Fanconi anemia (FA) is a DNA repair disorder resulting from mutations in genes encoding for FA DNA repair complex components and is characterized by variable congenital abnormalities, bone marrow failure (BMF), and high incidences of malignancies. FA mosaicism arises from reversion or other compensatory mutations in hematopoietic cells and may be associated with BMF reversal and decreased blood cell sensitivity to DNA-damaging agents (clastogens); this sensitivity is a phenotypic and diagnostic hallmark of FA. Uncertainty regarding the clinical significance of FA mosaicism persists; in some cases, patients have survived multiple decades without BMF or hematologic malignancy, and in others hematologic failure occurred despite the presence of clastogen-resistant cell populations. Assessment of mosaicism is further complicated because clinical evaluation is frequently based on clastogen resistance in lymphocytes, which may arise from reversion events both in lymphoid-specific lineages and in more pluripotent hematopoietic stem/progenitor cells (HSPCs). In this review, we describe diagnostic methods and outcomes in published mosaicism series, including the substantial intervals (1-6 years) over which blood counts normalized, and the relatively favorable clinical course in cases where clastogen resistance was demonstrated in bone marrow progenitors. We also analyzed published FA mosaic cases with emphasis on long-term clinical outcomes when blood count normalization was identified. Blood count normalization in FA mosaicism likely arises from reversion events in long-term primitive HSPCs and is associated with low incidences of BMF or hematologic malignancy. These observations have ramifications for current investigational therapeutic programs in FA intended to enable gene correction in long-term repopulating HSPCs.

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Introduction: Pyruvate Kinase Deficiency (PKD) is a rare inherited hemolytic anemia that is caused by mutations in the PKLR gene leading to decreased red cell pyruvate kinase (RPK) activity and impaired erythrocyte metabolism. The disorder is characterized by anemia, reticulocytosis, splenomegaly and iron overload, and may be life-threatening in severely affected individuals. PKD represents a significant unmet medical need as current therapies are palliative and limited to chronic blood transfusions, iron chelation therapy, and splenectomy. The side effects of these supportive treatments include iron overload, end-organ damage and increased infection risks. AG-348, an allosteric activator of RPK, is under evaluation in clinical trials, predominantly in less severely-afflicted transfusion-independent patients. Allogeneic hematopoietic stem cell transplantation (HSCT) has been performed in selected cases and resulted in transfusion independence, suggesting that the disorder may be reversed when an adequate level of hematopoietic stem and progenitor cells (HSPCs) harboring a corrected PKLR gene engraft in the bone marrow (BM). The therapeutic efficacy of allogeneic transplant is limited by the availability of a suitable donor and transplant-associated toxicities. Preclinical studies conducted in a clinically relevant PKD murine model have demonstrated the safety and efficacy of Lin- BM cells transduced with the therapeutic lentiviral vector, PGK-coRPK-WPRE, in ameliorating the PKD phenotype. More specifically, transplantation of transduced cells resulted in increased erythrocyte survival, decreased reticulocytosis, and improvement in the secondary manifestations of hemolytic anemia, including splenomegaly and hepatic iron overload. Based on compelling preclinical data, a global Phase 1 clinical trial RP-L301-0119 (clinicaltrials.gov#NCT04105166) is underway to evaluate the feasibility and safety of lentiviral mediated gene therapy in adults and pediatric subjects with severe PKD. Methods: 6 subjects with severe PKD (defined as having a history of severe and/or transfusion-dependent anemia despite prior splenectomy) will be enrolled in the Phase 1 study; the first 2 subjects will be adults (age ≥18-<50 years), followed by 2 older pediatric subjects (age ≥12-17 years) once initial safety has been established. Subsequently, 2 younger pediatric subjects (age ≥8-11 years) will be enrolled. Peripheral blood (PB) hematopoietic stem cells are collected on two consecutive days via apheresis after mobilization with granulocyte-colony stimulating factor (G-CSF) and plerixafor. CD34+ HSPCs are enriched, transduced with PGK-coRPK-WPRE lentiviral vector (LV), and cryopreserved. Following final release testing of the investigational product (IP), RP-L301, myeloablative conditioning with therapeutic drug monitoring (TDM)-guided busulfan is administered over 4 days. RP-L301 is then thawed and infused. Patients are followed for safety assessments, including replication competent lentivirus (RCL) and insertion site analysis (ISA), and for efficacy parameters including PB and BM genetic correction, decrease in transfusion requirements, clinically significant improvement in anemia, and reduction of hemolysis. Results: An adult female PKD subject (age 31 years) with significant anemia and transfusion requirement has received treatment as of July 2020. Mobilization and apheresis procedures were performed successfully and busulfan conditioning was administered at the target area under the curve (AUC). IP consisted of 3.9×106 CD34+ cells/kg body weight, with a mean vector copy number (VCN) of 2.73. Safety and preliminary efficacy results will be available at the time of presentation. Conclusions: Efficacy in pre-clinical models indicates promising potential for clinical gene therapy in severe PKDHematopoietic stem cell mobilization using G-CSF and plerixafor appears feasible and effective in adult PKD patientsIP was successfully manufactured to meet the required specifications for the Phase 1 clinical study and administered without short-term infusion related complications Disclosures Navarro: Rocket Pharmaceuticals, Inc.: Current equity holder in publicly-traded company, Other: SN has licensed medicinal products and receives research funding and equity from Rocket Pharmaceuticals, Inc., Patents & Royalties, Research Funding. Sevilla:Rocket Pharmaceuticals, Inc.: Consultancy, Current equity holder in publicly-traded company. Glader:Agios Pharmaceuticals, Inc.: Consultancy. Beard:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Law:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Zeini:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Choi:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Nicoletti:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Bueren:Rocket Pharmaceuticals, Inc.: Consultancy, Current equity holder in publicly-traded company, Other: Consultant for Rocket Pharmaceuticals, Inc. and has licensed medicinal products and receives research funding and equity from this company., Patents & Royalties, Research Funding. Rao:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Schwartz:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Segovia:Rocket Pharmaceuticals, Inc.: Consultancy, Current equity holder in publicly-traded company, Other: Consultant for Rocket Pharmaceuticals, Inc. and has licensed medicinal products and receives research funding and equity from the Company., Patents & Royalties, Research Funding.

Introduction: LAD-I is a rare inherited disorder of leukocyte (primarily neutrophil) adhesion to endothelial cell surfaces, migration, and chemotaxis resulting from ITGB2 gene mutations encoding for the β2-integrin component, CD18. Severe LAD-I (i.e., CD18 expression on <2% of PMNs) is characterized by recurrent severe infections, impaired wound healing, and childhood mortality. Although allogeneic hematopoietic stem cell transplant (alloHSCT) is potentially curative, its utilization and efficacy are limited by HLA-matched donor availability and risk of graft-versus-host disease (GVHD). RP-L201-0318 (clinical trials.gov # NCT03812263) is a phase 1/2 open-label clinical trial evaluating the safety and efficacy of autologous CD34+ cells transduced with a lentiviral vector (LV) carrying the ITGB2 gene encoding for CD18 (Chim-CD18-WPRE) in severe LAD-I. Methods: Pediatric patients ≥ 3 months old with severe LAD-I (demonstrated by CD18 expression on <2% PMNs and at least 1 prior significant bacterial or fungal infection) are eligible. Peripheral blood (PB) HSCs are collected via apheresis after mobilization with granulocyte-colony stimulating factor (G-CSF) and Plerixafor. CD34+ cells are selected, transduced with Chim-CD18-WPRE LV, and cryopreserved. Myeloablative conditioning with busulfan (with therapeutic drug monitoring (TDM) to adjust dosing to enable target area under the curve (AUC)) is administered, followed by infusion of the investigational drug product (RP-L201). Patients are followed for safety assessments (i.e., replication competent lentivirus (RCL) and insertion site analysis (ISA)), and efficacy -- survival to age 2 and at least 1-year post-infusion without alloHSCT, increase in neutrophil CD18 expression to at least 10%, PB vector copy number (VCN), decrease in infections and/or hospitalizations, and resolution of skin or periodontal abnormalities. Results: Two patients (ages 9 and 3) were treated in Phase 1. Both have a history of recurrent severe infections, documented ITGB2 mutations, and baseline CD18, CD11a, and CD11b expression < 1%. Mobilization and apheresis procedures were performed successfully and busulfan was administered at the target AUC. Investigational product comprised of 4.2x106 CD34+ cells/kg with VCN of 3.8 copies/cell (liquid culture) for Patient 1 and 2.8x106 CD34+ cells/kg with VCN of 2.5 for Patient 2 and were infused without complications. No serious treatment-emergent adverse events were reported. Neutrophil engraftment was observed in < 3 weeks in both patients. For Patient 1, PB PMN CD18 expression 6 months post-treatment was 47% (sustained from 45% at 3-months, vs. < 1% at baseline), with PB VCN of 1.3. Skin lesions present at baseline were resolving with no new lesions. Replication competent lentiviral (RCL) testing at 3 months and 6 months post-infusion were negative. Safety and efficacy data 12-months post-treatment for Patient 1 and 6-months post-treatment for Patient 2 will be available at the time of presentation. Conclusion: Initial results from Phase 1 demonstrate preliminary safety and efficacy of RP-L201 for reversal of severe LAD-I. Enrollment of patients in the Phase 2 study is underway. Disclosures Kohn: Allogene Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees; Orchard Therapeutics: Consultancy, Patents & Royalties, Research Funding. Rao:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Almarza:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Nicoletti:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Law:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Beard:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Sevilla:Novartis: Other: Advisory Board; Sobi: Other: Advisory Board; Rocket Pharma: Consultancy; Amgen: Other: Advisory Board. Bueren:Rocket Pharmaceuticals, Inc.: Consultancy, Current equity holder in publicly-traded company, Other: Consultant for Rocket Pharmaceuticals, Inc. and has licensed medicinal products and receives research funding and equity from this company., Patents & Royalties, Research Funding. Schwartz:Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company.

Background: Pyruvate kinase deficiency (PKD) is a rare inherited hemolytic anemia caused by mutations in the PKLR gene resulting in decreased red cell pyruvate kinase activity and impaired erythrocyte metabolism. Manifestations include anemia, reticulocytosis, splenomegaly and iron overload, and may be life-threatening in severely affected individuals. PKD represents a significant unmet medical need as current treatments are palliative and limited to blood transfusions, chelation therapy, and splenectomy which are associated with significant side effects. Preclinical studies in a clinically relevant PKD murine model have demonstrated that infusion of gene-modified Lin− bone marrow (BM) cells may ameliorate PKD phenotype. Based on compelling preclinical data, a global Phase 1 clinical trial RP-L301-0119 (NCT04105166) is underway to evaluate the feasibility and safety of lentiviral mediated gene therapy in adult and pediatric subjects with severe PKD. Methods: Six adult and pediatric patients with severe PKD (defined as severe and/or transfusion-dependent anemia despite prior splenectomy) will be enrolled. Peripheral blood (PB) hematopoietic stem cells (HSCs) are collected on 2 consecutive days via apheresis after mobilization with granulocyte-colony stimulating factor (G-CSF) and plerixafor. HSCs are enriched, transduced with PGK-coRPK-WPRE lentiviral vector (LV), and cryopreserved. Following release testing of the investigational product (IP), RP-L301, myeloablative therapeutic drug monitoring (TDM) busulfan is administered over 4 days. RP-L301 is then thawed and infused. Patients are followed for safety assessments, including replication competent lentivirus (RCL) and insertion site analysis (ISA), and for efficacy parameters including PB and BM genetic correction, decrease in transfusion requirements, clinically significant improvement in anemia, and reduction of hemolysis for 2 years post-infusion. Results: As of May 2021, 2 adult patients with severe anemia have received RP-L301. Patient 1 (age 31 years) received 3.9x106 CD34+ cells/kg with mean vector copy number (VCN) of 2.73. Patient 2 (age 47 years) received 2.4x106 CD34+ cells/kg with mean VCN of 2.08. Despite baseline hemoglobin (Hb) levels in the 7.0-7.5 g/dL range, both patients displayed normal-range hemoglobin (Hb), improved hemolysis markers, and have required no red blood cell transfusions post-engraftment at 9- and 6- months follow-up. Both report improved quality of life. PB mononuclear cell VCNs for both patients were >2.0 at last evaluated timepoint (6- and 3-months post-treatment, respectively). No serious adverse events have been attributed to RP-L301. Updated safety and efficacy data will be presented. Conclusions: Hematopoietic stem cell mobilization using G-CSF and plerixafor is feasible and effective in adult PKD patients. RP-L301 was successfully manufactured to meet the required specifications for the Phase 1 clinical study and administered without short-term infusion related complications. Efficacy was demonstrated by normalized Hb associated with engraftment confirmed by PB and BM VCN. Disclosures Shah: OrchardTherapeutics: Membership on an entity's Board of Directors or advisory committees, Other: Dr. Shah currently serves on the medical advisory board for Orchard Therapeutics . Navarro: Rocket Pharmaceuticals, Inc.: Current equity holder in publicly-traded company, Other: Dr. Navarro has licensed medicinal products and receives research funding and equity from Rocket Pharmaceuticals, Inc., Patents & Royalties, Research Funding. Sevilla: Miltenyi: Consultancy; Novartis: Consultancy; Amgen: Consultancy; Rocket Pharmaceuticals, Inc.: Consultancy, Other: J.Sevilla is an inventor on patents on lentiviral vectors filed by CIEMAT, CIBERER and Fundación Jiménez Díaz, and may be entitled to receive financial benefits from the licensing of such patents.; SOBI: Consultancy. Glader: Agios: Consultancy. Quintana Bustamante: Rocket Pharmaceuticals, Inc.: Current equity holder in publicly-traded company. Beard: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Law: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Zeini: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Choi: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Nicoletti: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Rao: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Bueren: Rocket Pharmaceuticals, Inc.: Consultancy, Other: J.Bueren is an inventor on patents on lentiviral vectors filed by CIEMAT, CIBERER and Fundación Jiménez Díaz, may be entitled to receive financial benefits from the licensing of such patents and receives funding for research., Patents & Royalties, Research Funding. Schwartz: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Segovia: Rocket Pharmaceuticals, Inc.: Consultancy, Research Funding.

Background: Leukocyte Adhesion Deficiency-I (LAD-I) is a rare disorder of neutrophil adhesion resulting from ITGB2 mutations encoding for the β2-integrin component, CD18. Severe LAD-I (CD18 <2% of polymorphonuclear [PMN] cells) is characterized by severe infections, impaired wound healing, and childhood mortality. Allogeneic hematopoietic stem cell transplant (alloHSCT) is potentially curative; however, its efficacy is limited by donor availability and risk of graft-versus-host disease (GVHD) and graft failure. RP-L201-0318 (NCT03812263), a phase 1/2 open-label trial currently underway, employs autologous CD34+ cells transduced with a lentiviral vector (LV) carrying the ITGB2 gene (cDNA) Methods: Pediatric patients ≥ 3 months old with severe LAD-I are eligible. HSCs are collected via apheresis after mobilization with granulocyte-colony stimulating factor (G-CSF) and plerixafor and transduced with Chim-CD18-WPRE-LV. Myeloablative therapeutic drug monitoring (TDM) busulfan conditioning precedes RP-L201 infusion. Patients are followed for safety and efficacy (i.e., survival to age 2 and at least 1-year post-infusion, increase in PMN leukocyte CD18 expression to at least 10%, peripheral blood [PB] vector copy number [VCN], decrease in infections/hospitalizations, and resolution of skin or periodontal abnormalities). Results: Seven patients (ages 5mos-9yrs) have received RP-L201; all with follow-up ≥3 to 18 months. RP-L201 cell doses ranged from 2.8x106 to 6.5x106 CD34+ cells/kg with VCN from 1.8-3.8 copies/cell. No serious RP-L201 related treatment-emergent adverse events were reported. PB PMN CD18 expression in Patient 1 at 18-months post-treatment was 40% (vs. < 1% at baseline), with PB VCN of 1.44. Baseline skin lesions resolved with no new lesions reported. CD18 expression in the subsequent 6 patients has been 25-80% in neutrophils and stable for each patient from 3 to up to 18 months post-treatment. No new infections have been reported in patients post-infusion. The safety profile of RP-L102 remains highly favorable with no serious adverse events (SAEs) attributed to the investigational product (IP). Conclusion: RP-L201 leads to durable neutrophil CD18 expression and improved clinical course. Additional patient treatment is ongoing in 2021. Disclosures Kohn: UC Regents: Patents & Royalties; Pluto Immunotherapeutics: Membership on an entity's Board of Directors or advisory committees; Allogene: Membership on an entity's Board of Directors or advisory committees; ImmunoVec: Membership on an entity's Board of Directors or advisory committees; Lyrik Therapeutics: Membership on an entity's Board of Directors or advisory committees; MyoGene Bio: Membership on an entity's Board of Directors or advisory committees; Bluebird Bio: Membership on an entity's Board of Directors or advisory committees; Sangamo Biosciences: Membership on an entity's Board of Directors or advisory committees. Booth: GSK: Honoraria; Takeda: Honoraria; SOBI: Consultancy, Honoraria; Orchard Therapeutics: Consultancy, Honoraria; Rocket Pharmaceuticals, Inc.: Consultancy. Sevilla: Miltenyi: Consultancy; Novartis: Consultancy; Amgen: Consultancy; Rocket Pharmaceuticals, Inc.: Consultancy, Other: J.Sevilla is an inventor on patents on lentiviral vectors filed by CIEMAT, CIBERER and Fundación Jiménez Díaz, and may be entitled to receive financial benefits from the licensing of such patents.; SOBI: Consultancy. Rao: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Almarza: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Nicoletti: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. de Oliveira: Orchard Therapeutics: Research Funding; Bluebird Bio: Research Funding. Law: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Beard: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Choi: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Zeini: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company. Thrasher: Orchard Therapeutics: Consultancy, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Generation bio: Consultancy, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; 4Bio Capital: Consultancy, Membership on an entity's Board of Directors or advisory committees; Rocket Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees. Bueren: Rocket Pharmaceuticals, Inc.: Consultancy, Other: J.Bueren is an inventor on patents on lentiviral vectors filed by CIEMAT, CIBERER and Fundación Jiménez Díaz, may be entitled to receive financial benefits from the licensing of such patents and receives funding for research., Patents & Royalties, Research Funding. Schwartz: Rocket Pharmaceuticals, Inc.: Current Employment, Current equity holder in publicly-traded company.

Fanconi anemia (FA) is characterized by chromosome fragility, bone marrow failure (BMF) and predisposition to cancer. As reverse genetic mosaicism has been described as "natural gene therapy" in patients with FA, we sought to evaluate the clinical course of a cohort of FA mosaic patients followed at referral centers in Spain over a 30-year period. This cohort includes patients with a majority of T cells without

C, Bruchfeld JB, et al. Impact of performance status on overall survival in patients with relapsed and/or refractory multiple myeloma: real-life outcomes of daratumumab treatment.