Ballmaier and colleagues (page 137) have identified the molecular cause of congenital amegakaryocytic thrombocytopenia (CAMT) in 9 patients: deficiency in expression or function of the thrombopoietin receptor c-mpl. Together with 2 earlier, smaller studies, this report adds CAMT to the growing list of congenital disorders of hematopoietic cytokine receptors, such as Laron dwarfism (growth hormone receptor) and severe combined immunodeficiency (γC chain of the IL-2, IL-4, IL-7, IL-15, and IL-21 receptors). The extensive cellular and molecular evaluation performed in the study adds to our understanding of the role of thrombopoietin in hematopoiesis, by verifying in humans important conclusions derived from studies of genetically engineered mpl-deficient mice. It is now clear that, like that of mice, the human thrombopoietin/mpl system is vital for hematopoietic stem cell function, as these patients have defects in all hematopoietic progenitors and develop pancytopenia, probably due to stem cell exhaustion.

In addition to defining another disease on the genetic level and extending our physiologic understanding of hematopoiesis, Ballmaier and colleagues raise a number of very interesting scientific and clinical questions. Five of the mutations identified by the authors were frame-shift or nonsense mpl mutations, but the remainder were more subtle. What do the point mutants in the extracellular domain of mpl tell us about the structure-function relationships of the receptor? If mpl deficiency can cause CAMT, why have thrombopoietin mutations not been identified as an etiology? And should CAMT patients be considered for gene therapy? As CAMT is a severe disorder that eventuates in aplastic anemia, that can only be treated by stem cell transplantation, and that requires a low level of gene expression to remedy and as mpl-transduced stem cells will have an obvious growth advantage in patients, it seems time to add the disease to the list of candidate diseases for gene therapy.

The concept of introducing “suicide” genes into cells via gene-transfer vectors in order to allow subsequent controlled destruction of the cells was conceived initially as a method to treat aggressive brain tumors. The prototype suicide gene encodes the herpes simplex virus thymidine kinase enzyme gene (HS-tk).Introduction of this gene into target cells renders them susceptible to specific killing by nucleoside analogs such as ganciclovir. Bonini, Bordignon, and colleagues reasoned that introduction of a suicide gene into allogeneic T lymphocytes could be used to control posttransplantation EBV lymphoproliferation or leukemia relapse but allow destruction of the cells if significant GVHD occurred. Their 1997 pioneering study reported encouraging data in 3 patients receiving DLI after transplantation, with evidence for a graft-versus-leukemia effect and effective control of GVHD with subsequent administration of ganciclovir.

In this issue, Tiberghien and colleagues report on the application of this concept to a larger group of patients, moving the administration of HS-tk gene–modified T cells back to the time of initial allogeneic marrow transplantation (see page 61). This detailed study illustrates both the clinical potential and the practical difficulties of this complex technology. Successful transduction and selection ofHS-tk T cells from 12 donors was documented, and circulating gene-modified cells were detectable in all patients after transplantation. Three-quarters of patients with significant acute or chronic GVHD were treated successfully with ganciclovir alone, accompanied by reduction in the number of circulating gene-modified T cells. But it was of concern that 3 of 12 patients developed posttransplantation EBV lymphoproliferation, suggesting that the prolonged ex vivo culture period necessary to allow transduction and selection of HS-tk donor T cells may have compromised function of these cells. Future studies will be needed to explore alternative transduction and selection approaches and suicide genes. In the setting of alternative donor transplants, the ability to deliver a “designer” graft containing optimal stem cell numbers along with T cells able to facilitate engraftment and deliver a graft-versus-tumor effect but also sensitive to killing via a suicide gene is very attractive, especially as an alternative to high-dose systemic immunosuppression, which is often ineffective in treating established severe GVHD.

The cytokine receptor molecular family members demonstrate a particular propensity to exist in both anchored and soluble forms. In this issue Meißner and colleagues (page 181) provide solid evidence that the murine γc, the subunit common to the receptors for IL-2, IL-4, IL- 7, IL-9, and IL-15, now joins the growing list of cytokine receptors that can escape their membrane tether. Their data provides much insight into the regulated nature of the production of the soluble γc (sγc), the structure of the soluble receptor, the biological impact of sγc, and the mechanism by which the soluble isoform arises.

The soluble γc joins a short list, including soluble gp130 and soluble βc, of soluble versions of cytokine receptor subunits that have no intrinsic affinity for ligand. Despite this, Meißner's data suggests that sγc can antagonize the biologic activity of its ligands by participating in the assembly of the larger cytokine–cytokine receptor complex on the cell surface, presumably usurping the action of the membrane spanning version of γc. Given that other subunits of the extended IL-2R family are known to exist in soluble forms, there arises the possibility that a whole series of multisubunit ligand-receptor complexes assemble in solution each contributing to the modulation of cellular response. Furthermore, knowing that severe combined immunodeficiency can arise from loss of function defects of γc, one wonders what role this antagonistic soluble γc might play in immunodeficiency states; a question that provides impetus for a further understanding of the potentially unique proteolytic mechanisms which the authors show seem to control the release of γc from the cell surface. Your humble correspondent is left once again amazed at the diversity that Nature is able to generate from variations on the simple theme of a cytokine receptor.

The rare leukocyte adhesion deficiency type II (LADII) syndrome is associated with absence of selectin ligand activity on leukocytes and defective selectin-dependent leukocyte adhesion. Defective selectin ligand activity is thought to be consequent to faulty fucosylation of the glycans that decorate leukocyte selectin counterreceptors and that are essential to selectin ligand activity. Faulty fucosylation in LADII has been associated with defects in the constitutive, mannose-dependent cytosolic pathway for synthesis of the fucosyltransferase substrate GDP-fucose, or in transport of this molecule into the lumen of the Golgi apparatus where fucosylation takes place. The fucosylation defect exhibited by cultured LADII cells may be overcome by growing the cells in media supplemented with fucose. This maneuver supplies a fucose-dependent salvage pathway for GDP-fucose synthesis and may also overcome defects in GDP-fucose transport. In a 1999 Bloodarticle (Marquardt et al, 94:3976-2935), this approach was tried in one LADII patient. This individual experienced a restoration of leukocyte selectin ligand activity and amelioration of infectious complications following chronic oral administration of fucose. But these studies could not exclude a noncausal association between re-expression of selectin ligands and fucose administration. Lühn and colleagues (see page 328) now extend these studies to demonstrate that leukocyte selectin ligand expression in this LADII patient is dependent upon fucose administration. In this study, leukocyte selectin ligand activity rapidly disappeared when fucose was withheld from the patient and reappeared following chronic readministration of the monosaccharide. This work confirms that fucosylation is essential to selectin ligand activity and to leukocyte trafficking processes responsible for host defense mechanisms. These studies also support prior evidence that selectin ligands contribute to homeostasis in blood leukocyte number, since this patient's leukocyte counts varied inversely with selectin ligand expression and serum fucose concentration. Although these studies further advance our understanding of the important role for fucosylation in selectin ligand activity, much remains to be learned about the pathophysiology of this disease and about the genes that control GDP-fucose synthesis. In contrast to the patient studied by this group, an LADII patient from a different kindred studied by another group did not respond to oral fucose. The 2 patients appear to exhibit distinct biochemical defects, although it is not yet clear if these correspond to mutations in the same locus, nor how these can account for the contrasting therapeutic responses to fucose. Molecular cloning of the defective locus in these LADII patients and characterization of the mutant alleles responsible for defective fucosylation should clarify the situation. These studies, considered together with several recent reports implicating fucosylation in the regulation of Notch-dependent signal transduction, should also eventually shed light on how defective fucosylation may contribute to the short stature, dysmorphology, and mental retardation also characteristic of LADII.

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