Thalassemia: molecular pathology and management

Novel potential therapeutics to modify iron metabolism and red cell synthesis in diseases associated with defective erythropoiesis

Under normal conditions, iron metabolism is carefully regulated to sustain normal cellular functions and the production of hemoglobin in erythroid cells. Perturbation to the erythropoiesis-iron metabolism axis can result in iron imbalances and cause anemia or organ toxicity. Various congenital and acquired diseases associated with abnormal red cell production are characterized by aberrant iron absorption. Several recent studies have shown that improvements in red blood cell production also ameliorate iron metabolism and vice versa. Many therapeutics are now under development with the potential to improve a variety of hematologic diseases, from β-thalassemia and iron-refractory iron deficiency anemia to anemia of inflammation and polycythemia vera. This review summarizes selected mechanisms related to red cell production and iron metabolism and describes potential therapeutics and their current uses. We also consider the potential application of the discussed therapeutics on various diseases, alone or in combination. The vast repertoire of drugs under development offers new opportunities to improve the clinical care of patients suffering from congenital or acquired red blood cell disorders with limited or no treatment options.

Allogeneic bone marrow transplant in the absence of cytoreductive conditioning rescues mice with β-thalassemia major

β-thalassemia is a group of inherited blood disorders that result in defects in β-globin chain production. Cooley anemia (CA), or β-thalassemia major, is the most severe form of the disease and occurs when an individual has mutations in both copies of the adult β-globin gene. Patients with CA fail to make adult hemoglobin, exhibit ineffective erythropoiesis, experience severe anemia, and are transfusion dependent for life. Currently, allogeneic bone marrow transplantation (BMT) is the only cure; however, few patients have suitable donors for this procedure, which has significant morbidity and mortality. In this study, a novel humanized murine model of CA is rescued from lethal anemia by allogeneic BMT in the absence of cytoreductive conditioning. A single intravenous postnatal injection of allogeneic bone marrow results in stable, mixed hematopoietic chimerism. Five months after transplantation, donor cells accounted for approximately 90% of circulating erythrocytes and up to 15% of hematopoietic stem and progenitor cells. Transplanted mice are transfusion independent, have marked improvement of hematological indices, exhibit no growth retardation or signs of graft-versus-host disease, and are fertile. This study describes a method for the consistent engraftment of allogeneic donor hematopoietic cells that rescues a humanized mouse model of CA from lethal anemia, all in the absence of toxic cytoreductive conditioning.

Hydroxyurea for lifelong transfusion-dependent β-thalassemia: A meta-analysis

OBJECTIVE

Chronic blood transfusion remains the most feasible therapeutic option for lifelong transfusion-dependent β-thalassemia (lifelong TDβT). However, it is associated with serious risks and complications. Hydroxyurea (HU), an oral chemotherapeutic drug, is expected to increase hemoglobin levels, thereby minimizing the burden of blood transfusion and its complications. Growing literature over the last twenty years suggests promising results of the use HU in lifelong TDβT; however, its role and safety remain unanswered questions. The objective of this study was to evaluate the clinical efficacy and safety of HU in patients with lifelong TDβT.

METHODS

MEDLINE, EMBASE, Cochrane databases, and major preceding conferences for studies that assessed HU in lifelong TDβT patients were searched. The effect size was estimated as a proportion (responder/sample size).

RESULTS

Eleven observational studies, collectively involving 859 patients, fulfilled eligibility criteria. HU was associated with a significant decrease in transfusion need with complete and overall (≥50%) response rates of 26% [95% confidence interval (CI), 13-41%] and 60% (95% CI, 41-78%), respectively. No serious adverse effects were reported. All of the studies had several limitations, such as lack of a comparison group.

CONCLUSION

HU appears to be effective, well tolerated; however, large randomized clinical trials should be done to confirm such findings.

Humanized Mouse Model of Cooley's Anemia

A novel humanized mouse model of Cooley's Anemia (CA) was generated by targeted gene replacement in embryonic stem (ES) cells. Because the mouse does not have a true fetal hemoglobin, a delayed switching human gamma to beta(0) globin gene cassette (gammabeta(0)) was inserted directly into the murine beta globin locus replacing both adult mouse beta globin genes. The inserted human beta(0) globin allele has a mutation in the splice donor site that produces the same aberrant transcripts in mice as described in human cells. No functional human beta globin polypeptide chains are produced. Heterozygous gammabeta(0) mice suffer from microcytic anemia. Unlike previously described animal models of beta thalassemia major, homozygous gammabeta(0) mice switch from mouse embryonic globin chains to human fetal gamma globin during fetal life. When bred with human alpha globin knockin mice, homozygous CA mice survive solely upon human fetal hemoglobin at birth. This preclinical animal model of CA can be utilized to study the regulation of globin gene expression, synthesis, and switching; the reactivation of human fetal globin gene expression; and the testing of genetic and cell-based therapies for the correction of thalassemia.