Telomeropathies, also referred to as telomere biology disorders (TBD), correspond to a spectrum of diseases characterized by genetic defects in the maintenance mechanisms of telomeres and telomerase. Individuals carrying variants in the genes involved in telomeres machinery may have critically short or dysfunctional telomeres, which leads to poor cell regeneration and predisposition to cancer. The commonly affected tissues are those that renew rapidly, such as the bone marrow, lungs, and skin. The most effective therapeutical approach to treat bone marrow failure in patients with TBD is allogeneic bone marrow transplantation, restricted by the low availability of compatible donors and complications of the conditioning regimen. Fares et al. (2013) demonstrated that the small molecule UM171 expands the CD34+ cell subpopulations from cord blood with superior engraftment potential. We demonstrated that UM171 expands primitive CD34+ cells from patients with immune aplastic anemia. The expanded hematopoietic progenitors showed no observable chromosomal, genetic, or telomeric changes. Here we assessed the potential of the HSC agonist UM171 to expand the HSPC compartment of patients with telomeropathies.

Bone marrow aspirate samples were collected from five patients, and CD34+ cells were enriched using immunomagnetic labeling with human CD34 MicroBeads and a magnetic separator. Cells were cultured for 7 days in ACF medium supplemented with cytokines that support expansion of HSPC and UM171 or DMSO (negative control). To assess the capacity of the cells cultured either with DMSO or UM171 to generate hematopoietic progenitors, 1,000 cells/mL were resuspended in Methocult and seeded onto 35 mm dishes in triplicate. After 14 days, the colonies were counted and classified according to their morphology.

After a 7-day expansion, the percentage of CD34+ cells was higher with UM171 in comparison to control (UM171, 48 ±5.1% vs. DMSO, 25.3% ±6.2% [mean ±standard error] n = 5; p = 0.003). The cell surface EPCR is a reliable marker for the purification of the HSPC compartment. Therefore, we evaluated the CD34+EPCR+ subpopulation, which was increased after the UM171 treatment (UM171, 14.4 ±4.1% vs. DMSO, 1.8% ± 0.6%; n = 5; p = 0.03). Additionally, cells expanded with UM171 gave rise to more progenitor cells, as observed by the CFU assay (UM171, 142 ± 18 vs. DMSO, 94% ± 9; n = 4; p = 0.01). No significant telomere attrition was observed with the expansion.

Bone marrow failure is a common manifestation in patients with telomere disorders since the telomerase complex is essential for maintaining the self-renewal capacity of the HSPC pool. Our data suggest the possible future use of UM171 to enhance ex vivo production of HSPC from TBD patients, making it feasible to use these cells in autologous transplants. These findings should improve our understanding of the effect of UM171 on these cells and help to develop protocols for scaling up its production for future therapies.

Our studies reveal UM171 to be a potent molecule to expand HSPC in patients with telomeropathies. Further experiments are required to confirm these results besides the assessment of the engraftment potential of the expanded cells in a murine model.