Purpose of review

Hematopoietic cell transplantation (HCT) is a curative therapy for sickle cell disease (SCD) that is utilized very rarely because of limited allogeneic donor availability, limited healthcare resources needed to expand the treatment to regions in the world where most affected individuals reside, and by a view among SCD experts that HCT lacks the evidential rigor with short and long-term toxicity profiles that together might support its broader application.

Recent findings

In this update, recent advances focused on donor selection, reduced toxicity preparation for HCT, and treatment of young adults will be presented. The current status of conventional bone marrow transplantation with a human leukocyte antigen-identical sibling donor is summarized.

Summary

HCT for SCD is curative in almost all children who have a human leukocyte antigen-matched sibling donor. The future of this therapy will hinge on expanding the number of individuals who might be treated.

This article has a companion Counterpoint by DeBaun and Clayton.

In utero hematopoietic cell transplantation (IUHCTx) is a promising strategy to circumvent the challenges of postnatal hematopoietic stem cell (HSC) transplantation. The goal of IUHCTx is to introduce donor cells into a naïve host prior to immune maturation, thereby inducing donor-specific tolerance. Thus, this technique has the potential of avoiding host myeloablative conditioning with cytotoxic agents. Over the past two decades, several attempts at IUHCTx have been made to cure numerous underlying congenital anomalies with limited success. In this review, we will briefly review the history of IUHCTx and give a perspective on alpha thalassemia major, one target disease for its clinical application.

Background

It is well established that umbilical cord blood and bone marrow are biologically different stem cell sources.

Patients and methods

We analyzed the feasibility and outcome of hematopoietic stem cell transplantation (HSCT) in 13 children (median age 5.9 years) with hemoglobinopathies after the co- infusion of cord blood (CB) and bone marrow (BM) from the same human leucocyte antigen (HLA) identical sibling donor. We also compared outcomes of children with co-transplantation to outcomes in children with hemoglobinopathies who had received a BM (n = 21) or CB (n = 22) transplant alone.

Results

Compared to CB transplant (CBT) recipients, the co-transplant group had more rapid neutrophil (17 vs. 25 days, P = 0.013) and platelet (29 vs. 48 days, P = 0.009) recovery and less transplant related mortality. Patients who received a co-transplant had a lower incidence of ≥ grade II acute (0% vs. 26.3%) and chronic (0% vs. 21%) graft versus host disease (GVHD) compared to BM transplant (BMT) recipients (P = 0.055 and 0.045, respectively). With a median follow-up of >60 months in each treatment group, the 5-year probability of event free survival (EFS) was 100% in the co-transplant group, 90% after BMT and 86% after CBT (P = 0.42).

Conclusion

Co-transplantation of CB and BM from HLA-identical sibling donors appears to be a feasible and effective strategy to further optimize outcomes of HSCT for hemoglobinopathies.

Although a number of published trials exist of HLA-identical sibling hematopoietic cell transplantation (HCT) for sickle cell disease (SCD) that span 2 decades, when and for whom this therapy should be pursued is a subject of debate. Assessments of the risks of transplant-related complications that include infertility and debilitating graft-versus-host disease and long-term quality of life after successful HCT are difficult to perform without prospective trials in transplant and nontransplant cohorts. However, it is possible to assess the risk of mortality and to compare published rates of survival in individuals with SCD treated and not treated by HCT. In this brief review, projections about mortality risk based on recent published reports are reviewed and summarized. The published data show overall survival and event-free survival rates of 95% and 92%, respectively, in children treated by HLA-identical sibling HCT. The overall survival rates in the Center for International Blood and Marrow Transplant Research (N = 412) and European Blood and Marrow Transplant (N = 487) registries were 91% and 95%, respectively. These results provide broad support for the therapeutic value of HLA-identical sibling HCT for children with SCD and serve as the basis for a strong recommendation in favor of the option of HCT when a suitable donor is available. The experience of HLA-identical sibling HCT in adults with SCD is limited but appears to be similar to results in children. These preliminary observations, however, warrant further investigation.

The California Institute for Regenerative Medicine (CIRM) Alpha Stem Cell Clinic (ASCC) Network was launched in 2015 to address a compelling unmet medical need for rigorous, FDA-regulated, stem cell-related clinical trials for patients with challenging, incurable diseases. Here, we describe our multi-center experiences addressing current and future challenges.

Ability to accurately attribute adverse events post–gene therapy is required to describe the benefit-risk of these novel treatments. A SCD patient developed myelodysplastic syndrome post-LentiGlobin treatment; we show how insertional oncogenesis was excluded as the cause.

Graft-versus-host disease (GVHD) is a common complication of allogeneic bone marrow transplantation (BMT). Upregulation of inflammatory cytokines precedes the clinical presentation of GVHD and predicts its severity. In this report, thiol/redox metabolomics was used to identify metabolic perturbations associated with early preclinical (Day+4) and clinical (Day+10) stages of GVHD by comparing effects in Syngeneic (Syn; major histocompatibility complex- identical) and allogeneic transplant recipients (Allo BMT) in experimental models. While most metabolic changes were similar in both groups, plasma glutathione (GSH) was significantly decreased, and GSH disulfide (GSSG) was increased after allogeneic compared to syngeneic recipient and non-transplant controls. The early oxidation of the plasma GSH/GSSG redox couple was also observed irrespective of radiation conditioning treatment and was accompanied by significant rise in hepatic protein oxidative damage and ROS generation. Despite a significant rise in oxidative stress, compensatory increase in hepatic GSH synthesis was absent following Allo BMT. Early shifts in hepatic oxidative stress and plasma GSH loss preceded a statistically significant rise in TNF-α. To identify metabolomic biomarkers of hepatic GVHD injury, plasma metabolite concentrations analyzed at Day+10 were correlated with hepatic organ injury. GSSG (oxidized GSH) and β-alanine, were positively correlated, and plasma GSH cysteinylglycine, and branched chain amino acids were inversely correlated with hepatic injury. Although changes in plasma concentrations of cysteine, cystathionine (GSH precursors) and cysteinylglycine (a GSH catabolite) were not significant by univariate analysis, principal component analysis (PCA) indicated that accumulation of these metabolites after Allo BMT contributed significantly to early GVHD in contrast to Syn BMT. In conclusion, thiol/redox metabolomic profiling implicates that early dysregulation of host hepatic GSH metabolism and oxidative stress in sub-clinical GVHD before elevated TNF-α levels is associated with GVHD pathogenesis. Future studies will probe the mechanisms for these changes and examine the potential of antioxidant intervention strategies to modulate GVHD.

Mixed donor chimerism after hematopoietic cell transplantation for sickle cell disease (SCD) can result in resolution of disease symptoms, but symptoms recur when donor chimerism is critically low. The relationship between chimerism, hemoglobin S (HbS) level, and symptomatic disease was correlated retrospectively in 95 patients who had chimerism reports available at day 100 and at 1 and 2 years after transplantation. Recurrent disease was defined as recurrence of vaso-occlusive crises, acute chest syndrome, stroke, and/or HbS levels > 50%. Thirty-five patients maintained full donor chimerism (myeloid or whole blood) through 2 years. Donor chimerism was less than 10% (defined as graft failure) in 13 patients during this period. Mixed chimerism was reported in the remaining 47 patients (range, 10% to 94%). The lowest documented donor chimerism without symptomatic disease was 26%. Of 12 surviving patients with recurrent disease, 2 had recurrence of symptoms before documented graft failure (donor chimerism of 11% and 17%, respectively). Three patients underwent second transplantation for graft failure. None received donor leukocyte infusion to maintain mixed chimerism or prevent graft failure. We conclude stable donor chimerism greater than 25% is associated with resolution of SCD-related symptoms, and HbS levels in transplant recipients should be interpreted in context of the sickle trait status of the donors.

Genetic diseases of blood cells are prime candidates for treatment through ex vivo gene editing of CD34⁺ hematopoietic stem/progenitor cells (HSPCs), and a variety of technologies have been proposed to treat these disorders. Sickle cell disease (SCD) is a recessive genetic disorder caused by a single-nucleotide polymorphism in the β-globin gene (HBB). Sickle hemoglobin damages erythrocytes, causing vasoocclusion, severe pain, progressive organ damage, and premature death. We optimize design and delivery parameters of a ribonucleoprotein (RNP) complex comprising Cas9 protein and unmodified single guide RNA, together with a single-stranded DNA oligonucleotide donor (ssODN), to enable efficient replacement of the SCD mutation in human HSPCs. Corrected HSPCs from SCD patients produced less sickle hemoglobin RNA and protein and correspondingly increased wild-type hemoglobin when differentiated into erythroblasts. When engrafted into immunocompromised mice, ex vivo treated human HSPCs maintain SCD gene edits throughout 16 weeks at a level likely to have clinical benefit. These results demonstrate that an accessible approach combining Cas9 RNP with an ssODN can mediate efficient HSPC genome editing, enables investigator-led exploration of gene editing reagents in primary hematopoietic stem cells, and suggests a path toward the development of new gene editing treatments for SCD and other hematopoietic diseases.