Chaf1a is required for cardiomyocyte karyokinesis and cardiac endowment

Cardiac endowment and CM ploidy state are both critical for adult cardiac function. While polyploid CMs differ in nuclearity across species, mostly multinuclear in rodents versus mononuclear in humans, the functional implications remain unclear. Notably, halving CM number causes neonatal lethality in binuclear-dominant hearts, but whether mononuclear polyploid (MNPP) CMs are functionally distinct is unknown. We generated a mouse model with predominant MNPP CMs by conditionally deleting Chaf1a in perinatal CMs. Chaf1a deficiency caused G2/M arrest and failed karyokinesis, reducing CM number by 50% while yielding hearts with 89% MNPP CMs. Despite severe endowment deficit, mutant mice maintained normal cardiac function for 3 months through robust physiological hypertrophy, sharply contrasting to the binuclear-dominant hearts, where comparable CM loss is lethal. Thus, MNPP CMs possess superior adaptive capacity under endowment stress. These findings establish Chaf1a as essential for CM karyokinesis and cardiac endowment, demonstrate that MNPP configuration provides substantial functional compensation, and offer insights into species-specific CM nuclearity evolution.