Declined presentation loss of tRNA-methylation at cytosine 38 by Dnmt2 induces a hematopoietic stem cell aging-like phenotype

During hematopoietic aging, stem cells undergo a variety of molecular changes leading to genomic instability, epigenetic alterations and stem cell exhaustion. Within the last years, it has been reported that the loss of proteostasis also contributes to the development of age-related diseases. We have shown previously that the t-RNA methyltransferase Dnmt2 modifies several tRNAs at cytosine 38 and that Dnmt2-deficiency leads to a reduced bone marrow cellularity in newborn mice, which is accompanied by a decreased LSK cell population and a cell-autonomous differentiation defect compared to wildtype (wt) littermates. As serial BM transplantation experiments revealed reduced engraftment efficiency of young Dnmt2-/- BM in 3° recipients with reduced lymphoid lineage output, we hypothesized that Dnmt2 plays an important role during hematopoietic aging. To subsequently investigate the function of Dnmt2 in aged HSCs, we analyzed PB and BM samples of 2 year old Dnmt2-/- (n=12) and wt mice (n=9). RBC and PLT counts were slightly increased (1.23-fold and 1.65-fold, respectively) as well as the proportion of LT-HSCs upon Dnmt2 deficiency (1.22-fold). Interestingly, the ratio of Dnmt2-/- MPP4 cells, reported to give rise to lymphoid cells, is less (p=0.041) compared to wt, which is in line with a reduced lymphoid (D2-/-: 57.4 ± 9.3 [col. ± SEM]; n=5 vs. wt: 97.9 ± 22.1; n=6) but similar myeloid colony forming capacity (D2-/-: 82.3 ± 5.6 [col. ± SEM]; n=8 vs. wt: 82.8 ± 7.5; n=9) of old Dnmt2-/- BM assessed in differentiation assays in vitro. Moreover, aged Dnmt2-/- BM cells engraft 1.8-fold (p=0.016) less in primary recipients (n=20) with reduced neutrophil and lymphoid blood cell counts as well as a delayed RBC and PLT repopulation within the first eight weeks. Our results suggest that Dnmt2 may play a role during HSC aging and further research is needed to investigate mistranslation of specific codons by tRNAs lacking Dnmt2-dependent methylation to elucidate its role in age-related processes.