A gene dosage‐dependent effect unveils NBS1 as both a haploinsufficient tumour suppressor and an essential gene for SHH‐medulloblastoma

Abstract

Aims Inherited or somatic mutations in the MRE11, RAD50 and NBN genes increase the incidence of tumours, including medulloblastoma (MB). On the other hand, MRE11, RAD50 and NBS1 protein components of the MRN complex are often overexpressed and sometimes essential in cancer. In order to solve the apparent conundrum about the oncosuppressive or oncopromoting role of the MRN complex, we explored the functions of NBS1 in an MB‐prone animal model. Materials and methods We generated and analysed the monoallelic or biallelic deletion of the Nbn gene in the context of the SmoA1 transgenic mouse, a Sonic Hedgehog (SHH)‐dependent MB‐prone animal model. We used normal and tumour tissues from these animal models, primary granule cell progenitors (GCPs) from genetically modified animals and NBS1‐depleted primary MB cells, to uncover the effects of NBS1 depletion by RNA‐Seq, by biochemical characterisation of the SHH pathway and the DNA damage response (DDR) as well as on the growth and clonogenic properties of GCPs. Results We found that monoallelic Nbn deletion increases SmoA1‐dependent MB incidence. In addition to a defective DDR, Nbn+/− GCPs show increased clonogenicity compared to Nbn+/+ GCPs, dependent on an enhanced Notch signalling. In contrast, full NbnKO impairs MB development both in SmoA1 mice and in an SHH‐driven tumour allograft. Conclusions Our study indicates that Nbn is haploinsufficient for SHH‐MB development whereas full NbnKO is epistatic on SHH‐driven MB development, thus revealing a gene dosage‐dependent effect of Nbn inactivation on SHH‐MB development.

The Multiple Faces of the MRN Complex: Roles in Medulloblastoma and Beyond

“…It was demonstrated that, independently of the DDR, NBS1 deletion up-regulates the NICD protein level, as well as NOTCH activity in neurons, causing in turn repression of neurite outgrowth and neuronal migration in post-mitotic neurons [118]. Moreover, in line with Cheung et al's study which found NOTCH pathway upregulation in lymphoblastoid cell lines from NBS heterozygous carriers [119], we recently published that Nbn heterozygosity increases the Notch pathway and a Notch-dependent clonogenicity in cerebellar granule cell progenitor (cGCPs) primary cultures, isolated from a Medulloblastoma (MB)-prone mouse model [120].…”

“…Collectively our data and those found in the literature converge to the concept that MRN low levels or truncating or missense mutations in MRN genes may play an oncopromoting role as much as the overexpression of functional MRN proteins. Notably, they also reveal that a single wild-type allele (in hemizygous condition) may be sufficient to produce high levels of protein in full-blown tumors [120]. Moreover, in principle, a mutated allele may also be overexpressed.…”

“…Despite paradoxical, collectively, clinical, preclinical, and in vitro studies indicating that, due to their multiple functions in DDR and beyond, both reduced and increased levels of MRN proteins may play a role in carcinogenesis and tumor progression. In order to validate this concept, we recently addressed the pleiotropic role of the MRN complex in cancer by using the model of Medulloblastoma (MB) [120], the most common malignant brain tumor affecting children [150]. Here, exploring the effects of different levels of NBS1, we discovered that alterations in the expression of the MRN genes, both in the sense of decrease and increase, impact cancer development and tumor full-blown survival/progression [120].…”

“…In order to validate this concept, we recently addressed the pleiotropic role of the MRN complex in cancer by using the model of Medulloblastoma (MB) [120], the most common malignant brain tumor affecting children [150]. Here, exploring the effects of different levels of NBS1, we discovered that alterations in the expression of the MRN genes, both in the sense of decrease and increase, impact cancer development and tumor full-blown survival/progression [120]. In particular, we found that mono-allelic KO of Nbn, reducing the total amount of NBS1 available for the cell, significantly increases the probability of developing a tumor in an SHH-MB-prone mouse model.…”

Effect of silencing SSB1 gene on the expression of NBS1 in irradiated rat submandibular gland cells

“…MRN complex has the function of inhibiting and promoting tumor growth. Mre11, Rad50, NBS1 and other protein components in the MRN complex are often highly expressed, which is very important for the occurrence and development of tumors [55]. SBB1 existed at the site of DNA double-strand break in the early stage after DNA injury and could combine with MRN complex, suggesting that it was involved in the initial process of DNA double-strand break [56].…”