Cellular and molecular mechanisms underlying the maintenance of genomic integrity in epidermal stem cells

homeostasis and tissue repair after injury. SCs reside and self-renew in the adult body

throughout the life of the organism. In rapid self-renewing organs, such as the skin, the

intestine and the blood, SCs divide many times during the life of the animal in order to sustain

the homeostatic needs of the tissue.

All cells of the body, including SCs, are constantly subjected to DNA assaults arising from

endogenous sources, such as reactive oxygen species (ROS) generated by cellular

metabolism, or exogenous assaults arising from the environment. The DNA damage response

(DDR) and DNA repair mechanisms protect cells from accumulating DNA damage by

inducing transient cell cycle arrest allowing DNA repair, triggering senescence or apoptosis.

DNA damages trigger the activation of the effectors of the DDR inducing a transient cell

cycle arrest, allowing DNA repair, or triggering a permanent arrest of the cell cycle or

apoptosis if damages are too extensive.

As skin is the outermost barrier of the body, epidermal cells, including SCs, are

continuously subjected to genotoxic stress, such as UV rays, ionizing radiation (IR) and

chemicals. The skin epidermis is composed of hair follicles (HFs), its associated sebaceous

gland (SG) and the surrounding inter-follicular epidermis (IFE). Different types of SCs

maintain the homeostasis of the skin; multipotent adult bulge SCs ensure the cyclic

regeneration of the HF and the repair of the epidermis after injury, while individual unipotent

SCs ensure homeostasis of the SG and the IFE.

In tissues with high cellular turnover, such as the epidermis, the numerous divisions that a

SC undergoes could result in the accumulation of replication-associated DNA damage. It has

been suggested that adult SCs may undergo asymmetric divisions in which the daughter SC

retains the older (thus “immortal”) DNA strand, while the daughter cell committed to

differentiation inherits the newly synthesized strand that may have incorporated replicationderived

mutations. The in vivo relevance of this mechanism is still a matter of intense debate.

We used multiple in vivo experimental approaches to investigate precisely how bulge SCssegregate their chromosomes during HF morphogenesis, SC activation and skin homeostasis.

Using pulse-chase experiments with two different uridine analogs together with DNAindependent

chromatin labelling, we showed that multipotent HF SCs segregate their

chromosomes randomly, and that the label-retention observed in the skin epidermis derives

solely from relative quiescence of skin SCs 1.

We investigated the in vivo response of multipotent adult HF bulge SCs to DNA damage

induced by IR. We showed that bulge SCs are profoundly resistant to DNA damage-induced

cell death compared to their more mature counterparts. Interestingly, we demonstrated that

resistance of bulge SCs to IR-induced apoptosis does not rely on their relative quiescence.

Moreover, we showed that DDR in SCs does not lead to premature senescence. We found that

two intrinsic cellular mechanisms participate in the resistance of bulge SCs to DNA damageinduced

cell death. Bulge SCs express higher level of the anti-apoptotic Bcl-2 and present

more transient activation of p53 due to a faster DNA repair activity mediated by a nonhomologous

end joining (NHEJ) mechanism. Since NHEJ is not error free, this property

might be a double-edged sword, supporting short-term survival of bulge SCs but impairing

long-term genomic integrity 2.

While we unveiled the relevance of DSBs repair by NHEJ in the skin epidermis, little is

known about the role of homologous recombination (HR) during the morphogenesis of the

skin epidermis. Brca1 is an essential protein for HR. Conditional deletion of Brca1 in the

developing epidermis leads to congenital alopecia accompanied by a decreased density of hair

placodes. The remaining HFs never produce mature hair and progressively degenerate due to

high levels of apoptosis. Multipotent adult HF bulge SCs cannot be detected in adult HF in

the Brca1 cKO epidermis. Brca1 deletion in the epidermis triggers p53 activation throughout

the epidermis, which activates apoptosis. Interestingly, IFE and the isthmus region of the HF

do not present any pathological phenotype by constitutive deletion of Brca1. Our results

demonstrated the critical role of Brca1 during HF morphogenesis. Future studies will be

required to understand the molecular mechanisms controlling this phenotype