Impact of Dkc1 Deficiency on BM Niche Function during Hematopoietic Stem Cell Transplantation in a Conditional Murine Model of X-Linked Dyskeratosis Congenita

Dyskeratosis congenita (DC) is a telomere biology disorder associated with bone marrow failure (BMF) and the triad of dystrophic nails, skin rashes, and leukoplakia. X‐linked DC (XDC) is caused by mutations in DKC1, which encodes dyskerin, a nucleolar protein associated with rRNA processing and telomerase complex formation. Hematopoietic stem cell transplantation (HSCT) is currently the only definitive curative treatment for BMF in XDC patients, though many patients exhibit only partial restoration of hematopoietic function. Our previous studies demonstrated that molecular and cellular defects in BM niches contribute to poor donor HSC engraftment in murine models of other BMF syndromes, including Shwachman‐Diamond Syndrome (SDS), a disease whose pathogenesis intersects with the role of dyskerin in ribosome biogenesis. These studies led us to hypothesize that BM niche dysfunction contribute to poor graft function in DC patients post-HSCT. Defining specific deficits would enable development of novel targeting strategies to improve HSCT outcomes for patients with XDC. Here, we report that in a novel murine model of XDC driven by conditional Dkc1 deletion, Dkc1 deficiency within mesenchymal BM niche cells causes significant impairment of healthy donor HSC engraftment after HSCT. We have identified defects in multiple pathways within Dkc1-deficient BM niche in response to myeloablative conditioning that may drive this reduction in HSC supportive capacity. To generate a murine model with conditional Dkc1 deletion in BM niche, we crossed Dkc1l/lmice with Mx1 Cre+mice, inducing Cre expression in hematopoietic cells and Mx1‐inducible niche cells by polyinosinic-polycytidylic acid administration. At baseline, the resulting Mx1 CreDkc1 Exc mice exhibit mild hematopoietic abnormalities versus Dkc1 l/l controls, including variable thrombocytopenia, increased blood monocytes, and reduced BM lymphoid progenitors. To assess capacity of BM niches from Mx1 CreDkc1 Exc mice to efficiently engraft donor HSC, we transplanted Mx1 CreDkc1 Exc mice and Dkc1 l/l controls with healthy GFP + wild‐type donor BM and assessed efficiency of donor HSC engraftment at 2 weeks following HSCT using competitive secondary HSCT. The Mx1 CreDkc1 Exc recipients exhibited significantly impaired efficiency of donor HSC engraftment compared with controls, as indicated by a 25-30% reduction of long‐term GFP + cell reconstitution in mature blood lineages in secondary recipients (Figure 1). We next sou