Patient Derived Neomorphic SEPTIN6 Mutations Induce Hematopoietic Stem Cell Senescence Underlying Congenital Neutropenia and Myelodysplasia

Septins are RAS-like GTP-binding proteins involved in cytoskeletal organization and cellular division. We previously described two pediatric patients harboring SEPT6 germline mutations (GM1:SEPT6 c.1282T>C and GM2:SEPT6 c.1282T>A) presenting with congenital neutropenia associated with the development of myelodysplastic syndrome. In one patient, an additional somatic mutation (SM:SEPT6 c.43C>T) was detected at a low allelic fraction in the BM. Here, we describe the consequences of these mutations on hematopoiesis. Transgenic expression or gene editing of GM1 or GM2 in human CD34+ cells led to enlarged, multinucleated cells associated with accelerated exhaustion and premature senescence of hematopoietic stem and progenitor cells (HSPCs). Senescence defects were associated with the upregulation of mTOR phosphorylation; and pharmacologic inhibition of the mTOR pathway resulted in rescue of senescence. In parallel, HSPCs exhibited marked mitochondrial abnormalities, including increased expression of OPA1, a key regulator of mitochondrial fusion, and enhanced oxygen consumption during mitochondrial respiration. Introduction of SM in cis with GM1 mitigated all observed defects. Collectively, our findings demonstrate a gain-of-function mechanism underlying marrow failure in patients carrying germline SEPT6 mutations and suggest that acquisition of a secondary somatic SEPT6 variant may confer a compensatory effect within hematopoietic cells.