Imatinib to Increase RUNX1 Activity in Participants With Germline RUNX1 Deficiency

Background: Runt-related transcription factor 1 (RUNX1) gene regulates the formation of blood cells. People with mutations of this gene may bleed or bruise easily; they are also at higher risk of getting cancers of the blood, bone marrow, and lymph nodes. Objective: The purpose of the study includes determining which dose of imatinib is best for people with pathogenic or likely pathogenic RUNX1 mutations without blood cancers, and to determine whether there are any changes in platelet function and inflammatory markers. Eligibility: Adults aged 18 and older with RUNX1 mutations. Healthy people without this mutation, including family members of affected participants, are also needed. Design: Participants with the RUNX1 mutation will be screened. They will have a physical exam with blood tests. They will have a test of their heart function. They may need a new bone marrow biopsy if they haven't had one in the past year. Imatinib is a tablet taken by mouth once a day, every day, at home. Affected participants in different parts of the study will take imatinib for either 42 days or up to 98 days. They will fill out questionnaires about how they are feeling. For the first part of the study, participants will have blood tests every 2 weeks, either at home or at the NIH, while they are taking the imatinib. They will have a follow up visit, at home or at the NIH, when they are done taking imatinib on Day 42. Participants on the second part of the study will come to NIH on days 1 and days 98. They will have blood tests every 2 weeks (at home or the NIH) while they are taking imatinib. They may opt to have a bone marrow biopsy repeated after they finish their course of imatinib. Participants will have a follow-up visit (at home or the NIH) 30 days after they stop taking imatinib. Participants who do not have the RUNX1 mutation will have 1 clinic visit. They will have blood tests. They will fill out questionnaires. They may opt to have a bone marrow biopsy.