Thrombocytopenia, congenital amegakaryocytic, 1

Base Editing (ABE8e) via RNP electroporation (ex vivo) delivery is a rationale-driven therapeutic strategy for Thrombocytopenia, congenital amegakaryocytic, 1 targeting the MPL c.304C>T (p.Arg102Cys) variant (Pathogenic, missense variant). The editing system (ABE8e-nSpCas9 (adenine base editor)) converts the pathogenic A back to G on the target strand, restoring the wild-type codon. Target tissue: Blood/HSC. Therapeutic goal: Correct biallelic loss-of-function MPL variants in hematopoietic stem and progenitor cells to restore thrombopoietin signaling, normalize megakaryopoiesis, and prevent progression to bone marrow failu. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Medium, immunogenicity Low.

1. Molecular basis: MPL NM_005373.3(MPL):c.304C>T (p.Arg102Cys) is classified as Pathogenic (ClinVar variation ID 644406). Molecular consequence: missense variant. Protein change: R102C. 2. Epidemiology: Ultra-rare inherited bone marrow failure syndrome with fewer than 100 reported cases worldwide and estimated prevalence <1/1,000,000; most cases present in the neonatal period with severe thrombocytopenia and progress to pancytopenia and marrow failure in early childhood (WEB-01, WEB-03, WEB-09, PAP 3. Standard of care: Supportive care with platelet transfusions and aggressive management of bleeding, followed by early allogeneic hematopoietic stem cell transplantation (HSCT) which is the only established curative option for MPL-associated CAMT1. Thrombopoietin receptor agonists (eltrombopag, romiplostim) show littl 4. Pipeline: No registered interventional trials or approved gene therapies specifically targeting MPL in CAMT1 were identified in ClinicalTrials.gov or major regulatory databases (WEB-11, WEB-12). Preclinical proof-of-concept studies have demonstrated successful CRISPR-Cas9 correction of MPL mutations in cell s 5. ABE clinical validation: ABE8e (Richter et al. 2020, Nat Biotechnol) achieves ~1.7x higher editing efficiency than ABE7.10. VERVE-101 demonstrated first-in-human LNP-ABE liver editing with 55-66% PCSK9 reduction (Raal et al. 2025, NEJM). Beam Therapeutics is advancing multiple ABE programs.

1. No published data specifically correcting MPL c.304C>T (p.Arg102Cys) with Base Editing (ABE8e); strategy is based on general principles and must be validated preclinically. 2. PAM availability and bystander base analysis for the specific genomic context have not been performed. If no canonical NGG PAM positions the target within the editing window, PAM-flexible variants (SpRY) may be needed. 3. Long-term durability, off-target genome-wide effects, and immunogenicity in the target patient population require thorough preclinical and clinical evaluation.

For MPL c.305G>C (p.Arg102Pro), a pathogenic missense variant in the thrombopoietin receptor that impairs TPO-mediated megakaryopoiesis, allogeneic HSCT remains the only curative treatment for congenital amegakaryocytic thrombocytopenia (CAMT). TPO receptor agonists (romiplostim, eltrombopag) — approved for immune thrombocytopenia — are sometimes used as supportive therapy in CAMT, but their efficacy is inherently limited because they act through the defective MPL receptor itself. Arg102 is located in the extracellular domain of MPL and participates in TPO binding; the R102P substitution (classified as CAMT type II) typically allows some residual receptor signaling, resulting in transient platelet recovery in infancy before progressive bone marrow failure. This residual signaling may explain why some CAMT-II patients show partial responses to supraphysiological TPO agonist doses.

CAMT patients with type II MPL mutations (missense variants with residual receptor function, including R102P) generally have a milder early course than type I (null variants) but ultimately progress to pancytopenia and aplastic anemia by age 3-5 years, necessitating HSCT (Ballmaier & Germeshausen, 2009, PMID: 19846852). Allogeneic HSCT achieves >85% overall survival when performed before aplastic transformation with a matched donor. Case reports have documented transient responses to romiplostim (Fc-peptibody TPO mimetic) and eltrombopag (small molecule TPO agonist) in CAMT patients, with temporary platelet increases, though responses are incomplete and unsustained. Ex vivo lentiviral MPL gene therapy in CD34+ HSPCs is in preclinical development — proof-of-concept in Mpl-knockout mice has shown restoration of megakaryopoiesis and platelet production following transplantation of corrected cells.

TPO receptor agonists cannot overcome severe MPL loss-of-function — for type I (null) variants, they are completely ineffective. Even for type II variants like R102P with residual receptor function, agonist responses are partial and temporary, providing at best a bridge to transplant. HSCT carries transplant-related mortality (5-15% with matched donors, higher with mismatched/haploidentical donors), GVHD risk, and long-term complications including infertility and secondary malignancies. Gene therapy (lentiviral MPL correction of autologous HSPCs) remains preclinical with no clinical trials reported. The progressive nature of CAMT — evolving from isolated thrombocytopenia to pancytopenia as HSCs fail — means that the therapeutic window for gene therapy or transplant closes as bone marrow reserve diminishes. Newborn or early infant diagnosis via genetic testing is essential for optimal outcomes.

For MPL c.305G>C (p.Arg102Pro), a pathogenic missense variant in the thrombopoietin receptor extracellular domain, the therapeutic relevance of TPO receptor agonists (romiplostim, a peptibody; eltrombopag, a small-molecule) depends critically on the residual receptor function. Romiplostim is an Fc-peptide fusion ("peptibody") that mimics TPO and activates MPL through a binding site partially overlapping with native TPO. If p.Arg102Pro retains sufficient membrane localization and conformational integrity for romiplostim binding, even partial receptor activation could stimulate residual megakaryopoiesis. However, for variants causing complete loss of surface expression, TPO mimetics are ineffective.

MPL (thrombopoietin receptor, c-Mpl) is essential for megakaryocyte differentiation and platelet production. Biallelic MPL loss-of-function variants cause congenital amegakaryocytic thrombocytopenia (CAMT), which progresses to pancytopenia/aplastic anemia as HSCs gradually deplete. Type I CAMT (null variants, complete MPL loss) presents with severe thrombocytopenia at birth and early marrow failure; Type II (hypomorphic variants, partial MPL function) has later onset and milder course. The p.Arg102Pro substitution in the extracellular cytokine receptor homology domain (CRH) likely disrupts local secondary structure but may not completely abolish surface expression. Romiplostim has been used off-label in CAMT case reports with variable success: patients with hypomorphic MPL variants (type II) showed platelet responses, while those with null variants (type I) did not respond (Ballmaier & Germeshausen, Blood 2009; PMID:19005177). Eltrombopag binds the MPL transmembrane domain (a different site from TPO) and has theoretical advantages for variants affecting the extracellular domain while preserving transmembrane/intracellular signaling.

TPO receptor agonists are palliative — they do not correct the underlying genetic defect or prevent the progression to aplastic anemia in type I CAMT. The only curative therapy is allogeneic HSCT. Response to romiplostim/eltrombopag is strictly genotype-dependent: null MPL variants cannot respond regardless of dose. The p.Arg102Pro variant has not been functionally characterized for surface expression or TPO/romiplostim binding capacity, making response prediction uncertain. CAMT is ultra-rare (estimated <100 known patients), so evidence for TPO mimetics comes from individual case reports, not trials. Long-term TPO agonist use raises theoretical concerns about marrow fibrosis and potential for clonal hematopoiesis in a genetically predisposed marrow. Eltrombopag's hepatotoxicity is a concern in pediatric patients requiring chronic therapy.