Telomeres Fuse during Cardiomyocyte Maturation

Although polyploidization is a hallmark of adult mammalian cardiomyocytes and may constrain their proliferation, the mechanisms leading to ploidy increase in cardiomyocytes remain elusive.1 Our laboratory and others have reported the formation of DNA bridges between daughter nuclei as a potential route to cardiomyocyte polyploidization.2,3 These earlier in vivo studies found DNA bridges on thin tissue sections, but this approach does not cover all events because not all cardiomyocytes were oriented in the same 2-dimensional plane. To detect all DNA bridges, we dispersed cardiomyocytes obtained from Myh6-H2B-mCh mice4 at postnatal day 4 on a culture plate and labeled them with the mitosis marker phospho-histone H3 (pH3) and DAPI (4’6-diamidino-2-phenylindole). No apparent mitotic errors were identified in prophase or metaphase, but DNA bridges were apparent in 54±7.9% of cardiomyocytes in early anaphase. The percentage of cardiomyocytes with DNA bridges decreased rapidly to 17.8±9.5% in late anaphase. On the contrary, the percentage of cardiomyocytes with resolved or broken DNA bridges (cardiomyocytes with discontinuous DNA fibers between sister chromatids) increased from 11.7±6.4% in early anaphase to 28.9±16.3% in late anaphase. DNA bridge remnants were present in 8±3.7% of telophase cardiomyocytes. This analysis indicates that DNA bridges form in early anaphase and are resolved in the subsequent mitotic stages. Time-lapse videomicroscopy of postnatal day 4 cardiomyocytes reinforces the finding that DNA bridges between sister chromatids resolve as mitosis advances.