo therapy include locus-specific alteration of patient hematopoietic stem cell genomes, lack of allogeneic immune response, and avoidance of insertional mutagenesis. Despite exciting progress, many aspects of this approach remain to be optimized for ideal clinical implementation, including the efficiency and specificity of gene modification, delivery to hematopoietic stem cells, and robust and nontoxic engraftment of gene-modified cells. This review highlights genome editing as compared to other genetic therapies, the differences between editing strategies, and the clinical prospects and challenges of implementing genome editing as a novel treatment. As the world's most common monogenic disorders, the β-hemoglobinopathies are at the forefront of bringing genome editing to the clinic and hold promise for molecular medicine to address human disease at its root.