Expanding the genetic spectrum of TUBB1-related thrombocytopenia

β1-tubulin plays a major role in proplatelet formation and platelet shape maintenance, and pathogenic variants in TUBB1 lead to thrombocytopenia and platelet anisocytosis (TUBB1-RT). To date, the reported number of pedigrees with TUBB1-RT and of rare TUBB1 variants with experimental demonstration of pathogenicity is limited. Here, we report 9 unrelated families presenting with thrombocytopenia carrying six β1-tubulin variants: p.Cys12Leufs12*, p.Thr107Pro, p.Gln423*, p.Arg359Trp, p.Gly109Glu, and p.Gly269Asp, the last of which novel. Segregation studies showed incomplete penetrance of these variants for platelet traits. Indeed, most carriers showed macrothrombocytopenia, some only increased platelet size and a minority no abnormalities. Moreover, only homozygous carriers of the p.Gly109Glu variant, displayed macrothrombocytopenia, highlighting the importance of allele burden in the phenotypic expression of TUBB1-RT. The p.Arg359Trp, p.Gly269Asp and p.Gly109Glu variants deranged β1-tubulin incorporation into the microtubular marginal ring in platelets, while had negligible effect on platelet activation, secretion or spreading, suggesting that β1-tubulin is dispensable for these processes. Transfection of TUBB1 missense variants in CHO cells altered β1-tubulin incorporation into the microtubular network. In addition, TUBB1 variants markedly impaired proplatelet formation from peripheral blood CD34+ cell-derived megakaryocytes. Our study, using in vitro modeling, molecular characterization, and clinical investigations provides a deeper insight into the pathogenicity of rare TUBB1 variants. These novel data expand the genetic spectrum of TUBB1-RT and highlight a remarkable heterogeneity in its clinical presentation, indicating that allelic burden or combination with other genetic or environmental factors modulate the phenotypic impact of rare TUBB1 variants.

Peripheral blood hematopoietic stem cell pool in individuals chronically exposed to radiation over a long-term period

Changes in the peripheral blood cellular composition were observed in the long term period in the residents of the Techa riverside villages chronically exposed to radiation, which may be the consequence of structural and functional disorders in the pool of hematopoietic stem cells (HSC) and progenitor cells. Therefore, the study was aimed to quantify peripheral blood CD34+ cell pool in individuals chronically exposed to radiation over a long-term period. Sixty years after the onset of exposure, a total of 153 individuals were examined, who were divided into four groups: individuals exposed in utero and postnatally (the average postnatal absorbed dose was 570 mGy); individuals exposed only postnatally (the average postnatal absorbed dose was 790 mGy), and two comparison groups, in which the average postnatal absorbed dose to red bone marrow did not exceed 70 mGy. Absolute and relative peripheral blood CD34+ cell counts in chronically exposed individuals were assessed by flow cytometry. No changes in CD34+ cell counts compared to comparison group were revealed in the group of individuals exposed in utero and postnatally; no age-related changes were registered as well. However, a significant decline in absolute HSC and progenitor cell counts with increased absorbed dose to red bone marrow was observed. In the group of individuals exposed only postnatally, there was a significant increase in peripheral blood CD34+ cell counts compared to comparison group (p = 0.004 for absolute cell count; p = 0.009 for relative cell count), dose-dependent increase in peripheral blood HSC and precursor cell counts (p = 0.02 for absolute cell count; p = 0.03 for relative cell count), along with age-related decline in these cells’ counts (р = 0.02 for absolute cell count; p = 0.04 for relative cell count).

To Predict Success of Postapheresis Yield and Post–Autologous Transplant Engraftment Based on Preapheresis Peripheral Blood CD34+ Cell Counts: An Indian Scenario–Based Study

Introduction The use of hematopoietic stem cells for autologous and allogeneic transplantation has increased in the recent past significantly, due to introduction of newer chemotherapeutic drugs, immunological techniques, and better stem cell technology. Among the bone marrow and peripheral blood stem cells, collection of the latter being more convenient to the patient and associated with faster granulocyte and platelet engraftment has been known as preferred method for mobilization. Peripheral blood stem cells can be extracted from the autologous or allogeneic donor. Mobilization of the stem cells for autologous stem cell transplant is traditionally done using growth factors alone or in combination with chemotherapy, with or without an additional mobilizing agent. A significant number of hematological malignancy patients are poor mobilizers, (i.e., they are unable to achieve the minimal target cell dose during their first round of mobilization).Therefore, a prediction for a successful stem cell mobilization ideally should be made before initiating any apheresis procedure to spare those with a low rate of success from the risks associated with apheresis procedure. Preapheresis CD34 cell count can predict postapheresis yield and hence, can help to reduce the collection sessions. Reduction of apheresis sessions decreases the discomfort, inconvenience, time, and monetary expenses. Objectives This study was aimed to analyze preapheresis and postapheresis CD34+ cell counts. Materials and Methods Patients of any age and gender with diagnosis of hematological malignancies admitted for autologous stem cell transplantation for hematological malignancies (including Hodgkin lymphoma, non-Hodgkin lymphoma, and multiple myeloma) and germ cell tumors in our institute from July 2008 to July 2016 were included in the study. The post-GCSF CBC, preapheresis CBC, CD34+ cell counts, and postapheresis CBC, CD34+ cell counts, mononuclear cell counts to predict the outcome of amount of yield. The effect on engraftment will be measured according to the defining criteria of achieving a sustained peripheral blood neutrophil count of >500 × 106/L (Wolff 2002) and a platelet count of more than >20 × 109/L (Teltschik et al. 2016) independent of platelet transfusion for at least 7 days. Collection of stem cells was done using apheresis machine (COBE SPECTRA). Complete peripheral blood counts using automated analyzers. Peripheral blood CD34 + cell counts and postapheresis CD34+ cell count using BD FACS CANTO II flow cytometer. To calculate postapheresis yield, the related CD34 count measured by flow cytometer was multiplied by the apheresis product volume and divided by the recipient’s body weight (kg). Number of CD34+ cells collected = (CD34 cell concentration in final product) × (final product volume). Results A total of 100 patients who underwent a total of 320 apheresis sessions were included in the study. There were 78 males and 22 females. We also found a significant correlation between preapheresis CD34 + cell count and postapheresis CD34 percentage on days 1, 2, and 3 of the apheresis sessions. In our study, to obtain more than 1.31 × 106 cells (median = 1.04, range: 0.15–4.70), an absolute count of pre apheresis CD34 + cells ≥14 cells would be necessary. A target of CD34 + cells ≥ 2 × 106/kg was obtained in majority of patients if a concentration of ≥25 CD34 + cells was present in postapheresis collection. Conclusion Compiling our results with the previous published data, we conclude that there is a strong correlation between preapheresis absolute CD34 + cell counts and postapheresis CD34 + cell count. Our study also suggests that the minimum absolute cell count of >10 cells/μL is required, to achieve a target of >2–5 × 106 cells for postapheresis yield.

Optimization of Plerixafor Utilization Based on Peripheral Blood CD34+ Count for Autologous Peripheral Blood Stem-Cell Collection

Introduction: Peripheral CD34+ cells may be mobilized using filgrastim (G-CSF) alone or in combination with chemotherapy. However, some patients also require plerixafor, an inhibitor of C-X-C chemokine receptor type-4, for adequate mobilization. Given its cost, judicious utilization of plerixafor is warranted. Material and Methods: A retrospective analysis of autologous stem-cell mobilization was performed at a tertiary-care medical center in adult patients with multiple myeloma and lymphoma; here we will focus on the utility of repeat plerixafor dosing. Patients were mobilized at the treating physician's discretion with filgrastim plus plerixafor or chemotherapy plus filgrastim plus plerixafor. Collections were initiated once peripheral CD34+ counts reached 20/µL (or 10/µL if chemotherapy mobilized); plerixafor was administered if these counts were not reached after 4 or 8 days, respectively, of filgrastim treatment. Results: Patients with multiple myeloma (86) or lymphoma (30) were evaluated. One hundred five were mobilized by filgrastim plus plerixafor and 11 by chemotherapy plus filgrastim plus plerixafor. No patient that received plerixafor with a CD34+ count <5/µL after chemotherapy mobilized the next day. The end collection goal was achieved in 86 (81.9%) of the filgrastim plus plerixafor group and 7 (63.6%) of the chemotherapy plus filgrastim plus plerixafor group. Patients given at least one dose of plerixafor were divided into groups based on collection goal, peripheral blood CD34+ cell count after 1 dose and the first day collection yield: Group 1) Goal of 3x10^6/kg and CD34+ count ≥ 30 cell/µL vs < 30 cell/µL; Group 2) Goal of 6x10^6/kg and ≥ 50% of collection goal after 1 day of collection vs CD34+ count < 50 cell/µL or < 50% of collection goal. Forty of 42 (95%) patients in Group 1 with a CD34+ count ≥ 30 cell/µL achieved their end collection goal after one plerixafor dose. Eighteen of 19 (95%) patients in Group 1 with a CD34+ count <30 cell/µL received a second dose of plerixafor and 8 (44.4%) achieved their end collection goal. Twenty-eight of 32 (87.5%) patients in Group 2 with ≥ 50% of collection goal achieved on the first day of collection reached their end collection goal after one plerixafor dose. Nine of 12 (75%) patients in Group 2 with a CD34+ count of < 50 cells/µL or <50% collection goal received an additional dose of plerixafor and 6 (66.7%) achieved their end collection goal. Conclusion: Based on these data, we have developed the following repeat plerixafor dosing algorithm: 1) for a collection goal is 3x10^6/kg, administer a second dose of plerixafor if the CD34+ count on the first day of collection is < 30 cell/µL, and 2) for a collection goal of 6x10^6/kg, administer a second dose of plerixafor if the CD34+ count on the first day of collection is < 50 cell/µL or if the first day of collection yields <50% of the end goal. This algorithm optimizes pharmacy, apheresis and stem cell processing resources. Disclosures No relevant conflicts of interest to declare.

Timing of Daratumumab Administered Pre-Mobilization in Multiple Myeloma Impacts Pre-Harvest Peripheral Blood CD34+ Cell Counts and Plerixafor Use

Background: Daratumumab (DARA) is a monoclonal antibody which targets CD38 on plasma cells and B cell progenitors. DARA has been effectively combined with other therapies in newly diagnosed and relapsed/refractory multiple myeloma (RRMM), while DARA-based induction regimens in transplant-eligible patients (pts) are increasingly being used in clinical practice. Given that hematopoietic stem cells also express CD38, DARA may potentially affect stem cell mobilization and hematopoietic reconstitution following autologous stem cell transplant (ASCT). Although no clinically significant impact of DARA on stem cell mobilization or hematopoietic recovery was described in large phase 3 trials of triplet induction regimens +/- DARA in newly diagnosed MM, stem cell yields were lower and plerixafor more commonly used in the DARA-containing arms [Moreau et al, Lancet 2019; Voorhees et al, Blood 2020]. Significantly longer time to neutrophil (PMN) engraftment was also reported in pts receiving DARA-based induction who underwent upfront ASCT [Al Saleh et al, Am J Hematol 2020]. In this study, we examine the impact of timing of DARA administration pre-mobilization on day 4 pre-harvest peripheral blood CD34 cell count, stem cell apheresis yield, and post-ASCT engraftment. Methods: Between 1/1/2016 and 12/31/2019, newly diagnosed and RRMM pts receiving DARA-based induction regimens with ≥1 dose of DARA administered within 1 month prior to stem cell mobilization were identified retrospectively and compared to matched controls receiving similar induction regimens without DARA. Granulocyte colony-stimulating factor (G-CSF) was administered per institutional standards and plerixafor added based on day 4 pre-harvest peripheral blood CD34 counts. PMN and platelet engraftment post-ASCT was defined as the first of 3 consecutive days with sustained PMN count >500 x 106/L and independence from platelet transfusion in the preceding 7 days with a count >20 x 109/L, respectively. Pre-harvest peripheral blood CD34 counts and stem cell apheresis yields were obtained from the Cellular Therapy Laboratory at NewYork-Presbyterian Hospital. The study was approved by the Weill Cornell Medicine IRB. Results: We identified 16 pts who received DARA-based induction with ≥1 dose of DARA administered within 1 month of apheresis (DARA group) and 16 non-DARA-containing regimen-matched controls (non-DARA group). Demographics of the DARA and non-DARA groups were well matched (Table 1). DARA pts received their last dose of DARA a mean of 17.3 days prior to the first day of apheresis, with 8 pts receiving their last dose within 2 weeks and the remaining 8 pts between 2 weeks and 1 month prior. Overall, mobilization outcomes were inferior in the DARA group (Table 2). DARA pts had significantly lower day 4 pre-harvest peripheral blood CD34 counts compared to non-DARA pts (17.2 vs 35.4 cells/µL; P=0.0146). Institutional algorithm required plerixafor to be given for day 4 CD34 count ≤40 cells/µL. Fifteen of the 16 DARA pts received plerixafor vs. 11 non-DARA pts (P=0.07). Additionally, DARA pts required significantly more apheresis days (2.4 vs 1.6 days; P=0.0279). Differences in stem cell yield were not significant (8 vs 10 x106cells/kg; P=0.1391). Hematopoietic recovery post-ASCT was not affected by DARA administered in the month preceding mobilization. Conclusions: In summary, we report lower day 4 pre-harvest peripheral blood CD34 count, increased requirement for plerixafor, and longer apheresis duration in newly diagnosed and RRMM pts receiving DARA within 1 month ofstem cell mobilization. These limitations are largely overcome by plerixafor usage which, combined with G-CSF, resulted in successful stem cell collection in all patients. Limitations in our study include small sample sizes, retrospective control selection, and fewer pts in the DARA group achieving ≥VGPR prior to mobilization. Nevertheless, our findings are consistent with inferior mobilization outcomes reported in the DARA-containing arms of phase 3 trials of triplet induction +/- DARA and highlight the nearly universal requirement for plerixafor usage when DARA is administered within a month prior to apheresis. Prospective study of day 4 pre-harvest peripheral blood CD34 counts and other predictors of stem cell yield should be incorporated into future clinical trials of CD38 monoclonal antibody-based induction regimens for transplant-eligible MM pts. Disclosures Rossi: Janssen: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees. Niesvizky:GSK: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; BMS: Consultancy, Honoraria; Karyopharm: Consultancy, Honoraria; Takeda: Consultancy, Honoraria. Rosenbaum:Amgen: Research Funding; GlaxoSmithKline: Research Funding; Akcea: Honoraria; Celgene: Honoraria; Janssen: Research Funding.

Peripheral Blood CD34+ Donor Chimerism Is Superior to CD3+ Donor Chimerism for Predicting Relapse Following Allogeneic Stem Cell Transplantation for Myeloid Malignancies

Background: Disease relapse remains the main cause of mortality after allogeneic stem cell transplantation (allo-SCT) for patients with myeloid malignancies. Loss of donor chimerism (DC) is commonly used as a biomarker of impending relapse, which allows the initiation of pre-emptive therapies such as withdrawal of immunosuppression or donor lymphocyte infusion (DLI). Surprisingly, there are few if any direct comparisons of peripheral blood CD34+ and CD3+ DC as biomarkers to predict relapse. We hypothesized that loss of CD34+ DC may be a more direct measure of impending relapse given most myeloid malignancies express CD34. Methods: We prospectively measured peripheral blood CD34+ and CD3+ DC on days 30, 60, 90, 120 and 180 following allo-SCT for patients with AML (n=113) or MDS (n=23) transplanted at a single centre between July 2011 and November 2019. Chimerism analysis was performed using purified cell subsets isolated from 60 mL peripheral blood using PCR-based amplification of short tandem repeats (STRs). The goal of this retrospective analysis was to compare the value of CD3+ and CD34+ DC for predicting relapse. Institutional practice for CD34+ DC below 80% included a bone marrow biopsy to identify morphologic relapse and donor lymphocyte infusion. Statistical analysis was performed with R 3.5.2 (The R project for Statistical computing) or GraphPad (v8.2.0). Results: Overall, 41 of 136 (30%) patients had morphologic relapse at a median time of 153 days after allo-SCT (range 50-1742). The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of CD3+ and CD34+ donor chimerism for morphologic relapse is shown in Figure 1A. CD34+ DC outperformed CD3+ DC in all criteria, irrespective of the percentage chosen. Furthermore, a concurrent reduction in CD34+ DC was seen in almost all patients (14/16 at the 80% level) that had a fall in CD3+ DC. A fall in CD34+ DC below 80% was highly predictive for relapse, with a 5 year relapse free survival of only 17% compared with 80% for those that maintained DC > 80% for the first 180 days (Figure 1B). To determine the clinical utility of CD34+ DC, we measured the time to relapse in those patients that had a fall in DC without morphologic relapse at the time of DC measurement (Figure 1A). Based upon our institutional trigger of CD34+ DC < 80%, the median time to relapse was 49 days in 22 patients. In contrast, a CD34+ DC < 90% had a much longer time to relapse (71 days). However, this longer lead time would come at the price of unnecessary intervention in almost half of all patients as the PPV for CD34+ DC < 90% was only 55%. Loss of CD3+ DC had the longest lead time (> 70 days), however it was only useful up to 10 (24%) of all relapses. Finally, DLI administered for CD34+ DC < 80% maintained durable remission (> 12 months) in only 2 of 19 patients. Conclusion: This is the largest comparison of peripheral blood CD34+ and CD3+ DC following allo-SCT. Our results show that monthly monitoring of CD34+ DC in the first 6 months after allo-SCT is a more useful biomarker than CD3+ DC for predicting relapse in patients allografted for AML or MDS. The level of CD34+ DC chosen to trigger intervention should be guided by characteristics of the intervention such as toxicity and expected response time. Given the relative ineffectiveness of DLI for CD34+ < 80%, we suggest that 90% may provide greater time for immunologic responses. Figure Legend (A) Characteristics of different levels of CD3 and CD34 DC at any time in the first 180 days post-allo-SCT. Number of patients that fulfil the level in the total cohort of 136 patients. Number of patients before relapse indicates the number that do not have morphologic relapse at the time of DC measurement. Median days before relapse is shown for those patients. (B) Relapse free survival of patients according to CD34+ DC > 80% (blue line) or < 80% (yellow line) in the first 180 days. Disclosures Spencer: AbbVie, Celgene, Haemalogix, Janssen, Sanofi, SecuraBio, Specialised Therapeutics Australia, Servier and Takeda: Consultancy; Celgene, Janssen and Takeda: Speakers Bureau; Amgen, Celgene, Haemalogix, Janssen, Servier and Takeda: Research Funding; AbbVie, Amgen, Celgene, Haemalogix, Janssen, Sanofi, SecuraBio, Specialised Therapeutics Australia, Servier and Takeda: Honoraria. Wei:Celgene: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria; Astellas: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; MacroGenics: Consultancy, Honoraria; Servier: Consultancy, Honoraria, Research Funding; Walter and Eliza Hall Institute: Other: former employee and receives a fraction of its royalty stream related to venetoclax; Pfizer: Honoraria; Genentech: Honoraria; Astra Zeneca: Honoraria, Research Funding; AbbVie Inc.: Consultancy, Honoraria, Research Funding.