Opposite modifying effects of HR and NHEJ deficiency on cancer risk in Ptc1 heterozygous mouse cerebellum

Abstract

Heterozygous Patched1 (Ptc1 þ /À ) mice are prone to medulloblastoma (MB), and exposure of newborn mice to ionizing radiation dramatically increases the frequency and shortens the latency of MB. In Ptc1 þ /À mice, MB is characterized by loss of the normal remaining Ptc1 allele, suggesting that genome rearrangements may be key events in MB development. Recent evidence indicates that brain tumors may be linked to defects in DNA-damage repair processes, as various combinations of targeted deletions in genes controlling cell-cycle checkpoints, apoptosis and DNA repair result in MB in mice. Non-homologous end joining (NHEJ) and homologous recombination (HR) contribute to genome stability, and deficiencies in either pathway predispose to genome rearrangements. To test the role of defective HR or NHEJ in tumorigenesis, control and irradiated Ptc1 þ / À mice with two, one or no functional Rad54 or DNA-protein kinase catalytic subunit (DNA-PKcs) alleles were monitored for MB development. We also examined the effect of Rad54 or DNA-PKcs deletion on the processing of endogenous and radiation-induced double-strand breaks (DSBs) in neural precursors of the developing cerebellum, the cells of origin of MB. We found that, although HR and NHEJ collaborate in protecting cells from DNA damage and apoptosis, they have opposite roles in MB tumorigenesis. In fact, although Rad54 deficiency increased both spontaneous and radiation-induced MB development, DNA-PKcs disruption suppressed MB tumorigenesis. Together, our data provide the first evidence that Rad54-mediated HR in vivo is important for suppressing tumorigenesis by maintaining genomic stability.

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“…Despite the critical role of the HR system for embryonic development, attenuated HR by deletion of Rad54 also did not result in lethality in Ptc1 +/-mice, implying redundancy in function of proteins involved in HR; however, in previous work, by exposing double Rad54/Ptc1 mutants to 2 Gy, we detected significantly increased MB tumorigenesis, possibly through the generation of viable but genetically rearranged neural cell precursors due to repair of DNA lesions by error-prone mutagenic pathways [21]. An analogous increase was not seen here, likely due to the lower dose used and relative radioresistance of Rad54 -/-mice [23].…”

“…Our previous work showed that neonatal irradiation dramatically increases the frequency of MB [19,20]. We also showed that combined loss of Ptc1 and Rad54 or of Ptc1 and Parp-1 increased radiation induced MB [21,22]. Here, we analyzed the effects of combined loss of Rad54 and Parp-1 in the Ptc1 mouse model of spontaneous and radiation-induced cancer, highlighting novel synthetic lethal interactions during development and tumorigenesis, and dissecting the underlying cellular and molecular mechanisms.…”

In vivo Importance of Homologous Recombination DNA Repair for Mouse Neural Stem and Progenitor Cells

“…By contrast, Rad54 inactivation generates a very low increase (approximately 1 apoptotic cell/1000 cells) in apoptosis in the mouse developing cerebellum at postnatal day 1 [46] and here we show the absence of significant increase in apoptosis in the developing cortex of Rad54 −/− mice compared to WT controls. Rad54 inactivation does not totally abrogate HR [34].…”

Developmental and oncogenic radiation effects on neural stem cells and their differentiating progeny in mouse cerebellum

“…F1 mice of the desired genotypes (DNA-PKcs 1/2 /Ptc1 1/2 3 DNA-PKcs 1/2 /Ptc1 1/1 and Rad54 1/2 / Ptc1 1/2 3 Rad54 1/2 /Ptc1 1/1 ) were intercrossed to produce F2 populations. Genotyping of mice was performed as described previously [24].…”