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MYBL2 is a transcription factor with roles in the cell cycle and genome integrity. MYBL2 is located on chromosome 20, within a region commonly deleted in human blood disorders (del20q). Our published data shows that reduced levels of MYBL2 predispose to development of myelodysplastic syndrome (MDS)-like disease in mouse models during ageing, indicating that MYBL2 could be acting as a tumour suppressor gene within del20q abnormality. Moreover, our previous work demonstrated that regardless of del20q deletion, MYBL2 expression is reduced in CD34⁺ bone marrow cells from MDS patients with worse prognosis. Because it has been shown that the cell of origin of MDS is the haematopoietic stem cell (HSC) and given the role of MYBL2 in DNA replication fork progression and maintenance of genome integrity, we hypothesised that low MYBL2 levels in HSC could contribute to elevated somatic mutations through changes in DNA repair pathways and drive disease development.

The aim of this study was to determine if low MYBL2 levels affect the double strand break (DSB) DNA repair damage response in HSC. 

In this study we used our mouse model in which animals express ~50% normal levels of MYBL2 (Mybl2^+/∆). We characterised the ability of HSCs from young (7 weeks) and old (70 weeks) animals to respond to in vivo ionising radiation (2Gy) in terms of proliferation, apoptosis and colony forming ability. We measured the activation of the two main DNA repair pathways operating in the cells to deal with DSB: the error prone non-homologous-end-joining (NHEJ) and the error-free homologous recombination (HR) by assessing 53BP1 and Rad51 recruitment by immunofluorescence, respectively. Finally, we analysed the frequency of chromosome abnormalities present in the progeny of Mybl2^+/∆ HSC that have previously been irradiated to determine the long term effects of changes in DNA repair. 

We observed that Mybl2^+/∆ HSCs had limited proliferative potential and displayed an increased sensitivity to ionizing radiation which increased during ageing. Mybl2^+/∆ HSCs also displayed altered kinetics of 53BP1 and Rad51 recruitment and clearance, including retention of 53BP1 foci at later time points following irradiation and decreased levels of Rad51 foci when compared to Mybl2^+/+ HSCs. Using plasmid functional assays, we showed that Mybl2^+/∆ HSCs repair quite efficiently by NHEJ, but this efficiency is disrupted when cells are challenged with ionising radiation. Furthermore, Mybl2^+/∆ HSCs have increased sensitivity to inhibition of DNA-PKC (required for NHEJ) but not ATM (required by HR). We also observed that after ionizing irradiation Mybl2^+/∆ HSCs progeny displayed an increased percentage of chromatids with fragile telomeres. Moreover, by making use of publically available RNA-seq datasets from MDS patients, we have identified a clear association between low MYBL2 levels and low expression of DNA-repair genes in patients with worse prognosis.