Biological Age and Circulating Progenitor Cell Levels as Predictors Heart Disease Events

Identifying patients with stable coronary artery disease at high risk for future heart attacks or death remains a challenge for all cardiologists. The study entitled “Telomere Shortening, Regenerative Capacity, and Cardiovascular Outcomes” in this issue has shed new light in the field¹. Hammadah and co-workers focused on the relationship between circulating CD34+ cell levels, telomere length, and cardiovascular outcomes in subjects with stable coronary artery disease (CAD)¹. Their study yielded two novel findings: 1) 10% shorter telomeres correlated with 5% lower levels of circulating CD34+ cells and 2) both short LTL and low CD34+ cell levels independently predict adverse cardiovascular disease (CVD) outcomes (death, myocardial infarction (MI), coronary revascularization, or cerebrovascular events). Measurements of LTL and circulating CD34+ cells may facilitate risk stratification of patients with stable CAD distinct from traditional CVD risk factors. Prior studies reported associations between shortened LTL and CVD², and postulated that decreased regenerative capacity may be a primary cause of CVD events. The study by Hammadah and co-workers is the first and largest study to test the hypothesis that LTL may be linked to the levels of CD34+ cells as a surrogate for regenerative capacity, and correlate these measures with CVD events¹.

Telomeres prevent chromosome ends from being recognized as a DNA break and activating DNA damage response pathways causing cell cycle arrest³. The length of the telomere can be viewed as a replicative history of cells, where shorter telomeres occur from high cell divisions or impaired telomere repair mechanisms. Telomere attrition occurs predominantly during childhood and largely explains inter-individual variation among adults⁴. Extrinsic factors influencing telomere length include socioeconomic status, stress, smoking, and obesity³. In hematopoietic stem cells (HSCs), telomerase is critically important for their regeneration. Proliferative stress of serial bone marrow transplantation is a known trigger for telomere shortening, and deficiency of telomerase impairs HSC function through cell intrinsic effects⁵ and extrinsic stromal cell impairment, resulting in impaired HSC engraftment upon transplantation⁵. Collectively, LTL provides an index of replicative stress and telomere repair balance influenced by factors distinct from those affecting circulating CD34+ cell levels.

Hammadah and co-workers¹ measured circulating CD34+ cells as an index of regenerative capacity. CD34+ cells are heterogeneous, containing multiple cell populations including HSCs and lineage specified progenitors of both the innate and adaptive immune systems. Inflammation in atherosclerosis and repair of tissue injury are influenced by all cell types derived from CD34+ cells⁶. How CVD risk factors reduce CD34+ cell levels remains of seminal importance. Several recent publications indicate a direct role for HSCs and their inflammatory progeny in both atherosclerosis and repair of MI⁶. Degeneration of the hematopoietic system is associated with aging-associated changes including decreased lymphocyte levels, reduced competence of the adaptive immune system, increased myeloid cells, and reduced regenerative potential with transplantation⁷. Chronological age alone does not impair stem cell function in human bone marrow transplant studies⁸, indicating additional factors are involved. HSC aging involves multiple factors including activation of inflammatory signaling, DNA damage accumulation, and epigenetic changes that alter cell function⁷. The decline in circulating CD34+ cells in subjects with CAD reported by Hammadah and co-workers¹ may be the result of degeneration through similar mechanisms. Al Mheid and co-workers⁹ showed CVD risk factors reduce the levels of circulating CD34+ cells. How CVD risk factors impact CD34+ cell levels remain an open question, but some potential insights from animal studies are provided below.

The effect of hypercholesterolemia (HC) on HSC biology involves effects of inflammatory cytokines elaborated from cholesterol crystal mediated activation of the inflammasome¹⁰. Inflammasome activation causes chronically elevated levels of IL-1β resulting in impaired HSC self-renewal, and increased production of inflammatory monocytes¹¹. Chronic exposure of HSCs to HC results in increased reactive oxygen species, reduced numbers of HSCs, decreased telomere length, and reduced re-population capacity¹². These findings provide mechanistic insight to understand the findings of Hammadah and co-workers¹. A prolonged period of chronically stimulated HSC proliferation may lead to transition of HSCs to the aged functional profile, resulting in reduced levels of circulating CD34+ cells. In diabetes mellitus, altered bone marrow niche function may reduce the number of circulating CD34+ cells through impairment of HSC mobilization¹³. Collectively, the levels of circulating CD34+ cells may be thought of as an integrative sensor of CVD risk factors.

There a few limitations of the study by Hammadah and co-workers¹ to be considered. The study focuses on telomere length in total leukocytes and not CD34+ cells, or more purified cell populations. Prior studies indicate LTL correlates with telomere length in HSCs, however this remains to be determined. Telomeres attrition is not uniform throughout bone marrow¹⁴. Telomere length declines with age, with the most dramatic drop occurring in the first year of life in both lymphocyte and granulocyte lineages suggesting replicative telomere shortening in their precursors, the HSCs¹⁴. With increasing age, telomere length in lymphocytes decreases more than in granulocytes¹⁴. Additional insights may be garnered in future studies using single-cell approaches to measure telomere length such as flow-FISH¹⁴. A second limitation of the study is a practical consideration. To apply this approach in routine care of patients, a reference range for circulating CD34+ cells from age and sex matched subjects without CAD is needed. Studies to validate such an approach would need to be defined in a study tailored to this purpose.

To summarize, circulating CD34+ cells and LTL measures were predictive of CVD events in patients with stable CAD. Circulating CD34+ cells may represent a sensor for CVD risk factors, as CVD risk factors have biological effects on HSCs that results in a decline in available CD34+ cells and shortening of telomeres. The role of CVD risk factors on LTL remains less clear and may be more influenced by inherited factors, and stressors encountered during youth. LTL may be viewed as a measure of the replicative history of cells, where shorter telomeres are incurred by higher levels of cell replication. Future studies may focus on the utility of these two measures to predict CVD event risk in younger patients without known CAD. Most importantly, these two measures of aging and regenerative capacity appear closer to clinical use to risk stratify patients with stable CAD.