Abstract
Cancer is a multi-stage disease caused by sequential mutations; however, the molecular mechanism that generates cancer-initiating mutations is not well understood. Using the developing cerebellum as a model system, here we show the molecular mechanism of tumor initiation in Sonic hedgehog medulloblastoma (SHH-MB), a common subtype of the most frequent pediatric brain tumor. The most common initiating event of SHH-MB is Ptch1 loss of heterozygosity (LOH), which occurs in cerebellar granule cell progenitors (GCPs) and leads to preneoplasia formation. Since GCPs proliferate in response to Shh, we tested whether the normal proliferative effects of Shh can lead to genomic instability and DNA lesions responsible for Ptch1 LOH in the SHH-MB cell-of-origin. We found that Shh profoundly alters DNA replication dynamics in GCPs, leading to DNA damage, formation of DNA breaks in S-phase and hyper-recombination. Shh led to DNA helicase loading and activation, resulting in high levels of replication origin firing. Shh-dependent origin firing was required for Shh-induced DNA damage and recombination. Moreover, reducing origin firing decreased recombination and tumor initiation in a pre-clinical model of MB. These results show that reduction of Shh-dependent, DNA replication-associated DNA damage in tumor-prone Ptch1+/- GCPs before cancer initiation is capable of preventing MB-initiating mutations. Thus, we demonstrate that a developmental mitogen can cause cancer-initiating mutations through an increase in origin firing, and attenuating origin firing can prevent cancer initiation.