The mitochondrial DNA polymerase gamma degrades linear DNA fragments precluding the formation of deletions

Double-strand breaks in the mitochondrial DNA (mtDNA) result in the formation of linear fragments that are rapidly degraded. However, the identity of the nuclease(s) performing this function is not known. We found that the exonuclease function of the mtDNA polymerase gamma (POLG) is required for this rapid degradation of mtDNA fragments. POLG is recruited to linearized DNA fragments in an origin of replication-independent manner. Moreover, in the absence of POLG exonuclease activity, the prolonged existence of mtDNA linear fragments leads to increased levels of mtDNA deletions, which have been previously identified in the mutator mouse, patients with POLG mutations and normal aging

Effect of Population Doublings on Differentiation Capacity of Human Adipose-Derived Stem Cells: Establishing Standard Guidelines for Clinical Translational Applications

Human adipose-derived stem cells (ASCs) are providing promise in clinical soft tissue replacement and repair. This study investigated the effect of multiple population doublings (PD) on ASCs growth rates and cell specific differentiation capacities (DC) to evaluate potential inter-individual specimen growth rates and DC for in vitro cell expansion use in future clinical applications. A decreased cell growth rate and DC were observed in long term cell culture when PD reached to 10.1. Early PD specimens with PD range from 1.6 to 6.2 showed the average population doubling time (PDT) as 5.1 days. There was no significant difference in ASCs growth rates among individual specimens (n=4, p=0.184). The DC over PD range 1.6 to 6.2 also showed no significant changes (osteogenic, n=3, p=0.472; chondrogenic, n=3, p=0.878; adipogenic, n=3, p=0.256). Our preliminary data showed the importance of PD for in vitro ASCs expansion. The cell growth rate and DC of ASCs were relatively stable up to PD 6.2. Beyond this PD, there was a trend of decreased growth rate and DC. Therefore, our study suggested that future in vitro expansion of ASCs for clinical application should consider limiting expansion of ASCs to PD 6.2 or less to ensure the cell quality. Use of specific PD instead of the non-specific passage numbers would be a better standardization of in vitro ASCs expansion for their clinical translational applications