Characterization of pathogen-inactivated COVID-19 convalescent plasma and responses in transfused patients.

BACKGROUND: Efficacy of donated COVID-19 convalescent plasma (dCCP) is uncertain and may depend on antibody titers, neutralizing capacity, timing of administration, and patient characteristics. STUDY DESIGN AND METHODS: In a single-center hypothesis-generating prospective case-control study with 1:2 matched dCCP recipients to controls according to disease severity at day 1, hospitalized adults with COVID-19 pneumonia received 2 × 200 ml pathogen-reduced treated dCCP from 2 different donors. We evaluated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies in COVID-19 convalescent plasma donors and recipients using multiple antibody assays including a Coronavirus antigen microarray (COVAM), and binding and neutralizing antibody assays. Outcomes were dCCP characteristics, antibody responses, 28-day mortality, and dCCP -related adverse events in recipients. RESULTS: Eleven of 13 dCCPs (85%) contained neutralizing antibodies (nAb). PRT did not affect dCCP antibody activity. Fifteen CCP recipients and 30 controls (median age 64 and 65 years, respectively) were enrolled. dCCP recipients received 2 dCCPs from 2 different donors after a median of one hospital day and 11 days after symptom onset. One dCCP recipient (6.7%) and 6 controls (20%) died (p = 0.233). We observed no dCCP-related adverse events. Transfusion of unselected dCCP led to heterogeneous SARS CoV-2 antibody responses. COVAM clustered dCCPs in 4 distinct groups and showed endogenous immune responses to SARS-CoV-2 antigens over 14-21 days post dCCP in all except 4 immunosuppressed recipients. DISCUSSION: PRT did not impact dCCP anti-virus neutralizing activity. Transfusion of unselected dCCP did not impact survival and had no adverse effects. Variable dCCP antibodies and post-transfusion antibody responses indicate the need for controlled trials using well-characterized dCCP with informative assays.

Suitability of haematopoietic cell donors: updated consensus recommendations from the WBMT standing committee on donor issues.

The contribution of related donors to the globally rising number of allogeneic haematopoietic stem cell transplantations (HSCT) remains increasingly important, particularly because of the growing use of haploidentical HSCT. Compared with the strict recommendations on the suitability for unrelated donors, criteria for related donors allow for more discretion and vary between centres. In 2015, the donor outcome committee of the Worldwide Network for Blood and Marrow Transplantation (WBMT) proposed consensus recommendations of suitability criteria for paediatric and adult related donors. This Review provides updates and additions to these recommendations from a panel of experts with global representation, including the WBMT, the European Society for Blood and Marrow Transplantation donor outcome committee, the Center for International Blood and Marrow Transplant Research donor health and safety committee, the US National Marrow Donor Program, and the World Marrow Donor Association, after review of the current literature and guidelines. Sections on the suitability of related donors who would not qualify as unrelated donors have been updated. Sections on communicable diseases, clonal haematopoiesis of indeterminate potential, paediatric aspects including psychological issues, and reporting on serious adverse events have been added. The intention of this Review is to support decision making, with the goal of minimising the medical risk to the donor and protecting the recipient from transmissible diseases.

Frequency, reactivity and evolution of human leukocyte antigen and human platelet antigen antibodies in the setting of hematopoietic cell transplantation.

BACKGROUND AND OBJECTIVES: Antibodies (Ab) against HLA and HPA antigens play an important role in HCT. In this prospective study we evaluated prevalence and kinetics of HLA- and HPA-Ab after HCT, including a possible donor-recipient transfer and their clinical relevance in respect to platelet transfusion refractoriness (PTR). MATERIALS AND METHODS: Patients were consecutively recruited. Ab were determined by microbead assay technique and a mean fluorescence intensity cut-off of 1,000. RESULTS: At baseline, 21 donors (42 %) and 27 patients (54 %) had HLA-Ab with a mean panel reactivity (cPRA) of 34.9 ± 29.4 % and 46.1 ± 36.5 %, respectively. We observed a significant higher number of HLA-Ab specificities in female donors and patients and a predominance of HLA-class I Ab. At day 0 we detected an increase of HLA-Ab (from 526 to 673) and cPRA (55.2 ± 31.9 %). Thirty-six patients (72 %) developed new HLA-Ab, mainly 3 weeks after HCT. In 7 patients an HLA-Ab with the same specificity as detected in the corresponding donor emerged, suggesting a possible transfer from the donor to the recipient. Overall, MFI showed a high variation. Type and number of transfusions were not associated with number and intensity of HLA-Ab (ρ: -0.05 - 0.02). Number of HLA-Ab, cPRA and intensity were not associated with PTR, which occurred in 9 patients (18 %) and none had bleeding WHO > 2. CONCLUSIONS: Although a considerable number of patients have and develop HLA-Ab before and early after HCT, we found no association with PTR and bleeding and management should be individualized.

Immune cytopenia after allogeneic haematopoietic stem-cell transplantation: challenges, approaches, and future directions.

Immune-mediated cytopenia after allogeneic haematopoietic stem-cell transplantation is rare. The pathophysiology of immune-mediated anaemia, thrombocytopenia, and neutropenia, which occur alone or in combination with other cytopenias, is unclear and most probably a consequence of immune dysregulation. Risk factors for this complication have been identified in retrospective studies but these should be interpreted with caution and should not be generalised to this heterogeneous patient population. Diagnosis is challenging, requires awareness of such complications, and has to be differentiated from a multitude of other, and sometimes overlapping, possible complications. The clinical course of immune-mediated cytopenia is highly variable. Treatment requires an interdisciplinary approach and ranges from observation to symptomatic measures and directed therapies. Intensive immunosuppression is associated with an increased risk of infections and relapse, and current treatments are based on approaches in patients who have not undergone transplantation. Plasma cell-directed therapies, immunomodulation, and receptor-stimulating agents can be used to treat immune-mediated cytopenia.

Targeting compensatory MEK/ERK activation increases JAK inhibitor efficacy in myeloproliferative neoplasms.

Constitutive JAK2 signaling is central to myeloproliferative neoplasm (MPN) pathogenesis and results in activation of STAT, PI3K/AKT, and MEK/ERK signaling. However, the therapeutic efficacy of current JAK2 inhibitors is limited. We investigated the role of MEK/ERK signaling in MPN cell survival in the setting of JAK inhibition. Type I and II JAK2 inhibition suppressed MEK/ERK activation in MPN cell lines in vitro, but not in Jak2V617F and MPLW515L mouse models in vivo. JAK2 inhibition ex vivo inhibited MEK/ERK signaling, suggesting that cell-extrinsic factors maintain ERK activation in vivo. We identified PDGFRα as an activated kinase that remains activated upon JAK2 inhibition in vivo, and PDGF-AA/PDGF-BB production persisted in the setting of JAK inhibition. PDGF-BB maintained ERK activation in the presence of ruxolitinib, consistent with its function as a ligand-induced bypass for ERK activation. Combined JAK/MEK inhibition suppressed MEK/ERK activation in Jak2V617F and MPLW515L mice with increased efficacy and reversal of fibrosis to an extent not seen with JAK inhibitors. This demonstrates that compensatory ERK activation limits the efficacy of JAK2 inhibition and dual JAK/MEK inhibition provides an opportunity for improved therapeutic efficacy in MPNs and in other malignancies driven by aberrant JAK-STAT signaling.

The value of the post-thaw CD34+ count with and without DMSO removal in the setting of autologous stem cell transplantation.

BACKGROUND: CD34+ cell count correlates with engraftment potency after autologous stem cell transplantation. Assessment of CD34+ mainly occurs after apheresis and before cryopreservation with dimethyl sulfoxide (DMSO). The influence of postthaw CD34+ cell numbers over time to engraftment is not well studied, and determination of postthaw CD34+ cell counts is challenging for a variety of reasons. The aim of this retrospective study was to systematically assess the value of postthaw CD34+ cell counts in autologous grafts with and without DMSO removal. STUDY DESIGN AND METHODS: Between January 2008 and December 2015, 236 adult patients underwent a total of 292 autologous stem cell transplantations. Median age at transplantation was 56 years, and the main indication was multiple myeloma (60%). DMSO removal was done in 96 grafts (33%), either by centrifugation or by Sepax method. RESULTS: Patients receiving grafts containing DMSO showed a significantly faster platelet (p = 0.02) and RBC (p = 0.001) engraftment. DMSO removal was not associated with fewer infusion-related adverse events. We observed a good correlation between CD34+ cell count after apheresis and CD34+ cell count after thawing/washing (r = 0.931). Ninety grafts (31%) showed a significant loss of viable CD34+ cells, which translated into a delayed engraftment. CONCLUSION: DMSO removal was associated with delayed platelet and RBC engraftment without preventing adverse events. CD34+ cell enumeration after thawing remains difficult to perform, but grafts showing higher cell loss during cryopreservation and thawing are associated with slower engraftment. Prospective studies on the role of DMSO removal and postthaw CD34+ enumeration using defined protocols are needed.

Human platelet antigen antibody induction in uncomplicated pregnancy is associated with HLA sensitization.

BACKGROUND: Alloimmunization against human platelet antigens (HPAs) during pregnancy is rare but can lead to severe bleeding disorders, such as fetal and neonatal alloimmune thrombocytopenia. STUDY DESIGN AND METHODS: In a cohort of 241 uncomplicated pregnancies, we investigated the immunogenicity of HPA mismatches and correlated HLA sensitization with HPA antibody formation. HPA antibodies were measured with a Luminex-based multiplex assay. RESULTS: HPA mismatches were observed in 109 of 241 pregnancies (45%), but child-specific HPA antibodies were only found in two of 109 cases (2%), indicating a low immunogenicity. Only nine of 241 women (4%) had detectable HPA antibodies. HLA sensitization was identified as a strong and independent predictor for HPA antibody formation (hazard ratio, 10.2; 95% confidence interval, 1.8-193; p = 0.006), whereas the number of pregnancies was not. CONCLUSION: Our observational data indicated a low immunogenicity of HPA and suggest that a broader immune response-inferred by HLA sensitization-is probably associated with HPA antibody induction.

Prognostic impact of posttransplantation iron overload after allogeneic stem cell transplantation.

In patients referred for allogeneic hematopoietic stem cell transplantation (HSCT), iron overload is frequent and associated with increased morbidity and mortality. Both the evolution of iron overload after transplantation and its correlation with late posttransplantation events are unknown. We studied 290 patients undergoing myeloablative allogeneic HSCT between 2000 and 2009. Serum ferritin, transferrin saturation, transferrin, iron, and soluble transferrin receptor were determined regularly between 1 and 60 months after HSCT, and values were correlated with transplantation outcome. Ferritin levels peaked in the first 3 months posttransplantation and then decreased to normal values at 5 years. Transferrin saturation and iron behaved analogously, whereas transferrin and soluble transferrin receptor increased after an early nadir. Landmark survival analysis showed that hyperferritinemia had a detrimental effect on survival in all periods analyzed (0 to 6 months P < .001; 6 to 12 months P < .001; 1 to 2 years P = .02; 2 to 5 years P = .002). This effect was independent of red blood cell transfusion dependency and graft-versus-host disease. Similar trends were seen for other iron parameters. These data show the natural dynamics of iron parameters in the setting of allogeneic HSCT and provide evidence for a prognostic role of iron overload extending beyond the immediate posttransplantation period. Interventions to reduce excessive body iron might therefore be beneficial both before and after HSCT.

Comparison of transfusion efficacy of amotosalen-based pathogen-reduced platelet components and gamma-irradiated platelet components.

BACKGROUND: Trials of transfusions of platelets (PLTs) treated with amotosalen-based pathogen reduction (PR) showed lower corrected count increments (CCIs) compared to conventional PLT components (PCs). However, PR-PLTs and conventional PCs often differed in various factors besides PR. We compared transfusion efficacy of single-donor apheresis PCs treated with PR or gamma irradiation. STUDY DESIGN AND METHODS: Hematologic patients were assigned to receive PR-PLTs or gamma-irradiated conventional PCs, both prepared in PLT additive solution (PAS). One-hour CCI (primary endpoint), 24-hour CCI, time to next PLT transfusion, and transfusion requirement of red blood cells and plasma were analyzed. RESULTS: Forty-four patients assigned to PR-PLTs received 220 PR-PLTs and 136 conventional PCs; 72 controls received 517 conventional PCs. No differences between patient groups were observed for mean (±standard deviation [SD]) 1-hour CCI (11.4 [±4.9] for PR-PLT vs. 11.0 [±4.9] for controls), mean (±SD) 24-hour CCI (6.1 [±4.4] for PR-PLTs vs. 6.2 [±4.8] for controls), and for the other evaluated outcomes. No differences between PC types were observed for mean (±SD) 1-hour CCI (10.6 [±6.7] for PR-PLTs vs. 9.9 [±6.2] for conventional PCs) and mean 24 hour-CCI (3.3 [±3.9] for PR-PLTs vs. 4.2 [±5] for conventional PCs). Thirty-five percent of PR-PLTs and 38% of conventional PCs (p = 0.63) were associated with 1-hour CCIs of less than 7.5. Inadequate 24-hour CCIs were observed for 72% of PR-PLTs and 64% of conventional PCs (p = 0.002). CONCLUSIONS: Transfusion efficacy of single-donor apheresis PCs in PAS treated with amotosalen PR versus gamma irradiation is comparable.

Lung resection in hematologic patients with pulmonary invasive fungal disease.

BACKGROUND: Pulmonary invasive fungal disease is a frequent complication in patients with hematologic malignancies. Surgical resection in addition to antifungal therapy is an option for selected cases but often feared because of immunosuppression. METHODS: We analyzed the outcome of 71 patients undergoing lung resection for pulmonary invasive fungal disease. Most patients had leukemia, 44 underwent high-dose chemotherapy, and 18 underwent stem cell transplantation. RESULTS: On the day of surgery, 44 patients were neutropenic, and 41 had a platelet count < 50 × 109/L. Forty-five nonanatomic (atypical) resections and 26 lobectomies were performed. Fungal infection was histologically proven in 53 patients. Reoperation was needed in four patients (bronchial stump dehiscence, persistent air leak, chylothorax, and seroma). Minor complications at the site of surgery occurred in 14 patients. In only two, there was an uncontrolled disseminated fungal infection. Overall, mortality at 30 days was 7% (five of 71). Long-term survival was mainly influenced by the underlying hematologic disease. CONCLUSIONS: Lung resection is a therapeutic option for hematologic patients with pulmonary fungal infection. Despite immunosuppression, the perioperative morbidity and mortality is acceptable, and, therefore, the prognosis is not determined by the surgical intervention.

The association of preoperative anemia and perioperative allogeneic blood transfusion with the risk of surgical site infection.

BACKGROUND: The purpose of the study was to investigate allogeneic blood transfusion (ABT) and preoperative anemia as risk factors for surgical site infection (SSI). STUDY DESIGN AND METHODS: A prospective, observational cohort of 5873 consecutive general surgical procedures at Basel University Hospital was analyzed to determine the relationship between perioperative ABT and preoperative anemia and the incidence of SSI. ABT was defined as transfusion of leukoreduced red blood cells during surgery and anemia as hemoglobin concentration of less than 120 g/L before surgery. Surgical wounds and resulting infections were assessed to Centers for Disease Control standards. RESULTS: The overall SSI rate was 4.8% (284 of 5873). In univariable logistic regression analyses, perioperative ABT (crude odds ratio [OR], 2.93; 95% confidence interval [CI], 2.1 to 4.0; p < 0.001) and preoperative anemia (crude OR, 1.32; 95% CI, 1.0 to 1.7; p = 0.037) were significantly associated with an increased odds of SSI. After adjusting for 13 characteristics of the patient and the procedure in multivariable analyses, associations were substantially reduced for ABT (OR, 1.25; 95% CI, 0.8 to 1.9; p = 0.310; OR, 1.07; 95% CI, 0.6 to 2.0; p = 0.817 for 1-2 blood units and >or=3 blood units, respectively) and anemia (OR, 0.91; 95% CI, 0.7 to 1.2; p = 0.530). Duration of surgery was the main confounding variable. CONCLUSION: Our findings point to important confounding factors and strengthen existing doubts on leukoreduced ABT during general surgery and preoperative anemia as risk factors for SSIs.

The allele burden of JAK2 mutations remains stable over several years in patients with myeloproliferative disorders.

In a retrospective single center study we determined the time course of the JAK2-V617F or JAK2 exon 12 allele burden in DNA from purified granulocytes from 48 patients with myeloproliferative disorders. The percentage of change between the first and last sample in JAK2-V617F positive patients without cytoreductive therapy (n=16) was only +9% during a follow-up of 36+/-13 months, reflecting a remarkably stable mutant allele burden. When treatment with hydroxyurea was initiated during the course of the study, we observed a significant decrease of the JAK2-V617F allele burden (n=6). However, in JAK2-V617F positive patients who were already on hydroxyurea treatment before the first blood sampling (n=14), we observed stable allelic ratios with a variance of only +3% during a follow-up of 34+/-16 months. Our data suggest that in untreated myeloproliferative disorders patients, from whom samples at diagnosis are not available, the JAK2 allele burden determined at later stages could be equally informative.

A de novo splice donor mutation in the thrombopoietin gene causes hereditary thrombocythemia in a Polish family.

BACKGROUND: Hereditary thrombocythemia is an autosomal dominant disorder with clinical features resembling sporadic essential thrombocythemia. Germline mutations in families with hereditary thrombocythemia have been identified in the gene for thrombopoietin (TPHO) and its receptor, MPL. DESIGN AND METHODS: Here we characterized a THPO mutation in a hereditary thrombocythemia pedigree with 11 affected family members. RESULTS: Affected family members carry a G --> C transversion in the splice donor of intron 3 of THPO that co-segregated with thrombocytosis within the pedigree. We previously described the identical mutation in a Dutch family with hereditary thrombocythemia. Haplotype analysis using single nucleotide polymorphisms surrounding the mutation indicated that the mutations arose independently in the two families. MPL protein levels, but not mRNA levels, were low in platelets from affected family members. Bone marrow histology showed features compatible with those of essential thrombocythemia, but the megakaryocytes were unusually compact, as assessed by planimetric analysis. Impaired microcirculation resulting in brief episodes of fainting and dizziness that responded well to aspirin were the predominant clinical features in a total of 23 affected family members studied. Disease onset is earlier in patients with hereditary thrombocythemia than in those with essential thrombocythemia, but the frequencies of thrombotic, vascular and hemorrhagic events are similar in the two groups. CONCLUSIONS: A mutation in THPO occurred de novo in the same position as in a previously described family with hereditary thrombocythemia. Patients with this mutation have elevated serum levels of thrombopoietin and a phenotype that responds to aspirin and does not require cytoreductive treatment.

Autoimmunity and malignancy in hematology--more than an association.

Several associations between hematological malignancies and autoimmunity directed against hematopoietic cells exist. Antibody mediated elimination of mature blood cells such as autoimmune hemolytic anemia (AIHA) and immune thrombocytopenia (ITP) are frequent complications of non-Hodgkin lymphomas, most prominently chronic lymphocytic leukemia. Autoimmunity directed against hematopoietic precursor cells is the hallmark of aplastic anemia, but many features of this disease are shared by two related disorders, paroxysmal nocturnal hemoglobinuria (PNH) and myelodysplastic syndrome (MDS). While the clinical associations between hematological malignancy and autoimmunity have been described many decades ago, only in the last several years have the common pathogenetic mechanisms been elucidated. We summarize the recent progress made in understanding how hematological malignancy gives rise to autoimmunity directed against blood cells and vice versa, and illustrate parallels in the etiology of malignant and autoimmune hematological disorders. Specifically, recent progress in the recognition of the association of lymphoproliferative disorders and autoimmunity against mature blood cells, and common pathogenetic background of aplastic anemia, paroxysmal nocturnal hemoglobinuria, and myelodysplastic syndrome are discussed.

Acquisition of the V617F mutation of JAK2 is a late genetic event in a subset of patients with myeloproliferative disorders.

An acquired gain-of-function mutation in the Janus kinase 2 (JAK2-V617F) is frequently found in patients with myeloproliferative disorders (MPDs). To test the hypothesis that JAK2-V617F is the disease-initiating mutation, we examined whether all cells of clonal origin carry the JAK2-V617F mutation. Using allele-specific polymerase chain reaction (PCR) assays for the JAK2 mutation and for the X-chromosomal clonality markers IDS and MPP1, we found that the percentage of granulocytes and platelets with JAK2-V617F was often markedly lower than the percentage of clonal granulocytes determined by IDS or MPP1 clonality assays in female patients. Using deletions of chromosome 20q (del20q) as an autosomal, X-chromosome-independent clonality marker, we found a similar discrepancy between the percentage of cells carrying JAK2-V617F and del20q. Our results suggest that in a proportion of patients with MPDs, JAK2-V617F occurs on the background of clonal hematopoiesis caused by a somatic mutation in an as-yet-unknown gene.

Altered gene expression in myeloproliferative disorders correlates with activation of signaling by the V617F mutation of Jak2.

We identified 13 new gene expression markers that were elevated and one marker, ANKRD15, that was down-regulated in patients with polycythemia vera (PV). These 14 markers, as well as the previously described PRV1 and NF-E2, exhibited the same gene expression alterations also in patients with exogenously activated granulocytes due to sepsis or granulocyte colony-stimulating factor (G-CSF) treatment. The recently described V617F mutation in the Janus kinase 2 (JAK2) gene allows defining subclasses of patients with myeloproliferative disorders based on the JAK2 genotype. Patients with PV who were homozygous or heterozygous for JAK2-V617F exhibited higher levels of expression of the 13 new markers, PRV1, and NF-E2 than patients without JAK2-V617F, whereas ANKRD15 was down-regulated in these patients. Our results suggest that the alterations in expression of the markers studied are due to the activation of the Jak/signal transducer and activator of transcription (STAT) pathway through exogenous stimuli (sepsis or G-CSF treatment), or endogenously through the JAK2-V617F mutation.

A gain-of-function mutation of JAK2 in myeloproliferative disorders.

BACKGROUND: Polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis are clonal myeloproliferative disorders arising from a multipotent progenitor. The loss of heterozygosity (LOH) on the short arm of chromosome 9 (9pLOH) in myeloproliferative disorders suggests that 9p harbors a mutation that contributes to the cause of clonal expansion of hematopoietic cells in these diseases. METHODS: We performed microsatellite mapping of the 9pLOH region and DNA sequencing in 244 patients with myeloproliferative disorders (128 with polycythemia vera, 93 with essential thrombocythemia, and 23 with idiopathic myelofibrosis). RESULTS: Microsatellite mapping identified a 9pLOH region that included the Janus kinase 2 (JAK2) gene. In patients with 9pLOH, JAK2 had a homozygous G-->T transversion, causing phenylalanine to be substituted for valine at position 617 of JAK2 (V617F). All 51 patients with 9pLOH had the V617F mutation. Of 193 patients without 9pLOH, 66 were heterozygous for V617F and 127 did not have the mutation. The frequency of V617F was 65 percent among patients with polycythemia vera (83 of 128), 57 percent among patients with idiopathic myelofibrosis (13 of 23), and 23 percent among patients with essential thrombocythemia (21 of 93). V617F is a somatic mutation present in hematopoietic cells. Mitotic recombination probably causes both 9pLOH and the transition from heterozygosity to homozygosity for V617F. Genetic evidence and in vitro functional studies indicate that V617F gives hematopoietic precursors proliferative and survival advantages. Patients with the V617F mutation had a significantly longer duration of disease and a higher rate of complications (fibrosis, hemorrhage, and thrombosis) and treatment with cytoreductive therapy than patients with wild-type JAK2. CONCLUSIONS: A high proportion of patients with myeloproliferative disorders carry a dominant gain-of-function mutation of JAK2.

Comparison of molecular markers in a cohort of patients with chronic myeloproliferative disorders.

Decreased expression of c-MPL protein in platelets, increased expression of polycythemia rubra vera 1 (PRV-1) and nuclear factor I-B (NFIB) mRNA in granulocytes, and loss of heterozygosity on chromosome 9p (9pLOH) were described as molecular markers for myeloproliferative disorders (MPDs). To assess whether these markers are clustered in subgroups of MPDs or represent independent phenotypic variations, we simultaneously determined their status in a cohort of MPD patients. Growth of erythropoietin-independent colonies (EECs) was measured for comparison. We observed concordance between EECs and PRV-1 in MPD patients across all diagnostic subclasses, but our results indicate that EECs remain the most reliable auxiliary test for polycythemia vera (PV). In contrast, c-MPL, NFIB, and 9pLOH constitute independent variations. Interestingly, decreased c-MPL and elevated PRV-1 also were observed in patients with hereditary thrombocythemia (HT) who carry a mutation in the thrombopoietin (TPO) gene. Thus, altered c-MPL and PRV-1 expression also can arise through a molecular mechanism different from sporadic MPD.