Small molecules that delay S phase suppress a zebrafish bmyb mutant

Bmyb is a ubiquitously expressed transcription factor involved in cellular proliferation and cancer(1-4). Loss of bmyb function in the zebrafish mutant crash&burn (crb) results in decreased cyclin B1 expression, mitotic arrest and genome instability(5). These phenotypic observations in crb mutants could be attributed to the decreased expression of cyclin B1, a cell-cycle regulatory protein that is responsible for driving cell progression from G2 through mitosis. To identify small molecules that interact with the bmyb pathway, we developed an embryo-based suppressor screening strategy. In 16 weeks we screened a diverse similar to 16,000 compound library, and discovered one previously unknown compound, persynthamide (psy, 1), that suppressed bmyb-dependent mitotic defects. Psytreated embryos showed an S-phase delay, and knockdown of the cell-cycle checkpoint regulator ataxia telangiectasia-and Rad-related kinase (ATR) abrogated the suppression of crb. The DNA synthesis inhibitors aphidicolin (2) and hydroxyurea (3) also suppressed crb. S-phase inhibition upregulated cyclin B1 mRNA, promoting the progression of cells through mitosis. Our study demonstrates that chemical suppressor screening in zebrafish can identify compounds with cell-cycle activity and can be used to identify pathways that interact with specific cell-cycle phenotypes.