CD34+ Circulating Progenitor Cell Frequency Is Highly Heritable

Circulating blood CD34+ cells are a heterogeneous population of cells comprising hematopoietic stem/progenitor cells, circulating angiogenic cells, and a small population of endothelial/endothelial progenitor cells. CD34+ cells are used clinically for hematopoietic stem cell transplants and have also been studied as cellular biomarkers of response to anti-angiogenic therapy in clinical trials and as markers of cardiovascular disease (CVD) risk. Understanding factors that regulate circulating CD34+ cell frequency has important implications for the fields of stem cell transplantation, oncology, cardiology, and vascular biology. Therefore, we initiated a large, community-based cohort study to examine clinical characteristics associated with circulating CD34+ cell frequency, as well as to define the heritable contribution to this phenotype.

We assessed CD34+ cell frequency in 1,786 participants attending the eighth examination cycle (2005-2008) of the Framingham Offspring Study (mean age 66 years, 54% women). 1,595 participants had no known history of cardiovascular disease. Participants underwent a standardized medical examination and laboratory assessment of cardiovascular risk factors. These included systolic and diastolic blood pressure, body mass index, glucose, total and high-density lipoprotein, cholesterol, triglycerides, medications (lipid-lowering, antihypertensive, and hormone replacement therapies), cigarette smoking, and diabetes (defined as a fasting glucose >126 mg/dL or the use of medications to treat diabetes). For CD34+ cell phenotyping, buffy coat samples were diluted with PBS and mononuclear cells isolated by Ficoll density-gradient centrifugation. Mononuclear cells were stained with an anti-human CD34 antibody and cells were analyzed by FACS analysis (Becton-Dickenson FACSCalibur). Data was analyzed as the frequency of CD34+ events within the cellular gate. Results: In multivariable-adjusted analyses adjusting for familial (sibling) correlations, CD34+ frequency was inversely associated with older age, female gender, and cigarette smoking. CD34+ frequency was positively associated with weight, total cholesterol, and statin therapy. Framingham risk score was inversely correlated with CD34+ frequency in men (coefficient -0.037, P=0.004) but not in women (coefficient 0.08, P=0.59). CD34+ frequency was not associated with prevalent CVD (P=0.69). Notably, the clinical covariates associated with CD34+ frequency only accounted for 6.3% of CD34+ variability. Therefore, we performed an assessment of CD34+ frequency as a heritable trait. Using variance component methods implemented in SOLAR on sibship-pair and spouse-pair data, the estimated heritability (h²) for CD34+ cell variability was 53.9 +/− 9.2 % (mean +/− standard error) in age- and sex-adjusted analyses and was highly statistically significant (P<0.00001). Conclusions: This study represents the largest assessment of clinical correlates of circulating CD34+ cell frequency to date. The inverse correlation between CD34+ cell frequency and Framingham risk score in a population predominantly free of CVD was confirmed; however, this finding was limited to male study subjects, and was driven by the inverse association with age. Surprisingly, previously identified clinical covariates accounted for only 6.3% of CD34+ variability, while the proportion of unexplained variance attributable to heritable factors accounted for 53.9% of CD34+ cell variability. These data, therefore, support efforts to define genetic variants that may regulate CD34+ frequency. Such investigations would have important implications for the fields of stem cell transplantation, as well as the study of hematologic and cardiovascular diseases more broadly.

Martin:Genzyme: Employment.