Figure 1. TopBP1 deletion in the nervous system.

(a) TopBP1 contains eight BRCA1 C-terminal (BRCT) domains. These encompass an amino terminal TRESLIN/Ticrr interacting domain, a C-terminal ATR-activating, a BACH1-binding region and an ATM phosphorylation site in BRCT domain 5. A conditional TopBP1 allele has LoxP sites flanking exons 3 and 6 and generates an out-of-frame mutation after cre-mediated recombination. (b) TopBP1 protein is absent in P0 TopBP1^Nes-cre brain tissues but is present at normal levels in the thymus and spleen, while Nbs1 and actin levels are similar in all mutant tissues. (c) TopBP1^Nes-cre neonates (asterisks) are initially similar to littermate controls, but become runted by P4. (d) The TopBP1^Nes-cre cerebellum lacks granule neurons resulting in a disorganized Purkinje cell placement as shown at P0 using calbindin staining and at P12 using the neuronal differentiation marker βtubulin III (Tuj1). (e) Quantitative real-time PCR shows TopBP1 deletion occurs by E12.5 in the cortex but not the liver. (f) Western blot analysis of E14.5 TopBP1^Nes-cre tissues shows that TopBP1 is deleted in the cortex but not the liver; p53pSer18 expression is also elevated in TopBP1^Nes-cre tissue. Nbs1 and Ponceau staining are loading and blotting controls, respectively. (g) Apoptosis (arrows) occurs in proliferating cells in the developing cortex at E13.5.