Contagious Cancer
Nancy Andrews, MD, PhD
Dr. Andrews indicated no relevant conflicts of interest.
Murgia C, Pritchard JK, Kim SY, et al. Clonal origin and evolution of a transmissible cancer. Cell 2006;126:477-87.
Cancer is generally considered a private disease which arises in an individual with inherited, spontaneous, or induced genetic changes that predispose to malignant transformation. However, there are rare examples of transmissible cancers in non-human species. It has been appreciated for 130 years that canine transmissible venereal tumor (CTVT) spreads among dogs through sexual and oral contact. Living histiocytic tumor cells pass from one individual to another and establish new tumors in the recipient. In this paper, Murgia and colleagues present evidence that cases of CTVT in diverse dog breeds and geographic locations are descendants of a single rogue tumor estimated to have arisen between 200 and 2500 years ago. The investigators took advantage of modern DNA analysis to show that tumors in 40 dogs on five continents over 20 years all had the same genetic signatures. Although the tumors could be assigned to two subclades, they appear to have arisen from the same ancestral tumor, which gave rise to the subclades early after its establishment as a transmissible malignant parasite. All carry a characteristic insertion of a LINE DNA repetitive element close to the c-myc proto-oncogene. They share other distinct genetic markers that distinguish the tumor from its hosts. The authors conclude that the CTVT cells represent the oldest mammalian cell line in continuous propagation – the widely used HeLa cell line is quite young by comparison.
There are several important lessons to be learned from this study. First, the relative genomic stability of CTVT is surprising – although aneuploid, the karyotype is unexpectedly preserved. This may be relevant to its success as a parasitic tumor. CTVT regresses spontaneously and does not kill its hosts, suggesting that there is selective pressure on the tumor cells to maintain their immortality by ensuring that they can continue to be transmitted to others. However, the fact that the tumor cells must propagate asexually suggests that, over a long enough time, they will acquire deleterious mutations that will impair their viability. Second, the tumor has escaped from the normal allo-recognition mechanisms of its MHC-incompatible hosts. The authors raise the interesting notion that the highly polymorphic MHC system may have evolved to protect animals from transmissible tumors as well as infectious agents. But they point out that MHC antigens are suppressed by many types of tumors, raising the interesting questions of why transmissible allograft tumors have not emerged more frequently and why they have not (yet?) been observed in human populations.
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Does Janus Have Three Faces?
Vahid Afshar-Kharghan, MD, and Josef Prchal, MD
Drs. Afshar-Kharghan and Prchal indicated no relevant conflicts of interest.
Shi S, Calhoun HC, Xia F, et al. JAK signaling globally counteracts heterochromatic
gene silencing. Nat Genet 2006;38:1071-6.
The importance of JAK2 in erythropoiesis, specifically in erythropoietin/erythropoietin-receptor signaling and in modulation of receptor expression, has been highlighted after the seminal role of the JAK2 mutant in myeloproliferative disorders (JAK2V617F) was described. The JAK/STAT pathway is evolutionarily conserved and plays an important role in cell proliferation and organism development. In this paper, Shi et al. used a Drosophila melanogaster hematopoietic tumor model to show that the role of JAK/STAT in tumorogenesis is mediated by suppression of epigenetic gene silencing, suggesting yet a third possible mechanism of action for mutated JAK2. D. melanogaster possesses a single JAK, known as Hopscotch (hop), and a single STAT, which are similar to JAK2 and STAT5 in higher organisms. Interestingly, a hyperactive mutant of hop, known as Tumorous lethal (Tum-l), displays a leukemia-like phenotype with hematopoietic tumors manifested as blood cell aggregates in the body cavities of drosophila and globally disrupts heterochromatic gene silencing. Shi and collegues found that several enhancers of Tum-l encoded mutations in proteins involved in chromatin remodeling. These proteins, including HP1 (heterochromatin protein-1), Su(var)3-9 (suppressor of variegation methylating histone 3 at lys 9), and Rpd3 (a histone deacetylase) silence genes by stabilizing heterochromatin structures. The presence of the mutant HP1, Su(var)3-9, and Rpd3 failed to induce hematopoietic tumors by themselves, but their tumorogenesis was enhanced in the presence of hyperactive JAK Tum-1. Furthermore, the authors showed that hopTum-l (gain-of-function JAK) disrupts heterochromatin-induced gene silencing in genes that are not normally a target of the JAK/STAT pathway. These findings demonstrate that disruption of heterochromatic gene silencing is essential for JAK-induced increased cell proliferation and tumorogenesis. Finally, the authors showed that overexpression of HP1 in the hopTum-l flies prevents formation of hematopoietic tumors and counteracts the tumorogenesis of mutant JAK.
In the early 20th century, Emil Heitz studied chromatin structure under the microscope and discovered the presence of heterochromatin (condensed chromatin) and euchromatin (decondensed chromatin) in eukaryotes1. Later, it was discovered that genes in euchromatin were actively transcribed while genes in the heterochromatin remained largely silenced. Chromatin structure is comprised of nucleosome units, each formed by DNA and histone proteins. The histone proteins in heterochromatin are heavily acetylated and demethylated, and, as a result, genes located in the heterochromatin area of chromosomes are less accessible to the transcription machinery of the cell2. Deacetylation, mediated by histone deacetylase, and methylation by Su(var)3-9 with subsequent binding of HP1, play an important role in the stabilization of D. melanogaster heterochromatin structure and the silencing of genes. A similar array of enzymes and nuclear chromatin-binding proteins are involved in the epigenetic silencing of genes in higher eukaryotes, including humans. The findings of Shi et al., which identify a role for the JAK/STAT pathway in “global” reversal of heterochromatic gene silencing, are significant because they show a direct link between signaling events and gene expression. The tumorogenesis seen in the gain-of-function JAK mutant (hopTum-l) might be due to removal of the epigenetic silencing of tumor suppressor genes, a removal that could be reversed in D. melanogaster by overexpression of the heterochromatin stabilizing protein HP1. The existence of a similar mechanism in JAK2V617F-induced myeloproliferative disorders is a tantalizing possibility that demands further investigation. The work of Shi and colleagues might also explain the dominant inhibitory, regulatory role of wild-type JAK2 co-expressed with JAK2V617F in vitro (these cells lose the erythropoietin independence that is seen in cells transfected with JAK2V617F)3. Perhaps, wild-type JAK2 signaling counteracts the epigenetic changes induced by JAK2V617F and reverses the phenotype of cells carrying both wild-type and mutant JAK2 back to normal. Further, the mutagenesis-promoting properties of hyperactive JAK such as JAK2V617F may explain the frequent mitotic crossover in PV (present in 30 percent of PV patients) resulting in uniparenteral disomy and a high rate of homozygosity for somatic JAK2V617F mutation4.
- Heitz E. Das Heterochromatin der moose,1. [The heterochromatin in moss.] Jahrb Wiss Botanik 1928;69 762-818.
- Struhl K. Histone acetylation and transcriptional regulatory mechanisms. Gene Dev. 1998;12:599-606.
- James C, Ugo V, Le Couedic JP, et al. A unique clonal JAK2 mutation leading to constitutive signaling causes polycythemia vera. Nature 2005;434:1144-8.
- Kralovics R, Guan Y, Prchal JT. Acquired uniparental disomy of chromosome 9p is a frequent stem cell defect in polycythemia vera. Experimental Hematology 2002;30:229-236.
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Down the Slippery Slope: Neutrophils Suppress T Cells by Depletion of Arginine
Stephen Emerson, MD, PhD
Dr. Emerson indicated no relevant conflicts of interest.
Munder M, Schneider H, Luckner C, et al. Suppression of T-cell functions by human granulocyte arginase. Blood 2006;108:1627-1634.
Why do some patients seem to accumulate large numbers of medical problems while others stay essentially healthy? Is it genetics, is it what they eat, or does one problem directly trigger the next? In particular, why do patients with chronic inflammatory conditions seem to be at higher risk for cancer and for poor prognoses after cancer diagnosis? Does the inflammatory process promote cancer through increased oxidative damage or direct suppression of anti-tumor immune responses?
In this paper, Munder et al. show that activated human neutrophils directly suppress the ability of T cells to be activated through the T-cell receptor. This brake occurs because of a sudden depletion of the local T-cell milieu of the amino acid arginine, which is caused by the local release of arginase from neutrophil granules. Not only is overall T-cell metabolism and cell proliferation repressed, but the ζ chain of CD3 is specifically downregulated, so that no T-cell activation signals are delivered from the cell surface after antigen binding. So in a locally inflamed environment, T cells are essentially paralyzed. One could easily imagine that local or chronic generalized inflammation would lead to depressed T-cell responses, including both response to infectious pathogens and tumor antigens.
This link to arginase secretion had been previously detected in the mouse, but many mouse cells secrete arginase, including macrophages and dendritic cells1. Not so in humans, where arginase production is strictly limited to neutrophils. Munder and colleagues demonstrate every step of the arginase-T-cell paralysis link in human neutrophils. In addition, by showing that neutrophils from a patient with congenital arginase deficiency do not suppress T cells, they show that arginase is specifically required for this T-cell paralysis. This is important, because it suggests that specific pharmacologic inhibitors of arginase activity could be developed to specifically disrupt inflammation-induced T-cell anergy. For now, it suggests yet another reason why aspirin and anti-inflammatory homeopathic interventions might be of real benefit to our patients. And of course, this reinforces the lesson that basic scientists would be well served to closely consider folk wisdom for possible unsuspected physiology, if we are to truly uncover nature’s mysteries.
- Bronte V, Zanovello P. Regulation of immune responses by L-arginine metabolism. Nat Rev Immunol 2005;5:641-654.
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PTEN Scores a “10” in Stem Cell Discrimination
Peter Emanuel, MD
Dr. Emanuel indicated no relevant conflicts of interest.
Zhang J, Grindley JC, Yin T, et al. PTEN maintains hematopoietic stem cells and acts in lineage choice and leukemia prevention. Nature 2006;441:518-522.
Yilmaz OH, Valdez R, Theisen BK, et al. Pten dependence distinguishes hematopoietic stem cells from leukemia-initiating cells. Nature 2006;441:475-482.
Don’t we wish sometimes that cancerous stem cells would just hold up a sign and say “Here I am?” Wouldn’t it make life a lot easier, especially for transplant physicians, if normal hematopoietic stem cells could be distinguished from malignant stem cells? Well, we’re not quite there yet but we may be starting to unlock some of the clues. Two mouse models recently reported in Nature implicate PTEN as a major regulatory switch in maintaining normal hematopoietic stem cell function. In their mouse model, Zhang and colleagues deleted exon 5 of the PTEN gene, which contains the lipid phosphatase domain, leading to PTEN deficiency. Their mice displayed a decline in the number of hematopoietic stem cells, a diminished self-renewal capacity, an increase in the number of colony-forming unit cells in the spleen and peripheral blood (with stable numbers of colony-forming unit cells in the bone marrow), a decrease in the number of common lymphocyte progenitors, and an increase in the peripheral blood monocytes and granulocytes. The end result culminated in PTEN-deficient mice developing a myeloproliferative disorder. Further, the PTEN-deleted cells could be transferred to irradiated recipient mice with subsequent disease development, evidence that they are “cancer stem cells.” Yilmaz and colleagues demonstrated that the leukemia-initiating stem cell could be distinguished from a normal hematopoietic stem cell by a differential response to rapamycin. In other words, dependency on normal levels of PTEN just might be the neon sign that says normal vs. leukemic.
What is PTEN anyway? PTEN is a tumor suppressor gene which is defective in a vast array of human cancers. PTEN is a phosphatase that negatively regulates signaling through the phosphatidylinositol-3-OH-kinase (PI3K) pathway, inhibiting survival and proliferation. More and more studies are demonstrating that both the Ras and the PI3K pathways are able to converge on and activate mTOR (mammalian target of rapamycin) to stimulate cell growth1,2. Rapamcyin has long been approved for clinical use as an immunosuppressant. A number of newer rapamycin analogues are in various clinical testing phases. The study by Yilmaz and colleagues showed that normal hematopoietic stem cells were unable to maintain themselves without PTEN, in contrast to their leukemic counterparts. But these effects were mediated almost exclusively through mTOR, because they could be inhibited by rapamycin. Rapamycin, administered to these mice, not only depleted the leukemia-initiating cells, but also restored function to the normal hematopoietic stem cells. Could it be that simple in human leukemias? Likely not as PI3K signals through a multitude of other effectors besides mTOR. But it will certainly be intriguing to find out in future human clinical trials whether mTOR inhibitors can, in fact, differentially affect normal vs. leukemic stem cells.
- Shaw RJ, Cantley LC. Ras, PI(3)K and mTOR signalling controls tumor cell growth. Nature 2006;441:424-430.
- Cully M, You H, Levine AJ, et al. Beyond PTEN mutations: the PI3K path-way as an integrator of multiple inputs during tumorigenesis. Nat Rev Cancer 2006;6:184-192.
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It’s About Time: A New Prognostic Tool for Acute Graft-Versus-Host Disease
Michael Linenberger, MD
Dr. Linenberger indicated no relevant conflicts of interest.
Leisenring WM, Martin PJ, Petersdorf EW, et al. An acute graft-versus-host disease activity index to predict survival after hematopoietic cell transplantation with myeloablative conditioning regimens. Blood 2006;108:749-55.
Acute graft-versus-host disease (aGVHD) affects at least 40–60 percent of allogeneic hematopoietic stem cell transplant (HSCT) patients after conditioning with myeloablative preparative regimens and is the major cause of early morbidity and nonrelapse mortality (NRM) in this population. One barrier to improving strategies for aGVHD prophylaxis and therapy is the lack of a clinical grading system that reliably predicts outcomes attributable to aGVHD across all severity grades and that adjusts the mortality risk based on changes in aGVHD activity and treatment over time. To provide a more precise and dynamic prognostic tool, Leisenring and colleagues developed an aGVHD activity index (aGVHDAI) to predict the risk of day-200 NRM. Using training and validation clinical data sets, the aGVHDAI proved to be accurate over a full range of threshold values when calculated either as an average aGVHDAI score across time or as a current aGVHDAI score at specific time points during the first 100 days after transplant. The data sets were retrospectively obtained from patients < 55 years old with chronic myeloid leukemia who underwent allogeneic HSCT from unrelated donors after myeloablative conditioning with total body irradiation and cyclophosphamide. For each 10-day period after transplant, aGVHD activity was quantitatively staged according to the extent of skin rash, serum bilirubin elevation, and diarrhea volume. In addition, daily caloric intake, immunosuppressive drug usage, fever, and performance status were scored. Logistic regression models were fitted to these clinical parameters from a training data set of 193 patients and coefficients for each factor were used to assign a weight that optimized the ability of the aGVHDAI score to predict NRM by day 200. The six most significant factors for both the average aGVHDAI score and the real-time, current aGVHDAI score included two different ranges of serum hyperbilirubinemia, oral caloric intake < 40 percent of requirements with poorly controlled anorexia, nausea, and vomiting, any prednisone dose or secondary immunosuppressive therapy, and two different levels of reduced performance status. Notably, patient age, HLA disparity, severity of skin GVHD, and diarrhea volume did not improve the models. The final scaled weight factors and the aGVHDAI were then validated with an independent data set of 193 patients. When using the validation cohort data set to predict day-200 NRM, the average aGVHDAI score significantly outperformed the prognostic accuracy of both the conventional Glucksberg scale1,2 and IBMTR Severity Index3. The validation data set was also used to generate three discrete contour lines with 95 percent confidence intervals that reflected the probability of day-200 NRM as a function of the current aGVHDAI score calculated at days 0-19, 20-39, or 40-100 after transplant.
The aGVHDAI offers a number of improvements over the conventional aGVHD clinical grading systems. Most exciting is the ability to extrapolate a real-time mortality risk estimate based on the current aGVHDAI score. This prognostic information could greatly facilitate clinical decision making, especially in regard to tailoring treatment choices, and considering experimental interventions. While not directly applicable for day-to-day management, the average aGVHDAI score may also prove to be a more useful research tool than the conventional systems in assessing aGVHD severity, treatment responses, and outcomes. The aGVHDAI must still be validated in prospective studies and at other institutions. It must also be determined whether the scaled weight factors defined for the aGVHDAI are applicable to other diseases, age groups, donor sources, and conditioning regimens. Because it has been 30 years since the introduction of the original aGVHD grading system and almost 10 years since the last modifications, the aGVHDAI is a welcome and innovative advancement.
- Glucksberg H, Storb R, Fefer A, et al. Clinical manifestations of graft-versus-host disease in human recipients of marrow from HL-A-matched sibling donors. Transplantation 1974;18:295-304.
- Przepiorka D, Weisdorf D, Martin P, et al. Consensus conference on acute GVHD grading. Bone Marrow Transplant 1995;15:825-28.
- Rowlings PA, Przepiorka D, Klein JP, et al. IBMTR Severity Index for grading graft-versus-host disease: retrospective comparison with Glucksberg grade. Br J Haematol 1997;97:855-64.
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Strategies to Improve Outcomes in Follicular Lymphoma
Michael Williams, MD
Dr. Williams indicated no relevant conflicts of interest.
Press OW, Unger JM, Braziel RM, et al. Phase II trial of CHOP chemotherapy followed by tositumomab/iodine I-131 tositumomab for previously untreated follicular non-Hodgkin's lymphoma: five-year follow-up of Southwest Oncology Group Protocol S9911. J Clin Oncol 2006;24:4143-9.
An optimal therapeutic strategy for initial treatment of follicular lymphoma remains undefined. In this paper, Press and colleagues have updated the findings of the SWOG 9911 study of sequential CHOP chemotherapy (cyclophosphamide, doxorubicin, vincristine, prednisone) followed by radioimmunotherapy (RIT) with the anti-CD20131I-tositumomab regimen (Bexxar™). Ninety eligible patients were treated with six cycles of standard-dose CHOP followed by restaging four to eight weeks after cycle six. RIT was then administered for those achieving a partial or complete response, using the standard 131I-tositumomab treatment protocol. Ninety-six percent of patients had stage III-IV disease, 27 percent had “B” symptoms, 23 percent had bulky adenopathy > 10 cm, and 65 percent had intermediate- or high-risk FLIPI scores. Eighty-six of the patients completed CHOP therapy, and 77 (90 percent) of these completed RIT1. The complete response rate improved from 39 percent following CHOP to 69 percent following RIT; 22 percent achieved partial response. With 5.1 years median follow-up, the estimated five-year progression-free survival rate is 67 percent and overall survival is 87 percent. Acute toxicities included expected levels of transient cytopenias following each phase of therapy. Late toxicities have included elevated TSH in 9 percent of patients (despite the use of Lugol’s solution or potassium iodide to block thyroid uptake of 131I), one patient with myelodysplasia (MDS) 1.4 years after trial registration, and three patients with second cancers (3 percent, said to approximate the expected rate for the study age group).
Follicular lymphoma, a highly prevalent subtype of non-Hodgkin lymphoma, remains incurable for the vast majority of patients. The clinical course historically has been typified by response and later progression following various cytotoxic therapies. However, the advent of chemoimmunotherapy approaches in the past decade appears to have changed the natural history and improved survival for the disease. The updated results presented by Press and colleagues show that progression-free and overall survival represent “the best ever observed in a study of FL by the SWOG,” according to the authors. The low rates of late toxicities observed to date, including MDS and second cancers, is reassuring. Nonetheless, it remains important for hematologists/oncologists to enroll eligible patients in the ongoing SWOG/Intergroup S0016 trial comparing the CHOP -> RIT regimen with CHOP-rituximab to prospectively verify these findings. Other front-line strategies currently being tested in high-priority clinical studies include the ECOG 4402 indolent lymphoma trial of front-line rituximab followed by scheduled maintenance rituximab vs. observation and rituximab retreatment at progression. The European PRIMA study includes randomization to a four-arm rituximab-chemotherapy induction regimen followed by a second randomization of responders to rituximab maintenance therapy vs. observation. Correlative analyses of clinical risk parameters such as the FLIPI score, and biomarkers of higher- or lower-risk disease promise to further refine our ability to provide risk-adapted treatment approaches and improved outcomes for these patients.
- Press OW, Unger JM, Braziel RM, et al. A phase 2 trial of CHOP chemotherapy followed by tositumomab/iodine I 131 tositumomab for previously untreated follicular non-Hodgkin lymphoma: Southwest Oncology Group Protocol S9911. Blood 2003;102:1606-12.
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Heparins – is the Lite Stuff Really Healthier?
Charles Abrams, MD
Dr. Abrams has served as an ad hoc consultant to GlaxoSmithKline and Portola Pharmaceuticals.
Kearon C, Ginsberg JS, Julian JA, et al. Fixed-Dose Heparin (FIDO) Investigators. Comparison of fixed-dose weight-adjusted unfractionated heparin and low-molecular-weight heparin for acute treatment of venous thromboembolism. JAMA 2006;296:935-42.
Low-risk patients with thromboembolic disease can be safely treated with low-molecular-weight heparin — a more expensive cousin of standard unfractionated heparin. It is true that the cost of low-molecular-weight heparin is high. However, this approach does save money since it saves on the even higher cost of inpatient hospitalization and monitoring required for treatment with unfractionated heparin. In this paper, Kearon et al. analyzed whether a fixed dose of weight-adjusted subcutaneous unfractioned heparin was as effective as low-molecular-weight heparin for treating thromboembolic disease in outpatients. The purpose of this trial was to ascertain whether patients could be safely treated as outpatients with a fixed dosage of unfractionated heparin without adjusting the prescribed amount to aPTT results.
In this multi-institution trial, 708 patients were randomized to receive a fixed dose of unfractionated heparin subcutaneously (333 U/kg initial dose followed by 250 U/kg every 12 hours) or low-molecular-weight heparin (100 IU/kg every 12 hours). Nineteen percent of the patients in both treatment groups had pulmonary embolisms at enrollment into the trial. After a few days of treatment, all patients were transitioned to standard warfarin therapy for three months. Approximately 70 percent of patients in either arm of the trial remained as outpatients for the entire length of the study. Recurrent thromboembolism occurred in 3.8 percent of the 345 unfractionated heparin-treated patients, and in 3.4 percent of the 352 low-molecular-weight heparin-treated patients. This met the non-inferiority primary objective of the study. The rate of major bleeding was also similar in the two treatment strategies (1.1 percent in those treated with unfractionated heparin, and 1.4 percent in those treated with low-molecular-weight heparin). There were no patients who developed heparin-induced thrombocytopenia in this study. Therefore, in this one trial, it appears that a fixed dose of unfractionated heparin (not monitored for aPTT) was as efficacious and safe as low-molecular-weight heparin.
Metanalysis of multiple trials has demonstrated that subcutaneous administration of unfractionated heparin is actually superior to the more commonly used intravenous delivery1. However, these previous trials adjusted the dosage of subcutaneous heparin based on aPTT monitoring. The need to monitor the aPTT several times a day has prevented the widespread use of subcutaneous unfractionated heparin for outpatient treatment of thromboembolic disease. However, there is substantial literature that suggests that aPTT results do not correlate well with either recurrent thromboembolism or bleeding. This implies that a fixed dosage of subcutaneous unfractionated heparin might be appropriate for the initial treatment of deep vein thromboses or pulmonary embolisms. Consistent with this idea, a few small trials have suggested that fixed doses of subcutaneous unfractionated heparin can lead to good outcomes2,3. However, it is notable that all of these trials (including the recently published JAMA article) converted the subcutaneous therapy to oral warfarin within a few days. Therefore, it is not known whether prolonged administration of a fixed dose of subcutaneous unfractionated heparin is either safe or effective.
There is now an ever-increasing variety of pharmacologic products that may be appropriate for the outpatient treatment of thromboembolic disease (Figure). Low-molecular-weight heparin and fondaparinux can easily be used for this indication. There is a small amount of data supporting the use of subcutaneous direct thrombin inhibitors (such as hirudin) for outpatient anticoagulation. Oral inhibitors of Factor IIa (such as dabigatran etexilate) and Factor Xa (such as Rivaroxaban) appear to be promising antithrombotics. The data by Kearon and colleagues suggests that standard unfractionated heparin may be able to compete with these pricey alternative agents. Does this mean that we can all use a heparin preparation that is approximately one-twentieth the cost of low-molecular-weight heparin? Not yet, but there may still be another life left for our old friend unfractioned heparin.
- Hommes DW, Bura A, Mazzolai L, et al. Subcutaneous heparin compared with continuous intravenous heparin administration in the initial treatment of deep vein thrombosis: A meta-analysis. Ann Intern Med 1992;116:279-284.
- Belcaro G, Nicolaides AN, Cesarone MR, et al. Comparison of low-molecular-weight heparin, administered primarily at home, with unfractionated heparin, administered in hospital, and subcutaneous heparin, administered at home for deep-vein thrombosis. Angiology 1999;50:781-787.
- Kearon C, Harrison L, Crowther M, et al. Optimal dosing of subcutaneous unfractionated heparin for the treatment of deep vein thrombosis. Thromb Res 2000;97:395-403.
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The Devil is in the Details
Lilli Petruzzelli, MD, PhD
Dr. Petruzzelli indicated no relevant conflicts of interest.
Almeida AM, Murakami Y, Layton DM, et al. Hypomorphic promoter mutation in PIGM causes inherited glycosylphosphatidylinositol deficiency. Nat Med 2006;12:846-51.
Somatic cell mutation of the gene that encodes PIGA results in the loss of formation of GPI-anchored proteins and is associated with PNH. In this paper, the investigators describe an autosomal recessive disease that results in GPI deficiency. In uncovering the etiology of childhood thrombosis and seizures in two unrelated families (but each consanguineous), a HAM test was found to be positive. GPI expression was reduced in both patients in some but not all hematopoietic cells. By homozygosity mapping and SNP analysis of the two families, they closed in on the PIGM gene as the possible culprit. Cell lines were isolated from the affected patients and their parents and radiolabeled glycolipids were isolated and analyzed. The products reached the Gln acyl PI stage (Figure), but the cell lines failed to add additional mannose and PEtN residues — implicating either PIGM or PIGX as the defective step. The combination of genetic information with the biochemical findings pointed to PIGM as the mutated gene.
The analysis put forth by this group to explain the clinical course of two families with children who presented with thrombosis at a young age is not only an admirable piece of detective work but also a lesson in the application of biochemical insight into their genetic findings. Although it would be attractive to place the defect that the investigators uncovered in the context of more general clinical application, the thoroughness of their studies (which enabled them to discern a cellular defect) is the major take-home lesson from this work. The mutation is not in the expressed enzymatic portion of the gene, but rather in the upstream promoter that binds the transcription factor Sp-1. In explaining why a gene, PIGM, that is important in development, may lead to only mild neurological defects, acetylation status of the PIGM locus is suggested to play a role in promoting sufficient biosynthesis of the PIGM product to overcome the defect in Sp-1 binding. Alternatively, other promoters were detected (e.g., GATA) that may induce expression in a more tissue-restricted manner. Since these enzymatic pathways play a critical role in development across a number of tissues, it is difficult to surmise whether other defects will be uncovered to explain some similar thrombotic complications or whether this enzymatic step is at the threshold of sustaining development but still yields clinically significant findings when affected.
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