Response to Molkentin’s Letter to The Editor Regarding Article, “The Absence of Evidence Is Not Evidence of Absence: The Pitfalls of Cre Knock-Ins in the c-kit Locus”

In the article referred to1 in Jeffery D. Molkentin’s letter,2 we evaluated the article from his laboratory entitled c-kit+ Cells Minimally Contribute Cardiomyocytes to the Heart.3 We pointed out that the van Berlo et al’s article (hereafter van Berlo) has serious methodological weaknesses and lacks some crucial controls that together do not allow reaching many of their conclusions and certainly do not support the title.Molkentin challenges most, if not all, of the scientific basis for our critiques, yet his letter does not provide any new experimental information or data, which was missing from the original article.3 van Berlo did not identify, characterize, or isolate a single bona fide c-kit+ cardiac stem/progenitor cell (abbreviated either as CSC or CPC). The statements in Molkentin’s letter2 and van Berlo3 about the analysis of CPCs are misleading. Whether “the lungs are green, the gastrointestinal tract was green, the bone marrow was green, but the heart was not”2 has no bearing on the issue under discussion because of the recombination in the lungs and other tissues is neither correlated nor a predictor of what happened to the CPCs; the van Berlo is focused on the fate of the cardiomyocyte precursors. The statement that “van Berlo, used fluorescence-activated cell sorter imaging cytometry to show that ≈80% of cardiac CPCs labeled by the c-kit antibody are recombined by the Cre approach and expressed…eGFP and well >80% in the bone marrow”2 is flawed because the authors did not isolate or identify CPCs or hematopoietic stem cells, a necessary condition to determine the degree of recombination. The imaging cytometry used in van Berlo3 gated …