Regulation of stem cell (SC) proliferation is central to tissue homoeostasis, injury repair, and cancer development. Accumulation of replication errors in SCs is limited by either infrequent division and/or by chromosome sorting to retain preferentially the oldest 'immortal' DNA strand. The frequency of SC divisions and the chromosome-sorting phenomenon are difficult to examine accurately with existing methods. To address this question, we developed a strategy to count divisions of hair follicle (HF) SCs over time, and provide the first quantitative proliferation history of a tissue SC during its normal homoeostasis. We uncovered an unexpectedly high cellular turnover in the SC compartment in one round of activation. Our study provides quantitative data in support of the long-standing infrequent SC division model, and shows that HF SCs do not retain the older DNA strands or sort their chromosome. This new ability to count divisions in vivo has relevance for obtaining basic knowledge of tissue kinetics.

Aml1/Runx1 controls developmental aspects of several tissues, is a master regulator of blood stem cells, and plays a role in leukemia. However, it is unclear whether it functions in tissue stem cells other than blood. Here, we have investigated the role of Runx1 in mouse hair follicle stem cells by conditional ablation in epithelial cells. Runx1 disruption affects hair follicle stem cell activation, but not their maintenance, proliferation or differentiation potential. Adult mutant mice exhibit impaired de novo production of hair shafts and all temporary hair cell lineages, owing to a prolonged quiescent phase of the first hair cycle. The lag of stem cell activity is reversed by skin injury. Our work suggests a degree of functional overlap in Runx1 regulation of blood and hair follicle stem cells at an equivalent time point in the development of these two tissues.

Several experimental in vivo systems exist that generate reciprocal translocations between engineered chromosomal loci of yeast or Drosophila, but not without previous genome modifications. Here we report the successful induction of chromosome translocations in unmodified yeast cells via targeted DNA integration of the KAN(R) selectable marker flanked by sequences homologous to two chromosomal loci randomly chosen on the genome. Using this bridge-induced translocation system, 2% of the integrants showed targeted translocations between chromosomes V-VIII and VIII-XV in two wild-type Saccharomyces cerevisiae strains. All the translocation events studied were found to be non-reciprocal and the fate of their chromosomal fragments that were not included in the translocated chromosome was followed. The recovery of discrete-sized fragments suggested multiple pathway repair of their free DNA ends. We propose that centromere-distal chromosome fragments may be processed by a break-induced replication mechanism ensuing in partial trisomy. The experimental feasibility of inducing chromosomal translocations between any two desired genetic loci in a eukaryotic model system will be instrumental in elucidating the molecular mechanism underlying genome rearrangements generated by DNA integration and the gross chromosomal rearrangements characteristic of many types of cancer.

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