Correction of sickle cell disease in transgenic mouse models by gene therapy

Baroreceptor reflex pathways and neurotransmitters: 10 years on

The central nervous system plays a critical role in the management of blood flow to the tissues and its return to the heart and lungs. This is achieved by a complex interplay of neural efferent pathways, humoral mechanisms and afferent pathways. In this review, we focus on recent progress (within the past 10 years) that has been made in the sympathetic control of arterial blood pressure with a special emphasis on the role of baroreceptor mechanisms and central neurotransmitters. In particular, we focus on new features since 1991, such as neurotransmission in the nucleus tractus solitarius, the role of neurons in the most caudal part of the ventrolateral medulla oblongata and the increasing understanding of the exquisite control of different sympathetic pathways by different neurotransmitter systems.

New therapies in sickle cell disease

CONTEXT

New therapies have evolved from our improved understanding of the biology of sickle cell disease (SCD) and the availability of a useful transgenic animal model. Several therapeutic options are available that interrupt the sickling process at various key pathways. Nitric oxide (NO)is a critical factor in the pathophysiology of SCD and is a promising antisickling agent with vasodilation properties. NO regulates blood vessel tone, endothelial adhesion, and the severity of ischaemia-reperfusion injury and anaemia in SCD. Although NO is difficult to administer, its precursor, L-arginine, is an oral supplement.

STARTING POINT

J R Romero and colleagues recently demonstrated in sickle transgenic mice that oral arginine supplementation induced NO production and reduced red-cell density by inhibiting the Gardos channel, which modulates cell hydration and polymerisation of haemoglobin S (Blood 2002; 99:1103-08). Haemoglobinopathies can be cured by stem-cell transplantation. This therapy is now accepted treatment in symptomatic children. However, most patients lack a genotypically identical family donor. G La Nasa and colleagues demonstrated unrelated-donor stem-cell transplantation may give similar results to related-donor stem-cell transplantation when extended phenotypic matching is used (Blood 2002; 99: 4350-56). This pilot study offers the possibility of cure to patients without a family donor.

WHERE NEXT

Although potential opportunities to prevent morbidity in SCD through new therapies are exciting, most patients do not have access to standard multidisciplinary specialty care. Patients require both.

Dangerous liaisons: the role of "danger" signals in the immune response to gene therapy

Recent studies in gene transfer suggest that the innate immune system plays a significant role in impeding gene therapy. In this review, we examine factors that might influence the recruitment and activation of the innate system in the context of gene therapy. We have adopted a novel model of immunology that contends that the immune system distinguishes not between self and nonself, but between what is dangerous and what is not dangerous. In taking this perspective, we provide an alternative and complementary insight into some of the failures and successes of current gene therapy protocols.

Expression of a human beta-globin transgene in erythroid cells derived from retrovirally transduced transplantable human fetal liver and cord blood cells

Transfer of therapeutic genes to human hematopoietic stem cells (HSCs) using complex vectors at clinically relevant efficiencies remains a major challenge. Recently we described a stable retroviral vector that sustains long-term expression of green fluorescent protein (GFP) and a human beta-globin gene in the erythroid progeny of transduced murine HSCs. We now report the efficient transduction of primitive human CD34(+) fetal liver or cord blood cells with this vector and expression of the beta-globin transgene in the erythroid progeny of these human cells for at least 2 months. After growth factor prestimulation and then a 2- to 3-day exposure to the virus, 35% to 55% GFP(+) progeny were seen in assays of transduced colony-forming cells, primitive erythroid precursors that generate large numbers of glycophorin A(+) cells in 3-week suspension cultures, and 6-week long-term culture-initiating cells. In immunodeficient mice injected with unselected infected cells, 5% to 15% of the human cells regenerated in the marrow (including the erythroid cells) were GFP(+) 3 and 6 weeks after transplantation. Importantly, the numbers of GFP(+) human lymphoid and either granulopoietic or erythroid cells in individual mice 6 weeks after transplantation were significantly correlated, indicative of the initial transduction of human multipotent cells with in vivo repopulating activity. Expression of the transduced beta-globin gene in human cells obtained directly from the mice or after their differentiation into erythroid cells in vitro was demonstrated by reverse transcriptase-polymerase chain reaction using specific primers. These experiments represent a significant step toward the realization of a gene therapy approach for human beta-globin gene disorders.

Developing treatment for sickle cell disease

Sickle cell disease pathophysiology results from sickle haemoglobin polymerisation and its effects on the sickle erythrocyte and the vasculature. Many of the abnormalities of sickle cell disease are secondary to the damage caused by the polymer and the injured red cell. Pharmacological treatment of the disease is focused on the inhibition of sickle haemoglobin polymerisation, prevention or repair of red cell dehydration and interruption of the interaction of sickle cells with the endothelium.

Hematopoietic stem cell gene therapy

Gene therapy applications that target hematopoietic stem cells (HSCs) offer great potential for the treatment of hematologic disease. Despite this promise, clinical success has been limited by poor rates of gene transfer, poor engraftment of modified cells, and poor levels of gene expression. We describe here the basic approach used for HSC gene therapy, briefly review some of the seminal clinical trials in the field, and describe several recent advances directed toward overcoming these limitations.

Biotechnologies and therapeutics: chromatin as a target

As alterations in gene expression underlie a considerable proportion of human diseases, correcting such aberrant transcription in vivo is expected to provide therapeutic benefit to the patient. Attempts to control endogenous mammalian genes, however, face a significant obstacle in the form of chromatin. Aberrant gene repression can be alleviated by using small-molecule inhibitors that exert nucleus-wide effects on chromatin-based repressors. Genome-wide chromatin remodeling also occurs during cloning via nuclear transfer, and causes the deregulation of epigenetically controlled genes. Regulation of genes in vivo can be accomplished via the use of designed transcription factors - these result from a fusion of a designed DNA-binding domain based on the zinc finger protein motif to a functional domain of choice.

Limitations of a mouse model of sickle cell anemia

We aimed to use an established murine model of sickle cell anemia to develop an unambiguous method for testing new therapies, with survival as an end point. Survival rates following various challenges were compared for three different groups of mice: (a) sickle cell mice expressing human hemoglobin-S exclusively ((h)beta(s)); (b) littermates that expressed both human hemoglobin S and murine beta major globin ((h)beta(s)(m)beta); and (c) wild-type C57BL/6 mice (wt). Two types of challenge were tested. The first set of studies was based upon recent observations indicating that granulocyte-colony stimulating factor (G-CSF) can precipitate severe complications in patients with sickle cell disease. While (h)beta(s) mice had higher neutrophil counts than (h)beta(s)(m)beta mice at baseline, (h)beta(s) mice tolerated several different doses and schedules of either human or murine G-CSF without adverse effects. A second type of challenge tested whether sickle cell mice exhibit an enhanced susceptibility to hemoglobin deoxygenation. Acute hemoglobin deoxygenation was accomplished either by a single intraperitoneal injection of sodium bisulfite or by a 1-h exposure to hypoxia. Neither intervention resulted in a significantly different survival rate for (h)beta(s) mice compared to either (h)beta(s)(m)beta or wt mice. Chronic twice-weekly exposures to hypoxia (1 h per exposure) also failed to produce significant differences in survival rates between (h)beta(s) mice, (h)beta(s)(m)beta, and wt mice over a period of 12 weeks. Our results demonstrate that neither G-CSF administration nor hypoxia accentuates survival differences between this model of sickle cell mouse and normal controls.

Globin gene transfer for the treatment of severe hemoglobinopathies: a paradigm for stem cell-based gene therapy

The prospect of treating blood disorders with genetically modified stem cells is highly promising. This therapeutic approach, however, raises a number of fundamental biological questions, spanning several research fields. Further investigation is required to better understand how to isolate and efficiently transduce hematopoietic stem cells (HSCs), while preserving optimal homing and self-renewing properties; how to design safe vectors permitting controlled expression of the transgene products; and how to promote host repopulation by engrafted HSCs. This article addresses basic issues in stem cell-based gene therapy from the perspective of regulating transgene expression, taking globin gene transfer for the treatment of severe hemoglobinopathies as a paradigm.

Sickle cell anaemia: progress in pathogenesis and treatment

The phenotypic expression of sickle cell anaemia varies greatly among patients and longitudinally in the same patient. It influences all aspects of the life of affected individuals including social interactions, intimate relationships, family relations, peer interactions, education, employment, spirituality and religiosity. The clinical manifestations of sickle cell anaemia are protean and fall into three major categories: anaemia and its sequelae;pain and related issues; andorgan failure including infection. Recent studies on the pathogenesis of sickle cell anaemia have centred on the sequence of events that occur between polymerisation of deoxy haemoglobin (Hb) S and vaso-occlusion. Cellular dehydration, inflammatory response and reperfusion injury seem to be important pathophysiological mechanisms. Management of sickle cell anaemia continues to be primarily palliative in nature, including supportive, symptomatic and preventative approaches to therapy. Empowerment and education are the major aspects of supportive care. Symptomatic management includes pain management, blood transfusion and treatment of organ failure. Pain managment should follow certain priniciples that include assessment, individualisation of therapy and proper utilisation of opioid and nonopioid analgesics in order to acheive adequate pain relief. Blood selected for transfusion should be leuko-reduced and phenotypically matched for the C, E and Kell antigens. Exchange transfusion is indicated in patients who are transfused chronically in order to prevent or delay the onset of iron-overload. Acute chest syndrome is the most common form of organ failure and its management should be agressive, including adequate ventilation, multiple antibacterials and simple or exchange blood transfusion depending on its severity. Preventitive therapy includes prophylactic penicillin in infants and children, blood transfusion (preferably exchange transfusion) in patients with stroke, and hydroxyurea in patients with frequent acute painful episodes. Bone marrow and cord blood transplantation have been successful modalities of curative therapy in selected children with sickle cell anaemia. Newer approaches to preventative therapy include cellular rehydration with agents that inhibit the Gardos channel or the KCl co-transport channel. Curative gene therapy continues to be investigational at the level of the test tube and transgenic mouse models.