p53 is the single most frequently mutated gene in cancer. Our discovery of the niche cells’ regulation of the p53 response in stem cells has major clinical implications for stem cell function and tissue homeostasis during aging. Understanding the mechanisms of niche control might reveal new concepts for tumour therapy targets as in contrast to cancer cells, niche cells have low mutation rates and a low adaptive potential.

DNA damage causes cancer development and drives the aging process. The DNA damage checkpoint signalling arrests the cell cycle to allow time for repair or induces apoptosis. The tumour suppressor p53 is a central mediator of the DNA damage response (DDR) and the single most frequently mutated gene in human cancer.

We recently discovered that the p53-mediated DDR in stem cells is not regulated cell autonomously but instead through signalling via the niche cells. We uncovered that in C. elegans the translation initiation factor IFE-4 is activated in somatic gonad precursor (SGP) niche cells that surround the primordial germ cells (PGCs), when the latter carry DNA damage. Upon DNA damage in PGCs, IFE-4 is induced in SGPs and required for the p53 induction in the damaged PGCs. We determined  that the interaction between PGCs and SGPs is regulated through FGF-like signalling. The IFE-4 target FGFR-like EGL-15 receives the signal in the SPG, while the reverse signal is received through FGFR-related SERF receptor tyrosine kinases in the PGC.

We demonstrated that the IFE-4 ortholog eIF4E2 in mammals is induced in niche cells upon UV-induced DNA damage and is required for the induction of p53 in hair follicle stem cells (HFSCs). Our data thus indicate a highly conserved mechanism of the niche cell regulation of the p53-mediated DDR in stem cells. We wish to explore here the mechanisms of the eIF4E2 control of the stem cell DDR and investigate the role of the niche cell regulation for skin homeostasis and carcinogenesis.

1. Assess the eIF4E2 function in skin homeostasis
2. Investigate eIF4E2 in the regulation of p53 in HFSCs
3. Investigate the biochemical mechanism of eIF4E translational regulation
4. Assess the role of eIF4E2 in UV-induced skin carcinogenesis

DNA damage causes cancer when it leads to mutations that can activate oncogenes or inactivate tumor suppressor genes. The p53 tumor suppressor, also referred to as the “guardian of the genome” is the single most frequently mutated gene in human cancers and often associated with therapy resistance. The activity of p53 is tightly regulated on multiple levels and we previously discovered mechanisms of translational and posttranslational control of p53 (1, 2).

Upon detection of DNA damage, the p53 protein is activated by the checkpoint signaling cascades operating through the ATM and ATR kinases. The most well understood function of p53 is the transcriptional induction of cell cycle arrest regulators such as p21 and pro-apoptotic BH3 only domain proteins such as PUMA and NOXA. We recently demonstrated that p53 and p21 function synergistically to protect from tumorigenesis (3). Thus far, it was thought that the activation of p53 was regulated cell-autonomously within the cell depending on the type and extent of the DNA damage. Our recent discovery of the non-cell autonomous regulation of the p53-mediated DNA damage response has challenged that long-held believe. 

We previously identified the C. elegans p53 homolog, CEP-1, and established that in a highly conserved fashion, CEP-1 induces the BH3 only domain proteins EGL-1 and CED-13 that then trigger apoptosis (4, 5). We recently established that global-genome nucleotide excision repair (GG-NER) mutations that in humans lead to the skin cancer susceptibility syndrome Xeroderma pigmentosum (XP) leads to genome instability in proliferating germ cells in C. elegans (6, 7). As genome instability in proliferating cells is the causal event in human tumorigenesis, the nematode thus provides a model for this cancer-igniting event. In the first larval L1 stage of the worm, the primordial germline is comprised of two primordial germ cells (PGCs), which start dividing when the animals sense a food cue.

When the animals are exposed to UV irradiation, the PGCs in wild type (WT) worms transiently arrest to repair the DNA damage and then proliferate to form the germline. In contrast, PGCs of animals with defective GG-NER due to a mutation in the xpc-1 gene permanently arrest resulting in sterile worms. However, a mutation in cep-1/p53 suppresses the PGC arrest and a significant number of animals grow a germline despite the GG-NER defect. 

Having established this experimental system, we employed the powerful C. elegans genetics to conduct a forward genetic screen aimed to identify novel genes operating in the p53 pathway. We isolated a mutant in the ife-4 gene that we determined to be epistatic with cep-1/p53 (8). Surprisingly, IFE-4 is exclusively expressed in somatic but not in germ cells. We employed several experimental strategies including tissue specific RNAi and cell type specific restoration of IFE-4 to prove that IFE-4 operates specifically in the two niche cells that surround the PGCs. These niche cells are the somatic gonad precursor (SGP) cells. Here, IFE-4 is induced when PGCs carry unrepaired DNA lesions. IFE-4 in SGPs is required for the induction and activation of CEP-1/p53 in PGCs (Figure 1). 

In C. elegans distinct roles of target mRNA regulation are executed through the five different eIF4E homologs. IFE-4 specifically binds 7-monomethyl guanosine caps and has been demonstrated to regulate the translational initiation of a specific subset of 33 mRNAs in somatic tissues of the animals. We conducted an RNAi screen through all known IFE-4 target mRNAs and identified the FGF receptor ortholog EGL-15 as critical IFE-4 target. We determined that EGL-15/FGFR is required in the SGPs for inducing IFE-4 upon DNA damage in PGCs. Moreover, the FGF-like EGL-17 is required in both PGCs and SGPs for the induction of IFE-4 in SGPs and subsequent CEP-1/p53 activation in PGCs. To reveal how the PGC receives the niche signal, we RNAi screened all receptor tyrosine kinases (RTKs) and identified 3 small extracellular region family (SERF) RTKs that are also related to human FGF receptors and are specifically required in the PGC to mediate the activation of CEP-1/p53 in these cells.

Taken together, we could show that DNA damage in PGCs leads to an EGL-17/FGF signal that is received by the niche SGP cells through the FGFR EGL-15, which through ERK 1/2 MAPK signaling induces IFE-4 that elevate the translation of EGL-15/FGFR to further amplify the PGC-niche signal. The SGP signals back to the PGC through EGL-17/FGF through the FGFR-rated SERF RTKs, which then activates CEP-1/p53 that in turn arrests the PGC cell cycle.

Our findings have at least two major implications: First, the p53-mediated DNA damage response in stem cells is regulated non-cell-autonomously through FGF signals from niche cells and second the integrity of heritable genomes is regulated through the somatic niche. The latter conclusion challenges the Weismann barrier concept that has pertained over more than hundred years that heritable genomes are protected from any somatic influence thus excluding any somatic (“Lamarckian”) impact on inheritance.