3' UTR-truncated HMGA2 overexpression induces non-malignant in vivo expansion of hematopoietic stem cells in non-human primates

Chromatin modifier Hmga2 promotes adult hematopoietic stem cell function and blood regeneration in stress conditions

The molecular mechanisms governing the response of hematopoietic stem cells (HSCs) to stress insults remain poorly defined. Here, we investigated effects of conditional knock-out or overexpression of Hmga2 (High mobility group AT-hook 2), a transcriptional activator of stem cell genes in fetal HSCs. While Hmga2 overexpression did not affect adult hematopoiesis under homeostasis, it accelerated HSC expansion in response to injection with 5-fluorouracil (5-FU) or in vitro treatment with TNF-α. In contrast, HSC and megakaryocyte progenitor cell numbers were decreased in Hmga2 KO animals. Transcription of inflammatory genes was repressed in Hmga2-overexpressing mice injected with 5-FU, and Hmga2 bound to distinct regions and chromatin accessibility was decreased in HSCs upon stress. Mechanistically, we found that casein kinase 2 (CK2) phosphorylates the Hmga2 acidic domain, promoting its access and binding to chromatin, transcription of anti-inflammatory target genes, and the expansion of HSCs under stress conditions. Notably, the identified stress-regulated Hmga2 gene signature is activated in hematopoietic stem progenitor cells of human myelodysplastic syndrome patients. In sum, these results reveal a TNF-α/CK2/phospho-Hmga2 axis controlling adult stress hematopoiesis.

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.

Gene Therapy and Gene Editing for β-Thalassemia

After many years of intensive research, emerging data from clinical trials indicate that gene therapy for transfusion-dependent β-thalassemia is now possible. Strategies for therapeutic manipulation of patient hematopoietic stem cells include lentiviral transduction of a functional erythroid-expressed β-globin gene and genome editing to activate fetal hemoglobin production in patient red blood cells. Gene therapy for β-thalassemia and other blood disorders will invariably improve as experience accumulates over time. The best overall approaches are not known and perhaps not yet established. Gene therapy comes at a high cost, and collaboration between multiple stakeholders is required to ensure that these new medicines are administered equitably.

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.

Lentivector cryptic splicing mediates increase in CD34+ clones expressing truncated HMGA2 in human X-linked severe combined immunodeficiency

X-linked Severe Combined Immunodeficiency (SCID-X1) due to IL2RG mutations is potentially fatal in infancy where 'emergency' life-saving stem cell transplant may only achieve incomplete immune reconstitution following transplant. Salvage therapy SCID-X1 patients over 2 years old (NCT01306019) is a non-randomized, open-label, phase I/II clinical trial for administration of lentiviral-transduced autologous hematopoietic stem cells following busulfan (6 mg/kg total) conditioning. The primary and secondary objectives assess efficacy in restoring immunity and safety by vector insertion site analysis (VISA). In this ongoing study (19 patients treated), we report VISA in blood lineages from first eight treated patients with longer follow up found a > 60-fold increase in frequency of forward-orientated VIS within intron 3 of the High Mobility Group AT-hook 2 gene. All eight patients demonstrated emergence of dominant HMGA2 VIS clones in progenitor and myeloid lineages, but without disturbance of hematopoiesis. Our molecular analysis demonstrated a cryptic splice site within the chicken β-globin hypersensitivity 4 insulator element in the vector generating truncated mRNA transcripts from many transcriptionally active gene containing forward-oriented intronic vector insert. A two base-pair change at the splice site within the lentiviral vector eliminated splicing activity while retaining vector functional capability. This highlights the importance of functional analysis of lentivectors for cryptic splicing for preclinical safety assessment and a redesign of clinical vectors to improve safety.

Evolution of Gene Therapy, Historical Perspective

The earliest conceptual history of gene therapy began with the recognition of DNA as the transforming substance capable of changing the phenotypic character of a bacterium and then as the carrier of the genomic code. Early studies of oncogenic viruses that could insert into the mammalian genome led to the concept that these same viruses might be engineered to carry new genetic material into mammalian cells, including human hematopoietic stem cells (HSC). In addition to properly engineered vectors capable of efficient safe transduction of HSC, successful gene therapy required the development of efficient materials, methods, and equipment to procure, purify, and culture HSC. Increased understanding of the preparative conditioning of patients was needed to optimize the engraftment of genetically modified HSC. Testing concepts in pivotal clinical trials to assess the efficacy and determine the cause of adverse events has advanced the efficiency and safety of gene therapy. This article is a historical overview of the separate threads of discovery that joined together to comprise our current state of gene therapy targeting HSC.