Letter by Opolski et al Regarding Article, “Patient Characteristics and Cell Source Determine the Number of Isolated Human Cardiac Progenitor Cells”
To the Editor:
We have read with great interest the recent study published by Itzhaki-Alfia et al1 defining the optimal source of human cardiac progenitor cells (hCPCs) and the influence of patient characteristics on their number. Hereby, we would like to congratulate the authors on a novel and highly efficient method of hCPC isolation; however, the main conclusion regarding the right atrium as the best source for c-kit+ hCPCs seems to differ from previous observations and therefore requires further commentary.2
First, a careful look at the tissue sample data reveals that the mixed-effects model was based on 41 myocardial samples, with 22 samples obtained from the right atrium, 10 from the left atrium, 7 from the left ventricle, and only 2 from the right ventricle. However, the recommended sample size for multilevel mixed models preferably should be 50, with a fall-off in statistical power as the number of observations decreases below the specified threshold.3 Furthermore, the number of subjects donating >1 sample from different areas, thus enabling within-subject comparison by a mixed-effects model, was limited to 4. Therefore, the reported within-subject comparison of different sample origins with regard to the c-kit+ hCPCs may reveal a methodological flaw derived from analysis of various patients.
Second, as opposed to right atrial appendage collection during open heart surgery, right ventricular biopsy has gained acceptance as a clinically safe interventional technique and remains the “gold standard” to assess myocardial disease in the living patient.4 However, in the study by Itzhaki-Alfia et al,1 the number of specimens collected from the right ventricle was limited to 2 samples. Significantly, the unbalanced and very low number of observations (especially for the right ventricle) can result in a risk of unmet assumptions about the percentage of normal distribution of c-kit+ hCPCs. Thus, statistical inference is not sound enough to consider the right atrium as a better source for c-kit+ hCPCs than the right ventricle.
Finally, the use of a heterogeneous patient population that included subjects with various primary heart diseases, such as coronary artery disease, valvular dysfunction, atrial fibrillation, and intrinsic cardiomyopathies, resulted in a fixed sample collection from specific origins during cardiac procedures. In addition, with a high prevalence of left or right atrial samples, various factors related to different levels of atrial fibrosis should have affected c-kit+ levels. However, despite the documented association between atrial fibrosis and atrial fibrillation,5 the percentage of c-kit+ cells was similar in patients with versus those without atrial fibrillation, valvular disease, or atrial dilatation. We assume this may be explained by noncomparable patient characteristics with mostly a single fixed-specimen origin and different levels of sample fibrosis.
Although the study by Itzhaki-Alfia et al1 represents a milestone in the development of a highly efficient hCPC isolation protocol, the main conclusion regarding the right atrium as the best source for hCPCs failed to consider important methodological confounders. For these reasons, the main conclusion should be regarded as more hypothesis generating and exploratory than definitive. In our opinion, the very practical question of the optimal source of hCPCs in a patient scheduled for myocardial biopsy remains unanswered.