Aspm sustains postnatal cerebellar neurogenesis and medulloblastoma growth

Abstract

Alterations in genes that regulate brain size may contribute to both microcephaly and brain tumor formation. Here, we report that Aspm, a gene that is mutated in familial microcephaly, regulates postnatal neurogenesis in the cerebellum and supports the growth of medulloblastoma, the most common malignant pediatric brain tumor. Cerebellar granule neuron progenitors (CGNPs) express Aspm when maintained in a proliferative state by sonic hedgehog (Shh) signaling, and Aspm is expressed in Shh-driven medulloblastoma in mice. Genetic deletion of Aspm reduces cerebellar growth, while paradoxically increasing the mitotic rate of CGNPs. Aspm-deficient CGNPs show impaired mitotic progression, altered patterns of division orientation and differentiation, and increased DNA damage, which causes progenitor attrition through apoptosis. Deletion of Aspm in mice with Smo-induced medulloblastoma reduces tumor growth and increases DNA damage. Co-deletion of Aspm and either of the apoptosis regulators Bax or Trp53 (also known as p53) rescues the survival of neural progenitors and reduces the growth restriction imposed by Aspm deletion. Our data show that Aspm functions to regulate mitosis and to mitigate DNA damage during CGNP cell division, causes microcephaly through progenitor apoptosis when mutated, and sustains tumor growth in medulloblastoma.

ATR maintains chromosomal integrity during postnatal cerebellar neurogenesis and is required for medulloblastoma formation

“…ATR is activated in response to stalled DNA replication forks (Nam and Cortez, 2011), which can be caused by exogenous (Harley et al, 2016;Harper et al, 2010) or endogenous (Willis et al, 2013) factors. We have previously shown that normal CGNP proliferation produces endogenous DNA damage, detectable as small γH2A.X + foci (Williams et al, 2015). We now show that ATR mitigates endogenous, proliferation-associated DNA damage in CGNPs in order to maintain replication fidelity.…”

“…Prior studies showed that deletion of the apoptosis gatekeeper Bax is sufficient to stabilize CGNPs with radiationinduced or proliferation-associated DNA damage (Chong et al, 2000;Garcia et al, 2013;Williams et al, 2015). We found, however, that apoptosis of Atr-deleted CGNPs and cerebellar hypoplasia were not prevented by Bax co-deletion in Math1-Cre;Atr loxP/loxP ; Bax loxP/loxP (Atr;Bax M-cre ) animals ( Fig.…”

“…Apoptosis in Atr-deleted CGNPs, in contrast to irradiated Atrintact CGNPs (Chong et al, 2000;Williams et al, 2015), was not prevented by co-deletion of Bax, but was prevented by co-deletion of both Bax and Bak. In other cell types, BAK-driven apoptosis can be activated by genotoxic stress during mitosis (Chu et al, 2012;Flores et al, 2012).…”

“…Stained slides were digitally imaged and positively stained cells were counted using Aperio Software (Aperio Technologies) for chromogenstained slides or Tissue Studio (Definiens) for fluorescence, as previously described (Williams et al, 2015). The entire EGL region in each section was manually annotated and used for quantifications, which were normalized to the total number of nucleated cells in the designated region.…”

“…Diverse mutations that increase DNA damage can restrict brain growth, causing microcephaly McMahon et al, 2014;Orii et al, 2006;Rosin et al, 2015;Williams et al, 2015). Combined with p53 deletion, mutations in DNA repair genes may produce unrestricted growth in medulloblastoma (Frappart et al, 2009;Lee and McKinnon, 2002;Tong et al, 2003), a malignant tumor of neural progenitors.…”