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Fig. 7
(A) Our study reveals two major roles of Banp in cell-cycle regulation of retinal progenitor cells. In general, DNA replication stress, DSBs, and chromosome segregation defects induce genomic instability, which causes cancer or cell death. In S phase, Banp may suppress DNA replication stress by promoting transcription of the wrnip1, which protects stalled replication forks and promotes replication restart after replication stress. Failure of recovery from DNA replication stress induces DSBs, which activate ATM/Chk2/tp53-mediated DNA damage response. Banp may promote DNA damage repair in concert with tp53. ATR also activates tp53 through activation of Chk1 or crosstalk with ATM. Banp may suppress genotoxic stress-mediated DNA replication stress and DSBs. In M phase, Banp is required for chromosome segregation by promoting transcription of two mitotic regulators, cenpt and ncapg. Since a Banp motif is found near TSSs of the cenpt, ncapg and wrnip1 genes, it is likely that these genes are direct targets of Banp. Dotted arrows indicate proposed regulatory pathway, which will be necessary to investigate in the future. (B) banprw337 mutant phenotypes along the cell-cycle progression and neurogenesis. In banp mutants, stalled DNA replication and subsequent DSBs are accumulated, leading to increase in γ-H2AX+ cells. These DNA damaged cells undergo apoptosis. In M phase, mitosis is markedly prolonged due to chromosome segregation defects; however, almost all mitoses are eventually completed, so it is less likely that M phase defects directly link to apoptosis. It is possible that a fraction of γ-H2AX+ cells survive and enter M phase, so DNA damage is inherited by daughter cells, which reenter the cell-cycle or differentiate into postmitotic neurons. Neuron with a high level of γ-H2AX+ signals may undergo apoptosis. Thus, S phase defects and M phase defects influence each other through cell-cycle progression in banp mutants. Red lines indicate path of γ-H2AX+ cells.
A possible model of Banp functions in stalled fork recovery, DNA damage repair, and chromosome segregation during mitosis.