In vivo expression of mutant Samd9l collaborates with inflammation to induce bone marrow failure

SAMD9 and SAMD9L germline mutations have recently emerged as a new class of predispositions to pediatric myeloid neoplasms. Patients commonly have hypocellular bone marrows and a greater risk of developing clonal chromosome 7 deletions. We recently demonstrated that expressing SAMD9/SAMD9L mutations in hematopoietic cells suppresses their proliferation and induces apoptosis. Here we generated a mouse model that conditionally expresses mutant Samd9l to assess the in vivo impact on hematopoiesis. We showed that mutant Samd9l cells are less fit relative to wild-type counterparts, which is exacerbated by inflammatory stimuli, and ultimately leads to marrow hypocellularity. Genomic and phenotypic analyses recapitulated many of the hematopoietic cellular phenotypes observed in patients with germline SAMD9 or SAMD9L mutations, including lymphopenia, and pinpointed TGF-b  as a potential causative and targetable networkpathway. Further, we observed non-random genetic deletion of the mutant Samd9l locus on mouse chromosome 6, mimicking the pathologic features observed in patients. For experiment 1, WBM cells were extratced from 3-month old Samd9l-KO, Samd9l-Wt and Samd9l-Mut mice (n=1 per group). For Experiment 2, Samd9l-Wt and Samd9l-Mut mice were treated with vehicle or pI:pC twice a week for 4 weeks (n=1 per group). WBM was then extracted and Lineage-cKit+ cells were sorted to run 10x csRNA-seq.