Radiation sensitivity and the DNA damage response in  planarian stem cells
Keywords: Genome instability
Adult stem cells
DNA damage response
Planaria

Ionizing radiation (IR) can inflict various types of DNA lesions which if not repaired, can induce genomic instability and subsequent oncogenic transformation. Potentially immortal and highly regenerative animals are hypothesized to have enhanced genome maintenance mechanisms to protect their stem cells. One such example, the freshwater planarian, Schmidtea mediterranea, contains a large population of collectively pluripotent adult stem cells called neoblasts that are completely ablated following exposure to lethal doses of IR (30 Gy). We identified a non-lethal dose of IR (15 Gy) that leads to a significant decrease in neoblasts but where full recovery of the stem cell number occurs over time. However, there is no evidence that DNA repair is required during regeneration and normal neoblast function. Here we show, exposure to 15 Gy of IR following knockdown of DNA repair gene is lethal, proof of principle that well-known DNA damage response (DDR) genes have a role in stem cell survival and repopulation post IR.
We provide evidence that a new non-canonical role of DDR is to combat DNA damage during stem cell migration and that in the absence of a fully functioning DDR machinery, stem cells fail to migrate. Using an in-vivo shielded-irradiation assay, that allows cell migration to be tracked, we observed that neoblasts pre-exposed to IR migrate much slower, in a dose dependent manner, but eventually reach the wound. Migrating neoblasts were also more sensitive to IR than stationary cells suggesting that the mechanical stress due to changes in nuclear shape during migration represents a significant load on repair mechanisms. Our results provide an in vivo demonstration that a major novel role of DNA repair mechanisms may be to allow stem cell migration.
Despite enormous efforts to treat cancer, radiotherapy is still the major treatment to kill cancerous cells. There is growing evidence that tumour-initiating cancer stem cells survive and adapt to repeated rounds of IR eventually leading to cancer-recurrence. This radio-tolerance is dependent on an efficient DDR signalling. However, the molecular basis of variations in IR resistance is not well understood. The extraordinary capacity of neoblasts to tolerate high doses of IR offer an opportunity to get novel mechanistic insights into radiation resistance. Using RNA-sequencing we delineate the transcriptional response to IR in planarian stem cells. We identified genes that were differentially expressed in response to IR and characterized the role of transcription factors (FHL-1) and a tetraspanin family of genes in stem cell repopulation post IR. We further extended our investigation by comparing the transcriptome of irradiated planarian stem cells with a human fibrosarcoma cell line, HT1080. We identified conserved transcriptional responses to IR providing a rich resource to identify radiation responsive genes. Given the conservation between pASCs and mammalian stem cells these conserved genes may include novel druggable targets for combining with radiotherapy.