Rationale: Hypercholesterolemia, a major risk factor for cardiovascular disease, also increases all-cause mortality. Previous work from our lab and others has shown that hypercholesterolemia induces an oxidant stress-dependent accelerated ageing of hematopoietic stem cells (HSCs) that shortens the telomeres of both HSCs and WBCs. However, little is known about the underlying epigenetic mechanism. Objective: We found that hypercholesterolemia causes an oxidant stress-dependent downregulation of Tet1 and so we hypothesize that hypercholesterolemia accelerates the ageing of HSCs through a Tet1 dependent epigenetic pathway. Identifying the mechanisms by which hypercholesterolemia accelerates the ageing of HSCs may provide clues to understanding how hypercholesterolemia increases all-cause mortality in divergent clinical studies. Methods and Results: RT-PCR showed that the expression of Tet1 was specifically downregulated in HSCs isolated from ApoE -/- mice. FACS analysis showed that the frequency of total HSCs was significantly higher in Tet1 -/- mice than those in WT control, while the long-term and side populations of HSCs in Tet1 -/- mice were significantly fewer than those in WT mice. The reconstitution capacity of total HSCs in Tet1 -/- mice was significantly less than that of WT mice. In contrast, long term HSCs from Tet1 -/- mice showed higher reconstitution capacity than that of long term HSCs from WT mice. Restoration of the Tet1 expression in Tet1 -/- HSCs effectively restored the LT-HSC population as well as the reconstitution capacity of total HSCs. RT-PCR showed that the expression of p19 and p21 were upregulated in HSCs from Tet1 -/- mice. DNA pyrosequencing did not show significant change in DNA methylation status in the promoter region of p19 and p21, while ChIP-PCR indicated that Tet1 deficiency facilitated the accumulation of the H3K4m3 modifications of p19 and p21. Conclusion: Tet1-dependent epigenetic regulation is critical to maintain a healthy HSC compartment. Hypercholesterolemia induced downregulation of Tet1. Tet1 increased the expression of p19 and p21, and thereby reduces the long-term HSC population, resulting accelerated ageing in HSCs.